EFFICIENCY, EFFECTIVENESS AND USES OF CONCRETE CURING COMPOUND

CONCEPT OF CURING 

Curing is the process for maintaining moisture in concrete within a reasonable temperature range and following 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, volume 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, pozzolanic 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.)

NECESSECITY OF CURING

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 technology 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 following 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 tool 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.

THE CURING PROCESS

The water is held in concrete two primary ways:

1. Wet curing the concrete by keeping it constantly wet for a minimum of 7 days. This is done by ponding, immersion, spraying or fogging, or applying saturated wet coverings such as burlap.

2. Sealing the surface of the fresh concrete to prevent water from leaving. Typical methods include covering the concrete with impervious paper or plastic sheets or by applying a curing compound to form a membrane on the surface. Each curing method has advantages and disadvantages. The method or combination of methods used may depend on availability, size and shape of the concrete, the location, environment or economics.

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.

TYPE OF CURING COMPOUNDS

Liquid membrane-forming compounds are classified according to the colour of the compound and the type of solid constituent present for forming the membrane. Table 1 shows the American Society for Testing and Materials (ASTM) C 309 and American Association of State Highway 3 and Transportation Officials (AASHTO) M 148 classifications for membrane-forming compounds. The ASTM and AASHTO classifications are equivalent. Membrane forming curing compounds is of two general types; clear or white pigmented. Clear curing compounds may contain a fugitive dye (usually red) that makes it easier to visually check for complete covering of the concrete surface when the compound is applied. The dye will fade after several days. White pigmented curing compounds have the added benefit of light reflectivity to aid in keeping the concrete cool improving the hydration process. All the resin based curing compounds confirming to ASTM C309 class B can also be tested as per BS 7542-1992. High efficiency Polymer based  film forming curing agent and   90% or  greater curing efficiency when tested to BS 7542-1992. The properties and use of Liquid Membrane-forming Curing Compounds are also described in AS 3799.

Membrane-forming curing compounds that meet the requirements of ASTM C 309 permit the loss of some moisture and have a variable capacity to reduce moisture loss from the surface, depending on field application and ambient conditions (Mather 1987, 1990; Shariat and Pant 1984; Senbetta 1988). Compounds formulated to meet the requirements of ASTM C 1315 have special properties, such as alkali resistance, acid resistance, adhesion-promoting qualities, and resistance to degradation by ultraviolet light, in addition to their moisture retention capability as measured by ASTM C 156. Products meeting the requirements of ASTM C 1315 are often referred to as “breathable membrane

sealers” after they have performed the function of a curing membrane used during the final curing. When these products are tested in accordance with ASTM C 156, the allowable moisture loss is 0.40 kg/m2 (0.08 lb/ft2) in 72 h when applied at a curing compound coverage rate of 5.0 or 7.4 m2/L (200 or 300 ft2/gal.) for Type I or Type II compounds, respectively. When performing the ASTM C 156 test in a laboratory to verify compliance with either ASTM C 309 or C 1315, the surface of the test specimen is relatively smooth, and the laboratory-applied coverage rates specified in the test method can be readily obtained. ASTM C 1315 notes that agencies can require a substantially greater application rate on deeply textured surfaces. Further, the specified evaporative environment for the ASTM C 156 test corresponds to an evaporation rate from a free water surface varying between about 1/2 to 1 kg/m2/h (0.1 to 0.2 lb/ft2/h), which may be less severe than encountered in a given construction environment. For these reasons, the water loss experienced by concrete in the field may vary from the values specified by ASTM for compliance with the requirements of C 309 or C 1315.

Curing materials can be composed of wax or other organic material thinned with a solvent. The solvent can make the use of the curing compound subject to various restrictions or regulations governing the transport, storage, or use of hazardous materials. When appropriate, adequate ventilation should be provided and other safety precautions should be taken when using solvent- based compounds. Other similar curing compounds based on water-soluble solids or a water emulsion are available. When the concrete surface is to receive paint, finishes, or toppings that require positive bond to the concrete, it is critical that the curing procedures and subsequent coatings, finishes, or toppings be compatible to achieve the necessary bond. It should be noted that all water-based products appear white when first applied. Unless they include a white pigment such as titanium dioxide, all water based curing compounds will dry clear. Water based cures require soap and water clean-up while solvent based materials require mineral spirits. It is important to not mix solvent-based and water based cures. Most applicators who apply both types will have different spray equipment for each type to avoid the extensive clean out required when switching from one type to the other. Curing compounds meeting the requirements of ASTM C 1315 have been formulated to promote such adhesion, and ASTM C 1315 includes references to test methods for evaluating the bonding of tile and other floor coverings. Testing to establish compatibility among the curing compound, subsequent surface treatments, concrete moisture content, and the actual finished surface texture of the concrete is recommended when performance is critical. Such testing is beyond the scope of this document, but useful references include Suprenant and Malisch (1998d,1999a,b).In FOSROC Range there are many curing compound like Concure WB is a white, low viscosity wax emulsion which incorporates a special alkali reactive emulsion breaking system. This system ensures that the emulsion breaks down to form a non penetrating continuous film immediately upon contact with a cementitious surface. This impervious film prevents excessive water evaporation which in turn permits more efficient cement hydration, thus reducing shrinkage and increasing durability. Once formed, the membrane will remain on the concrete surface until eventually broken down and eroded by natural weathering. Where it is required to apply a further treatment to such concrete surface, it may be necessary to remove the membrane remaining after curing by wire brushing or other mechanical means. The use of curing membranes on internal floor slabs is generally to be avoided where additional surface finishes are to be applied. Concure WB is however ideal where the concrete surface of a floor slab is to be left as 'finished'.This concure WB complies to ASTM C309-90 Standards. Concure 1315 is an acrylic polymer based non-degrading, single component, clear curing compound. It is also suitable for use as a sealer and dust proofer for floors and walls.Concure 1315 is resistant to UV, abrasion and a range of chemicals. Concure 1315 is applied by spray at a coverage rate of 5- 10m²/litre.  Complies to ASTM C1315 Type 1, class A. Concure LP90 concrete curing compound is polymer resin based continuous film forming curing compound and is supplied in various pigmented grades. Clear: Straw liquid curing to a clear film and Blue: As clear grade but with blue fugitive dye with Specific Gravity : 0.82 - 0.85 .When first applied to a fresh cementitious surface the product forms a continuous, non-penetrating coating. This coating dries to a form a continuous film which provides a barrier to moisture loss ensuring more efficient cement hydration, improved durability and reduced shrinkage. All grades of Concure LP90 give 90% or greater curing efficiency when tested to BS 7542-1992.

