T/f: poured concrete shrinks during the curing process, which usually results in small cracks.
Curing plays an important role on strength development and durability of concrete. Curing takes place immediately after concrete placing and finishing, and involves maintenance of desired moisture and temperature conditions, both at depth and near the surface, for extended periods of time. Properly cured
concrete has an adequate amount of moisture for continued hydration and development of strength, volume stability, resistance to freezing and thawing, and abrasion and scaling resistance.
The length of adequate curing time is dependent on the following factors:
Slabs on ground (e.g. pavements, sidewalks, parking lots, driveways, floors, canal linings) and structural concrete (e.g. bridge decks, piers, columns, beams, slabs, small footings, cast-in-place walls, retaining walls) require a minimum curing period of seven days for ambient temperatures above 40 degrees Fahrenheit1. American Concrete Institute (ACI) Committee 301 recommends a minimum curing period corresponding to concrete attaining 70 percent of the specified compressive strength2. The often specified seven-day curing commonly corresponds to approximately 70 percent of the specified compressive strengths. The 70 percent strength level can be reached sooner when concrete cures at higher temperatures or when certain cement/admixture combinations are used. Similarly, longer time may be needed for different material combinations and/or lower curing temperatures. For this reason, ACI Committee 308 recommends the following minimum curing periods3:
Effect of curing duration on compressive strength development is presented in Figure 1. Figure 1. Moist Curing Time and Compressive Strength GainHigher curing temperatures promote an early strength gain in concrete but may decrease its 28-day strength. Effect of curing temperature on compressive strength development is presented in Figure 2. Figure 2. Effect of Curing Temperature on Compressive StrengthThere are three main functions of curing: 1) Maintaining mixing water in concrete during the early hardening process Ponding and immersion Spraying and fogging Saturated wet coverings Left in Place Forms 2) Reducing the loss of mixing water from the surface of the concrete Covering concrete with impervious paper or plastic sheets Applying membrane-forming curing
compounds 3) Accelerating strength gain using heat
and additional moisture Live steam Heating coils Electrical heated forms or pads Concrete blankets Other forms of curing include internal moist curing with lightweight aggregates or absorbent polymer particles. For mass concrete elements (usually thicker than 3 feet), a thermal control plan is usually developed to help control thermal stresses. Additional information can be found in ACI Committee 308 report Guide to Curing Concrete3. For specialty concretes, it is recommended to refer to other ACI reports as follows:
Curing in either cold or hot weather
requires additional attention. In cold weather, some of the procedures include heated enclosures, evaporation reducers, curing compounds, and insulating blankets. The temperature of fresh concrete shall be above 50 degrees Fahrenheit. The curing period for cold weather concrete is longer than the standard period due to reduced rate of strength gain. Compressive strength of concrete cured and maintained at 50 degrees Fahrenheit is expected to gain strength half as quickly as concrete cured at 73
degrees Fahrenheit. In hot weather, curing and protection are critical due to rapid moisture loss from fresh concrete. The curing actually starts before concrete is placed by wetting substrate surfaces with water. Sunscreens, windscreens, fogging, and evaporation retardants can be used for hot weather concrete placements. Since concrete strength gain in hot weather is faster, curing period may be reduced. Additional information can be found in ACI 306.1, Standard Specification for Cold
Weather Concreting, ACI 306R, Cold Weather Concreting, ACI 305.1, Specification for Hot Weather Concreting, and ACI 305R, Hot Weather Concreting Curing Concrete Test SpecimensCuring of concrete test specimens is usually different from concrete placed during construction. American Society for Testing and Materials (ASTM) has developed two standards for making and curing concrete specimens. ASTM C1926 is intended for laboratory samples while ASTM C317 is intended for field samples. Both documents provide standardized requirements for making, curing, protecting, and transporting concrete test specimens under field or laboratory conditions, respectively. ASTM C192 provides procedures for evaluation of different mixtures in laboratory conditions. It is usually used in the initial stage of the project, or for research purposes. ASTM C31 is used for acceptance testing and can also be used as a decision tool for form or shoring removal. Depending on its intended purpose, the standard defines two curing regimes: standard curing for acceptance testing and field curing for form/shoring removal. Variation in standard curing of test specimens can dramatically affect measured concrete properties. According to the National Ready Mix Concrete Association8 (NRMCA), strength for concrete air cured for one day followed by 27 days moist cured will be approximately 8 percent lower than for concrete moist cured for the entire period. The strength reduction is 11 percent and 18 percent for concrete specimens initially cured in air for three days and seven days, respectively. For the same air/moist curing combinations, but 100 degrees Fahrenheit air curing temperature, the 28-day strength will be approximately 11 percent, 22 percent, and 26 percent lower, respectively. ReferencesSteve Kosmatka et al, Design and Control of Concrete Mixtures, 15th Edition, EB001, PCA Engineering Bulletin EB 001, Portland Cement Association , Skokie, IL 2002 Specifications for Structural Concrete, ACI 301 (www.concrete.org) Guide to Curing Concrete, ACI 308R-01
(www.concrete.org) ASTM C309, Standard Specification for Liquid Membrane-Forming Compounds for Curing Concrete (www.astm.org) ASTM C1315, Standard Specification for Liquid Membrane-Forming Compounds Having Special Properties for Curing and Sealing Concrete (www.astm.org) ASTM C192 / C192M, Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory (www.astm.org) ASTM C31 / C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field (www.astm.org) |