Chipping of concrete is a phenomenon that occurs when water trapped in concrete goes through multiple freeze-thaw cycles. This causes microcracks to form, and the surface begins to separate from the body of the slab. There are several reasons why chipping occurs, including freeze-thaw cycles, the expansive effects of the alkaline silica reaction, or exposure to fire. However, the most common cause of chipping is corrosion of embedded steel reinforcing bars or steel sections.
Corrosive steel can expand up to ten times its original volume, putting stress on the surrounding concrete. This is a type of weathering erosion that occurs in colder climates, where ice can grow. Carbonation is another natural process that can lead to concrete flaking. Water immediately seeps into concrete and causes other elements, especially reinforcing bars embedded in concrete, to corrode.
This creates bulges and cracks on the surface of the concrete. Poor quality steel can also lead to chipping. The flaking is caused by certain chemical reactions inside the concrete that lead to the formation of foreign products of large volume. These new products due to lack of space inside concrete will increase internal pressure.
Chipping is more common in colder climates where freeze-thaw cycles and thawing chemicals are prevalent. It is important to ensure that the coefficient of thermal expansion of the old concrete and the new filling material is the same. This refers to improper mixing of ingredients when pouring concrete at the time of construction or when the structure is subjected to high pressure from overload. If left unchecked, the chipping will tend to accelerate and spread, so that eventually the structure could become unstable.
Fortunately, there are ways to avoid chipping in concrete structures. It is essential to use an appropriately qualified contractor and diligent preparation of the substrate and selection of a repair material compatible with the host concrete. Additionally, air entrainment should be introduced into concrete by adding an air retention agent, a surfactant (a type of chemical that includes detergents), or a portland cement that incorporates air into concrete in an appropriate amount. Chips in concrete can be effectively avoided if concrete is handled with care and when proper techniques are used during the pouring process.