The Impact-Echo Method: An Overview
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paper_title: Impact-Echo: The Complete Story

paper_content:

Impact-echo's history is an interesting story of how a real need for nondestructive test methods for flaw detection in concrete structures led to a systematic and sustained basic and applied research effort to develop such a method, beginning in 1983, at the National Bureau of Standards, and continued since 1987 at Cornell University. This paper discusses the contributions of the people and the organizations who carried out the theoretical, numerical, laboratory, and field studies that established the method and who developed the software and instrumentation that gave rise to a patented impact-echo field system. It also documents how this effort was undertaken and sustained with government and industry funding. Subsequently, this paper draws on knowledge gained over 12 years of research to provide, for the first time, a unified explanation of impact-echo theory as it applies to the testing of structural elements, including plates (slabs, walls, bridge decks, and pavements), bars (beams and columns), and hollow cylinders (pipes and tunnel and mine shaft liners) and to the detection of flaws within these elements. The last key pieces fell into place in 1995, and it is now possible to explain in a concise and coherent way the principles upon which impact-echo testing is based.
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paper_title: Impact-Echo: The Complete Story

paper_content:

Impact-echo's history is an interesting story of how a real need for nondestructive test methods for flaw detection in concrete structures led to a systematic and sustained basic and applied research effort to develop such a method, beginning in 1983, at the National Bureau of Standards, and continued since 1987 at Cornell University. This paper discusses the contributions of the people and the organizations who carried out the theoretical, numerical, laboratory, and field studies that established the method and who developed the software and instrumentation that gave rise to a patented impact-echo field system. It also documents how this effort was undertaken and sustained with government and industry funding. Subsequently, this paper draws on knowledge gained over 12 years of research to provide, for the first time, a unified explanation of impact-echo theory as it applies to the testing of structural elements, including plates (slabs, walls, bridge decks, and pavements), bars (beams and columns), and hollow cylinders (pipes and tunnel and mine shaft liners) and to the detection of flaws within these elements. The last key pieces fell into place in 1995, and it is now possible to explain in a concise and coherent way the principles upon which impact-echo testing is based.
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paper_title: EFFECTS ON IMPACT-ECHO SIGNALS CAUSED BY STEEL REINFORCING BARS AND VOIDS AROUND BARS

paper_content:

The paper explains the interaction of transient stress waves with steel bars in concrete and shows the effects that bars have on impact-echo signals. Numerical (finite element) and experimental results are shown to demonstrate the effects that different variables have on impact-echo results. Equations for the relationships between bar diameter, bar depth and the measured impact echo response are presented. It is shown that these relationships can be used to identify the effects caused by bars on impact-echo signals obtained from reinforced concrete structures. Finally, the changes in the impact echo response caused by the presence of a void around the top or bottom of a bar are also explained.
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paper_title: A finite element study of transient wave propagation in plates

paper_content:

Studies of transient wave propagation in plates were carried out to establish a ba­ sis for the impact-echo technique as a nondestructive test for flaw detection in concrete. The surface displacements caused by stress waves generated by point impact on a plate were calculated using both the Green's function solution and the finite element method; displace­ ment waveforms obtained by the two approaches showed good agreement. Displacement and stress fields in a plate were studied using finite element analy­ sis. It was shown that transient point load applied normal to a stress-free boundary gives rise to P- and S­ "wakes" -disturbances trailing the P­ and S-waves. The displacement and stress fields in each wake resemble those in the preceding wave.
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paper_title: A finite element study of the interaction of transient stress waves with planar flaws

paper_content:

This paper presents a finite element study of transient wave propagation in plates containing planar flaws. The ef­ fects on displacement waveforms caused by waves diffracted from the sharp edges of a flaw are determined. Dis­ placement fields within a plate contain­ ing a flat-bottom hole show the interaction of transient stress waves with a planar flaw. Waveforms obtained from the finite element analysis were compared with experimentally obtained waveforms.
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paper_title: IMPACT-ECHO RESPONSE OF CONCRETE SHAFTS

paper_content:

