Experimental and numerical study of ultra-high performance cementitious composites filled steel tube (UHPCC-FST) subjected to close-range explosion

Highlights

• UHPCC-FST specimens were prepared with the concrete compressive strength of 141.5 MPa.

• Field close-range TNT charge explosion test on UHPCC-FST specimens were conducted.

• Global and local deformations of UHPCC-FST specimens were experimentally recorded.

• Applicability of ALE, velocity and SDOF methods to predict global responses is discussed.

Abstract

The blast performance of ultra-high performance cementitious composites filled steel tube (UHPCC-FST) subjected to close-range explosion were investigated experimentally and numerically. Firstly, the field test of four circular UHPCC-FST specimens under the close-range TNT charge explosion with the scaled standoff distance Z of 0.12≤Z ≤ 0.14 m/kg^1/3 were conducted, the global and local deformations of specimens were experimentally examined. Then, three methods were applied to analyze the dynamic responses of those specimens, i.e., numerical simulations based on Arbitrary-Lagrangian-Eulerian elements method (ALE method) and velocity loading method (velocity method), as well as the equivalent single degree of freedom method (SDOF method). By comparing the experimental data of the present and existing total nineteen close-range explosion test on CFST (0.14≤Z ≤ 0.58 m/kg^1/3), the velocity method is recommended by considering both the accuracy and efficiency of computation. The derived conclusions could provide helpful references for evaluating the blast resistance and designing the CFST under potential close-range explosions.

Introduction

From 1970 to 2017, there have been more than 1,80,000 terrorist attacks occurred around the world, including more than 88,000 bombings, 19,000 assassinations, and 11,000 kidnappings [1]. It is obvious that the bomb attack is the most common terrorist action. Besides, the public infrastructures have becoming the main potential targets by terrorists in recent years [2]. As one of the important public infrastructures, the bridges, especially the landmark and memorable ones, are prone to be vulnerable to the terrorist bomb attack. Bridge columns are the main load-bearing members of bridge, the blast-resistance of which has much great significance to study.

The exiting work are mainly concentrated on the dynamic responses of normal reinforced concrete (RC) building or bridge columns under explosions [3], [4], [5], [6], [7], [8], [9], [10]. Taking the bridge columns for instance, Zong et al. [7] performed the contact and non-contact explosion (0.5 ≤ Z ≤ 2.56 m/kg^1/3, Z = a/W^1/3 is the scaled standoff distance, a is the standoff distance from the center of the explosive to the surface of structure, W is the charge weight) test on eleven bridge column specimens. It was derived that the blast-resistance capacity of RC column will be enhanced by increasing the sectional area and employing the square section, spiral stirrup and steel fiber reinforced concrete. Supported by the National Cooperative Highway Research Program (NCHRP), Williamson et al. [8], [9], [10] conducted a systematic research on the blast-resistance capacity of bridge columns, and it was observed that the concrete cover is easily cracked and spalled under blast loadings. Obviously, those failures have negative effect on the blast performance of the column.

