Precast Inc. - November/December 2008 - (Page 26) SCSFRC mix compositions vary depending on specific applications. Table 1 lists mix proportioning based on trial and error. Different types of steel fibers include: straight, hooked-end and crimped. Fibers were of circular, rectangular or even elliptic cross section. Fiber lengths varied from 5 to 60 mm and aspect ratios from 30 to greater than 100. Dosage of fibers varied from: 25 to 30 kg/m3 (0.32 percent to 0.38 percent by volume) for slabs on grade and facade panels; 40 to 50 kg/m3 and 80 kg/m3 (0.64 percent to 1.02 percent by volume) for housing units, tunnel segments and prestressed elements; up to 100 kg/m3 in roof slabs; and 123 kg/m3 for sheet piles. In the case of precast applications, proper mix protocol and quality control during the manufacturing process are crucial to ensure the specified structural performance. Mix design methodology should be based on reliable predictive tools that take into account the effects of steel fibers. Prompt adjustments to material composition may be required based on the fiber type, geometry and dosage. Figure 1 shows the flow chart for the proposed mix-design procedure. performance of SCSFRC. The ability of the mix, when at rest (static state), to hold fibers in place can be easily assessed by measuring the amount of fibers found along the height of a cylinder. Proposals have been made to measure dynamic segregation of fibers or the ability of the mix to drive the fibers when filling the formworks. One proposal is to cast prototypes of the precast SCSFRC piece and measure fiber content for different sections of the replica. Alternately, a Plexiglas mold set into the formwork would allow a visual inspection of fiber distribution during casting. An example of fiber dispersion testing is shown in Figure 2 with reference to girders for slope stabilization. A replica was made at a 1:1 scale of the central, most congested region in the form to check for fiber dispersion in the mix. Testing of fiber dispersion in hardened state The homogeneity of fiber dispersion in the hardened concrete state is crucial to structural performance. Testing for fiber dispersion can be nondestructive or destructive. Figure 3 shows an example of fiber dispersion monitoring in precast SCSFRC elements after two months of continuous manufacturing. Fiber content may be measured nondestructively by washing the fibers out of small batch samples from the precast facility. This is an easy way to assess the effectiveness of the mixing procedure and monitor fiber dispersion throughout the manufacturing process (Figure 3a and b). A “slightly destructive” method of checking for homogeneous fiber dispersion can also be employed. It consists of drilling small cores at different positions along the hardened precast concrete element. Cores are then crushed and, after separation by a magnet, the fiber content in each core is determined (Figure 3c and d). Tests using this method revealed a remarkable sensitivity of SCSFRC to temperature. This test was also instrumental in highlighting the need for cost-effective Measuring SCSFRC fluidity (also called “flowability”) Validation of a SCSFRC mix-design methodology is based on tests that measure fresh state performance. In addition to the common slump-flow test for mix fluidity, other test methods are available. These tests for plain SCC are: V-funnel, L-box, U-box and J-ring. Fluidity tests assess the passing ability of the mixture and identify openings or obstacles that may block flow of the fibers. Fluidity tests are calibrated on the maximum typical aggregate size used in plain SCC (less than 1 inch) and on typical reinforcing bar arrangements. Use of steel fibers in a mix reduces the amount of reinforcing steel required and thus helps to reduce congestion within the form. A valid assessment of fiber distribution in the SCSFRC mix is a major issue. The mix resistance (versus the static and dynamic segregation of coarser particles and fibers) factors into the fresh state 26 NOVEMBER/DECEMBER 2008 | PRECAST INC. A B C D E F FIGURE 2: GEOMETRY OF A SCSFRC PRECAST CONCRETE GIRDER FOR SLOPE STABILIZATION (A); CAST ELEMENTS (B); PLEXIGLAS REPLICA (C); MOLD FILLING (D, E); AND SECTOR SUBDIVISIONS FOR FIBER COUNTING (E, F).
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