On January 4th, 2010, the world’s tallest building opened in Dubai. It’s the Burj Khalifa, which stands 2,722 ft. (829.8 m) tall. Its primary building material, reinforced concrete, made this architectural marvel possible.
Concrete is the most versatile building material ever invented. It’s used everywhere, from simple sidewalks to massive dams and skyscrapers. While the concrete’s composition is critical, the curing process is equally important. Proper curing of concrete allows it to develop its full strength, ensuring it can withstand the test of time.
This informative guide will cover the concrete curing process stages, the different curing methods, and the steps to ensure your concrete structures will last.
What is The Science Behind Concrete Curing?
The Romans invented concrete in the 3rd century A.D. They ground two parts volcanic ash, the pozzolan, with one part powdered lime or calcium oxide. Roman structures, like the Pantheon, still exist over 1,400 years later.
Modern concrete is a mixture of cement, water, and aggregates. Aggregates are sand, gravel, or crushed stone. The cement is primarily lime or calcium oxide mixed with silica or silicon dioxide and alumina or aluminum oxide.
Calcium oxide reacts with water causing a heat-releasing or exothermic reaction. This reaction, called hydration, generates heat, which is why concrete can become hot to the touch during curing. Heat is a byproduct and is necessary for the reaction to occur. However, if the concrete temperature gets too high, it can cause the concrete to crack or weaken. It’s critical to control the temperature and moisture levels during curing.
Hydration transforms calcium oxide into a hard stone-like material called calcium hydroxide. The cement in the mixture reacts with the water to form a crystalline structure, hardening and strengthening the concrete. The reaction begins immediately after the water and calcium oxide mix, but it can take weeks or months to cure completely. The curing time of concrete depends on several dynamics, including the cement type, the environmental conditions, and the thickness of the concrete.
Importance of Proper Concrete Curing
The freshly mixed concrete is in a plastic state and can be molded and shaped into any desired form. However, it is also weak and vulnerable to damage.
Through the process of curing, the concrete gains strength and durability, becoming more resistant to cracking, shrinkage, and other forms of damage.
Proper curing also ensures that the concrete has a consistent and uniform strength throughout its entire structure, essential for ensuring its long-term durability.
The Stages of Curing Concrete
The American Concrete Institute (ACI) recommends a minimum curing period to achieve 70 percent of the specified compressive strength. The specific concrete curing times depend on the concrete mix and environment.
Here are the general curing times required during the three concrete curing stages.
Initial Curing
The initial curing stage begins immediately and takes 24 to 48 hours. Workers can remove forms and walk on the concrete surface but avoid heavy traffic and weather to prevent damage.
Intermediate or Partial Curing
The intermediate curing stage takes about 7 days. At this point, the surface gains most of its final strength, and vehicles can drive on it.
Final Curing
The final curing stage takes from 14 to 28 days. The concrete achieves over 90% of its compressive strength during this stage.
Concrete strength relies on the growth of crystals within the concrete matrix. They grow like tiny rock-hard vines intertwining with the aggregate in the mix due to the hydration reaction between cement and water.
Factors That Impact the Curing Process
The concrete curing process is not uniform. Four primary physical and chemical factors can expedite or retard the hardening process, ultimately affecting the concrete’s strength.
- Temperature – The ideal temperature for curing is between 50°F and 90°F (10°C and 32.2°C).
- Moisture – Moisture is crucial for concrete strength. If the concrete is too dry, the chemical reaction will slow down, and the concrete will not achieve its full strength. A higher water-cement ratio makes weaker, more porous, and less durable concrete.
- Cement Type – Various cement mixtures have different curing times and strengths.
- Concrete Thickness – Thicker concrete will take longer to cure and may require additional measures to control its temperature and moisture levels.
Types of Concrete Curing Methods
Several methods for curing concrete work best for specific environments and situations. They are:
Wet or Moisture Curing
After pouring fresh concrete, wet curing involves moistening the concrete surface for 7 to 14 days. Methods include using water sprinklers, wet burlap, or a curing blanket. This method is best for hot and dry climates with a high risk of premature drying. The benefits include reduced shrinkage, increased strength, and improved durability.
Keeping concrete moist within the first 24 hours after the casting prevents rapid evaporation that causes plastic shrinkage cracking and weak and dusty concrete surfaces.
Dry or Thermal Curing
Dry curing involves keeping the concrete surface dry for 7 to 14 days after casting. This method uses the addition of heat, such as concrete curing blankets. Dry curing is best for cold and wet climates, with a high risk of freeze damage.
