What happens when you add steel fibers to concrete?
How to Use Steel Fibers in Concrete What happens when you add steel fibers to concrete?
A 22,000-square-foot gym floor was placed at Marquette University’s Al McGuire Center in Milwaukee in 2003. The design/build contractor, Opus North, Milwaukee, wanted a floor with no joints, no cracks, and no curling. To meet those requirements, the contractor used a concrete mix that included 46 pounds of steel fiber (see “Marquette’s No-Crack, No-Curl Floor” Concrete Construction January 2004, http://go.hw.net/cc-marquette). From that experience and many others since the introduction of steel fiber reinforcement in the 1960s, the advantages and limitations when fibers are added to concrete are beginning to be understood.
The largest application for steel fiber reinforced concrete is floor slab construction, although its use as a replacement for or complement to structural reinforcement in other applications is growing fast. Steel fiber floor/slab applications can save money when compared to other reinforcing systems. In addition, joint spacing can be increased and they can be used as a replacement for structural reinforcement in some cases.
In some ways, the role polymer macro fibers and steel fibers play in concrete is similar. Each product can be used to extend joint width in floor slabs and each can reduce curling. Both types of fibers can be mixed successfully in concrete at high dosage rates without interfering with placing and finishing conditions, and they can both be pumped successfully. Steel fibers, however, have other advantages.
The types of steel fibers are defined by ASTM A820:
Type I fibers have tensile strength from 145,000 to 445,000 psi, while Types II, III, IV, and V have tensile strength as low as 50,000 psi. Fiber shapes range from round wires with deformed ends in different diameters (Type I), rectangular or square rod shapes with dimples (Type II), triangular cross-section and twisted (Type V), or crescent cross-section and corrugated (Type V), as well as other shapes. They also come in different lengths, ranging from 1/4 inch to more than 2 inches. Michael Carter, manager of key accounts for Propex (Fibermesh), Chattanooga, Tenn., says there is a tradeoff with length. Longer fibers tend to perform better but they can be more difficult to blend and mix well into concrete. To solve this problem, manufacturers often bundle fibers using water-soluble glue to achieve better dispersion in concrete during mixing.
Diameter or perimeter dimensions between products differ and fiber manufacturers sell different shapes. Jimm Milligan, regional manager-Midwest, Muncie, Ind., for Bekaert (Dramix), says the challenge is to deform fiber ends in such a way as to achieve maximum anchorage with concrete and good cement paste bond along the length of the fiber.
You also can gage fiber effectiveness by aspect ratio—the length divided by the diameter. The higher the aspect ratio the better the performance. Longer fibers have higher aspect ratios. Use aspect ratios to compare fibers of equal length.
Some manufacturers blend steel fibers with polymer plastic macro and micro fibers in order to get a synergistic effect.
Joint maintenance is a big deal, says Mike McPhee, technical support manager for Fibercon, Charlotte, N.C. For owners of floors, cracks and control joints represent future maintenance problems, so fewer joints is a mark of quality. Joints in floors, as necessary as they are, typically deteriorate first, costing owners money for repairs as a floor ages. So owners often are willing to pay for higher dosages of steel fibers in exchange for increased joint spacing and joint life. If they could afford it, owners would construct floors with no joints at all.
How much steel fiber is added to a concrete mix depends on the objectives: cost savings, increased joint spacing, or structural improvement. Steel fiber dosages can be as light as 8 pounds to as much as 200 pounds per cubic yard. Increasing the percentage of fibers in a mix allows specifiers to increase the distance between joints. Floors are reinforced to control cracking between sawcuts using the ACI joint spacing guidelines, or fully reinforced for no sawcut joints between construction joints. These are the same guidelines ACI maintains for reinforced floors.
Fibers are sometimes listed as a percentage of concrete volume. So for instance, 66 pounds of fiber per cubic yard is about 0.5% by volume. A 1% fiber addition is approximately 132 pounds.
Just adding steel fibers to a load of concrete doesn’t ensure success. Steel fibers in concrete represents only one part of the system. There are other important elements to consider, including subgrade preparation, concrete mix design, and the total water in a mix.
The condition of the subbase is critical. The subgrade under a slab must have adequate drainage, be properly compacted, and have a flat, smooth surface. The installation of a good vapor barrier system also is recommended. Concrete placed over mud and water puddles shouldn’t be allowed. These areas should be removed, replaced with suitable material, and compacted before concrete is placed. The goal is to create a smooth surface for the underside of a concrete slab to freely move on when shrinking—slabs that get caught by irregularly shaped subgrade can become stressed enough to crack.
Michael Carter, manager of key accounts for Propex, says it’s wise to work out good aggregate distributions for a mix. Well-graded mixes require less cement, yielding stronger concrete. They also require less total water, so there is less shrinkage. The compressive, flexural, and tensile strength of concrete is largely due to the concrete mix design, not the addition of steel fiber. High flexural strength is especially necessary for a quality concrete steel fiber installation.
Deciding on the dosage of steel fiber to include for an application is important. For instance, to increase joint spacing on a project while still providing crack control might require 40 pounds per cubic yard of steel fibers added to a good low-shrinkage mix. Increasing joint spacing can be achieved by adding the right amount of fibers (and the right type) to a good concrete mix, adding the right amount of water, and placing it on a well-prepared subbase
Most fibers today are added at the ready-mix plant. The most popular method is to use a conveyor to load them into the truck just after the concrete ingredients are loaded. If they are mixed into concrete on a jobsite, either conveyors or machines that can blow them into the mixer are used. Either way, mixing is easily accomplished.
