CONCRETE

                                A MATERIAL FOR
                               THE NEW STONE AGE

                                                                  A MAST Module

                                               Materials Science and Technology

                                                                           1995
                               Acknowledgments:

The authors would like to thank the following people for their advice and support in the development of this module:

Dr.  Jennifer Lewis
Director of the Materials Science Workshop

Dr. James Adams
Assistant Director

Dr. Francis Young
Department of Materials Science and Engineering
University of Illinois at Urbana-Champaign, Urbana, IL

Dr. David Lange
Department of Civil Engineering
University of Illinois at Urbana-Champaign, Urbana, IL

Authors:

Beth Chamberlain
Tuscola HS., Tuscola, IL

Newell Chiesl
University of Illinois at Urbana-Champaign, Urbana, IL

Jerald Day
Turkey Run HS., Marshall, IN

Lesa Dowd
Bishop Noil Institute, Hammond, IN

Betty Overocker
Antioch HS., Antioch, IL

Denise Pape
Harlem HS.,  Machesney Park, IL

Marcia Petrus
Danville HS., Danville, IL

Mary Swanson
University of Illinois at Urbana-Champaign, Urbana, IL

John Toles
Sycamore HS., Sycamore, IL
                                   Foreword

This is one in a series of MAST modules developed and revised during the Materials Technology Workshop held at the University of Illinois at Urbana-Champaign during 1993-95.

A combination of university professors, high school science teachers, and undergraduate students, came together to create and revise this module over a three year period.

This module is to be used as a curriculum aide by high school science teachers who would like to introduce their students to concepts of Materials Science and Technology.  Teachers are urged to use one, some, or all of the MAST modules.  Some teachers may wish to implement this module in its entirety as a subject unit in a course.  Others may wish to utilize only part of the module, perhaps a laboratory experiment.  We encourage teachers to use these materials in their classrooms and to contact the workshop with any assessments, comments, or suggestions they may have.

Financial support for the Materials Technology Workshop was provided by the National Science Foundation (NSF) Education and Human Resource Directorate (Grant #ESI 92-53386), the NSF Center for Advanced Cement Based Materials, the Dow Chemical Foundation, the Materials Research Society, the Iron and Steel Society, and the Peoria Chapter of the American Society for Metals.  The University of Illinois at Urbana-Champaign Department of Materials Science and Engineering and the College of Engineering and the College of Engineering Office of Extramural Education provided organizational support.

                               Table of Contents

Acknowledgments....................................................................................	ii
Foreword...............................................................................................	iii
Introduction............................................................................................	1
A Short Story:  A Hard Lesson to Learn...........................................................	2
What is Concrete?.....................................................................................	3
The History of Concrete: Pictorial Timeline.......................................................	9
The History of Concrete: Textual Timeline........................................................	11
Scientific Principles...................................................................................	15
References.............................................................................................	25
Resources..............................................................................................	26
Master Materials and Equipment Grid..............................................................	27
Demonstrations
	1: Making a Silt Test.........................................................................	29
	2: Conducting an Organic Matter Test.....................................................	30
	3: Effect of Aggregate on Workability of Concrete......................................	31
	4: It's Heating Up...........................................................................	32
	5: pH of Cement.............................................................................	33
Laboratory Activities
	The Basic Mix................................................................................	34
	Lab 1:  What's the Matter?..................................................................	36
	Lab 2:  How Dense Is It?....................................................................	40
	Lab 3:  Hot and Cold pHun!................................................................	44
	Lab 4:  The Fleet Afloat!....................................................................	49
	Lab 5:  Stress and Strain!...................................................................	55
	Lab 6:  Make and Take!.....................................................................	63
Assessments...........................................................................................	66
Glossary................................................................................................	74

                                 Introduction

Module Objective:

Students will learn about cement hydration, material properties, and making concrete.

Key Concepts:

	::  Importance and widespread uses of concrete.
	::  Component materials used to make concrete.
	::  Macroscopic and microscopic structure of concrete.
	::  Role of water in the preparation of a concrete mixture.
	::  Role of water in the hardening of concrete.
	::  Effective ratios of component materials in various concrete structures.
	::  Effect of porosity and aggregates on the strength of concrete.
	::  Building and testing various concrete structures.
	::  Derivation of an optimum ratio of component materials for different concrete
	applications.
	::  Chemical reactions which take place when concrete is mixed.

Prerequisites:

It is assumed that students will have studied the following concepts prior to beginning this module:
	::  Calculation of ratios and using fractions
	::  Using a laboratory balance to measure mass
	::  Using a graduated cylinder to measure volume
	::  Making and interpreting graphs

Placement of Module in Current Curriculum:

The material covered in the concrete module  may be adapted for use in classes of general science,  chemistry,  and physics. The basic concepts which may be taught or enhanced by using this module include:
	::  Matter classification
	::  Density
	::  Compression and tension
	::  Hydration
	::  Heats of reaction
	::  pH

The materials and activities in this module are intended to introduce the student to the topic of concrete.  Further lecture, audiovisual, reading, speakers, field trips, worksheets, and evaluation instruments could be used to support and expand upon the materials provided.  An excellent activity would involve a field trip to a local ready-mix concrete plant or a cement manufacturing plant.  This would give students the opportunity to not only see how concrete is produced but also the many applications of concrete.   There is a list of professional concrete organizations in the Resources section which can provide you with further information.

                            A Hard Lesson to Learn
                                 A Short Story
                              By Betty Overocker

	"Hey you!  Let's go for a walk."  said a gruff voice behind me as I sat on a concrete park bench.  Without question, I stood up and walked along the concrete sidewalk in the direction the dark suited man pointed to.  The heat of the day was intense as it radiated off the concrete building that lined the concrete street we were walking beside.  I paused momentarily to lean against a concrete lamp post and concentrate my slurry of thoughts into a more rigid mass.  The men in dark suits continued to move me along this set journey.
	At  a large concrete archway, the men told me to enter.  The entrance was very steep, and contained two concrete statues of vicious looking dogs guarding the way.  The door opened into a large room. The walls were made of concrete blocks arranged in an off-set pattern.  My mind tried to trace an escape route in  the mortar trails between the bricks, but I kept running into dead ends much like the situation I was in, there were no concrete answers.
	The room was arranged in a court hall formation consisting of massive poured concrete chairs and benches.  A man of questionable character sat in the front of the room, in the largest of the concrete thrones.  The men in dark suits motioned for me to approach the front.  Being tired from the long walk, I leaned against the concrete pillars that outlined the path that I least wanted to travel.
	I approached the domineering godfather. He told me to place my legs into a cylindrical container that came up to my knees.
	The two goons that had been watching me headed to a concrete box nearby.  One of the goons carried a bag of pre-mix concrete.  The other, a container of water.  As they began to mix these ingredients, I began to realize what was about to occur.  I broke the code of silence and asked the godfather if I could have one last request.  He nodded.  I surveyed the scene.  Knowing that this long look of the tall concrete skyscrapers may be one of my last, I asked for a large soda and a large cotton candy.  Not bad, I thought for a diabetic on his supposed last binge.
	As my last requests arrived, the goon's transferred the contents of the concrete mix into the bucket.  I could feel the weight of the wet mass entering my shoes.  I squirmed just as the deliverer was handing me the food.  In the shuffle, the large soda slipped out of his hand and spilled into the bucket in which I was standing.  The cotton candy also was lost to the mess on my feet.  The dom thought nothing of the new additives in the mixture.  I, on the other hand, was pleased that my plan had worked.
	What followed was the usual take-and-get-rid-of-the-guy routine.  The goons were not too bright on the ways of concrete.  They just followed directions.  As for me, the training I received in a high school module on concrete had taught me all about the effects of admixtures on the curing processes of concrete.  The sugar in the candy and the soda would prevent the concrete from setting.  As the assistants carried me and my "hardened" boots to the water's edge, I hoped that all that modular information was accurate because my life now depended on it.  The buckets and I were set on a dolly for ease of movement.  The dolly was rolled to the edge of the drop off and I was released into the water.
	Due to the retardation effect of sugar on the setting of concrete, I managed to wiggle my legs out of the fresh concrete anchor and rise to the surface.  No one was in sight and I decided to learn more about other industrial materials so I could save my life again some other time.
                               What is Concrete?

Brain Storming Activity  1:  Concrete Survey

      1.  When was concrete first made?

      	9000 BC     500 BC    100 AD    1756    1824

      2.  Circle the possible components of concrete.

      	water      cement     gravel     sand     air    steel rods

      3.  What is the purpose of cement in concrete?

      4.  What role does water play in producing concrete?

      5.  Why does concrete harden?

      6.  Why does concrete set (harden) slowly?

      7.  How can you make concrete set:
           faster?
           slower?

      8.  Is concrete stronger in compression, tension, or the same in either?

      9.  How strong can concrete or cement be (in pounds per square inch (psi))?

      	50,000      20,000     5000     2000

      10. How long can concrete last (in years)?

      	50,000     5000     500     50

scores:	8-10 materials science major;    5-7 concrete contractor;    2-4  concrete laborer;
	0-1  home owner

Concrete Survey  (Key)

      1.  When was concrete first made?
      	9000 BC     500 BC  100 AD    1756    1824

      2.  Circle the possible components of concrete.
      	 water      cement     gravel     sand     air

      3.  What is the purpose of cement in concrete?
      	It acts as a primary binder to join the aggregate into a solid mass.

      4.  What role does water play in producing concrete?
      	Water is required for the cement to hydrate and solidify.

      5.  Why does concrete harden?
      	The chemical process called cement hydration produces crystals that 	interlock and bind together.

      6.  Why does concrete set (harden) slowly?
      	It takes time for the hydrated cement crystals to form

      7.  How can you make concrete set:
      	faster?  add calcium chloride or "accelerator"
      	slower? add sugar or "set retarder"

      8.  Is concrete stronger in compression, tension, or the same in either?
      	It is stronger in compression.

      9.  How strong can concrete or cement be (in pounds per square inch (psi))?
      	50,000      20,000     5000     2000

      10. How long can concrete last (in years)?
      	50,000    5000     500     50

scores:	8-10 materials science major;    5-7 concrete contractor;    2-4  concrete laborer;
	0-1  home owner

(Note:	Correct answers are given in bold.)
Concrete-	An artificial stone-like material used for various structural purposes.  It is made by
            mixing cement and various aggregates, such as sand, pebbles, gravel, shale, etc., with water and allowing the mixture to harden by hydration.

Here are just a few facts to help convince you that the topic of concrete deserves to become a part of your science curriculum:

	:: Concrete is  everywhere!!  Roads, sidewalks, houses, bridges, skyscrapers, pipes, dams, 		canals, missile silos, and nuclear waste containment.  There are even concrete canoes and
	Frisbee competitions.

	:: It is strong, inexpensive, plentiful, and easy to make.  But more importantly, it's versatile.  It 		can be molded to just about any shape.

	:: Concrete is friendly to the environment.  It's virtually all natural.  It's recyclable.

	:: It is the most frequently  used material in construction.

	:: Slightly more than a ton of concrete is produced every year for each person on the planet, 		approximately 6 billion tons per year.

	:: By weight, one-half to two-thirds of our infrastructures are made of concrete such as:  roads, 		bridges, buildings, airports, sewers, canals, dams, and subways.

	:: Approximately  60% of our concrete highways need repair and 40% of our concrete highway 		bridges are structurally deficient or functionally obsolete.

	:: Large cities lose up to 30% of their daily water supply due to leaks in concrete water pipes.

