Site constraints, delivery routes along the edge of downtown Louisville, Ky., and quality control concerns led the joint venture building the new McAlpine lock on the Ohio River to consider a multi-purpose batch plant for varying types of structural and roller-compacted concrete mixes. The job is the latest under the Army Corps of Engineers to incorporate a horizontal twin shaft mixer-equipped batch plant as a single unit in lieu of separate central mixed tilt-drum plant and continuous pugmill RCC mixers.

The agency gave the nod for twin shaft mixer-derived RCC based on provisions for individual weigh batching (versus pugmill mixers' cumulative weighing) and experience with other RCC jobs dating to 1989. Officials from the Corps and McAlpine lock contractor TGM Constructors also considered the success of southern California's Olivenhain Dam (2001-02), which drew 1.3 million yd. of RCC from a batch plant built with four twin shaft mixers. Acceptance of the mixers for the McAlpine structural concretes hinged on results of a Corps uniformity test measuring consistency of samples from the first and final 10 percent of a load, along with a specimen from the middle.

TGM Constructors began work on the lock chamber in fall 2003 and started concrete production last spring. Built adjacent to a service ramp accessing the lock site, the batch plant features tandem 6-yd. horizontal twin shaft mixers, extensive mix-temperature controls, and storage sufficient for 5,000 yd. of production. The location enables mix transportation vehicles and conventional mixer trucks to load up to 9 yd. in 90 seconds and reach placement points in four minutes.

A joint venture of Traylor Bros. in Evansville, Ind.; Granite Construction Co. in Watsonville, Calif.; and Massman Construction Co. in Kansas City, TGM is building a 1,200-ft.-long main lock chamber to replace an existing chamber half the size. The new structure will require 220,000 yd. of higher strength and 35,000 yd. of lean structural mixes, plus 140,000 yd. of RCC mixes. All concrete on the job is being produced according to a 70°F placing temperature spec.

Material placement during 2004 included 30,000 yd. of RCC. Off-season work will be confined to placing mass-concrete sections. Part of a $350 million undertaking along the Louisville waterfront, the lock is scheduled for completion in 2008. At the peak of construction, TGM expects to have upwards of 300 workers on site. The expanded lock will include a new crossover bridge built of prestressed girders from Prestress Services Inc. in Lexington, Ky.