Elsevier

Structures

Volume 14, June 2018, Pages 220-229
Structures

The Failure Behaviour of Reinforced Concrete Panels Under Far-field and Near-field Blast Effects

https://doi.org/10.1016/j.istruc.2018.03.009Get rights and content

Abstract

The distribution and intensity of blast pressure from far-field and near-field blasts can produce remarkably different patterns of loading on a reinforced concrete (RC) panel and can cause the panel to experience different modes of failure. Behaviour of RC panels, designed without consideration for extreme loading was investigated to analyse their damage and failure modes under different blast scenarios. The damage and failure modes of RC panels tested in two different full scale open air blast trials are presented. One of the tests was conducted in far-field condition with 40 m standoff distance. The other test was conducted on panels in near-field condition with standoff distances up to 2 m. These tested RC panels were subsequently modelled and analysed using the non-linear finite element (FE) code, LS-DYNA. Developed finite element models were used to conduct further parametric study to investigate damage behaviour and failure modes of RC panels under different blast loading conditions. The effect of blast pressure distribution and intensity on the failure modes of different RC elements were studied and blast pressure distribution and shock density were found to influence the mode of failure of a RC element significantly. Based on parametric study results, specific values of incident angle and shock density of blast pressure were suggested which can be used to predict the transition between failure modes of RC panels from shear to flexure for far-field and near-field blasts.

Introduction

The structural response of concrete elements subjected to blast loading is complicated, as the impulse load caused by the explosion is highly non-linear and occurs in an extremely short duration of time. Concrete damage and fragmentation under blast loading is mainly caused from stress waves generated by the pressure pulse loading of an explosion [1]. Tests on concrete elements under impact load for damage assessment and failure mode prediction are not scarce in literature. Due to the cost and complexity involved in conducting full scale tests, open air blast results are very limited. Although, some investigations have been reported in literature, many of these studies were focused on the study of the performance of different strengthening techniques applied to concrete panels [[2], [3], [4], [5]]. Some of the tests were conducted to study the performance of ultra-high strength concrete elements under blast effects [[6], [7], [8], [9]]. Some of the relevant blast trials reported by various researchers are briefly presented in the following paragraphs.

Ellis and Tsui [10] studied the behaviour of two-way panels under explosive loading and these panels were subjected to gas pressure with significantly lower peak pressure (below 150 kPa) to produce substantial damage to the panels. Meanwhile, number of concrete beams with the same cross-sectional dimensions but with different types of concrete have been tested in shock tubes by Magnusson and co-researchers in order to study their responses [[11], [12], [13]]. Schenker et al. [14] conducted full-scale field explosion tests on concrete slabs mainly to study the ability of aluminium foam to mitigate blast wave loads, while Iqbal [15] reported on external blast tests on a scaled down containment structure.

Wang et al. [16] undertook experiments to study the explosion resistance of one-way square reinforced concrete slabs under a closed-in explosion and two major damage levels, i.e., spallation with a few cracks and moderate spallation were identified. The spallation process were then modelled and compared with the experimental findings in a subsequent study by the authors [17]. Pantelides et al. [18] reported on the experimental findings of reinforced concrete panels tested under detonations with scaled distances ranging from 0.41 to 0.57 kg/m1/3. The performance of the panels was classified based on different reinforcement details in terms of damage resulting from blast event into three protection levels. Damage was defined in terms of the amount of concrete spalling, crack width, and panel deflection. The finding of the study also revealed that fibre reinforced concrete panels reinforced with steel bars performed better than any other type of panels, for all three panel thicknesses [18].

Shin et al. [19] studied close-in detonations of high explosive using computational fluid dynamics code AUTODYN. The focus of this research was to understand the blast load nature and its propagation for close-in detonations. In this study, numerical analysis was performed to examine the influence of mesh size of air medium, expansion of detonation product and afterburning, and the presence of reflecting and transmitting boundaries, on the blast overpressures and impulses [19]. Li and Hao [20] conducted numerical study to developed three-dimensional models to predict spall damage of reinforced concrete under explosive loads. Numerical findings by Lin et al. [21] indicated that the mass of explosive charge and the standoff distance have great influence on the blast response of reinforced concrete panels. The maximum deflection at the middle of a panel was greatly affected by the choice of charge mass and stand-off distance, and also the deflection of reinforced concrete panel can be reduced by increasing the panel thickness and the ratio of reinforcement [21].