	:: It has been estimated that the necessary repairs and improvements to our infrastructures will 		cost $3.3 trillion over a nineteen-year period.  $1 trillion of that is needed for repairing
		the nation's concrete.

	:: Cement has been around for at least 12 million years and has played an important role in 		history.
Brainstorming Activity 2:  Why is Concrete Important?

Objective:  	Students will create a list of the importance of concrete and explain how it affects 	their lives.

Procedure: 	1.	"Why concrete is important?"  In a large group students will create a list of the 		importance of studying concrete.

	2.	Upon completion of their list,  students will  develop acronyms for concrete 		based on their list of concrete's importance.  (See example below.)

	3.	Students will discuss the implications that would occur if we could no longer 		make concrete. (i.e. increasing levels of  CO2 production or federal regulations)

Brainstorming Activity 3:  Applications of Concrete

Objective:	Students will create a list of the past, present, and future applications of concrete 	and how these applications affect their lives and lifestyles.

Procedure:

In small groups, the students will list applications for concrete:

 	1.	In the past:
		Students will create a list of past applications for concrete that has influenced 			their lives and/or lifestyles.

	2.	Currently:
		Students will describe common applications of concrete that they 				encounter daily.  Label these as present applications of concrete.

	3.	In the future:
		Students will create a list of future applications of concrete by predicting 			how concrete will affect their lives in the future.

	4.	Students will present their lists to the class in the form of a collage or a mobile 			displaying the correlation between their lives and lifestyles with the applications of 		concrete throughout their lives.

APPLICATIONS OF CONCRETE
Past, Present, and Future

roads	sidewalks	houses
bricks/blocks	bridges	walls
beams	foundations	floors
sewer pipes	water mains	computer chip backing **
canals	missile silos	containment of nuclear waste
dams	churches	automobile brake lining **
caskets	monuments	solidification of hazardous wastes
tombs	indoor furniture	garden ornaments
swimming pools	airport runways	sailing boats
canoes	barges	subways
tunnels	parking garages	patio bricks
holding tanks	cement "overshoes"	sculptures
flower pots & planters	chimneys	mantels
ballast	bath tubs	grave vaults
bank vaults	basements   	lamp posts
telephone poles	electric light poles	Frisbees
headstones	steps	fence posts
business/credit cards **	fertilizer	bone replacement  **
insulating tiles/bricks	corn silos	park benches
parking stones	roof tiles	water troughs
water tanks	curb & gutters	nuclear reactor containment structures
artificial rocks	office buildings	parking lots
railroad ties	airports	monorails
picnic tables	swimming pools	break waters
wharves & piers	bird baths	barbecue pits
stadium seats	fountains	lunar bases  **

**  Denotes future applications.

                           The History of Concrete:
                                  A Timeline

Cement has been around for at least 12 million years.  When the earth itself was undergoing intense geologic changes natural, cement was being created.  It was this natural cement that humans first put to use.  Eventually, they discovered how to make cement from other materials.

12,000,000 BC	Reactions between limestone and oil shale during spontaneous
			combustion occurred in Israel to form a natural deposit of cement
			compounds.  The deposits were characterized by Israeli geologists in the
			1960's and 70's.

3000 BC		Used mud mixed with straw to bind dried bricks.  They also used gypsum
Egyptians		mortars and mortars of lime in the pyramids.

Chinese		Used cementitious materials to hold bamboo together in their boats and in
			the Great Wall.

800 BC		Used lime mortars which were much harder than later Roman mortars.
Greeks, Crete
& Cyprus

300 BC		Used bitumen to bind stones and bricks.
Babylonians
& As Syrians

300 BC - 476 AD	Used pozzolana cement from Pozzuoli, Italy near Mt. Vesuvius to build the
Romans		Appian Way, Roman baths, the Coliseum and Pantheon in Rome, and
			the Pont du Gard aqueduct in south France.  They used lime as a
			cementitious material.  Pliny reported a mortar mixture of 1 part lime to 4
			parts sand.  Vitruvius reported a 2 parts pozzolana to 1 part lime.  Animal
			fat, milk, and blood were used as admixtures (substances added to cement
			to increase the properties.) These structures still exist today!

1200 - 1500		The quality of cementing materials deteriorated.  The use of burning lime
The Middle Ages	and pozzolan (admixture) was lost, but reintroduced in the 1300's.

1678			Joseph Moxon wrote about a hidden fire in heated lime that appears upon
			the addition of water.

1779			Bry Higgins was issued a patent for hydraulic cement (stucco) for exterior
			plastering use.

1780			Bry Higgins published "Experiments and Observations Made With the
			View of Improving the Art of Composing and Applying Calcereous
			Cements and of Preparing Quicklime."

1793 			John Smeaton found that the calcination of limestone containing clay gave
			a lime which hardened under water (hydraulic lime).  He used hydraulic
			lime to rebuild Eddystone Lighthouse in Cornwall, England which he had
			been commissioned to build in 1756, but had to first invent a material that
			would not be affected by water.  He wrote a book about his work.

1796			James Parker from England patented a natural hydraulic cement by
			calcining nodules of impure limestone containing clay, called Parker's
			Cement or Roman Cement.

1802			In France, a similar Roman Cement process was used.

1810			Edgar Dobbs received a patent for hydraulic mortars, stucco, and plaster,
			although they were of poor quality due to lack of kiln precautions.

1812 -1813		Louis Vicat of France prepared artificial hydraulic lime by calcining 			synthetic mixtures of limestone and clay.

1818			Maurice St. Leger was issued patents for hydraulic cement.
			Natural Cement was produced in the USA.  Natural cement is limestone			that naturally has the appropriate amounts of clay to make the same type of 			concrete as John Smeaton discovered.

1820 - 1821		John Tickell and Abraham Chambers were issued more
			hydraulic cement patents.

1822			James Frost of England prepared artificial hydraulic lime like Vicat's and
			called it British Cement.

1824			Joseph Aspdin of England invented portland cement by burning finely
			ground chalk with finely divided clay in a lime kiln until carbon dioxide
			was driven off.  The sintered product was then ground and he called it
			portland cement named after the high quality building stones quarried at
			Portland, England.

1828			I. K. Brunel is credited with the first engineering application of
			portland cement, which was used to fill a breach in the Thames Tunnel.

1830			The first production of lime and hydraulic cement took place in Canada.

1836			The first systematic tests of tensile and compressive strength took place in
			Germany.

1843			J. M. Mauder, Son & Co. were licensed to produce patented portland
			cement.

1845			Isaac Johnson claims to have burned the raw materials of portland cement
			to clinkering temperatures.

1849			Pettenkofer & Fuches performed the first accurate chemical analysis of
			portland cement.

1860			The beginning of the era of portland cements of modern composition.

1862			Blake Stonebreaker of England introduced the jaw breakers to crush
			clinkers.

1867			Joseph Monier of France reinforced William Wand's (USA) flower pots
			with wire ushering in the idea of iron reinforcing bars (re-bar).

1871			David Saylor was issued the first American patent for portland cement.  He
			showed the importance of true clinkering.

1880			J. Grant of England show the importance of using the hardest and densest 			portions of the clinker.  Key ingredients were being chemically analyzed.

1886			The first rotary kiln was introduced in England to replace the vertical shaft
			kilns.

1887			Henri Le Chatelier of France established oxide ratios to prepare the
			proper amount of lime to produce portland cement.   He named the
			components:  Alite (tricalcium silicate), Belite (dicalcium silicate), and
			Celite (tetracalcium aluminoferrite).  He proposed that hardening is caused 			by the formation of crystalline products of the reaction between cement and
			water.

1889			The first concrete reinforced bridge is built.

1890			The addition of gypsum when grinding clinker to act as a retardant to the 			setting of concrete was introduced in the USA.  Vertical shaft kilns were 			replaced with rotary kilns and ball mills were used for grinding cement.

1891			George Bartholomew placed the first concrete street in the USA in
			Bellefontaine, OH.  It  still exists today!

1893			William Michaelis claimed that hydrated metasilicates form a gelatinous
			mass (gel) that dehydrates over time to harden.

1900			Basic cement tests were standardized.

1903			The first concrete high rise was built in Cincinnati, OH.

1908			Thomas Edison built cheap, cozy concrete houses in Union, NJ.
 			They still exist today!

1909			Thomas Edison was issued a patent for rotary kilns.

1929			Dr. Linus Pauling of the USA formulated a set of principles for the
			structures of complex silicates.

1930			Air entraining agents were introduced to improve concrete's resistance to 			freeze/thaw damage.

1936			The first major concrete dams, Hoover Dam and Grand Coulee Dam, were			built.  They still exist today!

1956			U.S. Congress annexed the Federal Interstate Highway Act.

1967			First concrete domed sport  structure, the Assembly Hall, was constructed 			at The University of Illinois, at Urbana-Champaign.

1970's		Fiber reinforcement in concrete was  introduced.

1975			CN Tower in Toronto, Canada, the tallest slip-form building, was  				constructed.

			Water Tower Place in Chicago, Illinois, the tallest building was constructed.
1980's		Superplasticizers were introduced as admixtures.

1985			Silica fume was introduced as a pozzolanic additive.

			The "highest strength" concrete was used in building the Union
			 Plaza constructed in Seattle, Washington.

1992			The tallest reinforced concrete building in the world was constructed at
			 311 S. Wacker Dr., Chicago, Illinois.
                             Scientific Principles

What is in This Stuff?

The importance of concrete in modern society cannot be underestimated.  Look around you and you will find concrete structures everywhere such as buildings, roads, bridges, and dams.  There is no escaping the impact concrete makes on your everyday life.  So what is it?

Concrete is a composite material which is made up of a filler and a binder.  The binder (cement paste) "glues" the filler together to form a synthetic conglomerate.  The constituents used for the binder are cement and water, while the filler can be fine or coarse aggregate.  The role of these constituents will be discussed in this section.

Cement, as it is commonly known, is a mixture of compounds made by burning limestone and clay together at very high temperatures ranging from 1400 to 1600 ˚C.

Although there are other cements for special purposes, this module will focus solely on portland cement and its properties. The production of portland cement begins with the quarrying of limestone, CaCO3.  Huge crushers break the blasted limestone into small pieces.  The crushed limestone is then mixed with clay (or shale), sand, and iron ore and ground together to form a homogeneous powder.  However, this powder is microscopically heterogeneous.  (See flowchart.)

Figure 1:	A flow diagram of Portland Cement production.
The mixture is heated in kilns that are long rotating steel cylinders on an incline.  The kilns may be up to 6 meters in diameter and 180 meters in length.  The mixture of raw materials enters at the high end of the cylinder and slowly moves along the length of the kiln due to the constant rotation and inclination.  At the low end of the kiln, a fuel is injected and burned, thus providing the heat necessary to make the materials react.  It can take up to 2 hours for the mixture to pass through the kiln, depending upon the length of the cylinder.

Figure 2:	Schematic diagram of rotary kiln.

As the mixture moves down the cylinder, it progresses through four stages of transformation.  Initially, any free water in the powder is lost by evaporation.  Next, decomposition occurs from the loss of bound water and carbon dioxide.  This is called calcination.  The third stage is called clinkering.  During this stage, the calcium silicates are formed.  The final stage is the cooling stage.

The marble-sized pieces produced by the kiln are referred to as clinker.  Clinker is actually a mixture of four compounds which will be discussed later.  The clinker is cooled, ground, and mixed with a small amount of gypsum (which regulates setting) to produce the general-purpose portland cement.

