A forward-looking fixture commands center stage at the main Denver plant of Rocky Mountain Prestress (RMP), a company soon to reflect on decades of progress
Don Marsh
A forward-looking fixture commands center stage at the main Denver plant of Rocky Mountain Prestress (RMP), a company soon to reflect on decades of progress leading to a 50th anniversary celebration in 2008.
Common in land-locked European concrete factories, but only recently adopted by North American precast producers, a new tower-style batch plant rises prominently at RMP’s 55-acre structural operation. Meticulous engineering at all material transfer points, an abundance of galvanized steel surfaces and components throughout the exterior and interior, and redundant safety mechanisms guarding moving parts and vessels will keep concrete flowing from the $3.5 million plant to double tee and flat beds well toward RMP’s centennial year.
We began looking at a new batch plant around 2003, when high maintenance and scarcity of parts for our 35-year-old Rex equipment became real concerns, says RMP Manager of Special Projects Mike Caron. By early 2006, the need for a new plant became more critical as business picked up and the order log for at least a couple years out was strong. We came up with around 70 features to discuss with manufacturers prepared to deliver a turnkey project.
Dubbing the ideal solution a flagship facility, he adds, the goal was a batch plant that would not limit future production to the consistency of current concrete mixes. We also wanted a design to tie with an adjoining office structure built of product from the immediate and [RMP’s nearby satellite] architectural facility.
RMP awarded a turnkey contract to Skako Inc., San Diego, which coordinated construction and equipment delivery through tower plant-savvy Skako A/S, Denmark. The U.S. and home offices handled all plant engineering and controls, plus the steel structure and shell of the circular, bin-topped tower, hovering above an approximately 36-ft.-square footprint. At the heart of the facility are tandem 4-yd. twin shaft mixers, which Skako sourced through Japan’s KYC Machinery. A second subcontractor, Minnesota’s Belgrade Steel Tank Co., delivered four 900-bbl. silos.
The tower design eliminates much of the inefficiencies associated with ground-level aggregate storage, and nearly all of the traditional wheel loader and dump hopper handling process, explains Caron. We will keep some material in conventional stockpiles, but the new plant can store up to 90 percent of the aggregate on hand. The tower holds 1,300 tons in 10 bins and is configured so multiple bins can be kept idle for a day or more to drain, allowing better moisture consistency when material is required.
Aggregate is conveyed to the bins and weigh batch hopper at lightning speed, thanks to vibratory feeders running on quiet, electromagnetic vibrators. The first vibratory feeder transfers incoming material from a 30-ton dump hopper below a driveover grizzly. Smaller vibratory feeders at each of the tower’s 10 bins convey coarse and fine aggregate at rates up to 600 lbs./second, and impart extreme accuracy by operating in tilted or flat modes during primary and final ( < 700 lbs.) charging phases. The feeders transfer coarse and fine aggregate to the steep wall of a 12-ft.-diameter, 9-ton cone batch hopper charging a Y-shaped mixer chute.
Increasingly a staple among Precast/Prestressed Concrete Institute Certified Plants, the twin shaft mixers operate more efficiently than RMP’s old pan models. However, hourly output was not among plant design parameters the company sought from Skako. Caron notes that existing equipment could usually outpace material requirements for the yard’s 10 double tee lines and specialty product forms, plus a new 24- _ 120-ft. flat bed. But, the twin shaft mixing technology enables high yield of any range of concrete that current or future products and customers require, from zero slump to maximum spread self-consolidating mixes.
TIMELY ARRIVAL
RMP effected the batch plant contract in November 2006 and began trial mix production 11 months later. Foundation work commenced in April and saw placement of four 60-in.-diameter caissons to support the tower’s ingenious structural elements. The aggregate bins and mixer platform below bear on four members shaped like upside-down Vs, with rounded bases and tips. Each of the eight tips is connected with robust, rounded base units in a design so efficient as to require only four bolts per member.
With the batch plant rising, RMP crews fabricated two-story wall panels and double tee roof members to create the adjacent control office. As a precast company, we needed to demonstrate production capabilities and belief in our product, says Caron, who refused to economize on office options that included modular steel structures.
