ACI 304.6R-09 Guide for Use of Volumetric-Measuring and Continuous-Mixing Concrete Equipme.
3.3—Equipment condition All proportioning and mixing equipment should be well maintained in accordance with the manufacturer’s instructions. This point cannot be overemphasized. Equipment condition directly impacts the quality of the finished product. Attention should be paid to the following areas: • The cement dispenser should be clean and free of any buildup; • Valves should operate smoothly and not leak; • Any accumulation of materials on any controlling surface or opening in the system will alter the calibrated flow of materials; • Mixer augers should not be allowed to wear beyond the manufacturer’s recommended limits; • There should be no buildup of concrete on the mixer auger surfaces; • Delivery mechanisms should be properly adjusted and kept in good repair; • There should be no leaks in the hydraulic or air systems; • There should be no cut or damaged insulation on electrical wires; and • All covers and guards should be securely in place. ASTM C685/C685M provides guidance for evaluating mixer uniformity.
3.2—Mixers Most continuous mixers use an auger rotated in a sloped trough or tube. Materials are introduced at or near the lower end, and the mixed concrete is discharged at the other. This basic principle is the same for all VMCM equipment, although there are many individual variations. Augers are available in different lengths and diameters, can operate at different speeds, and can have continuous or interrupted flights. Troughs may have flexible or rigid bottoms and covered or open tops. The slope of the mixer may be fixed or adjustable. Lowering the trough (which is normally set at about a 15-degree inclination) may reduce the mixing time, while raising the trough may extend it. Mixing time can be adjusted by modifying the mixer angle of inclination, mixer rpm, mixer flighting configuration, throughput rate, or a combination of these. Actual mixing time from input to output is usually less than 20 seconds. Mixing times as long as 45 seconds have been achieved, but this comes at a considerable reduction in production throughput.
5.1—General VMCM equipment lends itself to a wide variety of applica- tions. While many of these applications involve relatively low- volume production of concrete (less than 1 yd 3 [0.8 m 3 ]), some very large jobs in excess of 1000 yd 3 (800 m 3 ) have also been completed with VMCM equipment. In addition to producing conventional concrete, VMCM equipment is well suited for a number of special applications. Some of these are discussed in the following sections. 5.2—Mixtures with short working times Concretes made with rapid-setting cements, special rapid- setting admixtures, or polymeric materials have a relatively short working life. Applications include repairs to hydraulic and highway structures, pavement, and precast concrete products. Because VMCM equipment proportions and mixes at the job site, the maximum possible working time is obtained.ACI 304.6R pdf download.