Taking a chance on new technology takes guts. But Juan Alvidrez, president and owner of Plaza Construction, Denver, felt it was worth the risk to try self-consolidating concrete for his Broomfield, Colo., project. SCC is a highly fluid concrete that does not segregate. This is achieved by using a well graded aggregate to prevent segregation and high-range water reducers (a superplasticizer—typically one of the new polycarboxylates) to get the fluidity. This new kind of concrete has been used extensively in precast plants, but less often in the field.
The SCC mix was pumped into the top of the core forms and allowed to drop.
Alvidrez, however, was willing to try it since the fluidity would allow him to skip vibrating the concrete. “Getting rid of the vibrators is a great advantage,” said Alvidrez as he watched the concrete going into the forms. “Usually I would have three vibrator operators on this jobsite, and I would spend half the day yelling at them not to miss any spots. And just when you get a good vibrator guy, he takes off. If this concrete works the way it should, it will be a really good thing for this job. We did a test pour last week using this mix, and things went very well, so we'll see what happens today.”
The project included three cores— two for stairs and one for an elevator—at a two-story office building. The stairwell cores are about 20x15 feet in plan, the elevator is about 20x10 feet, and all three are about 27 feet high with 8-inch-thick walls—a total of about 100 cubic yards of concrete. Alvidrez planned to pour each core in three lifts. To account for the extra pressure on the forms created by high lifts of very fluid concrete, he had placed ties every 12 inches vertically (as opposed to the more typical 24 inches) and at 24 inches horizontally, although he was using job-assembled HDO (plywood with a high-density overlay) forms. Most contractors who have poured SCC use heavier, steel-framed gang forms to resist the higher form pressures developed with SCC. Based on the test pour, Alvidrez knew that the concrete would set up quickly enough to prevent excessive pressure on the forms.
Above: Workers tapped on the forms to check the level of the lift. Right: During the pour there was one worker on scaffolding inside the core and a second with the pump operator on a scissor lift on the outside of the core.
Lafarge produced the concrete, a proprietary SCC mix it calls Agilia. This design used a ¾- to 1-inch aggregate, rather than the 3/8;-inch aggregate used in most SCC mixes. The aggregate size for SCC is typically -selected based on rebar density to ensure that the concrete can pass through without developing any blockages. In this case, rebar density was not an issue, so the larger aggregate was used. Lafarge's Cole Jacobs, who had convinced Alvidrez to try it, was on hand first at the ready-mix plant and then at the jobsite to keep an eye on the mix. “Juan is the perfect customer for a product like this,” said Jacobs, “enthusiastic and willing to try new things.” Since it was expected to be a cold day the Tuesday before Thanksgiving, workers had set up kerosene heaters inside each of the three cores to keep the forms warm enough for the concrete to set up quickly once in place. To get the required set time with the cold weather, Lafarge had added 3% nonchloride accelerator to the mix at the plant. The first truck arrived about 15 minutes later than expected (“We were doing a little last-minute adjusting,” said Jacobs.) and began dumping its load into one of O'Brien Concrete Pumping Company's 42-meter-boom Putzmeister pumps. “We only use Putzmeister pumps,” said O'Brien's Randy Sutton. “They are very dependable and can pump a very harsh mix. This stuff should be easy, though.”
Above: The mix pumped easily into the forms. Right: An SCC mix is tested using a slump cone, but spread is measured rather than slump.
Testing SCC on a jobsite is a bit different from testing a normal mix. Rather than running a slump test, the field technician turns the slump cone upside down on a level board, fills the larger end with concrete, then pulls the slump cone to measure how far it spreads across the board. Some SCC mixes will spread as much as 29 inches, but this mix was a compromise between high slump and rapid strength development. In this case they wanted about a 22-inch spread, which the first truckload achieved. With the second truck, however, they were not getting the fluidity they wanted; the accelerator had worked too fast because the day ended up about 15 degrees warmer than anticipated. Jacobs added some superplasticizer to that truckload to bring the slump up and called the plant to lower the dose of accelerator to 1½% on trucks not already batched. Alvidrez turned off the heaters in the core, and the pour proceeded.
Workers, including the pump operator, were positioned at the top of the forms on scissor lifts to guide the pump hose into the form cavity, allowing the concrete to drop about 20 feet during the first lift. Alvidrez walked around the form tapping with a hammer to detect the level of the concrete and to assure that it was consolidating below the many form blockouts placed to create doors and service access into the elevators and stairs. Generally, workers poured from one location and let the concrete run completely around the forms, which it did well. For one ground-floor door, however, Alvidrez became concerned that the concrete was piling up on one side and would displace the door blockout. He had the workers move the pump hose and place concrete equally on both sides of the block-out. He was also concerned that the concrete was not consolidating under blockouts and at the corners, but in the end it did very well.
After the concrete in the forms reached 7 to 8 feet, Alvidrez moved his crew to the second stairwell core. Initially he planned to place a lift for each of the three cores, return to place the second lift in each of the three forms, and then go back for the third and final lift. With the warmer weather, though, the concrete was setting up too quickly, so after the first lift in the second stairwell core, workers returned to place the second lift on the first stair core. “Since it was setting so fast, we decided to concentrate on the bigger pours and leave the smaller elevator core until the end,” said Alvidrez.
Below: Concrete was pumped in from one location and ran all the way around the form.
Alvidrez was concerned that it he let the first lift in the first stairwell go too long he could have ended up with a cold joint where the second lift started. But since the last truck load used in the first lift had not been setting quite as quickly as earlier loads, he was confident that the lift line would be minimized if he poured the second lift before advancing to the elevator shaft. The crews, therefore, finished off both stair cores before beginning the elevator core. The only significant problem occurred there. Because the first truck-load that went into the elevator core still had the 3% accelerator, it didn't flow quite as well as anticipated. As a result, a small section ended up with less-than-perfect surface finish.
On Monday after the Thanksgiving holiday, Alvidrez was back with his crew to strip the forms and clean up. The concentrate surface quality was generally excellent —even slick—with no visible lift lines. “The forms were a little old so the concrete showed every little imperfection”, said Alvidrez. “But the only significant problem we had was with the accelerator. We had a tight schedule on this job and were able to complete it in three weeks when it was scheduled for four weeks. That wouldn't have been possible with vibrated concrete.”
Alvidrez is happy enough with the results to have two more jobs planned with Agilia. First he is doing a test pour of a couple of slabs at a Lafarge ready-mix plant to see how the high slump concrete works as a self-leveling mix. “Then, we have a job coming up to pour two 24-inch-diameter 20-foot-high columns that will be exposed concrete at a cemetery. There is no way to get the quality this job needs with a vibrator so we'll use the SCC and steel forms to get a good finish, but we won't use any accelerator. The SCC is a good way to go for walls. I'm always looking for new technology and am willing to take a chance if it ends up giving my customers a better product.”