Concrete is a composite FOURTH class construction material, composed of cement (commonly Portland cement) and other cementious materials such as fly ash and slag cement, aggregate (generally a coarse aggregate made of gravel or crushed rocks such as limestone, or granite, plus a fine aggregate such as sand), water and chemical admixtures.
In evaluating concrete problems, one of the important decisions home inspectors must make is determining whether a problem is the result of conditions that have stabilized with a low chance of continuing future problems, or whether the conditions that caused the problem are such that there is a high probability that problems will continue or worsen.
Concrete additives have been used since Roman and Egyptian times, when it was discovered that adding volcanic ash to the mix allowed it to set under water. Similarly, the Romans knew that adding horse hair made concrete less liable to crack while it hardened and adding blood made it more frost-resistant.
Concrete, as the Romans knew it, was a new and revolutionary material. Laid in the shape of arches, vaults and domes, it quickly hardened into a rigid mass, free from many of the internal thrusts and strains that troubled the builders of similar structures in stone or brick. Modern structural concrete differs from Roman concrete in two important details. First, its mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand-layering together with the placement of aggregate, which, in Roman practice, often consisted of rubble. Second, integral reinforcing steel gives modern concrete assemblies great strength in tension, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension.
Combining water with a cementitious material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it and allows it to flow more freely. Less water in the cement paste will yield a stronger, more durable concrete; more water will give a freer-flowing concrete with a higher slump. Impure water used to make concrete can cause problems when setting or in causing premature failure of the structure. Hydration involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bond together the individual sand and gravel particles and other components of the concrete, to form a solid mass.
Movement of the sub-grade, such as settling or heaving, can crack concrete. This may be caused by changes in soil volume in response to changes in the soil’s moisture content, or it may be caused by subsidence. Subsidence is settling that can have a number of causes. Sub-surface mining, extraction of natural gas, the dissolution of limestone or conditions related to groundwater can all cause soil to settle. An example is when groundwater dissolves the carbonate cement holding sandstone particles together, and then carries away the particles, creating a void in the soil.
The advantage to inspectors in being able to accurately determine the source of cracking is in understanding whether the condition that caused the cracking has stabilized so that it is no longer likely to cause additional cracking or encourage the propagation of existing cracks. Many cracks, like those caused by concrete shrinkage, are shallow cracks caused by forces that allow conditions to stabilize relatively quickly and do not lead to structural problems. Others, like those caused by soil subsidence or changes in soil volume, are caused by forces that can continue to affect concrete for a long time. This long-term instability can continue to cause serious structural problems over the long term.
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