Concrete Cube 150mm x 150mm x 150mm

Why we test Concrete Compressive Strenth after 28 days?

Concrete gains strength with time after casting. The rate of gain of concrete compresessive strength in higher during the first 28 days of casting and then it slows down. 

Concrete is used mostly for structural purposes such as foundations, columns, beams and floors and therefore must be capable in taking the loads that will be applied to it, for which the compression test is performed with the cubes prepared on the time of casting.

Cubes are tested on 7th day and 28th day, but if we want to remove shuttering priory than the test is conducted on cubes to determine the strength and the safety of removal of formwork.

Cube moulds are usually 150mm x 150mm x 150mm or 100mm x100mm x100mm and can be made from steel or polyurethane.

When using a 150mm mould, each layer compacted is tampered using a certified compacting rod /bar (genereally 16mm diameter and 600mm long), 35 tamps per layer is required. Once the 3 layers have been tampered, tap the side of the mould with a hammer. Tampering helps in removing the air content which usually inside the concrete mix, resulting in making the mix compacted.

The strength of concrete increases with age. Table shows the strength of concrete at different ages in compression. 

Age	Strength in %
1 Day	16%
3 Day	40%
7 Day	60%
14 Day	90%
28 Day	99 %

 

From above table, 
We can see that concrete gain strength of 16% in 1 day, 40% in 3 days, 65% in 7 days, 90% in only 14 days and 99% in 28 days. Concrete gains its strength rapidly in initial days after casting i.e, 90% in only 14 days. From day 14 to day 28 concrete just gain a strength of 9%. So, we can say that the rate of strength decrease as the time increases. 
It assumed it may gain its final strength after 1 year.