As per Technical Code Explanation ACI 318-11 5.6 - Evaluation and acceptance of concrete 5.6.3.3 - Strength level of an individual class of concrete shall be considered satisfactory if both of the following requirements are met: (a) Every arithmetic average of any three consecutive strength tests (see 5.6.2.4) equals or exceeds fc ′ ; (b) No strength test (see 5.6.2.4) falls below fc′ by more than 500 psi when fc′ is 5000 psi or less; or by more than 0.10fc′ when fc′ is more than 5000 psi. 5.6.3.4 - If either of the requirements of 5.6.3.3 is not met, steps shall be taken to increase the average of subsequent strength test results. Requirements of 5.6.5 shall be observed if requirement of 5.6.3.3(b) is not met. 5.6.5 - Investigation of low-strength test results 5.6.5.1 - If any strength test (see 5.6.2.4) of laboratory-cured cylinders falls below fc′ by more than the values given in 5.6.3.3(b) or if tests of field-cured cylinders indicate deficiencies in protection and curing (see 5.6.4.4), steps shall be taken to ensure that load[1]carrying capacity of the structure is not jeopardized. 5.6.5.2 - If the likelihood of low-strength concrete is confirmed and calculations indicate that load-carrying capacity is significantly reduced, tests of cores drilled from the area in question in accordance with ASTM C42 shall be permitted. In such cases, three cores shall be taken for each strength test that falls below the values given in 5.6.3.3(b). 5.6.5.3 - Cores shall be obtained, moisture condi[1]tioned by storage in watertight bags or containers, transported to the laboratory, and tested in accor[1]dance with ASTM C42. Cores shall be tested no earlier than 48 hours and not later than 7 days after coring unless approved by the licensed design profes[1]sional. The specifier of tests referenced in ASTM C42 shall be the licensed design professional. 5.6.5.4 - Concrete in an area represented by core tests shall be considered structurally adequate if the average of three cores is equal to at least 85 percent of fc′ and if no single core is less than 75 percent of fc′. Additional testing of cores extracted from locations represented by erratic core strength results shall be permitted. 5.6.5.5 - If criteria of 5.6.5.4 are not met and if the structural adequacy remains in doubt, the responsible authority shall be permitted to order a strength evaluation in accordance with Chapter 20 for the questionable portion of the structure or take other appropriate action. GUIDE TO EVALUATION OF STRENGTH TEST RESULTS OF CONCRETE (ACI 214R-11) 5.3.1 Criterion No. 1-The engineer may specify a maximum percentage of individual, random strength test results permitted to fall below the fc ′. This is the form of many international specifications, which tend to use a reliability factor of 1.65, implying acceptance of only a 5% failure rate. This criterion is no longer used in ACI 318, but may occur in situations where the average strength is a fundamental part of the design methodology, such as in some pavement specifica[1]tions. A typical requirement permits no more than 10% strength tests to fall below fc ′. Specified strength in these situ[1]ations generally ranges from 3000 to 5000 psi (21 to 35 MPa) 5.3.2 Criterion No. 2-The engineer can specify a probability that an average of n consecutive strength tests will fall below fc ′. For example, ACI 318, Section 5.6.3.3, stipulates that the average of any three consecutive strength test results should equal or exceed fc ′. The fcr′ should be established where failure to meet fc ′ is anticipated at no more than 1 in 100 times (0.01). 5.3.3 Criteria No. 3 and 4-The engineer may specify a certain probability that a random individual strength test result will be no more than a certain amount below fc ′. For example, Criterion No. 3 is used in ACI 318 by stipulating that no individual strength test result falls below fc ′ by more than 500 psi (3.5 MPa). An alternative Criterion No. 4 is appropriate for concrete with fc ′ > 5000 psi (35 MPa), requires that no individual strength test result falls below 90% of fc ′ . These two criteria are equivalent at 5000 psi (35 MPa). The minimum value of fcr′ is established so that nonconformance of an individual, random test is anticipated no more often than 1 in 100 times in either case. ASTM C 94-06, Chapter 18 - Strength 18.4.1 The average of any three consecutive strength tests shall be equal to, or greater than, the specified strength, f ‘c, and 18.4.2 When the specified strength is 5000 psi [35 MPa] or less, no individual strength test (average of two cylinder tests) shall be more than 500 psi [3.5 MPa] below the specified strength, f ‘c. NOTE 19-Due to variations in materials, operations, and testing, the average strength necessary to meet these requirements will be substantially higher than the specified strength. The amount higher depends upon the standard deviation of the test results and the accuracy with which that value can be estimated from prior data as explained in ACI 318 and ACI 301. Pertinent data are given in Appendix X1. 