Why do concrete surfaces flake and spall?
Answers thus far are correct as far as they go. Freeze Thaw Damage may be responsible for a spall condition. Coorosion of embedded reinforcing steel is another reason for spalling of concretes.
When corrosion cells form on rebar, the rust on the surface increases the size of the corroding steel member. This expansion causes the concrete to crack under pressure and loose the bond to the steel. This condition is usually much more serious than a surface spall or the appearance of flaking on a surface, as it involves structural issues and loss of strength in tension of the concrete.
While use of air entrained concretes and proper placement procedures are recognized to avoid Freeze Thaw effects, too often, not a lot of importance is placed on on waterproofing a concrete to stop contaminates from entering the matrix. As water enters carrying contaminates to the depth of the steel another very important event in the preservation of concrete is overlooked! The high pH of new concrete [13-13.2 pH] forms an oxide on the steel surface that is called the "Passivating Layer". This layer, when present does not allow corrosion cells [Rust] to form. As a pH drops to lower levels the passivating layer is lost and corrosion is able to start. At a pH of 13, 7,000 ppm of chlorides are required to initiate corrosion. When the pH drops down to 11.5 that chloride threshold is now only 70 ppm before corrosion can commence. Once this type of corrosion starts, it's a matter of time in a high moisture/oxygen environment the rust develops to the point it begins to spall. Concretes need to be waterproofed as soon as possible to avoid this type of spall activity. StableCrete is a product applied to concrete, that is absorbed into the matrix where it forms a solid Inside the concrete. In addition to waterproofing, this application stops carbonation of a concrete surface. Carbonation starts as soon as the atmosphere, laden with Co2 reaches a concrete surface. This instantly begins to lower the pH of a concrete material.
Concrete surfaces can flake or spall for one or more of the following reasons: • In areas of the country that are subjected to freezing and thawing the concrete should be air-entrained to resist flaking and scaling of the surface. If air-entrained concrete is not used, there will be subsequent damage to the surface. • The water/cement ratio should be as low as possible to improve durability of the surface. Too much water in the mix will produce a weaker, less durable concrete that will contribute to early flaking and spalling of the surface. • The finishing operations should not begin until the water sheen on the surface is gone and excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because the finishing operations are begun too soon, the concrete on the surface will have too high a water content and will be weaker and less durable. Will concrete harden under water? Portland. cement is a hydraulic cement which means that it
Concrete surfaces can flake or spall for one or more of the following reasons: In areas of the country that are subjected to freezing and thawing the concrete should be air-entrained to resist flaking and scaling of the surface. If air-entrained concrete is not used, there will be subsequent damage to the surface. The water/cement ratio should be as low as possible to improve durability of the surface. Too much water in the mix will produce a weaker, less durable concrete that will contribute to early flaking and spalling of the surface. The finishing operations should not begin until the water sheen on the surface is gone and excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because the finishing operations are begun too soon, the concrete on the surface will have too high a water content and will be weaker and less durable.
In areas of the country that are subjected to freezing and thawing the concrete should be air-entrained to resist flaking and scaling of the surface. If air-entrained concrete is not used, there will be subsequent damage to the surface. The water/cement ratio should be as low as possible to improve durability of the surface. Too much water in the mix will produce a weaker, less durable concrete that will contribute to early flaking and spalling of the surface. The finishing operations should not begin until the water sheen on the surface is gone and excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because the finishing operations are begun too soon, the concrete on the surface will have too high a water content and will be weaker and less durable. Will concrete harden under water? Portland cement is a hydraulic cement which means that it sets and hardens due to a chemical reaction with water. Consequently, it will harden
