According to the ACI CT-18 ACI Concrete Terminology, concrete compressive strength is the specified resistance of a concrete specimen to axial compressive loading use in design calculations by structural engineers and the basis for acceptance of concrete used in work. It has several purposes – it is needed for design, evaluation purposes and also for acceptance purposes.
According to ACI 318-19, the evaluation of hardened concrete shall be based on strength tests. The definition of a strength test is the “average of the compressive strengths of at least two 150 x 300 mm cylinders or at least three 100 x 200 mm cylinders”. Test cylinders must be made from the same batch of concrete, sampled in accordance with ASTM C172 at the point of delivery, handled and standard-cured in accordance with ASTM C31, and tested in accordance with ASTM C39 at 28 days or at a test age designated for specified compressive strength.
When you do a test on cylinders that are field cured or on cylinders from laboratory trial batches, in the language of ACI 318-19 or ACI 301M-16, those are not considered a strength test.
How do you prepare samples for testing?
When considering how to sample concrete, a sample of concrete must be taken in accordance with ASTM C172 at the point of delivery. When you go through the standard, it tells you that a composite sample must be made and there are some requirements regarding minimum volume, number of portions and portions of the batch. There are also different requirements depending on the type of mixer.
All specimens for testing need to be handled and standard-cured in accordance with ASTM C31, which highlights the testing requirements, sampling concrete and molding specimens.
What factors affect strength?
Smaller cylinder sizes yield higher strengths; therefore, you can typically expect 4% higher strength in 100 x 200 mm than in 150 x 300 mm if they are made from the same batch of concrete. Either moist curing or lower temperatures will result in higher long-term strength results.
Who can conduct the tests?
ACI 318-19 mandates that acceptance testing can be done by a testing agency that is compliant with ASTM C1077. Meanwhile, the specimen preparation for the strength test has to be made by a certified field-testing technician and laboratory testing by certified laboratory technicians. ACI can provide these certifications.