Everyone in the concrete industry knows the importance of curing and uses a wide range of products such as curing compounds, plastic membranes, or saturated cotton blankets to accomplish the task. Good curing procedures result in concrete that has:

* increased impermeability and durability

* harder, more abrasion-resistant surfaces

* reduced shrinkage

* higher concrete strength

* better protection of steel reinforcement from rusting since the depth of carbonation is reduced, which helps maintain higher pH levels, and because the paste is less permeable to aggressive compounds including chlorides.

To say how much curing--that is, how much hydration of the cemen--is necessary for concrete to meet the performance and durability requirements of a project, or to determine what is "adequate" hydration, is difficult. Petrographers can, however, detect whether the concrete has been cured.

The basics

When water is mixed with portland cement, it occupies spaces (referred to as water spaces) between cement particles. As hydration proceeds, some water evaporates and some is consumed in the hydration process (creating hydration products or hydrates). Hydration stops when the water spaces are filled with hydrate or when no water is available. Low water-cement (w/c) concrete mixes have fewer water spaces, making it more likely that there will be enough hydrate to fill the spaces. A minimum w/c of 0.35 to 0.40 is required for essentially complete hydration of the cement particles. With high w/c mixes, the cement hydrates are unable to densely fill the water spaces, leaving a fine porous structure. The higher the w/c is raised, the weaker the structure and the greater the drying shrinkage.

When hydration is adequate

Complete hydration of the cement and "adequate" hydration are not the same thing. Though it's theoretically possible (but not practically possible) to hydrate all of the cement in a mix, the result wouldn't be desirable because such a concrete would be brittle and would have a high modulus of elasticity, meaning it would easily crack.* This paste is brittle because the leftover, unhydrated cement particles, which normally act as micro-aggregates, are gone so there is nothing within the paste to help restrict shrinkage or distribute some of the drying shrinkage stresses. Therefore, complete hydration (which realistically isn't possible) is actually not "adequate."

Let's look at an industrial concrete floor with a hard-troweled finish. The surface of this floor is highly compacted and densified because most of the air and some of the water are removed in the finishing process. That results in a very low water-cement ratio paste at the surface. Hydration is low because there isn't enough water left to continue the hydration of the cement and because the hydration products have completely filled the water spaces. The large number of partially hydrated cement particles act as a pigment, making the concrete surface darker in color. But this surface region is harder and tougher than the rest of the concrete, so the performance and life of the surface are greatly extended. This is because the paste is denser, and the many partially hydrated cement particles act as "hard" micro-aggregates. So in the surface of hard-troweled floors, even though the amount of cement hydration is relatively low, performance is greatly enhanced, all because cement hydration, although far from complete, is "adequate."

Cement paste consists of cement particles, water, and a skeletal structure created by cement hydration products. As the cement hydrates and the paste gains rigidity, the water spaces become filled more and more by hydration products. The amount of space that must be filled depends on the amount of water in a mix. When too much water is added to concrete when it is first mixed (or afterwards), there are many water spaces to be filled with cement hydration products. But even under the best hydration conditions (the best curing practices), there aren't enough hydration products to densely fill the water spaces so the paste that forms is weak, porous, permeable, and low in strength. Resistance to abrasion is poor, other aspects of durability are poor, and drying shrinkage is greatly increased. So, even though there is more complete hydration of the cement at high water/cement ratios, there aren't enough hydration products, and therefore "adequate" hydration cannot be achieved.

Concrete with a lower water-cement ratio can perform much better, even with less cement hydration, because the cement hydration products fill the water spaces. Hydration stops when the water spaces become filled with hydration products, leaving the concrete dense and impermeable. Thus "adequate" hydration can be achieved with good curing practices.

If hydration stops, for example by rapid loss of water from the surface (poor curing), then cement particles in the surface region cannot hydrate, and the skeletal structure of the paste, slightly filled by small amounts of early-formed hydration products, is very weak. That allows atmospheric carbon dioxide to infiltrate the weak paste and deeply carbonate the calcium hydroxide and other cement hydrates present. These carbonation products encapsulate the cement particles, keeping them from hydrating--forever. Additional curing doesn't help because water cannot penetrate the carbonation skin around the particles. So, the large amount of unhydrated cement particles in this case is not beneficial because hydration is not "adequate."

Assessing the adequacy of curing

When petrographers are asked to determine whether concrete was adequately cured, they have several avenues of approach, In the surface region of a sample, they check both the intensity and depth of carbonation--which give them clues to the water-cement ratio. The age of the concrete makes a difference, too. At a given w/c, older concrete is usually more carbonated.

Petrographers will also note the physical properties such as the hardness and absorption characteristics of the paste. The color of the surface region as compared to the matrix is also recorded. A darker surface region usually indicates higher amounts of unhydrated cement particles. This can happen with a hard troweled finish, or when there was no attempt at curing, or when too much water was allowed to leave the surface of a slab (inadequate curing). Surface regions can become soft and crumbly as a result, and water is more quickly absorbed into the surface, filling spaces that should have been filled with hydration products.

When finishing occurs before bleeding stops, or when water is applied to the surface during bleeding, the surface region has a higher w/c and is lighter in color, soft, absorptive, and usually less durable. Soft surfaces lead to rapid erosion of the paste so that aggregate particles are easily seen on the surface.

There are many instances where concrete is not purposely cured, yet performance, including durability, is not compromised. Petrographic examinations of these surface regions reveal that cement hydration was not affected by the lack of curing and was adequate. That probably means ambient conditions were not too adverse and, as normal drying occurred, water movement through the slab was relatively slow. Therefore, the relative humidity in the top part of the concrete stayed greater than 80 percent for a long enough time for adequate hydration to occur. (Cement hydration essentially stops at an internal concrete relative humidity below about 80 percent.)

When is concrete ideally cured? You can't, unfortunately, answer this question by directly observing concrete in the field. A simple answer is that it is adequately cured when it performs according to the specification. But the theoretical answer is that it is adequately cured when the correct amount of water is added to a concrete mix and enough is retained during curing to enable cement hydrate to adequately fill the spaces formerly occupied by water (hydrate does not fill air voids, capillaries, or gel pores). To arrive at this point, you must first decide upon a good mix design with the right w/c ratio (in the neighborhood of 0.40 to get the right amount of water spaces), and then properly cure your work. If you do that, petrographic analysis can reveal whether concrete was cured to a near optimum level.

Curing tips

However you choose to cure your concrete, the best properties can't be attained without good curing