Twin Steers to Twin Shafts

Fleet management is one of the competencies positioning Raleigh, N.C., ready mixed, aggregate and asphalt producer Sunrock Group among the Carolinas’ stronger independent operators. A 2001 change to Allison 4600s in its rear discharge mixer truck specs made the group’s concrete brand, Carolina Sunrock, an early adopter of fleetwide automatic transmissions.

It then tested a twin steer chassis now spec’ed on 25 mixers, management confirming improved overall durability against conventional models, plus the benefit of 10-yd. loads’ weight distribution—especially on low tonnage roads.

The Durham plant was first up for the transit-to-central mixed conversion.

In its latest primary capital equipment test, Carolina Sunrock sees improved fleet efficiency stemming from conversion of two dry alley plants to central mixed, using fast and compact Liebherr Concrete Technology twin shaft mixers. The 6-yd. models load in two cycles, versus the single cycle of tilt drums that remain the standard in U.S. central mixed plants, and are equipped with automatic wash out. “Having worked in a central mixed plant, standing on the platform you feel materials hitting the drum wall and the tilting action even more. The twin shaft mixer loads as quickly as the drum, but without the stress on the plant structure,” says Carolina Sunrock Manager Ron Taylor. “At first I was a little skeptical with two cycles instead of one, then realized it is a smoother operation with the twin shaft.

“The truck loading cycle is about the same as transit mixed, but there is no slump adjustment or rack time. The twin shaft has a clean loading cycle and no mix spillage. With less wash time required, trucks exit faster than from a dry alley plant.”

Slump adjustment time is the biggest area of savings Carolina Sunrock has observed during nearly a year of twin shaft mixer operation at Durham and North Raleigh plants, two of three integrated ready mixed, asphalt and recycling sites serving the Raleigh-Durham/ Research Triangle markets. Quality control and binder efficiency are other factors validating the dry plant conversions, notes Sunrock Group Vice President John A. Tankard III. “In transit mixed, you have different drivers, different mixers, different slump adjustment levels, and limited potential for consistency from one load to the next,” he says.

In terms of binder efficiency, he adds, “The twin shaft mixer has shearing instead of the tilt drum’s folding action. The shearing coats all materials well, to a point where you might rework a mix design. We have been testing historical mix performance and strength trends with concrete from the new mixers, and see the effects of the improved particle coating and greater air content consistency. The twin shaft equipment gives us greater overall flexibility and will allow us to move into roller compacted concrete pavement, which has started to get attention in the state.”

The Durham and North Raleigh plant upgrades have Carolina Sunrock preparing to convert a third integrated site, the RDU Distribution yard, near the Raleigh-Durham International Airport, from transit to central mixed. The producer stands to realize stepped up return on the twin shaft mixer investments as Raleigh-Durham home building picks up after a slow five to six years, and normal development activity resumes among the market-driving North Carolina State University, University of North Carolina, Duke University, and Research Triangle Park interests.

Liebherr DW 4.5Installation of the Liebherr DW 4.5 twin shaft mixer at the North Raleigh plant has relieved trucks from most mixing duty, while positioning Carolina Sunrock to optimize cement usage; reduce vehicles’ peak production yard time toward 10 minutes; and, competitively bid orders for roller compacted concrete pavement. RCC mix production capability is a natural for Carolina Sunrock, which has integrated ready mixed and asphalt production at three of six central-east North Carolina sites. A removable chute will allow fast dump truck loading for RCC or conventional concrete pavement jobs.

The twin shaft mixers equip the North Raleigh and Durham (opposite page) plants for 180 yd./hour output. Carolina Sunrock mechanics and welders tackled much of the installation work at each site over 10 days. The use of a twin shaft model is less likely to require alley reconfiguration than tilt drum equipment most often used for central mixed plants. Carolina Sunrock converted North Raleigh and Durham without alley work; a tilt drum mixer installation at the latter plant would have been more costly and complicated PHOTO: Liebherr Concrete Technology than the twin shaft model selected.