Water is the key ingredient, which when mixed with cement, forms a paste that binds the
aggregate together.  The water causes the hardening of concrete through a process called hydration.  Hydration is a chemical reaction in which the major compounds in cement form chemical bonds with water molecules and become hydrates or hydration products.  Details of the hydration process are explored in the next section.  The water needs to be pure in order to prevent side reactions from occurring which may weaken the concrete or otherwise interfere with the hydration process.  The role of water is important because the water to cement ratio is the most critical factor in the production of "perfect" concrete.  Too much water reduces concrete strength, while too little will make the concrete unworkable.  Concrete needs to be workable so that it may be consolidated and shaped into different forms (i.e.. walls, domes, etc.).  Because concrete must be both strong and workable, a careful balance of the cement to water ratio is required when making concrete.

Aggregates are chemically inert, solid bodies held together by the cement.  Aggregates come in various shapes, sizes, and materials ranging from  fine particles of sand to large, coarse rocks.  Because cement is the most expensive ingredient in making concrete, it is desirable to minimize the amount of cement used.  70 to 80% of the volume of concrete is aggregate keeping the cost of the concrete low.   The selection of an aggregate is determined, in part, by the desired characteristics of the concrete.  For example, the density of concrete is determined by the density of the aggregate.  Soft, porous aggregates can result in weak concrete with low wear resistance, while using hard aggregates can make strong concrete with a high resistance to abrasion.

Aggregates should be clean, hard, and strong.  The aggregate is usually washed to remove any dust, silt, clay, organic matter, or other impurities that would interfere with the bonding reaction with the cement paste.  It is then separated into various sizes by passing the material through a series of screens with different size openings.

Refer to Demonstration 1

Table 1:	Classes of Aggregates

		class			examples of aggregates used			uses

	ultra-lightweight		vermiculite				lightweight concrete which
					ceramic spheres			can be sawed or nailed, also
					perlite					for its insulating properties

	lightweight		expanded clay				used primarily for making					shale or slate				lightweight concrete for 					crushed brick				structures, also used for its 										insulating properties.

	normal weight		crushed limestone			used for normal concrete
					sand					projects
					river gravel
					crushed recycled concrete

	heavyweight		steel or iron shot			used for making high density
					steel or iron pellets 			concrete for shielding against
										nuclear radiation

Refer to Demonstration 2

The choice of aggregate is determined by the proposed use of the concrete.  Normally sand, gravel, and crushed stone are used as aggregates to make concrete.  The aggregate should be well-graded to improve packing efficiency and minimize the amount of cement paste needed.  Also, this makes the concrete more workable.

Refer to Demonstration 3

Properties of Concrete

Concrete has many properties that make it a popular construction material.  The correct proportion of ingredients, placement, and curing are needed in order for these properties to be optimal.

Good-quality concrete has many advantages that add to its popularity.  First, it is economical when ingredients are readily available.  Concrete's long life and relatively low maintenance requirements increase its economic benefits.  Concrete is not as likely to rot, corrode, or decay as other building materials.  Concrete has the ability to be molded or cast into almost any desired shape.  Building of the molds and casting can occur on the work-site which reduces costs.

Concrete is a non-combustible material which makes it fire-safe and able withstand high temperatures.  It is resistant to wind, water, rodents, and insects.  Hence, concrete is often used for storm shelters.

Concrete does have some limitations despite its numerous advantages.  Concrete has a relatively low tensile strength (compared to other building materials), low ductility, low strength-to-weight ratio, and is susceptible to cracking.  Concrete remains the material of choice for many applications regardless of  these limitations.

Hydration of Portland Cement

Concrete is prepared by mixing cement, water, and aggregate together to make a workable paste.  It is molded or placed as desired, consolidated, and then left to harden.  Concrete  does not need to dry out in order to harden as commonly thought.

The concrete (or specifically, the cement in it) needs moisture to hydrate and cure (harden).  When concrete dries, it actually stops getting stronger.  Concrete with too little water may be dry but is not fully reacted.  The properties of such a concrete would be less than that of a wet concrete.  The reaction of water with the cement in concrete is extremely important to its properties and reactions may continue for many years.  This very important reaction will be discussed in detail in this section.

Portland cement consists of five major compounds and a few minor compounds.  The composition of a typical portland cement is listed by weight percentage in Table 2.

	Cement Compound	       Weight Percentage	 Chemical Formula
	Tricalcium silicate			      50 %		Ca3SiO5  or 3CaO.SiO2
	Dicalcium silicate			      25 %		Ca2SiO4 or 2CaO.SiO2
	Tricalcium aluminate			      10 %		Ca3Al2O6 or 3CaO .Al2O3
	Tetracalcium aluminoferrite	      	      10 %		Ca4Al2Fe2O10 or
									     4CaO.Al2O3.Fe2O3
	Gypsum				        5 %		CaSO4.2H2O

Table 2:	Composition of portland cement with chemical composition and weight percent.

When water is added to cement, each of the compounds undergoes hydration and contributes to the final concrete product.  Only the calcium silicates contribute to strength.  Tricalcium silicate is responsible for most of the early strength (first 7 days).  Dicalcium silicate, which reacts more slowly, contributes only to the strength at later times.  Tricalcium silicate will be discussed in the greatest detail.

The equation for the hydration of tricalcium silicate is given by:

Tricalcium silicate + Water--->Calcium silicate hydrate+Calcium hydroxide + heat
 2 Ca3SiO5   +   7 H2O ---> 3 CaO.2SiO2.4H2O     +    3 Ca(OH)2  +  173.6kJ

Upon the addition of water, tricalcium silicate rapidly reacts to release calcium ions, hydroxide ions, and a large amount of heat.  The pH quickly rises to over 12 because of the release of alkaline hydroxide (OH-) ions.  This initial hydrolysis slows down quickly after it starts resulting in a decrease in heat evolved.

The reaction slowly continues producing calcium and hydroxide ions until the system becomes saturated.  Once this occurs, the calcium hydroxide starts to crystallize.  Simultaneously, calcium silicate hydrate begins to form.  Ions precipitate out of solution accelerating the reaction of tricalcium silicate  to calcium and hydroxide ions.  (Le Chatlier's principle).  The evolution of heat is then dramatically increased.

The formation of the calcium hydroxide and calcium silicate hydrate crystals provide "seeds" upon which more calcium silicate hydrate can form.  The calcium silicate hydrate crystals grow thicker making it more difficult for water molecules to reach the unhydrated tricalcium silicate.  The speed of the reaction is now controlled by the rate at which water molecules diffuse through the calcium silicate hydrate coating.  This coating thickens over time causing the production of calcium silicate hydrate to become slower and slower.

Figure 3:	Schematic illustration of the pores in calcium silicate through different stages of 		hydration.

The above diagrams represent the formation of pores as calcium silicate hydrate is formed.  Note in diagram (a) that hydration has not yet occurred and the pores (empty spaces between grains) are filled with water.  Diagram (b) represents the beginning of hydration.  In diagram (c), the hydration continues.  Although empty spaces still exist, they are filled with water and calcium hydroxide.  Diagram (d) shows nearly hardened cement paste.  Note that the majority of space is filled with calcium silicate hydrate.  That which is not filled with the hardened hydrate is primarily calcium hydroxide solution.  The hydration will continue as long as water is present and there are still unhydrated compounds in the cement paste.

Dicalcium silicate also affects the strength of concrete through its hydration.  Dicalcium silicate reacts with water in a similar manner compared to tricalcium silicate,  but much more slowly.  The heat released is less than that by the hydration of tricalcium silicate because the dicalcium silicate is much less reactive.  The products from the hydration of dicalcium silicate are the same as those for tricalcium silicate:

Dicalcium silicate + Water--->Calcium silicate hydrate + Calcium hydroxide +heat
2 Ca2SiO4    +  5 H2O--->   3 CaO.2SiO2.4H2O      +     Ca(OH)2   + 58.6 kJ

The other major components of portland cement, tricalcium aluminate and tetracalcium aluminoferrite also react with water.  Their hydration chemistry is more complicated as they involve reactions with the gypsum as well.  Because these reactions do not contribute significantly to strength, they will be neglected in this discussion.  Although we have treated the hydration of each cement compound independently, this is not completely accurate.  The rate of hydration of a compound may be affected by varying the concentration of another.  In general, the rates of hydration during the first few days ranked from fastest to slowest are:
tricalcium aluminate > tricalcium silicate > tetracalcium aluminoferrite > dicalcium silicate.

Refer to Demonstration 4

Heat is evolved with cement hydration.  This is due to the breaking and making of chemical bonds during hydration.  The heat generated is shown below as a function of time.

Figure 4:	Rate of heat evolution during the hydration of portland cement

The stage I hydrolysis of the cement compounds occurs rapidly with a temperature increase of several degrees.  Stage II is known as the dormancy period. The evolution of heat slows dramatically in this stage.  The dormancy period can last from one to three hours.  During this period, the concrete is in a plastic state which allows the concrete to be transported and placed without any major difficulty.  This is particularly important for the construction trade who must transport concrete to the job site.  It is at the end of this stage that initial setting begins.  In stages III and IV, the concrete starts to harden and the heat evolution increases due primarily to the hydration of tricalcium silicate.  Stage V is reached after 36 hours.  The slow formation of hydrate products occurs and continues as long as water and unhydrated silicates are present.

Refer to Demonstration 5

Strength of Concrete

The strength of concrete is very much dependent upon the hydration reaction just discussed.  Water plays a critical role, particularly the amount used.  The strength of concrete increases when less water is used to make concrete.  The hydration reaction itself consumes a specific amount of water.  Concrete is actually mixed with more water than is needed for the hydration reactions.  This extra water is added to give concrete sufficient workability.  Flowing concrete is desired to achieve proper filling and composition of the forms.  The water not consumed in the hydration reaction will remain in the microstructure pore space.  These pores make the concrete weaker due to the lack of strength-forming calcium silicate hydrate bonds.  Some pores will remain no matter how well the concrete has been compacted.

Figure 5:	Schematic drawings to demonstrate the relationship between the water/cement ratio 		and porosity.

The empty space (porosity) is determined by the water to cement ratio.  The relationship between the water to cement ratio and strength is shown in the graph that follows.

Figure 6:	A plot of concrete strength as a function of the water to cement ratio.

Low water to cement  ratio leads to high strength but low workability.  High water to cement ratio leads to low strength, but good workability.

The physical characteristics of aggregates are shape, texture, and  size.  These can indirectly affect strength because they affect the workability of the concrete.  If the aggregate makes the concrete unworkable, the contractor is likely to add more water which will weaken the concrete by   increasing the water to cement mass ratio.

Time is also an important factor in determining concrete strength.  Concrete hardens as time passes.  Why?  Remember the hydration reactions get slower and slower as the tricalcium silicate hydrate forms.  It takes a great deal of time (even years!) for all of the bonds to form which determine concrete's strength.  It is common to use a 28-day test to determine the relative strength of concrete.

Concrete's strength may also be affected by the addition of admixtures.  Admixtures are substances other than the key ingredients or reinforcements which are added during the mixing process.   Some admixtures add fluidity to concrete while requiring less water to be used. An example of an admixture which affects strength is superplasticizer.  This makes concrete more workable or fluid without adding excess water.  A list of some other admixtures and their functions is given below.  Note that not all admixtures increase concrete strength.  The selection and use of an admixture are based on the need of the concrete user.

