Concrete is a widely used construction material that is known for its durability and strength. In order to ensure that the concrete used in a construction project is strong enough to withstand the anticipated load and environmental conditions, it is necessary to perform strength testing. One common method of strength testing involves measuring the compressive strength of concrete cubes or cylinders.
The testing of concrete strength is typically carried out after 28 days from the time of casting, as this is considered to be the optimal time for obtaining accurate results. During this period, the concrete undergoes a process known as hydration, during which the water in the mix reacts chemically with the cement to form a strong and durable material. This process can take several weeks to complete, and it is important to allow sufficient time for the concrete to reach its maximum strength before testing.
By waiting 28 days before testing, the concrete is given sufficient time to develop its full strength potential. Testing the strength of the concrete too early may result in inaccurate results, as the concrete may not have fully hardened or cured yet. Conversely, waiting too long to test the concrete may also result in inaccurate results, as the concrete may have already started to deteriorate or weaken over time.
Overall, the decision to test concrete strength after 28 days is based on the need to obtain accurate and reliable results, while also ensuring that the concrete is strong enough to meet the requirements of the intended application. By following this standard protocol for strength testing, builders and engineers can have confidence in the quality and durability of their concrete structures.
Why do we test concrete compressive strength after 28 days?
Concrete is a material that gains strength over time after it has been cast. The duration required for concrete to attain its full strength is still unknown. However, it is known that the rate at which concrete gains compressive strength is highest during the first 28 days after casting. After this period, the rate of strength gain begins to slow down.
In order to gain a better understanding of how the compressive strength of concrete changes over time, we can look at the table provided. The table shows the compressive strength that concrete attains after 1, 3, 7, 14, and 28 days, depending on the grade of the concrete used. By analyzing this data, we can see that the strength gain of concrete is influenced by the grade of the concrete used.
| Age | Strength per cent |
| 1 day | 16% |
| 3 days | 40% |
| 7 days | 65% |
| 14 days | 90% |
| 28 days | 99% |
Based on the data in the table, it is evident that concrete experiences significant strength gains during the initial days after it is cast. For instance, it gains 16 percent strength in just one day, 40 percent in three days, and 65 percent in seven days. This rapid increase in strength continues, with concrete reaching 90 percent of its ultimate strength within 14 days. Even after 28 days, when concrete has achieved 99 percent of its strength, it continues to gain strength, albeit at a much slower rate compared to the initial 28-day period. Therefore, it is clear that while concrete gains strength rapidly in the early days after casting, the rate of gain decreases after 28 days.
After the initial 14 days of casting, the rate of gain of strength for concrete decreases significantly, with only a 9% increase in strength over the next 14 days. It is assumed that concrete may reach its final strength after 1 year, although there is no clear indication of when this occurs, and it could potentially take up to 2 years. As a result, the concrete’s strength at 28 days, which is approximately 99% of its final strength, is often relied upon as the base for design and evaluation purposes. However, there are also rapid methods of testing concrete compressive strength that provide a relationship between rapid test results and the 28-day strength. These rapid tests are useful in situations where construction time is limited and the strength of structural members needs to be determined quickly for further construction work.