Numerical and experimental studies of the transient response of concrete shafts subjected to elastic impact were carried out using the finite element method and the impact-echo testing technique. Two- and three-dimensional finite element studies of concrete shafts were performed for: (a) solid shafts; (b) shafts containing cracks, voids, layers of low-quality concrete, and changes in cross section, such as bulges and necks; and (c) shafts in soil. These studies were carried out to gain an improved understanding of the impact response of concrete shafts containing flaws, problems for which there are currently no analytical solutions. Laboratory studies of solid concrete shafts were carried out to verify the finite element models. Also, experimental studies were carried out on concrete shafts embedded in soil. Results show that additional information about the integrity of drilled shafts and piles can be obtained using impact techniques. The numerical studies also show that the finite element method is a powerful tool for studying the impact response of concrete foundation elements.
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paper_title: Flaw Detection in Concrete Using the Impact-Echo Method

paper_content:

A technique called the impact-echo method has been developed for flaw detection in concrete. The technique is based on a simple concept using transient stress wave propagation. Mechanical impact is used to generate a short duration stress pulse which travels into the test object as P- and S-waves. The waves are reflected by internal discontinuities or external boundaries. A displacement transducer located close to the impact point measures the time history of the surface displacement caused by the arrival of reflected waves. This paper discusses the theoretical basis of the method and gives representative examples of the results of analytical and experimental studies. A procedure called spectral peak plotting is described which simplifies interpretation of data from scans across a structure. By using this procedure a profile view of the interior of the test object can be constructed.
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paper_title: Caisson Evaluation by Stress Wave Propagation Method

paper_content:

The theoretical aspects of wave propagation were verified in the laboratory by experiments on bars of aluminum and concrete which showed that it is possible to detect discontinuities and abrupt changes in the cross-sectional areas of long members. The theory is based on stress wave propagation in elastic solids and on the Rayleigh theory of surface waves. Field tests demonstrated that the method could be used for in-situ installations of caissons and piles, resulting in very little interference with normal construction operations. Basic data and records are presented for laboratory experiments on aluminum and concrete bars, and field tests on cast-in-place caissons and piles. The results indicate that the stress wave propagation method has the potential for providing a simple, rapid, inexpensive, and reliable method for detecting irregularities in caissons and piles.
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paper_title: Transient response of thick circular and square bars subjected to transverse elastic impact

paper_content:

The objective of the studies presented in this paper was to understand the transient response of thick circular and square bars subjected to transverse elastic point impact. It is shown that the transient response is composed of a number of resonant frequencies caused by cross‐sectional modes of vibration. The individual cross‐sectional modes and their corresponding natural frequencies were determined using plane strain and three‐dimensional finite‐element models of circular and square cross sections. The first few modes for both circular and square cross sections are shown. Subsequently, three‐dimensional finite‐element models of circular and square bars were used to determine the transient response caused by transverse point impact. To verify the results obtained from the numerical models, experimental studies were performed on a 0.4‐m‐diam circular bar and a 0.3‐m square bar. These specimens were representative of full‐size concrete columns—typical barlike structural elements. The results of these expe...
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paper_title: IMPACT-ECHO RESPONSE OF CONCRETE SHAFTS

paper_content:

Numerical and experimental studies of the transient response of concrete shafts subjected to elastic impact were carried out using the finite element method and the impact-echo testing technique. Two- and three-dimensional finite element studies of concrete shafts were performed for: (a) solid shafts; (b) shafts containing cracks, voids, layers of low-quality concrete, and changes in cross section, such as bulges and necks; and (c) shafts in soil. These studies were carried out to gain an improved understanding of the impact response of concrete shafts containing flaws, problems for which there are currently no analytical solutions. Laboratory studies of solid concrete shafts were carried out to verify the finite element models. Also, experimental studies were carried out on concrete shafts embedded in soil. Results show that additional information about the integrity of drilled shafts and piles can be obtained using impact techniques. The numerical studies also show that the finite element method is a powerful tool for studying the impact response of concrete foundation elements.
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paper_title: Transient response of thick rectangular bars subjected to transverse elastic impact

paper_content:

The objective of the research presented in this paper was to study the transient response of thick rectangular bars subjected to transverse elastic point impact. It is shown that the transient response is composed of a number of resonant frequencies caused by cross‐sectional modes of vibration. The individual modes and their corresponding natural frequencies were determined using plane‐strain, finite‐element models of rectangular cross sections. The effect of the cross‐section aspect (depth/width) ratio on the natural frequencies is shown. Three‐dimensional, finite‐element models of rectangular bars were analyzed to determine the transient response caused by transverse point impact. To verify the results obtained from the numerical studies, experimental studies were performed on rectangular bars with aspect ratios of 0.60, 0.75, 1.33, and 1.67. The bars used in the experimental study were representative of full‐scale concrete beams and columns—typical barlike structural elements. Excellent agreement was o...
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paper_title: Using Impact-Echo to Assess Tendon Ducts

paper_content:

