Experimental study of effects of water-cement ratio and curing time on nonlinear resonance of concrete

Experimental techniques based on the theory of nonlinear acoustic resonance have been proved highly sensitive to the development of damage in concrete, particularly during the early age. As a multiphase composite material, the microstructure of concrete has a significant influence on damage development of concrete when subjected to external loading. Hence, it is important to understand the effect of main parameters associated with material microstructure such as water-cement (w/c) ratio and curing period on the nonlinear resonance response of concrete. In this research, theoretical background is first introduced for the implemented nonlinear impact resonance acoustic spectroscopy (NIRAS) technique, in which a parameter representing material nonlinearity is designated as the overall indicator for discontinuous material features of concrete samples. Applying the NIRAS technique, the nonlinearity parameter is analyzed for two sets of concrete samples mixed with different w/c ratios and cured for 7 and 28 days, respectively. The results show that under the increased loading, the longer curing time generally reduces the nonlinear response of concrete because the late filling effect of hydrated products in voids of interfacial transition zone could limit the development of microcracks. The higher w/c ratio will accelerate the nonlinear response of concrete due to the interconnection effect of material porosity on the propagation of microcracks. It is also noteworthy that the nonlinear response of samples in intact state corresponding to a lower w/c ratio is, however, enhanced. This effect is explained by the fact that less capillary water existing in the samples mixed with a lower w/c ratio work as lubricating fluid in voids to mitigate the opening-closing motion of initial material defects.