Strain-sensing concrete improved by carbon fiber surface treatment

Strain-sensing concrete is concrete capable of sensing its own strain, as provided by using short carbon fibers las little as 0.2 vol.%) as an admixture. The sensing ability is related to the change in volume electrical resistivity of the concrete upon straining, as resulted from the change in contact electrical resistivity between fiber and matrix. The gage factor (fractional change in volume resistivity per unit strain) is up to 700. This paper reports that surface treatment of the carbon fibers by using ozone prior to incorporation in concrete improves the strain sensing ability, in addition to improving the mechanical properties. These effects are due to the increase in fiber-matrix bond strength, which is in turn due to the decrease in the contact angle between fiber and water to zero. The contact angle decrease indicates wetting of fiber by water and is due to the increase in fiber surface oxygen concentration and the change from C-O to C=O functional groups on the surface of the fiber. The improvement in strain sensing ability relates to the increase in gage factor and the absence of an irreversible resistivity decrease from cycle to cycle. The improvement in mechanical properties relates to increases in tensile strength, modulus and ductility. The ozone treatment has no effect on the tensile strength, volume electrical resistivity or morphology of the fiber itself.