26 july 2009 / Concrete international

By Heidi Helmink and james e. scHiBley

Batching, Mixing, Placing, and

Finishing Steel Fiber- Reinforced Floors

correct finishing methods are an important key to producing a high-quality floor

Steel fiber-reinforced concrete (SFRC) contains short discrete fibers that are uniformly distributed and

randomly oriented throughout the concrete to improve its structural properties. The steel fibers are typically used in slabs-on-ground to increase impact strength, flexural strength, energy absorption, toughness, fatigue strength, and crack resistance. At typical dosage rates, construction procedures for slabs-on-ground don’t differ greatly from those used in conventional concrete. But there are some key SFRC slab installation items that should be kept in mind to make the project go smoother.

SeleCting the CoRReCt ConCRete MixtuReFrom past experience, concrete containing coarse

aggregate with a nominal maximum size up to 1-1/2 in. (38 mm) can be used, but larger aggregates generally result in more fibers at the surface of the slab. The mixture proportions must provide enough paste to coat both the aggregates and the steel fibers. Specifications commonly require a 4 to 7 in. (100 to 175 mm) slump after the fibers have been added, and this may require a slump before fiber addition 1 to 3 in. (25 to 76 mm) greater than the final slump desired, depending on the fiber type and dosage. If slump adjustment is required, a water-reducing or high-range water-reducing admixture should be used to maintain the specified water-cementitious material ratio. To determine how the proposed concrete mixture proportions will interact with steel fibers, some fiber suppliers evaluate the mixture with a proprietary program based on industry documents such as “Guide for Specifying, Proportioning, and Production of Fiber-Reinforced Concrete (ACI 544.3R-08).”1

Mixing MethodSFibers must be distributed uniformly to impart the

desired reinforcement to the hardened concrete, so care is required during mixing to ensure that fiber bunching doesn’t occur. Bunching typically initiates as fibers are added to the concrete, but it can be avoided by using collated fibers (clips of fibers held together with glue) or controlling the rate of fiber addition.

High-performance steel fibers typically have aspect ratios (length/diameter) greater than 60, and they therefore tend to interact and nest together (bunch) in the mixer. To alleviate this problem, high-performance fibers are collated into clips. Collated fiber clips have low aspect ratios relative to the aspect ratios of individual fibers, so the clips disperse well within the mixture. As the mixing action continues, individual fibers break off the clip and are dispersed throughout the mixture. Once the fibers are properly dispersed, they generally remain dispersed.

Bekaert suggests adding collated fibers to a truck mixer after it is operating at the normal charging speed (12 to 18 RPM). This charging speed is required to carry the fibers away as they enter the mixer. After all the fibers have been added, set the mixer to the highest mixing speed and continue to mix for 70 revolutions—about 4 to 5 minutes—until the concrete-fiber combination is homogeneous. Other steel fiber manufacturers may recommend different procedures. Manufacturers of low-aspect-ratio fibers, for example, may recommend loading fibers into a truck mixer prior to the other ingredients. Always ask for written instructions.

Concrete international / july 2009 27

In a batch plant arrangement, the fibers can be added a multitude of ways, as long as the fibers are not the first component, including:

■ Adding to the aggregates on the conveyor belt and mixing in a normal manner. The fibers should not pile up or bunch on the way to the mixer;

■ Adding to the mixer after the aggregates are introduced; and

■ Adding to the aggregate-weighing hopper after the aggregates have been weighed. Usually, this arrangement works best with a conveyor belt because the fibers and aggregates flow into the mixer together. Bekaert recommends ribbon feeding the fibers into the mixer on a belt conveyor (Fig. 1).

PlaCing and FiniShingSFRC can be placed and finished with standard

equipment. If the concrete will be pumped (Fig. 2), overly wet mixtures—slumps greater than 7 in. (175 mm)—are likely to result in line blockages caused by pump pressures squeezing paste and mortar out ahead of the fibers and coarser aggregate. The best cure for blockages of SFRC is reducing the slump or wetness of the mixture so a mat of fibers and coarse aggregate can’t form in the line.

The steel fibers’ specific gravity is greater than that of the coarse aggregate, so the fiber will naturally tend to lay below the top surface, but surface vibration is still needed after placement. Laser screed vibrators work well for consolidating the concrete without pushing the fiber too far below the surface, but optimal settings should be coordinated with the screed supplier. Handheld vibratory screeds resting on wet screeds (wet pads of concrete, struck off at the required finished floor elevation) can also be used.

Once concrete is placed and screeded to the desired elevation, use a check rod (also known as a modified straightedge), like the one in Fig. 3, with the head in a flat position to cross check the floor, perpendicular to the direction of the laser screed or handheld vibratory screed. This is done not only to check for highs and lows, but to close up any tears or open areas that may have developed during screeding. It also helps keep the fibers from protruding out of the concrete surface. Avoid using wooden bullfloats or other wooden finishing tools. They tend to tear the SFRC surface, while magnesium tools will develop a smoother surface.

