Novel protective covering to enhance concrete resistance against projectile impact

Highlights

• Novel protective covering was designed to protect concrete against projectile impact.

• The protective covering had a layered and staggered structure.

• Penetration depth in the protected concrete was highly reduced.

• Protective covering with epoxy adhesive interlayers can effectively protect concrete.

• Protective covering with silicone sealant interlayers suffered little severe damage.

Abstract

A novel protective covering with a layered and staggered structure was proposed to protect concrete against projectile impact. Experimental study was conducted to investigate the ballistic behaviour of the concrete targets against 12.7 mm armour-piercing incendiary projectile at velocities ranging from 537.7 to 596.5 m/s. The results showed that the concrete targets with protective covering exhibited superior integrity with no damage on the distal surface, whereas the concrete targets without protective covering were fractured with penetrating cracks throughout the thickness of the target. Moreover, the protected concrete targets displayed significantly reduced penetration depth compared with the concrete targets without protective covering. The protective covering with epoxy adhesive interlayers had a bigger protection factor than that with silicone sealant interlayers, but the former suffered more severe damage than the latter.

Introduction

High strength concrete with large thickness is usually used in protective structures such as military facilities, government buildings and civil air defence works. However, high strength concrete is generally more brittle than normal strength concrete and usually exhibits extensive damage when subjected to ballistic impact loading. Incorporating a small amount of steel fibre at 0.5–1.5 vol.% into high strength concrete can significantly reduce the impact crater damage through controlling crack development [1], [2], [3], [4], but it cannot effectively reduce the penetration depth even using much higher steel fibre dosages up to 7–11 vol.% [5], [6].

A protective covering could be used on concrete to provide better protection against projectile or fragment attack. The ideal material of the covering should be a cost-effective, hard/stiff, and tough material. However, such material is not commonly available. Ceramics with high mechanical properties such as hardness, strength and Young’s modulus are usually used as face plates in armour systems [7], but they are costly to be used in structural protection. Building ceramic mosaic tiles (CMTs) is one of the major decorative materials used in external walls, bathroom, swimming pools, etc. The tiles sintered in a high temperature of over 1200 °C possess higher hardness and lower porosity than concrete of the same compressive strength, but they have rarely been used in protective structures due to their inherent brittleness.

The existing publications on layered ceramics can provide some inspiration for the design of protective covering on concrete. The layered ceramics with weak interfaces have been reported to possess improved fracture toughness over the monolithic ceramics through delaying crack propagation at the interface regions and deflecting crack growth along the weak interfaces [8], [9], [10], [11]. However, such layered pattern shows less increased fracture toughness than the pattern of layered ceramics with strong interfaces due to the improved crack growth resistance and the residual compressive stresses (generated during cooling down from sintering) of the latter pattern [12], [13], [14], [15]. With strong interfaces, a crack crossing through the first layer may be arrested and/or bifurcate at the strong interface (compressive layer), thus the reliability of the system can be enhanced. Based on such findings, Bermejo and Danzer [12] proposed an optimal layered ceramic design which can combine the crack deflection and the crack bifurcation mechanisms through optimising the thickness and stresses of the compressive layers. Dutta and Tekalur [16] first synthesised staggered ceramic–polymer composites using freeze-casting technique and found that the fracture toughness of the composites is significantly higher than that of the ceramic and polymer counterpart respectively.

Although many studies have been performed on the impact resistance of concrete, limited attention has been given to the use of anti-penetration shield or covering on concrete which just like the face plate of ceramics in lightweight armor systems. If CMTs are arranged in a layered and staggered structure, they could be possible to exhibit high strength and satisfactory toughness, that means, besides traditional decorative function, CMTs may have the potential to serve as a protective covering on some concrete elements such as concrete walls or columns. Therefore, the present work aimed to explore the possibility of using the layered and staggered CMTs as a protective covering on concrete for ballistic protection. The effects of different types of adhesives on the failure behaviours of the protective covering and the protected concrete were also investigated. The investigations on the impact resistances of the novel protective covering and the protected concrete by it can be helpful in the optimisation of the anti-penetration shield or covering of protective structures, also, this work becomes more significant due to its innovation in exploring and improving the concrete resistance against high-speed projectile.