Concrete normally cures in 24–48 hours, at which point it is safe for normal foot activity. After one week, concrete is usually sufficiently cured to withstand additional building, including heavy machinery. Concrete is considered to have gained maximum strength 28 days after it is placed. At this time, it can be tested for its compressive strength.
The strength of concrete decreases as it ages because water moves into the concrete pores with enough force to push some of the sand out. This reduces the concrete's weight bearing capacity over time. Concrete that has not been exposed to any weather conditions will continue to deteriorate over time due to the growth of mold and bacteria. However, if the concrete is kept dry, this deterioration process is slowed down considerably.
Concrete's durability is also affected by how it is treated after it has been poured. The type of surface treatment will determine how well it resists wear-and-tear. If untreated, concrete will naturally age to a gray color and will require repainting about every ten years. Concrete that is painted will look good for much longer than that. The lifespan of a concrete driveway depends on how it is maintained. If left untouched for several decades, it will eventually become dark colored from all the dirt kicked up by vehicles driving on it. This can be removed with sootless soap and water. A concrete patio will need to be resealed every few years to keep it looking new.
How Long Does Concrete Take to Cure? The standard rule of thumb for curing time in concrete is 28 days. It can take up to 28 days for each inch of thickness, and only under perfect curing and drying circumstances, such as an enclosed location with low humidity, good air circulation, and the HVAC system turned on. If you will be placing concrete before the sun gets high in the sky, or if there are signs of rain during the curing process, then you should wait until all is dry and cured before proceeding.
Concrete cures over time, not immediately after pouring. There is a period of time where the concrete remains workable but not hard yet. This is called its "green" state. The length of time that elapses between casting concrete and when it can be used depends on the type of concrete and the curing method used. Curing methods include ambient temperature (the normal sun) and accelerated (with additives). For example, if you add 5% cement to 95% sand, then the green strength is about the same as regular concrete. But if you want the mix to set up harder and faster, you can use accelerated curing which makes the concrete ready for placement within 24 hours.
Green strength is important to allow workers to work safely without damaging the fresh concrete while it's still soft. Once it reaches full strength, you no longer need to keep it in its green state.
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 stresses of regular use.
In practice, concrete sets up early and therefore has a short usable life. For example, if water is not allowed to penetrate deep enough into the concrete to reach the alkali metal oxide (such as sodium oxide or potassium oxide) then it will form a hard shell around the salt particle which prevents any further hydration of the cement paste and therefore inhibits the development of high-strength concrete. This in turn means that the concrete must be maintained in a condition where it can be re-wetted periodically to allow this process to occur.
The practical limit for most concretes is about 10 years. After this time, the concrete begins to deteriorate due to the action of environmental factors such as air pollution, water penetration, and temperature change. The rate at which this occurs depends on several factors such as the type of concrete, its age, and how well it was constructed in the first place. Concrete that is placed in poor conditions or used for temporary structures will usually need to be replaced before it becomes too old for useful purposes.