SEGMENT METHOD REIGNS AMID POURS

Feb 1, 2005 12:00 PM

Thanks to precast technology, a deluge that produced widespread flooding and landslides throughout Southern California posed no more than a minor inconvenience for the installation of an 18-ft.-diameter, 29⅓-ft.-deep sewer pump station in Camarillo. As an alternative to the original cast-in-place design, Simi Valley, Calif.-based Precon Products fabricated 180° precast segments with 16-in.-thick walls, connecting the halves on site by means of Splice Sleeve rebar couplers. Precon President David Zarraonandia estimates that precast provided cost savings of as much as 20 percent compared to cast-in-place methods and cut approximately two months of construction time.

Undertaking improvements for a new 343-acre subdivision, with 1,000 homes, an elementary school, three recreation centers, a YMCA, and a 55-acre park, developer D.R. Horton solicited design proposals for a single-structure sewer pump station. An initial cast-in-place design was rejected ultimately in favor of the precast alternative.

According to Zarraonandia, joining and sealing the segments, which ranged in height from 49 to 63 inches, were accomplished flawlessly. What could have been an arduous task, he notes, was performed smoothly due to meticulous preparation, thorough instruction by Splice Sleeve representatives, and precision molding. Each section featured one joint projecting rebar and one joint with a complementary cast-in-place sleeve. The halves were positioned and temporarily fastened by means of angle brackets and coil rods. With positions secured, vertical joints and couplings were grouted using Splice Sleeves' SS Mortar. Setting time was approximately a half hour per segment.

The package of 12 segments, plus two sections comprising the cover, was produced in four weeks, casting Mondays, Wednesdays, and Fridays. The concrete mix was a 4,500-psi Type II/V with 4 percent air and 4-in. slump, formulated to attain design strength in six to seven days; admixtures included ADVA and Darex II. Molds were manufactured by Helser Industries to a specification of ±1/16-in. Ameron's T-Lock provided a PVC lining for the components, whose exteriors were plant-waterproofed with Xypex Concentrate and Modified agents.

While segments were being cast, the footing was poured on location. A partial excavation of the pump station site had been performed by open cut. The last 16 feet were vertically cut and shored. With shoring in place, the base was formed and poured within three days. After the concrete reached design strength in four days, the first six segments were installed. Joints were grouted and sealed, shoring was removed, and the structure was backfilled. Time of open excavation was less than two weeks.

A couple days after installation of the first segment, a series of storms dropped over 9¼ inches of rain. Located at the lowest point of the 343-acre site, the partially installed pump station was submerged under four feet of water. Removing the shoring and backfilling, however, averted what could have been a disaster.

In the three-day window between storms, the installed sections were pumped out and an additional 9½ feet were placed, grouted, sealed and backfilled — just in time for another 6¾ inches of rain. Zarraonandia contends that, if a cast-in-place design had been used, the structure would have been only partially complete during the rain and flood; the formwork would have surely been lost or damaged; the rebar would have been coated with mud; and, additional costs would have been significant. Further, as the rental cost of shoring was well over a thousand dollars per week, he adds, the savings in shoring alone justified the selection of a precast design.

The 12 segments, ranging in weight from 21,500 lb. to 31,500 lb., were transported individually by truck 30 miles from the Precon facility to the job site. Total deliveries of precast, including the two cover components, comprised 13 truck loads. Placement of the segments occurred on two different days. For the first set, a 170-ton crane was used due to reach requirements. Once shoring was removed and the lower section back filled, a 70-ton crane was sufficient to install the remaining units.

Overall, the project went smoothly, Zarraonandia reports, and an additional time savings of three to four weeks for future projects is anticipated. “We expect to submit this economic design on a regular basis as an alternate to large cast-in-place rectangular structures of the same capacity,” he affirms. — Precon Products, 805/306-2488, www.pre-conproducts.com