18.4.3 When the specified strength is greater than 5000 psi [35 MPa], no individual strength test (average of two cylinder tests) shall be less than 0.90 f ‘c. Designing Concrete Structures: Acceptance of Concrete Compressive Strength Test Results According to ACI 318M-19 ACI 318M, 26.12.3.1(b) Note: Test 9 has one cylinder test result below 24 MPa (22.1 MPa). The average of the two cylinders tested is 23.4 MPa (the strength test result), which is above 20.5 MPa, so the concrete is acceptable. Test 11 has a cylinder test result of 20.5 MPa. The strength test result is 23.0 MPa, so the concrete is acceptable. ACI 318M, R26.12.3.1(b) Steps to be taken because the arithmetic averages were low (arithmetic average of Tests 13, 14, and 15) and because the strength in the Test 14 strength test result was below fc′ by more than 3.5 MPa. 1. Increase Average of Strength Test Results Steps to be taken will be dependent on circumstances and could include one or more of the following: · Increase in cementitious materials content · Reduction in, or better control of, water content · Use of a water‐reducing admixture to improve the dispersion of cementitious materials · Other changes in mixture proportions · Reduction in delivery time · Closer control of air content · Improvement in the quality of testing, including strict compliance with ASTM C172, ASTM C31, and ASTM C39. Note: Changes in operating procedures or small changes in cementitious materials content or water content should not require a formal resubmission of mixture proportions; however, changes in sources of cement, aggregates, or admixtures need to be accompanied by evidence that the average strength level will be improved. DPWH BLUE BOOK -Series of 2016 3.20 Acceptance of Concrete No acceptance and final payment shall be made for the completed concrete pavement unless core test for thickness determination is conducted, except for Barangay Roads where the Implementing Office is allowed to waive such test, and for National Primary Roads where the concrete pavement shall also meet the International Road Roughness (IR!) criteria at the time of completion. The concrete pavement shall only be considered accepted if it meets the specified IRI value at the time of completion. The strength level of the concrete will be considered satisfactory if the averages of all sets of three (3) consecutive strength test results equal or exceed the specified strength, fe' and no individual strength test result is deficient by more than 15% of the specified strength, fe'.A set shall consist of a minimum of three (3) concrete beam specimens. Concrete deemed to be not acceptable using the above criteria may be rejected unless the Contractor can provide evidence, by means of core tests, that the quality of concrete represented by failed test results is acceptable in place. At least three (3) representative cores shall be taken from each member or area of concrete in place that is considered deficient. The location of cores shall be determined by the Engineer so that there will be at least impairment of strength of the structure. The obtaining and testing of drilled cores shall be in accordance with MSHTO T 24, Standard Method of Test for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete. Concrete in the area represented by the cores will be considered adequate if the average strength of the cores is equal to at least 85% of, and if no single core is less than 75% of, the specified strength, f’c. If the strength of control specimens does not meet the requirements of this Subsection, and it is not feasible or not advisable to obtain cores from the structure due to structural considerations, payment of the concrete will be made at an adjusted price due to strength deficiency of concrete specimens as specified hereunder: