By Sue McCraven
The precast concrete industry has emerged as a major player in public construction projects throughout North America, as increasing numbers of architects and engineers learn more about the substantial benefits derived from manufactured concrete products. Some of the best educators are the precast manufacturers themselves, who are showing municipalities and contractors how the precast option can save time, increase service life, reduce site labor, gain LEED points and meet exacting specifications.
Here are two examples where precasters presented their cases to decision-makers and showed them how to improve their projects with precast.
What engineers can learn from a Jersey project
Our first case study, a post-Superstorm Sandy road rehab project from the Jersey Shore, illustrates how communication pays off and includes some seasoned advice for architects and engineers.
“We continuously mine public advertisements for bids,” said Paul Heidt, engineer manager with Garden State Precast, Farmingdale, N.J. “This is where it starts. From our staff member who follows all potential work in our service area, we learned that nine stormwater pumping stations were going to be part of NJDOT’s rehab of 10 miles of Route 35.” Route 35 is near the Jersey Coast, the area ravaged by Superstorm Sandy in 2012.
Heidt and the Garden State Precast staff quickly completed the legwork, including precise engineering of a precast concrete pump station with a 2 ft by 3 ft concept drawing that could be presented to the resident NJDOT engineer. “We put a detailed plan together prior to the bid and presented it to the successful bidder, telling the contractor that we could offer an engineered plan that was the best, most efficient solution for watertight pumping stations,” Heidt said.
The Route 35 project presented difficult conditions, including a very high water table, restricted work site access and restricted installation hours. The photo on page 28 shows the reason why NJDOT’s specifications made watertightness a priority for the pumping stations. A high water table also impacted buoyancy designs for precast element size and weight.
Armed with engineering plans that met NJDOT’s exacting specifications, Heidt was able to address the specific concerns of the project engineer, which included differential settlement, watertightness and installation schedule. Precast offered NJDOT better quality, a faster installation and longer service life than cast-in-place (CIP). A CIP alternative would have required driving steel sheeting 60 ft into the ground in addition to forming and curing time requirements.
For this challenging construction project, good communication among the manufacturer, contractor and engineer of record resulted in the smartest solution for Route 35’s underground infrastructure. What does Heidt think is the most important thing to relay to engineers and architects? “Whether construction calls for CIP or precast, there’s no difference – the engineering concepts are the same, with the exception that precast is more amenable to design,” he said.
The Rockingham Trail tunnel: aluminum or precast?
Proposed as a paved trail to connect the City of Manchester to the coastal town of Auburn in New Hampshire, The Rockingham Trail would run along the right-of-way of the abandoned Boston & Maine Railroad corridor.
“Original trail specifications called for a metal tunnel section where the bike path would run under Peabody Avenue,” said Mike Worden, president of CSI Concrete Systems in Hudson, N.H. “Specs showed an aluminum, multiple-plate arch with CIP footings.” Worden’s first thought was: “Precast’s service life would be much longer than a three-plate aluminum arch, and a metal arch would require CIP footings and depend on proper backfill to be structurally sound,” he said. “We could manufacture an SCC precast box culvert for the project that would be delivered as a load-bearing structure, ready to install, with a 75- to 100-year life.”
CSI approached Dubois Excavation Co., a contractor bidding on the tunnel work. “We gave them an alternate quote for 120 linear ft of 11-ft span by 12-ft rise box culvert with sloped and tapered end sections on inlet and outlet, approximately 19-ft long, to serve as wingwalls, which allowed the structure to be installed in one day,” Worden said.
Michael Bean, P.E., and vice president for Dubois, explained what happened next. “The (tunnel) area was in an older, well-established neighborhood, and my feeling was that a precast box culvert installation would have a much more appealing appearance to the residents,” said Bean. “I knew that the culvert approach would probably run at least 25% more than the metal pipe, but I also knew bolting and installation of pipe sections would take two weeks, thus increasing project costs. We worked with CSI on successful projects in the past, and I knew that the precast option would offer the city more than twice the design life.”
When Bean and Worden presented their alternative precast tunnel proposal to the city engineer, he received their estimate as very well thought out. The city engineer asked CSI to proceed with shop drawings, which required only three days. As a result, the final precast tunnel proposal was approved by the city and the contract was signed.
“We took a lot of pride in this project. CSI delivered all the precast sections in one day, and all the tunnel work was complete in four weeks,” said Bean. “In fact, the precast was installed in one day.”
As these projects illustrate, great things happen when engineers and architects get together with precast concrete manufacturers at the start of a project. Taking advantage of the precast concrete solution requires ongoing communication, upfront planning and a well-prepared proposal. But after that, precast concrete sells itself.
Sue McCraven, NPCA technical consultant, is a civil and environmental engineer.