Multifunctional concretes for structural, strain sensing, vibration damping, thermal and electrical applications

Multifunctional concretes capable of both structural and non-structural functions are made possible by appropriate admixtures. The use of short 10 {dollar}mu{dollar}m-diameter carbon fibers (0.5% by weight of cement) gave the strain sensing function (with gage factor up to 700). The strain sensing is associated with change in electrical resistance (DC) or reactance (AC) upon straining. The ability of carbon fiber reinforced cement was increased by ozone treatment of the fibers, such that the gage factor and repeatability upon repeated loading was improved. The reactance was a more sensitive indicator of strain than the resistance. The addition of latex to cement paste increased the vibration damping ability (with loss tangent up to 0.14 at 0.2-2 Hz), as well as increasing the storage modulus and decreasing the thermal conductivity. Silica fume, latex and methylcellulose were effective for decreasing the thermal conductivity of cement paste by up to 46%. The specific heat of cement paste was increased by up to 10% by addition of these admixtures. Carbon fibers as admixture did not increase the thermal conductivity. The flexural storage modulus was found to decrease gradually and reversibly with increasing temperature from 30 to 150{dollar}spcirc{dollar}C for cement pastes. Silica fume gave the highest flexural storage modulus at all temperatures, but also the greatest fractional decrease in modulus upon heating. Methylcellulose gave higher modulus than latex. The modulus increased with increasing latex/cement ratio. The degree of dispersion of latex particles in latex-modified cement paste was assessed by measurement of the volume electrical resistivity, which increased with latex content. Carbon filaments of diameter 0.1 {dollar}mu{dollar}m were found to be a much more effective additive than conventional carbon fibers of diameter 10 {dollar}mu{dollar}m in providing cement pastes capable of electromagnetic interference shielding (with shielding effectiveness of 40 dB for 1 GHz radiation), which is potentially useful for lateral guidance in automatic highway system. The bond strength between carbon fiber and cement was enhanced by oxidizing chemical treatments, with ozone treatment giving the greatest effect. The effect was accompanied by an increase in the electrical contact resistivity of the interface. The contact electrical resistivity of the steel fiber-cement interface was found to correlate strongly with the shear bond strength, so that it provides a non-destructive method of bond strength assessment. The bond between concrete and steel rebar was evaluated by electromechanical pull-out testing. The bond strength was increased by steel rebar surface treatment (acetone, water, ozone or sand blasting, with ozone being most effective and acetone being least effective), polymer addition to concrete (methylcellulose or latex, the methylcellulose with or without silica fume), increase in water/cement ratio of concrete (particularly from 0.45 to 0.50), and decrease in curing age (particularly from 14 to 7 days).