A Discussion on Curing and SealingBy: Hank HaugeThere is much discussion in our industry about curing and sealing concrete placements. Unfortunately, it is common to hear curing and sealing described as being synonymous. Nothing can be further from the truth. Curing and sealing are two distinct processes. Curing is a temperature and moisture control process that ensures proper development of the engineering properties of a concrete placement. Sealing, on the other hand, is a process in which compounds are applied to the surface of hardened concrete to reduce the penetration of contaminants into the concrete placement.Sealers are typically not applied until the concrete placement has had a chance to "cure out" for 28 days. At 28 days of age the concrete has reached its design strength and characteristically the relative humidity within the concrete matrix is less than 80%. This is important with respect to alkalis silica reaction and sulfate attack, which generally require greater than 80% relative humidity in order to have enough moisture for the negative reactions to occur. Applied too soon, it is possible for a sealer to "lock in" enough moisture for these destructive reactionsWhy is Curing is So Important?The need for adequate curing cannot be overemphasized. Improper curing can easily cut the strength of even the best concrete by 50%. Curing simply means keeping the water in the concrete where it can do its job of chemically reacting with the cement to form the hydration products that will create strong, durable concrete. Good curing means keeping the concrete damp and above 50 degrees F until the concrete is strong enough to do its job. Recommended practice calls for a minimum of seven days curing or the time necessary to attain 70% of the specified compressive or flexural strength, whichever period is less. (Refer to ACI - 308, "Standard Practice for Curing Concrete".)All concrete must be cured to attain maximum strength. Correctly cured concrete is best from every standpoint: It shrinks less, cracks less, and dusts less. It is stronger, more durable, and is more wear resistant.Freshly placed concrete normally contains more water than is necessary for the hydration of the cement particles; however, excessive loss of water by evaporation can prevent adequate hydration. If the water loss due to evaporation exceeds the amount of water available for hydration, the hydration process can actually stop. In such a case, the engineering properties of the concrete do not fully develop. The surface of a concrete placement is particularly susceptible to insufficient hydration because it dries first.Loss of water will also cause the concrete to shrink, thus creating tensile stresses within the concrete matrix. If these stresses develop before the concrete has attained enough strength, surface cracking can result.Start curing the concrete as soon as possible. Early drying - especially in hot, windy weather - must be prevented or the concrete will not attain its full potential quality. (Refer to ACI 305, "Hot Weather Concreting".)Results of Proper CuringMore Durable Concrete:Good concrete, properly cured, has a more complete crystalline structure due to an efficient hydration reaction. This concrete will have fewer pores and crevices where water can enter, freeze, expand and crack the concrete. Air entrainment is necessary to make a more durable exterior concrete, but its use must also be accompanied by proper curing.More Wear-Resistant Concrete:Well-cured concrete (28 day curing period) will develop a surface twice as wear resistant as a surface that is cured for only three days. Proper curing prevents dusting and means less cracking, crazing, and spalling of the concrete.Cure Concrete Longer In Low TemperaturesConcrete strengths develop more slowly at lower temperatures. Below 50 degree F, special precautions should be taken. Heaters, if used, must be vented so combustion gasses are exhausted outside the enclosure in order to avoid carbonation of the fresh concrete placement. (Refer to ACI 306, "Report on Cold Weather Concreting".)Methods of CuringFor practical purposes, concrete can be kept moist by two curing methods:1."Wet Curing" - maintaining adequate external water on the concreteThis method includes the techniques of ponding, spraying, fogging, and placing saturated wet coverings.Fogging / Water SprayThis is a good curing method only if the concrete is kept continually damp. Allowing the concrete surface to dry between sprinklings can create excessive wicking that results in crazing of the surface and cracking of slabs.The nozzle should produce a fine mist. This will avoid over-saturating any one area to the point of raising the water to cement ratio at the concrete surface.Saturated Wet CoveringsFabric coverings saturated in water, such as burlap, cotton mats, or other moisture-retaining fabrics, can be effective for curing. However, the fabric must be kept continually