SOME ADMIXTURES AND FUNCTIONS

		TYPE						FUNCTION
	AIR ENTRAINING				improves durability, workability, reduces 							bleeding, reduces freezing/thawing 							problems
							(e.g. special detergents)

	SUPERPLASTICIZERS			increase strength by decreasing water needed 							for workable concrete
							(e.g. special polymers)

	RETARDING				delays setting time, more long term strength, 							offsets adverse high temp. weather
							(e.g. sugar )

	ACCELERATING				speeds setting time, more early strength, 							offsets adverse low temp. weather
							(e.g. calcium chloride)

	MINERAL ADMIXTURES			improves workability, plasticity, strength
							(e.g. fly ash)

	PIGMENT					adds color
							(e.g. metal oxides)

Table 3:	A table of admixtures and their functions.

Durability is a very important concern in using concrete for a given application.  Concrete provides good performance through the service life of the structure when concrete is mixed properly and care is taken in curing it.  Good concrete can have an infinite life span under the right conditions.  Water, although important for concrete hydration and hardening, can also play a role in decreased durability once the structure is built.  This is because water can transport harmful chemicals to the interior of the concrete leading to various forms of deterioration.  Such deterioration ultimately adds costs due to maintenance and repair of the concrete structure.  The contractor should be able to account for environmental factors and produce a durable concrete structure if these factors are considered when building concrete structures.

                               Concrete Summary

Concrete is everywhere.  Take a moment and think about all the concrete encounters you have had in the last 24 hours.  All of these concrete structures are created from a mixture of cement and water with added aggregate.  It is important to distinguish between cement and concrete as they are not the same.  Cement is used to make concrete!

                 (cement  +  water)  +  aggregate =   concrete

Cement is made by combining a mixture of limestone and clay in a kiln at 1450˚ C.  The product is an intimate mixture of compounds collectively called clinker.  This clinker is  finely ground into the powder form.  The raw materials used to make cement are compounds containing some of the earth's most abundant elements, such as calcium, silicon, aluminum, oxygen, and iron.

Water  is a key reactant in cement hydration.  The incorporation of water into a substance is known as hydration.  Water and cement initially form a cement paste that begins to react and harden (set). This paste binds the aggregate particles through the chemical process of hydration.  In the hydration of cement, chemical changes occur slowly, eventually creating new crystalline products, heat evolution, and other measurable signs.

                 cement  +  water  =    hardened cement paste

The properties of this hardened cement paste, called binder, control the properties of the concrete.  It is the inclusion of water (hydration) into the product that causes concrete to set, stiffen, and become hard.  Once set, concrete continues to harden (cure) and become stronger for a long period of time, often up to several years.

The strength of the concrete is related to the water to cement mass ratio and the curing conditions.  A high water to cement mass ratio yields a low strength concrete.  This is due to the increase in porosity (space between particles) that is created with the hydration process.   Most concrete is made with a water to cement mass ratio ranging from 0.35 to 0.6.

Aggregate is the solid particles that are bound together by the cement paste to create the synthetic rock known as concrete.  Aggregates can be fine, such as sand, or coarse, such as gravel.  The relative amounts of each type and the sizes of each type of aggregate determines the physical properties of the concrete.

                         sand + cement paste =  mortar

                        mortar  +  gravel =   concrete

Sometimes other materials are incorporated into the batch of concrete to create specific characteristics.  These additives are called admixtures.  Admixtures are used to:   alter the fluidity (plasticity) of the cement paste;  increase (accelerate) or decrease (retard) the setting time;  increase strength (both bending and compression); or to extend the life of a structure.  The making of concrete is a very complex process involving both chemical and physical changes.  It is a material of great importance in our lives.
                                  References

Abercrombie, S.  Ferrocement:  Building with Cement, Sand, and Wire Mesh. Schocken Books,
	NY, 1977.

Bye, G. C.  Portland Cement:  Composition, Production and Properties. Pergamon Press, NY,
	1983.

Hewlett, P. C., and Young, J. F.  "Physico-Chemical Interactions Between Chemical Admixtures
	and Portland Cement,"  Journal of Materials Education.  Vol. 9, No. 4, 1987.

Introduction to Concrete Masonry.  Instructor's Edition, Associated General Contractors of 	America, Washington D.C., Oklahoma State Department of Vocational and 	Technical Ed., Stillwater, 1988.

Kosmatka, Steven H., and Panarese, William C.  Design and Control of Concrete Mixtures, 	Thirteenth edition, Portland Cement Association, 1988.

Materials Science of Concrete I, II, III.  edited by Jan P. Skalny, American Ceramic Society, Inc,
	Westerville, OH, 1989.

Mindess, S., and Young, J.F.  Concrete. Prentice-Hall, Inc., Englewood Cliffs, NJ, 1981.

Mitchell, L.  Ceramics:  Stone Age to Space Age.  Scholastic Book Services, NY, 1963.

Rixom, M. R., and Mailuaganam, N. P.  Chemical Admixtures for Concrete.  R. & F.N. Spon, 	NY, 1986.

Roy, D.  Instructional Modules in Cement Science.  Pennsylvania State University, PA, 1985.

Sedgwick, J.  "Strong But Sensitive"  The Atlantic Monthly  Vol. 267, No. 4, April 1991,
	pp 70-82.

Weisburd, S.  "Hard Science" Science News  Vol. 134, No. 2, July 9, 1988, pp 24-26.

                                   Resources

Portland Cement Association
5420 Old Orchard Rd.
Skokie, IL  60077
Tel.<PHONE_NUMBER>
Fax<PHONE_NUMBER>

NSF Center for Science and Technology of Advanced Cement-Based Materials
Northwestern University
Evanston, IL  60208-4400
Tel.<PHONE_NUMBER>
Fax<PHONE_NUMBER>

Materials Resources:  cement, sand, and, admixtures - contact your local concrete dealer.  Check the yellow pages.

Admixture sources:		Axim Concrete Technology
				ESSROC Co.
				7230 Northfield Rd.
				Walton Hills, OH
				Tel.<PHONE_NUMBER>
				Fax<PHONE_NUMBER>

				Master Builders
				23700 Chagrin Blvd.
				Cleveland, OH 44122
				Tel.<PHONE_NUMBER>

				W.R. Grace
				62 Whittmore Ave.
				Cambridge, MA 02140-1692
				Tel.<PHONE_NUMBER>

Note:	These companies supply admixtures to your local ready-mix concrete companies.  For 	quantities needed for your labs, it is best to contact a local concrete supplier.

Aberdeen's Concrete Construction is a periodical published monthly.  This would be a good resource for articles on applications and improvements in concrete.  1-800-323-3550 for subscriptions ($24 for a year).  Their address:
							  426 S. Westgate St.
							  Addison, IL  60101
							  Tel.<PHONE_NUMBER>
							  Fax<PHONE_NUMBER>

You may want to contact them requesting complimentary copies or subscriptions.
                     Master Materials and Equipment Grids

Material
                                    Demo 1
                                    Demo 2
                                    Demo 3
                                    Demo 4
                                    Demo 5
sand
                                      HIS
                                      HIS
                                      HIS

kitty litter
                                      DS

glass container and lid (pop bottle)
                                       G
                                       G

3 % NaOH solution

                                      LE

cement

                                      HIS
                                      HIS
                                      HIS
pea gravel

                                      HIS

thermometer

                                      LE

insulated container with lid

                                      DS

drinking straws

                                       G

plastic cup

                                      DS

glass petri dishes

                                      LE
pH hydrion paper or universal indicator

                                      LE
spatula

                                      LE
KEY FOR TABLE:
H = HARDWARE
G = GROCERY
DS = DISCOUNT STORE
HIS = HOME IMPROVEMENT STORE
LE = LAB EQUIPMENT / SCIENTIFIC CATALOG
O = OTHER

Material
                                     Lab 1
                                     Lab 2
                                     Lab 3
                                     Lab 4
                                     Lab 5
                                     Lab 6
baking soda
                                       G

air filled balloon
                                      DS

corn starch
                                       G

flour
                                       G

sulfur
                                      LE

pepper
                                       G

sugar
                                       G

perlite (or Styrofoam beads)
                                       H

                                       H

clay
                                      DS

iron filings
                                      LE

test tube
                                      LE

magnet
                                      LE

magnifying glass
                                      LE

wooden splints
                                      LE

cement
                                      HIS
                                      HIS
                                      HIS
                                      HIS
                                      HIS
                                      HIS
sand
                                      HIS
                                      HIS

                                      HIS
                                      HIS
                                      HIS
gravel
                                      HIS
                                      HIS

                                      HIS
                                      HIS
                                      HIS
banana split dishes

                                       O

trust spacers to make beams

                                      HIS

mixing containers (sm. buckets,
butter bowls, lg. plastic cups)

                                      DS
                                      DS
                                      DS
                                      DS
                                      DS
cylinder molds (paper or plastic
tubes, pipe insulation)

                                       O
                                       O

disposable gloves

                                       G
                                       G
                                       G
                                       G
                                       G
mixing utensils

                                      DS
                                      DS
100 ml graduated cylinder

                                      LE
                                      LE

balance

                                      LE
                                      LE
                                      LE

drying oven

                                      LE

refrigerator

                                      LE

Ziploc bags

                                      DS

pH paper or universal indicator

                                      LE

sealable container for 1 cylinder

                                      DS

petri dish

                                      LE

barge mold

                                       O

spreading utensil

                                      DS

cargo (washers or weights)

                                       H

finishing tools

                                      DS
form for flower pot

                                      DS
8' and 2' 2x4's

                                      HIS

large gate hinge

                                       H

sheet metal

                                       H

copper pipe caps

                                       H

rubber stoppers or washers

                                       H

2 - C-clamps (6'' or larger)

                                       H

PVC pipe

                                      HIS

hydraulic jack

                                    see lab

dowel rod

                                      DS
releasing agents

                                       G
wire mesh

                                       H
Demonstration 1:

Making a Silt Test

Objective:  The purpose of this demonstration is to determine the viability of an aggregate based on a silt test.

Materials and Supplies:

	Sample aggregate (sand and kitty litter work well for comparison)
	Glass container with lid
	Water
	Ruler

Procedure:

	1.	Place 5 cm of aggregate in the container.

	2.	Fill the container with water so the water level is 2 cm above the aggregate.

	3. 	Shake vigorously for 1 minute, making the last few shakes in a swirling 			motion to level off the aggregate.

	4.	It is suggested that this demonstration be done twice, once with sand and 			once with kitty litter to obtain various results.

	5.	Allow the container to stand for an hour, or until the liquid above the 				aggregate is clear.

	6.	The layer that appears above the aggregate is referred to as silt.  Measure the
		silt layer.  If this layer is more than 3 mm thick, the aggregate is not suitable for 		concrete work unless excess silt is removed by washing.

C. Expected Results:

The sand leaves a 3mm layer and therefore is a viable aggregate. Kitty litter, which is clay, leaves a thicker layer and is not a suitable aggregate.

Demonstration 2

Conducting an Organic Matter Test

Objective:  The purpose of this demonstration is to determine the viability of an aggregate based on the amount of organic matter present.

Materials Needed:

	Sand
	A 50:50 mixture of sand and dirt
	Glass container (10 oz. juice jar or similar size) with lid
	A 3% solution of sodium hydroxide NaOH (made by dissolving 9 grams of sodium 		hydroxide, household lye, or caustic soda, in 300 mL of water, preferably 			distilled).

