Evaluation of steel fiber distribution in cement-based mortars using active microwave thermography

Abstract

The non-uniform distribution of steel fibers in fiber-reinforced cement-based mortars (FRCMs) can lead to heterogeneous properties of hardened material with direct impact on mechanical properties. Among various nondestructive testing techniques, the active microwave thermography (AMT) has shown good potential for inspection of cement-based materials. AMT utilizes combination of microwave energy to generate controlled and localized heating and uses commercially-available infrared cameras to capture surface thermal images in real-time. Utilizing AMT, four FRCM samples made with different steel fiber volumes ranging from 0 to 3 % were investigated to evaluate the feasibility of this method for detecting and quantifying fiber distribution. Full-wave coupled electromagnetic-thermal numerical modeling was also conducted to evaluate the effect of dielectric properties, fiber depth, and fiber clumping on surface thermal profile. The results of simulations indicate that increase in fiber depth results in lower surface temperature, due to lower heating associated with induced surface current. Based on AMT measurement results, samples with higher fiber contents were shown to undergo greater increase in surface temperature, as observed for samples with 1 and 2 % fibers. However, this tendency was reversed with further fiber addition from 2 to 3 %, due to the potential of fiber clumping. Furthermore, the fiber clumping and lower level of fiber homogeneity for FRCM with 3 % fiber resulted in 55 % lower flexural toughness compared to the FRCM containing 2 % fiber. The results of fiber homogeneity measured from freshly cast prism were found to correlate well with those obtained from AMT technique determined on hardened samples.