Yifei Hao

Review of the current practices in blast-resistant analysis and design of concrete structures

In contemporary society, industrialization and rising of terrorism threats highlight the necessity and importance of structural protection against accidental and intentionally malicious blast loads. Consequences of these extreme loading events are known to be catastrophic, involving personnel injuries and fatalities, economic loss and immeasurable social disruption. These impacts are generated not only from direct explosion effects, that is, blast overpressure and primary or secondary fragments, but also from the indirect effects such as structural collapse. The latter one is known to be more critical leading to massive losses. It is therefore imperative to enlighten our structural engineers and policy regulators when designing modern structures. Towards a better protection of concrete structures, efforts have been devoted to understanding properties of construction materials and responses of structures subjected to blast loads. Reliable blast resistance design requires a comprehensive knowledge of blast loading characteristics, dynamic material properties and dynamic response predictions of structures. This article presents a state-of-the-art review of the current blast-resistant design and analysis of concrete structures subjected to blast loads. The blast load estimation, design considerations and approaches, dynamic material properties at high strain rate, testing methods and numerical simulation tools and methods are considered and reviewed. Discussions on the accuracies and advantages of these current approaches and suggestions on possible improvements are also made.

Finite element modelling of mesoscale concrete material in dynamic splitting test

Dynamic tensile strength is one of the key factors of concrete material that needs to be accurately defined in analysis of concrete structures subjected to high-rate loadings such as blast and impact. It is commonly agreed that dynamic testing results of concrete material are influenced by the inertia effect, which is very much dependent on the specimen size and loading rate. It is therefore very important to remove the inertia effect in testing data to derive the true dynamic concrete material properties. On the other hand, coarse aggregates in concrete material are usually neglected due to testing limitation or numerical simplification. It has been acknowledged that neglecting coarse aggregates might not necessarily give accurate concrete dynamic material properties. In this study, a three-dimensional mesoscale model of concrete specimen consisting of cement mortar and coarse aggregates is developed to simulate splitting tensile tests and investigate the behaviour of concrete material at high strain rate. The commercial software LS-DYNA is used to carry out the numerical simulations of dynamic splitting tensile tests. The reliability of the numerical model in simulating the dynamic splitting tensile tests is verified by comparing the numerical results with the laboratory test data from the literature. The influence of inertia effect in dynamic splitting tensile tests is investigated and removed. An empirical formula to represent the true dynamic increase factor relations obtained from dynamic splitting tensile test is proposed and verified.

Damage Assessment of Two-Way RC Slab Subjected to Blast Load using Mode Approximation Approach

Significant research efforts have been invested on studying the response and damage of structures subjected to blast loads for better life and property protections. The single-degree-of-freedom (SDOF) approach has been widely adopted to simplify the structural response analysis for engineering design purpose. However, such an approach under certain circumstances oversimplifies the structural behavior and might not give reliable predictions of structural responses to blast loads. On the other hand, although detailed high fidelity finite element (FE) approach is able to give relatively accurate predictions of structural response, it is unfortunately not straightforward for application and very time-consuming, which impedes its application among engineers. Therefore, a method that can assure not only reliability but also efficiency is highly needed for design practice. In the present study, mode approximation method with Pressure–Impulse (P-I) diagrams is applied to analyze response and damage of RC slab due to blast load. Slab under analysis is assumed rigid-plastic and simply supported. Shear failure, bending failure and combined failure modes are considered based on different failure modes. Critical equations for structural shear and bending failures are derived respectively with appropriate failure criteria. P–I diagrams are then developed for quick damage assessments. The analytical results are verified by comparing with high fidelity numerical simulations. The reliability and efficiency of using this approach for design and analyzing RC slab response under blast loads are demonstrated.

Effects of curing conditions and sand-to-binder ratios on compressive strength development of fly ash geopolymer

AbstractThis paper investigates the effects of curing conditions on a high-strength geopolymer material synthesized by activating different combinations of Class F fly ash (FA), ground-granulated b...

Mesoscale modelling of concrete reinforced with spiral steel fibres under dynamic splitting tension

The addition of discrete steel fibres into concrete has been widely recognised as an effective measure to enhance the ductility, post-cracking resistance and energy absorption of the matrix subjected to impact loads. Despite useful information from experimental studies that investigate the macro-scale performance of steel fibre–reinforced concrete under dynamically applied loadings, results from a series of tests or from tests by different researchers are often found to be scattered. Besides variations in testing conditions, random variations of size, location and orientation of aggregates and fibres in steel fibre–reinforced concrete are deemed the fundamental reason of the scattering test data. High-fidelity modelling of concrete and steel fibre–reinforced concrete in mesoscale has been widely adopted to understand the influence of each component in the composite material. Numerical studies have been published to discuss the behaviour of steel fibre–reinforced concrete under dynamic splitting tension. Different shapes, for example, circles, ovals and polygons, of coarse aggregates were considered in different studies, and different conclusions were drawn. This study investigates the influence of the shape of aggregates on numerical prediction in mesoscale modelling of steel fibre–reinforced concrete materials with spiral fibres under dynamic splitting tension in terms of the strain distribution, cracking pattern and strength. The numerical model is validated by experimental results. It is found that the shape of aggregates in mesoscale modelling of splitting tensile tests has negligible influence. Furthermore, steel fibre–reinforced concrete specimens with different volume fractions of spiral fibres from 0.5% to 3.0% under various loading rates are simulated. Results from parametric simulations indicate the optimal dosage of spiral fibres in steel fibre–reinforced concrete mix with respect to the construction cost and mechanical property control.