Steel fibers do not tend to affect the evaporation rate of the bleed water. When bleed water has disappeared and a finisher can walk on the floor while leaving a barely perceptible footprint depression, begin floating with a riding trowel equipped with pans. A walk-behind machine with a 3 ft (0.9 m) diameter pan works well in areas that are too restricted for riding trowels. As with any concrete placement, the finishers should avoid starting pan floating operations too soon. Premature floating would not only have a negative effect on the overall flatness of

the floor, but it would also bring the steel fibers to the surface, creating more problems for the finishers as they try to create a highly polished shine. If the pans are producing large windrows or slinging mortar, and movement of the paste at the top of the slab is revealing fibers, discontinue pan floating until these symptoms of premature finishing go away. If there is a danger that discontinuing floating will result in crusting of the surface, however, finishers can cover the revealed fibers by repanning the floor at a 90-degree angle to the original direction. Once the floor has been pan floated in two

Fig. 1: The Dramix Booster is a fully automated transport and dosing device for steel fibers

Fig. 2: A crew uses a pump and a laser screed to place a steel fiber-reinforced concrete (SFRC) slab. Unlike concrete without steel fibers, pump line blockages often occur because the concrete is too wet, rather than too dry

Fig. 3: Check rods (modified straightedges) are used by finishers to reduce bumps and fill in low spots when the concrete is still plastic. They can be fitted with various pitch-adjusting tools (Photo courtesy of Wagman Metal Products)

28 july 2009 / Concrete international

directions, the steel fibers will be covered with enough paste and mortar to avoid problems during troweling. If only a few fibers appear on the surface during pan floating, pick them up immediately. Continued power floating will not bury the fibers below the surface. Some contractors assign a worker to walk on the slab and pick up loose fibers as the slab is being finished. Magnets can also be placed on riding trowels for this purpose.

The Sundt Company finishers use the pan machine to bring the surface to a very tight consistency before switching to a riding or walk-behind trowel with combination or trowel blades. The choice of a riding or walk-behind trowel depends on the size of floor and the situation, but the blades should be kept as flat as possible for as long as possible to avoid exposing fibers on the surface.

Although combination or trowel blades can be used for finishing an SFRC floor, combination blades flex more during hard troweling, and this helps keep the steel fibers from popping out. Trowel blades have a slightly higher tendency to pop out the steel fibers.

There may be a few fibers that protrude from the surface of the concrete when finishers highly polish a floor. As the trowel or combination blades catch these fibers, they may rotate and create tiny pockmarks in the floor. A finisher should walk the floor and either clip the fibers with wire cutters or push them down with a pointing trowel, then patch these marks as the floor is being finished. On a heavy industrial slab with a high dosage of steel fibers, a shake hardener is effective in reducing the number of visible fibers on the surface of the concrete.

Only metal hand tools should be used during finishing. Hold the hand tools flat, and move them back and forth in short quick movements across the surface of the concrete. If a steel trowel follows the hand floats, keep the trowel flat. The edge of the steel trowel can cause the steel fibers to come out of the slab surface.

During the last phase of finishing, a finisher should once again walk the floor and patch any steel fiber marks that are still on the surface. The final finishing steps are chalking sawcut lines and applying a good curing compound.

SawCutting ContRaCtion jointSEarly-entry dry-cut saws can be used on SFRC. The

depth of the sawcut should be 1 in. (25 mm) minimum for slabs up to 9 in. (225 m) thick.2 A newer blade should be used with the skid plate tight and properly aligned. Choose the saw blade based on the aggregate hardness. Early-entry sawcutting typically starts as soon as the concrete has hardened enough to prevent aggregates from being dislodged by the saw blade and the edges of the cut from raveling. A good rule is waiting about 1 hour in hot weather and 4 hours in cold weather after final finish of the slab, before starting the early-entry cut. Time the sawing so steel fibers or large aggregate are not

Heidi Helmink is a Technical manager for dramix steel Fiber with the Bekaert corporation, marietta, Ga. she has been in the construction and engineering industry for 21 years, working on heavy industrial and commercial projects. she previously held positions as a Regional engineer for Rinker materials, and senior engineer and marketing manager for cmm engineering llc.

she has a Bs in marketing from kent state university and a Bs in civil/structural engineering from cleveland state university.

aci member James E. Schibley is a General superintendant in charge of placing and finishing for the concrete division of sundt construction, inc., Phoenix, aZ. He has been in the concrete industry for 35 years and has 25 years of experience in industrial floors, including superflat floors; floors with traprock, emery, metallic, light-reflective, and colored dry shake bonded toppings;

and steel fiber-reinforced floors. He is a member of aci committee 302, construction of concrete Floors.

pulled up by the saw. If fibers are pulled up, delay sawing until no fibers are uncovered.

Conventional wet-blade sawcutting can also be used on SFRC. Timing of sawcutting can vary due to temperature and humidity, but traditionally the waiting period is 4 hours in hot weather and 12 hours in cold weather after final finish of the slab. At high steel fiber dosages or for thick slabs, the sawcut may have to extend to 1/3 of the slab depth to activate the joint.

PReConStRuCtion Meeting ReCoMMendedA preconstruction meeting attended by representatives

of the fiber manufacturer, concrete contractor, and concrete producer is highly recommended. The correct fiber-reinforced concrete system for any project also requires correct installation. Thoroughly mixed fibers, a consistent slump, and experienced finishers all contribute to a successful project.

References1. ACI Committee 544, “Guide for Specifying, Proportioning, and

Production of Fiber-Reinforced Concrete (ACI 544.3R-08),” American

Concrete Institute, Farmington Hills, MI, 2008, 16 pp.

2. ACI Committee 302, “Guide for Concrete Floor and Slab

Construction (ACI 302.1R-04),” American Concrete Institute,

Farmington Hills, MI, 2004, 76 pp.

Selected for reader interest by the editors.