What Is Rescue Breaching?
Rescue breaching is a technique used to create access openings in reinforced-concrete walls or floors so that rescuers can reach people trapped inside collapsed structures. At confined disaster sites where heavy machinery cannot enter, the work must often be performed mainly with hand tools, requiring considerable time and physical effort to create an opening.
In this study, we are developing a compact controlled-fracture device for disaster rescue. A small amount of nitromethane is distributed among multiple charge holes and deflagrated simultaneously, causing cracks to propagate between the holes and form a fracture plane. The device is not intended to remove an entire wall or floor in a single operation. Instead, it forms a fracture plane between the charge holes, making subsequent breaking and removal with hand tools or a hydraulic breaker easier. This approach is intended to shorten the time required to create an opening and reduce the physical burden on rescue personnel.
The video shows the complete process used to create a triangular rescue opening, approximately 90 cm on each side, in a 120 mm-thick reinforced-concrete floor. The process included drilling the holes, installing the devices, placing blast-protection sheets, activating the devices, and breaking and removing the concrete with a hydraulic breaker. The rescue opening was completed in less than 30 minutes from the start of the operation.
We have begun collaborating with fire and rescue teams and aim to put the technology into practical use within three years.
Principle of Selective Fracture-Plane Formation by Simultaneous Activation
A large quantity of explosive material can cause extensive damage to reinforced concrete. However, the damage may spread beyond the intended opening, breaking areas that do not need to be removed and scattering concrete fragments. Conversely, reducing the amount of explosive material limits the effects on the surrounding structure but may not provide sufficient energy to fracture the concrete.
Our previous study demonstrated that a fracture plane could be formed between two charge holes by simultaneously inducing the deflagration of two nitromethane charges [1]. In the present study, this principle of simultaneous activation is applied to multiple charge holes to form a fracture plane within the intended area.
The deflagration of nitromethane is induced using an initiator of the type employed in automotive airbags. The initiator is electrically activated and ignites the gas-generating material. Variations in its activation time are extremely small, and the difference in activation time between initiators energized simultaneously is within 3 μs. The device developed in this study combines this initiator with a nitromethane charge, enabling multiple nitromethane charges to be deflagrated almost simultaneously with high temporal accuracy.
Considering the spacing between the charge holes and the propagation velocity of stress waves in concrete, an activation-time difference of 3 μs is sufficiently small. The stress waves generated from the individual charge holes therefore overlap almost simultaneously between the holes. As a result, tensile stress is concentrated between the charge holes, initiating cracks. The cracks then propagate under the gas pressure within the holes, forming a fracture plane that connects the charge holes.
The numerical-analysis animation shows the initiation and propagation of cracks when multiple nitromethane charges are deflagrated simultaneously. The cracks are concentrated between the charge holes and within the area enclosed by them, demonstrating the process by which simultaneous activation selectively forms the fracture plane.
[1] Tanaka S., Nishi M., Yamaguchi M., Bataev I., Hokamoto K., “Simultaneous Initiation of Nitromethane in Two Holes by Pulsed Wire Discharge for Crack Control of a Concrete Block,” Journal of Dynamic Behavior of Materials, Vol. 6, No. 1, pp. 53–63, 2020. DOI: 10.1007/s40870-019-00227-6
News Coverage Video
This research was featured by Kumamoto Kenmin Television. The report introduces the controlled-fracture device developed for disaster rescue and shows the device being used during training by a fire and rescue team.
Controlled Fracture of a 300 mm-Thick Reinforced-Concrete Member
The device can also be applied to reinforced-concrete members thicker than those used to create the rescue opening described above. The video shows the fracture of a 300 mm-thick reinforced-concrete member. By appropriately selecting the device arrangement and activation conditions, a fracture plane can be formed in a thick member and, under certain conditions, even the reinforcing bars inside the concrete can be severed.
In small structures in urban areas and at confined work sites, it may be difficult to bring in large heavy machinery. This technology may therefore also be used to support partial demolition with compact construction equipment in such locations.