Procedure:

	1.  Fill container with sand to the 150 mL mark.

	2.  Add 120 ml of  3 %  NaOH solution.

	3.  Shake thoroughly for 1 or 2 minutes and allow to stand for 24 hours.

	4.  Repeat the procedure using the sand-dirt mixture.

	5.  Indicate the color of the liquid remaining on the top of the aggregate.

The color of the liquid will indicate whether or not the aggregate contains too much organic matter.  A colorless liquid indicates a clean aggregate, free from organic matter.  A straw-colored solution, not darker than apple-cider vinegar, indicates some organic matter bur not enough to be seriously objectionable.  Darker colors mean that it contains too much organic matter and should not be used unless it is washed and tested again.

Expected Results:

The sand leaves a colorless liquid.  The sand-dirt mixture produces a yellow to orange liquid.
Demonstration 3:

Effect of Aggregate on Workability of Concrete

Objective:  The purpose of this demonstration is to determine the effect of different aggregates on the workability of the resulting concrete.

Materials and Supplies:

	cement					two containers labeled:
	water						"limestone chip aggregate"
	small  limestone chips(pea gravel)	 	"sand  aggregate"
	sand

Procedure:

	1.  Add 1 part of cement and 1/2 part water to each container.  Suggested 				amounts include 50 grams of concrete and 25 ml of water.

	2.  Mix to make the cement paste.

	3.  To the appropriate container, add 3 parts (150g) of limestone chips and mix.

	4.  To the second container, add 3 parts (150 g) of sand and mix.

	5.  Using gloved hands, knead the concrete to determine its workability.

	6.  Which concrete mixture is more workable?  Why?

Expected Results:

The sand aggregate is more workable, because the smaller particles facilitate flow. The larger particles of the gravel hinder it.

Demonstration 4

It's Heating Up!

Objective:  The purpose of this demonstration is to track the temperature changes that occur during the curing process of concrete.

Materials and Supplies:

	fresh cement-- use at least 100 grams for best results
	thermometer
	insulated container with a cover
	drinking straw
	plastic cup to hold cement

Suggestions:

	1.	Use 150 grams of cement and 75 mL of water in a 6 ounce yogurt container or 			other  plastic container.

	2.	Use an insulated 1 quart drinking mug or place the sample in a plastic bottle which is 		set inside a child's thermos. The thermometer is inserted into a one-holed rubber 		stopper which fits the thermos.  Alternatively, the sample bottle can be placed in a box 		filled with Styrofoam.  Another option would be to use a coffee can.  The space inside 		the can could be packed with insulation, and the outside could be wrapped in pipe 		insulation.  A hole can be cut in the  coffee can lid to accommodate the thermometer.

Procedure:

	1.	Fill the container with fresh concrete, using aggregate is not necessary.

	2.	Fold over an inch of the drinking straw and tape closed. Insert thermometer into straw.

	3.	Place the filled container into an insulated container.  Insert the drinking straw housing 		the thermometer into the center of the concrete.  Place the lid securely on the container.

	4.	Record the temperature every 5 minutes for 20 minutes.  Most of the change will occur 		in the first 15 minutes but will continue throughout the whole curing period.

Optional Procedure:

	1.	Repeat experiment using an admixture, such as calcium chloride,  which speeds up 		the process  (Use 2 % of CaCl2 by weight of cement )

	2.	Attach the apparatus to a computer thermocouple that will record the temperature 		changes for the class over a day.

Expected Results:

You should see an increase for 4 hours, most of which is observed within the first 15-20 minutes.  The larger the mass of concrete the higher the temperature rise.  500 grams of concrete should give a rise of about 10˚C if well insulated.  150 grams of cement gave a 4 ˚C change.
Demonstration 5

pH of Cement

Objective:  The purpose of this demonstration is to show the reaction of water with cement and the accompanying change in pH.

Materials and Supplies:

	distilled water
	cement
	petri dish
	overhead
	universal indicator or pH hydrion paper
	spatula

Procedure:

	1. Fill a Petri dish half full of distilled water.

	2.	If using universal indicator, place a drop in the water and mix.  If solution is not 		yellow, add a drop of dilute acid to make it yellow.  If using the pH paper, lay 2 strips 		on the bottom of the dish so the diameter of the dish is covered.

	3. 	Place the dish on the overhead.

	4. 	Into the solution or on top of the pH paper, place a spatula full of cement.

	5. 	Observe the change.

Expected Results:

As the cement mixes with the water, hydroxide ions are formed, thus changing the indicator solution to blue.  Emphasis should be placed on the fact that a chemical reaction is occurring not a dissolving process.
The Basic Mix:

A general teacher's guide for concrete preparation

The physical properties of density and strength of concrete are determined, in part, by the proportions of the three key ingredients, water, cement, and aggregate.  You have your choice of proportioning ingredients by volume or by weight.  Proportioning by volume is less accurate, however due to the time constraints of a class time period this may be the preferred method.

A basic mixture of mortar can be made using the volume proportions of 1 water : 2 cement : 3 sand. Most of the student activities can be conducted using this basic mixture.  Another "old rule of thumb" for mixing concrete is 1 cement : 2 sand : 3 gravel by volume.  Mix the dry ingredients and slowly add water until the concrete is workable.  This mixture may need to be modified depending on the aggregate used to provide a concrete of the right workability.  The mix should not be too stiff or too sloppy.  It is difficult to form good test specimens if it is too stiff.  If it is too sloppy, water may separate (bleed) from the mixture.

Remember that water is the key ingredient.  Too much water results in weak concrete.  Too little water results in a concrete that is unworkable.

Suggestions:

	1.	If predetermined quantities are used, the method used to make concrete is to dry blend 		solids and then slowly add water (with admixtures, if used).

	2.	It is usual to dissolve admixtures in the mix water before adding it to the concrete. 		Superplasticizer is an exception.

	3.	Forms can be made from many materials.  Cylindrical forms can be plastic or paper 		tubes, pipe insulation, cups, etc.  The concrete needs to be easily removed from the 		forms.  Pipe insulation from a hardware store was used for lab trials.  This foam-like 		material was easy to work with and is reusable with the addition of tape.  The bottom of 		the forms can be taped, corked, set on glass plates, etc.  Small plastic weighing trays or 		Dairy Queen banana split dishes can be used as forms for boats or canoes.

	4.	If compression tests are done, it may be of interest to spread universal indicator over 		the broken face and note any color changes from inside to outside.  You may see a 		yellowish surface due to carbonation from CO2  in the atmosphere.  The inside may be 		blue due to calcium hydroxide.
	5.	To answer the proverbial question, "Is this right?" a slump test may be performed.   		A slump test involves filling an inverted, bottomless cone with the concrete mixture.  A 		Styrofoam or paper cup with the bottom removed makes a good bottomless cone.  		Make sure to pack the concrete several times while filling the cone.  Carefully remove 		the cone by lifting it straight upward.  Place the cone beside the pile of concrete.  The 		pile should be about 1/2 to 3/4  the height of the cone for a concrete mixture with good 		workability.
		(SEE DIAGRAM)

	6.	To strengthen samples and to promote hydration, soak concrete in water (after it is set).

	7.	Wet sand may carry considerable water, so the amount of mix water should be reduced
      		to compensate.

	8.	Air bubbles in the molds will become weak points during strength tests.  They can be 		eliminated by:
		i.	packing the concrete.
		ii.	Tapping the sides of the mold while filling the mold.
		iii.	"rodding" the concrete inside the mold with a thin spatula.

	9.	Special chemicals called "water reducing agents" are used to improve workability at low 		water to cement ratios and thus produce higher strengths.  Most ready-mix companies 		use these chemicals, which are known commercially as superplasticizers.  They will 		probably be willing to give you some at no charge.

	10. You can buy a bag of cement from your local hardware store.  A bag contains 94 lb. 		(40kg) of cement.  Once the bag has been opened, place it inside a garbage bag (or two) 		that is well sealed from air.  This will keep the cement fresh during the semester.  An 		open bag will pick up moisture and the resulting concrete may  be weaker.  Once 		cement develops lumps, it must be discarded.  The ready mix company in your area 		may give you cement free of charge in a plastic pail.
Experiment 1

What's the Matter

Introduction to the Physical Properties of Matter

Objective:  The objective of this lab is to identify different classes of matter based on physical properties.  This lab introduces the key ingredients of concrete.  It provides a deeper understanding of the physical properties of concrete.

Scientific Principles:

Matter is divided into the four basic states of solid, liquid, gas, and plasma.  Matter is classified based on composition.  Homogeneous matter is matter that appears the same throughout a mixture.  Heterogeneous matter is matter that has differing appearances throughout the mixture.  The concept map below shows the relationship between some of the primary classes of matter.

Matter is identified by its characteristic physical properties.  Physical properties are those that can be determined without altering the composition of the substance, such as, color, odor, density, strength, elasticity, magnetism, and solubility.  Chemical properties are descriptions of the substance and its reactions with other substances to create new substances with new properties.  These chemical properties are identified through chemical reactions.  Evidence of a chemical reaction possibly occurring can be seen through a color change, temperature change, evolution of a gas, and the formation of a new substance.  This lab will only focus on the physical properties of matter.

Time: 45-50 minutes

Materials and Supplies:

test tubes
magnet
magnifying glass
water
wooden splints
Different samples of matter (any of the following):
	baking soda
	balloon filled with air
	iron filings
	flour
	sulfur
	corn starch
	sugar
	vermiculite or perlite
	Styrofoam beads
	salt
	pepper
	cement
	aggregates
	clay

General Safety Guidelines:

	::	Wear safety goggles.
	::	Some of the materials may cause skin irritation.  Wear gloves.

Procedure:

	1.	Examine each sample.  Record color, odor, and relative particle size in the data table.  		Use a magnifying glass if necessary.

	2.	With a magnet, test each sample for magnetic properties.  Record whether the sample is 		magnetic or not.

	3.	Test the solubility in water of each sample by adding 5 mL of water to a small test tube. 		Add some of the sample to the water.  Flick the test tube with your finger to help mix 		the sample in the water.  (Note:  If mixing does not occur, use a wooden splint.)   		Record observations.

Data and Calculations:

Fill in the data table based on your observations.
                                    Sample
                                     Color
                                     Odor
                                 Particle Size
                                   Magnetic
                                  Solubility
                                State of Matter
                                Class of Matter

Questions:

	1	a.	How do you determine which sample is the most soluble?

		b.	List the samples from highest to lowest solubility.

	2.	Which of the samples would be classified as a mixture?

	3.	What physical properties of matter were tested in this lab?

	4.	What physical properties of concrete would be important to consider when making a 		structure?

	  	Why?

Notes for Teacher:

	::	This is a version of a common classification experiment conducted in many chemistry 		and general science courses.  It is included in this module to provide a means for the 		introduction of concrete and its key ingredients.

	::	The total number of samples is left to the teacher's discretion.  Sand, water, gravel, 		cement, and concrete should be samples of matter to be tested.  This will provide for 		the introduction of the topic concrete and its key ingredients.

	::	Be sure the magnet is protected from directly picking up any of the magnetic fragments 		in the samples.  The magnet may be covered with tape or students may place a piece of 		paper between the magnet and the sample.

	::	It is strongly recommended that the teacher do a trial run of this experiment before 		using it with the students.

	::	This experiment could be used as a lead in to density by asking, "Of the materials that 		didn't dissolve in water, which was the most dense and least dense?"

