Effect of the sample size on the mechanical and acoustic characteristics of concrete

Abstract

One of the important features of the fracture of heterogeneous materials with a stress concentrator is the scale effect associated with the dependence of the nominal stress on the specimen size. This effect must be considered when studying the fracture process of concrete to improve the operation safety of concrete structures. The purpose of this work is to study the size effect of the specimens with a notch made of a concrete (class B25, R = 28 MPa) on the strength characteristics and parameters of the acoustic emission(AE) recorded in the process of fracture. Three-point bend tests with registration of AE signals were carried out using specimens of three sizes (1075, 465, and 215 mm long), their geometry corresponded to two-dimensional similarity, since the thickness of the specimens remained unchanged, and both length to width ratio for the specimen and notch remained constant. The size of the zone of fracture localization at different stages of crack development were estimated on the larger specimens using planar AE location and the time dependence of the zone size was plotted. The size of the localization zone of small specimens was determined by measuring the ultrasound attenuation coefficient. Analysis of the loading diagrams combined with the time dependences of the AE parameters revealed that the specimen size affects the fracture staging. The dependences of the rated strength, fracture energy, b AE -parameter, and the total number of AE signals on the specimen size are plotted. It is shown that the size effect is associated with a decrease in the rated stress for the large-sized specimens and number of accumulated acoustic emission signals, as well as with an increase in the fracture energy and b AE -parameter. It is assumed that change in the AE characteristics with increasing size of the specimen is attributed to a decrease in the relative fraction of structural heterogeneity (the ratio of the average size of granite aggregate to the specimen size) in large specimens. New data were obtained regarding the effect of the size of concrete specimens on the acoustic characteristics.