What Causes Flaking on the Top of Concrete?

Properties of Concrete

• Concrete is made from a mixture of cement, an aggregate of gravel and sand, plus water. When these ingredients are mixed, the concrete begins to cure. Curing is a chemical process that requires consistent hydration over a period of five to seven days. Concrete continues to cure and harden for more than a year but reaches sufficient strength after four weeks. Once the concrete is poured, bleed water rises to the surface and the heavier particles of sand and cement sink to the bottom of the slab. Although it may seem intuitive to allow concrete to dry, curing is not a drying process and it is most important that concrete remain wet during the curing period.

Weather Conditions

• Installing concrete during extremely hot or cold days may cause flaking, scaling or cracking. Concrete should be poured when temperatures are expected to remain between 60 and 90 degrees. High heat and wind will cause the top layer of concrete to dry out, harden and shrink while the concrete beneath remains hydrated. The top layer will fail to bond with the rest of the slowly curing concrete, causing a weak surface that is susceptible to flaking, delaminating and scaling. Cold weather stops the hydration process, and therefore the curing process. Concrete that does not remain hydrated will be weak and fragile, causing it to flake and scale.

Improper Mix

• Using proper ratios of aggregate, cement and water is crucial to the strength and integrity of concrete. Consult the cement bag for the proper proportion. Too much water will cause weak concrete susceptible to flaking and cracking. Choosing premixed concrete allows you to eliminate the step of measuring aggregate and cement. The only step necessary is to add water as specified. Premixed blends are labeled for various specific applications.

Freezing

• Freezing and thawing creates internal stresses on concrete, causing flaking and scaling. Mixing concrete with an air-entrainment additive will create minute air spaces in cured concrete to provide relief from the pressure of freezing and thawing. Deicers containing ammonium sulfate or ammonium nitrate will also cause flaking and scaling, and are not recommended.

Improper Finishing

• After concrete is poured, excess water rises to the top. This bleed water should be allowed to evaporate before the surface is finished. If the bleed water is worked back into the surface concrete, the top layer will contain too much water, causing a weak, fragile surface layer that will break, scale and flake. The surface should still be damp but not covered with water before finishing work begins.