{QUESTION} We have been asked by our customer to allow them to add fibers at the point of delivery. What precautions should we take? What happens if the slump or air or both changes, or the strengths are low?

{ANSWER} It is becoming more common for some ready-mix producers to sell their concrete additives to the contractor, in the hope that the margins will be better and that the “sales pitch” can be project-specific. Many admixtures and additives, like fibers, can be used to make good concrete. The issues arise when the concrete doesn’t perform—the question asked quickly will be “whose concrete is it?” Everyone will be clear that it is your concrete, but are you responsible?

Rather than let the courts decide or argue it out in the trailer onsite, you should take several precautions before making any concrete. The knee-jerk reaction is to say that once the material is added it’s not your problem, but this confrontational style can lead to lost jobs. Taking a collaborative approach is usually better.

Adding materials

Start with a trial batch under the site conditions. Make sure there is transit time to reflect what is anticipated for the project. Add the materials as they will be added onsite, including the bags. Some paper bags are pulpable, but take care where they are stapled, as they may not break up. Make sure the mixer is turned at the mixing speed as recommended by the fiber manufacturer.

At the trial placement, run slump and air before and after the addition, and cast a set of cylinders each time. Compare the seven-day strengths before and after. More than a 15% difference in strength, a 1-inch change in slump, or 1% change in air would be cause for concern. If there are significant changes in the properties, then there is a potential problem. In that case, you should take steps to ensure that the responsibility for concrete manufacturing is clear.

Make sure to have the fiber manufacturer at the trial placement. They should be prepared to comment if there are differences as noted above. You should also make sure that your contracts are clear that for onsite additions the fiber manufacturer and the contractor are responsible for ensuring that the concrete stays in spec and makes the required strength.

Purchasing fibers

One approach is to require the fiber manufacturer to be present at all of the pours. Another is to arrange to purchase the fibers directly, as that makes any potential claim easier to address. Having the contractor present allows the fiber manufacturer’s “customer” to ask questions as well. Although many fibers have good properties, few are unique. It is likely that your regular supplier of admixtures can also provide you with an equivalent fiber.

These comments are also applicable to hardeners or other materials that might be added to the concrete at the site. Make it clear who is responsible before the project goes forward.

This article was contributed by Kevin MacDonald, of Beton Consulting Engineers. E-mail kmac.donald@betonconsultingeng.com.