How does polyurethane foam injection work step by step?

The technician tap-tests the slab to locate voids then drills 5/8-inch holes at strategic points over the sunken area. Two-part polyurethane foam is injected through each port and the components mix at the tip, react immediately, and expand beneath the slab to fill voids and generate upward pressure. The technician monitors slab movement and stops injection when the target elevation is reached. Ports are patched with hydraulic cement and the area is ready for use in 15 minutes.