Dynamic splitting tensile behaviour of engineered geopolymer composites with hybrid polyvinyl alcohol and recycled tyre polymer fibres

Partial replacement of the widely used polyvinyl alcohol (PVA) fibre in engineered geopolymer composites (EGC) with recycled fibres can reduce the material cost and improve sustainability. This study investigates the effect of hybrid PVA and recycled tyre polymer (RTP) fibre content on the quasi-static and dynamic splitting tensile behaviour and microstructure of ambient-cured fly ash-slag based EGC through split Hopkinson pressure bar, scanning electron microscopy and X-ray computed tomography tests. Results indicate that the presence of PVA or RTP fibres can considerably improve the quasi-static and dynamic splitting tensile behaviour of geopolymers. All investigated mixtures are characterised by remarkable strain rate sensitivity within the considered test range, which can be well described using the proposed relationship between dynamic increase factor and strain rate for predictions of dynamic properties. Replacing PVA fibre with 0.25–0.5% RTP fibre can lead to better dynamic splitting tensile properties of EGC compared to that with 2.0% PVA fibre, which can be mainly ascribed to the improved synergistic effect of hybrid fibres in controlling the cracks. The microscopic images reveal that the failure mode of RTP fibres is not sensitive to the strain rate due to its hydrophobic surface feature, which could benefit the energy absorption capacity of EGC under dynamic loading. EGC containing hybrid PVA and RTP fibres holds promise as a cost-effective and sustainable material for applications against dynamic loadings

Multiscale modelling of ionic diffusivity in unsaturated concrete accounting for its hierarchical microstructure

This study presents an integrated multiscale framework for modelling ionic diffusivity in unsaturated concrete accounting for its microstructural features and 3D moisture distribution. The hierarchical microstructure of concrete at multiscale from nano- to meso-scale is mimicked, based on which the fluid-solid interaction and moisture distribution in pore network of concrete with various saturation levels are simulated using a lattice Boltzmann multiphase model. A lattice Boltzmann-finite difference model for diffusion is developed to mimic the ionic diffusion and predict the ionic diffusivity in unsaturated concrete. Results indicate that ionic diffusivity in unsaturated concrete highly depends on moisture content and distribution, pore structure, and aggregate content. As the water saturation level drops to around 90%, interfacial transition zone starts to retard ionic diffusion. Voids have a great contribution to water saturation level but less effect on ionic diffusivity. The simulation results of ionic diffusivity at each scale agree well with experimental data

3D meso-scale modelling of tensile and compressive fracture behaviour of steel fibre reinforced concrete

This paper presents a novel meso-scale modelling framework to investigate the fracture process in steel fibre reinforced concrete (SFRC) under uniaxial tension and compression considering its 3D mesostructural characteristics, including different types of fibres, realistic shaped aggregates, mortar, interfacial transition zone and voids. Based on a hybrid damage model consisting of cohesive element method and damage plasticity method, a cost-effective finite element approach was proposed to simulate the fracture behaviour of SFRC in terms of stress-strain response, energy dissipation and crack morphology. The results indicated that under given conditions, the straight and hooked-end fibres improved the compressive damage tolerances of concrete over 11.5% while the spiral fibres had a negligible effect of 2.6%. The tensile macro-damage level index introduced was reduced over 15% by all fibres. Compared to straight fibres, the higher anchoring capacity of spiral fibres reduced the reinforcement performance while hooked-end fibres did not exhibit a significant influence

Effect of recycled polymer fibre on dynamic compressive behaviour of engineered geopolymer composites

To enhance the cost-effectiveness and sustainability of engineered geopolymer composites (EGC), polyvinyl alcohol (PVA) fibres in EGC can be partially replaced with recycled tyre polymer (RTP) fibres. This paper presents a systematic experimental study on the effects of PVA fibre volume fraction (1.0%, 1.5% and 2.0%) and RTP fibre content (0.25%, 0.5%, 0.75% and 1.0%) on the dynamic compressive behaviour of EGC under various strain rates (54.43–164.13 s−1). Results indicate that the flowability, quasi-static compressive strength and elastic modulus of EGC reduce with the increase of PVA fibre content, where the reductions can be effectively mitigated by adding RTP fibres. The dynamic compressive properties of all investigated mixtures including dynamic compressive strength, dynamic increase factor (DIF) and energy absorption capacity show a pronounced strain rate dependency which can be well described using the proposed equations for DIF against strain rate ranging from 10−5 s−1 to 103 s−1 with values of mostly greater than 0.9. The dynamic compressive properties of EGC are enhanced with the increasing PVA fibre dosage under various strain rates while replacing PVA fibre with a certain amount of RTP fibre (0.25% and 0.5%) can result in better dynamic compressive properties compared to EGC with 2.0% PVA fibre. EGC containing 1.75% PVA fibre and 0.25% RTP fibre can be considered as the optimal mixture given its superior quasi-static and dynamic compressive properties in comparison with EGC with 2.0% PVA fibre

Micromechanical modelling of fracture processes in cement composites

Cement composites are the most popular and widely used construction material in the world. Understanding and predicting fracture processes in these materials is scientifically challenging but important for durability assessments and life extension decisions. A recently proposed microstructure-informed site-bond model with elasticbrittle spring bundles is developed further to predict the elastic properties and fracture process of cement paste. It accounts for microstructure characteristics obtained from high resolution X-ray computed microtomography (micro-CT). Volume fraction and size distribution of anhydrous cement grains are used to determine the model length scale and pore-less elasticity. Porosity and pore size distribution are used for tuning elastic and failure properties of individual bonds. The fracture process is simulated by consecutive removal of bonds subject to failure criterion. The stress-strain response and elastic properties of cement paste are obtained. The simulated Young’s modulus and deformation response prior to peak stress agree very well with the experimental data. The proposed model provides an effective tool to simulate micro-cracks initiation, propagation, coalescence and localization

