Methods For Curing Of ConcretePublished bySuryakantaonMay 16, 2014|Leave a responseThere are various methods of curing. The adoption of a particular method will depend upon the nature of work and the climatic conditions. The following methods of curing of concrete are generally adopted.Curing of Concrete Shading concrete work Covering concrete surfaces with hessian or gunny bags Sprinkling of water Ponding method Membrane curing Steam curingShading Of Concrete WorkThe object of shading concrete work is to prevent the evaporation of water from the surface even before setting. This is adopted mainly in case of large concrete surfaces such as road slabs. This is essential in dry weather to protect the concrete from heat, direct sun rays and wind. It also protects the surface from rain. In cold weather shading helps in preserving the heat of hydration of cement thereby preventing freezing of concrete under mild frost conditions. Shading may be achieved by using canvas stretched on frames. This method has a limited application only.Covering Concrete Surfaces With Hessian or Gunny BagsThis is a widely used method of curing, particularly for structural concrete. Thus exposed surface of concrete is prevented from drying out by covering it with hessian, canvas or empty cement bags. The covering over vertical and sloping surfaces should be secured properly. These are periodically wetted. The interval of wetting will depend upon the rate of evaporation of water. It should be ensured that the surface of concrete is not allowed to dry even for a short time during the curing period. Special arrangements for keeping the surface wet must be made at nights and on holidays.Sprinkling of WaterSprinkling of water continuously on the concrete surface provides an efficient curing. It is mostly used for curing floor slabs. The concrete should be allowed to set sufficiently before sprinkling is started. The spray can be obtained from a perforated plastic box. On small jobs sprinkling of water may be done by hand. Vertical and sloping surfaces can be kept continuously wet by sprinkling water on top surfaces and allowing it to run down between the forms and the concrete. For this method of curing the water requirement is higher.Ponding MethodThis is the best method of curing. It is suitable for curing horizontal surfaces such as floors, roof slabs, road and air field pavements. The horizontal top surfaces of beams can also be ponded. After placing the concrete, its exposed surface is first covered with moist hessian or canvas. After 24 hours, these covers are removed and small ponds of clay or sand are built across and along the pavements. The area is thus divided into a number of rectangles. The water is filled between the ponds. The filling of water in these ponds is done twice or thrice a day, depending upon the atmospheric conditions. Though this method is very efficient, the water requirement is very heavy. Ponds easily break and water flows out. After curing it is difficult to clean the clay.Membrane CuringThe method of curing described above come under the category of moist curing. Another method of curing is to cover the wetted concrete surface by a layer of water proof material, which is kept in contact with the concrete surface of seven days. This method of curing is termed as membrane curing. A membrane will prevent the evaporation of water from the concrete. The membrane can be either in solid or liquid form. They are also known as sealing compounds. Bituminised water proof papers, wax emulsions, bitumen emulsions and plastic films are the common types of membrane used.Whenever bitumen is applied over the surface for curing, it should be done only after 24 hours curing with gunny bags. The surface is allowed to dry out so that loose water is not visible and then the liquid asphalt sprayed throughout. The moisture in the concrete is thus preserved. It is quite enough for curing.This method of curing does not need constant supervision. It is adopted with advantage at places where water is not available in sufficient quantity for wet curing. This method of curing is not efficient as compared with wet curing because rate of hydration is less. Moreover the strength of concrete cured by any membrane is less than the concrete which is moist cured. When membrane is damaged the curing is badly affected.Steam CuringSteam curing and hot water curing is sometimes adopted. With these methods of curing, the strength development of concrete is very rapid.These methods can best be used in pre cast concrete work. In steam curing the temperature of steam should be restricted to a maximum of 750C as in the absence of proper humidity (about 90%) the concrete may dry too soon. In case of hot water curing, temperature may be raised to any limit, ay 1000C.At this temperature, the development of strength is about 70% of 28 days strength after 4 to 5 hours. In both cases, the temperature should be fully controlled to avoid non-uniformity. The concrete should be prevented from rapid drying and cooling which would form cracksCuring of Concrete Techniques for Different StructuresAdding water to Portlandcementto form the water-cement paste that holdsconcretetogether starts a chemical reaction that makes the paste into a bonding agent. This reaction, called hydration, produces a stone-like substancethe hardened cement paste. Both the rate and degree of hydration, and the resulting strength of the finalconcrete, depend on the curing process that follows placing and consolidating the plastic concrete.Hydration continues indefinitely at a decreasing rate as long as the mixture contains water and the temperature conditions are favorable. Once the water is removed, hydration ceases and cannot be restarted. Curing is the period of time from consolidation to the point where the concrete reaches its design strength. During this period, you must take certain steps to keep the concrete moist and as near 73F as practical.The properties of concrete, such as freeze and thaw resistance, strength, water-tightness, wear resistance, and volume stability, cure or improve with age as long as you maintain the moisture and temperature conditions favorable to continued hydration. The length of time that you must protect concrete against moisture loss depends on the type of cement used, mix proportions, required strength, size and shape of the concrete mass, weather, and future exposure conditions. The period can vary from a few days to a month or longer.For most structural use, the curing period for cast-in-place concrete is usually 3 days to 2 weeks.This period depends on such conditions as temperature, cement type, mix proportions, and so forth. Bridge decks and other slabs exposed to weather and chemical attack usually require longer curing periods. Figure (1) shows how moist curing affects the compressive strength of concrete.CONCRETE CURING METHODSSeveral curing methods will keep concrete moist and, favorable hydration temperature.