damp. Allowing the fabric to dry out will create excessive wicking that results in crazing and cracking of the concrete placement. Caution should be exercised when using saturated fabrics because staining of the concrete surface may result. (Refer to ASTM C 171.)2."Membrane Curing" - reducing the loss of the initial mix water utilizing membrane-forming materialsMembrane-forming materials provide a controlled moisture loss from the surface of the concrete. Methods would include membrane curing compounds, plastic sheets, or waterproof paper.Membrane Curing Compounds:These compounds consist of waxes, resins, chlorinated rubber and other materials that retard the evaporation of the water from the concrete surface. Membrane curing compounds are of two general types: clear, and white pigmented. White pigmented compounds have the advantage of reducing solar heat gain, thus reducing the concrete temperature. If a membrane compound is used, make sure it meets the moisture retention requirements of ASTM C-309 at the specified coverage rate.Plastic sheets / Waterproof Curing PaperThis method holds moisture in the concrete by preventing evaporation. Water is sprayed on the concrete surface and covered with plastic, or a non-staining, waterproof paper. Surface mottling can occur if plastic sheets are not evenly placed.Concrete SealersSealing compounds are liquids applied to the surface of hardened concrete to keep harmful substances from penetrating the concrete placement. Sealers are generally classified as either film-forming or penetrating.Film-Forming SealersWith film forming sealers, penetration of durable concrete will be slight at best, with most of the product remaining on the surface to form a barrier. The ability of these products to effectively penetrate concrete is restricted by the relatively large molecular structure of the materials. The use of solvents, therefore, will not enhance penetration capabilities. In addition to sealing out water, these products also offer some protection against mild chemicals and prevent the absorption of grease, some oils, and other liquid spills.Urethanes and epoxies are two of the most commonly used film-forming sealers. In general, urethanes typically demonstrate greater abrasion resistance while epoxies sealers have shown superior chemical resistance. Retreatment intervals will vary depending on traffic and exposure.Penetrating SealersThe effectiveness of penetrating sealers is a result of their very small molecular size in comparison with urethanes and epoxies. These materials are able to infiltrate and coat the pores and capillary structure of concrete. Penetrating sealers may achieve penetration depths of as much as 1/8 inch, depending on the density and finish of the concrete. Unlike film-forming sealers, the effectiveness of penetrants is not dependant upon the continuity of an exposed surface film.Penetrants produce a durable sealing film embedded within the concrete itself that is relatively unaffected by abrasion or ultraviolet induced deterioration.The two most common penetrating sealers, the silanes and the siloxanes, are both derived from the silicone family. When catalyzed by moisture, these silicon materials react with the silica available in concrete to form an interstitial, hydrophobic siloxane resin film that repels water without loss of vapor transmission properties.Despite being very closely related, the two silicon-based materials have significant performance differences. Silane monomers are small molecules that have not yet reacted with similar molecules to form polymer chains and require a substrate with a high pH. Siloxanes are produced when silane molecules are partially reacted to form a larger chain structure that is not as dependent on the substrate pH. Because of this, siloxanes are ideal for treating brick, block, stucco, and stone.Due to their very small molecular size, silanes will typically obtain deeper penetration and as a result are less subject to loss of effectiveness caused by abrasion and weathering. A consequence, however, of this very small molecular size is that the molecule is relatively volatile. The solids content in a finished silane product should be high enough to compensate for the loss of reactive material due to evaporation during the application process.For normal concrete placements that have a wear surface, such as pavements, decks, and flatwork, treatment with a silane will provide longer lasting protection. In regard to surface texture and color, treatment with silane sealers typically cannot be detected. Siloxane products may slightly darken the treated surfaces.Concrete sealers are designed to supplement, not replace, the weathering characteristics of a durable, properly cured concrete surface. Best results will be obtained when applied to concrete surfaces proportioned, placed, and finished in accordance with standards defined in ACI Committee Report 201, "Guide to Durable Concrete"

