Durability of Concrete Structures

About this course.

Durability is the ability of a concrete element to resist the deterioration processes of its environment and continue to perform its intended function for the whole of its design service life, with only reasonable, planned maintenance. Strength — the load a specimen can carry before it crushes or cracks — is a property measured in the first weeks of a concrete's life. Durability is about the decades that follow. The two are related, because the same things that generally produce dense, low-permeability concrete also tend to produce strong concrete, but they are not the same property, and treating a strength number as a durability guarantee is one of the most common and costly misconceptions in the industry.

The reason strong concrete is not automatically durable is that most deterioration is driven by transport processes rather than by load. Water, dissolved salts, carbon dioxide, oxygen and aggressive ions move into concrete through its interconnected pore network and through cracks. A mix can be proportioned to hit a high strength result on a laboratory cylinder yet still be relatively permeable in the actual structure — for example if it was placed with too much water, poorly compacted so that it is honeycombed, or cured badly so that the near-surface layer never developed a tight pore structure. It is this outer "cover" layer, not the strong core, that faces the environment first, and it is where durability is won or lost.