8.3 : Ready Mixed Concrete

Ready-mixed concrete, also known as pre-mixed concrete, is prepared in a centralized plant and then transported in trucks to construction sites where it is ready for placement. This type of concrete is categorized into central-mixed, truck-mixed (or transit-mixed), and shrink-mixed. Central-mixed concrete is entirely prepared at a plant and moved to the site in agitator trucks that rotate at a speed of 2 to 6 rpm. Truck-mixed concrete, on the other hand, has the ingredients batched at the plant but mixed within the truck at a speed of 4 to 16 rpm, either during transit or just before discharging the concrete. Shrink-mixed concrete is prepared by batching the ingredients and partially mixing them at the plant, then completely mixing them in the truck rotating at 4 to 16 rpm while transiting to the site.

Ready-mixed concrete is widely used over on-site mixing as it is dictated by its advantages, such as enhanced uniformity due to controlled batching, the capacity to avoid segregation during transportation in agitator trucks, suitability for congested sites, and projects that require small and intermittent pours of concrete. ASTM C 94-05 guidelines stipulate that the concrete must be discharged before the truck mixer completes 300 revolutions or within 90 minutes from the start of mixing to ensure the quality of the concrete.