Detecting Under Slab Voids using Ground Penetrating Radar

by Dr. Brian Pailes January 30th, 2015 in News
GPR Signal for Void Under Concrete Slab

Ground penetrating radar (GPR) is an electromagnetic nondestructive test (NDT) method used to evaluate various dielectric materials.  Most commonly, GPR is used to investigate concrete and soils.

GPR has many applications, such as determining the location and depth of concrete cover over reinforcing steel in concrete, however, one of its most effective applications for detecting voids underneath a concrete slab-on-grade.  Voids under concrete slabs can be caused by many factors including subsidence, improper subgrade compaction, and washout due to water drainage.

When concrete is in contact with soil the boundary between the concrete and soil is not easily identifiable.  When an air or water void is between the concrete and soil, the boundary of the slab can be identified using GPR.  Thus, GPR scans of a concrete slab-on-grade can be conducted to identify locations of voiding. 

A limitation of GPR is that it cannot identify the thickness of the observed void.  Therefore, drilling a small hole through the slab for video borescope inspection will provide more information regarding the void condition and thickness.  This information can be used to determine the proper repair technique such as under slab cement grouting or slab jacking if the slab needs to be raised and to estimate the quantity of repairs.

For more information on non-destructive testing for concrete structures, please contact Vector Corrosion Services at info@vcservices.com.