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.

弯曲裂缝在钢筋混凝土 (RC) 梁中很常见。在使用载荷下，由弯矩引起的拉伸应力导致梁截面开裂，导致刚度损失和梁挠度随之增加。RC 梁设计中的适用性极限状态包括最大挠度和最大裂纹宽度。裂缝通过破坏超声波的传播路径来影响超声波的传播，这会导致超声波的强烈散射。因此，超声波能流到达的延迟可以通过尾波的增加来观察到。这个合成的非相干波场可以通过扩散超声方法来近似。与超声脉冲速度相比，扩散超声方法可以更好地描述 RC 元件更大区域上的裂纹效应，在具体应用中最常用的超声参数。扩散超声参数（扩散率、耗散和 ATME）的变化可能与 RC 元件中的裂纹程度有关。本研究的目的是应用漫射超声方法来评估由于 RC 梁的弯曲开裂引起的刚度损失。浇筑具有不同纵向抗弯配筋率的梁并进行弯曲试验。在测试过程中监测跨中挠度，从而监测梁刚度。超声波换能器安装在梁的中心区域，在测试期间进行超声波读数，以获取不同加载阶段的波形。对于每个波形，使用时频分析恢复扩散超声参数。分析了随着弯曲开裂引起的渐进式损伤的扩散参数的行为，并将其与梁的刚度损失相关联。结果，据观察，扩散系数和 ATME 是识别开裂开始的最敏感参数，并且还被视为与早期开裂阶段的梁刚度变化有关。当与高达 70% 的刚度损失值相关时，扩散率和 ATME 呈现出高平均相关系数，从而可以得出结论，可以通过开裂过程开始后的间隔内的扩散超声参数来估计刚度损失。据观察，扩散系数和 ATME 是识别开裂开始的最敏感参数，并且还被认为与早期开裂阶段的梁刚度变化有关。当与高达 70% 的刚度损失值相关时，扩散率和 ATME 呈现出高平均相关系数，从而可以得出结论，可以通过开裂过程开始后的间隔内的扩散超声参数来估计刚度损失。据观察，扩散系数和 ATME 是识别开裂开始的最敏感参数，并且还被认为与早期开裂阶段的梁刚度变化有关。当与高达 70% 的刚度损失值相关时，扩散率和 ATME 呈现出高平均相关系数，从而可以得出结论，可以通过开裂过程开始后的间隔内的扩散超声参数来估计刚度损失。