By Mark Rea

When the University of Texas Health Science Center at Houston planned to build a new School of Nursing and Student Community Center, the university wanted it to be an environmentally friendly facility.

As a result, the $57 million multipurpose million building is being constructed under the U.S. Green Building Council's Leadership in Energy and Environmental Design requirements, including extensive use of high volume fly ash concrete.

The joint venture of Houston-area firms Jacobs Engineering and Vaughn Construction is serving as construction manager at risk for the project.

Fly ash is a byproduct of coal-burning power plants. A limited amount of fly ash has been utilized in the production of concrete for years, Vaughn Construction structural estimator Lenny Enderle said that many structural engineers have historically allowed up to 25 percent of the cementituous materials in concrete to be fly ash.

"Ready mix concrete producers require the use of fly ash to produce the higher-strength concrete that is becoming more and more common in larger projects," Enderle said. "However, even though cement production has fallen behind demand, the majority of fly ash produced in the United States is disposed of in landfills. It makes much more sense to use a valuable product rather than throw it away."

The original goal of UTHSC was for the concrete to contain a minimum of 51 percent fly ash. For nearly two years beginning in 2000, using input from the owner, project architects, structural engineers, quality control personnel from Houston-area ready mix suppliers and concrete consultants from the University of Texas, more than 50 different mixes were tested.

Mixtures ranged in cement content from 4.75 sacks to 9.25 sacks in 0.75-sack intervals. They also ranged in fly ash content from 30 percent in the 4.75- to 7-sack mixes to 65 percent in the 7- to 9.25-sack mixes. Low and medium dosages of midrange water reducers and normal dosages of high-range water reducers were scattered throughout the testing plan.

Carbon dioxide ratings were also given to each mix based on a study conducted for the project by the Austin-based Center for Maximum Potential Building Systems.

Strength tests were then conducted at intervals of one, two, three, four, seven, 14, 28 and 56 days. Concrete was batched at the Hanson Concrete plant in Houston with Houston-based testing agent PSI conducting early stage testing at Hanson's testing facility. Later-stage cylinder breaks were performed at PSI's laboratory.

After the 28-day strength results were obtained, mixes with the best test results were chosen for retesting.

"Several mixes had their water cement ratios' modified to quantify the impact of additional water while others were retested with different water reducing agents," Enderle said.

Quality control personnel from Houston-based TXI Concrete also agreed to conduct tests of the same or similar mixes at one of the company's batch plant facilities.

Concrete Specs

For the School of Nursing and Student Center project - an eight-story, 187,011-sq.-ft. classroom building and a two-story, 8,150-sq.-ft. service building - engineers determined the most practical way to issue the concrete specifications was to specify minimum fly ash percentage requirements to the various concrete elements based on the test results obtained.

"Though testing revealed strength requirements could be met in a 28-day time frame, due to slower strength gains expected with the various fly ash mixes, strength requirements were changed to 56 days," Enderle said.

The classroom building is being constructed on a site previously occupied by a drainage channel, and the foundation required a 5-ft.-thick mat with drilled straight shaft piers in certain areas below the old site drainage channels. Flat deck slabs supported by exposed round columns and shear walls comprise the structure.

The service building is designed as a structural steel building with slabs supported on metal deck over bell-bottom pier foundation elements.

The mat foundation consists of more than 5,200 cu. yds. of 5,000-psi concrete with 60 percent fly ash., while a 2,500-psi seal slab was poured below the mat and contains 30 percent fly ash. Piers were specified for a 65-percent fly ash requirement and a 4,000-psi mixture.

Walls and columns are 50 percent fly ash with 5,000- to 6,000-psi mixes, while metal decks over structural steel framing have 3,000-psi slabs containing 30 percent fly ash because of the minimal amount of concrete involved and because of the relationship of the slabs to the work of other trades, Specifications for the elevated slab and beam members proved more complex. "These required a hard trowel finish thereby requiring that the initial set time be as close as possible to conventional mixes," Enderle said.

With Vaughn Construction acting as the concrete subcontractor to the Jacobs-Vaughn joint venture, piers were drilled with the use of a 65-percent high volume fly ash concrete mix. The mat foundation was placed in a single June 2002 pour from midnight to 3 p.m. and utilized five Putzmeister concrete boom pumps.

"Unlike conventional mat pours, this one was brought up to final elevation gradually over the entire 28,500-sq.-ft. mat area," Enderle said. "This shortened the time frame to three hours that any portion of the top surface would be exposed before the flooding operation could occur."

TXI supplied the mat pour from four separate batch plants.

Cost Comparisons

Enderle said concrete material prices received on the project were all within $1 per cu. yd. of those received on similar projects bid in the Houston area within the time frame of the project.

"Placement costs on the project were not significantly different than those of conventional mixes," he added. "However, set times for slabs were two to three hours longer than with conventional concrete mixes in warm weather and even longer in the fall and winter months. Overall finishing costs were estimated at approximately 15 percent higher than on conventional mixes."

The longer set times also required use of evaporation control measures consisting of misting equipment and evaporation control compounds. The misting equipment was a one-time cost item with only minimal setup cost requirements for each pour. The evaporation control compounds added an additional 4 to 5 cents per sq. ft. of slab area. Curing costs are not significantly different than those on conventional concrete mixes.

Extended set times also required the mobilization and rental of an additional set of column and wall forms than would have been required. Also, an extra construction joint was added to each floor so additional soffit materials would not be required.
Formwork costs associated with the use of high volume fly ash concrete were approximately 2 percent higher than traditional concrete.

Enderle added that the overall cost of the concrete construction portion of the project was approximately 1.3 percent higher due to the use of fly ash in the mixes. "On future projects, however, actual savings may be realized as concrete vendor prices reflect the material savings achieved and engineer designs realize the significant strength differentials from those of conventional mixes," he said.

Once the project is complete next spring, the university hopes the facility will receive gold certification under the LEED program. In addition to the extensive use of fly ash concrete, the facility will also feature several innovative design concepts, including rooftop rainwater collection, a "green" roof planted with grass and other foliage, a future photo-voltaic system to supplement power usage and use of natural light and reflective surfaces for light direction at work surfaces.

Additionally, the facility's exterior will feature reclaimed brick from a 100-year-old building as well as "floating" stairwells, which will be visible through the glass curtainwall.

The School of Nursing and Student Community Center will finally offer nursing students a campus facility of their own.

"Over the years, our nursing students and faculty have worked in places that were borrowed or rented, not a place that you could identify as a nursing school," said Dr. James T. Willerson, president of the University of Texas Health Science Center at Houston. "Nevertheless, our school is ranked in the top 10 percent of graduate nursing schools in the country. Imagine what may happen once we are in our new facility."