subject: Concrete Spalling: Causes & Cures [print this page] Spalling can take different forms, but it generally refers to surface deterioration where concrete crumbles or flakes away in patches or sheets. "Flaking" and "scaling" are also used to describe different types of spalled concrete.
Although spalling is more likely to occur in areas where concrete can be exposed to freezing temperatures, it's a defect that can turn up anywhere. Sometimes it's just a cosmetic issue that can be corrected by a homeowner. In other situations, spalled concrete should be checked out by a concrete or foundation repair specialist to make sure that it won't pose structural problems. The main causes of spalling include the following:
- Freezing temperatures. Concrete that is not air-entrained, or prepared so that it contains tiny air bubbles, can spall when it freezes. Air entrainment is a commonly used strategy to improve concrete's durability in freezing conditions, but it's not always used when it should be.
- Improper Use of De-Icers. When too much de-icer is used on stone steps, driveways, walkways, or other concrete structures, water and salt will mix into salty brine that will seep beneath the concrete surface. As the water portion of the brine evaporates, the de-icer salt will crystallize beneath the concrete, creating pressure on the surface that can lead to spalling.
- Too much water. Too high a proportion of water will weaken concrete. This is a mistake that can occur when the concrete is mixed, when it's poured under wet conditions or when it's being spread and leveled. When an overly wet mix cures, the concrete will be weaker than normal and prone to spalling.
- Rebar corrosion. In poured concrete walls, slabs and other structural elements, steel reinforcing bars (rebar) and/or steel mesh are usually embedded in the concrete to add strength. Normally, this steel is fully encased in concrete, so it isn't exposed to enough moisture to cause rusting. But if the steel begins to corrode for some reason, the surrounding concrete can break away. Once steel is more directly exposed to moisture, the corrosion and spalling cycle will accelerate.
- Poor workmanship. Sound techniques and specific tools are required to complete a successful concrete pour. As concrete is placed in forms, it often requires vibration using special tools. This step helps the mix flow into all areas of the form, eliminating voids (empty spaces) that can be a cause of spalling. Other installation errors that can cause spalling include introducing impurities (like dirt, leaves or construction debris) into the mix.
Spalling can often be repaired by a homeowner with basic masonry tools and concrete patching materials. But if other defects are present, like large cracks or chunks of loose concrete, a concrete repair specialist should be consulted. This is also the case when rusting steel reinforcement is exposed in spalled concrete.
