Tuesday, November 28, 2006

2 Self Sensing "Smart" Concrete - Part 3

A sample of self sensing concrete (160mm long and 40 mm wide) using the procedures briefed above was prepared for a laboratory experiment. A 3 point testing was done to measure simultaneous compressive/tensile flexular strength and electrical resistivity along the stress line. The results confirmed that flexural strength increased, and sensing ability was achieved. It was also noticed that with different compositions, different results were yielded. The best linearity for fractional change of resistance and strain vs stress was acheived with methylcellulose. In any sensing device, linearity is an attractive feature which makes calibration easy.

The question will arise as to why continuous fibres cannot be used instead of discontinuous ones. One main reason is that it is expensive, and difficult to incorporate into the concrete. The workability is negatively affected, and the same sensing ability is not acheived. Also, it is not necessary to have all that electrical conductivity for making concrete smart.

Another method that could be done for flexural testing is to coat conventional concrete on the tension or compression side with smart concrete. This scheme will yield higher sensitivity on the tension side than on the compression. An implementation of this procedure is coating ceilings of buildings for self sensing behavior. Through out my life, I have noticed cracks on ceilings in concrete that can increase seepage of water. Imagine if you were the victim and the leaking water came from a dirty washroom on the floor above you!

In a real world application as in the levees we talked about, Dr. Chung explained that structural flaw such a micro-crack would increase the resistance of the structure and this change could be continously monitored by probes on the outside of the structure. She also explains that the outside shell of the levee can be made from this composite (similar to what is done in the airline industry nowadays). This will mean reinforcement with sensing capability.

Its an interesting question as tohow small these probes are and what fashion these probes will be placed in. Won't they be subjected to the elements as well? Another question being thrown is whether plants can grow through smart concrete. I've seen tiny plants emerging from regular concrete.

It can be immediately deduced, however, that this sort of technology holds great promises. In situations like earthquakes, or evacuation emergencies in high rise buildings this will greatly help. The weak spots in a structure can be found out without having to actually have a full scale disaster to prove it. A crack is a surface defect and the carbon fibres (impurity) can interfere with the propogation of this flaw, reducing chances of catastrophic failure.

In a post 9/11 world where our borders and important infrastructure (that means a lot economically) are in danger, this technology can provide another avenue of solutions. The idea for this invention adds to other 21st century smart technologies such as cargo analysing at border checkpoints and seaports using magnetic sensors, and "self healing" nano structures.

The 21st century is a dangerous one, as well as one that holds amazing potential for growth in technology. I have to careful about thinking in this manner, but I have often thought that sometimes, disasters are the impulse (the need or want) for newer technologies. In this way, they may be good or bad, but the passion of discovery is never a bad one, whatever may set it off. I'm proud of the fact that Dr. Chung teaches me. I am also confident of the fact that the day is not far when this technology will be adopted in construction after proving to be commercially viable.





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