Coping with Ready-Mix Test Anxiety
Even small producers can benefit from performing basic concrete testing in-house.
The first step is to find someone who knows how to perform the tests. This can be an outside laboratory, but it’s a good idea to have at least one employee on staff who knows the appropriate ASTM International, department of transportation (DOT), and American Association of State Highway Transportation Officials (AASHTO) test procedures. Ideally, this person is certified by the American Concrete Institute or local DOT. Training and certification is typically available from ACI chapters or state ready-mixed associations for around $500 (fee varies depending on location).
Raw Materials Due Diligence
Not all testing must be performed in-house. Most material information can be obtained from your supplier. At a minimum, the supplier should provide the density or specific gravity of sand, stone, cement, fly ash, and admixtures. You’ll use this information to convert batch weights to volume to make certain a true cubic yard or cubic meter of concrete is being produced. (For more information on this procedure, read “Profiting from Quality Concrete”) If the mix design under yields, the concrete will have a higher strength but won’t fill forms as fast as the customer expects. If the mix design over yields, the customer won’t have to order as much concrete, but strength will be lower than expected.
Aggregate suppliers can also provide gradation information, which you’ll need when supplying projects with combined aggregate grading specs, like the big box retailers. You can then compile this information for use on projects that require submittals.
Testing the Concrete
At a bare minimum, your technician should be able to perform a slump test (ASTM C143) and make cylinders (ASTM C31). If you don’t own a compression machine, send cylinders to a lab for compression testing. If you’re making air-entrained concrete, the technician should be able to test for air content (ASTM C231 for normal-weight concrete or ASTM C173 for lightweight concrete). If possible, the technician should also be able to test for concrete unit weight (ASTM C138) because this test is required by ASTM C94, “Specifications for Ready-Mixed Concrete” and because unit weight can act as a double check for air content and added water. The equipment for these tests typically costs less than $1,000.
Based on my experience, verifying yield and confirming slump, air content, and strength saves the average producer 25 cents to $1.50 per cubic yard. You’ve spent $1,500 on certification and equipment, so full payback should take about a month. Even producers that only provide residential concrete, which is rarely tested, can see savings.
Admittedly, knowing how to perform a test isn’t enough to establish a quality control department. It’s also necessary to know what to test and why it should be tested. However, even the best technical expert can’t do his job without knowing the basics about the materials involved and the concrete performance. For more information on quality control, visit the National Ready Mixed Concrete Association’s Quality Initiatives page.
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