Volumetric Mixers Enable Green Pour at San Francisco Airport

In this article, you will learn:

How a volumentric mixer handles green concrete.
How San Francisco Airport pioneered structural application of green concrete.
How recycled aggregate fosters sustainability.

As part of its Terminal 3 West Expansion project, San Francisco International Airport (SFO) became one of the first airports in the world to use a 100% Portland cement-free alternative in the concrete. This innovative, low-carbon material was used to build the structural footings for the terminal’s interim corridor, a vital link connecting United Airlines gates during the renovation.

Breaking new ground isn’t always easy. Success depended upon close collaboration between the general contractor, Turner Construction; the cement manufacturer, C-Crete; and the concrete producer, Bauman Redi-Mix. Batches of the low-carbon concrete were tested and fine-tuned with test pours. Early coordination ensured the project remained on solid ground.

Volumetric mixer handles green concrete

It was up to Bay Area-based Bauman Redi-Mix to supply an on-site volumetric mixer to produce C-Crete. The company has experience developing innovative solutions.

“We were the first ones in the city to have a volumetric mixer,” said Mike Bauman, CEO. Initially, the City of San Francisco was hesitant to approve the use of the volumetric mixers, but that changed after they saw the results on a major project. “We used the volumetric mixers for about 2,000 to 3,000 yards of concrete on-site. After running tests, the results were outstanding.”

Bauman Redi-Mix quickly gained a reputation as a leader in green construction through its ability to use 100% recycled aggregate in the volumetric concrete mix. “It makes us more attractive to general contractors who need to meet green building requirements,” said Bauman. This demand has allowed Bauman Ready-Mix to expand its volumetric mixer fleet to 18 Cemen Tech C60 automated mixers.

Recycling makes a lot of sense in the San Francisco market. “There are a lot of roadways here in the city. We will take them out, recycle the concrete and put it back in the mix,” said Bauman.

The company uses its own crusher to convert used concrete into three-quarters minus aggregate. “Recycling works just as well as virgin materials, and it is way more cost effective,” Bauman said. This is an important consideration when aggregates are bringing over $45 a ton. Recycling aggregate cuts this price in half.

Their automated volumetric mixers allow Bauman to offer a wide diversity of mixes. Mix design can be switched on the job as a contractor transitions from curbs and gutters to handicap ramps.

This versatility also applies to specialty mixes. “We’ve started doing shotcrete, gunite, and slurry mixes that are popular here,” said Bauman. With their volumetric mixers, they can meet customer needs quickly without waiting weeks for specialized mixes from a plant. The company also offers standard mixes in addition to their green mixes that use recycled aggregates and sand.

Another benefit of their automated mixers is simplified operation for the drivers. “It is hard to train drivers to use a manual mixer. With the automated mixer, they just set up and go,” said Bauman. “If you know how to run an iPad, you can run one of these mixers. You can go back and see how much cement, aggregate, and water was added.”

San Francisco International Airport specifies low-carbon concrete

Workers finish C-Crete mixed on-site by Bauman Redi-Mix.

At the Terminal 3 interim corridor, structural footings stepped toward sustainability with the use of C-Crete, a low-carbon cement alternative. This marked one of the first structural applications of the material on a public project.

Turner Construction was selected to place the C-Crete that was mixed on-site by Bauman Redi-Mix. The aggregate for the C-Crete mix was supplied by Martin Marietta. With C-Crete, locally sourced, non-carbonate rocks are pulverized and then used as a binder.

“We poured close to 100 yards,” said Bauman. “It was a good experiment.”

Volumetric mixers adapt to fast set-up time

Unfamiliarity with the new material presented plenty of challenges. Bauman has not seen anything comparable to C-Crete. The material mixes like traditional cement, yet it has unique characteristics. “It came out like a gel and had a white color,” said Bauman. “It set up really fast.”

Bauman learned through experimentation. “It was the first time we poured it. I believe volumetric mixers are the only way you can put it down because it sets up so fast. We ran some through a pump and almost lost the pump. We couldn’t get it out fast enough. If you stop, it automatically sets up. It has to be constantly worked. You have to keep moving.”

Green C Crete is applied at the San Francisco International Airport.

Equipment cleanliness became a priority. “We had to constantly keep the chute clean, otherwise it would stick to the paddles and everything else. It is not something that you can put down and stop for five minutes without cleaning, otherwise, you will have a problem. You have to pay attention. You don’t want to start and stop.” The volumetric truck facilitated the process since material is mixed as it is used, and the conveyor and chute can be easily cleaned out.

Since completing the project, Bauman Redi-Mix gained interest from surrounding contractors. “We get a lot of calls since we poured it. We’ve poured it a few times since then,” said Bauman. “There are a couple of big contractors trying to use C-Crete in the Bay Area. We are the only concrete supplier here who has used it. The material is difficult because you need a separate silo to store the material.”

Volumetric mixers give Bauman Redi-Mix incredible flexibility. One of the key advantages is their ability to use 100% recycled aggregate in the concrete mix. This helped cement the company’s reputation as a leader in green construction. Volumetric mixers give Bauman Redi-Mix the flexibility to handle a wide variety of mixes such as C-Crete.

Bauman Redi-Mix is all about sustainable solutions that help contractors take on environmental challenges. The flexibility of its Cemen Tech C-60 automated volumetric mixers allows the company to offer unique mix designs with no waste.