The compressive strength of concrete is determined by the following steps:
- Testing the concrete in terms of compliance with characteristic strength (the cured samples in the laboratory) according to ASTM C192, C39 standard
- Testing the mixing uniformity according to ASTM C94 standard (for centrally-mixed concrete)
- Testing the concrete curing (providing cured samples on the site) according to ASTM C31, C39
- Testing the concrete strength at different times under the conditions of on-site curing (control sample) for demolding and curing
- Testing the strength of the concrete cores, the hardened concretes of the construction according to ASTM C42 standard
- Sampling the freshly mixed concrete according to ASTM C172 and ISIRI 489 standards
- Providing and curing the concrete samples in the laboratory according to ASTM C192 and ISIRI 581 standards
Important Items for Sampling the Freshly Mixed Concrete
- There should not be more than 15 minutes interval between the first and last samples.
- The taken samples should be mixed well again with a shovel or spade to make them uniform in the shortest possible time.
- Slump and air content tests of concrete, should be started in less than 5 minutes after the last section of concrete is made.
- Molding of the test specimens should be started within 15 minutes of preparation of the mixed sample (according to the mold preparation instructions).
- The specimen should be protected from wind, the Sun and the other flash (rapid evaporation) factors as well as proximity to damaging substances and harmful agents.
- The minimum required amount of sample for strength tests is 25 liters (at least 5 times of the volume of the specimens). Smaller specimens allowed to be used in consistency and air content tests.
- Sampling from the fixed concrete mixers is made by mixing 2 parts or more of the samples taken at regular intervals when emptying the middle parts of the concrete mix. Samples should never be taken from the first and last parts of the mixture. The sample should be taken from the whole surface of the mixed stream and no separation should occur in the stream.
- Sampling from the truck concrete mixers is made by mixing 2 parts or more of the samples taken at regular intervals when emptying the middle parts of the concrete mix. Samples should never be taken from the first and last parts of the mixture. They should not also be taken before the complete mixing of water or additive. The concrete consistency test can be done after draining 3 cubic meters of concrete from the mixer truck.