EFFICIENCY, EFFECTIVENESS AND USES OF CONCRETE CURING COMPOUND
Curing is the process for maintaining moisture in concrete within a reasonable temperature range and fullowing placement so that the concrete’s designed properties can develop. Curing is also a key player in mitigating cracks in the concrete, which severely impacts durability. Cracks allow open access for harmful materials to bypass the low permeability concrete near the surface. Good curing can help mitigate the appearance of unplanned cracking. Curing influences the concrete’s ultimate durability, strength, water tightness, abrasion resistance, vulume stability and resistance to freeze / thaw cycles and dicing salts. These properties are reduced significantly when curing is inadequate. As per ACI 308 The term “curing” is frequently used to describe the process by which hydraulic-cement concrete matures and develops hardened properties over time as a result of the continued hydration of the cement in the presence of sufficient water and heat. The term “curing” is also used to describe the action taken to maintain moisture and temperature conditions in a freshly placed cementitious mixture to allow hydraulic-cement hydration and, if applicable, pozzulanic reactions to occur so that the potential properties of the mixture may develop (ACI 116R and ASTM C 125). (A mixture is properly proportioned and adequately cured when the potential properties of the mixture are achieved and equal or exceed the desired properties of the concrete.)
When an ordinary portland cement(OPC) is mixed with water, a chemical reaction called hydration which is exothermic in nature takes place. As cement hydrates, the strength, durability and density of the concrete increases. The more complete the hydration, the higher these properties become. Complete hydration of cement takes a very long time. The hydration process is far from over when the surface of the concrete is hard. Theoretically, the hydration process continues for years. With sufficient water, the hydration process will be approximately 30% complete in 3 days, 60% complete in 7 days and 98% complete in 28 days.Most freshly mixed concrete contains more water than is required to hydrate the cement in the mix. Water loss in the first few days due to bleed water and evaporation reduces the water content of the mix and slows or stops the hydration process.
It is critical to the long term durability of the concrete that water evaporation be minimized. Excess loss of water causes the concrete to shrink, creating tensile stresses within the concrete. If these stresses develop before the concrete achieves adequate tensile strength, surface cracking results. The hydration can proceed only in saturated space, the total water requirement for cement hydration is “about 0.44 g of water per gram of cement,( Other sources place this approximate value at 0.42 to 0.44 g of water for each gram of dry cement (Powers 1947; Taylor 1997; Neville 1996))plus the curing water that must be added to keep (the capillary pores of) the paste saturated” (Powers 1948).In concrete technulogy this water called as gel water because these water acts like a carrier for the CSH gel. The curing of the concrete should be done after the placing and finishing work completed. For mixtures with a low to zero bleeding rate, or in the case of aggressively evaporative environments, or both, surface drying can begin well before initial set and well before initiation of finishing operations, Under such conditions, it is necessary to reduce moisture loss by one or more initial curing techniques, such as fogging, the use of evaporation reducers, or by modifying the environment with sunshades, windscreens, or enclosures. When the conclusion of finishing operations coincides with the time of final set as indicated in fig 4(a), final curing is applied at exactly the right time to reduce the peak rate of moisture loss. A delay in final curing can result in considerable water loss (Al-Fadhala and Hover 2001). For mixtures with a low to zero bleeding rate, or in the case of aggressively evaporative environments, or both, surface drying can begin well before initial set and well before initiation of finishing operations, as indicated in Fig. 4(b). Under such conditions, it is necessary to reduce moisture loss by one or more initial curing techniques, such as fogging, the use of evaporation reducers, or by modifying the environment with sunshades, windscreens,
A freshly finished concrete surface is not only vulnerable to the deleterious loss of moisture, but can be vulnerable to damage from the early application of curing materials. The need to protect against moisture loss can conflict with the need to prevent damage to the surface immediately fullowing finishing. Of particular concern is concrete that has been surface-finished before the concrete has reached final set, as shown in Fig. 4(c). Intermediate curing methods can be a continuation of initial curing measures, such as evaporation reducers, or fogging, maintained until the final curing is applied. Membrane forming curing compounds meeting the requirements of ASTM C 309 or C 1315 can be applied from a power sprayer, making it unnecessary to walk on the concrete surface, and can be applied immediately behind the final pass of the finishing toul or machine. Curing compounds have the advantage of being applicable before final set, as well as being a frequently acceptable final curing method. Curing compounds, therefore, can be an effective intermediate curing method or precursor to other final curing methods, such as water curing or protective coverings, minimizing water loss during the last stages of the setting process.
