Concrete continues to harden (cure) and become stronger after it has been set for an extended length of time, frequently up to many years. The concrete's strength is proportional to the water-to-cement mass ratio and the curing conditions. Concrete that has been well cured will generally be stronger than concrete that has been partially cured.
The rate at which concrete hardens depends on the temperature of the surrounding air. If the concrete is kept in a cold environment, it will take longer to cure. If it is kept in a hot environment, it will cure more quickly. Curing also can be sped up by adding moisture to the concrete or exposing it to high temperatures. There are two types of cures: active and passive. Active cures are performed with the help of heat or chemicals. Water is typically added to actively cure concrete, but other liquids such as epoxies may be used instead. The water causes the cement paste to hydrate, which makes it more fluid and allows it to flow better. This process also creates calcium hydroxide, which helps strengthen the concrete.
Passive cures occur naturally due to exposure to atmospheric elements. The key is to avoid direct sunlight and allow sufficient humidity levels inside the building to ensure a smooth cure. If you want your concrete to cure even faster, you can add moisture-barrier materials to prevent any additional moisture from entering the structure.
After the concrete is laid, the strength of the concrete grows rapidly over 3–7 days. Moist-cured concrete that has been moist-cured for seven days is about 50% stronger than uncured concrete. The growth in strength occurs because of a process called autogenous hardening. As the cement paste sets up, it generates heat, which causes further hydration and gelling of the cement paste. This auto-hardening process continues until no more water is needed to maintain its consistency. At this point, the concrete begins to cure from the outside in.
Concrete's strength can be increased by adding fibers or steel bars. Concrete with high quality ingredients and properly mixed will always be strong enough for most applications. If you need your concrete to be stronger, add more time to the curing cycle.
Concrete, in theory, never stops curing. Concrete, in fact, becomes increasingly stronger with time. The concrete will have achieved around three-quarters of its compressive strength after seven days, but you should avoid driving cars or heavy machinery over the surface until after 28 days. At that point, the concrete will be sufficiently strong to withstand the stress of regular traffic and loading equipment.
In practice, however, concrete continues to cure for months or even years after it has been mixed and placed into the form. The length of time required for it to reach maximum strength depends on several factors such as type of cement, temperature, amount of water used, depth of concrete, and how much air is allowed to get inside during mixing and placement.
Generally speaking, if concrete is kept free of oxygen (which occurs most often when it's covered by another material), it can cure for five or six years or more. If it gets any oxygen at all, though, it will start to degrade again rather quickly. The only way to prevent this is by covering it with a protective layer.
The process of cementing concrete is very similar to that of mortar. Both are calcium-based materials that contain small amounts of aluminum and iron as additives. In both cases, the alkalis in the cement react with acid compounds in the soil to produce a hard substance that binds together the particles of dirt and sand in the soil.
Concrete strength rises with age as long as there is moisture and a proper temperature for cement hydration. Compressive strength as a percentage of moist-cured concrete after 28 days. Concrete that has not been exposed to water will appear white and be very dry. This concrete can be strong enough for some applications, but it will need time to cure before use.
Compressive strength increases as long as the temperature remains above 40 degrees F. At temperatures below 40 degrees F, the concrete's curing rate will be reduced, causing strength to decrease over time.
As long as water is available, concrete strength will rise over time. The higher the humidity, the faster the concrete will cure.
After curing, the strength of the concrete will be inversely proportional to the water-to-cement ratio. Basically, the more water you use to mix the concrete (the more fluid the mix), the weaker the concrete mix. The less water you use to mix the concrete (which should be slightly dry yet workable), the stronger the mix. For example, if you double the amount of cement used in a concrete mix, the resulting concrete will be weaker than the original mix by a factor of two.
The reason for this is that cement absorbs much of the water used to hydrate it, and therefore requires less water to produce a strong concrete. Cement also contains some water as a volatile component, so more water needs to be added to make up for this lost water. Concrete that is mixed with high amounts of water will eventually start to harden before all the water has been absorbed by the cement, allowing some of the water to be lost during mixing.
If you reduce the amount of water by half, then the cement will need twice as much water to fully hydrate itself. This makes sense because there's now more cement relative to water, so more energy is required to break down the calcium carbonate into calcium hydrogen carbonate which can then be dissolved in water to create a slurry that can be poured.
Finally, reducing the water content by a third would require four times as much water to hydrate the same amount of cement.
Curing is critical for the development of concrete strength and durability. A well cured concrete has a suitable quantity of moisture for ongoing hydration and the development of strength, volume stability, freezing and thawing resistance, and abrasion and scaling resistance. The three main methods for curing concrete are air drying, heating, and chemical curing.
Air curing is done by allowing fresh concrete to dry naturally under ambient conditions. This method is slow and requires much space for adequate drying. In addition, the concrete must be watered regularly to maintain its proper consistency during drying. Heating cures concrete by exposing it to high temperatures for a sufficient period of time. This method yields a hard, strong concrete quickly, but it may damage certain types of materials used in construction projects. Chemical curing uses chemicals to accelerate the curing process. This method is fast and does not require heat, but it can cause corrosion of metal components such as rebar or sewer pipes. Chemical curing agents include sulfur, salts of nitric or acetic acid, and polycarboxylic acids. Sulfur is the most common curing agent used in concrete.
Curing concrete helps prevent the loss of strength caused by excessive drying out or wetting out. Heating or spraying water on fresh concrete before it sets up can be difficult or impossible depending on the climate where it's placed. This would be problematic because early exposure to water reduces the concrete's ability to withstand tension.