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Center deflection (mm) Figure 16.5. Typical flexural test curves of fiber-reinforced concrete [78].

mechanism, the energy absorption during flexural failure was significantly higher than that for plain concrete. The flexural toughness indices (I5 and I20) were calculated according to ASTM C1018.

Shaw Industries, Inc., the largest carpet manufacturer in the world, constructed a 11,000 m2 R&D Center in Dalton, Georgia, which used concrete reinforced with carpet waste fibers in the construction project [80]. The amount of waste fiber included was 5.95 kg/m3, and about 20 tons of carpet production waste was consumed in the project. Mixing was done by adding fibers to the mixing truck directly, after which the fibers were found to be uniformly dispersed in the concrete without balling or clumping. Mixing, pouring, and finishing followed standard procedures, used conventional equipment, and went smoothly. The com-pressive and flexural strengths exceeded specifications, and reduced shrinkage cracking was observed. Such concrete containing waste fibers was used for floor slabs, driveways, and walls of the building. The project demonstrated the feasibility of using large amount of carpet waste for concrete reinforcement in a full-scale construction project.

Other studies on the use of polypropylene fibers from carpet waste in concrete [81], used tire cords in concrete [82, 83], and using recycled nylon fibers to reduce plastic shrinkage cracking in concrete [84] have also been reported and reviewed [85]. Gordon et al. [86] used the waste nylon fibers and ground carpet to stabilize asphalt concrete. Increase of asphalt content in asphalt concrete is favorable because it leads to more durable roads. But it is limited by the resultant flushing and bleeding of pavements and possible permanent deformation of the pavement. Addition of 0.3 wt% waste fibers increased the allowed asphalt content by 0.3-0.4 wt%.

Fiber-Reinforced Soil Studies reported in the literature have shown that fiber reinforcement can improve the properties of soil, including the shear strength, compressive strength, bearing capacity, postpeak load strength retention, and the elastic modulus. Recycled-fiber-reinforced soil was studied by laboratory evaluation, field trials, and ranking of potential applications [86-88]. Carpet waste, apparel waste, and virgin fibers were used for this study at different dosage rates. Additionally, the effect confining pressure and saturation were investigated. Compaction tests were performed to determine the moisture density relationships of the fiber-reinforced soil.

The tests were conducted on a silty to clayey sand. The soil-fiber mixture was allowed to hydrate for 24 h prior to compaction and triaxial testing. The triaxial test specimens were compressed hydraulically with a static-loading machine in a metal split mold. To determine the effect of reinforcing fibers on the moisture density relationship, standard compaction tests (ASTM D698) were conducted for soil reinforced with carpet fibers and polypropylene fibers at fiber contents of 1, 2, and 3%. It was observed that the reinforcing fibers impeded the compaction process, and thus increasing the fiber content had the same effect as reducing the compactive effort (energy). With the addition of fibers, more water was required to lubricate the soil grains during compaction, resulting in a higher optimal moisture content.

The triaxial compression tests were performed on as-compacted and soaked samples. Fiber type, fiber content, and confining pressure were varied, and for each specimen the dry density and moisture content was maintained at 1597 kg/m3 (100 lb/ft3) and 19.0%, respectively. In the unconfined compression tests performed on soil reinforced with carpet fiber, polypropylene fiber, and apparel fibers, the peak compressive stress increases with increasing fiber content for all three fiber types with the exception of the 0.3% polypropylene-reinforced specimen, which showed a decrease in peak compressive stress. Fiber reinforcement also resulted in a reduction of postpeak strength loss with increasing fiber content for all fiber types. The control, 0% fiber content specimen exhibits strain-softening behavior whereas strain-hardening behavior is exhibited at higher fiber contents. Triaxial compression tests on soil reinforced with carpet fibers under confining pressures of 34.5 and 69.0 kPa were also performed (Fig. 16.6, Table 16.12). The fiber-reinforced specimens showed significant increases in peak stress ranging from over 121 to 303%. More importantly, with increasing fiber content, the soil behavior changed from strain softening to strain hardening. The compressive stress at an axial strain of 10% is given in Table 16.12. From these tests, it was generally observed that the enhancement of soil properties generally occurs at large deformation levels. At very small strains, the stiffness of the soil is actually decreased due to a reduction in soil compaction.

The specimens in the soaked triaxial compression tests were allowed to absorb water for 48 h prior to testing. These tests were used to simulate in-service saturation that can occur during periods of heavy rainfall or due to other natural or man-made events. The soaked tests showed reduced strength at all strain levels as compared to the as-compacted condition. However, the 1 and 2% fiber content soaked specimens confined at 34.5 and 69.0 kPa exhibited increases in strength over the unreinforced soaked specimens. Thus, the use of fiber reinforcement can greatly reduce the strength losses associated with in-service saturation.

Axial strain, %

Figure 16.6. Stress-strain relationships for triaxial compression test of carpet-fiber-reinforced soil confined at 34.5 kPa [87].

Axial strain, %

Figure 16.6. Stress-strain relationships for triaxial compression test of carpet-fiber-reinforced soil confined at 34.5 kPa [87].

Table 16.12 Compressive Stress (kPa) at 10% Axial Strain for Soil with Carpet, Polypropylene, and Apparel Fibers [87, 88]

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