Hao and Hao [22] performed numerical analysis of a reinforced concrete wall subjected to different blast loading conditions. The findings demonstrated that neglecting strain rate effect on concrete strength always lead to overestimation of the performance of the RC structures. Thiagarajan et al. [23] undertook investigations to determine the response and behaviour of both high-strength concrete and normal-strength concrete slabs doubly reinforced with high-strength low alloy vanadium (HSLA-V) reinforcement and conventional steel reinforcing bars subjected to explosive loads. The findings indicated that the use of high-strength materials, both steel and concrete, improved the level of protection of offered to the structural members when subjected to blast loads.

Li et al. [24] conducted an experimental study on the behaviour of Ultra-high performance concrete (UHPC) panels under blast loading. The equivalent TNT charge weights varied between 1.0 and 14.0 kg at scaled distances ranging from 0.41 to 3.05 m/kg1/3. This study studied the performance and structural behaviour of UHPC panels. In a separate study, Li et al. [25] investigated the blast resistance of a composite slab design with steel wire meshes along with conventional rebars. These high strength self-compacting concrete panels with steel mesh displayed higher flexural capacity as expected. Park et al. [26] conducted a study on two reinforced slabs exposed to blast loading generated from explosive placed at a distance of 20 m. The objective of far-field blast experiment [26] was to utilize the strain values in rebars and in concrete to predict reinforced slab behaviour. Although, the experiment was aimed at obtaining the one-way slab behaviour but the selected slab dimensions are not consistent with that aspect.

The summary research presented above shows that many aspects related to blast loading and corresponding damage has been investigated in the past, but detailed study to understand the possible failure modes and related damage for near-field and far-field blast is desirable. This paper presents the results of parametric study based on two separate open-air blast trials conducted on normal-strength reinforced concrete panels by the Advanced Protective Technologies for Engineering Structures (APTES) Research Group of the University of Melbourne. The experimental programmes were designed to determine the damage patterns and the failure modes of reinforced concrete panels designed for standard loading conditions and exposed to blast loading from full-scale open-air blasts. Finite element modelling of the tested panels were performed using explicit non-linear finite element code LS-DYNA and various significant aspects for realistic simulation of concrete elements under blast loading were considered. The finite element models, after verification with the experimental results, were used in the parametric study of reinforced concrete element behaviour under blast loading. The objective of the parametric analysis was to investigate the effect of blast pressure distribution on the failure modes of one-way reinforced concrete panels with different reinforcement configuration and dimensions. The study also focussed to find a limiting value to restrict the use of flexural and shear-based damage criteria for various blast loading situations to achieve realistic damage assessment. This constitutes the main contribution of this study in the optimised analysis of the response of reinforced concrete elements under blast effects.

After the introductory discussion, this paper presents the findings from the experiments conducted as a part of this research. The subsequent section presents the details, parameters and findings obtained from the numerical analysis performed in this study. The parametric analysis, results and findings from it was covered in the next section. The final section of this paper provides an examination on the major findings obtained from this research.

Section snippets

Open air blast trials

The first test was conducted at a large open air blast trial site in South Australia and the other tests were conducted at Hanoi, Vietnam. The objective of the first blast trial (Trial 1) was to obtain the damage of a concrete panel subjected to uniformly distributed blast pressure through a far-field blast. Meanwhile, a number of RC panels were tested in the second blast trial (Trial 2) at close range explosions. In both field tests, the test panels were housed in the support structure in such

Numerical modelling

The non-linear finite element code, LS-DYNA [28] was used for the numerical analysis in this study. This program has been widely used for modelling impact and blast-related problems of structural engineering and has the capability to perform advanced structural dynamics analysis with sophisticated material constitutive models. Strain rate effects on concrete and reinforcement bars, and erosion of concrete elements from the structure during response analysis to simulate the spalling of concrete

Parametric study

The primary focus of the parametric study was to investigate the effect of blast pressure distribution on the failure modes of one-way RC panels with different reinforcement configuration and dimensions. The finite element models of panels B3 and A1 were employed for this parametric study. The study also focused to find a limiting value to restrict the use of flexural and shear-based damage criteria for various blast loading situations where other modes of failure will control the failure.

Results and discussion

The mode of failure was predicted based on the plastic strain profile of the RC panels at different stages under blast loading. The maximum principal strain value of 0.05 was taken to identify the spalling in the RC concrete elements. Fig. 12 shows the response of panel B3 to a 5-kg charge weight at 0.3 m stand-off distance without the inclusion of erosion criteria, while Fig. 13 presents the response of the same panel under similar blast scenario, but with maximum principal strain value of

Conclusion

Damage patterns and the failure modes observed for two one-way RC panels under open air blast trials were presented. In the first trial, the stand-off distance was high and the uniform pressure distribution resulted in yield line failure of the simply-supported panel. In the second trail, due to less uniform distribution of blast pressure, distributed flexural cracks were observed in the panel. Non-linear transient finite element analyses of those RC panels were performed. Those finite element

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