	::	Note that when cement is added to water to determine solubility, the students may 		conclude that cement is insoluble because its rate of dissolution is relatively slow.  		However, a quick check of pH will demonstrate that something is happening.

	::	Note:  Vermiculite is a compound.

Answers to Questions:

	1	a.  The sample that has the highest mass dissolved per volume of water is the most
			soluble.
		b.  Answers will vary.

	2.	This is dependent upon the samples used.  However, mixtures are usually obvious 		from appearance except solutions.

	3.	color, odor, size, magnetism, solubility

	4.	Size of particles and solubility of substances used to make it.  The physical properties 		help determine the purity of concrete's ingredients which greatly affect the produced 		concrete's characteristics.
Experiment 2

How Dense Is It?

An Introduction to Concrete Density and Aggregates

Objective:  The objective of this experiment is to determine the density of a concrete sample and to learn the effect of various types of aggregates on concrete's density.

Scientific Principles:

Density is the physical property of matter that measures the quantity of a substance per unit of space.  Density is recorded in units of grams per cubic centimeter (g/cm3) for solids, grams per milliliter (g/mL) for liquids, and grams per liter (g/L) for gases.  Density is a way of determining how compact one substance is compared to another.  Density is also the property that enables one object to be buoyant or another to sink.  The object that is less dense will float in a more dense substance.  The density of a typical concrete is 2.3 g/cm3.

Time:  45-50 minutes

Materials and Supplies:

      cured 1/2" x 2" cylinders of concrete samples (each consisting of a different aggregate)
      sample concrete recipes

                                     water
                                     25 mL
                                     15 mL
                                     25 mL
                                     20 mL

                                    cement
                                     50 g
                                     50 g
                                     50 g
                                     50 g

                                   aggregate
                                  150 g sand
                                35 g pea gravel
                              4 g dry vermiculite
                                     none

      100 ml graduated cylinder
      water
      balance

General Safety Guidelines:

	::	To avoid cracking the graduated cylinder, gently slide the concrete cylinder down the 		side.
	::	Wear goggles.
	::	Concrete can be caustic.  Wash hands after direct contact.

Procedure:

	1.	Estimate which sample will have the highest and which will have the lowest density.
		Record your ideas on the data chart.

	2.	Mass each cured cylinder on the balance.  Record in data table.

	3.	Half-fill a graduated cylinder with water.  Record the initial volume of water.

	4.	Gently slide the concrete cylinder into the graduated cylinder so as to not splash out any
		water or break the glass bottom.

	5.	Record the final volume of water with the concrete cylinder in it.
		Subtract the final volume from the initial volume of water to obtain the volume of the 		cylinder.

	6.	Remove the concrete cylinder from the water.

	7.	Repeat steps 1-6 for all other cylinders.

	8.	Calculate the density of each cylinder by dividing cylinder's mass by its volume.

Data and Analysis:

Estimates

	highest density cylinder_________________
	lowest density cylinder _________________

Data Table

Questions

	1.	a.  What type of cylinder was the least dense?

			b.  What type of cylinder was the most dense?

	2.	Give a use for the cylinder in question 1a.

	3.	Give a use for the cylinder in question 1b.

	4.	Were your estimates correct?  Did any of the results surprise you? Why?

	5.	How do your values compare to the typical density of concrete (2.3 g/cm3) ?

Notes for Teacher:

Many science courses already have a density experiment as part of the course.  The use of concrete cylinders will add a new touch to these experiments.  It is assumed the concrete cylinders were previously prepared by the teacher.

This experiment could be performed as an investigation in which students prepare concrete mixtures of different ingredient proportions to study their effect on density.  These cylinders of varying composition could later be used in the Stress and Strain  experiment.  This could also be done with different kinds of aggregates.  Aggregate densities could be determined, as well as the density of hardened paste.  Lightweight aggregates are usually available from a ready mix company.  Horticultural vermiculite or perlite can be used.  The density of aggregates could be measured and correlated with the density of concrete.  (A good project for advanced students.)  When measuring the density of aggregates, it is advisable to measure the density of saturated aggregates (soak in water for at least one hour or overnight).  Otherwise the aggregates will absorb water during the displacement measurement and give erroneous results.  Remember, aggregates in concrete become fully saturated.

This could be turned into a materials science competition in which the students must make a cylinder with the greatest or least density.

Expected Results:

The four cylinder types listed from most dense to least dense:
	gravel, sand, paste, vermiculite

Answers to Questions:

	1.	a. vermiculite
		b. gravel

		2.	Any objects that float.  Answers will vary.

		3.	Answers will vary.  Roads, bridges, and underwater structures.

	4.	Answers will vary.

		5.	Sand or gravel should be closest to this value.

Experiment 3

Hot and Cold pHun!

Cement Hydration and pH Evolution during Curing

Objective:  Students will calculate the amount of water that reacts during hydration when cement becomes concrete under differing curing temperatures and observe the pH change that occurs in the curing process.

Scientific Principles:

Water is the reactant that makes concrete hard. The degree of hydration, or maturity, of the concrete determines the porosity of the concrete. Porosity is the amount of empty space in the concrete.   Low porosity concrete has high strength and lasts for a long time.

During the hydration of the cement, the compound calcium hydroxide, Ca(OH)2,  is produced.  Calcium hydroxide is a basic compound (alkaline).  Bases are caustic (eats skin tissue), feel slippery on your skin (like soap), and have a bitter taste (don't taste it!).   Calcium hydroxide is a mild base, but can irritate sensitive skin, so be careful!

Bases also affect indicators (chemicals that change color).  There are many different indicators.  Universal indicator is a mixture of different indicators so that colors can be achieved all along the pH scale.  The pH scale is a numerical range used to determine the acidity or alkalinity of a substance.  The scale ranges from 1 to 14.  pH's from 1 to 6 are acidic.  A pH of 7 is neutral.  Values ranging from 8 to 14 are basic.

The environmental conditions, such as temperature and humidity, under which the concrete is cured can also affect the concrete's processing and properties. Cooler surroundings results in concrete hydrating at a slower rate.

Time: 100 - 120 minutes (2 1/2 class periods)

Materials and Supplies for Groups of 2-3:

	Fresh cement
	mixing container
	stir stick
	balance
	oven
	refrigerator
	labels
	pH paper
	3  glass or plastic petri dishes (see Teacher's Notes)
	plastic wrap or Ziploc bags
	gloves
	graduated cylinder
	releasing agent (PAM cooking spray, cooking oil, aluminum foil, plastic wrap)

General Safety Guidelines:

	::	Prolonged exposure may cause severe chemical burns.
		Wash exposed skin after contact.
	::	Gloves should be worn to protect the skin, especially if your skin is sensitive.
	::	All heating must be done in a oven only.

Procedure:

Day 1:
	1.	Prepare a batch of fresh paste using 100 g of cement and 40 mL of water.

	2	a.	Place a releasing agent (PAM cooking spray, aluminum foil, plastic wrap, cooking 		oil) in all three petri dishes.  Be sure to cover the entire inside of the dish.

		b.	 Mass the three petri dishes and record in the data table.

	3.	a.	Fill each petri dish 1/4 full with the paste and level it off.
		b.	Mass each dish separately and record in the data table.

		c.	Wrap two dishes in plastic wrap or place in Ziploc bags.  Set one aside at room 			temperature.  Set the second in a refrigerator.

		d.	Put the third dish into a sellable container with water.  Be sure the water level 			is well below the edge of the dish.  Tightly seal the container and place it in 			an oven set at 40˚ C.

	4.	Using pH paper, test the pH of tap water and any leftover fresh concrete paste.  Record 		your values on the data page.

	5. 	Allow forms to cure 24 hours.

Day 2:
	1.	Remove the concrete samples from their controlled environments.

	2.	Mass each dish separately and record in the data table.

	3.	Determine the mass of original water by using the equation found in the data and		calculations section.

	4. 	Determine the mass of the original cement by using the equation found in the data and 		calculations section.

	5.	Place each sample in an oven set at 100 -110 ˚ C.  Allow the samples to remain in the 		oven for 24 hours.

Day 3:
	1.	Remove the samples from the oven and allow to cool.

	2.	Weigh each petri dish and then determine the mass of only the cement paste.

	3.	Determine the mass of water driven off in the oven by using the equations in the data 		and calculations section.

	4.	Calculate the mass of water combined during hydration using the equations in the data 		and calculations section.

	5.	Calculate the percent of water reacted using the equations in the data and calculations 		section.

Data and Analysis:

Calculation Hints:

Data:

pH of mixed cement _________________
pH of  tap water ____________________

Day 1
                                     ROOM
                                 REFRIGERATOR
                                     OVEN
mass of petri dish

mass of concrete and dish

Day 2

mass of concrete and dish

mass of sample

mass of original water

mass of original cement

Day 3

mass of dish and dry sample

mass of dry sample

mass of water driven off

mass of water combined in reaction

% of water reacted (hydration)

Questions:

	1.	Which curing environment yielded the greatest percent of water reacted?

	2.	a. What is the relationship between curing environment and amount of hydration?

		b. Do your results support your answer?

	3.	Which concrete sample would you hypothesize to have the greatest strength?
		Explain your reasoning.

	4.	What proof do you have that hydration is a chemical change?

	5.	Suppose you came across a container filled with a substance that looked like concrete.
		When you tested it with pH paper, the paper turned red.  Was the substance concrete?
		Explain your reasoning.

Notes for Teacher:

      ::	Students may use an universal batch or make their own mixtures to study water to 	cement ratio effect on pH and hydration.

      ::	If plastic petri dishes are used, place a few sheets of paper on the oven rack before 	setting the dishes in the oven.  Then they shouldn't melt.  If plastic dishes are used, a 	releasing agent is unnecessary since the dish is disposable.

      ::	Students may investigate the effect of admixtures.  Calcium chloride can be used to 	accelerate setting (2% CaCl2::2H2O by weight of cement).  Sugar can be used to retard 	setting (0.1% by weight of cement).  Commercial accelerators and retarders can be 	obtained from your local ready mix company.

      ::	An alternative experiment is to vary the amounts of water available for hydration and 	determine the results.  Samples are all kept at room temperature.  One is covered with 	wet paper towels and plastic sheeting, one with plastic sheeting only, and one is 	covered with only a wet paper towel.   The test specimens would then be treated as 	described in Day 2.

      ::	Students could perform the same procedure on cylinders of different ages to prove that 	hydration continues over a long period of time.

Expected Results:

Hydration of the sample in the oven is accelerated by a warm environment.  Therefore, the sample in the oven will use more water in its hydration products.  So it will have the highest water reacted and the cooler environment in the refrigerator will have the least.

Answers to Questions:

	1.	Oven

	2.	a. Colder environment results in less hydration occurring, more free water is available.

		b. Answers will vary

	3.	Oven sample, because more hydration could have occurred.

	4.	The pH paper changed color and new products were formed, because water was 		retained, not evaporated.

	5.	No.  pH paper changes to blue, indicating a base is produced during the formation of 		concrete.  A change to red indicates an acidic change, not basic.

Experiment 4

The Fleet Afloat!

A Design Project

Objective:  Students will design a concrete barge which will float and carry a maximum cargo.  In this experiment the students will be learning to work with the component materials used to make concrete and building and testing a concrete structure for physical properties.

Scientific Principles:

Density is a measure of the compactness of a material.  It is a measure of how much matter is squeezed into a given space.  Density is the amount of matter per unit of volume.

