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.”