At the ACI spring convention, the Hot Topic session focused on acceptance testing of concrete. Someone pointed out that we’ve been having the same discussion for the last 40 years. That is, we’re still talking about how important testing is, how expensive bad testing ends up being, and how hard it is to get right. Ready-mix concrete suppliers depend on reliable testing; their product is rejected if it doesn’t pass the test. Yet how many times does the owner select the low bidder to do the testing? As the owner’s representative, the Engineer of Record has a lot to say about the testing. Here’s what engineers should know about acceptance testing.
Why does testing matter?
Acceptance testing by an independent third party is what determines whether the concrete delivered to the site meets the specification. It is not a measure of the in situ strength of the concrete. Usually the Engineer specifies the compressive strength at 28 days, but strengths at other ages may be significant, too. For example, if you have a lot of fly ash or slag cement in the concrete, it will take longer to gain strength. A 56- or even 90-day strength may make more sense. On the other hand, you may need an earlier-age strength for posttensioning. Or maybe you just want an early indication that everything is going well.
Make enough cylinders
One thing that engineers should know about acceptance testing is what constitutes a test. According to ACI 318,
A strength test shall be the average of the strengths of at least two 6 x 12 in. cylinders or at least three 4 x 8 in. cylinders made from the same sample of concrete and tested at 28 days or at test age designated for f’c.
It’s a common practice to break one cylinder at 7 days “for information”. But if you want a test, specify a test. One cylinder break is not a test, so the “information” you get from it is questionable.
Once the technician is on site, the cost of making a few additional cylinders is minor. So make sure you have enough cylinders to do a proper test at every age that matters to you. An extra cylinder or two is also good to have. If a cylinder is obviously defective, you can break one of the spares. And if something goes badly wrong on the project, you have an intact cylinder for the petrographer to examine.
Getting it right
ASTM C172, Standard Practice for Sampling Freshly Mixed Concrete, specifies how to obtain a representative sample of concrete from a truck. ASTM C31, Standard Practice for Making and Curing Concrete Test Specimens in the Field, specifies the particulars of making the cylinders and handling them in the field. ASTM C39, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens, specifies the laboratory procedures. As we’ve discussed previously, what engineers should know about acceptance testing is that just about any deviation from these standards will yield an artificially low test result.
In practice, it’s a lot easier to do things right in the laboratory than in the field. However, what happens in the field is at least as important. Things that commonly go wrong include improper curing—too hot, too cold, too dry—and mishandling of the specimens. While concrete eventually becomes very strong, at early ages it’s easy to damage. If it rolls around in the back of a truck on the way to the lab, it may never reach the specified strength.
Sometimes technicians stop the test once the load reaches the specified strength. That’s not a proper test. It tells us that the concrete met the strength requirement, but not how strong it was. For the concrete producer, it’s essential to know the actual strength. If they’ve overdesigned the concrete, they may want to back off on the cement content. That reduces the carbon footprint and reduces pressure on the cement supply.
Who should do what?
ACI PRC-132.1-22, “Responsibility for the Care of Test Specimens for Acceptance of Concrete“, outlines who should do what to ensure reliable acceptance testing.
The Engineer of Record should:
- Specify the responsibility for initial curing in the construction documents.
- Ensure that the testing lab complies with ASTM C1077.
- Require that lab reports include all the information listed in ASTM C31 and C39, as well as exactly where the concrete was placed.
The contractor should:
- Schedule a pre-pour meeting to discuss responsibilities for maintaining the test specimens in accordance with ASTM C31.
- Inform the lab of the placement schedule.
- Notify them at least 1 day in advance of placement.
- Provide a secure location with water and power for initial curing.
- If the contract requires, provide an enclosed facility for initial curing.
- Provide access to the testing lab to sample the concrete and make and retrieve specimens.
The testing lab should:
- Sample the concrete in accordance with ASTM C172 and cast, cure, transport, and test the specimens in accordance with ASTM C31 and C39.
- Store the specimens without moisture loss and within the temperature range ASTM C31 requires. If special equipment or facilities are necessary to meet these requirements, include the cost in the bid for testing services.
- Report the data in compliance with ASTM C31 and C39, including the temperature range of the initial curing environment [emphasis added].
- Transmit these reports in a timely manner to the Engineer of Record, the contractor, the concrete producer, and any others as agreed at the pre-pour meeting.
How to read the lab report
When you get the lab report, read it right away. Verify that the initial temperature range was within the limits specified in ASTM C31: between 60 and 80 ˚F, or between 68 and 78 ˚F for strengths of 6000 psi or higher. Cylinders may remain on site for up to 48 hours. Verify that they were transported to the laboratory within that time.
Next, look at the test results. ACI 301 has two requirements for concrete strength tests:
- 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.
Naturally you’ll want to make sure that the strength isn’t too low, and follow up quickly if it is. However, be aware that low test results may be due to improper testing, not necessarily defective concrete.
Also consider the variability among individual cylinder breaks. If the measurements are too consistent, the technicians may have stopped loading once they verified that the concrete met the strength. They need to break the cylinder to get the actual strength.
Takeaways: what engineers should know about acceptance testing
- In the end, bad testing costs a lot more than good testing: investigations, coring, and project delays cost far more than you could possibly save by choosing the low bidder.
- Make field curing of cylinders a pay item. Specify the requirements of ASTM C31 in the construction documents.
- In the pre-pour meeting, verify who is responsible for what.
- Be suspicious of too-consistent results. Did they really do the test? Did they break the cylinder or just load to the required strength?
- A single cylinder break is not a test. There’s no sense in breaking one cylinder “for information”.