In today’s economic climate nothing is as important as saving money. With respect to the construction industry, it is vital that actions taken to save money will not hinder the structural and design integrity. Typically, the greatest savings are achieved through the development of new materials and processes. One development expected to save money in the future is fiber reinforced concrete (FRC). Although, the concept itself is dated; recent advances have allegedly created lighter concrete with an increased crack resistance. Aside from increased performance, FRC is also thought to decrease labor costs commonly associated with traditional steel reinforced concrete (SRC). The following report is a review and comparisons of each system characteristic.
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Concrete is arguably one of the most commonly used construction materials. The success of the material is due to its ability to resist upward and downward loads known as compressive strength. However, tensile strengths of plain concrete are relatively low. Tensile strength is considered a materials ability to resist pulling forces. To compensate for this, concrete is reinforced using various methods depending on the application. The most common method of reinforcing is steel reinforced concrete (SRC). Steel reinforced systems have been utilized since the early 1900’s and have proven to be successful. Despite the success, the construction community is showing an increasing amount of interest in fiber reinforced concrete (FRC). The following sections dissect the characteristics of each system and reveal the inherent distinctions.
For the purposes of investigation and clarity research was conducted on concrete slab construction only. The systems have been evaluated and compared with respect to mechanical and design characteristic. Cost and labor practices as well as other concrete building systems such as; beams, foundations, and bridge decks have not been considered for this study. This provides a uniform comparison of both systems.
Description of Design Concepts
Steel Reinforced Concrete
A steel reinforced slab is a composite system consisting of steel and concrete. The steel is typically rods factory welded in a mesh pattern known as welded wire fabric. For larger slabs, and most other applications, the steel is manufactured rods commonly referred to as rebar. Unlike welded wire fabric, the rebar must be tied together. Depending on the application the steel can be unfinished, galvanized, or epoxy coated. Continuous steel primarily provides tensions resistant. The assessment of steel reinforced systems is done with respect to cast-in-place one, and two way slabs reinforced with continuous steel only. Corrugated and waffle slabs have not been considered in this evaluation.
Fiber Reinforced Concrete
Fiber systems are the addition of natural or man maid fibers to the concrete. The most popular fibers today are; nylon, steel, glass, and natural. The fibers are typically added to the concrete mix either as a monofilament or fibrillated fibers. Monofilament fibers are individually added to the mix and are used where preservation of the finish is a priority. Fibrillated fibers are added as large bundles which break down in to smaller bundles of connected by perpendicular fibers. Fibrillated bundles typically yield a stronger bond than monofilament fibers. “The major benefit derived from the use of FRC is improved concrete durability.” (Committee, 2006). All FRC systems reviewed consist of only simple fiber reinforcing. Systems using a combination of continuous steel and fibers or any manufactured products containing fiber reinforcing have not been considered in the evaluation.
With respect to the mechanical behavior of concrete in slab construction FRC and SRC slabs are fundamentally different. As, SI Concrete Systems representative, Mel Galinat explains, “The current methodology for reinforced concrete is based on the steel rebar’s continuous reinforcing function and tensile strength characteristics.” (Marsh, 2001). The bar is laid continuously in each direction to accept and distribute tensile loads to balance the system. The grid pattern ensures that tension in either direction is resisted. Additionally, the pattern segregates the aggregates and prevents cracks from spreading. When using rebar the grid is tied at the intersections and overlapping lengths. The slab becomes a composite system of steel and concrete composite system. Depending on the slab size, control joints are strategically installed throughout the slab to further minimize cracking.
In a fiber reinforced slab system the concrete itself is manipulated. The fibers vary in size depending on the application, however, when setting a related standard, ACI considers, “Common lengths of discrete fibers range from 10 mm (3/8 in.) to a maximum of 75 mm (3 in.).” (E-701Committee, 2006). The fibers are added directly to the concrete ingredients while mixing; resulting in a random distribution of reinforcing fibers. Consequently, the fibers do not align continuously throughout and prevents the system from working together.
As proven with the steel system tension loads are successfully resisted with continuous reinforcement. The lack of synergy among the fibrous members provides minimal tensile strength. An experiment conducted by the ACI in 2006 looks at the characteristics of fiber reinforced concrete in order to establish uniform design criteria for the concept. The study reviewed eight concrete slabs, one with no reinforcement and the remaining slabs were reinforced with various types, sizes, and combinations of fibers. When compared to an unreinforced concrete slab on grade, the fiber reinforced concrete provided better resistance to concentrated loads.
For this reason, even at relatively low volume fractions (<1%), steel fibers effectively increase the ultimate load and can be used as partial (or total) substitution of conventional reinforcement (reinforcing bars or welded mesh) of slabs on ground. (Sorelli, 2006).
Although advantageous to concrete slabs, concentrated loads are only one of the many forces exerted on a slab. Other loads, common of concrete slabs, were not considered in the ACI experiment. These results demonstrate the fibers ability to increase the concretes flexural strength. Materials with high flexural strength resist deformation caused by loads. Flexural stress is caused by concentrated loads such as; heavy equipment or industrial machinery. Therefore, fibers are commonly added to concrete mixes for large industrial slabs and airport runways. In an elevated slab system, where loads are high and unsupported spans are common, current fiber reinforcing cannot efficiently replace continuous steel.
Another characteristic which differentiates fiber and steel reinforcing is each systems approach to crack control. As detailed in the ACI Committee 302 documents;
Polypropylene, polyethylene, nylon, and other synthetic fibers can help reduce segregation of the concrete mixture and formation of shrinkage cracks while the concrete is in the plastic state and during the first few hours of curing. As the modulus of elasticity of concrete increase with hardening of concrete, however, most synthetic fibers at typical dosage rates recommended by the fiber manufacturers will not provide sufficient restraint to inhibit cracking. (ACI Committee 302, 2010).
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Moreover the fibers reduce the spread of cracks caused by shrinkage and temperature change rather than increasing the overall resistance. As expanded further in the ACI Materials Journal, “It is usually assumed that fibers do not influence the tensile strength of the matrix, and that only after the matrix has cracked do the fibers contribute by bridging the cracks.” (Shah, 1991). Therefore the fibers work reactively by responding to loads, whereas continuous steel works proactively by resisting loads. The continuous steel is strategically positioned in anticipation of certain loads, thus providing a resistance.
As explained by the Portland Cement Association (2010); “Fibers should not be expected to replace wire mesh in a slab on ground.” The current experimental results show no evidence of a fibrous additives providing equal, or superior, strength when compared to traditional steel reinforcement. The effects of fiber reinforcing on a concrete slab are inherently different than traditional steel. Continuous steel resists particular stresses while fiber reinforcing responds to different stress. The traditional methodology of designing and constructing concrete slabs using continuous steel reinforcement has slowly developed overtime and has become a highly proven and widely accepted system. Fiber reinforcing is still a young concept, however, design criterion are slowly being developed and studied. The system does show potential for crack control and increased flexural strength. Combining the flexural strength of fibers and tensile strength of continuous steel one can see that such systems would be helpful for slabs enduring high concentrated loads. The fibers help maintain the flat surface by resisting flexural stress while the continuous rebar resists tension stress
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