Curing of Concrete | Reasons | Duration | Types | Weather

 

CURING OF CONCRETE

 

 

WHAT IS CURING?

Curing is the name was given to the procedures used to improve the hydration of cement and temperature control and the movement of moisture from the concrete. It plays an important role in the strength development and durability of concrete.

 

WHY CURING OF CONCRETE IS IMPORTANT?

Curing is done to control the rate and amount of moisture loss from the concrete to ensure the uninterrupted hydration of the Portland cement after the concrete has been placed and finished in its final stage.

"Drying" concrete does not reach its design strength or meet the specifications. The longer the Curing treatment, the better the concrete.

 

REASONS TO CURE CONCRETE

There are several important reasons why concrete should be cured:

·       Increases Strength Of Concrete

·       Increases The Durability Of Concrete.

·       Harder And More Abrasion Resistant Surfaces

·       Improved Serviceability

·       Improved Microstructure

 

Increases Strength Of Concrete

Strength of concrete increases - Concrete increases in strength with moisture and has a favourable temperature for hydration of cement.

“Cement, Concrete & Aggregates Australia" (CCAA) conducted an experimental inquiry into the importance of curing

Comparing the strength of concrete in 180 days of moist curing with various periods of moist curing (0, 3, 7, 14 & 28 days) and then allowing it to dry is illustrated in below picture.

From the diagram below, it can be seen that the concrete is allowed to dry immediately, reaching only 40% of the water strength of the same concrete cured for the entire period of 180 days.

 

 

Increases The Durability Of Concrete

The life of concrete is affected by many factors including its permeability, porosity and absorption.

Well-cured concrete reduces heat, plastic and drying shrinkage cracks, making concrete more watertight, thus preventing moisture and water-dispersing chemicals from entering the concrete, thus increasing its durability.

 

Improved Serviceability

Concrete that is allowed to dry quickly undergoes considerable shrinkage at an early age. Improper curing contributes to weak, dusty surfaces having poor abrasion resistance.

 

Improved Microstructure

Material properties are directly related to their microstructure. The curing cement hydration reaction progresses smoothly and forms a calcium silicate hydrate gel, which binds the aggregates leading to a rock-solid mass, reducing the porosity of the concrete and improving the physical and mechanical properties of the concrete.

 

DURATION OF CURING

The curing time of concrete depends on the Grade and type of cement, the mixing ratio, the desired concrete strength, the shape and size of the concrete member and the environmental and exposure conditions. The duration can vary from a few days to a month.

In the case of ordinary Portland cement (OPC), the exposed concrete surfaces shall be kept continuously wet or wet conditions by by covering them with sacks hessian cloth or other similar material or ponding, and shall be kept continuously moist for at least 7 days at as of the date of pouring.

Therefore, concrete used with mineral admixtures or mixed cement, it is recommended to extend the above minimum periods to 10-14 days to facilitate the secondary reaction.

 

 

Curing Requires Adequate Moisture, Temperature & Time

If any of these factors are neglected the desired properties will not develop.

 

 

METHODS TO CURE CONCRETE

Methods of curing concrete are widely included in the following categories:

Water Curing: the loss of moisture from the concrete surface prevented by constantly wetting the exposed surface of the concrete.

Membrane Curing: Reducing Moisture Loss from Concrete Surface by covering it with an impermeable membrane.

Steam Curing: Raising the temperature of concrete and by keeping surface wet will accelerate accelerate the rate of strength gain

 

1. WATER CURING

Ponding: This is the most common and cost-efficient method of curing flat surfaces such as concrete slab, 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 filled with water to create a shallow pond. Care must be taken to ensure that the pond water does not dry out as it can lead to an alternate drying and wetting condition.

Wet Coverings: Water-absorbent cloth such as hessian, burlap, cotton mats, rugs, etc. can be used to cover the entire surface and maintain water on the concrete surface as soon as the concrete is set. Due to the capillary action, they must be constantly moist to prevent water from absorbing from the concrete body.

