For more than a decade, volumetric producers have been trying to earn respect in the engineering community by promoting the versatility of their equipment and quality products their machines can produce.
But when a specifier asked for an ACI document with which to reference these attributes in a bid, there was a disconnect. In the document that described the best industry practices for producing volumetric concrete, producers were forced to supply a report that was based on the 1970s techniques. As all producers know, the volumetric production industry has made great strides in incorporating technology to its equipment and procedures.
Fortunately, this dilemma is about to change. The “Guide for the Use of Volumetric-Measuring and Continuous-Mixing Concrete Equipment” (ACI 306.6R), published by ACI Committee 304, is in the final stages of re-approval, the first major re-write since its inception. This document was originally published in 1991 and re-approved in 1997 with little update.
The revised ACI Document 304.6R is an excellent guide for the use of volumetric-measuring, continuous mixing (VMCM) equipment. Many changes were incorporated to bring this document up to date. The flexibility of VMCM equipment will enable its use in additional applications as the industry continues to grow along with its acceptance.
The guide offers needed information to the specifier, and provides aid to users because it describes the proper operation and calibration of VMCM equipment. The guide covers all VMCM equipment, which is manufactured in two basic configurations. The mobile unit that is mounted on a truck or trailer is often referred to as a mobile mixer. The stationary (static) unit is designed to be mounted on a floor in a pre-cast plant.
As accepting and using VMCMs has increased, so have the applications for which this equipment is used. The revised guide includes 16 specific applications, four more than the original document.
One of the more significant changes reflected in the document is the establishment of the Volumetric Mixer Manufacturers Bureau (VMMB) under the auspices of the NRMCA. The bureau has published standard VMMB 100-01. This standard reflects the agreement by the volumetric mixer manufacturers who are members of the bureau to sell VMCM equipment that meets certain minimum industry requirements. A certification plate is attached to eligible equipment manufactured by VMMB member companies, assuring the owner that the equipment meets minimum requirements.
Another important feature of the new guide is a revised, more generic and user-friendly calibration form. This will help producers and engineers standardize the process and should settle concerns that have been raised in the past.
Other items updated in the new guide include:
Production Rates: Production rates on many units have increased more than three-fold since the original guide. VMCM units have produced quality standard concrete mixes at rates that exceed 100 yd/hr. The document also suggests that to achieve these high production rates, the concrete producer must pay close attention to planning material logistics and in-feed raw materials, as well as freshly mixed concrete output.