A total of seven cantilever beams (each containing two ducts) were evaluated during the preliminary site investigation. In a number of the beams, cracks which extended through the webs of the beams were observed along the tendon trajectory. Although in these cases the impact-echo responses did not always follow the "ideal" patterns, it was still possible to identify the presence of voids in the tendon ducts.
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paper_title: Finite element studies of the impact-echo response of plates containing thin layers and voids

paper_content:

Quantitative nondestructive evaluation based on the use of transient stress waves generated by point impact is hindered by the fact that the governing partial differential equations admit closed-form solutions for only the most trivial cases, an infinite half-space and an infinite plate. In previous studies carried out by the authors, the finite element method has been shown to provide useful numerical solutions for a variety of cases involving bounded solids containing flaws. Numerical results have been verified with carefully controlled experiments. Currently, the method is being used to establish the basis for a new nondestructive evaluation technique for civil engineering structures. This technique is called impact-echo, and it is based on the use of low frequency, transient stress waves generated by elastic point impact. In this paper, the impact-echo response of plates containing thin layers is studied using finite element models. The purpose of these studies was to determine the applicability of using the method for detecting voids in layered civil engineering structures. Results of the numerical studies show that it is feasible to use the impact-echo method for this application.
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paper_title: EFFECTS ON IMPACT-ECHO SIGNALS CAUSED BY STEEL REINFORCING BARS AND VOIDS AROUND BARS

paper_content:

The paper explains the interaction of transient stress waves with steel bars in concrete and shows the effects that bars have on impact-echo signals. Numerical (finite element) and experimental results are shown to demonstrate the effects that different variables have on impact-echo results. Equations for the relationships between bar diameter, bar depth and the measured impact echo response are presented. It is shown that these relationships can be used to identify the effects caused by bars on impact-echo signals obtained from reinforced concrete structures. Finally, the changes in the impact echo response caused by the presence of a void around the top or bottom of a bar are also explained.
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paper_title: The impact-echo response of concrete plates containing delaminations: numerical, experimental and field studies

paper_content:

This paper describes the use of the impact-echo technique— a non-destructive testing technique based on the use of transient stress waves— for detecting delaminations in concrete plate-like structures such as bridge decks, slabs, and walls. Results obtained from numerical (finite element) analysis and controlled-flaw laboratory studies are presented and used to explain the elastic impact response of delaminated plates. Current impact-echo instrumentation is described, and results obtained from concrete bridge decks containing delaminations caused by corrosion of reinforcing steel are presented. These results provide a better understanding of the impact response of delaminated plates.
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paper_title: Detecting Delaminations in Concrete Slabs With and Without Overlays Using the Impact-Echo Method

paper_content:

The paper demonstrates the feasibility of detecting delaminations in reinforced concrete slabs using the impact-echo method, a nondestructive testing technique based on transient stress wave propagation. The results of two laboratory studies are discussed. One study involved detecting artificial delaminations embedded at unknown locations in a reinforced concrete slab. All the artificial delaminations in the slab were located. The second study was aimed at showing the feasibility of detecting delaminations in reinforced concrete slabs with asphalt concrete overlays. Two reinforced concrete slab specimens with corrosion-induced delaminations were tested. Prior to overlaying the slabs with asphalt concrete, the depths of delaminations as determined by impact-echo testing were verified by drilling at selected points. After the asphalt concrete overlays were applied, the slabs were retested. It was found that the impact-echo method could successfully locate the delaminations in the slabs through the asphalt concrete overlays.
---
paper_title: A finite element study of transient wave propagation in plates

paper_content:

Studies of transient wave propagation in plates were carried out to establish a ba­ sis for the impact-echo technique as a nondestructive test for flaw detection in concrete. The surface displacements caused by stress waves generated by point impact on a plate were calculated using both the Green's function solution and the finite element method; displace­ ment waveforms obtained by the two approaches showed good agreement. Displacement and stress fields in a plate were studied using finite element analy­ sis. It was shown that transient point load applied normal to a stress-free boundary gives rise to P- and S­ "wakes" -disturbances trailing the P­ and S-waves. The displacement and stress fields in each wake resemble those in the preceding wave.
---
paper_title: A finite element study of the interaction of transient stress waves with planar flaws

paper_content:

