7.7 Rigid Pavement Construction

7 Rigid - Plant Operations

	Major Topics on this Page
	7.1	Truck Mixed PCC
	7.2	Central Mixed PCC
	7.3	Shrink Mixed PCC

All PCC intended for pavement use is ready-mixed concrete. Ready-mixed concrete refers to PCC that is delivered to the customer in a freshly mixed and unhardened state (NRMCA, 2002). Therefore, PCC production involves the batching and mixing of portland cement, aggregate, water and admixtures to form a ready-mixed concrete in accordance with the approved mix design. There are three basic production methods: truck mixing, central mixing and shrink mixing. Other methods of PCC mixing do exist but they are not common in pavement applications and are therefore not covered in this Guide. Truck mixed PCC is dry batched into a concrete mixing truck then blended in the truck either at the batching plant or in route to the job site. Central mixed PCC is batched and mixed in a central facility and then loaded into a truck for transport to the job site. Shrink mixed PCC is partially mixed in a plant mixer and then discharged into a truck mixer where the mixing is completed. This section provides a brief overview of truck, central and shrink mixing. More detailed information on plant operations can be found in:

American Concrete Pavement Association. (1995). Construction of Portland Cement Concrete Pavements. National Highway Institute Course No. 13133. AASHTO/FHWA/Industry joint training. Federal Highway Administration, Department of Transportation. Washington, D.C.
National Ready Mixed Concrete Association (NRMCA). (2002). NRMCA web site, Concrete Basics home page. National Ready Mixed Concrete Association. Silver Spring, MD. http://www.nrmca.org.

7.1 Truck Mixed PCC

Truck mixed PCC is usually proportioned by batching the separate ingredients directly into the concrete mixing truck. The ingredients are usually charged in a certain order to ensure good mixing of all ingredients. Although the specific order depends upon local practice, mixing plant procedures and possibly job specifications, a typical charging order is shown in Figure 7.67.

Figure 7.67: Typical Charging Order for Truck Mixed PCC (redrawn from ACPA, 1995)

Truck mixed PCC (see figure 7.68) is appropriate for all types of pavement construction, but is particularly well-suited for instances that can take advantage of a concrete mixing truck's monitored, continuously agitated storage volume (the mixing drum) and its ability to precisely deliver PCC through its chute (see Figure 7.69). These situations might include: intersection paving, street paving, pavement repair, urban environments, high traffic areas and staged/phased construction. Disadvantages of truck mixed PCC are longer load and unload times (due to the nature and opening size of the mixing drum) and the higher operating cost of a concrete mixing truck when compared to an end or bottom dump truck. Truck mixer specifics are covered in Section 8, Mix Transport.


Figure 7.68: Loading PCC for Truck Mixing	Figure 7.69: Using the Mixing Truck Chute to Deliver PCC to a Precise Location

7.2 Central Mixed PCC

Central mixed PCC is usually proportioned by batching the separate ingredients into a central plant mixer (see Figures 7.70 through 7.73) where they are completely mixed before discharge into a transport vehicle. Transport vehicles can be concrete mixing trucks or conventional end and bottom dump trucks depending upon travel distance and other requirements. About 20 percent of the concrete plants in the U.S. use a central mixer (NRMCA, 2002). Central mixing plants can either be permanent or mobile (see Figure 7.74) and offer the following advantages (NRMCA, 2002):

High production volume. A high production volume would be on the order of 3000 m³/day (3950 yd³/day). Most central mix drums can mix about 9 m³ (12 yd³) in a single batch and produce fully mixed PCC in excess of 150 m³/hr (200 yd³/hr). Mixing times are on the order of 30 to 90 seconds (ACPA, 1995).
Improved quality control. Since mixing is controlled by a central facility and not by individual truck, PCC quality can be more closely monitored and controlled.
Reduced wear on truck mixer drums. If used, concrete mixing trucks are used as an agitating haul unit rather than a mixing unit. Agitating rotation speeds are much slower than mixing speeds and thus, produce less wear on drum components.


Figure 7.70: Central Mixing Plant	Figure 7.71: Tilting Drum Mixer Showing Material Charging Apparatus


Figure 7.72: Aggregate Feed Bins	Figure 7.73: Loading a Bottom Dump Truck from a Central Mixing Plant


Figure 7.74: Mobile Central Mixing Plant

7.3 Shrink Mixed PCC

Shrink mixed PCC is partially mixed in a plant mixer before discharging into a truck mixer. The short mixing period in the plant mixer reduces the bulk volume of the overall ingredients by allowing fine aggregate, portland cement and water to fill the large void space in bulk coarse aggregate. Typically, 1 m³ (1.3 yd³) of fully mixed PCC requires about 1.58 m³ (2.07 yd³) of individual ingredients (ACPA, 1995). Thus, with shrink mixing, more PCC can be loaded into each truck mixer. Many central mixing plants use a stationary plant-mounted mixer to shrink mix PCC before charging their truck mixers. The amount of mixing that is needed in the truck mixer varies in these applications and should be determined via mixer uniformity tests. Generally, about thirty turns in the truck drum, or about two minutes at mixing speed, is sufficient to completely mix shrink-mixed concrete (NRMCA, 2002).