This is a complete guide to the curing of concrete.
In this in-depth guide, you’ll learn everything about concrete curing. That includes:
- Importance of concrete curing
- When and how long concrete should be cured
- Methods of curing concrete
- The technical aspects of concrete curing
- Curing in hot and cold weather, and
- Lots more
So if you want to sharpen your concrete construction knowledge and produce durable concrete structures every time, you’ll love this easy-consumable guide.
Let’s dive right in.
What Is The Curing Of Concrete?
First thing first.
Before going deep down, let’s learn the basics of curing.
Basically, concrete is a mix of cement and aggregates.
When water is added to the mix, cement particles start a chemical reaction. This reaction is called the hydration of cement.
Fundamentally, concrete gains strength from this hydration process of cement.
This hydration process isn’t a single-time action. It continues for a long time. But at a decreasing rate.
That means the rate of hydration is fast at the beginning.
And thus produce a good amount of heat.
This heat evaporates water from fresh concrete.
And thus hinders the hydration process of cement.
Besides this, since the concrete is open to the atmosphere, the water from the concrete evaporates and prevents the effective hydration process.
If you want to get desired concrete strength, you have to make sure uninterrupted hydration of cement.
For this, you need to keep the concrete moist and warm.
And, the process of keeping the concrete moist and warm enough to ensure uninterrupted hydration is called curing of concrete.
Simple!
If you want a text-book-like definition, here it is:
Now, the question is:
How Much Water Is Required For Curing?
The right question is:
How much water is required for the proper hydration of cement?
Theoretically, cement requires a water/cement ratio of:
- 0.23 for hydrating all particles of cement,
And
- 0.15 for filling the Voids in the gel pores.
In total, a water/cement ratio of about 0.38 is required. And this is sufficient. You won’t require any more water for curing.
However, it differs practically.
If you make concrete in a sealed container, you’ll see that a water-cement ratio of 0.50 will be required for complete hydration.
But, it’s a different story in the field and in actual work.
Because the water used in the concrete evaporates due to atmospheric actions, especially on the top surface.
When the water evaporates from the top surface, the water inside the concrete comes out.
And, this impedes the hydration process.
Even a higher water-cement ratio can’t solve this problem.
Why Is It Important To Cure Concrete?
Why I’m writing this long guide about the curing of concrete.
Because curing is very important for concrete products. Proper curing is crucial for the long-term performance, durability, and strength of concrete structures.
As construction professionals, we need to be versatile and knowledgeable in this area.
So let’s learn the importance of concrete curing.
Moisture Retention: For developing the strength and durability of concrete proper hydration of cement is essential.
And, proper hydration of cement depends mostly on adequate moisture.
Curing helps to retain that moisture.
Reducing Cracks: Cracks compromise the structural integrity and aesthetics of the concrete.
By mitigating early-age thermal and drying shrinkage stresses, curing helps to reduce concrete cracks.
Increasing Durability: Curing helps to increase the durability of concrete by forming a dense and impermeable concrete matrix.
Strength Development: Curing plays a vital role in achieving desired strength of concrete.
Moist curing allows a hydration process over a long period, which enables concrete ingredients to form strong chemical bonds and create a dense structure.
Properly cured concrete can achieve higher compressive and flexural strength.
Time Efficiency: Proper curing accelerates the initial strength gain of concrete.
It allows faster formwork removal.
And thus reduces the project’s timeline.
Note that proper curing depends on the concrete mix design, environmental conditions, and project specifications.
Based on these, you may need to select a specific curing method or combination of them.
So, let’s discuss the various…
Methods Of Curing
Boardly, the curing method of concrete can be divided into four types:
1. Water curing
2. Membrane curing
3. Application of heat, and
4. Miscellaneous curing method
Let’s discuss all these methods elaborately.
1. Water Curing
This is the most common and by far the best curing method.
And, it satisfies all the requirements of curing. Such as:
- Promotion of hydration
- Elimination of shrinkage, and
- Absorption of the heat of hydration
Water curing is done in the following ways:
- Immersion
- Ponding
- Wet covering
- Spraying or fogging
Immersion: Precast concrete members are basically cured in this way.
For this, a curing tank is used and concrete members are immersed in the tank for a certain duration.
Ponding: In this way of curing, somehow a pond is made and water is poured into the pond to cover the concrete surface.
Horizontal members, such as concrete slabs, are cured in this way.