CALCULATION OF CURING EFFICIENCY

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 following 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.

PROPER APPLICATION METHODOLOGY OF CURING COMPOUND

In accordance with the manufacturer recommendations, liquid membrane-forming curing compounds should be stirred or agitated before use and applied uniformly at the manufacturer’s recommended rate. The curing compound should be applied in two applications at right angles to each other to ensure uniform and more complete coverage. On very deeply textured surfaces, the surface area to be treated can be at least twice the surface area of a trowelled or floated surface (Shariat and Pant 1984). In such cases, two separate applications may be needed, each at 5 m2/L (200 ft2/gal.), with the first being allowed to become tacky before the second is applied. A curing compound can be applied by hand or power sprayer, using appropriate wands and nozzles with pressure usually in the range of 0.2 to 0.7 MPa (25 to 100 psi). If the job size is large, application by power sprayer is preferred because of speed and uniformity of distribution. For very small areas such as repairs, the compound can be applied with a wide, soft-bristled brush or paint roller. Application rates are most readily verified by recording the number of containers of compound used, or the number of sprayer tanks or buckets of compound applied to the surface. When the evaporation rate exceeds the rate of bleeding of the concrete, the surface will appear dry even though bleeding is still occurring. Under such conditions, finishing the concrete or the application of curing compound, or both, can be detrimental because bleed water can be consequently trapped just below the concrete surface. Clearly identifying this condition in the field is difficult, and it is always risky to delay finishing and curing. In such cases where it is important to diagnose this problem, a transparent plastic sheet can be placed over the unfinished, uncured concrete to shield the test area from evaporation, and any bleed water can be seen accumulating under the plastic. Another consequence of applying curing compound to a freshly cast concrete surface that appears dry is that evaporation will be temporarily stopped, but bleeding might continue, resulting in map cracking of the membrane film with the subsequent reduction in moisture-retention capability. This situation would require reapplication of the curing compound. When using curing compounds to reduce moisture loss from formed surfaces, the exposed surface should be wetted immediately after form removal and kept moist until the curing compound is applied. The concrete should be allowed to reach a uniformly damp appearance with no free water on the surface, and then application of the compound should begin at once. As with flatwork, dampening the concrete prevents absorption of the curing compound, which would prevent the formation of a membrane.

HOW TO CHOOSE A GOOD CURING COMPOUND

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 solids in the cure affects the thickness of the film and its effectiveness as a vapour barrier. The higher the solids, the thicker the film, the more moisture the film will hold in the concrete. A minimum 15% solids is usually required to pass ASTM C-309. Other common solids levels for curing compounds like Concure 1315 are contain more than 30% solids. 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 solvent 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 discolour with continued exposure to ultraviolet 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.

HOW LONG WILL A CURING COMPOUND LAST ON THE SURFACE?

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 ultraviolet light and abrasion.

CONCLUSION

“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.

REFERENCES

1. ACI 308
2. ASTM C-309
3. ASTM C-1315
4. ASTM C 156
5. AASHTO M 148
6. Identification of Compliance Testing Method for Curing Effectiveness By Seongcheol Choi and Moon Won(February 2008, Rev. June 2008)
7. A Review of the Curing Compounds and Application Techniques Used by the Minnesota Department of Transportation For Concrete Pavements By Julie M. Vandenbossche, P.E. November 1999
8. A Study of the Effectiveness of PCC Curing Compounds By Nancy M. Whiting and Mark B. Snyder, Ph.D., P.E.,
9. A STUDY ON THE INFLUENCE OF CURING ON THE STRENGTH OF A STANDARD GRADE CONCRETE MIX By M.V. Krishna Rao, P. Rathish Kumar, Azhar M. Khan(UDC 666.942.3/.7:691.32:620.175/.176(045)=111)
10. Curing Of High performance concrete for strength By N.J.Carino and K.W.Meeks September 2000
11. EFFECT ON CONCRETE BY DIFFERENT CURING METHOD AND EFFICIENCY OF CURING COMPOUNDS – A REVIEW ,Nirav R Kholia, Prof. Binita A Vyas, Prof. T. G. Tank,( Kholia et al., International Journal of Advanced Engineering Technology E-ISSN 0976-3945)
12. EFFECT OF APPLICATION TIME OF WATER-BASED CURING COMPOUND ON STRENGTH, HARDNESS, SORPTIVITY AND POROSITY OF BLENDING CONCRETES By G. E. ABDELAZIZ