 

Sprinkling, fogging & mist curing: The use of a good spray or mist or water mist is an efficient method of water supplying to the concrete surface especially during hot weather, which helps to reduce the temperature of the concrete and ultimately retains moisture inside the concrete body.

 

2. IMPERMEABLE MEMBRANE CURING

This process done Occasionally, concrete work is carried out in areas with severe water shortages.

The amount of water normally mixed to make concrete is more than enough to hydrate the cement, and this water is not allowed to leave the concrete body.

The concrete is covered by a membrane, which effectively clogs the vapour from the concrete.

 

Formwork Leaving the formwork at an early age of the concrete is one of the most effective methods of healing, especially for columns. However, turning the time of the formwork is significantly reduced.

Plastic Sheeting: Plastic sheeting forms an effective barrier to control moisture loss from the concrete surface, provided they are secured in place and protected from damage. They should be placed immediately after the final set of the concrete without causing any damage to the surface.

On flat surfaces such as 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 gusts of wind.

 

 

Membrane curing compounds: 

The curing compounds are wax, acrylic and water-based liquids, which are sprayed onto the newly finished concrete, forming a membrane that reduces the loss of moisture content from the concrete. These are low-cost methods of healing that are difficult to follow with standard procedures.

When used to cure concrete, application time is critical for maximum performance. They should be used when the free water on the surface evaporates and there is no water sheen on the surface. Very early application dilutes the membrane, where too late application is absorbed into the concrete. Care should be taken to avoid foot, machinery and vehicle traffic on the concrete surface to prevent damage to the finish.

 

3. STEAM CURING

Steam curing is a process of accelerating the initial hardening of concrete and mortar by exposing them to steam and moisture.

This type of system is commonly used for precast concrete products that are manufactured in the factory and have a very fast turning time of formwork.

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 WEATHER

1.   In hot weather, concrete should be protected from over-drying and direct sunlight and wind.

2. Curing materials that reflect sunlight should be used to reduce the temperature of the concrete.

3.  Wet curing is recommended and care should be taken to avoid excessive stress.

4.   Caused by alternate wetting and drying or by cold water on hot concrete. canvas tarpaulins or sun shades framed enclosures can be used to protect concrete from direct sunlight.

CURING IN COLD WEATHER

1.   Some problems associated with a temperature below 4 ° C are:

2.   Freezing of concrete before adequate strength has developed

3.   Slow development of concrete strength

4. Thermal stresses induced by cooling hot concrete to colder ambient temperatures.

5.  In cold weather, certain procedures such as heated enclosures, insulating blankets and curing products may be used.

6. The temperature of fresh concrete should be kept above 100 ° C using heated raw materials and hardening should be continued for a longer period until the concrete acquires the desired strength.

CONCLUSION

1.  Chemical reactions between cement and water form the C-S-H gel, which binds to the concrete material. Coarse and fine aggregates, mineral compounds etc. turn these pieces into a rock-solid mass.

2. This is only possible if the treatment is continuous for at least 14 days; Regardless of the type of cement used.

3.   With a dense microstructure and flawlessness, prolonged curing can lead to improved longevity.

4.   Well-formed concrete can give poor life if not properly cured, on the other hand moderately designed concrete will give better life if well treated. So the importance of curing should never be ignored.

5. At many sites, concrete curing is left to the end and comfort of the unskilled worker.

6.   Site engineers and supervisors should make extra effort to ensure that on-site healing is not neglected and that they provide the resources needed for a satisfactory level of healing using the best technique available on site.

7.   Just like a newborn baby, when it comes to this world, it needs great care for its growth and protection from this new environment, similarly, newly placed concrete needs proper protection and care from the environment and aggressive environment.

8. Strict adherence to good curing practices on site will help the concrete achieve the properties of designed strength, improved durability, improved microstructure and longer service life.

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