The tower and office construction coincide with a continuation of strong order volume into 2008, primarily for mountain condominium and resort work; a handful of patent-pending products RMP has developed for commercial buildings, especially in more remote, high-elevation sites; and, completion of a gantry crane-accessible structure to enclose the flat panel bed. The batch plant, a major steel shop renovation, and other recent projects are the largest upgrades RMP has undertaken at the site. The company occupied the property, located about five miles northwest of downtown Denver, in 1988.
Two decades later, as it approaches a golden anniversary next year, RMP is the embodiment of a modern, regional precast/prestressed operator with gray and colored product specialties. With a payroll of about 350, it has evolved from a series of high-profile corporate owners who helped shape precast/prestressed during its first four decades, but eventually proved unwilling to weather the industry’s cyclical tendencies. RMP acquired its current structural and architectural facilities from Stanley Structures, a subsidiary of a company that proved better suited to making tools and hardware than double tees. Located about two miles from the flagship property, the architectural operation was once owned by Salt Lake City’s highly skilled Otto Buehner Co., whose legacy remains through still-functioning Schokbeton casting tables.
Acquisition of the Stanley Structures Denver plants occurred while RMP was operating under Phelps Inc. The producer migrated to the Phelps umbrella after being part of (pre-Hanson) ARC America and, before then, the Hydro Conduit family under Montgomery Ward and Mobil Oil. At the height of the corporate owner era, RMP had Denver headquarters, plus Albuquerque, N.M.; Burlington, Iowa; Kansas City; and Oklahoma City and Tulsa satellite plants, some of which continue operation within Cretex Cos. and Coreslab Structures.
Currently, RMP is privately owned by Phelps Tointon Inc. (PTI). A companion 1997 start-up business, RMP/Hawaii, was merged in early 2006 with Grace Pacific to form a two-plant joint venture, GPRM Prestress. It continues independent of PTI’s Colorado properties.
MATERIAL HANDLING MASTERY
Underlying the tower’s eye-catching spaceship element is an astutely executed material-handling plan. A sheltered grizzly (A, far right row, top photo) is designed for bottom-dump truck deliveries, but can also receive material through more conventional end-dump vehicles. A 30-ton underground dump hopper transfers aggregate to the main conveyor via a vibratory feeder, with a low-noise electromagnetic vibrator (B).
At a 30-degree incline, the conveyor is equipped with a chevron-style belt (C) to enable transfer of wet material with minimal roll back. Like the dump hopper and vibratory feeder, the conveyor is fully galvanized. Side screens (D) reduce noise during operation, curtail fugitive dust, and along with a shut down trip wire, augment safety. An additional galvanized touch can be found above the fly ash (one silo) and cement (three silos) tanker alley, where a gutter pan and downspout (E) capture and convey belt material away from the air above trucks to the ground below them.
Deployment of vibratory feeders in lieu of gates continues in the main plant, where 10 bins charge an inverted-cone batch hopper whose geometry approximates that of the tower roof. The feeders operate in tilted and level positions, with quiet, fully adjustable electromagnetic vibrators programmed to operate at 85 percent capacity in two sequences (tilted/coarse and level/medium) and 35 percent capacity in a final sequence (level/fine).
Abundant aggregate storage enables RMP to idle bins for a day or more to drain excess moisture. Contributing to mix consistency and quality control in a plant RMP expects to carry it well into the next generation of concrete mix technology are Hydronix moisture probes in each bin.
The inverted cone hopper feeds two 4-yd. twin shaft mixers, which combine for 125-yd./hour-rated output and are ideally suited to self-consolidating mix production. RMP has had limited use of SCC so far, but based plant specifications around current and projected concrete technology.
Compared to its predecessor, the new plant is closer to volume-heavy double tee beds, and features a fully enclosed, column-free loading area with independent, dual-discharge points. The facility has five mix transport vehicles. In addition to bread-and-butter gray members, RMP has built the structural plant around custom products, and will soon begin regular casting on a new enclosed flat bed.