Micro-cracking pattern recognition of hybrid CNTs/GNPs cement pastes under three-point bending loading using acoustic emission technique

The generation of microcracks has an important influence on the behaviour of concrete structures. In this study, the acoustic emission (AE) technique was used to investigate the fracture phenomena and micro-cracking behavior of hybrid carbon nanotubes (CNTs, the 1-D allotrope of carbon atoms) and graphene nanoplatelets (GNPs, 2D monolayer of sp²-hybridized carbon atoms), cement composites under three-point bending loading. In addition, dispersion level by ultraviolet visible spectroscopy (UV–Vis), mechanical performance, and micro-structure properties by scanning electron microscopy (SEM) of hybrid cement composites were studied. Different typical AE parameters like AE hits, rise time, frequency and amplitude distribution, and b-value were used for the AE analysis. The cracking pattern and failure mechanism of the specimens were observed into three stages: initial elastic stage, pre-fracture stage, and post-fracture stage. The conclusions were drawn as follows: (1) through the UV–Vis spectrometry, it is found that GNPs act as dispersion agents for CNTs; (2) hybrid specimens experienced large deformation with the increasing wt.% of CNTs + GNPs during post fracture phase, thus showing more ductile behavior as compared to the control sample; (3) based on the average frequency (AF) and RA value (rise time divided by the amplitude), the addition of CNTs + GNPs content in cement composites shifted the failure mode from shear to tensile; (4) the AE parameters are feasible to analyze and differentiate the cracking pattern between micro and macrocracks; (5) the field emission scanning electron microscopy (FESEM) image evidence that hybrid CNTs + GNPs particles are well dispersed within cement composites. Based on the obtained results, it was concluded that the acoustic emission technique is a favorable approach to monitor and identify the microcracking pattern in brittle concrete structures.