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Holes in concrete driveway - Lignite or Shale Problem

Holes in concrete driveway - Lignite or Shale Problem

Posted by Mike A. Hernandesz on Feb 22nd 2016

Lignite Holes in concrete

Problem: Small pop outs or holes that occur on concrete slabs are often the result of lignite or shale carbons that were in the sand when the concrete was mixed. The lignite or shale particles are lightweight and float near the surface of the slab when the concrete is installed and troweled.  These particles are very absorbent.  When water penetrates and freezes, these particles expand more than water and pushes the top off of the concrete creating a small hole. At first, it is a small hole (picture) The dark material inside the hole is lignite and/or shale if it is white. The surface of the concrete has now been compromised and the hole will continue to grow from the effects of freeze/thaw, usage, stress, etc. The surface of the concrete is like our skin - it prevents damage to the interior of the concrete.  Once the surface has been broken, it needs to be sealed to prevent damage to the interior of the slab.

Many people want to repair this damage by filling the small holes.  This solution does not work for a couple of reasons: 1) concrete does not bond well to other concrete so the repairs will pop out again after a very short time; and 2) the color of the repairs will not match the original slab and the concrete could end up looking like it has chicken pox.

Solution:  The best solution is to reduce water penetration by sealing the concrete with a chemically-reactive, penetrating sealer which will bond the edges of the hole and seal the pores.. Filling the holes or pop outs will not stop the problem.