Therefore, it is clear that concrete gains its strength rapidly in the first few days after casting, that is, when its strength has reached 99% in 28 days, yet concrete continues to gain strength after that period, but that rate of gain in compressive strength is much lower compared to From 28 days. The specification of the strength of concrete is normally made with a minimum compressive strength (psi) at a given age (days). The specified compressive strength of concrete is the minimum compressive strength at which concrete must fail in standard 28-day-old concrete cylinder tests. A typical compressive strength specification for concrete requires 4,000 to 5,000 psi in 28 days.
Some go a step further and demand that concrete products cannot be installed or used until 28 days after the date of manufacture. This, wrongly, has given some prescribers a reputation for being weak or inferior if they haven't healed for the full 28 days. After laying the concrete, the strength of the concrete grows rapidly for 3 to 7 days. Moisture-cured concrete that has been wet-cured for seven days is approximately 50% stronger than uncured concrete.
The increase in strength occurs due to a process called hardening. As concrete cures, it becomes less fluid and more solidified. This makes it more difficult for additional water to dissolve from the cement paste. According to the American standard ACI codes, the strength of concrete and Portland stone cement mix, fine aggregate and coarse-grained aggregate, are designed according to the cylinder test.
Before using concrete for its intended purpose, you must know its current strength so as not to exceed its design limits. The 28-day period is an arbitrary sample age, although it was chosen for many good reasons, to test the compressive strength of concrete. Finally, when its strength has reached 99% in 28 days, concrete continues to gain strength after that period, but that rate of increase in compressive strength is much lower compared to 28 days. This reduction in strength is another reason why concrete needs to be reinforced sooner rather than later after placement.
For more information on working in extreme temperatures, obtain a copy of ACI 305, Concreting in hot climates or ACI 306, Concreting in cold climates. After 7 days, the concrete will have gained about three-quarters of its compressive strength, but you should refrain from driving vehicles or heavy machinery on the surface until after 28 days. In reality, seawater does not affect the curing process of precast concrete products, since it is completed long before the product is placed under water. If water is available, concrete will cure automatically if it has a suitable mixture of cement and sand or gravel.
Ensuring that concrete achieves the minimum compressive strength required is not sufficient to ensure that it performs as required throughout its useful life. Simply put, as concrete cures and hydrates, concrete becomes stronger, and hydration can continue long after reaching the required minimum compressive strength. When concrete contains coarse-grained and fine aggregates, it can continue to be used for a longer period if it is watered regularly. For household mixes (vehicle entrances, for example), you can expect the concrete to harden within 24 to 48 hours.
This is not to say that the concrete has stopped curing completely and there is no gain, but as mentioned at this point, the relationship between strength gain and time is significantly reduced. .
