What:  Slab stabilization, also called undersealing, subsealing, or pressure grouting, is performed to restore support beneath concrete slabs by filling voids.  Material is injected through holes drilled in the slab to fill the voids.

Slab Jacking is the pressure insertion of a grout or polyurethane material beneath the slab to slowly raise the slab until it reaches a smooth profile at the elevation of adjacent slabs.

In both cases, a thorough pavement analysis first determines the cause of the problem.  Several 1.25 to 2” holes are drilled at predetermined locations into the slabs, and material is injected through the holes and underneath the slabs as indicated by the analysis and project design.  The material chosen for slab stabilization must be able to penetrate into very thin voids (typically less than 1/8”) while having the strength and durability to withstand pressures caused by traffic, moisture, and temperatures.  Pozzolan-cement grout and polyurethane are the most common materials, but asphalt cement, limestone dust-cement grouts, and silicone rubber foam are also used.  Slightly stiffer cement grout or polyurethanes are used for slab jacking.

Where:  Slab stabilization is done on slabs still in good condition, but where the underlying support is deficient.  It is most often necessary at areas where pumping and loss of support occur, such as beneath transverse joints and cracks.

Slab Jacking is appropriate on slabs that are still in good condition, but that have settled slightly below adjacent slabs.  The need is most common on pavements that exhibit localized areas of settlement, usually over fill areas, over culverts and at bridge approaches.  These are typically caused by poor or inadequate compaction of the underlying fill.

Why (advantages):  Slab stabilization restores support to slabs by filling voids to reduce deflections and retard additional deterioration.  The process provides a smoother riding surface.  As with most pavement preservation methods, it is a much less expensive and time consuming procedure than repair after damage is caused.

Slab Jacking raises settled slabs to decrease the stresses in the slab that causes cracking and rough rideability.  Slab jacking is less expensive and a quicker process than replacement of a concrete slab.

Why not (disadvantages):  Slab stabilization should not be performed after the onset of pavement damage due to loss of support.  It is also not recommended for areas with widespread pumping and highly plastic fine-grained subgrade soils with high in-situ water contents.

Slab jacking is not recommended for repairing faulted joints, which are more effectively addressed with diamond grinding.  The slab should not be lifted more than 1/4” at a time to prevent the development of excessive stresses in the slab.  If not done properly, there is danger of cracking and slab drift.

When:  Both processes should be done prior to the onset of pavement damage caused by loss of support.  A detailed pavement analysis should be done to determine the cause of the problem.  Using either method when there is no instability can damage the slab.

Estimated life extension:  5-10+ years.