This paper presents a finite element study of transient wave propagation in plates containing planar flaws. The ef­ fects on displacement waveforms caused by waves diffracted from the sharp edges of a flaw are determined. Dis­ placement fields within a plate contain­ ing a flat-bottom hole show the interaction of transient stress waves with a planar flaw. Waveforms obtained from the finite element analysis were compared with experimentally obtained waveforms.
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paper_title: DETECTING VOIDS IN GROUTED TENDON DUCTS OF POST-TENSIONED CONCRETE STRUCTURES USING THE IMPACT-ECHO METHOD

paper_content:

A nondestructive testing technique that uses transient stress waves is part of an ongoing research program at Cornell University, which is aimed at developing the theoretical basis and practical applications for the impact-echo method. This paper describes a phase of the program involving detection of voids in grouted tendon ducts in post-tensioned concrete structures. Findings from the numerical finite element studies, controlled-flaw laboratory studies, and a field study are presented. Three-dimensional dynamic finite element analyses were performed to examine the response of fully grouted, partially grouted, and ungrouted tendon ducts to transient stress waves. Laboratory specimens were constructed and tested to confirm the numerical results. Numerical and laboratory studies showed that the impact-echo method could be used successfully to detect both complete and partial voids in grouted tendon ducts. A field study was then conducted on an existing post-tensioned bridge. Fully grouted, partially grouted, and ungrouted tendon ducts located in actual bridge girders were examined. The results of the impact-echo tests were verified by invasive testing; the ducts were opened up and visually inspected. The impact-echo results correctly located fully grouted, partially grouted, and ungrouted tendon ducts.
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paper_title: Determining the minimum crack width that can be detected using the impact-echo method Part 2. Numerical fracture analyses

paper_content:

To determine the minimum crack width that can be detected using the impact-echo method, two types of laboratory experiment on concrete plate specimens were conducted. In each experiment, a crack was propagated through the plate parallel to the plate surfaces, while surface displacements caused by the crack opening were measured and impact-echo tests were performed along the crack trajectory. The relationship between the surface displacements and the real crack-opening displacements was established using the results of nonlinear fracture analyses. The minimum crack width was determined by estimating the crack-opening displacements for each impact-echo test location. As a result of this work, the minimum crack width which can be detected was found to be 0.025 mm, and the minimum crack width for which stress waves are not transmitted across the crack faces was found to be 0.08 mm. This paper describes how crack-opening displacements were determined using nonlinear fracture analyses.
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paper_title: A PROCEDURE FOR DETERMINING P-WAVE SPEED IN CONCRETE FOR USE IN IMPACT-ECHO TESTING USING A P-WAVE SPEED MEASUREMENT TECHNIQUE

paper_content:

The dilatational or P-wave speed in concrete is needed in impact-echo testing if the dimensions of the structural elements or the location of flaws are to be determined. Previously, the P-wave speed was determined from cores or from performing a test on a portion of the structure having known dimensions and no flaws. In cases where neither approach was possible, an estimate had to be made of the wave speed. This paper presents the details of a method for independently determining P-wave speeds in concretes by determining the time it takes the P-wave to travel between two points along the surface of a structure. This procedure is more accurate than procedures that involve the use of Rayleigh wave speed measurements, and thus it is a better technique for use in quality control situations and other testing situations where more accurate results are needed.
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Title: The Impact-Echo Method: An Overview

Section 1: Introduction
Description 1: Provide an introduction to the impact-echo method and the history behind its development.

Section 2: Background
Description 2: Discuss the origins of stress wave testing methods and give an overview of stress wave propagation concepts.

Section 3: Impact-Echo Method
Description 3: Explain the fundamentals of the impact-echo method, including how it operates and the principles behind it.

Section 4: Instrumentation
Description 4: Describe the equipment used in impact-echo testing, including impactors, receivers, and data acquisition systems.

Section 5: Impact Duration
Description 5: Discuss the importance of impact duration in the test, and the factors influencing it.

Section 6: Smallest Detectable Flaw
Description 6: Address the factors that affect the detectability of flaws and explain the limitations of the method.

Section 7: Applications
Description 7: Highlight various applications of the impact-echo method in detecting defects in concrete structures.

Section 8: ASTM Test Method C 1383
Description 8: Summarize the ASTM C 1383 standard method for measuring the thickness of plate-like concrete members using impact-echo.

Section 9: Summary
Description 9: Provide a summary of the impact-echo method, its reliability, success factors, and the importance of experience in interpreting results.