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 cured in dry conditions will be about half as strong as if it had been placed in a pond or body of water. Curing also helps prevent the growth of harmful organisms such as insects and algae.
The rate at which concrete cures depends on its moisture content. If allowed to dry out, concrete will continue to cure but at a much slower rate. Curing also can be sped up by using heat; this is done during construction of buildings with hot climates as a requirement for speedy completion of projects. Heating cables are embedded within the concrete during its formation and then removed when it is finished. The cables provide an internal source of heat that speeds up the drying process and also increases the temperature of the surrounding concrete.
Concrete continues to gain weight over time due to the evaporation of water from its pores. This is called "creep" and it is why concrete foundations must be sloped toward the center to allow for some drainage. Concrete that has dried out and no longer contains any free water will experience no further increase in weight. However, if it starts to rain again, the fresh water will enter those same pores and cause more water loss.
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 the hardening of the cement paste. As more and more water is needed to dissolve the sand and gravel, less and less is available to dissolve the aggregate, so the cement paste becomes harder and stronger.
The rate at which fresh concrete gains strength is known as the "ageing process". Ageing takes about 28 days for a concrete slab on grade ground, but can be as long as 90 days for a concrete bridge pier. Concrete's ageing capacity is limited by its water/cement ratio and by the amount of air entrained during mixing. High-strength concretes may not age as quickly as ordinary concretes but they still gain strength over time.
Concrete's average strength increases as it ages because new surface areas are exposed as some of the original surface cracks fill in. But if you use fresh concrete, the initial strength is what will matter most when you need it most - right after casting. Any additional strength gained due to ageing will come at the cost of reduced ductility (the ability to bend without breaking) until it reaches its ultimate strength.
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 concrete remains wet. For example, if you pour fresh concrete into an empty form and then let it cure, then the strength will increase as the hydration process proceeds. After about one year, the concrete should be strong enough for most applications. However, concrete strength will continue to rise as long as the concrete is kept in a damp environment. For example, if concrete is placed into forms and covered with plastic sheeting before it has cured, then the strength will increase more once the plastic sheeting begins to deteriorate.
Tensile strength increases as long as the concrete remains wet.
Concrete increases 16% of its original strength in 24 hours and 65% of its goal strength by the time it is cast and cured. After 14 days, concrete reaches 90 percent of its goal strength; after that, the strength development slows and it takes 28 days to reach 99 percent of its strength. Concrete's strength improves as it cures or hardens.
Cement paste becomes stronger as it cures. The early-age strength is mainly due to the formation of a solid network structure through gelation and polymerization of water-based cement paste. As the paste continues to cure, internal forces increase due to hydration products forming within the paste. These internal forces are responsible for the rise in strength at later ages.
Concrete's strength develops slowly during initial curing because sufficient heat is not transferred to the concrete immediately after casting. The transfer of heat occurs as the concrete begins to dry out. If the concrete is covered with a plastic film, it will remain moist while drying out, which helps the concrete to gain strength more quickly.
The strength of concrete can be increased by adding steel bars or fibers. Concrete's durability can be improved by using aggregate with smaller holes or pores (less than 2mm) or by using a pozzolanic material. Pozzolans are substances such as fly ash, slag, and ground limestone that when mixed with water and cement produce compounds that bind any remaining free acid in the mixture.
Over the course of several months, the concrete hardens. Hardening is not a drying process and may happen in water. Heat accelerates the setting and hardening of cement, whereas cold slows or even halts the process. Concrete that has hardened completely (within one year) is called "dry hardened cement".
Concrete hardens slowly at first and then faster as time goes by. The first few days to weeks after it's mixed, the mixture needs to be kept out of the way so it doesn't get wet. During this period, it's useful to let it set up in a dry place with no water damage.
After a few weeks, the concrete starts to gel. The gels come in two forms, an active gel stage and a passive gel stage. Active gels are more flexible while passive gels are less flexible. As long as the mixture isn't stirred too much, it will stay in its initial state for many months.
Concrete begins to harden as soon as it's mixed. However, depending on the temperature, it can take from a few hours to several days before it's ready for use. Cement sets quickly at warm temperatures and more slowly at cold temperatures. Concrete that has hardened completely within a year is called "dry-hardened cement".