Scott M. Tarr, P.E. CTL
Steven H. Gebler, P.E. CTL
While by no means the only step in the construction process which plays a part in the performance of concrete, the need for proper curing cannot be overlooked. If all other steps are executed to perfection, but adequate curing is not provided, the concrete element may not attain its intended serviceability potential. In short, optimal curing improves all properties of concrete and can be considered the "cure" for many concrete performance woes.
The American Concrete Institute (ACI) defines curing as the action taken to maintain moisture and temperature conditions in a freshly placed cementitious mixture to allow hydraulic cement hydration and (if applicable) pozzolanic reactions to occur so that the potential properties of the mixture may develop. In the 1940's, The Portland Cement Association (PCA) determined that the chemical process of hydration virtually stops when the internal relative humidity in the concrete drops to near 80 percent. In addition, the temperature of the concrete affects the rate of hydration. Hydration becomes sluggish below a temperature of about 50° F, is greatly retarded below 40° F, and effectively stops below the freezing point.
Therefore, curing involves controlling the environment within the concrete element such that sufficient moisture is available at a temperature which allows the continued hydration of cement. The critical regions of the concrete element are the surfaces exposed to external conditions (surrounding ambient temperature/humidity, moving air, and absorptive media such as a dry subgrade or porous formwork). To maintain sufficient early-age temperature, heat generated by the hydration process can be retained with the use of insulating coverings.
For moisture control, there are two general methodologies:
1) the addition of water to exposed concrete surfaces, and
2) the minimization of water loss from exposed concrete surfaces.
The curing method selected needs to be evaluated on a project-specific basis after consideration of the applicable set of circumstances involved. While most technical authorities generally agree that the most complete method of curing is the continuous application of moisture, this technique it is not always economically feasible or technically advisable. And as ever-tightening building construction schedules urge the contractor to shorten the curing period so subsequent stages can be initiated, proactive curing is regularly discontinued once specified compressive strength is achieved. And let's face it, with the current state of concrete technology, it is not difficult to achieve the 2,500 to 4,000 psi compressive strength typically specified. But compressive strength is not the only concrete property affected by curing. Once construction is complete and acceptable strength is achieved, how will the concrete actually perform? What are the real consequences of an abbreviated curing period?
As curing has a direct influence on the hydration of cement, literally every desirable property of concrete is affected by the degree of curing provided. While all concrete cures to a certain percentage of its full potential, the degree of curing is affected by the combined impact of the duration of the curing period, amount of evaporation permitted to occur, and the temperature maintained. As various concrete properties determine certain performance characteristics, the degree of curing plays a vital role in the expected serviceability of the constructed element. Proper curing decreases the extent of cracking, crazing, scaling, surface dusting, permeability, curling, popouts, and thermal-shock effects and also increases resistance to abrasion, acid attack, and freeze-thaw deterioration.
Many of these performance characteristics are related to the strength of the paste portion of the concrete. At very early ages, concrete allowed to dry or cool rapidly on the surface may exhibit plastic shrinkage cracking prior to final finishing. Subsequent to finishing, less-than-ideal curing can lead to unsightly crazing or map cracking of concrete surfaces not protected from rapid drying. As the concrete tensile strength increases, its ability to sustain drying shrinkage and thermal contraction restraint stresses (subsequent to the curing period) is increased. Extending the period not only increases the tensile strength while delaying the onset of drying shrinkage, but also enables the hydration reaction to consume a greater amount of free water which would otherwise evaporate from the concrete and contribute to the magnitude of the shrinkage. For moist-cured concrete, a membrane-forming curing compound should be applied after the curing period to reduce the rate of drying shrinkage.
A lack of proper curing can also lead to or exacerbate surface dusting, a form of distress particularly problematic for concrete slabs on grade. This is especially true when the surface is allowed to dry rapidly, which halts cement hydration and corresponding paste strength gain. As moisture retention and temperature control become more effective and the curing period duration is extended, the process of hydration becomes more complete and the potential for dusting that creates problems for both covered and uncovered floors is reduced.
The prevention of moisture loss also contributes to a reduction in the permeability of the concrete and therefore, provides greater corrosion protection of embedded reinforcing steel. The cement paste includes a system of pores. In fresh concrete, the mix water occupies these pores. Without curing, water within the concrete evaporates leaving empty, permeable, interconnected pores. If the water is retained in the pore system, it reacts with unhydrated cement particles to form hydration products that subsequently narrow and fill the pores. This pore-filling of hydration reduces the permeability of the concrete.