Whether an object will sink or float in water depends on its density.  An object will float if it is less dense than water.  An object will sink if it is more dense than water.  If an object has a density equal to that of water, it will neither sink nor float.  The density of water is 1.00 g/cm3.  The apparent density of an object can be changed by either changing the mass of the object, the shape of the object, or both.   For a given mass of concrete, the apparent density can be altered by changing the volume it occupies (i.e. volume displaced when placed in water).  Concrete can be made to float if it is shaped like a boat.  A boat-shaped or hollow object will displace a volume of water greater than the actual volume of solid material in the object.  The object is said to be "buoyant" when it floats due to low density.  By spreading out the concrete used to make the boat over a larger volume, the apparent density of the boat becomes less than that of water.   Hence the boat floats!

Time: 90 min. (1 class period for making the barges and 1 class period for testing)

Materials and Supplies:

The following materials are needed for each group of 2-3 students:
	mold to make barge
	cement
	water
	various aggregates
		sand
		pea gravel
		vermiculite
		small Styrofoam balls
		perlite
		crushed corn cobs
	2-3 pairs of gloves for concrete handling
	newspapers
	spreading utensil for spreading concrete in barges
	boat or tray to demonstrate buoyancy with cargo
	1-2 pounds of washers, weights or lead shot

General Safety Guidelines:

	::	It is easiest to form the canoe by hand, but cement is caustic.  Wear plastic  gloves.
	::	Wash your hands well after use.

Design Competition Parameters:

	1.	The barge or canoe must be constructed from materials (cement and various  			aggregates) provided by the instructor.  The form used to create the barge or canoe is to 		be provided by the students.

	2.	The proportions of the ingredients used in the cement mixture are to be chosen by 		the students using the following as a guide, not a recipe:
		3 parts aggregate: 2 parts cement : 1 part water
		100 grams of cement is sufficient for one Dairy Queen boat.

	3.	Once constructed, the barge or canoe should be cured for at least 24 hours.

Design Testing Procedures:

	1.	The mass of each empty barge or canoe will be measured.  The lighter the ship the 		more points it receives.  Any vessel less than 150 grams earns the maximum points 		value.

	2.	Each barge or canoe will be placed in the water.  The empty barge or canoe must float 		for one minute before the cargo will be added.   A cargo will be added in increments 		until the point when the ship begins to take on water.  Each ship must float for 1 		minute prior to the addition of  each additional cargo increment.  For the canoe, 		whether the canoe leans or floats level will be noted.

	3.	The mass of the cargo will be measured.  For every 50 grams the ship holds, it will 		receive one point.

	4.	A ratio of 	mass of cargo   will be calculated to determine each barge's class rank.
				mass of ship

	5.	Barges which fail to float without any cargo will be assigned a predetermined point
		value.

	6.	Points will be awarded to each barge based on its class rank.

	7.	The cost of construction will be calculated for each boat which successfully floats 		for 1-2 minutes to determine its class rank. (optional)

	8.	The amount of leakage will be determined and point values will be given for the 		different levels of leakage.

Other Procedures:

	1.	Suppose you and a friend are in a canoe with a load of rocks.  If you throw half of 		the rocks overboard, will the water level of the pond go up, down, or stay the 			same?  Make note of your prediction.

	2.	In a container of water, float a boat loaded with a cargo of washers or other uniform 		weight.  Make sure the water's height is greater than the combined height of the 		washers and the boat.

	3.	Mark the water level on the container and on the side of the boat.

	4.	Put the cargo of washers in the water.  Mark the new water levels on the container 		and the side of the boat.

Questions

	1.	What happens to the water level on the side of the boat when the cargo is moved from 		the boat to the water?

	2.	If you see a boat riding high in the water, which is it more likely to be carrying:  lead
		bricks,  glass marbles, or Styrofoam cups?

	3.	What happens to the water level on the side of the container when the cargo is moved 		from the boat to the water?  Was your prediction correct?

	4.	Describe the shape, dimensions, and composition of your barge.  Draw an illustration.

	5.	What was the limiting factor in your barge design?

		b. How would you change your barge's design to improve this ratio?

	7.	How could you change the density of your barge/canoe without changing its mass?

Notes for Teacher:

	::	Several criteria are listed so that the teacher may choose the guidelines upon 			which to rank the barges/canoes.

	::	This experiment emphasizes the physical geometry of the barge and therefore may be 		most suitable for a physics course when discussing principles of density and buoyancy.

	::	The cargo could be lead shot, washers, weights, or any other material of uniform mass.  		The one minute waiting period prior to the addition of more cargo may need to be 		shortened depending on class period length and the number of barges being tested.  		However, a waiting period is necessary to provide time for water to enter the barge.

	::	The form materials could be provided or the students could be directed to bring in their 		own forms, allowing their imaginations to wander.  Limits might need to be set in 		terms of dimension, weight, or amount of concrete/cement available.  Possibilities for 		the barge forms are travel soap dish or a travel toothbrush holder or even an old tennis 		shoe.  Dairy Queen banana split boats, foam insulation, plastic weighing dishes, old 		butter/margarine containers are suitable for the canoe.

	::	Foam meat trays or plywood boards make good bases for the barges/canoes to be set 		on and are very helpful when moving them from the building site to the curing site (if 		different).  The ships should be covered with wet rags or plastic while curing to help 		retard evaporation and improve hydration.

	::	A wash tub, 5 gallon bucket, or a small child's swimming pool could be used for the 		testing waters.  Do not use ponds or swimming pools.  Cement is caustic to living 		things.  A video recording of the testing could also be made.  The video could be set 		out in the hallway and replayed during parent conferences for the parents to view while 		waiting to speak with you - great public relations.

	::	This activity could be tied into a holiday such as Labor Day, Columbus Day or 			Washington's Birthday observance.  Decorations could be added to the barges and a 		competition held to determine the most aesthetically pleasing barge.  The art classes 		might be asked to judge the barges.

	::	Note:  Pre-assign dollar amounts to quantities of materials in order to calculate the cost 		of the boat. (optional)  This option allows discussion of economy and efficiency.

Answers to Questions:

	1.	Will go down.  i.e., boat rides high in the water

	2.	Styrofoam cups

	3.	The level goes down!
		Removing the cargo lowers the water level by a volume, V1, equal to the cargo's mass 		over the apparent density of the boat and cargo.  The water volume, V2, raises from the 		cargo put into the water by its mass over its density.

		Simple math yields: V2-V1=-0.57*Cargo Mass
		Note that the negative sign means down.

	4.	Answers vary based on students design.

	5.	Possible answers:  heavy mass;  low surface area;  poor construction; setting time 		inadequate.

	6.	a.	Answers vary.
		b.	Possible answers:   change aggregates; change concrete component proportions; 			different  shaped form.

	7.	By changing its volume.
Experiment 5

Stress and Strain

An Introduction to Stress and Strain

Objective:  Students will design and make a concrete cylinder and beam that can withstand the greatest applied load.

Scientific Principles:

Solids have rigidity because the bonding forces between the atoms resist changes.  When an external force is applied to a solid, the interatomic forces allow for deformations to occur.  When the distorting external force is removed, the forces between the atoms bring them back to their original positions and the solid, as a whole, back to its original shape.  This property is called elasticity.  All solid materials are elastic to some extent, even concrete.  The amount of elastic deformation that can occur in a material is dependent on the type of stress applied.

Two common types of stress are compressive and tensile.  Compressive forces exist when you push together a material.  Walking on concrete is one such example.  Tensile stresses describe a material when it is pulled upon.  The cable that supports the load of a crane is a good example of tensile stresses.

While in elastic regions, compressive and tensile stresses do no damage to the material.  A material will return to its original dimensions once the stress has been removed if it is within this region.  If the stress becomes too great in concrete, microscopic cracks begin to form inside the concrete.   Once cracks begin to form, the concrete will not return exactly to its original dimensions and thus behaves inelastically.  These changes are very small and not easy to measure without expensive equipment.   As the load increases, these microscopic cracks come together so as to form one large macroscopic crack, and the concrete fractures or fails.  Concrete has very good compressive properties but very poor tensile properties unless steel reinforcements are added.

Time:	90 min. (1 class period for making the beams & cylinders and 1 period for testing)

Materials and Supplies:

The following materials are needed for each group of 2-3 students:
	cement
	various kinds of aggregate
	mixing containers
	stirring utensils
	forms for making beams and cylinders (enough for each student to work individually)

LOW-TECH COMPRESSION TESTER
	two 8' long 2x4's
	one 2' long 2x4
	large gate hinge
	sheet metal -cut into two 4 "squares
	2 copper pipe caps to fit over concrete cylinders
	2 rubber stoppers or washers to fit inside pipe caps

MEDIUM TECH COMPRESSION TESTER
	2 C-clamps ---6" span or larger
	2 copper pipe caps to fit over concrete cylinders
	PVC pipe slightly longer than concrete cylinders and slightly larger in diameter
	2 rubber stoppers or washers to fit inside pipe caps

HIGH TECH COMPRESSION TESTER
	hydraulic jack with pressure gauge
	2 copper pipe caps to fit over concrete cylinders
	2 rubber stoppers or washers to fit inside copper caps

Design Competition Parameters:

	1.	All cylinders and beams will be made using uniform molds which will be provided.

	2.	The concrete used to make each cylinder and beam must be made using the raw 			materials provided.

	3.	Suggested recipe for concrete is 3 parts aggregate, 2 parts cement, and 1 part water. 		Any combination of aggregate or none at all may be used.  The exact amounts of each 		component to be used is up to you!

	4.	The beams must span at least 30 centimeters but may be longer.

Beam Testing Procedure:

	1.	Support the beam between two tables of the same height.

	2.	Twist the ends of a wire together so that it forms a circle around a bucket handle.  Put 		this loop around the middle of the beam.

	3.	Add weight to the bucket in uniform increments.  Wait 30 seconds between 			each addition of weight.

	4.	Continue to add weight until the beam breaks.

Cylinder Testing Procedure:

	1.	Place the cylinder in the testing apparatus.

	2.	Apply force until the concrete is smashed.

Questions and Analysis:

	1.	If a horizontal concrete beam is supported at one end and a downwards force is applied 		to the other end, where would the microscopic cracks tend to be formed?  The top 		layer, the bottom layer, or the middle layer?

	2.	If a concrete beam is supported but not held at both ends and a load is applied in the 		middle,  which layer would be under compression? The top layer, the bottom layer, or 		the middle layer?

	3.	In the construction of a bridge, would concrete be better suited for the columns
		supporting the bridge or for the bridge decking?   Explain your answer.

	4. 	Write the formula needed to calculate the
		a. tensile strength of a beam
		b. compression strength of a cylinder

	5. 	Calculate the strength each of your structures exhibited in the competitions.

	6	a. Write the equation to calculate the force exerted on a cylinder.
	  	b. Calculate the force that was exerted on your cylinder.

	7. 	What changes would you make to improve your beam design?

	8. 	What  changes would you make to improve your cylinder design?

Notes for Teacher:

We used a 24" truss spacer as our beam forms.  These can be found at a hardware store.  Our truss spacers cost 69 cents each and are reusable. A toothpaste box reinforced with tape can be used as a form.   The plastic cases that toothbrushes come in could also be used.  However, obtaining a classroom set of these might prove to be a challenge.  The beam testing could also be set up as a cantilever style with only one end of the beam fixed and the load applied to the free end.

Free weights can be added to the bucket until breakage occurs.  If free weights are not available, water can be used.  Be careful of water spillage.

