Table of Contents for concrete curing-
- What is concrete curing?
- Why concrete curing is important?
- When to start curing?
- What are the concrete curing techniques?
- How long is the concrete curing time & what is a concrete curing time chart?
- What are the three stages of concrete curing?
- How long 4-inch concrete takes to cure?
What is concrete curing?
Concrete curing is the process of maintaining the moisture in the concrete and developing strength. The concrete requires curing immediately after laying and finishing. Therefore, the necessary curing at the early stages of strength development provides significant support to strength development.
The concrete gains strength due to the formation of chemical compounds. Subsequently, the chemical reactions between the cement and water produce strength-imparting chemicals. Further, the rate of reaction depends upon many factors. Such as-
- Availability of moisture
- Concrete temperature
- Air temperature
- Mix design of concrete
To explain further, the reaction proceeds fastest concrete temperature, and air temperature is 20 degrees Celsius. Consequently, the temperature above and below 20 degrees Celsius slow down the strengthening of concrete.
Why concrete curing is important?
Since strength is the supreme feature of the concrete. Therefore, it is necessary to ensure favourable conditions for strength development. In addition, the formation of strength imparting compounds is an exothermic process. As a result, the moisture present in the concrete evaporates during the process. Consequently, the system becomes moisture-deficient and slows the compound formation.
The strength reduces without curing, there exist other effects of improper curing. The uncured concrete is weak and susceptible to volume instability (shrinkage), freezing and thawing. Moreover, it has lesser abrasion and scaling resistance. An uncured surface is likely to have fine cracks over it.
In continuation to benefits of curing, cured concrete is lesser permeable than uncured concrete. It is because the complete hydration of cement forms compounds that plug the pores. Consequently, the concrete gets low permeability.
When to start curing?
It is no doubt that concrete needs curing as soon as the hydration starts. But concrete transporting, laying and finishing cause delays to the curing. At this stage, the bleeding water present in the concrete cures the concrete. Therefore, the artificial curing methods should start once the bleeding water evaporates and the surface is hard.
One of the quick methods to check for the curing requirement of concrete is a “thumb impression check”. Press the concrete surface with a thumb. If the concrete doesn’t get a thumb impression, start the curing.
What are the concrete curing techniques?
Curing aims to maintain the moisture in the concrete to aid the strength gain. The below listed three techniques of concrete curing are widely accepted.
- Maintaining the moisture in the concrete.
- Reducing the loss of water from concrete.
- Accelerating the strength gain.
Maintaining the moisture in the concrete
It is one of the most popular and cheapest concrete curing techniques. The concept is simple and easy to execute. The concrete is kept in contact with water that keeps the concrete moist.
The curing technique uses ponding, spraying and fogging methods for maintaining the moisture. Another method is the left-in-place form that supports the concrete structure as well as keeps it moist. A regular water supply is necessary for this technique.
Reducing the loss of water from concrete
This concrete curing technique works on the principle of reducing moisture loss from concrete. As we know, the hydration of cement is an exothermic process that evaporates the moisture from the concrete. This technique traps the water within the concrete and doesn’t allow it to evaporate.
The implementation of the said technique of concrete curing is by covering the concrete surface. Therefore, chemical curing compounds and covering by sheets are the solutions. However, both methods are used only after the concrete surface is set.
A chemical curing compound forms a membrane over the surface of the concrete. This curing compound membrane is often white to reflect maximum sunlight falling over it. Spraying, paint-brush or roller are the methods of application of concrete curing compound.
In addition, the covering sheets are impervious papers or plastic sheets that are laid over the surface. The covering sheets are lightweight and easy to use.
Accelerating the strength gain
It is evident from the below-given graph that temperature affects the strength gain of concrete. During the initial days of hydration, the higher temperature increases the rate of strength gain. In contrast, a lower temperature is preferable on the final days of curing. In simple words, the concrete gains strength faster if we raise its’ temperature in the initial days of hardening.
In addition, the following methods are effective in accelerating the strength gain of the concrete.
- Steam curing
- Heating Coils
- Electrical heating forms
- Concrete blankets
Steam curing has two methods- live steam at atmospheric pressure and high-pressure steam in autoclaves. The live steam temperature is at 60 degrees Celsius.
Heating coils, electrical heating forms and concrete blankets- all protect the concrete from freezing. The use of all these methods is in cold weather conditions. The heating coils are embedded into the concrete and protect the concrete by heating its’ core. In contrast, the heating forms heat the surface of the concrete.
Concrete blankets are insulating covers to protect the concrete from freezing temperatures. Also, the concrete blankets are used only after the surface is hard enough to resist scratches.
How long is the concrete curing time & what is a concrete curing time chart?
The concrete keeps gaining strength up to 1 year of the start of hydration. However, the concrete gains 50% strength in the first three days. The concrete strength is 66% in the seven days. Also, by the end of 28 days, the concrete achieves 99% of its’ target mean strength. Therefore, it is feasible and viable to cure concrete for the first 28 days of hydration.
However, curing for 28 days is also not convenient in tight time schedules. Hence, a curing period of 14 days should be a target.
In continuation, it is necessary to keep the concrete moist in the first seven days of hydration. Researches have shown that water cured concrete is 50% stronger than uncured concrete. To conclude, seven days should be the minimum time for concrete curing.
A concrete curing time chart is attached herewith to demonstrate the strength development with different curing conditions.
What are the three stages of concrete curing?
Concrete curing is a three-stage process. It includes the following-
- Initial stage
- Intermediate stage
- Final Stage
Initial Stage of Concrete Curing
The initial stage is the first stage of the three stages of curing. It is necessary to reduce the shrinkage of the concrete surface and prevent surface cracks. Covering the concrete surface and fogging are efficient techniques for initial curing.
Intermediate Stage of Concrete Curing
The intermediate curing is the second stage of curing. It succeeds the initial curing. The concrete is cured while finishing.
Final Stage of Concrete Curing
Application of curing compound, ponding or spraying are the final stages in concrete curing. The final curing starts after the concrete sets. It continues for up to 28 days.
How long 4-inch concrete takes to cure?
A walkway or a driveway has a 4-inch thick concrete layer. You should cure a 4-inch driveway for at least seven days before you can drive your car on it. However, for a 4-inch or a 6-inch heavy vehicle motorway, the curing period is extended to 28 days.
Moreover, if the traffic is only pedestrian a 4-inch walkway should be open after 24-hours of finishing the surface.
The curing period for 4-inch concrete may extend or shorten from the given time. It depends on the weather conditions.
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