Concrete strength rises with age as long as there is moisture and a proper temperature for cement hydration. Compressive strength as a percentage of damp cured concrete after 28 days. Concrete that has been exposed to sun and air but not wetted will lose its strength over time.
Compressive strength increases about 5% per year up to about 10 years, after which time it may decrease due to drying out of the concrete. Concrete that is fully immersed in water will continue to gain strength for many years.
Tensile strength increases more rapidly than compressive strength. After just one year, tensile strength will be about 20% higher than compressive strength. After ten years, tensile strength will be about 200% higher than compressive strength.
The type of aggregate used in concrete affects how it ages. Concrete containing coarse aggregates such as gravel is strong enough for most uses before it dries out, while concrete containing fine aggregates such as sand is not. When concrete contains both coarse and fine aggregates, it can remain usable for a longer period if it is watered regularly.
Coarse aggregate concrete tends to be heavier and use more cement than fine-aggregate concrete.
Concrete's 28-day compressive strength, with or without entrained air, increases as cement fineness increases. At one year, the variation in compressive strength owing to cement fineness is significantly reduced. 6.6 m (22 ft) per 100 kg (220 lb) increment in cement content.
Compressive strength increases as water/cement ratio decreases. However, at low values of water/cement ratio, the material may not be sufficiently strong for some applications.
Strength also increases as aggregate size reduces. Concrete containing coarse aggregates has high initial strength and takes longer to reach maximum strength than concrete containing fine aggregates.
Strength also increases as exposure time to atmospheric conditions increases. Concrete that is exposed to atmospheric conditions for several months or years can develop considerable strength.
Strength also increases as temperature increases. Concrete that is heated above its normal curing temperature will show an increased early age strength gain. This effect becomes significant when the difference between the maximum and minimum temperatures exceeds 10 degrees Celsius (18 degrees Fahrenheit).
Strength also increases as moisture content increases up to about 15 percent. Moisture causes concrete to expand, which leads to internal stress that can cause cracking. Excessive moisture content can also result in mold growth in concrete structures.
After placing concrete, 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 hardening-off. As the concrete cures, it becomes less fluid and more solidified. This makes it harder for any further water to dissolve out of the cement paste.
The best way to test the strength of fresh concrete is with the "bucket test." Pour a bucket of water into the wet concrete until it reaches a depth of at least 2 inches. Let the concrete cure for at least 24 hours before testing its strength. The strength of cured concrete should be tested either by the American Concrete Institute (ACI) or the European Committee for Standardization (CEN). These tests determine how much weight can be supported by a given area of concrete. Cement paste alone has a strength of about 20 MPa (3000 psi), but once it has hardened, the strength can be 30–50 MPa (5000–8000 psi)
Concrete's average strength decreases as it ages. This reduction in strength is another reason why concrete needs reinforcement sooner rather than later after placement. Reinforcement can be added to concrete structures during construction either internally or externally.
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