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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.

According to ACI 318-19, strength tests specimens must be made for each concrete mixture and must be done at least once a day, at least once for each 110 m³ of concrete, and at least once for each 460 m² of slabs or walls. If the concrete thickness is less than 240 mm, you go by the area, and if it is greater than 240 mm, you go by the volume.

You must conduct a minimum of five strength tests for each concrete mixture. They must either be from five randomly selected batches and, if you have less than five batches, you need to get a test specimen from each batch.

ACI 318-19 also states that a strength test is not required if the total quantity is less than 38 m³, provided there is evidence of satisfactory strength from previous test(s), and is approved by the building official.

According to ACI 318-19 and ACI 301M-16, each strength test for concrete with a specified strength of 35 MPa or less has to be equal to or greater than the specified strength less than 3.5 MPa. If it is for concrete with a specified strength greater than 35 MPa, it has to be or equal to or greater than 90% of that specified strength. A second requirement looks at the average of three consecutive strength tests, and those have to be equal to or greater than the specified oppressive strength. Both requirements must be satisfied.

What are field-cured cylinders needed for?

The caveat of ACI 318-19 is that a strength test is only applicable when a building is under construction. If any strength test of standard-cured cylinders falls below fc’ by more than the limit allowed for acceptance, or if tests of field-cured cylinders indicate deficiencies in protection and curing, steps shall be taken to ensure that the structural adequacy of the structure is not jeopardised. These steps include looking at structural analysis, field-cure cylinders, and the other option is conducting in-place tests (estimating in-place concrete strength) based on ACI 228.1R.

How to test cores and interpret results?

However, if the above is inconclusive, you conduct core testing. Core testing must be in compliance with ASTM C42 and cores have to be drilled from the area in question. The minimum number of cores is three for each failed strength test. The acceptance criteria are that each core strength has to be equal to or greater than 75% of the specified strength, and the average of those three cores has to be equal to or great than 85% of the specified strength. More details can be found in ACI 214.4R-10 Guide for Obtaining Cores and Interpreting Compressive Strength Results.

Strength is still low – now what?

All the following tests have failed: Standard-cured cylinders, field-cured cylinders, in-place strength evaluation and core testing. Structural adequacy is also in doubt. In this case, ACI 318-19 is very explicit in highlighting that “…responsible authority shall be permitted to order a strength evaluation accordance with Chapter 27 for the questionable portion of the structure.”

Who pays for testing?

ACI 301-16 -1.6.3 and 1.6.4 specify that the owner pays for a concrete strength test that is part of the quality assurance. However, the contractor pays for additional testing of materials or concrete because of failure to meet specification requirements. When it comes to testing to verify the strength of concrete in a structure, the onus is on the contractor to pay but only if subsequent testing confirms concrete does not meet acceptance criteria. However, as the specifications on every job are different, this is not always binding. In case of conflict, check the specific language in your project specifications.

According to the definition from ACI CT-18, the difference between specified strength and required average strength is that the required average strength is the average strength of concrete used in mixture proportioning to ensure a high likelihood that the concrete will meet specified strength acceptance criteria. The ACI 214R states that “the required average strength should be established where failure to meet the specified strength is anticipated at no more than 1 in 100 times (or 1%).”

In instances where strength test data is not available, you should refer to table 4.2.3.1 in ACI 301-16. However, when you have strength data available, you should refer to the formula in table 4.2.3.3(a) 1.