Curing of Concrete

Curing of concrete is a method of keeping fresh concrete in a moist and warm condition enough so that the hydration of cement can continue. The curing of concrete will increase its strength and decrease its permeability of concrete.  It is also helped in limiting thermal and plastic cracks, which can severely impact the durability of structures. As we know that concrete derives its strength from the hydration of cement particles. The hydration of cement is not sudden or short-time action but a process continuing for a long time, Of course, the rate of hydration is fast to start with, but continues over a very long time at a decreasing rate. The amount of hydration and consequently the amount of gel formed depends upon the extent of hydration. we also know that cement requires a water/cement ratio of about 0.23 for hydration and a water/cement ratio of 0.15 for filling the voids in the gel pores. In other words, the water/cement ratio of about 0.38 would be required to hydrate all the particles of cement and also to occupy the space in the gel pores.  If casted concrete sealed in a container, the water/cement ratio of 0.38 would satisfy the requirement of water for hydration and at the same time, no capillary cavities would be left. However, it is seen that practically a water/cement ratio of 0.5 will be required for complete hydration in a sealed container for keeping up the desired relative humidity level.

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Why Concrete Curing Is Important?

In the field and actual work, it is a different story. Even though a higher water/cement ratio is used since the concrete is the open atmosphere. The water used in the concrete evaporates and the available in the concrete will not be sufficient for effective hydration to take place, particularly hydration is to continue unabated, extra water must be added to replenish the loss of water on account of absorption and evaporation, Alternatively, some measures must be taken by way of the provision of impervious covering or the application of curing compounds to prevent the loss of water from the surface of the concrete. Therefore, curing is a process of creating a favorable environment during the early period for uninterrupted hydration. For these favorable conditions concrete needs a suitable temperature and ample moisture. "Curing of concrete can be defined as assuring the concrete in a moist and warm condition enough so that the hydration of cement can continue. More precisely, it can be defined as the process of maintaining satisfactory moisture content and a favorable temperature in concrete during the period immediately following placement. More precisely, it can be defined as the process of maintaining satisfactory moisture content and a favorable temperature in concrete during the period immediately following placement. The hydration of the cement process may continue until the desired properties are developed to a sufficient degree to meet the requirement of service. The curing of concrete is being given a place of increasing importance as the demand for high-quality concrete is increasing. If curing is neglected in the early period of hydration, the quality of concrete will experience a sort of irreparable loss. Proper curing at the early age of a concrete period of hydration can be compared to good and wholesome feeding given to a newborn baby. Concrete laid in the afternoon of a hot summer day in a dry climatic region is apt to dry out quickly. The surface layer of concrete, exposed to acute drying conditions, with the combined effect of hot sun and drying wind, is likely to be made up of poorly hydrated cement with an inferior gel structure. Read More: Estimation Of A Building With Plan

5 Methods Concrete Curing

The following are types of curing concrete methods used in construction, 1. Water Curing of Concrete This is by far the best method of curing concrete as it satisfies all the requirements of curing, namely, promotion of hydration, elimination of shrinkage, and absorption of the heat of hydration.   Even if there is a membrane method of curing that is used it is always desirable to apply water curing is done before the concrete is covered with membranes. The following are the curing of concrete by water method commonly adopted.

- Immersion  - Ponding - Spraying or fogging - Wet covering - Miscellaneous The concrete precast elements are normally immersed in curing tanks for a certain duration. Road slabs and roof slabs are generally immersed in water by creating small ponds. Columns, plastering surface and retaining wall, etc. are cured by spraying water. In some cases when there is a shortage of water for ponding wet covering such as wet gunny bags, hessian cloth, jute matting, straw, etc., are wrapped to a vertical surface for keeping the concrete wet. For horizontal surfaces, sawdust, earth, or sand are used as a wet covering to keep the concrete in a wet condition for a longer time so that the concrete is not unduly dried to prevent hydration. 2. Membrane Curing

Many times concrete work is carried out in a place where there is an acute shortage of water. The excess use of water for curing is not possible for reasons of the economy. As the curing of concrete does not mean the only application of abundant water it means also the creation of conditions for the promotion of uninterrupted and progressive hydration. As we know that the quantity of water, normally mixed for making concrete is more than sufficient to hydrate the cement, provided this water is not allowed to go out from the body of concrete. For the above reason, the membrane can be used to cover the concrete which will effectively seal off the evaporation of water from the concrete. It is found that the application of a membrane or a sealing compound, after a short spell of water curing for one or two days is sometimes beneficial. Read More: How Long Should Concrete Cure Before Removing Forms 3. Application of Heat The concrete strength-gaining process does not only function time but also temperature. As the temperature of concrete increases, it accelerates the hydration process resulting in faster development of strength. The application of dry heat accelerates the hydration process as the presence of moisture is also an essential requisite. Therefore, setting up the required temperature of concrete and maintaining the required wetness can be achieved by subjecting the concrete to steam curing.

A quick gaining of strength by concrete has many other advantages mentioned below. Concrete is likely to damage only for a short time. Concrete members can cure and handled very quickly. Less area is required in the casting yard. For curing a small tank will be sufficient. The structure can be constructed very quickly, the fewer number of formwork will be sufficient or alternatively with the given number of formworks, more outturn will be achieved. Prestressing beds can use for further casting. From the above-mentioned benefits, it is clear that the method of curing concrete by steam curing not only gives a quick result but also technical advantages in the matter of prefabrication of concrete elements. 5. Electrical Curing

Another method of curing concrete, which applies mostly to very cold climatic regions, is the use of electricity. This method is not likely to find much application in ordinary climates owing to economic reasons. The electrical curing can be done by passing alternating current (Electrolysis trouble will be encountered if the direct current is used) through the concrete itself between two electrodes either buried or placed on one concrete surface. While performing electrical care should be taken to prevent the moisture from going out leaving the concrete completely dry. As this method is not likely to be adopted in this country, for a long time to come, this aspect is not discussed in detail. 6. Miscellaneous Methods Chlorides are sometimes used as a surface coating or as an admixture. It can be used as a well-curing medium. This can be used because calcium chloride is salt, and shows an affinity for moisture. The salt available in chloride not only absorbs moisture from the atmosphere but also retains it at the surface. This moisture retained on the surface of concrete prevents the loss of water from evaporation and thereby keeps the concrete wet for a long time to promote hydration.  If formwork is not removed from the surface of the concrete it helps in creating a moist condition for concrete, particularly, in the case of beams and columns. Leaving the formwork as it is and sealing the joint with wax or any other sealing compound prevents the evaporation of moisture from the concrete. These are some processes of keeping concrete in moist conditions that can be considered as one of the miscellaneous methods of curing concrete.

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