The reinforcing effect of aluminum foam energy-absorbing layers on reinforced concrete structures

Reinforced concrete (RC) structures, as the most common form of Construction, often exhibit brittle spalling characteristics when subjected to terrorist attacks or industrial explosions. Covering the surface of RC walls with afoamed aluminum energy-absorbing layer can effectively improve their dynamic response.

MechaNism of Failure Mode Transformation

Unprotected RC walls typically experience concrete spalling on the back surface under close-range explosions. This is caused by tensile waves generated by the reflection of stress waves at the back surface exceeding the tensile strength of the concrete. With the addition of a Foamed aluminum sacrificial layer, the shock wave is attenuated in the foam before entering the concrete. Numerical simulations show thatfoamed aluminum with 87% porosity can significantly reduce the peak stress wave transmitted to the concrete. As the thickness of the energy-absorbing layer increases, the failure mode of the RC wall changes from overall bending failure to local indentation or slight damage, or even remains completely intact.

Studies have found that under the same explosive environment (e.g., 7.0 kg TNT, 1.0 meter distance), unprotected RC walls exhibit significant mid-span deflection and crack propagation. However, a protective wall covered with 5 cm thick foamed aluminum shows significantly reduced mid-span displacement. Further increasing the thickness (e.g., to 20 cm) shows an earlier rebound time, because the covering layer not only absorbs energy but also increases the overall stiffness of the structure.

Protection of Bridge Piers and Underground Structures

Bridge piers are highly susceptible to explosions or heavy object impacts during war or accidents, leading to the collapse of bridges. Experiments using closed-cell foamed aluminum (CCAF) to reinforce bridge piers show that when subjected to horizontal impacts less than 7258 joules, a foamed aluminum layer with a density of 0.45 g/cm³ can reduce the cumulative impact force and displacement of the bridge pier by 67% and 35%, respectively. For higher energy impacts, a higher density foam (e.g., 0.55 g/cm³) is required to provide sufficient support stress.

In underground places like tunnels and subway stations, the use of foamed aluminum is working on the distribution of explosive shock waves. The other design is new and it comprises prefabricated staggered ventilation holes on the foamed aluminum plate, with a honeycomb structure at a 45-degree angle in the middle layer. This design provides natural air flow but also uses a jagged surface structure of the walls of the ventilation duct and the shape of the wave path to provide a shock wave decrease of as much as 99.1%, which is a new way of defending the underground environment, which combines air flow and protection.