In San Francisco, California, Interstate 280 is a major piece of urban infrastructure. When an important section of bridgework on the highway was in need of repair, mobile mixers came to the rescue.

As the primary corridor for vehicles moving in and out of the city’s downtown core, the I-280 is vital for San Francisco traffic flow. To avoid major traffic disruptions for residents, repair work on a bridge hinge located on the highway’s Southern Freeway Viaduct required careful scheduling and logistics. Foremost among the planning concerns was concrete mixing, batching, and placement technique. The engineers and construction managers on the job—Ric Maggenti, Sergio Gomez, and Roberto Luena—explain the concrete work for the project: “Each hinge reconstruction consisted of four pours: stem and soffit on the seat side, stem and soffit on the support side, deck pour seat side and finally deck pour supported side. Barriers and joint seals were the last order of work, notwithstanding clean-up and traffic markings.”

After careful consideration, a contractor using volumetric mixers was chosen for the job. The continuous pour capabilities of volumetric mixers were advantageous, as the concrete materials could be constantly replenished during placement for as long as necessary. The versatility offered by being able to adjust the mix on demand was also a meaningful benefit: “the contractor was able to control the mix with admixture adjustment as the temperature varied and as the placement needs varied. There was no dead time waiting for strengths. The working time was engineered to be about an hour, so by the time the top of a stem was finished, the soffits achieved final set. The contractor’s crews were openly impressed has to how soon forms could be stripped so as to move on to the next operation.”

The mix design, which was based on an award winning freeway project on the Pomona Freeway, required reports detailing the volume of ingredients per time, which were supplied using automatic dispatching tickets from the mixer, which also displayed real-time metering continuously on a digital monitor. All proportion checks, samples, and tests passed easily and the concrete easily complied with all structural specifications. Thanks to the unique advantages of mobile mixers, the bridge reconstruction was a resounding success.

More about this project from Structure Magazine here