CHALLENGING THE SYSTEM
Acceptance of Concrete
Test Results
Ward R. Malisch, PE, PhD, FACI is concrete construction specialist for the American Society of Concrete Contractors. Reach him at wmalisch@ascconline.org. Bruce A. Suprenant, PE, PhD, FACI, is technical director for the American Society of Concrete Contractors. Reach him at bsuprenant@ bsuprenant.com.
Why is it still a struggle?
C
onsider the following scenarios for a project on which the specified compressive strength (fc’) is 3,500 psi at 28 days. • A single seven-day-old cylinder breaks at 2,250 psi. The Architect says this is less than 70 percent of fc’. and the concrete strength is suspect. • A 28-day test result is 3,150 psi and the testing laboratory states in its report that this result does not meet project specifications. • The average of three consecutive 28-day strength tests is 3,400 psi and the Owner wants the concrete removed and replaced. These are all examples of incorrect interpretations of the acceptance criteria for concrete strength test results in accordance with ACI 318 “Building Code Requirements for Structural Concrete” and ACI 301 “Specifications for Structural Concrete.” Both of these documents define a strength test as the average strength of two 6x12-in. or three 4x8-in. cylinders tested at 28 days or at a test age designated for fc’. ACI Acceptance Criteria The acceptance criteria for concrete strength tests has been the same since the early 1970’s but for more than 40 years they have often been interpreted incorrectly. Fortunately, ACI committee E702 published “Designing Concrete Structures: Acceptance of Concrete Test Results” in March, 2007 to
provide a step-by-step example of concrete test-result evaluation and an explanation of the acceptance criteria. The document is a free download on the ACI web site. The data used in this article is based on that ACI document. ACI has two requirements for the acceptance of concrete test results as shown below: Strength level of a concrete mixture shall be acceptable if (1) and (2) are satisfied: 1. Every arithmetic average of any three consecutive strength tests equals or exceeds fc’. 2. No strength test falls below fc’ by more than 500 psi if fc’ is 5,000 psi or less; or by more than 0.10 fc’ if fc’ exceeds 5,000 psi Note that ACI does not require: • A minimum strength at seven days. • A minimum strength for an individual cylinder that is part of the test. • All strength test results to exceed fc’. ACI 318 accounts for concrete strengths less than fc’ by multiplying the calculated strength of a member by a strength reduction factor, which is always less than one. ACI 318 Commentary states that one of the purposes of the strength reduction factor is “to allow for the probability of understrength members due to variations in material strengths.” Thus ACI has already considered that the concrete may be less than fc’ and that is why an individual strength test result may be below fc’ by up to 500 psi or by not more than 0.10 fc’ when fc’ exceeds 5,000 psi.
It’s unfortunate that after 40 years of use, the provisions for acceptance of strength test results are still incorrectly interpreted. BEFORE CHECKING THE ACCEPTANCE CRITERIA Don’t reject concrete represented by strength test results until you confirm that the test results are valid. Use the checklist below to ensure the strength test results meet ACI testing requirements. Sampling frequency is adequate (once a day, once every 150 cubic yards, once for each 5,000 square feet of surface area for slabs or walls) [ACI 318:26.12.2.1] ✔Samples taken on a random basis (concrete not sampled due to appearance, convenience or other possibly biased criterion) [ACI 318:R26.12.2.1(a)] ✔ Each set of cylinders comes from a different batch of concrete [ACI 318:R26.12.2.1(a)] ✔ For each strength test, is the average of at least two 6x12 in. cylinders or three 4x8 in. cylinders [ACI 318:26.12.1.1(a)]
6 Concrete Contractor | August/September 2015 | www.forconstructionpros.com/concrete
CONC0815_06-9_Challenging.indd 6
8/19/15 8:06 AM