Contractors often rely on the strength test result of concrete conducted at the age of 28 days. The strength results at 28 days are considered standard and contractors rely on these to determine the integrity of the resultant structure.
Concrete gains strength over time. It can take anywhere from a few weeks to a few years for different concrete mixes to reach almost 100% strength. Contractors generally perform strength tests four weeks after pouring – or 28 days. During this time, all concrete batches will have cured and strengthened rapidly. After 28 days, the rate of strengthening slows down.
How concrete gains strength over time
Standard grades of concrete will generally gain 16% of its strength after the first day, 40% after three days, 65% after seven days, 90% after 14 days and almost 100% after 28 days. This is why compressive strength tests are performed after 28 days. The concrete will be near-optimal strength and is unlikely to change much more after four weeks of curing.
The figures above clearly show how rapidly concrete sets and cures during the first four weeks. After this time, the rate of strengthening slows down considerably, gaining just one more percent of strength in a number of months after being poured. After the first two weeks, concrete only gains nine percent strength in the next two weeks, so the slowing of the rate begins after 14 days.
Concrete takes about 12 months to reach almost 100% strength
Most concrete will reach 100% strength about a year after pouring. Contractors only need to wait for 28 days before performing strength tests as it is close enough to maximum strength to form reliable results. The basis for evaluation is so close to perfect after four weeks that there is no point waiting for another 11 months to do the strength tests.
Although all concrete mixes are different in terms of how rapidly they cure, 28 days has become the standardised length of time to wait. Some rapid-setting concrete mixes may cure within half the time, contractors should still wait for four weeks to perform tests on the structure unless time is extremely limited and deadlines are threatening the project.
LafargeHolcim is a leading building materials and solutions company that has been operating in international markets for decades. We produce cement and aggregates for construction projects, ranging from small affordable housing developments to large-scale infrastructure projects such as high-rise buildings, dams and bridges.
LafargeHolcim Tanzania has been supplying the country and neighbouring countries with our world-class Tembo cement brand for over 30 years. Our head office and fully-integrated plant are located in Mbeya, Southwest Tanzania.
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