oncrete failures at site are associated to several reasons; right from concrete mix design, properties of materials used, mixing, placing, compaction, curing procedures and many more. There are many misconceptions about the duration of curing of concrete, especially when we refer to site conditions. On many occasions, it is found that the curing period of concrete elements, plasters, brickwork, etc is left to the discretion of the site staff. Improper curing is considered as one of the significant reasons for concrete failures in columns, beams, slabs, pavements, etc, evident in the form of cracks, which are easily noticeable by the naked eyes. The vertical member like a column, in particular, is one of the most victimized RCC elements which must be carefully cured, as the entire load from the slabs and beams are supported by columns and transferred to the foundations. Unfortunately, adequate curing is not given much importance at most of the sites leading to reduction in the durability of the structure.

Curing of concrete plays a major role in developing the microstructure and pore structure of concrete. Curing of concrete means maintaining moisture inside the body of concrete during the early ages and beyond in order to develop the desired properties in terms of strength & durability. A good curing practice involves keeping the concrete damp until the concrete is strong enough to do its job. However, good curing practices are not always religiously followed in most of the cases, leading to a weak concrete. This article summarizes various aspects of curing of concrete which can be of valuable assistance in adopting good construction practices at site.Importance of CuringCuring is the process of controlling the rate and extent of moisture loss from concrete to ensure an uninterrupted hydration of Portland cement after concrete has been placed and finished in its final position. Curing also ensures to maintain an adequate temperature of concrete in its early ages, as this directly affects the rate of hydration of cement and eventually the strength gain of concrete or mortars.

Curing of concrete must begin as soon as possible after placement & finishing and must continue for a reasonable period of time as per the relevant standards, for the concrete to achieve its desired strength and durability. Uniform temperature should also be maintained throughout the concrete depth to avoid thermal shrinkage cracks. Also protective measures to control moisture loss from the concrete surface are essential to prevent plastic shrinkage cracks.

In a nut shell, curing process is designed primarily to keep the concrete moist by controlling the loss of moisture from the body of concrete, during the given period in which it gains strength.Reasons to Cure ConcreteThere are several important reasons why one should cure concrete:

Figure 1: Effect of duration of water curing on strength of concrete(CCAA, data sheet, April 2006, figure-1, page2)

Concrete strength gain- Concrete strength increase with age as moisture and a favorable temperature is present for hydration of cement. An experimental investigation was conducted by "Cement, Concrete & Aggregates Australia" (CCAA) and the same was published in their data sheet on "Curing of Concrete," which has been included in this article for reference. Figure-1 illustrates a comparison of the strength of concrete at 180 days of moist curing with various periods of moist curing (0, 3, 7, 14 & 28 days) and then allowing it to dry out. From the graph below, it can be observed that concrete allowed to dry out immediately, achieves only 40% of the strength of the same concrete water cured for the full period of 180 days. Improved durability of concrete The durability of concrete is affected by a number of factors including its permeability, porosity and absorptivity. Well cured concrete can minimize thermal, plastic & drying shrinkage cracks, making concrete more water tight, thus preventing moisture and water borne chemicals from entering into the concrete and thereby increasing its durability. Enhanced serviceability- Concrete that is allowed to dry out quickly undergoes considerable early age shrinkage. Inadequate curing contributes to weak and dusty surfaces having a poor abrasion resistance. Improved microstructure- Material properties are directly related to their microstructure. Curing assists the cement hydration reaction to progress steadily and develops calcium silicate hydrate gel, which binds the aggregates leading to a rock solid mass, makes the concrete denser, decreases the porosity and enhances the physical and mechanical properties of concrete.

