by Alireza Biparva
Designing for durable concrete presents a special challenge for specifiers. Concrete is usually specified based on a particular physical property such as compressive strength, but there is no single test to evaluate ‘durability.’ There is a significant difference between measuring the durability of concrete structures and measuring the structures’ strength. Compressive strength is a clearly defined property that can be measured directly at a particular point in time using standardized test methods. On the other hand, the properties making concrete durable are many in number. Therefore, durability is more complex to define, measure, and specify.
Concrete durability is defined by the material’s ability to resist weathering action, chemical attack, and abrasion while also maintaining its desired engineering properties such as its compressive strength, permeability, soundlessness, and overall serviceability. To outline what makes concrete durable, many factors must be taken into consideration, including how long the structure is designed to last and whether the building will be subjected to hydrostatic pressure, abrasion, erosion, freeze-thaw cycles, or chemical exposure.
It is important to understand achieving durability is not a one-size-fits-all mechanism. A mix suitable for making a structure durable in the Midwest may be unsuitable for another project exposed to a different environment. For instance, air entrainment is a necessity when a structure is being built in freeze-thaw climates. However, it is not a requirement in dry, desert-like conditions.
Despite the complexity of the many factors affecting concrete durability, the main culprit of deterioration is moisture penetration. As a porous material, concrete allows water (and waterborne contaminants such as chloride ions and sulfates) to migrate through it. Therefore, to make a structure durable, permeation of water through concrete must be prevented. Lowering permeability is the first line of defense against sulfate attack, chloride ingress, corrosion of steel reinforcement, carbonation, alkali-aggregate reaction, and other deterioration mechanisms
When produced using good construction practices, concrete mixes have low permeability. Some good construction practices include using the right mix design, following the relevant building codes, and ensuring adequate measures are taken to place and cure the concrete properly. Well-designed, placed, and cured concrete that follows the American Concrete Institute’s (ACI’s) guidelines remains relatively impermeable and durable as long as it does not crack.
However, cracks and micro-cracks can occur due to various factors, such as early-stage plastic shrinkage, drying shrinkage, thermal changes, excessive stress due to loading, or other elements that can affect the service life of the structure. When water is introduced into the environment, the cracks create a direct pathway for moisture and harmful chemicals to enter the concrete and reach the reinforcing steel. Estimates of durability generally do not take the cracking into account. The underlying danger is the effects of cracks have not been accounted for, which compromises the long-term durability and structural performance of concrete.