Chemical Membrane Curing
Membrane curing involves applying a waterproof sealant or concrete curing compounds to the surface immediately after placement. The sealant acts like a waterproof film to prevent rapid evaporation in hot and dry climates.
Self-Desiccating Method
The self-desiccating method allows the concrete to cure naturally without additional moisture or covering. It is a method for smaller projects that don’t require high strength. Due to faster drying, more cracks may form.
Monitoring The Concrete Curing Progress
Every commercial or structural project using concrete must test it for quality and strength. Three primary methods for monitoring concrete curing are temperature monitoring, moisture monitoring, and strength testing.
1. Temperature Monitoring
Temperature monitoring involves measuring the temperature of the concrete surface, especially when pouring concrete in cold weather.
The concrete should not be too cold or hot. Hydration slows below 50°F (10°C) and virtually stops below 40°F (4.4°C). If concrete freezes during curing, it causes permanent strength loss due to the expansion of water within the concrete. Freezing makes the concrete porous.
Conversely, if the concrete gets above 90°F (32.2°C), the rapid dehydration causes the concrete to break down and crack.
2. Moisture Monitoring
Moisture monitoring involves measuring the moisture content of the surface and internally using a moisture meter. Inserting probes into test holes is the most accurate way to determine moisture levels.
If the water evaporates before the concrete gains maximum strength, inadequate water will be left to hydrate the cement completely. It will never reach full compressive strength.
3. Strength Testing
Strength testing involves testing the strength of the concrete at various stages of the curing process. Typically, most concrete achieves 70% of maximum strength in 7 days, 90% in 14 days, and 99% in 28 days. Strength testing is ultimately about safety and verifying if the concrete achieves its specified strength.
Best Practices for Concrete Curing in Different Weather Conditions
Concrete curing can be challenging in extreme weather conditions. Here are some best practices for different weather conditions:
Hot weather – Keep the concrete cool and moist to avoid it from drying out too soon. Employ sunshades or covers to provide shade and sprinklers for moisture.
Cold weather – Keeping the concrete around 70°F (21°C) ensures a proper cure. The best solution is heated concrete blankets that maintain a consistent temperature.
Wet weather – During wet weather, protect the concrete from rain using covers or temporary roofs to keep the concrete from becoming saturated.
Avoiding Damage During Curing
Several factors can affect the curing process and cause damage to the concrete. Some of the most common include:
- Overwatering or underwatering
- Improper temperature control
- Direct sun and windy conditions
- Exposure to freezing weather
- Physical disturbances during the curing process
To avoid damage during curing, follow the proper guidelines and procedures for each type of curing method. Monitor the curing process regularly and make any necessary adjustments.
Common Questions About Concrete Curing
Contractors and DIYers have similar questions about concrete curing. Here are a few answers.
How Long Does 4 Inches of Concrete Take to Cure?
Generally, it takes about 28 days for 4 inches of concrete to cure to full strength. The curing time depends on the thickness of the concrete, the type of cement used, and the environmental conditions.
Should You Wet Concrete While Curing?
Wet curing can improve the strength and durability of the concrete. Wetting the concrete in hot, dry weather may be necessary to prevent it from drying out too rapidly. It means keeping concrete moist for several days after pouring.
In cooler weather or high humidity, wet curing may not be necessary. In these conditions, the concrete will cure naturally without additional moisture.
Can Concrete Cure in 7 Days?
Under ideal conditions, most concrete can reach 70% of its compressive strength in 7 days. It generally takes 28 days to get 99% strength.
How Do You Know If The Concrete Has Fully Cured?
Testing is the only way to verify if the concrete has reached its full strength. There are seven testing methods. They include:
- Cast-in-place Cylinders (ASTM C873)
- Rebound Hammer or Schmidt Hammer (ASTM C805)
- Penetration Resistance Test (ASTM C803)
- Ultrasonic Pulse Velocity (ASTM C597)
- Pullout Test (ASTM C900)
- Drilled Core (ASTM C42)
- Wireless Maturity Sensors (ASTM C1074)
However, the most common method is the use of field-cured cylinders. These samples are cast and cured according to the American Society of Testing and Materials (ASTM) C31. Then they are tested for compressive strength by a lab specializing in concrete using the ASTM C873 test.
Understand The Curing Process for Lasting Concrete
Concrete sets and hardens due to a chemical reaction between hydraulic cement materials and water. This process is called hydration. Curing maintains the correct temperature range and moisture conditions in concrete for hydration to develop its potential strength and durability.
The guidelines and tips outlined in this educational guide ensure that your concrete structures achieve maximum strength and can withstand the test of time. Whether you are a construction professional or a DIYer, understanding the stages of concrete curing is essential for ensuring the quality and longevity of your concrete.
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