Effect of the Specimen Size on the Mechanical and Acoustic Characteristics of Concrete

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 should be taken into account by studying the fracture process of concrete to improve the operational safety of concrete structures. The aim of this work is to study the effect of sizes of the concrete specimens with a notch (concrete class B25, R̅ = 28 MPa) on the strength characteristics and parameters of acoustic emission (AE) recorded during the fracture process. Three-point bending tests with registration of AE signals were performed using specimens with three sizes (1075, 465, and 215 mm long), the geometry of which corresponded to two-dimensional similarity, since the specimen thickness remained unchanged, and both length to width ratios for the specimen and notch remained constant. The size of the zone of fracture localization at different stages of crack development was estimated on the larger specimens using the planar AE location and the time dependence of the zone size was plotted. For small specimens, the size of the localization zone was determined by measuring the ultrasonic attenuation coefficient. Analysis of the loading diagrams combined with the time dependences of the AE parameters showed that the specimen size affected the fracture staging. The dependences of the nominal strength, fracture energy, b AE parameter, and the total number of AE signals on the specimen size were plotted. It was shown that, for the large-sized specimens, the size effect was associated with the decrease in the nominal stress and number of accumulated acoustic emission signals, as well as with the increase in the fracture energy and b AE parameter. It is assumed that the change in the AE characteristics with the increase in the specimen size is determined by the decrease in the relative fraction of structural heterogeneity (the ratio of the average size of granite macadam to the specimen size) in large-size specimens. New data were obtained on the effect of the size of concrete specimens on the acoustic characteristics.