Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and comments on blog posts or magazine articles posted on precast.org.
If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/technical-services.
Jerry writes:
The term “hydration” is used to describe the loss of water from concrete during the curing process. My understanding is that water loss refers to “dehydration” and water gain to “hydration.”
Does the concrete industry use these two terms in reverse? If so, why?
NPCA Technical Services engineers answered:
This is a great question. In everyday use, hydration is generally understood as “water gain.” Dictionary.com specifically defines hydration as “the process of causing something to absorb water” and uses the following sentence as an example: “The human body requires adequate hydration to function properly.” But in chemistry, hydration is defined as “the process of combining a substance chemically with water molecules.” This is how the term is understood when it comes to cement’s role in concrete.
NPCA file photo
Cement hydration is a chemical reaction. Specifically, it’s an exothermic reaction, meaning that the chemical process produces heat as it progresses. Contrary to popular belief, water does not evaporate during the cement hydration process. Rather, it is consumed chemically as the exothermic reaction converts the paste into a hardened mass.1 Because heat is generated by the chemical reaction, water can reach evaporation temperatures. This is why a freshly placed concrete mixture is covered until at least initial set, and often longer. Precast concrete producers seek to prevent the evaporation of water, because water released into the atmosphere is then unavailable to continue to hydrate cement.
Producing concrete in the plant allows for better control of moisture loss during the early life of freshly placed concrete and is one of the many reasons precast producers can manufacture products of more consistent quality than cast-in-place alternatives.
Terry writes:
During a discussion with a homebuilder about our driveway slab and curing time, he said he had heard the Hoover Dam is still curing. Is that possible, given the time that has elapsed since its construction?
NPCA Technical Services engineers answered:
Yes, it’s possible. A common adage in the concrete industry is that the Hoover Dam is stronger today than it was yesterday. The hydration process continues in perpetuity if both water and un-hydrated cement are present, helping concrete to continually gain strength over time. The Hoover Dam was a gigantic concrete pour and if additional curing is occurring it would be deep within its mass. However, it is likely that additional strength gain after many years is infinitesimally small and thus difficult to quantify.
References:
1 https://precast.org/2016/03/cement-hydration-kinetics
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