Evaluation of stiffness loss of reinforced concrete beams using the diffuse ultrasound method

Flexural cracks are common in reinforced concrete (RC) beams. At service loads, the tensile stresses induced by the bending moments cause beam sections to crack, leading to loss of stiffness and a consequent increase in beam deflections. Serviceability limit states in RC beam design include maximum deflection and maximum crack widths. Cracks affect the propagation of ultrasound by disrupting its travel path, which leads to a strongly scattering of the ultrasonic waves. As a result, there is a delay in the arrival of the ultrasonic energy flux, which can be observed by the increasing formation of coda waves. This resultant incoherent wavefield can be approximated by the diffuse ultrasound method. The diffuse ultrasound method can better describe the cracking effects over a larger region of the RC element compared to the ultrasonic pulse velocity, the most used ultrasound parameter in concrete applications. Changes in the diffuse ultrasound parameters (diffusivity, dissipation and ATME) can be related to the extent of cracking in a RC element. The objective of this research was to apply the diffuse ultrasound method to evaluate the stiffness loss due to flexural cracking of RC beams. Beams with different longitudinal flexural reinforcement ratios were cast and submitted to a bending test. The deflection at mid-span, and thus beam stiffness, was monitored during the test. Ultrasound transducers were installed in the central region of the beams with ultrasound readings performed during the tests in order to acquire the waveforms at various loading stages. For each waveform, the diffuse ultrasound parameters were recovered using a time-frequency analysis. The behavior of the diffuse parameters with increasing progressive damage caused by flexural cracking was analyzed and correlated to the stiffness loss of the beams. As a result, it was observed that diffusivity and ATME were the most sensitive parameters to identify the onset of cracking and also were seen to be related to beam stiffness variation at early cracking stage. When correlated with stiffness loss values up to 70%, diffusivity and ATME presented high mean correlation coefficients, allowing to conclude that it is possible to estimate the stiffness loss through the diffuse ultrasound parameters in the interval following the beginning of the cracking process.