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…
Concrete that is prepared at a centralized facility and then delivered to the job site in trucks, ready for placement, is known as ready-mixed or pre-mixed concrete. It comes in three types: Central-mixed concrete, Truck or transit-mixed concrete, and Shrink-mixed concrete.
Central-mixed concrete undergoes batching and complete mixing of the ingredients at the plant. It is then carried to the site in an agitator truck that rotates between 2 to 6 rpm.
In truck-mixed or transit-mixed concrete, the ingredients are batched at the plant and mixed in the truck, rotating between 4 to 16 rpm, either during transit or just before discharging concrete on site.
Shrink-mixed concrete is partially mixed at the plant and completely mixed in the truck, rotating between 4 to 16 rpm en route to the site.
Ready-mixed concrete ensures uniform quality through controlled batching, utilizes agitator trucks to avoid segregation, and is adaptable for use in congested sites or when concrete is needed intermittently.
According to ASTM guidelines, the truck's mixer should not exceed 300 revolutions to avoid segregation, and the concrete must be placed within 90 minutes of the start of mixing.
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Q1: What are the three types of ready-mixed concrete?
Ready-mixed concrete comes in three types: central-mixed, truck-mixed, and shrink-mixed. Central-mixed concrete undergoes complete batching and mixing at the plant before transport. Truck-mixed concrete is batched at the plant but mixed in the truck during transit or before discharge. Shrink-mixed concrete is partially mixed at the plant and completely mixed in the truck en route to the site.
Q2: How does central-mixed concrete differ from truck-mixed concrete?
Central-mixed concrete is fully prepared at the plant and transported in agitator trucks rotating at 2 to 6 rpm. Truck-mixed concrete has ingredients batched at the plant but mixed in the truck at 4 to 16 rpm, either during transit or just before placement. The key difference is when mixing concrete occurs: at the plant versus in the truck.
Q3: Why is agitation important during ready-mixed concrete transportation?
Agitator trucks rotate at controlled speeds to prevent segregation of concrete ingredients during transport. Central-mixed concrete rotates at 2 to 6 rpm, while truck-mixed and shrink-mixed concrete rotate at 4 to 16 rpm. This continuous gentle agitation maintains uniform distribution of aggregates and cement, ensuring consistent quality when the concrete arrives at the job site.
Q4: What are the ASTM guidelines for ready-mixed concrete discharge?
According to ASTM C 94-05 guidelines, concrete must be discharged before the truck mixer completes 300 revolutions or within 90 minutes from the start of mixing. These limits prevent segregation and maintain concrete quality. Exceeding these thresholds can compromise the uniformity and workability of the concrete at the placement site.
Q5: What advantages does ready-mixed concrete offer for congested construction sites?
Ready-mixed concrete is ideal for congested sites because it eliminates the need for on-site mixing equipment and reduces space requirements. It also accommodates projects requiring small and intermittent concrete pours without maintaining permanent mixing facilities. The controlled batching at a centralized facility ensures uniform quality regardless of site conditions or space limitations.
Q6: How does shrink-mixed concrete combine the benefits of central and truck mixing?
Shrink-mixed concrete is partially mixed at the plant and completely mixed in the truck rotating at 4 to 16 rpm during transit. This hybrid approach reduces plant workload while ensuring thorough mixing before placement. It balances efficiency with quality control, making it suitable for projects where complete on-site mixing is impractical but full plant mixing is unnecessary.
Q7: Why is controlled batching critical for ready-mixed concrete quality?
Controlled batching at a centralized facility ensures precise proportioning of cement, aggregates, and water, guaranteeing uniform concrete quality across all deliveries. This standardized process eliminates variability from on-site mixing and ensures consistent strength and durability. When combined with agitator trucks, controlled batching prevents segregation and maintains the concrete's intended properties until placing concrete.