Concrete curing methods

Curing concrete is the process of preserving the requisite moisture and temperature in hardened concrete for continued hydration. Concrete’s curing is crucial for sustaining the material’s longevity and design strength. This article discusses the importance of curing and the various concrete curing methods prevailing on construction sites.

Curing of concrete methods

Curing is the process of retaining moisture to allow fresh concrete to reach its intended strength in a predetermined manner through a hydration reaction. Concrete is a mixture of cement, aggregates, and water in fixed designed proportions calculated based on specific strength requirements. The water to cement ratio is the most important factor in these proportions (Water cement ratio). In order to facilitate the hydration reaction, the water-cement ratio must be maintained. If the water evaporates quickly, there will not be enough water available for the hydration process. Because of this curing of concrete is required.

Curing assists in the retention of moisture until the hydration process is complete and it reaches the required strength. The American Concrete Institute (ACI) Committee 301 recommends a minimum cure duration equal to 70% of the required compressive strength. According to IS 456-2000 standards and construction norms, the curing period of 7 days/10 days is the time required to achieve at least 70% of the intended compressive strength. That is why concrete is cured for 7-10 days.

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Why Curing of concrete is important?

Perfect curing is necessary for the following reasons in order to achieve good strength and serviceability:

Curing prevents concrete from drying and maintains the acceptable temperature range by preventing moisture loss.
Curing increases the strength and decreases the permeability of hardened concrete.
Curing prevents the formation of cracks caused by thermal and plastic shrinkage.
Concrete curing maintains a strong link between the components and the reinforcement.
Curing can provide the desired strength and a durable concrete free of cracks.
Concrete curing assists in achieving high serviceability performance by improving abrasion resistance.

Also Read : Concrete Pumps Types – Application and advantages

Concrete Curing time as per IS 456-2000

According to the Indian Standard IS 456 – 2000, concrete should be allowed to cure for a minimum of seven days while using ordinary Portland cement and for a minimum of ten days while using blended cement or concrete with mineral admixtures. Additionally, it suggests that the curing time should not be less than 10 days for structures exposed to hot, dry weather and 14 days in case for or blended cement or cement with mineral admixtures.

OPC cement = 7 Days (Normal conditions) and 14 Days ( Hot and dry weather)

Concrete with mineral admixture or blended cement (PPC cement) = 7 Days (Normal conditions) and 14 Days ( Hot and dry weather)

The curing method and time primarily depend on structure type, site conditions, and ingredient parameters. Some of the curing methods adopted in constructions sites are as follows.

Water curing
Membrane Curing
Steam curing

Water Curing method

Water curing is the most popular and common method adopted in construction sites. Basically, this method maintains or retains water on the concrete surface by various methods. This includes ponding, sprinkling and fogging, and saturated wet coverings or left-in-place forms.
Similarly, these methods prevent water loss from the concrete surface by continuous wetting of the exposed surface of the concrete.

Membrane Curing method

The basic concept of membrane curing is reducing the loss of water from the surface of the concrete. Generally, membrane curing methods uses curing compounds or impervious plastic sheets. Curing compounds are available in water-based and acrylic-based types. They form an impermeable membrane and reduces the loss of moisture.

Steam curing method

The steam curing process accelerates the process of strength gaining by using heat and providing additional moisture. Generally, this speeds up the early hardening process. Basically, these methods are familiar in prefabricated structures and factory-made precast components for the speedy recovery of form works.
Accordingly, Steam curing keeps the surface moist and raises the concrete temperature to speed up the strength achievement rate.

Steam curing - Precast factory — Precast factory

Conclusion

Now a days curing activity is not taken seriously and this hampers the strength and quality. Likewise, this is an activity to be done with utmost care to ensure design strength and serviceability of structures.

The concrete curing methods depend on the nature of the structure, site conditions, and ingredient parameters. In this article, we will go through some common curing methods adopted in constructions sites. For getting a better understanding of the topic, refer to our earlier article Curing of concrete – Process and significance.

Methods of concrete curing

Curing is the process that helps in maintaining moisture to allow fresh concrete to attain its desired strength in a planned manner through a hydration reaction. If the water evaporates quickly, the requisite amount of water will not be available for the hydration process. The curing of concrete helps to retain the concrete moisture until the hydration process is complete and concrete attains the requisite strength.

The curing method and time primarily depend on structure type, site conditions, and ingredient parameters. Some of the curing concepts adopted in constructions sites are as follows.

Maintaining a water layer of water over the concrete surfaces (water curing)
Reducing the loss of water from concrete
Accelerating the initial strength gain

Let us go through these three basic concepts and curing methods adopted for each of these criteria.

Also Read : Non destructive tests ( NDT } on hardened concrete

Maintaining water over the concrete surfaces -Methods of concrete curing

Maintaining water over concrete surfaces or water curing is the most popular and conventional method of concrete curing. The methods of curing depends on the type of structure basically. Let us go through the water curing methods adopted in construction sites .

