Curing refers to the maintenance of satisfactory moisture and temperature within a PCC mass as it sets and hardens such that the desired properties of strength, durability and density can develop (PCA, 1988). The desired properties of strength, durability and density are related to the extent of hydration within the PCC mass; the more complete the hydration, the better a PCC’s properties. The extent and rate of hydration depend on two critical construction-controlled parameters: moisture and temperature. This subsection covers:
- Moisture considerations for curing
- Temperature considerations for curing
- Curing methods
Hydration requires portland cement and water. The extent of hydration is controlled by the limiting ingredient, which is usually portland cement. However, if any substantial portion of water is lost to evaporation, hydration may be limited by a lack of water, causing it to slow or virtually stop. Thus, inadequate moisture will inhibit hydration, which results in a weaker, less durable PCC. Rapid moisture loss will also cause excessive shrinking and cracking. Therefore, a high relative humidity around a hydrating PCC mass will ensure an adequate water supply for hydration and limit shrinkage cracking. Generally, some method of curing is specified in order to maintain the relative humidity within the hydrating PCC at an adequate level.
Hydration rate is also dependent upon temperature. Higher temperatures speed up hydration’s chemical reactions, while lower temperatures slow them down. Therefore, temperature will affect PCC strength gain. Often, minimum ambient temperatures for PCC construction are specified to ensure an adequate hydration rate and thus, strength gain.
Since hydration progresses over time, and the rate of this progression is dependent on temperature, it should be possible to estimate the extent of hydration by tracking time and temperature. “Maturity” is the term used to describe this concept. Most maturity measures are expressed as a function of the product of curing time and temperature (see Figure 1). For example, the Nurse-Saul expression is:
Figure 1. Compressive strength vs. maturity.Often, maturity is correlated to PCC strength gain by laboratory testing prior to PCC placement. A non-destructive maturity measurement can then be used to estimate strength and avoid destructive strength tests during construction. ASTM C 1074 defines the maturity method as “…a technique for estimating concrete strength that is based on the assumption that samples of a given concrete mixture attain equal strengths if they attain equal values of maturity index.” The maturity method is useful because it can provide strength estimates of in-place PCC subject to actual environmental temperatures rather than relying solely on controlled-environment laboratory tests. There are also a number of significant limitations when using maturity to estimate strength (Mindess and Young, 1981):
- The maturity method requires establishment of strength-maturity relationship in the laboratory prior to any field measurements. Because different PCC mixes mature at different rates, maturity meters are typically calibrated to actual compressive strength using laboratory test cylinders. Thus, any change in mix proportions from the laboratory design used for calibration will require a new calibration.
- Other characteristics affecting PCC strength. Items such as moisture content, portland cement chemical composition and fineness, and construction practices (e.g., consolidation, finishing, air content) are not accounted for.
- Maturity only accounts for ambient temperature. In large concrete volumes, the heat of hydration contributes significantly to the PCC mass temperature, and thus, strength gain. In typical PCC pavements, which are relatively thin, this heat is quickly lost to the environment and can be ignored.
- Maturity functions are not accurate at low maturities. This is probably because the point at which time should be measured from is poorly defined. Probably, the best time is not the time of mixing or casting, but rather the time that the PCC actually begins to gain strength.
- Maturity does not correlate well with strength when there are large temperature variations during curing. Typically, a low initial curing temperature followed by a high temperature will lead to higher strengths, while the opposite (high followed by low) leads to lower strengths.
In sum, the maturity method is not a physical law, but rather a convenient way to estimate strength gain. In PCC pavement applications, maturity meters (see Figures 2 and 3) can be used to estimate the appropriate time for form removal, joint cutting or opening a pavement to traffic, but should not be entirely substituted for basic laboratory strength tests.
Generally, curing is accomplished by one of two methods (Mindess and Young, 1981):
- Water curing. Methods that prevent moisture loss and supply additional water to the PCC surface. These methods usually involve ponding water on top of a slab, continuously spraying a slab with a fine mist or covering a slab with a water-retaining material such as burlap. These methods are labor intensive and are generally not used on PCC pavements any more.
- Sealed curing. Methods that prevent moisture loss but do not supply any additional water. These methods usually involve placing a waterproof covering over a slab (such as plastic) or using a liquid membrane-forming chemical compound. Curing compounds are typically formed using resins, waxes or synthetic rubbers with a dissolved volatile solvent. Once the solvent evaporates, the curing compound forms a near-impermeable membrane over the PCC. Pigments are often added to curing compounds in order to reduce (white pigment) or increase (dark pigment) heat absorption. Additionally, pigments allow workers to see where the curing compound has been applied, which helps to ensure complete coverage.