Temperature and moisture effects on electrical resistance and strain sensitivity of smart concrete

Cement composites, with enhanced electrical properties, have been investigated as multifunctional composites, with application as strain sensors, heating elements or anode in different electrochemical technique. In this work, several aspects regarding the practical application of smart concrete with the addition of brass fibers have been addressed. In general, electrical conductivity of cement composites is dependent on their moisture content and temperature. Therefore, for a real use of this type of sensors the influence of these parameters should be determined. The electrical resistance of the smart concrete decreased linearly with temperature up to 50 °C, therefore this effect can be compensated in strain sensing applications. On the other hand, at higher temperatures, especially after 150 °C, the mismatch strain between brass fibers, cement paste and aggregates resulted in damage, which was detected as an increase of the electrical resistance on the order of 613%. Thus, smart concrete can be used as fire alarm sensor. Also, the relationship between moisture and electrical resistance change was determined. After curing, the samples with moisture content of 5.2% were put in an electric furnace at 90 °C. After 60 min of 90 °C exposure the minimum resistance was measured as 702 Ohm for an average moisture content of 4.8%. At this optimal moisture content, water between fibers was lost and direct contact between fibers was achieved, maximizing the electrical conductivity of the composite. After this point, when the smart concrete was heated for a longer time, the electrical resistance increased almost 300% because water acting as electrolyte was lost. Thus, smart concrete is sensitive to moisture change and can be used as moisture sensor, especially at constructions exposed to harmful water. The effect of moisture content on the strain sensitivity of smart concrete was tested. The mechanism relating piezoresistive properties and moisture content were enlightened. Multifunctional smart concrete can be used as strain, fire and moisture sensor while acting as a load bearing element.