Prior to the sale of RMC Group to Cemex in August 2003, Metromont Corp. purchased the outstanding interest in five precast/prestressed manufacturing facilities
Prior to the sale of RMC Group to Cemex in August 2003, Metromont Corp. purchased the outstanding interest in five precast/prestressed manufacturing facilities from RMC in Metromont’s flagship operation in Greenville, S.C. This arrangement resulted in an unusual situation at the Greenville site, since both the Metromont facility and the would-be Cemex businesses (a ready mixed plant and a block plant) shared an aggregate-handling system.
We had a land easement agreement with Cemex, since the plant is actually on Cemex property, explains Nathan Nabors, the Metromont engineer in charge of coordinating erection of the new Greenville plant Û as well as the company’s future Florida facility, which is expected to finish permitting early in 2009. We shared the same long aggregate conveyor with Cemex and bought raw materials from them. It seemed more economical to build a new plant that didn’t rely on the long conveyor and have materials delivered from two to five miles away.
So, the search began for a plant design that could incorporate four, 150-ton aggregate bins; four, 180-ton cement silos; a 6-yd., twin-shaft mixer; and, aggregate and cement loading systems in a footprint of less than two acres. Having worked with them many times in the last 15 years, Metromont selected the design-build firms Plant Architects & Plant Outfitters to come up with the ideal design for the small space.
At first, it appeared Metromont was asking for the impossible. The ideal space for the production facility was 75 _ 50 feet. To complicate matters, soil conditions in the area were far from ideal, and seismic design restrictions restrained most options even further. All of these factors led Plant Architects to develop a low-pressure, full-skirted, dual-tank design to tackle the soil pressure issue and seismic conditions.
The twin-tank plant, as it came to be called, required only a 0.05 acres adjacent to the production hall (a 35-ft.-wide _ 65-ft.-long parcel). The monolithic concrete slab that provides the entire seismic foundation for the large-scale plant is only 24 in. deep. With only a few steps from the control room to the mixer or the weigh system, Nabors says it’s easy to forget that the plant is capable of 200 yd./hour of the highest-quality self-consolidating or high performance mixes. We’ve run it at about 85 yd./hour a couple of times, he says. But we’d like to get it up to 130 or 150 yd./hour. Our old plant [built by Metromont in 1996], which we turned off around this time last year, only ran about 60 yd./hour.
The twinning of the tanks also showed real savings for Metromont in real estate and construction costs, and it also benefited the company by providing a four-season enclosure for the fairly mild climate of northwestern South Carolina. We have no ground storage; all storage is overhead, explains Nabors. Since we could no longer use a long conveyor, we went with a bucket elevator. At first, no one was really gung ho about that option, but we all realized it was the best solution. We saw a similar setup at another plant, and it seemed to work well.
At the plant’s base are two drive-through openings. One is for cement delivery (via pneumatic transfer), while the other is for concrete loadout. Greenville’s main production bay is served by Tuckerbilt and front-discharge mixer trucks. Cement storage is located above the loadout entry, while the other bins are used to hold aggregate, which are fed by the drive-over hopper and the bucket elevator.
The plant features an OMG-Sicoma twin-shaft mixer model 6000 Û the sixth such mixer of this brand that Metromont has specified for its recent plant upgrades and new plant installations in the last two years. We found the Sicoma mixer and frame to have a much sturdier box, says Nabors.
At the top of the tanks, the scene is quite different from a conventional mixing plant. With this twin-tank configuration, one can survey all four aggregate bins and all four cement silo sections, including dust collections, powder fill pipes, cement level indicators, aggregate feed chute and turn head, and aggregate level indicators Û all by standing in a single spot.
Customizing a 20-ft. sea container, insulating it, and finishing it in stainless steel and aluminum resulted in a climate-controlled electrical/automation center. The center was placed on a freestanding seismic structure with access to both tanks. An attached full-height staircase affords access to the adjoining rooftops of each tank.