Cylindrical forms can be plastic or paper tubes, pipe insulation, or any other small diameter tubing.  The concrete needs to be easily removable from the forms.  Pipe insulation from a hardware store works well.  This foam material is easy to work with, reusable, and inexpensive.  The bottom of the forms can be taped, corked, or set on glass plates.  The goal is to achieve smooth top and bottom surfaces which are necessary when performing the compression test.  If the surface is not smooth, the force will be applied unequally.  We recommend the cylinders be no more than 1/2" in diameter.  Cylinders with a larger diameter will require a greater force to fail.  A good rule of thumb is the height of the cylinder should be at least twice the diameter of the cylinder.  However, if the cylinder is too tall it will tend to buckle.

The aggregate selection could be limited to just sand or could include gravel.  The strength of the specimen can be adjusted by changing the water to cement ratio of the concrete to suit the capacity of your test set up.  The length of curing can also be varied.

The amount of force needed to produce failure in a beam depends on the beam's dimensions.  The theoretical maximum tensile strength of a beam can be calculated by:

The amount of force needed to produce failure in a cylinder depends on the cylinder's cross-sectional area.  The theoretical maximum compressive strength of a cylinder can be calculated by:

Some common values needed for failure are:	Beam -      20MN/m2  (3000 psi)
						Cylinder - 35 MN/m2  (5000 psi)

The experiment could be used to show the development of strength over time.  The students could make several cylinders and beams from each of their concrete batches and the compression strength could be tested over a long period of time such as 1 day, 3 days, 7 days, and 28 days.

The principle of reinforced concrete could be demonstrated by placing a strip of hardware cloth (galvanized steel screen) or chicken wire into the beam in the tension zone.

A cost factor could be incorporated into this lab to make it a true engineering project.  The competition could involve assigning points based on  most strength for lowest cost.

We offer three testing apparatus designs for measuring the compression strength of the cylinders.  Each of these designs is pictured and described on the following pages.

Low Tech Compression Testing Apparatus - The Cement Cracker

Two 8 feet long 2 x 4's were hinged together using a large gate hinge.  The hinge plates will need to be bent so as to fit to the 2 x 4 resulting in a device similar to a nut cracker.   A separate 2 x 4 which is about two feet long should be nailed to the bottom side of one of the 8' long boards to provide stability.  Otherwise, the unit tends to roll to the side.  To provide additional strength to the 8' long boards, we attached two small pieces of sheet metal at the location where the cylinders would be seated for testing, about 2 or 3 inches from the hinge.  To insure uniform pressure acting on the cylinders, copper pipe caps were placed over the ends of the concrete cylinders.  Before placing the copper caps on the cylinders, rubber inserts were placed inside the copper pipe caps to cushion the ends of the cylinders.  Place the copper capped concrete cylinders on the sheet metal pads and apply a force on the end of the 8' long boards.  By calculating the mechanical advantage of the lever and knowing the applied force, the force exerted on the cylinders can be determined.

Sample calculation:   Assuming the cylinder is placed 3" from the hinge on an 8' (96") long board, the mechanical advantage of the lever is 32

If a 100 pound student stands at the end of the 8' long board, the force exerted on the cylinder is 3200 pounds.  If this is attempted, ensure the safety of the student.

Medium Tech Testing Apparatus - The C-Clamp

Rubber inserts are first placed inside copper pipe caps to provide a cushion for the concrete cylinders.  The copper pipe capped cylinder is placed inside a piece of 1" diameter PVC pipe (clear or white) which has been cut long enough to completely cover the concrete cylinder.  The PVC pipe is used as a safety means to prevent pieces of concrete from flying outwards when the cylinder breaks.  It also provides stability to the test cylinder.   Any other piece of pipe that has an inside diameter greater than that of the copper pipe caps could be used.  The PVC covered cylinder is placed in the throat of a large (6" or greater) C-clamp.  Tighten the threaded shaft onto the copper caps on the cylinder.   This entire set up is then clamped to a table top using a second C-clamp.  A socket which fits a torque wrench is fitted over the handle of the C-clamp.  This socket will need to have slots cut into it so that it snugly fits the handle of the C-clamp.  The handle of the C-clamp can be tightened further down onto the concrete cylinder.  The torque value can be recorded from the torque wrench.  Because the cylinders break rather quickly, we suggest that a video camera is used to record the readings on the torque wrench and played back to obtain more accurate readings.  By calculating the mechanical advantage of the C-clamp screw and knowing the torque applied, the force exerted on the cylinder can be determined.

Sample Calculation:  The mechanical advantage of a screw can be calculated by:

The mechanical advantage of the screw multiplied by the torque reading equals the maximum force applied to the cylinder, neglecting friction.

High Tech Testing Apparatus - Hydraulic Jack

A hydraulic jack with a pressure gauge provides the easiest method for determining the force exerted on the concrete cylinders.  Brodhead-Garrett Co. ( 223 S. Illinois Ave., Mansfield, OH  44901)  sells a hydraulic jack with a pressure gauge and platform for about $100.  (Dan Phillip 1-800-949-8324)

Place rubber inserts inside copper pipe caps to provide a cushion for the concrete cylinders.  Cap the concrete cylinder and place in the platform of the jack.  Begin applying pressure while observing the pressure gauge.  Allow time between pumps for the cylinders to fail - a minute or two.  Safety glasses are a must as pieces of the cylinder fly outward!!  A safety shield around the sample would also be a good idea.  A video camera may be used to record readings on the pressure gauge and replayed to obtain more accurate readings.

To calculate the compressive strength that is applied to the cylinder, use the following equation.

Expected Results:

For 1/2" cylinders, the cement paste broke at 2000+ psi, gravel at 2000 psi, and vermiculite at 400  psi.  The beams' maximum load will vary depending on the form used. Loads as heavy as 100 kilograms have been observed!

Answers to Questions:

	1.	Top layer, because it is in tension and concrete is week in tension.

	2.	The top layer will be in compression.  The bottom will be in tension.

	3.	The columns of bridges are concrete while the decking is usually steel.
		The columns are only under compression while the decking is under tension.

	4.	a. See equation #1
		b. See equation #2

	5.	Answers will vary, units will either be psi or N/meter2

	6. 	a. See equations #3 and #4 for low tech apparatus and equation #5  for medium and 		high tech apparatus.
		b. Answers will vary, units should be newtons or pounds

	7. and 8. answers might include: use different aggregate, vary the amounts of components 		used in the cement, use a different form and change the dimensions.

Experiment 6

Make and Take

A Take-home Project

Objective:  Students will develop an understanding of concrete as a construction material.  A concrete product will be designed and constructed.

Scientific Principles:

Concrete is everywhere.  Take a moment and think about all the concrete encounters you have had in the last 24 hours.  All of these applications of concrete are created from a material that is a mixture formed from the chemical reaction of cement and water (hydration) with added aggregate. It important to distinguish between cement and concrete as they are not the same.  Cement is used to make concrete.

Water is a key reactant.  Water and cement initially form a cement paste that begins to react through the chemical process of hydration.  In the hydration of cement, chemical changes occur slowly, eventually creating new crystalline products, heat evolution, and other measurable signs.  The hydration process hardens (sets) the paste which binds the aggregate particles together.

The aggregate is the solid particles that are bound together by the cement paste to create the synthetic rock known as concrete.  Aggregates can be both fine, such as sand, and/or coarse, such as gravel.  The relative amounts of each type and the sizes of each type of aggregate determines the physical properties of the concrete.

Time:  1 Class period for each object made. (45 min.)

Materials and Supplies for groups of 2-3:

      cement
      pea gravel
      water
      2 flower pots or suitable plastic containers
      wire mesh
      mixing bowl or wheelbarrow
      finishing tools
      damp sand
      1" greased dowel rod

Procedure:

	1.	Prepare enough concrete to fill the space between your two containers when they are 		stacked with a 1" separation.

	2.	Coat the inside of the bottom container and the outside of the top container with plastic 		wrap.

	3.	Place a  1" lubricated dowel (future drain hole) that is 1" long, in the bottom center of 		the first mold for a round based flower pot or four dowels if you are making a 			rectangular flower pot.  This will serve as a spacer now and a drain in the hardened pot.

	4.	Surround the dowels with concrete mix, place the top mold  on the mix being sure to  		allow for about 1"  wall thickness.  Add concrete mix between the forms until the 		bottom form is filled.   Tamp concrete to remove air pockets.  Allow to harden for 15-		20 minutes.  If decorations are desired on the outside of the pot, remove outer mold and 		add glass beads, colored stones, etc.  Allow to harden 24 hours.

	5.	Remove molds and place in water for three days to harden.  Be cautious of this step if 		your pot is decorated.

Notes for Teacher:

	::	Use larger sized aggregates to create a nice appearance.
	::	Use as a money making project.
	::	Many different shapes can be made and may be a great way to inspire some creativity.

CONCRETE						NAME________________________
QUIZ 1

1.	Distinguish between cement and concrete.

2.	Name at least three items you have encountered today which are concrete.

3.	What are the major ingredients for concrete, and what role do they play?

4.	What is meant by "workable?"  Why is it important for concrete to be workable?

5.	Give an example of an aggregate.  What is the practical use for this aggregate in making 	concrete?
CONCRETE 					NAME _____________________________
QUIZ 2

1.	What can be used to slow the hardening of concrete (give example?)  Why would slowing this 	process be desirable?

2.	What can be used to speed the hardening of concrete (give example?)  Why would speeding up 	this process be desirable?

3.	Suppose you were in charge of building a skyscraper.  What would be your choice for 	aggregate and why?
CONCRETE 					NAME______________________________
QUIZ 3

1.	What will happen to concrete if it dries out too quickly?

2.	Suppose you were to be the chief designer in charge of building a concrete ship to carry people 	overseas.  What aggregate might you choose to put in your concrete and why?

3.	Explain what the dormancy period of fresh concrete is.  How do contractors make use of the 	dormancy period?

4.	Explain how can you measure the consistency of freshly mixed concrete?

CONCRETE 						NAME________________________
QUIZ 4

1.	Briefly discuss the importance of a proper water to cement ratio.

2.	Explain the purpose of a superplasticizer in making concrete.

3.	Why should gloves be worn when mixing concrete?  Be specific.

4.	Water is important in making concrete, however, it can also be harmful to concrete.  Explain 	this statement.
CONCRETE						NAME	KEY
QUIZ 1

1.	Distinguish between cement and concrete.

		Cement is a component of concrete.  Cement and water make the "glue" which holds 		concrete together.

2.	Name at least three items you have encountered today which are concrete.

		Answers will vary.

3.	What are the major ingredients for concrete, and what role do they play?
		cement- reacts with water to form "glue"
		water- reacts with cement, the amount also determines strength
		aggregate- makes concrete stronger, more durable, and less costly

4.	What is meant by "workable?"  Why is it important for concrete to be 	workable?

		Cement which is workable is able to be poured into forms without difficulty.  A slump test 		is used to measure workability.

5.	Give an example of an aggregate.  What is the practical use for this aggregate 	in making concrete?

		gravel, sand, vermiculite, perlite,
		Aggregate makes the concrete stronger and cheaper.
CONCRETE 			NAME__KEY________________________
QUIZ 2

1.	What can be used to slow the hardening of concrete (give example?)  Why 	would slowing this process be desirable?

		Sugar can be added to the concrete to retard hardening.  For example, if the concrete needs 		to be transported a long distance, then a retarding admixture would be desired.

2.	What can be used to speed the hardening of concrete (give example?)  Why 	would speeding up this process be desirable?