Right Time to Cure Concrete After concrete has been placed in its final position and during the initial set, bleed water rises to the concrete surface as plastic settlement occurs. During this period, if the rate of evaporation of bleed water is greater than the rising water, plastic shrinkage of the concrete occurs. Initial mist curing is necessary to keep the surface moist to prevent the surface from drying out. Between initial set and final set, intermediate curing would be needed if the finishing is complete prior to final set. This may be in the form of a barrier which prevents the loss of moisture from the concrete surface. e.g. covering the concrete surfaces with plastic sheets, waterproof paper, etc. After final set, meticulous curing will have to be done as per the procedures selected. e.g. water curing methods-Ponding, Misting, wet coverings with hessian cloth, Impermeable membrane curing, Curing compounds, etc.Duration of CuringThe duration of curing of concrete depends on the grade & type of cement, mix proportion, desired concrete strength, shape and size of the concrete member and environmental & exposure conditions. The duration may vary from few days to a month.

IS-456:2000 provisions for duration of Curing (Indian Standard-Plain & Reinforced concrete-Code of Practice, 4th revision, page 27)

Exposed surfaces of concrete shall be kept continuously damp or in a wet condition by ponding or by covering with sacks, canvas, hessian or other similar material and kept continuously wet for atleast 7 days from the date of placing, in case of Ordinary Portland Cement (OPC) and atleast 10 days when mineral admixtures or blended cements are used. In case of concrete where mineral admixtures or blended cements are used, it is recommended that the above minimum periods may be extended to 14 days, for assisting the secondary reaction.

Methods to Cure ConcreteMethods of curing concrete broadly fall into the following categories: Water curing-preventing the moisture loss from the concrete surface by continuously wetting the exposed surface of concrete. Membrane curing-minimizing moisture loss from the concrete surface by covering it with an impermeable membrane. Steam curing-keeping the surface moist and raising the temperature of concrete to accelerate the rate of strength gain.

Figure 2: Mist curing of freshly placed concrete

Water Curing- is done by spraying or sprinkling water over the concrete surface to ensure that the concrete surface remains continuously moist. This prevents the moisture from the body of concrete from evaporating and contributes to the strength gain of concrete. PondingThis is the most common and inexpensive method of curing flat surfaces such as floor slabs, flat roofs, pavements and other horizontal surfaces. A dike around the edge of the slab, which may be sub-divided into smaller dikes, is erected and water is filled to create a shallow pond. Care must be taken to ensure that the water in the pond does not dry up, as it may lead to an alternate drying and wetting condition. Sprinkling, fogging & mist curing: Using a fine spray or fog or mist of water can be an efficient method of supplying water to the concrete surface especially during hot weather, which helps to reduce the temperature of concrete, eventually conserving moisture inside the body of concrete. Wet coverings: Water absorbent fabrics such as hessian, burlaps, cotton mats, rugs etc. may be used to maintain water on the concrete surface by completely covering the surface immediately after the concrete has set. They must be continuously kept moist to prevent the fabric from absorbing water from the body of concrete, due to capillary action.

Figure 3: Straw sprinkled with waterFigure 4: Curing of concrete pavements

In rural areas, straw sprinkled with water regularly can be used to cure concrete. Care must be taken when using straw, as dry straw can fly away if the wind velocity is very high and it can also cause fire hazards. Moist earth, sand or saw dust can be used to cure horizontal surfaces. However, staining of the surface can occur due to certain organic matter, if present.Impermeable Membrane CuringFormworkLeaving the formwork in place during the early age of concrete is one of the most efficient methods of curing, especially for columns. However, the turn around time of the formwork reduces considerably.

Plastic sheetingPlastic sheets form an effective barrier to control the moisture losses from the surface of the concrete, provided they are secured in place and are protected from damage. They must be placed immediately after the final set of concrete without causing any damage to the surface. On flat surfaces like slabs, pavements, etc they must be properly secured to the surface and must extend beyond the edges of the slab, so that they are not blown away by gusty winds. Also foot, machinery and vehicular traffic must be avoided over the plastic sheet, to prevent damage. For vertical surfaces, the member must be thoroughly wrapped and the edges taped to prevent loss of moisture from the concrete surface. Plastic sheet may be transparent or colored depending upon the ambient temperature prevailing during that particular season. The efficiency of this system can be enhanced by flooding the concrete surface of the slab with water, under the plastic sheet.

