A new standard allows use of synthetic macro fiber reinforcement on steel decks

It was welcome news. While planning construction for the Cristo Rey Jesuit High School and Colin Powell Youth Leadership Center in Minneapolis, the engineering and construction team learned that the rules of construction had changed—at least when it came to placing concrete on steel decks.

That’s because while synthetic macro fiber reinforcement has been successfully used for years in slab-on-ground applications, for the first time, synthetic macro fiber received approval for use by the American National Standards Institute/Steel Deck Institute. The C-1.0 Standard for Composite Steel Floor Deck requirement for minimum temperature and shrinkage reinforcement could now be met with synthetic macro fiber, opening up a new construction option.

Weighing the facts

With the recent approval, the project’s construction team reviewed the data and costs for Grace Construction Product’s STRUX 90/40 reinforcement versus the welded wire fabric that had been previously specified. The results pointed to STRUX, as it met the one-hour Underwriters Laboratory (UL) listing fire-resistance requirement for the project, and eliminated the need for welded wire fabric that can make placing and finishing the concrete a more difficult task.

"Ease of placement, safety for the workers, reduced labor costs—there are lots of reasons why using synthetic macro fibers in applications like this instead of welded wire fabric just makes good sense," said Scott Saunders at Ericksen Roed & Associates in St. Paul, Minn., the engineering firm on the project.

The construction team knew that using welded wire fabric would require more labor throughout the construction process. After all, a crane is needed to move the wire fabric to its location, where a crew has to set chairs and tie-off the fabric before pouring the concrete. Typically, a 20-percent overlap of wire fabric sheets is needed, resulting in wasted material. In addition, during the concrete construction phase, pump lines can get caught on or tangled in the metal, making the job more difficult and dangerous, and welded wire presents other safety risks, as well. Handling, cutting, and positioning the wire exposes workers to sharp edges. And workers face the constant hazard of tripping on the metal, possibly resulting in injury. The team recognized that eliminating welded wire fabric from the construction process would result in faster, less costly, and safer construction.

"We saved time and labor since we didn’t have to cut and install the wire fabric, which translates into a lower bottom-line cost for our customer," said Andrew Haarklau, project manager at Ryan Companies US, Inc., Minneapolis, the contractor on the project. Haarklau explained that placing the wire fabric also presents challenges if it’s moved during construction and not positioned properly. Re-positioning the wire at the correct level in the slab can be a tricky business. "With the fibers, I’m more confident in the integrity of the slab because the reinforcement is more uniform than with wire fabric," he said.

The macro picture

Macro synthetic fibers virtually eliminate the shortcomings inherent in welded wire fabric. Added to the mix at the batch plant, macro synthetic fiber requires minimal labor time or cost at the jobsite beyond pouring the concrete. Macro synthetic fibers also put less wear on pumping and finishing equipment than steel fibers and don’t cause edge chipping when control joints are sawed. And the flexible fiber eliminates the safety hazards associated with steel and any potential for corrosion.

With the green light to go ahead with the macro synthetic fiber mix, approximately 1,000 cubic yards of 6-inch slump normal weight concrete containing STRUX 90/40 was pumped onto the facility’s elevated steel decks to form two and a half floors, or 66,000-square feet. Each 6 1/2-inch-thick floor consists of a 3-inch-deep metal deck with a 3 1/2-inch top slab.

"Our crews didn’t have any problems placing and finishing the concrete with the STRUX," observed Haarklau.

Macro synthetic fiber also offers post-crack performance advantages compared with welded wire fabric, which doesn’t control cracking well since cracks need to propagate to the steel before they’re controlled. Because there’s only one layer of wire fabric running through the concrete, cracks can widen above and below this reinforcement layer, weakening the concrete. The tremendous density of macro synthetic fibers, by contrast, provides uniform reinforcement throughout the concrete, from subgrade to surface.

Smoothing the process

The result? "This concrete deck didn’t have the usual cracking we see when wire fabric is used. It was the most crack-free job I’ve ever seen. In fact, we had trouble finding any cracks at all," said Saunders. "We’re really pleased with the way the macro fibers performed. Using STRUX made sense from both a financial standpoint and in terms of ease of construction."

Saunders noted that with the new regulations and the success of this project, his firm has already used STRUX on several new projects with similar crack-free results.

Kerry Willert at Ryan Companies sees it the same way and said, "We plan on placing more STRUX concrete for composite decks because it’s easier, safer, and has a lower overall in-place cost compared to wire fabric."

A fitting foundation

Saving time and money on this project was indeed important. The impressive new $30.5 million facility is being built to help meet the needs of the children of Minneapolis, as the Cristo Rey Jesuit High School and Colin Powell Youth Leadership Center come together under one roof—literally. The new building will not only provide college preparatory education for city youth, but also offer a safe place where young people can gain leadership skills during non-school hours. The combined facility includes an auditorium, conference center, theater, classrooms, learning labs, a chapel, and basketball courts—all built on a state-of-the-art concrete floor.

It’s a high tech foundation—for a place that community leaders, kids, parents, and everyone involved in the project hope will be a foundation on which to build brighter futures for the youth of Minneapolis.

Joseph Balik, P.E., is a senior product specialist for Cambridge, Mass.-based Grace Construction Products, and Robert Madore is a field market development manager for Grace Construction Products.

Photo credit: Ryan Companies US, Inc.