Blisters on Concrete Slabs

Blisters are hollow, low-profile bumps on a concrete slab surface, that can range in size from ¼ to 4 inch in diameter with a dense skin of mortar about ⅛ inch thick covering an underlying void. The dense surface is typically caused by successive finishing with a trowel and generally when the surface is prematurely finished or if the concrete surface stiffens or sets faster than the underlying concrete. Blisters may occur shortly after the completion of the finishing operation. In poorly lighted areas, small blisters may be difficult to see during finishing and may not be detected until they break under traffic.

Blisters typically form on the surface of fresh concrete when either entrapped air or bleed water migrate through the concrete and become trapped under the surface that has been sealed to almost airtight due to premature and excessive finishing of the slab surface. Larger areas of this type of surface defect are referred to as delamination. These defects are not easily repaired after concrete hardens.

• Order the right concrete mixture for slab surfaces that have to be hard troweled
• Concrete should have uniform setting through the depth of the slab in cooler weather
• Do not add excessive water to the load before it is discharged
• Do not overwork the surface to trap air or bleed water while the concrete is still plastic. Use a wooden float on non-air-entrained concrete to prevent early sealing
• Avoid using dry shakes surface hardeners on air-entrained concrete

• Avoid placing slabs directly on vapor retarders or on saturated subgrade. Avoid premature finishing
• Protect surface from premature drying and evaporation that results in surface crusting
• Avoid excessive consolidation, such as by using a vibratory screed on slumps over 5 inches (125 mm)
• Air entrained normal weight concrete should not be steel troweled. If required by specifications, modify finishing procedures to allow air and bleed water to escape before final troweling