Wet Covering: In this way, vertical concrete members are wrapped with hessian cloth, gunny bags, jute matting, straw, etc. and, keep them wet by spraying water.
And, horizontal concrete members are covered with sawdust, sand, or earth to keep them wet.
Spraying Or Fogging: As the name says, curing is done by spraying water in this way.
Normally, vertical retaining walls and plaster surfaces are cured in this way.
2. Membrane Curing:
If water curing isn’t possible or costly, then this method of curing is applied.
For example, there is an acute shortage of water in middle east countries.
Actually, you don’t need to cure concrete if you can somehow prohibit the evaporation of water from concrete.
Because the water in fresh concrete is more than enough for cement’s hydration.
This mechanism is used in membrane curing.
For this, concrete is covered with membranes to seal off the evaporation of water.
As membranes, the following materials are commonly used:
- Polyethylene or polyester film
- Bituminous compounds
- Curing compounds
- Waterproof paper
- Rubber compounds, etc.
As I write for construction professionals directly engaged in the field, I feel compelled to highlight crucial aspects that require careful attention.
If you want to adopt membrane curing, take care of the following things:
- When using bituminous compounds, apply a lime wash over the black coating.
Because bituminous compounds are black in color. And, they absorb heat which is not desirable. Applying lime wash prevents heat absorption.
- Membrane curing is good if the water/cement ratio of concrete isn’t less than o.50.
- To get the best result, apply membrane curing after one-two days of actual wet curing.
- Apply two-three coats of good-quality membrane to effectively seal the surface. So that water can’t evaporate.
- When using polyethylene films or waterproofing papers, make sure proper laps are given in joints and they are airtight.
3. Application Of Heat
In this method of curing concrete, a higher temperature is applied.
We know, concrete develops its strength with time.
But the development of strength can be accelerated by applying higher temperatures.
Because heat accelerates the hydration process of cement.
But dry heat can’t be applied to concrete as the presence of moisture is also an essential requisite.
That’s why steam is applied in this method of curing.
This method of curing is done in the following ways:
- Steam curing at ordinary pressure, and
- Steam curing at high pressure
- Curing by infra-red radiation
- Electrical curing
Steam Curing At Ordinary Pressure: In this way of curing, concrete members are stored in a chamber and then the door is closed.
After that, steam is applied.
This way of curing is applied mostly in prefabricated concrete members.
And, they gain the 28 days strength of normal concrete in about 3 days.
Steam Curing At High Pressure: In this way, the curing is carried out in a closed chamber with the superheated steam at a high pressure and high temperature.
This process is practiced in the factory to cure precast concrete products.
In this way of curing, concrete attains the strength of 28 days’ of normal concrete in just one day.
In high steam curing, the temperature applied is about 175°c which corresponds to a steam pressure of about 8.5 kg/sq.cm
Curing By Infra-red Radiation: This curing is practiced in very cold climatic regions.
Mostly, this curing is applied in hollow concrete products.
And, the temperature in this curing is kept at about 90°c.
Electrical Curing: In this way of curing, concrete is cured by passing an alternating current through the concrete itself between two electrodes.
This curing method isn’t widely used. because it’s costly.
4. Miscellaneous Curing Methods
There are some other methods that can be used for curing. Such as:
Unstripping Formwork Method:
We often do this unconsciously for beams and columns.
If you don’t remove the formwork of concrete members, mixing water can’t evaporate, which, as a result, promotes the hydration process of cement.
But you have to make sure that the joints of the formwork are airtight.
Calcium Chloride Method:
This is another method of curing we can put under the miscellaneous curing method.
Calcium chloride is a salt. It absorbs moisture from the atmosphere and retains it at the surface.
This moisture held at the surface prevents the evaporation of mixing water from concrete.
And, this helps the hydration process of cement.
Calcium chloride can be used either as an admixture or as a surface coating.
Now that you have acquired knowledge about various curing methods, you can make informed decisions and select the appropriate method or combination of methods that aligns with your project’s specifications.
But, the question is…
When Should You Start Curing?
Let’s face it.
We are directly involved in practical construction fields.
Most of the time we face some problems in the practical field. The solutions to these problems aren’t found anywhere, not in theoretical lessons or textbooks.
And, many a time we wonder how early we should start curing by application of water.
You may have seen that concrete gets dried up in a short period of time (say 2 hours). This problem arises in hot weather concreting.