Figure (1) Moist curing effect on compressive strength of concrete.They fall into two categories: those that supply additional moisture and those that prevent moisture loss. Table below lists several of these methods and their advantages and disadvantages.METHODADVANTAGEDISADVANTAGES

Sprinkling with Water or Covering with BurlapExcellent results if kept constantly wetLikelihood of drying between sprinklings; difficult on verticalwalls

StrawInsulator in winterCan dry out, blow away, or burn

Moist EarthCheap but messyStains concrete; can dry out; removal problem

Pending on Flat SurfacesExcellent results, maintains uni- form temperatureRequires considerable labor; un- desirable in freezing weather

Curing CompoundsEasy to apply and inexpensiveSprayer needed; inadequate coverage allows drying out; film can be broken or tracked off before curing is completed; unless pig- mented, can allow concrete to get too hot

Waterproof PaperExcellent protection, preventsHeavy cost can be excessive; must drying be kept in rolls; storage and handling problem

Plastic FilmAbsolutely watertight, excellent protection. Light and easy to handleShould be pigmented for heat protection; requires reasonable care and tears must be patched; must be weighed down to prevent blowing away

Methods that Supply AdditionalMoisture:Methods that Supply Additional Moisture:Methods that supply additional moisture include sprinkling and wet covers. Both these methods add moisture to the concrete surface during the early hardening or curing period. They also provide some cooling through evaporation. This is especially important in hot weather. Sprinkling continually with water is an excellent way to cure concrete. However, if you sprinkle at intervals, do not allow the concrete to dry out between applications. The disadvantages of this method are the expense involved and volume of water required. Wet covers, such as straw, earth, burlap, cotton mats, and other moisture-retaining fabrics, are used extensively in curing concrete. Figure (2) shows a typical application of wet burlap. Lay the wet coverings as soon as the concrete hardens enough to prevent surface damage. Leave them in place and keep them moist during the entire curing period. If practical, horizontal placements can be flooded by creating an earthen dam around the edges and submerging the entire concrete structure in water.Figure (2) Curing a wall with wet burlap sacks.Methods that Prevent Moisture Loss.Methods that prevent moisture loss include laying waterproof paper, plastic film, or liquid- membrane-forming compounds, and simply leaving forms in place. All prevent moisture loss by sealing the surface. Waterproof paper (figure (3)) can be used to cure horizontal surfaces and structural concrete having relatively simple shapes. The paper should be large enough to cover both the surfaces and the edges of the concrete. Wet the surface with a fine water spray before covering.

Figure (3) Waterproof paper used for curing.Lap adjacent sheets 12 inches or more and weigh their edges down to form a continuous cover with closed joints. Leave the coverings in place during the entire curing period. Plastic film materials are sometimes used to cure concrete. They provide lightweight, effective moisture barriers that are easy to apply to either simple or complex shapes. However, some thin plastic sheets may discolor hardened concrete, especially if the surface was steel-troweled to a hard finish. The coverage, overlap, weighing down of edges, and surface wetting requirements of plastic film are similar to those of waterproof paper. Curing compounds are suitable not only for curingfresh concrete, but to further cure concrete following form removal or initial moist curing. You can apply them with spray equipment, such as hand-operated pressure sprayers, to odd slab widths or shapes of fresh concrete, and to exposed concrete surfaces following form removal. If there is heavy rain within 3 hours of application, you must respray the surface. You can use brushes to apply curing compound to formed surfaces, but do not use brushes on unformed concrete because of the risk of marring the surface, opening the surface to too much compound penetration, and breaking the surface film continuity. These compounds permit curing to continue for long periods while the concrete is in use. Because curing compounds can prevent a bond from forming between hardened and fresh concrete, do not use them if a bond is necessary. Forms provide adequate protection against moisture loss if you keep the exposed concrete surfaces wet. Keep wood forms moist by sprinkling, especially during hot, dry weather

Making and curing concreteRecommended mix proportions for good concreteGood concrete can be obtained by using a wide variety of mix proportions if proper mix design procedures are used. A good general rule to use is the rule of 6's: A minimum cement content of 6 bags per cubic yard of concrete, A maximum water content of 6 gallons per bag of cement, A curing period (keeping concrete moist) a minimum of 6 days, and An air content of 6 percent (if concrete will be subject to freezing and thawing).

Curing concreteCuring is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. Concrete hardens as a result of hydration: the chemical reaction between cement and water. However, hydration occurs only if water is available and if the concrete's temperature stays within a suitable range. During the curing period (from five to seven days after placement for conventional concrete) the concrete surface needs to be kept moist to permit the hydration process. New concrete can be wet with soaking hoses, sprinklers or covered with wet burlap, or can be coated with commercially available curing compounds, which seal in moisture.

Effects from temperatureTemperature extremes make it difficult to properly cure concrete. On hot days, too much water is lost by evaporation from newly placed concrete. If the temperature drops too close to freezing, hydration slows to nearly a standstill. Under these conditions, concrete ceases to gain strength and other desirable properties. In general, the temperature of new concrete should not be allowed to fall below 50 Fahrenheit (10 Celsius) during the curing period.

Adding air to concrete to help when it freezesAir-entrained concrete contains billions of microscopic air cells per cubic foot. These air pockets relieve internal pressure on the concrete by providing tiny chambers for water to expand into when it freezes. Air-entrained concrete is produced through the use of air-entraining portland cement, or by the introduction of air-entraining agents, under careful engineering supervision as the concrete is mixed on the job. The amount of entrained air is usually between 4 percent and 7 percent of the volume of the concrete, but may be varied as required by special conditions.

Concrete can harden under waterPortland cement is a hydraulic cement which means that it sets and hardens due to a chemical reaction with water. Consequently, it will harden under water.