Structural engineering software company SCIA recently released SCIA Engineer 2018, the latest version of its signature structural analysis and design software. The new software includes several updates, but the most notable is the new tool for evaluating steel fiber-reinforced concrete (SRFC).Reinforcing concrete with tiny steel wires—in addition to rebar—has been popular in Europe, where it’s commonly used to reinforce industrial floors. SRFC is only now gaining traction in the U.S.
Steel Fiber-Reinforced Concrete Basics
On its own, concrete is weak in tension. So, concrete is reinforced by adding material that is strong in tension. The material of choice is rebar, tendons of steel embedded in concrete shapes. But steel fiber-reinforced concrete (SFRC) is increasingly gaining traction in the world of construction.
Fiber-reinforced concrete is a type of concrete that includes smaller, free-floating fibers that will, if oriented properly, take up the tension. The fibers can be made of steel, polymers, glass or carbon. Carbon or stainless steel are currently the most popular choices for the fibers used in this type of concrete because of their availability, mechanical properties and durability.Generally, the fibers are between 45 to 60mm (1.8 to 2.4 inches), or enough to bridge across two large pieces of aggregate.
The American Concrete Institute (ACI) divides the steel fibers used for SFRC into five basic types:
- Type I: cold-drawn wire (the steel is drawn through a die at room temperature)
- Type II: cut sheet
- Type III: melt-extracted
- Type IV: mill cut
- Type V: modified cold-drawn wire
Type I fibers are the most popular because of their relatively high tensile strength (145,000 to 445,000 psi). These fibers come as “zig-zag” wires, or as straight wires with small deformations ("hooks") at the ends. The more "hooks" a fiber has, the stronger it is. “At a microscopic level, they will really design the shape of those fibers so that it's not just one strand but it has angles in it in order to transmit the forces between the surrounding concrete and the steel fiber itself," said Cyril Heck, chief product and marketing officer for SCIA. "So, it's a kind of anchorage."
The amount of fiber in a given area of concrete is called the dosage, and the dosage chosen depends on what the concrete will be used for and how important cost savings are to the project. Dosage is usually measured in pounds per cubic yard, kilograms per cubic meter, or (less commonly) in percentage of the final solution. In wide-slab construction, the dosage is typically about 30 to 50kg/m3. But dosage on its own isn’t all that’s taken into consideration: the thickness of the fiber also makes a difference. Though it may seem counter intuitive, thinner is often better. For any given fiber dosage, thinner fibers will have greater overall fiber length, making the material better at preventing cracking in the concrete.
These fibers are usually added by conveyor belt at the batch plant, directly after the aggregates are mixed in, and then mixed for a further 4-5 minutes. If necessary, the fibers can also be added on the jobsite.
Why Is SFRCPopular?
One of the reasons people use steel fiber in concrete is that it increases the post-crack strength of the concrete: how long concrete can prevent cracks from breaking a beam or slab. SFRC can be more effective than rebar at preventing significant cracking because the reinforcement is more evenly spread-out, and thus more likely to get in the way of a crack.
"The obvious advantage of fibers, especially in the shell type structure, is to offer a three-dimensional distribution of reinforcement,” said Marc Jolin, an engineering professor at Quebec’s Laval University. “That's something that not-always-perfectly-located reinforcing bars cannot offer."
The reduction in cracking has several important effects. It means that contractors can make slabs thinner, that the concrete will require less maintenance, and that the structure will require fewer joints to control cracking. Indeed, SFRC is particularly popular for industrial floors because it allows contractors to lay "jointless" floors (floors with 60-70 percent fewer joints), giving less exposed area for the concrete to start deteriorating.
Another reason SFRC is gaining popularity is that it has the potential to reduce project costs. While the concrete itself is more expensive than traditional concrete, it allows contractors to reduce the amount of rebar in a slab. "If you need to place rebars on the construction site, a worker will have to go about and put the reinforcements where they need to be and make sure they're well in place before the concrete is poured," said Heck. "So, it's a complex operation on the construction site."
With so many “pros,” what might be the “cons” of using SFRC?
Reinforcing concrete with steel fibers can lead to a lower “slump,” meaning that wet concrete with steel fibers piled in a cone will slump less. Since concrete needs a certain degree of slump to be workable, adding still fibers runs the risk of reducing its work ability. Additionally, fibers can sometimes float to the surface, which many building owners see as an eyesore or a liability. But the lower slump can be combatted by using a superplasticizer, and steel fibers are less likely than other kinds of fibers to float to the surface, especially when the fibers are shorter.
The other “con” is more serious: even though SFRC is approved for use in floors, it’s not yet cleared as a replacement for rebar for structural uses like supporting walls and arches. "A slab on grade is not per se a structural application; in this case, it is relatively easy to replace those reinforcing bars with fibers, and many manufacturers actually offer tools to easily do the conversion from bars to fibers,” Jolin said. “Although it could/would be possible to replace most if not all the reinforcing steel in such an application according to recent R&D efforts, the building codes do not yet go that far."
Crunching the Numbers
To create software that accurately models the behavior of fiber, SCIA partnered with Belgian steel wire company Bekaert, whose wires are frequently used in concrete reinforcement. Engineer 18 doesn't analyze construction on a fiber level, because buildings are too big to focus on an individual fiber—or even an individual rebar. Instead, it allows the user to perform a global analysis that will give them internal forces or stresses for the structure by assuming the fibers are evenly distributed and randomly oriented in the concrete.
The software allows users to design and optimize the required fiber dosage, meaning that they won’t mix in too few—or too many—fibers. Engineer 18can perform ultimate limit state (ULS) and serviceability limit state (SLS) tests, ensuring that structures won't collapse when they are subjected to the peak load they were designed for, and that they will remain functional under the ordinary load they were designed for.
The software can also perform linear and nonlinear calculations with real material behavior simulation to predict cracking and create multiple stress-strain diagrams for material behavior.
Competition
Engineering 18 is not the only software capable of performing SFRC concrete analysis. Competition comes in the form of DIACalc by Portugal’s DIACLASE, which can perform ULS and SLS tests for SFRC with fibers of certain specifications.