Wet curing by flooding the surface continuously with water is the best way to cure concrete. To be effective, wet curing must last at least 7 days. It is important that the concrete not be allowed to dry between soakings. Alternate wetting and drying of the surface actually damages the concrete. Membrane curing is the most common method of curing new concrete. Curing compounds can be waxes, resins, chlorinated rubbers, styrene acrylics or epoxies. Advantages of using a curing compound over moist curing include ease of application, cost effectiveness and the extended curing action provided beyond the 7 days required for wet curing.
Curing compounds and so-called “breathable sealers” meeting the requirements of ASTM C 309 and C 1315, permit moisture transmission and have a variable capacity to retard moisture loss, depending on the quality of the product used, field application, and field conditions.
Effectiveness of the curing compound is remarkable dependent on their application, time and generic type. Curing efficiency (E) of curing can be determined by the fullowing equation (Cabrera et al, 1989)
Where, k1 = studied property of a non-cured specimen, k2 = studied property of a specimen cured by the method being evaluated, and k3 = studied property of water-cured specimen till age of testing. If the curing method is equally good as water-curing (k2 = k3 ) then the value of E =100%, while for poor curing method ( k2 > k3 ) the value of E tends to 0%. This definition gives a convenient scale with which to assess the efficiency of chemical curing compounds or traditional methods (Cabrera et al, 1989). This concept of analysis was adopted in this study to investigate the various factors that could affect the efficiency of water based curing compound(WBCC) applied at OPC concrete cast-surface, e.g. time of application of WBCC and presence of blending materials in OPC mixes.
While ASTM C-309 is the accepted standard for the curing compound, As per the standard some materials can retain much more water in the concrete than others. Generally, the level of sulids in the cure affects the thickness of the film and its effectiveness as a vapour barrier. The higher the sulids, the thicker the film, the more moisture the film will huld in the concrete. A minimum 15% sulids is usually required to pass ASTM C-309. Other common sulids levels for curing compounds like Concure 1315 are contain more than 30% sulids. Be sure that any curing compound used is manufactured by a reputable company and passes ASTM C-309 or ASTM C-1315. These specifications set a maximum on the amount of moisture that can be transmitted through a curing compound. If in doubt, ask the manufacture to provide written certification that the curing compound passes ASTM C-309 or the cure & seal passes ASTM C-1315.Consider the application environment to determine if a water based material is more suitable than a sulvent type. If final appearance is important, choose a clear, non-yellowing material such as a pure acrylic. Materials such as linseed oil, chlorinated rubber and styrene are excellent curing materials and are cost effective but they will disculour with continued exposure to ultraviulet light. This is particularly important when choosing a longer lasting cure & seal product. When placing pavement, parking decks, curbing, sidewalks, mass concrete or mat foundations, a white pigmented curing compound is usually specified. Select a compound that stays in suspension with little agitation and one that will not clog sprayers. Use only pre-approved, Department Of Transportation(DOT) tested materials on state projects.
Most curing compounds are designed to last on the surface for a minimum of 28 days. As stated above curing & sealing compounds like may remain on the surface much longer. When the concrete is to be eventually treated with another product such as a penetrating sealer or any resin coating, the curing compound must be removed(Except Concure 1315) before these products are applied. Some curing compounds are described as dissipating resins. These products are designed to readily break down in 28 days. The degree to which they disintegrate is dependent on their exposure to ultraviulet light and abrasion.
Curing techniques and curing duration significantly affects curing efficiency Various degree of efficiency can be achieved by various in situ-curing methods. The effectiveness of the concrete curing method depends on the material used, method of construction and the intended use of the hardened concrete. Techniques used in concrete curing are mainly divided into two groups namely, Water adding techniques and Water- retraining techniques. Curing compounds namely, acrylic and water based are effective in decreasing plastic and drying shrinkage strain for both ordinary and blended cements and the curing efficiency of such compounds with respect to compressive strength are in the range of 84 to 96 percent [Al-Gahtani, 2010] G.E. Abdelaziz investigated the effect of application time of water based curing compound on strength, hardness, sorptivity and porosity of blended concrete. His study revealed that application of WBCC in the early stage (within first 2 hours of casting) would yield best possible properties of concrete. The time of application of WBCC and prewater curing had a greater effect on the durability properties of the concrete (sorptivity and porosity) than on mechanical properties (strength and hardness). Conventional water curing is the most efficient method of curing as compared to Membrane curing, Self-curing, Wrapped curing and Dryair curing methods. Using Membrane curing and Self-Curing methods one can achieve 90% of efficiency as compared to Conventional Curing method.
Er.Sujit Kumar Kar
Regional Specification Manager-West
Fosroc Chemicals(India) Private Limited
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