1 Self Sensing "Smart" Concrete - Part 2

Self sensing concrete is concrete that is able to sense (measure) its condition without the help of embedded sensors that are used in instrumentation. The patent claim for this invention documents that it is composed of electrically conductive, discontinous, randomly oriented carbon fibres dispersed in an electrically conductive concrete-matrix (or masonsry) base. The electrical conductivity of the fibres are different than that of the base, and as a result of the contact between the fibres and base, the "sensor" attains an electrical conductivity. The concrete acts as the sensor because of this difference in electrical conductivity.

Strain is the change in length of a material per unit length. For ductile materials like steel, aluminium etc, strain is proportional to stress and has a linear relationship until a yield point when strain takes place more easily with a given amount of stress (plastic behaviour). Consequently, the ultimate stress is crossed and the material breaks or ruptures. Concrete on the other hand is a ductile material. Its stress strain curve is almost a straight line without any significant yeield point. What this means is that in the real world, there is no signs or warnings that point to over loading. Hence, concrete structures can fail on a fine morning and will shock many people.

In smart concrete, the detection of strain that can subsequently lead to micro-cracking is done with the help of a slight variation of a scientific phenomenon known as Piezoelectric Effect. Piezoelectric effect is the ability of a material to generate a voltage proportional to an applied stress. In our case, the deciding principle is piezoresitivity, or the proportional change of electrical resistivity with application of strain. In the event of the development of a micro-crack, the contact area between the fibres and the concrete varies sending off a proportionate change in resistivity. This change can then be related to how much strain was encountered.

The change of resistance per unit strain is a quantity called gage factor. Dr. Chung claims that gage factors as high as 700 has been achieved in carbon fibre reinforced concrete.

Traditional methods of sensing were, as we talked about, embedding structures with sensors - peizoelectrics, strain gages, optical fibres etc. This has a significant disadvantage in that the strength of the entire structure can be compromised to an extent because of the heterogenity induced by the cavity. Moreover, surface mounted strain gages are exposed to the elements and can be damaged easily.

In smart concrete, the discontinous fibres in question are about 10-15 micro meters in diameter and nominally 5mm long. Dispersion is brought about through the use of agents such as methylcellulose, silica fume and latex. Preparation includes dissolving the dispersant in water, mixing the defoamer and fibres with this solution and stirring, and then mixing it with concrete, latex and a water reducing agent and they are all mixed in a stone mixer. The mix is then shaken to reduce air bubbles and kept in a mold for some time. After that, it is demolded and cured in room temperature for upto 7 days.