We arise from tranquility to once again write a column or two, or three, that we hope will provide some helpful information about concrete. This time, we have a two-part column, unceremoniously named Parts 1 and 2, about water and some of its roles. Water always plays a big role in the life of concrete. First, water is always a component of fresh concrete—sometimes an excessive component. Second, water can be retained by concrete (dams, pipes) or restrained by it (walls). And third, water can go into concrete but generally not through it.
As a component of paste, about 20 percent water by weight is needed to hydrate all the cement (w/c, 0.20), and about 15 to 20 percent more to provide space for the cement hydration products. And then, usually more water is needed to make the mix workable. At about 0.65 to 0.70 w/c the permeability of concrete increases exponentially, so it is usually best to limit the w/c to 0.60 if that is tolerable with respect to other concrete properties. However, any w/c higher than 0.40 leads to micro “holes” as uncombined water evaporates and strength drops progressively.
Nowadays, of course, chemical admixtures can provide workability at low w/c, and mineral admixtures, such as fly ash and ground granulated blast-furnace slag, allow higher strength at the same water content. Such strength, however, usually comes later, except for the slag mixes where higher strength, even before 28 days, may result during warm weather.
Hardened concrete is usually not significantly affected by water. Concrete dams and water and sewer pipes are testimonials to that, along with streets, highways, and sidewalks. However, very pure or very impure water can attack concrete. More about that in a later article!
As mentioned, water does not usually penetrate significantly because of concrete's low permeability. And when it does, its rate of evaporation is usually faster than the rate at which it migrates to the surface, so that it is seldom seen. What sometimes is seen is water that migrates along lift lines and through micro or macro cracks—and most concrete usually contains some cracks—or control joints that are, in essence, pre-designed “cracks.” A typical joint is simply a location where a major crack would have otherwise developed.
Cracks are often “healed” by water leaching the calcium hydroxide component of portland cement hydration from the paste and precipitating it in cracks, where it subsequently carbonates and provides a barrier to future passage of water. That process is called autogenous healing. The calcium hydroxide, after reacting with atmospheric carbon dioxide, becomes calcium carbonate (CaCO3, calcite). Bill, a chemist, says that everyone occasionally needs a slight refresher course in chemistry. Feeling refreshed?
Stalactites that hang from overhead cracks such as soffits of parking garage slabs have the same chemical composition as stalactites in caves—CaCO3, calcite. They are called stalactites because they hang tightly, a virtue needed to defy gravity. When they grow from the bottom up, they are called stalagmites, because they need might to stand tall. And when the two meet, they are called mightytites, or, more simply, columns. Bernie, a geologist, says that everyone occasionally needs a slight refresher course in geology. Feeling refreshed?
So, when you take a drink of water or a bath, remember the great role of water in the formation of concrete and in the uses of concrete as related to industrial and residential water distribution and control. More about water and concrete next month.