Experimentally validated multi-scale fracture modelling scheme of cementitious materials

Abstract

Cementitious materials are heterogeneous on mutliple length scales, from nanometres to metres. Consequently, their macroscopic mechanical properties are affected by material structures at all length scales. In pursuit of fundamental understanding the relationship between their multiscale heterogeneous material structure and mechanical properties, testing and modelling are required at all length scales. In this thesis, a series of experimental and modelling techniques for cementitious materials on multiple length scales (micrometre to millimetre) has been developed. This forms an experimentally validated modelling scheme in which experimental results are used to provide input and validation for numerical model at each length scale. The approach on micro-scale sized specimen preparation has been developed by combining thin-sectioning and micro-dicing techniques. Mechanical measurements on the prepared micro-scale sized specimens were performed using a nanoindenter under various test configurations. The micromechanical model has been developed by combining the micro X-ray computed tomography and discrete lattice fracture model. In terms of hardened cement paste (HCP), the micro-cube indentation splitting test technique offers experimental results for the calibration of the micromechanical model. The one-sided micro-cube splitting test was used to validate the calibrated model. Moreover, the one-sided splitting test can offer the nominal splitting strength of HCP. The micro-cube compression test was developed to validate the modelling results and to provide the compressive strength and Young’s modulus measurements of HCP at the micro-scale. The experimentally validated micromechanical model was further used to predict the uniaxial tensile fracture behaviour of HCP at the micro-scale. It is confirmed by both numerical modelling and experimental measurements that the micromechanical properties (such as compressive strength, tensile strength) of HCP are much higher than at the meso-scale properties. With respect to the interfacial transition zone (ITZ), micro-scale sized HCP-aggregate cantilever beams were fabricated and loaded by the nanoindenter. The measured load-displacement response was used to calibrate the microstructure informed lattice fracturemodel. This model was further used to predict the fracture behaviour of the ITZ under uniaxial tension. The volume averaging up-scaling approach has been adopted as a tool to pass the outcome from the micro-scale to the higher scale as input. The micro-beam three-point bending test has been developed to validate this modelling scheme on HCP. The good agreement between modelling and testing shows that this modelling approach can reproduce the experimental results in terms of fracture pattern, strength and elasticity well. This up-scaling approach was further validated by comparing the modelling and testing results of the 10 mm cubic mortar under uniaxial tension. As strength and fracture properties of cementitious materials are size dependent, a size effect study on HCP has been carried out using both one-sided splitting test configurations and the multiscale modelling approach. The size range of specimens that can be experimentally measured and numerically simulated are significantly improved by using these techniques. The experimentally validated multi-scalemodelling scheme developed in this thesis is fully quantitatively predictable at the meso-scale. This modelling scheme is generic. It can be used in the same or similar way for studying systems utilizing other binders or aggregates.