In recent years, the concrete-filled steel tube (CFST) has attracted lots of attentions due to its excellent strength, ductility as well as the fatigue, high temperature and impact resistance [11]. As for the studies of CFST members under blast loadings, Fujikura et al. [12] conducted a blast test (scaled standoff distance is not specified due to the secrecy considerations) on 1/4 scaled CFST (diameters of 102, 127 and 152 mm) with the capping beams to simulate the scenario of car bomb attack on bridge, and it was shown that the CFST bridge has the prominent blast-resistance capacity. Li et al. [13] conducted the field explosion test on twelve CFST specimens, in which the effect of explosive charge weight (25~50 kg), standoff distance (3.5~5 m), axial compression ratio (7.6%, 16% and 23%), concrete cubic compressive strength (37~68 MPa) and longitudinal reinforcement ratio (6.9%, 8.3% and 9.9%) on the blast performance of CFST were systematically studied. It was derived that, the blast resistance of CFST is enhanced by increasing the concrete strength and steel reinforcement ratio, as well as decreasing the axial compression ratio. By performing the contact explosion test on the concrete-filled double-skin steel tube (CFDST) (core concrete compressive strength is 40 MPa, charge weights are 0.6 and 1 kg) with different outer steel tube thickness (6, 7 and 8.5 mm), Li et al. [2] derived that the blast resistance of CFDST increases with increasing the outer steel tube thickness. Remennikov and Uy [14] carried out the field contact and close-range explosion test (0.05≤Z ≤ 0.15 m/kg^1/3) on five square sectional CFST with the core concrete compressive strength of 46 MPa, the damage and failure of specimens were recorded. Ritchie et al. [15] performed the large-scale (1000 kg, 500 kg) and far-field (27 m, 15 m) air blast test on concrete filled rectangular hollow sections (RHS), in which the scaled standoff distances are 2.7 m/kg^1/3 and 1.89 m/kg^1/3, respectively. By measuring the free-field and reflected overpressure, displacement and longitudinal strain of specimens, the impact resistance of hollow RHS and the concrete filled RHS were comparably examined. Zhang et al. [16] prepared twelve Ultra-high performance concrete (UHPC) filled (double-skin) steel tube with the averaging compressive and flexural tensile strength of core UHPC are 170 MPa and 33.8 MPa, respectively. Then, an experimental study on above UHPC-FST subjected to close-range explosion (the scaled standoff distances are 0.41, 0.46 and 0.58 m/kg^1/3) was performed, in which the sectional shape of outer tube (circular and square), charge weight (17.5, 35 and 50 kg) and the axial load (0 and 1000 kN) were considered. Besides, under the close-range explosion with two scaled standoff distances of 0.19 and 0.14 m/kg^1/3, Sun [17] and Cui et al. [18] experimentally studied the dynamic response of CFDST (core concrete compressive strength of 45.8~55.2 MPa) as well as the reflected overpressure distributions of blast wave acted on the bridge columns, respectively.

As for the analytical and numerical studies of the column under close-range explosion, Zhang et al. [19,20] and Zhang et al. [21] numerically studied the dynamic responses of CFDST subjected to close-range explosion based on the CONWEP codes embedded in the finite element program LS-DYNA. However, Remennikov and Uy [14] pointed out that the CONWEP code is not suitable for most practical threats in near-field detonation scenarios, and then they proposed a simplified engineering-level model for the near-field blast impulse to predict the dynamic responses of structural members. Furthermore, based on Arbitrary-Lagrangian-Eulerian element method in LS-DYNA, Ngo et al. [22] numerically reproduced the close-range explosion test on CFST specimens conducted in Ref. [14]. Moreover, the equivalent single degree of freedom method has been widely used to predict the dynamic response of structural members under the blast loadings [23].

Generally, the existing studies have the following limitations: (i) Ultra-high performance cementitious composites (UHPCC) and UHPCC filled steel tube (UHPCC-FST) have becoming the most prospective construction materials and bearing members for high-rise and long-span bridges. However, most of the blast tests mentioned above are concentrated on the normal strength concrete and RC members; (ii) Based on the plenty of the test data, Orton et al. [24] defined the scaled standoff distance Z ≤ 0.4 m/kg^1/3, 0.4<Z ≤ 1.0 m/kg^1/3 and Z>1 m/kg^1/3 as the close-, medium- and far-range explosion scenarios, respectively. Considering the accessibility of the bridge column and the threat of terroristic car bombs, the close-range explosion should be paid much more attention. However, the existing experimental studies for the close-range explosion are limited; (iii) Compared with the far-range explosions, the detonation mechanism and blast loading of close-range explosion are more complex and different. The duration of the reflected overpressure acted on the specimen is commonly very short comparing to the vibration period of specimen, thus the main dominant parameter which influences the structural response is the non-uniform distributed impulse rather than the overpressure. There is no consensus for the efficient analytical approach to predict the structural dynamic responses subjected to the close-range explosion, although several methods have been proposed previously.

At present, the blast performance of UHPCC-FST subjected to close-range explosion is studied both experimentally and numerically. In Section 2, the close-range explosion test on four UHPCC-FST is conducted with the scaled standoff distance of 0.12≤Z ≤ 0.14 m/kg^1/3. Then, in Sections 3 and 4, three existing approaches are applied to analyze the dynamic responses of CFST in the present and existing close-range explosion tests, i.e., numerical simulations based on Arbitrary-Lagrangian-Eulerian elements method (ALE method) and velocity loading method (velocity method), as well as the equivalent single degree of freedom method (SDOF method). By comparisons of the experimental and predicted mid-span deflections, the applicability of above three approaches are discussed.