Getting the Right Mix
According to Dale Even P.E., project manager for Lunda Construction, one of the biggest challenges of forming the box girders was achieving a mix design that would perform on a variety of levels.
“The precast girders’ large size and radial exterior webs required a mix that provided high early strength, durability and workability—something fluid enough that it could be placed in the uniquely shaped form work but have 4000 PSI in 18 hours,” said Even.
Mark Bintzler, technical services manager for AIUS, said the performance requirements of the ready-mix concrete were unique. “The standards called for an early strength of 4,000 PSI in 24 hours, 7,000 PSI in 14 days and 9,000 PSI in 120 days. Technically speaking, Lunda/Ames needed a high early mix that would also gain strength over time,” he said.
AIUS teamed up with Holcim Technical Services, as well as American Engineering and Testing to achieve the right mix.
“Often mixes won’t achieve both strength criteria, but through rigorous research and testing we achieved both of these objectives and met the expectations of MnDOT and our client,” Bintzler said.
“The science of the mix balanced the unique requirements, and ensured that we could strip out the forms and keep production of the girders on track,” Even added.
Logistics: A Crucial Part of the Mix
Each precast girder weighed up to180 tons, and would be lifted and connected with post-tensioning. Lunda/Ames knew logistics posed a real challenge and the success of the project hinged on finding a solution. The AIUS team offered single-source purchasing of cement, slag, sand, gravel and crushed stone, and also leased 20 acres of land to Lunda/Ames at the Nelson Sand & Gravel Plant. This lease allowed Lunda/Ames to set up their enclosed casting shop a short distance away from the jobsite, directly on the backwaters of the mighty Mississippi River.
“We would have run into permitting issues and time constraints if we trucked the girders to the site. Housing our casting facility on AIUS’ property meant we could barge the girders down the Mississippi River and then up the St Croix River right to the jobsite. We avoided major constraints, and river access allowed us to move eight girders at a time instead of just one,” said Even.
AIUS also provided an onsite portable ready-mix plant that ensured Lunda/Ames worked efficiently and avoided errors.
“We addressed quality issues right on the site, as we poured,” said Even.
A Successful Result
All in all, even through the harsh Minnesota winter, forming and placing the girders were synchronized and successful through dedicated teamwork.
“The team effort was incredible from the mix formulations down to pouring in the cold,” said Bintzler.
“Out of the 650 segments we produced, we didn’t throw away one segment. That speaks volumes of the quality of the mix and the people on the team,” Even added.