Ponding
Spraying and fogging
Saturated or wet coverings
Immersion curing

Ponding

Ponding is the method of wetting the concrete surface by creating a temporary containment area around the concrete.

The ponding method is one of the very well-known and widely adopted concrete curing methods. This method is possible only on horizontal or flat structures and is best suited for curing concrete slabs.

Small bunds of clay or lean cement sand are placed over the concrete surface. Water is filled in the bunds, refilled, and maintained as and when the level goes down.

This method is very efficient but requires a huge amount of water. The ponding method of curing is not preferred for large areas and areas where water is scarce.

Spraying and fogging method of curing

Spraying and fogging are conventional water curing methods for horizontal and vertical concrete surfaces. These methods are very effective and efficient in supplying additional moisture during hot weather. The spraying and fogging methods help in reducing the temperature of the concrete.

Spraying is a manual method of simply spraying water over the concrete surface. Even though the spraying method is effective, it tends to waste a lot of water.

Fogging is applicable for areas where the temperature is above freezing point and with low humidity. This process raises the humidity higher than curing concrete by spraying a fine mist of water regularly across the concrete surface. Fogging, or fog spraying is effective in reducing the chances of plastic shrinkage cracking in mixes.

Saturated or wet coverings

In this method of curing, saturated or wet coverings are placed over the hardened concrete. The wet covering material includes hessian cloths, cotton mats,moisture-retaining fabrics, etc. Vertical and inclined structures like columns, beams, sloped roofs, etc are cured in this method.

Curing by wetting -methods of concrete curing — Curing by wetting

Immersion curing

Immersion method of curing is for curing concrete specimens.

Reducing Water loss from concrete- Methods of concrete curing

This concept of curing is by reducing the moisture loss from the concrete surface by wrapping it with an impermeable membrane or plastic sheets. This method is the most practical and efficient way to cure concrete nowadays and is a much-needed option for areas where there is water scarcity. The common methods are..

Membrane curing
Covering concrete with impervious sheets or plastic sheets

Membrane curing

The application of curing compounds is through spraying or painting directly on the concrete surface. The compound dries and forms an impermeable membrane that retards or reduces the moisture loss from the concrete. The membrane curing method is a flexible and easy method of curing.

Curing compounds are applied once the concrete is hardened, but still having water content on it. The application has to be done before the water evaporates fully from the concrete. The membrane curing may not produce desired results, if applied after the water evaporates. Ensure uniform thickness and coverage during the application process.
Curing compounds are available in acrylic-based and water-based forms.
The curing compound should comply with ASTM C3094 or ASTM C13155.
Always go through the manufacture specification before application of curing compounds.

Covering with plastic sheets

In this method, Impervious paper and plastic sheets is applied on thoroughly wetted concrete. The concrete surface should be hard enough to take the possible damages while placing the sheets.

Accelerating strength gain method – Methods of concrete curing

Accelerating strength gain using heat or additional moisture is done to speed up the early hardening of concrete and mortars by subjecting them to steam and humidity. Following are the methods adopted in this method of curing.

Steam curing for concrete

The steam curing method uses water vapor or steam for curing the concrete elements. This method of curing is for cold weather conditions where the concrete needs accelerated early strength gain and additional heat for hydration. Prestressing and precast factories use the steam curing method for early strength gain of components. The early strength gain can enable rapid removal and reuse of forms.

The steam curing is done in two ways

live (or low pressure) steam at atmospheric pressure
high-pressure steam curing in autoclaves.

Steam curing at atmospheric pressure

Steam curing allows increased production due to rapid repetition and reuse of molds/forms in precast yards.

The steam temperature should be kept at around 140 degrees Fahrenheit or less for live steam at atmospheric pressure until the desired concrete strength is achieved.

By maintaining an optimum temperature, a 28 days strength of normal water cured concrete can be achieved in 3 days using the steam curing method.

High-pressure steam

High-pressure steam curing is carried out in a closed chamber at high temperature and high pressure. This process is also known as “Autoclaving”. Temperature should be maintained between 325°to 375°F and pressures should be around 80 to 170 psi. This method is used for manufacturing precast components, cellular concrete products, such as cellular blocks, precast panels, autoclaved aerated concrete (AAC) blocks, etc.

The main advantage of a high-pressure steam curing system is the capability of attaining 28 days of strength of normally cured concrete in 24 hours.

High-pressure steam cured concrete got less creep and shrinkage and better sulfate resistance. There will not be any efflorescence and moisture content after curing.

Heating coils

Heating coils are usually used as embedded elements near the surface of concrete elements. Their purpose is to protect concrete from freezing during cold weather concreting.

Conclusion

The type of curing to be suggested depend upon the structure, type, nature of the structure and climatic conditions. Curing is the most important activity to be done on concrete to maintain its design strength, durability,serviceability and life span.

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Curing of Concrete – Concrete Curing Methods explained