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| Nearly every concrete floor has joints, cracks or both. When finishing a floor it is important to understand what is happening below the surface to best address these areas. |
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Surface Cracks
Also called spider cracks and shrinkage cracks, these are very narrow openings that do not travel all the way down through the slab. Too small to fill with any patch, the thicker epoxy primer and/or base coat fill these. |
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Moving Cracks
Once the reinforcing steel mesh or bars (if present) break, cracks will move under load or thermal cycling. If the cracks are small and the movement is minor (less than 1/16”) then the solution is simple. The cracks are opened up to 1/4” wide and a flexible polyurea sealant is used to fill the crack. The epoxy system can be simply applied over the repair. Some reflection of the joint may show on thinner systems, but the joint remains sealed and the concrete protected.
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If the slab has shifted vertically at the crack (see figure on left) then patching may not be the best solution. Grinding down the high side and/or patching up the low side may prove to be a very temporary (and expensive) fix if the slab continues to shift. These may need to be opened wider and treated as an expansion joint (see below.) |
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Control Joints
Either tooled into the wet concrete or saw cut in after a short cure, control joints are typically about 1/4” wide and 1” deep or more. If there is little or no evidence of movement they are filled with sealant and covered with the epoxy system. Otherwise, they are treated as an expansion joint. |
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Expansion Joints
Any joint that may move more than 1/16’ in any direction is considered an expansion joint and should not be covered with coatings, overlays, mortars or anything rigid. A properly designed expansion joint is sufficiently wide to accommodate any movement, flush to the surface and uses an elastomeric sealant over a backer rod.
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| Asphalt impregnated filler strips are often placed by the concrete contractor to function as an expansion joint. They can be left in place if a waterproof joint is not required, but they cannot be coated over. |
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And Finally...
If much of your floor looks like the picture to the right, call a concrete contractor. No patching material spread over the top of this floor will hold it together if it wants to move.
You can't "weld" concrete slabs back together by pouring epoxy into the cracks. Sometimes a full depth repair is inevitable.
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