Part 10 - Curing

An adequate supply of moisture is necessary to insure that hydration is sufficient to reduce the porosity to a level such that the desired strength and durability can be attained. Concrete needs time to gain strength even when good curing methods are used, and strength should be checked before form removal.

Curing at Ambient Temperatures

Cement paste will never completely hydrate because of the thick layer of C-S-H around each cement grain; however, in practice, water is lost from the paste by evaporation or absorption by aggregate, formwork, or subgrade will further reduce the reaction. If the internal relative humidity drops below 80%, hydration and strength gain will stop. The rate of strength gain is directly related to the amount of moist curing.

• Interrupted Curing -- This is more of a problem during early hydration than later in the concrete's lifetime. Intermittent wetting and drying can cause the concrete to be susceptible to tensile cracking developed during drying.
  
  
• Effects of Relative Humidity -- As discussed earlier, if the relative humidity falls below 80%, hydration will stop. A fully saturated concrete will be able to provide water to localized areas in the paste that are starved for moisture. Concrete that is sealed against moisture loss will hydrate and gain strength more slowly than a continuously moist cured concrete.
  
  
• Effect of Temperature -- Increased temperature results in improved early strength and lower ultimate strength. The early strength gain is explained by the increase of the hydration process. The lower ultimate strength is more difficult to explain, but seems to be related to non-uniform development of the microstructure.
  

Time of Moist Curing

For concrete cured above 45 C, ACI suggests that 7 days of moist curing, or the time necessary to attain 70% of the specified compressive strength, whichever is less, is adequate for structural concrete. Unreinforced concrete requires longer times. At temperatures below 45 C, freezing can be a problem. The concrete should not be allowed to freeze until it has developed some strength (500 lb/in2).

Methods of Curing

• Water Curing -- This technique involves ponding, spraying, or sprinkling of water on the concrete surface or to saturate some form of cover of the concrete. The water should be continuously applied so that the concrete does not dry out.
  
  
• Sealed Curing -- Waterproof paper, plastic sheeting, and curing membranes are the most widely used material for sealed curing. Each of these materials simply reduces the amount of water lost to evaporation. The major advantage is the flexibility of application to any number of shapes and sizes of concrete structures.

Curing in Special Situations

• Mass Concrete -- In this case, temperature is as important as moisture control. The internal temperature should not rise 11^o C above the ambient temperature. Some form of internal cooling system might be necessary.
  
  
• Hot-Weather Concreting -- To prevent excessive drying, protect concrete from direct sun and wind. Curing materials should be used that reflect sunlight to reduce concrete temperatures. Water curing is recommended, and care should be taken to prevent excessive stress caused by wetting and drying or by cold water on warm concrete.
  
  
• Cold-Weather Curing -- Some problems associated with temperatures below 4^o C are: (1) freezing of concrete before adequate strength is developed; (2) slow development of strength; (3) thermal stresses induced by the cooling of warm concrete to cooler ambient temperatures.

Curing at Elevated Temperature

• Low-Pressure Steam Curing -- Steam at atmospheric pressure is used to increase the rate of strength development of pre-cast concrete products. Optimum temperatures range from 65^o to 80^o C and is a compromise of strength gain and ultimate strength. The curing cycle consists of a pre-steaming period, where the concrete is allowed to hydrate and improve its stability; a controlled heating period, where the concrete is slowly brought to the desired maximum temperature; steaming or soaking period, an amount of time that the concrete spends at the maximum temperature; a controlled cooling period; and finally, a secondary curing or storage period. The properties of concrete using low-pressure steam curing do not differ from those of concretes cured under ambient conditions except for lower ultimate strengths.
  
  
• High-Pressure Steam Curing -- If temperatures above 100^o C are desired, then saturated steam must be developed and a sealed vessel used. The temperature range is about 160^o to 210^o C at 6 to 20 atm. The products of this hydration are different from those cured below 100^o C. The most important improvements are: (1) products develop 28-day strength in 1 day; (2) substantially less creep and shrinkage; (3) better surface resistance; (4) lower moisture content after curing. The curing cycle is similar to that of low-pressure curing. A concrete using only Portland cement as a binder will not develop good strength when autoclaved. Reactive siliceous material must be added to the product to achieve the desired high strength. Dry shrinkage is reduced by a third over regular concrete and resistance to sulfate attack is improved. The resulting product is white in color and is suitable for pigmenting.
  

This website was originally developed by Charles Camp for his CIVL 1101 class.
This site is maintained by the Department of Civil Engineering at the University of Memphis.
Your comments and questions are more than welcome.