Excess strength versus sample CuSum chart.
Excess strength versus sample CuSum chart.

Question: Why are ASTM standards important and why should concrete producers pay attention and follow them?

Answer: ASTM standards are designed to allow a “meeting of the minds” between the purchaser and the vendor. Most project specifications will cite many material standards for the materials that go into making concrete — ASTM C150 Cement, C618 Fly ash, C989 Slag, C494 Admixtures, C33 for Aggregate, and so on.

The problems occur when there is a wide definition of what meets the standard, or when complying materials are put together and the results are less than satisfactory. When there is a genuine incompatibility that can be detected in trial batching is one example.

But what happens when the change to a material in production still falls within the specification, but there are production issues that lead to changes in water demand or strength gain? Last I wrote what that means in terms of aggregate, which is easy to monitor. Setting limits on gradation are easy to check.

When this change happens with a different material or on a larger scale at the cement plant or at the fly ash generator, the problem can be more widespread and more difficult to deal with. Many of these specifications rely on physical tests that are not that relevant to manufacturing concrete. The cube strength of cement, for example, does not translate directly to the strength of concrete. And if the change in slag or fly ash or cement affects the strength gain, the producer might not be able to address the change until a lot of concrete has been made.

Tracking air, slump, and strength

One way to see if this is occurring is to use CuSum charts to detect a significant change in concrete after as little as one test. Tracking air, slump, and strength can allow the producer to detect changes in the incoming materials by setting the limits in the cumulative sum analysis, or CuSum, chart to reflect the normal plant variation. Ken Day has a terrific website, www.kenday.id.au, where this type of control chart is detailed. A change in water demand or required admixture dose is another indicator that can be identified before the seven-day strengths.

Another method is to use semi-adiabatic calorimetry and 6 x 12 test cylinders. If the heat evolved at 48 hours differs by more than 10% of the control determined at trial batching then investigate the change.

We recently adopted this method to check on a fly ash supply that varies wildly in strength gain and temperature rise in large elements. The supplier will be informed of non-compliance if the heat evolved is out of control.

A foam index test is another good, rapid fly ash test that can detect changes in composition quickly. It involves using a known quantity of air entraining agent, fly ash, and water to see what’s required to produce a stable foam. If this changes, then there has been a change in the fly ash composition.

Similarly, the mini-slump cone can detect changes in the cement by monitoring the mini-slump loss with time. The grinding targets are set for strength, and some cement is ground finer, and sometimes grinding aids or other ingredients are altered to meet the economic needs of the plants. These changes are not enough to deviate from the standard, but they could change your concrete and place it out of control.

Place these items into purchase agreements. Working with material suppliers reduce problems that can arise when a substantial change affects the concrete. After a problem, the litany will be “it met the standard.” It’s more productive to have limits set before a problem.