Good curing practices become more important as the water-cement ratio (w/c) is decreased. While lowering the w/c reduces the pore space between cement particles, there is also the possibility of insufficient free water to form hydration products to fill the pores. This lack of water to hydrate available cement particles is related to the issue of self-desiccation where available mix water is consumed by hydration and the concrete dries internally. Proper curing will minimize moisture loss and maximize pore filling.
In addition to contributing to water-tightness of concrete tank structures, a lower permeability decreases the rate of vapor transmission through concrete floors scheduled to receive vapor sensitive coverings. Also, as the differential drying between the slab top and bottom causes floor slab warping, a reduction in the rate of moisture movement decreases the moisture gradient and associated curling.
Another common form of distress impacted by the quality and duration of curing is popouts. The potential for popouts is reduced as low permeability prevents water from penetrating to porous coarse aggregate particles. According to PCA, a minimum wet-curing period of 7 days can greatly reduce or eliminate popouts caused by alkali-silica reactivity (ASR).
Also, with a decrease in permeability, the potential for carbonation is decreased. In carbonation, carbon dioxide from the air penetrates into the concrete and reacts with the calcium hydroxide to form calcium carbonate. Carbonated concrete experiences greater shrinkage upon drying, lower surface durability and less protection against corrosion of steel reinforcement by reducing alkalinity.
The durability of concrete is also enhanced by prolonged moisture retention. In properly air-entrained concrete, curing increases the strength and reduces the porosity of the surface. A lower porosity prevents water saturation into the surface that can subsequently freeze and thaw and, eventually, can lead to scaling and mortar flaking. A higher strength enables any concrete that does become saturated to resist the stresses developed by the expansion and migration of developing ice crystals. In the same category, curing increases concrete's resistance to deicer chemicals. However, PCA recommends a 30-day drying period subsequent to curing before the application of deicing salts To new concrete placements.
These same beneficial porosity and strength attributes aid in the resistance to abrasion and acid attack. Abrasion resistance is dependent upon the surface aggregate and paste hardness. While curing cannot influence aggregate hardness, it does enhance paste properties as discussed previously.
The environment within and immediately surrounding a newly-placed concrete element has a direct influence on the hydration of cement in the concrete. Optimum hydration of the cementitious components improves the desirable properties of the concrete. Proper curing prevents moisture loss and maintains an adequate temperature for the hydration process. In addition, extending the duration of the curing period beyond that required to achieve the specified compressive strength allows for the maximum degree of hydration and will minimize the occurrence of defects that can impact serviceability. While long-term curing may not always be feasible using wet curing techniques, curing compounds can be applied after coverings are removed to extend the curing period. Subsequent construction operations should be reviewed before selecting an appropriate curing compound to prevent a conflict between adjacent materials. However, increasing the curing period will undoubtedly enhance the properties of the concrete element. And with enhanced concrete properties, the user should realize better concrete performance.
About The Authors:
Scott Tarr, Senior Engineer, CTLGroup
University of New Hampshire
B.S. Civil Engineering, 1990
M.S. Civil Engineering, 1993
Licensed Professional Engineer - Illinois
Publications and Professional Activities: In addition to numerous technical reports, Scott Tarr has authored and coauthored over 20 professional publications on portland cement and asphalt concrete pavement performance and waste material utilization. Mr. Tarr is an active member in the American Society of Civil Engineers and the Transportation Research Board.
Steven H. Gebler, Senior Principal Engineer, CTLGroup
The Ohio State University B.S. Civil Engineering, 1969
Stevens Institute of Technology M.S. Civil Engineering, 1973
Licensed Professional Engineer - Manitoba, Saskatchewan, Canada
Registered Professional Engineer - Illinois, Nevada, Ohio, Pennsylvania, Texas
Registered Professional Civil Engineer - California
Registered Professional Quality Engineer - California
ASME/ACI 359 Level III Concrete Inspector
Publications and Professional Activiities: Steven H. Gebler has authored more than 30 papers on a wide range of subjects. The topics covered include: use of fly ash, ground blast-furnace slag and chemical admixtures in concrete; durability of concrete; construction procedures and defects; repair of concrete; cooling towers; curing of concrete; high strength concrete; quality assurance; and shotcreting (gunite).
© 2001 L&M Construction Chemicals, Inc. | ConcreteNews Fall 2001.