Depending on the applied code, the test sample may be cylinder [15 cm x 30 cm is common] or cube [15 cm x 15cm x 15 cm is the most common]. After 28 days, different tests are usually performed to determine the strength gain of the concrete. The pressure in terms of stress is measured to find the compression stress that the concrete can bear.Compressive strength is the parameter that represents the concrete in the structural design. Based on the cube test results, the below graph was plotted. This test is considered as the most popular test performed on concrete in construction as it gives a general idea on all the characteristics of concrete. Concrete is bought and sold by the cubic metre volume of the freshly mixed ingredients. Therefore, the above method could be used for all types of elements.Both the method is common in the construction. In some cases additional reserve cylinders are made and can be tested if one cylinder of a set broke at a lower strength. For example, as per ASTM C39, a standard test method is given to get the compressive strength of concrete cylinders.1- Concrete is poured in the mould and compacted properly to reduce the number of voids.2- After 24 hours, moulds are removed and test specimens are then placed in water for curing.3- After the specified curing period [3, 7, 28, 56, or 91 days], specimens are tested by the compression testing machine.4- Load is applied gradually until specimen failure.5- Divide failure load by cross-sectional area of the specimen to get the compressive strength of concrete.Compression test machine, moulds, mixer, trowel, tamping rod (steel bar 16 mm diameter and 60 cm long)Samples are taken from the concrete patches used in the site or prepared with the same mixture as used in the field.Minimum three specimens should be tested at each selected age. Apply the load increasingly at a rate of 140 kg/cm² per minute until the cube collapse.7. They represent 1, 2 and 4 weeks respectively. 1. These facts have shown the importance of keeping the concrete moist to achieve the required strength.

A good concrete should not show less than the minimum Compressive strength at respective days. The characteristic strength is defined as the strength of the concrete below which not more than 5% … Finally, add water and mix until the concrete appears to be homogeneous and of the required consistency.1- Clean the moulds properly and finish it with oil so that concrete will not stick into the mould and make latter cleaning difficult.2- Put concrete in the moulds layer by layer. Keep the specimens in a place, free from vibration, in moist air and at a temperature of 27°C ±2° C for 24 hours.2. While traditional concrete normally has a compressive strength ranging anywhere from 2,500 to 5,000 psi, UHPC can have a compressive strength of up to 10 times that of traditional concrete. The compressive strength of the concrete and concrete testing is must known information in structural design. A 3 or 7-day test may help detect potential problems with concrete quality or testing procedures at the lab but is not a basis for rejecting concrete, with a requirement for 28-day or other age strength. Each layer is compacted with a tamping rod [35 strokes] “ The compressive strength of concrete is given in terms of the characteristic compressive strength of 150 mm size cubes tested at 28 days (fck). Note down the maximum load applied to the specimen and any other unusual activities at the time of failure.The surface area of specimen: = 150 x 150 = 22500mm² = 225cm²Compressive Strength of Concrete  = 450000 /22500 = 20N/mm² = 203Kg/cm²The same calculation is done for the specimen at different ages as stated belowAverage Compressive strength at 7 days = _____N/mm²The average Compressive strength at 28 days = _____N/mm²The following table gives the strength of concrete at different ages as a percentage of the strength at 28 days in order to accept the test result. The American Concrete Institute recognizes 28 days or “the test age designated for determination of the specified compressive strength.”5 Therefore, when a specifier calls for 5,000 psi concrete at 28 days, this tells the concrete producer to select a mix design that will attain a minimum of 5,000 psi 28 days after manufacture. If the cube or the cylinder is failed unsatisfactory, the test specimen is disregarded.The following criteria given in the BS EN 12309-3 can be used to identify the satisfactory/unsatisfactory failure modes.It is very important to examine the failure method after testing and before considering the strength of the specimen as a result.The following information is tabulated as per the information of the BS 5328 Part 4.The above conformity criteria can be further explained as follows.The following guideline is given for conformity criteria.Structural loads, structural analysis and structural design are simply explained with the worked example for easiness of understanding. However, during the construction concrete not cured mostly the dates specified above. Element designs with notes and discussions have added to get comprehensive knowledge. Hence concrete is safe to use.