Cantilevered Extravaganza
The design proposal of this space is a cantilevered structure with minimal ground contact. 2 U-shaped masses create the cantilevered space; one of them supporting the other. To keep the structure as thin and continuous as possible, steel fiber reinforced concrete (SFRC) is proposed instead of the traditional wire mesh used in concrete systems. The filleted surfaces are possible through the use of SFRC, since placement of rebar is no longer an issue. Had rebar been the primary reinforcement technique, the structure would need to be thicker, and reinforcing would have to be different on the walls than on the floor/roof (placement of rebar toward the bottom of horizontal surfaces, and toward the middle of vertical surfaces – to aid in tensile resistance and lateral resistance respectively).
There are many advantages to using steel fibers, including ductility, energy absorption, shear resistance, and stiffness. The Modulus of Elasticity (E) of steel is 29,000 ksi - higher than that of concrete, which is about 4,350 ksi. By selecting a fiber material with a higher E, the stiffness of the structure is increased, and there is a lower deflection under load. The addition of steel fibers into the concrete improves the performance of the concrete when tensile stresses are introduced. Because the fibers are small, they are also able to bear tensile stresses in the presence of cracks.
Fibers are added to the concrete as a percentage of the total volume of the composite mixture. Typically, fibers make up between 0.1% and 3% of the total volume. Steel fibers can be initially mixed in either as single fibers, or as magnetically or glued-together bundles in order to ensure uniform distribution throughout the mix.
There are a multitude of fiber shapes that can be mixed in with concrete, and each has different mechanical properties depending on its size, shape. Tensile strength, grade of mechanical anchorage, and capability of stress distribution and absorption all differ from one fiber to the next. Fibers with hooked ends and corrugated fibers are used to improve the anchorage and adhesion to the concrete mix. Fibers that are too long tend to ball up in the mix, which leads to workability problems. In the end, it is a mixture of fiber content, type, distribution and orientation that all determine the properties of the hardened concrete mix.
When working at larger scales with composite slabs (concrete + steel deck), it is traditionally the rebar mesh inside concrete slabs that reinforces the concrete, controls cracking, and deals with shear and tensile reinforcement in the event of a fire (concrete is weaker under high heat, thus more susceptible to failure). Using fiber reinforcement offers many benefits. Construction is quicker because time does not need to be spent putting the mesh together, then holding it in place during pouring; instead, with fiber reinforced concrete, the fibers are mixed in with the concrete, which can then be directly pumped into the forms. Safety issues with mesh include manual handling, tripping hazards, and storage concerns – all of these are eliminated with fiber reinforced concrete, making it much safer to work with. Quality control is also improved through the use of fiber reinforced concrete, since the fibers are evenly distributed during mixing, and manual installation of reinforcing mesh is not required.
Steel is not the only fiber available for use in reinforcing concrete. Carbon fibers and many synthetic fibers are being studied as well. One interesting synthetic fiber is polypropylene, which has been implemented in some tunnel projects because of its behavior under extremely high temperatures. Under intense heat, concrete undergoes a process known as spalling, which is when trapped water vapor causes a buildup of pressure in the concrete. The concrete will then crack and begin to lose its surface, which can lead to extensive damage in a tunnel, since in the event of a fire, the confined space causes temperatures to rise rapidly. Spalling is dependent on the aggregate used in the concrete mix, the moisture within the mix, and how porous the mix is. Tunnels typically have very dense concrete, which does not allow for the release of built up pressure. Polypropylene comes into play in the early stages of a fire. It will melt away creating millions of little voids through which built up pressure can escape. Although it is not anticipated that this situation would take place in a structure at the scale of the proposed cantilever, it is an intriguing look into the versatility of fiber reinforced concrete.