Figure 5&6: Concrete protection systems during initial stage using framed enclosures & plastic sheets

Membrane curing compounds- Curing compounds are wax, acrylic and water based liquids which are sprayed over the freshly finished concrete to form an impermeable membrane that minimizes the loss of moisture from the concrete. These are cost effective methods of curing where standard curing procedures are difficult to adopt. When used to cure concrete the timing of the application is critical for maximum effectiveness. They must be applied when the free water on the surface has evaporated and there is no water sheen on the surface visible. Too early application dilutes the membrane, where as too late application results in being absorbed into the concrete. Care must be taken to avoid foot, machinery and vehicular traffic over the concrete surface to prevent damage of the coating.

For concretes with low w/c ratio, the use of curing compounds may not be suitable for curing. When hydration takes place the relative humidity of interior concrete drops leading to self-drying of concrete. Under such circumstances, wet curing provides an external source of water to replenish the water utilized in the hydration process. Curing compounds may also prevent the bond between the hardened and the freshly placed concrete overlay. For example Curing compounds should not be applied to two lift pavement construction. Similarly, curing compounds should not be applied to concrete surface which will be receiving plasters, decorative & protective paints, etc, as it affects the adhesion.Steam CuringSteam curing is a process for accelerating the early hardening of concrete and mortars by exposing it to steam and humidity. This type of system is most commonly used for precast concrete products where standard products are manufactured in the factory and the turnaround time of the formwork is very quick. In the curing chamber, the control of temperature and humidity is of prime importance or else the concrete products are likely of fracture, crumble and develop other problems later in their service lives. This type of curing systems are generally adopted for railway sleepers, concrete blocks, pipes, manhole covers, poles, pipe culverts, prestressed precast concrete products, and so forth.

Curing in Hot and cold weather requires additional attention.

Hot weather

During hot weather, concrete must be protected from excessive drying and from direct sun and wind. Curing materials which reflect sunlight to reduce concrete temperature must be used. Water curing is recommended and care should be taken to prevent excessive stress caused by alternative wetting and drying or by cold water on warm concrete. Framed enclosures of canvas tarpaulins or sun shades may be used to protect the concrete from direct sunlight.

Cold weather

Some problems associated with temperature below 4o C are:1. Freezing of concrete before adequate strength is developed2. Slow development of concrete strength3. Thermal stresses induced by the cooling of warm concrete to cooler ambient temperatures.In cold weather, some procedures like heated enclosures, insulating blankets & curing compounds may be used. The temperature of fresh concrete must be kept above 100C by using heated raw materials and the curing shall be continued for a longer period of time till concrete gains the desired strength.ConclusionThe chemical reactions between cement & water produces C-S-H gel which bonds the ingredients of concrete, viz. coarse & fine aggregates, mineral admixtures, etc, and converts these fragments into a rock solid mass. This is possible only if continuous curing is done for atleast 14 days; irrespective of the type of cement used. It is understood that blended cements require prolonged curing to convert calcium hydroxide into C-S-H gel. However, in case of OPC as well, voids within the concrete mass gets filled up and disconnected by the formation of C-S-H gel after about 10 days of curing. To have a dense microstructure and impermeability, prolonged curing is a must which leads to enhanced durability. Well designed concrete may give poor durability if not properly cured and on the other hand a moderately designed concrete if well cured can give a better durability. Hence importance of curing should never be ignored.

It has been observed that at several sites in India curing of concrete is left to the decision and comfort of the unskilled laborer. Site engineers & supervisors should put an extra effort to ensure that curing is not ignored at site & they should provide the necessary resources to maintain satisfactory levels of curing, by using the best technique available at site. Just as a new born baby, when it comes into this world needs the utmost care for its development and protection from this new environment, in the similar manner, a freshly placed concrete requires proper protection and care from the encapsulating & aggressive environment. Strictly adopting good curing practices at site will help concrete to achieve the properties of designed strength, enhanced durability, improved microstructure and a long lasting serviceability