In some cases, fiber manufacturers employ structural engineers, however, their sales representatives are specialists who can assist in designing mixes using steel fiber reinforcement. They can help you decide on the fiber type, style, and amount of fiber to use for an application, offer advice on mix proportions, supply cost information, and sometimes even supply the conveyors needed to load the fibers into a ready-mix truck. Milligan says his company developed a proprietary software system to help develop designs for different applications. But he says he only assists those who are actually responsible for the concrete.
Contractor experience
When contractors are faced with installing steel fiber floor slabs they naturally have questions about how to place and finish, what happens when the dosage rates increase, or if their installation costs increase. Here are reports from two contractors about their experience.
Steve Lloyd, vice president of Lloyd Concrete Services, Forest, Va., currently places and finishes 10 million square feet of floor each year—slabs on ground and decks. Much of this work includes steel fibers. He says they have 17 years of experience with steel fibers in concrete floor construction. “My first job was a disaster; fibers were sticking through the floor surface everywhere and the crew spent the entire day following the placement picking fibers out of the surface.” But they learned how to work with them, as well as which types to use for best results. They install the dosage rates owners want in accordance with floor performance. They place as little as 25 pounds and as much as 75 pounds per cubic yard of concrete.
Increasing joint spacing and reducing cracking are the major reasons their customers give for wanting steel fibers included in their concrete, says Lloyd. On metal deck projects, they can reduce the amount of cracking. He reports the longest successful joint spacing they’ve installed so far is 100x100 feet. Their longest superflat F-min floor slab is 12 feet wide by 210 feet long. “Some of the rebar reinforcement was replaced with steel fiber for this installation,” he adds.
Lloyd says you must take your time with this kind of work. Sometimes they pass their screed over the concrete twice. Reducing the vibration rate on their screed helps too.
Tom Garza, a project manager with Barton Malow, Southfield, Mich., a contractor that specializes in industrial work, has installed concrete with steel fiber dosage rates as high at 55 pounds per cubic yard. The higher rates are specified by owners to increase floor properties such as impact resistance, higher load ratings, and reduced cracking and curling—but not to increase joint spacing. They still follow the ACI joint spacing rules previously stated.
As the dosage rate increases, Garza says they take steps to ensure fibers don’t show on the surface finish. Their finishers pass a roller bug over the freshly struck concrete to depress the fibers a little bit. They don’t do this when surface hardeners are applied.
“We haven’t noticed increased wear on float pans and trowel blades with higher dosage rates, but we do see increased saw blade wear when cutting control joints,” Garza adds.
You may be wondering how difficult it is to place and finish concrete with steel fibers added to a mix. A project I visited recently, where a steel fiber floor was being installed by Barton Malow, which has a lot of steel fiber experience, shed some light on this.
When an auto manufacturer decided to add 100,000 square feet of manufacturing space to its facility, they specified a 12-inch-thick light-reflective concrete floor with 23-pounds of 2-inch-long high-performance steel fibers per cubic yard of concrete. They hired Barton Malow to construct the building, including the concrete work.
Milligan says the auto manufacturer’s specification replaced other forms of reinforcement—welded wire fabric and rebar—with steel fibers, saving them money, reducing preparation time, and making placement easier. Steel fiber reinforcement is oriented in all directions and is dispersed throughout the concrete. So with no reinforcement on the ground, ready-mix trucks could discharge directly from the chute, eliminating the need for concrete pumps. Worker safety is improved as well, because there’s no reinforcement to trip over.
Milligan says these owners didn’t specify steel fibers to increase joint spacing; they used it to replace other forms of crack-control reinforcement. “Joint spacing honors the ACI guideline requiring joints not to exceed 2 1/2 times the slab thickness, expressed in feet,” he says. So for this project, Barton Malow used an early-entry saw to cut joints every 21 feet 6 inches in both directions about three hours after finishing was completed.
Garza says the concrete mix for this project included 540 pounds of portland cement, a 0.54 water-cement ratio, a well-graded 2-inch top-sized blended coarse aggregate, and a mid-range water reducer. This yielded concrete with 4000 psi compressive strength and 200 psi of post-crack flexural strength. Garza says they worked with the engineer and ready-mix supplier to develop this reduced shrinkage mix. The coarseness of the well-graded aggregate in the mix is the only thing that made finishing a little more difficult.
Tom Binkowski, a general foreman for the project, says placement and screeding operations are no more difficult than for concrete without steel fibers. He adds that the light-reflective color hardener specified for this project, placed at the rate of 1 1/2 pounds per square foot, covered the fibers and made it easy to finish the surface. On other projects, he says they often pass a “roller-bug” over the freshly struck-off surface to depress the large aggregates and fibers, bringing cement paste to the surface to achieve a better hard-troweled finish.
To place and finish this concrete, Barton Malow cast the floor in 20,000-square-foot sections—approximately 1000 cubic yards. All the concrete was placed from the trucks chutes, screeded with a laser screed, bull floated, and followed with the application of the light-reflective color hardener, spread with a material spreader. As soon as a finisher could walk on the fresh concrete, a walk-behind finishing machine equipped with float pads was used to make the first pass, floating the color and preparing the surface for rider trowels equipped with pan floats that would make the next pass. Troweling operations followed to provide the desired hard-trowel finish.
“Not easily” is the answer most heard. Carter says anything above 50 pounds of fiber per yard of concrete has to be sawed and lifted out. “You won’t get it out with a jackhammer.” McPhee agrees, “If you forgot to run a waterline under a slab you have to saw the lines of the trench completely through the concrete and then cut the concrete into manageable sections that can be lifted out.”
No person interviewed for this article knew the best way to demolish a slab because they didn’t know of anyone who has done it. Even early applications of steel fiber reinforced concrete continue to perform well and that speaks well for the product.
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