Fiber reinforcement makes the grade at the T.C. Williams High School

Design & Construction Team

Project Name: T.C. Williams High School
Owner: Alexandria City Public Schools (Virginia) General Contractor Hensel Phelps Construction Co.
Structural Engineer: Moseley Architects
Concrete Supplier: Florida Rock, formally Newington Concrete
Secondary Reinforcement: SI Concrete Systems, now part of Propex; Novomesh 850

On Dec. 4, 2004, city and school officials broke ground on Alexandria City Public Schools’ new T.C. Williams High School in Alexandria, Virginia. T.C. Williams High School was made famous after it was featured in the Walt Disney motion picture Remember the Titans in which Denzel Washington starred as the school’s legendary former football coach, Herman Boone.

The new, 461,000-square-foot high school, a $90 million project, will have a capacity to house 2,500 students in grades 10 through 12. The three-story building, designed by Moseley Architects of Richmond, Va., will feature a 1,200-seat auditorium, a 3,000-seat gymnasium, a planetarium, and a parking deck with 416 spaces.The state-of-the-art facility is the most ambitious and expensive public building project ever undertaken by the city of Alexandria, Va.

Reduced cracking in composite metal decks

Composite metal deck construction was chosen as the structure for the elevated floors of the school. The decks have four main functions: to work as a platform during construction, to act as a form, to provide positive bending reinforcement for a concrete slab, and to provide resistance to horizontal wind or earthquake loads. This type of decking takes advantage of the high tensile strength of steel and the high compressive strength of concrete to handle gravity and lateral loads the deck may be exposed to during construction and in service.This is especially important because a composite metal deck is usually put into service a day or two after the concrete has been placed.

Some of the heaviest loads these floors will ever encounter will be during the construction phase—pallets of drywall, a pallet of concrete masonry units, or scissor- lifts and industrial lift trucks.

Because of the risk of these superimposed loads to the T.C. Williams High School project and the fact that close to 250,000 square feet of concrete was to be used in the decking, it was critical to understand the stresses that this decking system would be subject to and the potential risks of cracking. The cracking can occur because of two reasons: either as a result of flexing of the metal deck or from the restraint of movement of the concrete. Concrete also has the propensity to crack both in its plastic (early-age) and hardened (long-term) state. Earlyage cracks are microscopic fissures caused by the intrinsic stresses created when the concrete settles and shrinks during the first 24 hours after being placed. Longterm cracking is in part caused by the shrinkage and drying of the concrete that transpires over time. In either case, these cracks can jeopardize the overall integrity of the concrete and not allow it to maintain—or possibly ever attain—its maximum performance capability.

The normal method used to control cracks in concrete over metal deck assemblies is secondary reinforcement, otherwise known as shrinkage and temperature reinforcement. Shrinkage and temperature reinforcement is non-structural reinforcement used to minimize crack-widths that are caused by thermal expansion and contraction. Traditionally, steel such as wire mesh or welded-wire fabric (WWF) has been the primary choice for temperature and shrinkage reinforcement in composite metal deck construction.

However, as a result of the recent additions to the Steel Deck Institute’s criterion for temperature and shrinkage reinforcement coupled with added performance benefits of an alternate, the contractor—with the approval of the structural engineer—decided to use Novomesh 850, a combination of a steel and polypropylene fiber blend manufactured by SI Concrete Systems, now part of Propex. The synthetic fibers worked to reduce the formation of plastic shrinkage and plastic settlement cracks, allowing the concrete to develop to its optimum long-term integrity. The steel fibers will work to reduce drying and restrained shrinkage cracking that may occur over time, resulting in redistribution of stresses to larger areas of the concrete and tighter held cracks, which will provide years of exceptional service. And because the fibers were distributed evenly throughout the concrete, this multi-dimensional, secondary reinforcement is ensured to always be positioned correctly, providing 100 percent positive placement. As with traditional construction practices, supplementary steel was used to provide continuity over the negative moments.

Also, the fibers provided other benefits to the concrete that traditional temperature steel cannot provide.

During the plastic phase after the concrete was placed, the fibers created a three-dimensional support network that resisted the downward pull of gravity, thus keeping the aggregate in suspension while promoting uniform bleeding. This network of fibers increased the tensile strain capacity also, minimizing plastic cracking while adding impact, abrasion, and shatter resistance to the concrete.

All of these benefits work to minimize concrete surface weaknesses and defects, making the concrete more durable.

Fiber-reinforced concrete speeds up multi-story construction While all types of projects benefit from fibrous concrete construction, multiple-story buildings, such as the T.C.

Williams High School, reap unique gains from its use.With the increased popularity of composite metal deck design, steel frame buildings can be completed sooner using concrete reinforced with engineered, blended fibers systems, such as Novomesh 850, as an alternate to traditional temperature steel for secondary reinforcement. The faster construction of the T.C. Williams project resulted from the fact that the secondary reinforcement was already included in each cubic yard of concrete. And because there was no extra installation, the contractor was able to cut several days from their placement schedule, speeding up the overall construction timeline of the project.

The fiber reinforcement used offered no problems to pumping the concrete and actually reduced line pressures. Also, no additional materials had to be hoisted or lifted. The fibers were pre-mixed with the concrete, pumped directly onto the decking, and finished with standard techniques. By eliminating an extra step, the contractor was able to reduce the hoisting costs and labor time that it would have taken to place conventional temperature steel.

A proven technology

SI Concrete Systems’ fiber solutions have been used in slab-on-ground construction for more than 20 years and have been accepted as an alternate system for secondary reinforcement in concrete/metal deck assemblies because they are thoroughly proven. A sponsored series of fire tests were conducted by Underwriters Laboratories (UL) using standard ASTM tests E84 and E119 on 700-, 800-, and 900-series metal deck assemblies. The conclusion of the UL tests, based upon full-scale fire tests used with fibers in a protected concrete/steel form unit assembly, was that fiber reinforcement is a suitable alternative to WWF in similar type construction.

SI Concrete Systems also conducted an engineering study at Virginia Polytechnic Institute and State University (Virginia Tech) under the supervision of W. Samuel Easterling, Ph.D., P.E., and Carin Roberts-Wollmann, Ph.D., P.E.

The evaluation program included fullscale testing to compare the performance of composite metal deck systems with concrete containing traditional temperature and shrinkage steel and composite metal deck systems containing fiber reinforced concrete. Results from this study demonstrated that steel fibers offered an attractive alternative for temperature and shrinkage reinforcement in composite slabs under distributed load.

In 2003, the Steel Deck Institute revised its Design Manual (publication number 30-ANSI/SDI-C.10 Specification for Composite Steel Floor Deck, section 5.5) and included fibers as an alternate system for shrinkage and temperature reinforcement. This revision includes a performance requirement that steel fibers meet 80 pounds per square inch of residual strength when tested in accordance with ASTM C 1399.

State-of-the art facility

Fibers are just one of the many stateof- the-art technologies used in the T.C.

Williams High School. In addition, the new school will feature a 450,000-gallon underground cistern that will collect rainwater from the building’s roof and store it for use in toilet flushing, airconditioning operations, and irrigation.

It will also feature a permanent measurement and verification system that will track water and energy use at the facility.

Data collected will be made available to students at the central "dashboard" located in the student commons. The new high school will also feature a garden roof that will clean itself with the runoff water and then drain it to the storm sewer system to provide a living laboratory for students. The new school will be completed in 2007.

Sandy Stewart is the marketing manager for SI Concrete Systems, now part of Propex. She can be reached at 423-553-2650 or via e-mail at sandy_stewart@sind.com.