QUESTION: We recently worked with a new customer, and the concrete contractor's owner arrived on the jobsite just as the pour was about to start. He unloaded several boxes containing pre-weighed packages of synthetic fibers. He told the first ready-mix driver to add a package into the drum. When the driver refused, the contractor told our quality control technician that they always add fibers to the truck just prior to placement. The owner even offered to provide a laborer to help our driver add the fibers.
When we refused, the owner was upset, cancelled his order, and called another producer who agreed to add the fibers onsite. Are there any guidelines or standards that prohibit adding fibers onsite?
ANSWER: Your decision not to comply with the contractor's request seems prudent. From the safety standpoint, many ready-mixed concrete producers have adopted policies that prohibit non-drivers from climbing on the truck. These same producers often erect slump racks or other safe elevated platforms at the plants to facilitate loading prepackaged fibers.
There may also be some additional quality and contractual concerns. If the project references ACI in its specifications, jobsite addition of fibers may not be allowed without obtaining the project owner's written approval.
ACI 304, Guide for Measuring, Mixing, Transporting, and Placing Concrete, states, “The method and sequence of charging mixers is of great importance in determining whether the concrete will be properly mixed.” In regard to synthetic fibers, the document recommends that “prepackaged units should be readily accessible so they can be added directly to the mixer during the batching process.”
A producer's policy requiring fibers to be added at the plant helps in batching quality concrete. After adding fibers, the driver and quality control staff can determine whether the mix is properly blended and assure it conforms to the coefficient of uniformity. The producer can also maintain tighter slump.
Adding fibers on the jobsite without proper documentation may conflict with the procedures outlined in ASTM C-94, Standard Specification for Ready-Mixed Concrete. According to this specification, the manufacturer of the concrete is to furnish the purchaser with a delivery ticket indicating the type, brand, and amount of fiber reinforcement added to the material for each batch of concrete, before unloading at the site.
When a contractor adds material to the mix, the contractor becomes the manufacturer of the ready-mixed material and therefore becomes responsible for all aspects of the mix's hardened and plastic performance.
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