Erosion Under Concrete

Cause of Erosion Under Concrete

We get a lot of calls for repairing erosion under concrete based on situations like the photo above.  Most of the calls are for homes with basements, and generally the suspected erosion is under stoops, carports and sidewalks near the foundations.  Voids (air spaces) under the concrete are the cause for concern.

In the vast majority of times, the cause of the voids under the concrete isn’t the result of erosion.  Soil consolidation is a natural process that occurs when soil is placed loose (no tamping).  After being placed, it can take up to 10 years or more for the soil to slowly consolidate.  As this happens, it causes a void fill form under the overhead concrete.

If the settlement is extensive enough, it can alter drainage patterns, which can cause some erosion.  So, settlement usually starts the process, which can then be worsened by erosion.

Sometimes, erosion is the only cause for voids under concrete.  This is usually a result of poor grading and landscaping which directs water to flow alongside or pond upstream of structures.  The flowing water washes out soil from under the concrete, resulting in voids.  If the voids are extensive enough, this can result in the concrete settling.

Fixing Erosion Under Concrete

The best way to fix erosion under concrete is to fill voids with material that’s resistant to erosion.  This is usually done with foam jacking, which uses high density polyurethane, a rigid, lightweight plastic not susceptible to erosion.  Alternatively, sand-based grout can be installed, using a process called mud jacking or slab jacking.

After foam jacking or mud jacking are used to fill voids from erosion under concrete, it is really important to correct drainage problems to direct water away from the affected area.  While the materials used for void filling are designed to not wash away, poor drainage can continue to wash out the underlying soils below the void filling material.

Ways to address drainage problems include extending or redirecting downspout discharges, backfilling alongside slabs, installing drains and other similar repairs.