A recent conversation between the publisher of LC/DBM and Cameron Morgan, of Pacific Formliner, sent us off on a search for some hard facts on concrete. Morgan, who attended a concrete trouble-shooting seminar presented by the American Concrete Institute (ACI) in the late '90s, told LC/DBM that he remembered hearing that 38 out of 40 batches of concrete selected for testing had improper batch ratios.
But upon contacting ACI, we were told the organization does not compile data on concrete batch inconsistencies. So LC/DBM did some digging on our own about concrete testing.
Concrete can be batched and mixed on site by the build team, in transit to the work site, or at a central batching and mixing plant. Centrally mixed concrete arrives at the job site ready to pour. It is typically higher in quality and consistency according to Morgan. For transit-mix concrete, all necessary dry ingredients including water are added to the drum of a mixer-truck, which is set to turn at a specified speed until the load is delivered.
In the United States, a licensed civil engineer, architect or landscape architect specifies the concrete to be used on public, private and commercial projects. There are a variety of ways to test concrete; here are a couple of the most common.
"One of the first tests I'd recommend a builder perform on a fresh load of concrete is a slump test," said Morgan. "It's the best way to get a sense of the loads' workability, or water to cement ratio."
This test measures a batch's cement to water ratio. It is performed by pouring wet concrete into a cylindrical mold coated with oil. After the mold has been filled, it is placed concrete side down and slowly lifted vertically. Slump is measured as the difference in the height of the mold and the height of the specimen.
Another method to test concrete is by pouring numerous cylinders of a batch of concrete and measuring the force needed to break the samples at prescribed intervals as they harden. This measures the compressive strength of a given batch of concrete. This test is done in a lab as it takes longer to conduct.
Although a contractor may follow all of the specifications handed down by the designers, it is the contractor's or builder's responsibility to ensure the proper concrete has been ordered, and that it is within set parameters. When asked how he makes sure his concrete is up to standard, Morgan didn't hesitate to share.
"When I am on the job I follow some simple rules," he said. "I keep a pocket thermometer handy and reject any load 90 degrees or hotter. These hot loads set in less than two hours and limit the time available to get the job done."