Behaviour of alkali-activated concrete at elevated temperatures: A critical review

Alkali-activated concrete (AAC) is recognised as a novel sustainable construction material to substitute Portland cement concrete with superior thermal and mechanical performance. However, AAC would suffer significant deterioration when subjected to elevated temperatures due to different damage mechanisms, including thermal incompatibility caused by different thermal coefficients between matrix and aggregates, pore pressure build-up and phase transformation. This paper presents a systematic and comprehensive review on the behaviour of different types of AAC such as alkali-activated fly ash, alkali-activated slag, alkali-activated metakaolin and alkali-activated fly ash-slag systems at elevated temperatures in terms of phase stability and microstructural evolution as well as thermal and mechanical performance. The effective strategies for improving the high-temperature resistance of AAC are reviewed and discussed from the perspectives of AAC matrix, aggregates and fibre incorporation, with special focus on how these strategies can tackle different damage mechanisms. This paper summarises the recent advances in the field and identifies the remaining challenges and opportunities for future research

Behaviour of strain hardening geopolymer composites at elevated temperatures

Strain hardening geopolymer composite (SHGC) is a sustainable fibre-reinforced material that exhibits superior strain-hardening and multiple cracking behaviour under tension. The geopolymer binder synthesised through alkali-activation of aluminosilicate precursors sourced from industrial by-products is chemically stable at elevated temperatures. This paper presents a systematic experimental study on behaviour of fly ash-slag based SHGC reinforced with polyvinyl alcohol (PVA) fibres exposed to elevated temperatures up to 800 °C through weight loss, uniaxial compressive, ultrasonic pulse velocity (UPV) and uniaxial tensile tests as well as thermal analysis using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and derivative thermogravimetry (DTG), X-ray diffraction (XRD), scanning electron microscope (SEM) and mercury intrusion porosimetry (MIP). Results indicate that the compressive strength of SHGC is increased until exposure to 250 °C, followed by a decline up to 600 °C and a regain at 800 °C. The strength gain mechanisms include further geopolymerisation that refines the microstructure, fibre bridging action and sintering that densifies the binder gels, while the strength reduction can be attributed to the damage induced by internal moisture removals and evaporation of fibres, decomposition of calcium compounds, and empty channels introduced by vanished fibres. Besides, the SHGC specimens exposed up to 250 °C exhibit high ductility performance with saturated microcracks when subjected to uniaxial tension, whilst those at 105 °C demonstrate the best strain-hardening degree because of the desired fibre-matrix interaction and enhanced matrix strength. Moreover, no spalling can be observed in SHGC due to its relatively porous structure

Micromechanical modelling of deformation and fracture of hydrating cement paste using X-ray computed tomography characterisation

Cement paste is the basic but most complex component in cement composites, which are the dominant construction material in the world. Understanding and predicting elastic properties and fracture of hydrating cement paste are challenging tasks due to its complex microstructure, but important for durability assessments and life extension decisions. A recently proposed microstructure-informed site-bond model with elastic-brittle spring bundles is developed further to predict the evolution of elastic properties and fracture behaviour of cement paste. It is based on microstructural characteristics of hydrating cement paste obtained from X-ray computed microtomography (micro-CT) with a spatial resolution of 0.5 μm/voxel. Volume fraction and size distribution of anhydrous cement grains are used to determine the model length scale and pore-less elasticity. Porosity and pore size distribution are used for tuning elastic and failure properties of individual bonds. The fracture process is simulated by consecutive removal of bonds subjected to surface energy based failure criterion. The stress–strain response and elastic properties of hardened cement pastes with curing ages of 1, 7 and 28 days are obtained. The simulated Young's modulus and deformation response prior to peak stress agree very well with the experimental data. The proposed model provides an effective tool to evaluate time evolution of elastic properties and to simulate the initiation, propagation, coalescence and localisation of micro-cracks

Microstructure-informed modelling of damage evolution in cement paste

AbstractCement paste is a binder for cementitious materials and plays a critical role in their engineering-scale properties. Understanding fracture processes in such materials requires knowledge of damage evolution in cement paste. A site-bond model with elastic-brittle spring bundles is developed here for analysis of the mechanical behaviour of cement paste. It incorporates key microstructure information obtained from high resolution micro-CT. Volume fraction and size distribution of anhydrous cement grains are used for calculating model length scale and elasticity. Porosity and pore size distribution are used to allocate local failure energies. Macroscopic damage emerges from the generation of micro-crack population represented by bond removals. Effects of spatial distribution, porosity and sizes of pores on tensile strength and damage are investigated quantitatively. Results show a good agreement with experiment data, demonstrating that the proposed technology can predict mechanical and fracture behaviour of cementitious materials based exclusively on microstructure information

Effect of limestone on engineering properties of alkali-activated concrete: A review

Alkali-activated concrete (AAC) is a promising sustainable alternative to cementitious materials concerning its environmental benefits and satisfactory engineering properties. Given the growing use of limestone and other forms of calcium carbonate as an additive or supplementary precursor in AAC, this review summarises the effect of limestone on the engineering properties of AAC synthesised from various precursors such as fly ash, slag, metakaolin, and blends of them. Due to the underlying mechanisms including filler effect, additional nucleation and dilution effect, and chemical reaction of limestone, the incorporation of limestone with physical and chemical modifications into AAC can result in changes in reaction kinetics, microstructure, fresh properties, and hardened properties of AAC, which are quantitatively discussed and compared. In addition, the sustainability and environmental impact of AAC containing calcium carbonate are also assessed, whilst the research gap and opportunities for the future are identified