Go Green with Fly Ash
Go Green with Fly AshWhen the University of Texas was building an eight-story School of Nursing facility in Houston, they decided to go “green.” This meant they sought environmentally friendly construction practices for the new building, demonstrating their beliefs in helping with a clean atmosphere and promoting good health.
One approach to achieving this objective was to create special concrete mixes that are more favorable to the environment. A project team was set up, consisting of the University of Texas Health Science Center, BNIM architects, Jaster/Quintanilla engineers and Jacobs/Vaughn acting as the construction manager.
First, architects from the University of Texas Health Science Center, Reeves Taylor and Brian Yeoman, needed, sought and got approval from the Office of Facilities Planning and Construction (OFPC) to change the guidelines and thereby incorporate this new mixture in the building design. They shared the philosophy that “one project won’t change the environment, but it can help change the mindset for others to think about going ‘green.’ Therefore, you have to start somewhere.”
Studied projects on the Internet
Lenny Enderle, chief estimator and project manager of Vaughn Construction, enlisted the help of Joe Lucas with Hanson Concrete, and they took the lead to develop the new mix. Via the Internet, they studied reports of “green” building projects done in California and Canada. However, after careful investigation, the two decided to lead the team on a different route. They chose to reduce the amount of cement rather than just concentrating on increasing the percentages of fly ash.
Lenny noted that “As companies become more conscious of the environment and government implements more restrictions, I foresee a lot of changes in the industry pertaining to the widespread use of fly ash in construction practices. The harm done by the manufacture of cement cannot be ignored.”
Because a huge amount of carbon dioxide is released into the atmosphere when manufacturing cement, several environmental problems, including global warming, result. Therefore, one answer to reducing the level of carbon dioxide was to decrease the amount of cement by using more fly ash. In conventional mixes, 20 percent of the cementous material is comprised of fly ash. However, the special mix for the mat pour ended up at three times that amount or 60 percent.
The mix design was decided upon after 47 different mixes and tests were run. The team started with a 4-1/2 sack mix with 20 percent fly ash and went up to an 8-1/2 sack mix with 65 percent fly ash. They used these various combinations with normal-, mid- and high-range water reducers and studied the break-and-set data.
TXI chosen read-mix supplier
The project team then re-tested the mixes that seemed to perform, working with two ready-mix companies – Hanson Concrete and Texas Industry Ready Mix (TXI). However, TXI’s “domestic” cement, which uses all Texas ingredients, performed better in the testing stages. Therefore, as the owners preferred to use domestic rather than imported materials, TXI was chosen as the ready-mix supplier.
In the end, the special seven-sack mix design for the mat pour was 263# cement and 395# fly ash with high-range water reducers or super plasticizers. It was poured at a 9-inch slump. Although it wasn’t an easy process to create and approve, the final mixture was proven to be environmentally friendly and much stronger than conventional mixes.
Calculations indicate that this particular mat pour saved 42.11 percent of carbon dioxide emissions that would have been released by conventional mixes. For comparison purposes, .01 ton of carbon dioxide was released into the atmosphere for every ton of fly ash; however, there would have been 1.35 tons for every ton of cement – a significant air emission savings of 13,500 percent between the two materials.
Plus, after 56 days, over 6600- to 9000-psi ratings were noted. These phenomenal strengths were much higher than the 5000-psi design-strength factor required and the 7000-psi poured.
Mat pour took 15 hours
The 15-hour mat pour started at midnight on Friday, May 31, and ended the following Saturday afternoon at 3 p.m. Instead of placing concrete from one end toward the other as commonly found on mat pours, the entire 30,000-square-foot area was pumped by bringing up the whole mat gradually.
Five Putzmeister concrete boom pumps – a 46-Meter, a 36-Meter and three 42X-Meter units – were supplied by S.T.A.R. Concrete Pumping of Tomball, Texas. The pumps were positioned on two sides of the mat because of access problems. They were strategically staged to take advantage of each unit’s full reach using the larger pumps the entire time. The equipment quickly pumped the 5,200 yards of concrete needed for the job faster than the ready-mix trucks could keep up.
Lenny Enderle praised both S.T.A.R.’s and the pumps’ performance in working with the special mix. He said, “As the particles are smaller and rounder than cement, they slide easier. However, the downturn is that because they’re smaller, water absorbs into them faster. This required site dosage of the HRWR. Yet S.T.A.R. and its crew had all bases covered and the pumps easily handled the special mix.”
Ice added to mix
It was imperative to watch the temperature with the special mix. Over 250,000 pounds of ice was put on order a week in advance; and as soon as the weather reached 90 degrees in the early morning hours, ice was added to the mix.
Bringing up the whole mat gradually with concrete was necessary because fly ash hardens at a slower pace, and thus water evaporates off the top surface. Therefore, a loss of moisture would cause it to shrink and crack at the surface. Consequently, crews had to quickly finish and grade off the entire area in less than three hours, getting it flat enough to eventually put dirt atop it at a later date. It was then flooded with three inches of water using fire hoses – a process that took two hours.
It took a full week to harden the mat before the next building process could start. To-date, this appears to be the only drawback. As it takes longer for mixes with high contents of fly ash to harden, problems in scheduling fast-track projects can result.
During the two-year building construction, the $43 million building will consume 19,000 yards of concrete. All mixes will contain large amounts of fly ash, as 65 percent fly ash was used in the drilled piers, 60 percent for the mat, 50 percent for the columns and walls, and 40 percent for the elevated slabs.
Owner: University of Texas School of Nursing – Houston
Architect: BNIM – Kansas City, Mo.
General contractor: Jacobs/Vaughn Construction – Houston
Pumping contractor: S.T.A.R. Concrete Pumping – Tomball, Texas
Ready-mix supplier: TXI – Houston
Equipment: Putzmeister 46X-Meter, 36-Meter and (3) 42X-Meter concrete boom pumps