A rocking slab is a concrete slab that moves, teeters, or pivots under load because a void beneath it, usually near a joint or edge, has removed part of its support.
In depth
When soil erodes or settles unevenly, one section of a slab loses contact with the ground while an adjacent section still bears. The slab then rocks as weight shifts across it, which you may feel as movement underfoot or under vehicle traffic and hear as a knock at the joint. The motion accelerates joint wear and cracking.
The fix is joint stabilization: injecting foam that spreads under the slab and refills the void so both sides regain even support and the rocking stops. A slower-reacting foam is used so it travels and fills rather than lifting one side.
How Acme applies it. Acme uses RR-501 stabilizing foam specifically for undersealing and stabilizing joints on rocking slabs, and has eliminated forklift-induced slab vibration in warehouses by filling the void beneath the affected area.
Related terms: Undersealing, Under-Slab Void, Differential Settlement, Concrete Stabilization
Frequently asked questions
What causes a slab to rock?
A void beneath one part of the slab, often near a joint, removes support there while the rest of the slab still rests on soil, so the slab pivots when load moves across it.
Can a rocking slab be fixed without replacement?
Usually. Injecting stabilizing foam to refill the void restores even support and stops the movement, without removing the slab.