Concrete acceptance testing refers to the testing of concrete sampled directly from the ready mixed concrete truck. Its purpose is to verify the compressive strength of the material as delivered to the site. It does not address the adequacy of the in-place concrete.
The variability of concrete acceptance testing has been a point of contention in the industry for decades. Testing of any sort necessarily entails variability. One person testing the same thing in the same way 10 times will get 10 slightly different results. You can statistically analyze the repeatability of the test using these data. If 10 people each perform the test in their own labs, they’ll get somewhat greater variability, as they may be doing things slightly differently with different equipment. We characterize this variability statistically by the reproducibility of the test.
With most test methods, deviating from the standard procedures and conditions will cause the results to be artificially either high or low. However, almost any deviation from ASTM C31 or C39 will cause the results to be artificially low. ASTM C31, Standard Practice for Making and Curing Concrete Test Specimens in the Field, pertains to the sampling, fabrication, and handling of concrete specimens until they arrive in the lab. ASTM C39, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens, pertains to their subsequent handling and testing. Not surprisingly, it’s a lot harder to get everything right in the field. Any deviations from these standards increase the producer’s risk of rejection of good concrete.
NRMCA position statement
In 2024, The National Ready Mixed Concrete Association has issued a position statement, “Reliable Concrete Acceptance Testing for Improved Sustainability and Performance.” It addresses conformance with ASTM C31 on job sites.
ASTM C31 specifies storage of cylinders for up to 48 hours at the job site under conditions that prevent moisture loss and maintain temperatures between 60 and 80 ˚F. For concrete with fc’ of 6000 psi or greater, the temperature range is 68 to 78 ˚F. NRMCA’s position statement summarizes the results of several studies of the effect on the 28-day compressive strength of various deviations from these conditions. The greatest reduction in strength, 28%, occurred for cylinders cured outdoors in sunlight at temperatures reaching 93 ˚F. An average strength reduction due to poor initial curing would be closer to 20%.
How common are deviations from ASTM C31? Our colleague, Julie Buffenbarger, and her coauthors reported on a survey of 103 projects under the jurisdiction of the Pennsylvania Uniform Construction Code. They found that the specimens received proper initial curing on only 21 of these projects. That is, on four projects out of five, the test specimens got no curing or improper curing. Overall, the survey showed proper adherence to both ASTM C31 and C39 on only 15 of the 103 projects.
Investigating low strengths
ACI 301 has two requirements for concrete compressive strength:
- Every average of three consecutive tests equals or exceeds the specified strength, fc’.
- No test falls below fc’ by more than 500 psi if fc’ is 5000 psi or less, or by more than 0.10fc’ if fc’ exceeds 5000 psi.
If the concrete fails either of these criteria, the Engineer of Record must decide what to do. They may decide that the lower strength is adequate for the members these cylinders represent. Even so, it’s good practice to determine why the strengths were low and make appropriate procedural changes.
However, if the strengths are unacceptably low, the Engineer must investigate the strength of the in-place concrete. Non-destructive and minimally destructive methods are available, but it’s usually necessary to take some cores. We’ve discussed how to evaluate the strength of concrete cores in a previous blog.
It isn’t cheap to take and test cores, but the real cost to the project is the delay. Even if you mobilize a coring crew right away, the cores must be conditioned to equalize the moisture content. Cutting cores nearly always involves cooling the drill bit and saw with water. That leaves the outside wetter than the interior, and the resulting moisture gradient induces stresses in the concrete. Once the cores arrive in the lab, you should cut the ends right away and then store the cores in a plastic bag for 5 days. All told, it will be at least a week from the time you take the cores until you have the compressive strengths. Contracts often specify liquidated damages for late completion of the project, so you can calculate the cost of a delay for low strength results.
The importance of accurate concrete acceptance testing
[Coring] efforts are expensive, can delay a project, and can result in contentious relationships among project stakeholders. Adherence to the standards for acceptance testing of concrete will limit the occurrence of low strength test results to genuine situations where there could be a problem with the mixture. This minimizes additional costs for evaluation and delays in project scheduling.—NRMCA position statement, 2024
Naturally everyone wants to avoid costly delays. Concrete producers have no control over the testing, but they can adjust the mean strength of their concrete to ensure it passes. Usually they do this by adding cement to their concrete above what they’d need otherwise. For them, the extra cement is cheap at the price if it prevents even one low strength investigation.
However, there are consequences. Portland cement is the component of the concrete with the greatest carbon footprint. By necessitating excessive overdesign, poor testing practices increase the carbon footprint of the concrete.
The performance of the concrete may also be adversely affected. In mass concrete, the heat of hydration of the extra cement makes it that much more difficult to control the temperature and temperature gradients. That is, the concrete is more susceptible to thermal cracking. In concrete slabs, the extra cement will increase the shrinkage, making the slab more susceptible to cracking and curling. In concrete beams, the extra cement can increase creep—and therefore deflections or loss of prestress.
As we’ve discussed previously, poor concrete acceptance testing practices can also exacerbate cement shortages. When cement is in short supply, excessive overdesign wastes cement. Removal and replacement of concrete due to low strength test results waste even more cement.
Who’s responsible for concrete acceptance testing?
ACI PRC-132.1-22, Responsibility for the Care of Test Specimens for Acceptance of Concrete, describes who should do what to ensure accurate acceptance testing. As a TechNote rather than a standard or a code, it’s not mandatory and so can’t be incorporated into a specification. Instead it lays out best practices for engineers and contractors to adopt. The Engineer of Record, the contractor, and the testing lab all play their parts.
The Engineer of Record should address responsibility for initial curing of concrete cylinders in the contract documents. The testing lab should include the cost of initial curing in its bid for testing services. Depending on the project, the contractor may need to supply a secure location for the initial curing, electricity and water, and access to the site. All these must be spelled out in the contracts.
It’s essential to have a pre-construction meeting including all parties to ensure that everyone understands their responsibilities. NRMCA has a pre-construction checklist to help you make sure you’ve covered everything you need to. Several pages address concrete acceptance testing, including everything from the qualifications of the testing lab to what to do if the concrete doesn’t meet the strength requirements. Discussing these matters ahead of time can help you avoid a lot of problems. And if something does go wrong, you won’t waste a lot of time pointing fingers or figuring out what to do.