During that time, we often have some questions:
- Should we pour water over the concrete surface within the first two hours?
- If we pour, will it interfere with the water-cement ratio?
- How soon can we apply water without hampering the water-cement ratio?
You’ll get answers to all of these questions if you keep this formula in mind:
“Concrete shouldn’t be allowed to dry in any situation.”
There is no definite time frame for starting water curing.
In fact, it’s difficult to set a time frame.
Because it depends on:
- Prevailing temperature
- Wind velocity
- Humidity
- Water-cement ratio
- Type of cement
- The fineness of cement, and
- Size of the member, etc.
What you’re actually going to do is:
First, observe the fresh concrete surface after haulage.
If you think that the water from the surface is drying fast, cover the surface with wet hessian cloths.
But the hessian cloth should be properly squeezed. So that water doesn’t drip. Otherwise, it will interfere with the water/cement ratio.
On the other hand, make sure the hessian cloth doesn’t allow the concrete to dry.
You should continue this till the final setting time of cement (at least).
If you pour water after this time, the water won’t interfere with the water-cement ratio.
But, it’s best to keep the concrete under the wet hessian cloth for 24 hours.
And then start curing by ponding or spraying water.
Now, the question is:
How Long Should Concrete Be Cured?
Again, it’s difficult to set a limit.
If you ask me, I would say, curing should be as long as practical.
Because curing improves all the desirable properties of concrete.
Yet we have a tight project schedule. So we need to start work on the next phase.
And, shorten the curing period.
But up to a limit.
A general rule is:
“Concrete must be cured till it attains about 70% of desired strength.”
But how can you make sure that concrete attains 70% of desired strength?
For this, some cubes are made and kept near the structure under the same atmospheric condition.
And, applied the same curing method.
These cubes are periodically tested to know their strength.
And thus you can know the strength of the structure.
Once you observe that the concrete has attained 70% of the desired strength, you can determine the estimated number of days required for the curing process.
You don’t have to do this every time you make a concrete structure.
Under the same atmospheric condition and the same types of concrete, you need to cure the concrete for the same duration.
That’s why you’ll see that everyone applies the same curing method for a definite set of days in the same region.
Temperature For Curing Concrete
So far, you have learned deeply about:
- The concept of curing concrete
- Various methods of curing, and
- The curing period.
You can now make a durable concrete structure with this knowledge.
All these will be in vain if you don’t fully understand the curing temperature of concrete.
Let’s learn about it.
What Is The Concrete Temperature?
Fresh Concrete faces three types of temperatures:
- The temperature of concrete ingredients
- The temperature of the hydration process, and
- The ambient temperature
All these affect the curing process.
So, let’s see…
How Temperature Affects The Curing Of Concrete?
- Lower temperatures slow the hydration process down.
As a result, the concrete gains strength more slowly.
- Higher temperatures accelerate the hydration process.
And, thus the concrete gains strength rapidly.
- Very low temperatures can freeze water inside the fresh concrete. Which can adversely affect the strength of the final product.
- High temperature influences the moisture loss from the surface of the concrete. It increased the risk of surface cracking.
As you can see, a favorable temperature is crucial for proper concrete curing.
But what is the…
Favorable Temperature Of Curing
The favorable temperature for curing concrete is a range that allows optimal strength development, durability, and long-term performance.
it’s:
- 10°C to 25°C for the initial curing period, and
- 5°C to 20°C for later curing days.
But those are in general.
You should follow the specific guidelines based on the desired concrete properties and project location.
These guidelines are often provided by regional building codes, industry standards, and project specifications.
But we often don’t get that recommended temperature in our practical work field.
In that case, what should you do?
Favorable Curing Environment In Hot Weather:
Use some techniques to prevent excessive moisture evaporation and overheating of the concrete. Such as:
- Shading
- Misting
- Using the evaporative cooling method
Favorable Curing Environment In Cold Weather:
To protect the concrete from freezing and ensure proper strength development in cold weather, use the following techniques:
- Use heated aggregates, and
- Insulate the concrete formwork
Conclusion
So, this is my well-researched guide on the curing of concrete.
As you saw, the curing of concrete is very important for durable structures.
And, you can apply the knowledge I’ve shared in this guide to make sustainable concrete structures.
But before you get started to apply the knowledge, I would like to hear from you.
What did you think of today’s guide?
Or maybe you have a question about something you read.
Let me know by leaving a comment below.