One of the major causes of cracks in concrete structures are what we call a “sulfate attack”. Without proper concrete testing, it can be difficult to determine the true reason behind the cracking.
Unlike shrinkage cracks, which occur early in newly placed concrete, a sulfate attack is caused by a chemical reaction that takes several months, or even years, to show its effects. Medium to large sized cracks in concrete slabs is a symptom of sulfate attack but only expert concrete consulting can adequately diagnose the cause.
Concrete cast on high sulfate soils is at risk for falling victim to a sulfate attack. Over time, the sulfate will react with some hydration products and original cement phases to form fairly voluminous products that initiate and propagate cracks in the concrete. Certain types of cement are prone to this reaction, e.g. Type III cements.
Evidence of sulfate attacks are also frequently found in wastewater pipes and tanks where bacterial action converts sulfur components in the waste stream into sulfates. The latter reacts with some hydration products in the concrete to form calcium sulfate. This type of attack shows up in the erosion of the inner surfaces of pipes and tanks.
In extreme situations, soils above such eroded pipes have caused “cave-ins”. High humidity regions with high oxides of sulfur content in the environment (e.g. industrial areas processing sulfur-bearing raw materials) tend to have mild condensates of mild acids of sulfuric acid attack in wide open concrete slabs (e.g. pavements, airport runways and aprons).
Proper concrete testing for a sulfate attack requires petrographic analysis and other forensic evaluation methods. Once a clear diagnosis has been provided, suggestions for cost-effective concrete rehabilitation can be provided. Contact C3S, Inc. for concrete consulting if you see any cracks in your structure.