Strength testing is a paramount concern to the concrete industry.
Strength testing is a paramount concern to the concrete industry.

We turned to Daniel Kokonowksi, who heads research and design for Building Works Inc. of Brookline, Mass., a consulting firm that helps companies introduce new construction products, to address the issue of concrete strength testing.

Concrete strength testing is a much larger problem for ready-mix producers than many in the industry realize. Findings of inadequate strength—many of which prove to be false—cost the producer lots of money and aggravation. That comes through in the angry words of QC managers.

But the problem goes beyond money and heartburn. Strength is concrete's most important performance specification. If it cannot be adequately tested, it is difficult to see how the industry can move from prescriptive to performance-based specifications.

In the December 2006 issue (“What's Worrying Your QC Manager,” page 45), we asked 47 QC managers what their biggest challenges are in mix design and operations. Most listed strength as one of their greatest and most persistent problems.

Additional interviews with managers at prominent testing companies revealed other sides of the issue, suggesting possible approaches to solving the problem.

Test methods

Many QC managers complained that technicians failed to follow specified test methods. They often pointed to inadequate curing of the specimens.

As Jim English from Grand Rapids Gravel Co. in Michigan stated, “Despite their certification training, many testing companies don't put cylinders in the right environment. Samples that are not cured properly will have poor results when they are tested. When we get a report that our concrete failed testing we now have to core, it ends up costing us money. Ninety nine times out of 100, our product was fine.”

Another common complaint was that the testing companies don't take the proper time or care to sample the concrete correctly. “Testing companies are very busy, and thus interested in speed,” said Frank Nickolaus of Titan America in Florida. “They may not follow specified procedures and end up sampling incorrectly. They are supposed to sample the concrete every 50 yards, but they end up taking samples from one truck, and claiming that they took them from three different ones. Whoever is paying for the testing is not getting their money's worth.”

But some testing managers considered these complaints unfair. When asked about improper curing methods, Nathaniel Hayes of Jeff Zell Consultants in Pennsylvania replied, “This argument is not always fair. In most cases, it's the contractor's responsibility to provide a curing box. Sometimes when we show up to a site, there isn't one. Samples still have to be taken, so all we can do is inform the contractor of the risk involved in not curing properly. Once we do this, it's on the contractor's shoulders. If the cylinders test low for this reason, it's really not our fault.”

Technician qualifications

Several QC managers suggested that the root of the problem was that some testing companies send uncertified testers to the site to collect samples. “The owners hire the labs and more often than not, prefer the lowest bidder over the most qualified lab. Many times these labs send people to the site who aren't even qualified, let alone certified,” said Milan Lipensky of Pennsy Supply in Pennsylvania. “They don't follow proper testing and curing methods because they don't know any better.”

Lab managers argued that they make serious efforts to ensure that all of their employees were certified. However, they conceded that not everyone is as diligent. “We know that there are some companies that don't use certified or licensed people in the field,” said Rich Hamilton of Advance Testing Company in New York. “We make sure that all of the testers in our company are ACI-certified and a level-one field technician. However, I'm not going to deny that using uncertified technicians does sometimes happen in the industry.”

Limited reporting of results

Getting the testing labs to send reports back to the ready-mix producers seems to be a whole other issue. More often, QC managers claimed that they only received reports on their product when the testers measured a failure. Even then, they would sometimes only hear about the negative finding and not get the numerical results.

“Though we always request the entire report, we usually only get the bad results, and even then it's not until the 28-day tests have failed,” explained John Webb of American Concrete Products in Iowa. “This is very unfortunate for us. We lose the chance to correct problems at the seven-day breaks. If we had information, we could catch problems early and fix them, instead of waiting a month to find out the product failed.”

But the lab managers see a different side. “The ASTM standards require that we send the test reports to our client,” said Dennis Ream of Bowser Morner in Ohio. “This means all of them, not just failures. Ready mix producers seem to overlook this. Often they are not our client. The only time we can send these reports to them is if the person who hired us tells us to do so. Sometimes the client is unaware that the producers even want a copy. Occasionally, we even get clients that don't want the producers to get it for one reason or another.”

Possible solutions

There are many approaches for dealing with the problems. Some QC managers took an aggressive approach to dealing with testing that they felt was performed incorrectly and provided incorrect results.

Naturally, most companies challenged such results. They would core samples from the walls and have them tested. If the samples were fine, then some producers would back-charge the owner for the extra cost. Unfortunately, this approach can also hurt the relationship between the producer and the customer. So other companies opted for more diplomatic solutions.

One preventive measure is to have people from the ready-mix company monitor the work. “Sometimes we have QC people onsite at the beginning of jobs to document the testing,” said David Wescott of Cemex in Florida. “If we think the testing was done improperly, we will alert the contractor. If the testing is really bad, we will request a meeting with the contractor and the testing company.”

Testing companies also seem open to this solution. Some technicians produce a checklist that they give to QC people onsite, showing the testing procedure according to ASTM standards. Each box can then be checked as this part of the process is done properly. When they are done, the list can then be mailed back to the testing company to inform them of proper or improper methods.

Education and communication are other approaches. “We started a free training program for the testers,” said Lipensky. “We feed them, and show videos and PowerPoint presentations. We not only try to teach them proper methods, but also just try to talk to them and make them understand where we're coming from.”