Concrete is very strong in compression but relatively weak in tension. It can and often does crack. Concrete is also fairly porous and subject to forces that absorb and release water. Absorbed water can freeze within the concrete and cause spalling and cracking.
Chemical attack can occur because concrete is alkaline and chemically reactive. It can be attacked by acids; some alkalis; numerous salt solutions; and organics such as fermenting liquids, sugars, and animal oils, especially if they contain free acids. Seawater will attack concrete. Corrosive solutions penetrating to the steel reinforcing rods may be particularly destructive because the large displacement of the corrosion products of the steel can cause cracking and spalling of the concrete. In addition to the general physical and chemical properties of concrete that make it subject to physical and chemical attack, several other factors influence the makeup of concrete and therefore must be considered before selecting a method of surface preparation. How the concrete will be used (e.g., as structural concrete or for floors), the method used to place the concrete, and the additives that may be present either on the concrete surface or incorporated into it all will affect the strength and the surface condition of the concrete. A discussion of structural concrete, concrete for flooring, and the surface conditions that accompany each follows.
This is a network of very small surface cracks usually spreading out over large areas or the entire surface. Crazing is caused by finishing the concrete with bleed water on top. The bleed water is forced down into the surface by the finisher’s trowel. This increases the ratio of water to cement, creating a weak surface layer.
Thin flakes of concrete come loose and flake or peel off the surface. The sizes of the flakes vary, but they usually increase over time and with traffic. Conditions that cause scaling and include freeze/thaw cycles, deicing agents with calcium or sodium chloride, fertilizers containing nitrates, working in bleed water, or improper curing. Any one of the can cause scaling, but it’s usually a combination of two or more that lead to severe scaling.
These are typically about ¼ to 1-inch diameter, but it’s not impossible to get 3 to 4 inch blisters. They are not easily seen until they are broken by traffic. Blisters are caused by working and finishing the surface while water or air is still working its way up through the mix to the surface.
Spalling is similar to scaling except large chunks instead of just flakes break loose. This indicates a severe weakness in some parts or the entire project. It is more likely to happen during freeze thaw conditions.
Also known as chalking, this is a fine loose powder caused by the deterioration of a weak surface. Causes of dusting include working in bleed water, improper curing, a bad sand-to-cement ratio, or exposure to carbon monoxide caused by using an unvented heat source to keep a project warm.
Foreign objects can often slip into the form prior to pouring and may not become evident until after the forms are stripped. If fully embedded, the foreign object does not create a coating problem. But a piece of rope on the surface disappearing into the concrete is a problem. It must be removed, usually by chipping, and the concrete must be restored.
Efflorescence is more likely to be found on concrete that has been in place for a while. Concrete contains water-soluble salts. As water from the interior of the concrete migrates to the surface and evaporates, salts are deposited on the surface, usually as a white stain. Efflorescence can occur with concrete, brick, or concrete block construction. It can be removed with acid etching. The best way to prevent its recurrence is with adequate waterproofing.