The influence of density on the fracture energy of AAC: From experimental investigation to the calibration of a cohesive law

Abstract

The paper investigates cracking development in Autoclaved Aerated Concrete (AAC) elements, and clarifies the effect of density and porosity on material mechanical properties. To this purpose, 4 material densities are analyzed, ranging from 300 kg/m3 to 580 kg/m3, corresponding to a compressive strength interval approximately ranging from 1.90 MPa to 5.50 MPa. Fracture mechanics of AAC is analyzed by carrying out three-point bending tests on notched beams, similar to those commonly used for normal concrete elements. In these tests, the onset and development of crack pattern is studied by means of Digital Image Correlation technique. An almost linear dependence of fracture energy from density (and consequently from strength) is derived based on test results. Experimental results are used to calibrate a bi-linear cohesive law, whose parameters vary with material density, so allowing to differentiate fracture properties for structural (high density) and non-structural (low density) AAC elements in finite element analyses. The cohesive law parameters are calibrated by exploiting a neural network algorithm and interfacing MatLab with ABAQUS Finite Element package. The curves obtained for the 4 investigated densities, normalized with respect to material tensile strength, are almost superimposed on each other as if the stresses were scaled with porosity. The displacements corresponding to the knee of the curve are nearly coincident, independently from material density. The good agreement between experimental and numerical results proves the reliability of the proposed approach.