Cement-based composite materials are widely used in engineering applications. The strength and damage patterns of such materials depend upon the properties of the constituent components as well as the microstructure. Three scale levels are generally recognized in the analysis of the mechanical behaviour of composites, namely, macro-scale, meso-scale, and nano- or atomistic scale. Modelling of the mechanical properties at the meso-level provides a powerful means for the understanding of the physical processes underlying the macroscopic strength and failure behaviour of the composite materials under various loading conditions. This thesis endeavours to develop effective and efficient mesoscale models for cement-based composites, especially concrete, with a focus on dynamic analysis applications and in a three-dimensional stress-strain environment. These models are subsequently applied to investigate the intrinsic microscopic mechanisms governing the behaviour of such material under complex and high rate loadings, such as those due to shock, impact and blast. To cater to the needs of dynamic analysis under complex stress conditions, a general 2-dimensinal (2D) mesoscale modelling framework is further developed with the incorporation of the 3-D effect. This framework integrates the capabilities of MATLAB programming for the generation of the mesoscale geometric structure, ANSYS-CAE for finite element mesh generation, and the hydrocode LS-DYNA for solving the dynamic response of the model. The 3D effect is incorporated via a novel pseudo-3D modelling scheme such that the crucial lateral confinement effect during the transient dynamic response can be realistically represented. With the above mesoscale model a comprehensive investigation is conducted on the dynamic increase factor (DIF) in the concrete strength under compression, with particular focus on the variation trend at different strain rate regimes, and the key influencing factors. The wave propagation effect under high strain rate is scrutinised from a strip-by-strip perspective, and the correlation between the externally measured stress-strain quantities and the actual processes within the specimen is examined. The contribution of the material heterogeneity, as well as the structural effect (inertia), in the dynamic strength enhancement is evaluated. The classical Brazilian (splitting) test for the dynamic tensile behaviour of concrete is also investigated with the aid of the mesoscale model. Of particular interest here is the validity of such an indirect setup in reproducing the tensile behaviour of the specimen under high strain rates, as well as the effect of the heterogeneity in the dynamic tensile strength. Complications are found to arise as the loading rate increases. The change of the damage patterns with increase of the loading rate and the implications on the interpretation of the results are discussed. As an ideal solution to modelling of the 3-D effects, a methodology for the creation of a complex real 3-dimensional mesoscale model is put forward in the last part of the thesis. A geometric concept, called convex hull, is adopted for the representation of aggregates, and this makes it possible to utilize the relevant algorithms in computational geometry for the present purpose of generation of random 3-D aggregates. A take-and-place procedure is employed to facilitate the generation of the complete 3-D meso-structure. Associated techniques are developed for fast detection of particle inclusion-intersection. An example 3D mesoscale model is presented and representative numerical simulations are carried out to demonstrate the performance of the 3-D mesoscale modelling scheme.

Heavy timber construction is emerging as a viable alternative to conventional building materials, such as steel and concrete, for mid- and high rise structures. With the increasing presence of timber structures at or near potential targets comes an increased risk for damage to the structure and more importantly human casualties. The current provisions related to wood in the blast code (CSA, 2012) are limited and based on general understanding of the material behaviour rather than thorough research studies. Also, the standard does not clearly distinguish between the various types of engineered wood products. A study was undertaken to assess the behaviour of cross-laminated timber panels subjected to simulated blast loading using a shock tube apparatus. More specifically, the aim of this study was to investigate the behaviour of CLT panels subjected to static and dynamic loads to determine a dynamic increase factor in order to quantify high strain rate effects on this material. Testing was completed on a total of 18 CLT panels, with panel thicknesses of 105 and 175 mm corresponding to a 3-ply and 5-ply panel, respectively. An average dynamic increase factor of 1.28 on the resistance and no apparent increase in stiffness from static to dynamic loading were observed. Two resistance material predictive models that account for high strain-rate effects and the experimentally observed post-peak residual behavior were developed. A single degree-of-freedom model was validated using full-scale simulated blast load tests, and the predictions were found to match well with the experimental displacement-time histories.

Experiments were conducted on ASTM A572 50 and A992 steel over a range of intermediate strain rates in order to determine material strength properties of structural members subjected to dynamic loadings. The yield and ultimate tensile stress (UTS) of the steels were determined at increasing strain rates using a hydraulic apparatus and compared to static values obtained from ASTM E8 standardized tensile experiments. Results revealed that A572-50 steel exhibited an increase in yield stress of up to 35% and UTS of up to 20% as strain rate increased from 0.002 to 2.0 s-1. A992 steel demonstrated a similar increase in yield stress of up to 45% and UTS of up to 20%. Ratios of dynamic-to-static strengths were used to develop dynamic increase factor curves spanning the range of strain rates studied. These curves provide designers with material property values required for accurate and economical design of protective structures.