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Bomb-Proofing Buildings

Rescue workers after a bomb blast

Rescue workers after a bomb blast in southern Thailand. Three bombs exploded in the business district of the city of Yala, with police reporting that the devices had been placed in a car and a motorcycle. Surapan Boonthanom / Reuters / Picture Media\RTR304WS\

By Chengqing Wu

A new form of reinforced concrete that can absorb the blast of an explosion is being developed for use in buildings that can withstand terrorist attacks.

The increasing number of terrorist attacks has generated considerable concern throughout the world. Not only do explosive devices used in terrorist attacks result in multiple deaths and casualties, they can also cause the collapse of buildings, bridges and other important infrastructure.

When a bomb is detonated outside a building it releases a huge amount of energy instantaneously. The resulting shock wave in the surrounding air expands rapidly to the front of the building, blowing its exterior walls and columns. The blast pressure inside the building moves floors upwards while the blast pressure on the outside of the building forces the side walls, roof and rear walls inwards.

The direct air-blast effects cause local failure of parts of the building. Flying debris generated from the direct blast causes further damage to the building and injury to its occupants. Local failure may progress to global failure and the collapse of the building.

To enhance the resistance of structures to both local and global failure due to an explosion, there has been a significant increase in both theoretical and experimental research into the behaviour of structural components subjected to blast loading. The major objective is to increase the energy dissipation capabilities of the structural components, with most of the focus on developing new materials to absorb blast energy.

In order to improve the capacity of building materials to absorb the high energy impacts of bomb blasts, we are developing an “ultra-high performance concrete” (UHPC) that substantially improves the mechanical properties and plasticity of hardened concrete. We are currently developing and testing new materials that incorporate nanoscale materials into steel fibre-reinforced self-compacting concrete. The steel fibres not only increase the concrete’s plasticity and tensile strength – the ability to withstand stretching or pulling – it also creates a homo­geneous mix that binds cracks and retards their propagation, thus preventing sudden compressive failure.

What Is Ultra-High Performance Concrete?

Ultra-high performance concrete is a mix of self-compacting concrete with 1–3% steel or synthetic fibres by volume. With low water to binder content and much smaller particle size, the self-compacting concrete is known for its high “flowability” and good cohesiveness.

Combining self-compacting concrete with end-hooked, waved and spiral steel fibres can enhance anchorage properties within the surrounding structure and significantly improve its performance. The further addition of nanoparticles of CaCO3, SiO2 and Al2O3 facilitate chemical reactions that can be used to improve the performance of UHPC.

Immediately after the mixing procedure, slumps of the fresh concrete are measured to ensure that the consistency and flowability of the UHPC is suitable for construction purposes.

Improved Blast Resistance

Tests with our Chinese collaborators at Tianjin Chengjian University have found that structural components made from UHPC have a greater ability to withstand the effects of blasts. The material is at least five times stronger than conventional concrete, with five times the ability to withstand compression, 10 times the tensile strength and improved plasticity. Furthermore, the energy dissipation capabilities of the UHPC structural components can be several orders of magnitude higher than normal concrete materials.

Full-scale blast tests have also been conducted to investigate the resistance of UHPC columns to nearby explosions. In order to resist this blast loading and minimise structural damage, the concrete column must be able to absorb or dissipate the energy.

Our blast tests revealed that while a conventional reinforced concrete column failed to withstand a 10 kg explosive, the new UHPC column could survive a 50 kg explosive. This indicates that columns made of the new UHPC materials are strong enough to prevent the collapse of buildings after terrorist attacks made with devices like contact suitcase bombs and nearby car bombs.

UHPC therefore has tremendous potential for use in high-rise buildings, bridges and other infrastructure under threat of seismic, impact and blast loads.

Ongoing Research

All military and buildings that may be a target should be designed for reduced vulnerability against terrorist attack. The superior strength and plasticity of UHPC make it an ideal material for these engineering challenges. The new UHPC we have developed has great potential for use in the design of future blast-resistant structures and ultimately to the development of blast-mitigating technologies.

Our research team has already carried out an extensive investigation, and both finite element analysis and blast testing techniques have been used to study the characteristics of UHPC under different loading conditions. The results will help us devise guidelines for using the new material in building designs.

The price of the new concrete is about five times that of conventional concrete, which is currently prohibitive for widescale use. However, we are working to improve the cost-effectiveness of the formula.

If costs can be reduced to enable its widespread use, there is no doubt that the new UHPC can help strengthen Australia’s defence and national infrastructure engineering capabilities and help safeguard against terrorist attacks.

Chengqing Wu is a senior lecturer in The University of Adelaide’s School of Civil, Environmental and Mining Engineering. He is also the Director of the TCU-UA Joint Research Centre on Disaster Prevention and Mitigation, and chairs the Australian Chapter of the International Association of Protective Structures.