Behaviour of high strength concrete subject to impulsive loading

© 2005 Dr. Tuan Duc NgoWhile high strength concrete (HSC) has been used in many applications including blast and impact resistance, the behaviour of HSC under severe impulsive loading has not been investigated in-depth before. Due to the military nature of the past research work there is a serious lack of blast tests of concrete structures. Strain rate effects on concrete strength have been found in many previous studies but systematic experimental data, theoretical interpretation and advanced modelling tools have been lacking. This thesis presents the results of experimental and theoretical investigations into the behaviour of HSC subjected to severe impulsive loading. The research has been divided into four parts: (a) Review of the impulsive loading and structural response, strain rate effect and methods for nonlinear dynamic analysis of reinforced concrete structures; (b) Experimental programs consisting of the Hopkinson bar test and the blast test of high strength concrete panels; (c) Development of a strain rate dependent lattice model and an explicit code for modelling RC structures subject to impulsive loads; (d) Validation of the computer code and numerical applications. The main objective of the two experimental programs is to obtain research data in both the material level and the structural member level of concrete in the high strain rate domain. The first experimental program studied the material dynamic behaviour of concrete and the strain rate effect. In this program, Split Hopkinson Pressure Bar (SHPB) tests were conducted on different types of concrete cylinder with strength varying from 32 MPa to 160 MPa to derive the dynamic properties of concrete at strain rates up to 300 s^{-1}. SHPB test data were analysed to obtain the stress-strain relationship and strength dynamic increase factors (DIF) for concrete specimens under dynamic axial compression. A dispersion correction program was developed for analysing the test data which has significantly improved the accuracy of the results. The second experimental program involved a real blast test in the Woomera trial in South Australia. Four concrete panels 2m x 1m with thicknesses of 75mm and 100mm were tested against an explosion of 5 tons Hexoline (equivalent to 6 tons of TNT) at the stand-off distances of 30 and 40m. Three of the panels were made of Reactive Powder Concrete (RPC), an ultra-high strength concrete with steel fibre. This is the first time RPC is used for blast resistant applications. These RPC panels were specially designed for blast resistance and prestressed using high strength steel tendons. The fourth panel was made of normal strength reinforced concrete for comparison. The blast wave characteristics including reflected pressures and impulses were measured. The central deflection and post-blast damage of each panel were recorded and observed. The test results and observations showed that the Ductal panels performed extremely well surviving the blast with minor cracks while the comparison panel made of NSC was breached. Although HSC is an obvious choice to improve the blast resistance, the brittleness of HSC compared to normal strength concrete (NSC) may make HSC members more vulnerable. However as shown in the thesis if the ductility of HSC is increased by using special treatment such as adding steel fibres, HSC can be a very effective material for blast resistance. The main focus of the theoretical program is the development of an effective finite element technique for nonlinear dynamic analysis of reinforced concrete structures subjected to impulsive loading. A finite element code, RC-IMPULSIVE, for modelling the dynamic response of two-dimensional reinforced concrete structures has been developed in this research using the explicit time integration scheme. A strain rate dependent lattice model of concrete and steel was proposed. The smeared crack approach is employed to model concrete cracking behaviour. Steel reinforcement is modelled as a strain rate dependent elasto-viscoplastic material. Material nonlinearities including crack propagation, concrete crushing, post-failure residual strength, as well as the strain rate effects on the response of concrete and steel can be modelled by the explicit code developed in this study. In particular the deflections, stresses, strains, cracking patterns, progressive fracture in concrete and yielding of steel at any stage of the loading-time history can be obtained with sufficient accuracy. The explicit code RC-IMPULSIVE was validated using test results from the Woomera blast trial and other test data available from literature. Numerical analyses and a parametric study of reinforced concrete structures subjected to impact and blast loading were also performed using the computer code. The capability of program RC-IMPLUSIVE in predicting accurately the nonlinear dynamic response of concrete structures under impulsive loading has been proven