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2017SpringShotcreteEMag

Plastic Shrinkage Cracking Test As previously mentioned, evaluation of the plastic shrinkage cracking characteristics of the plain concrete mixture (WP), compared to the synthetic fiber mixture (WSF) and natural fiber mixture (WNF), was conducted using the ASTM C1579 test procedure. In this test, two molds were shot, finished, and tested for each mixture. After finishing in a prescribed manner, the test specimens were placed in individual environmental chambers (essentially heated wind tunnels), which provided an environment of: temperature 97 ± 5°F (36 ± 3°C); wind velocity of 15.4 ft/s (4.7 m/s); and relative humidity 30 ± 10%, as required by the test method. In addition, a water sample in a beaker was placed in each environmental chamber to monitor the evaporation rate. ASTM C1579 specifies a minimum evaporation rate of 0.2 lb/ft2-h (1.0 kg/m2-h) and this requirement was met in all environmental chambers. A setting time test was conducted to ASTM C403, “Standard Test Method for Time of Setting by Penetration Resistance.” Once the shotcrete samples reached final set, the samples, still in their molds, were removed from the environmental chambers and placed in still air in a temperature- and humiditycontrolled room in the laboratory for curing at 73 ± 4°F (23 ± 2°C) and 50% relative humidity for 24 hours, as prescribed by the test method. Average crack widths in the specimens were then measured with a crack comparator. The test method requires the plain concrete control sample (WP) to develop a minimum crack width of 0.02 in. (0.5 mm). The average crack widths in the fiber-reinforced samples (WSF and WNF) were compared against the WP mixture specimen crack widths. A factor called the crack reduction ratio (CRR), expressed as a percentage (%), is then calculated. CRR = 1 – average crack width of fiber-reinforced shotcrete mixture/average crack width of plain control shotcrete mixture × 100%. In this study, the plain concrete control mixture (WP) developed an average crack width of 0.025 in. (0.63 mm), which satisfies the ASTM C1579 requirement. The synthetic fiber mixture (WSF) developed an average crack width of 0.004 in. (0.10 mm) and the natural fiber mixture (WNF) developed an average crack width of 0.002 in. (0.05 mm). The calculated CRR was 84% for the synthetic fiber mixture WSF and 92% for the mixture WNF. In other words, the synthetic fibers and natural fibers were effective in reducing plastic shrinkage crack widths by 84% and 92%, respectively. In short, both fiber types were effective in mitigating plastic shrinkage cracking, but when used at equal fiber volume addition rates (0.15%), the natural fiber was more effective than the synthetic fiber. Finishability While in some shotcrete applications, such as ground support in tunnels and mines and rock-slope stabilization, the final surface is left in the rough, natural, as-shot surface condition (such as can be seen in Fig. 7), there are many applications where the applied shotcrete is cut and trimmed to line and grade and then finished to a specified surface tolerance and finish texture. This is common in structural Fig. 9: Finishing tools used, from left to right: steel trowel, magnesium trowel, wood float, hard rubber float, textured rubber float, and sponge float shotcrete walls and other elements, canal linings, bobsleigh and luge tracks, resurfacing of concrete dams and spillways, swimming pools and other liquid-retaining structures, and architectural applications. Finishing is labor-intensive and can require three to five or even more finishers to keep up with one nozzleman, depending on the nature of the project. (For example, finishing of a bobsleigh/luge track may require as many as eight finishers for one nozzleman). Thus, anything that can be done to enhance the finishability of shotcrete can have significant impact on productivity and hence costs of a shotcrete operation. In this study, the wet-mix shotcrete mixtures were shot into 14 x 22 x 4 in. (355 x 550 x 100 mm) plywood boxes and finished using different hand-held finishing tools to evaluate the finishability of the different mixtures. The finishing tools used (in sequence from smoothest to most textured surface finish) are shown in Fig. 9 and were: steel trowel, magnesium trowel, wood float, hard rubber float, textured rubber float, and sponge float. The plain shotcrete mixture (WP) was relatively easy to finish with all the selected finishing tools, showing the expected sequence of smoothest to more textured surface finishes for the different tools, as described earlier. The mixture with synthetic fibers (WSF) proved to be the most difficult to finish, particularly with the tools producing more textured surface finishes, as they tended to pull fibers to the surface, leaving a rougher surface with many protruding fibers. Even the steel- and magnesium-troweled surfaces displayed some protruding synthetic fibers. This is attributed to the hydrophobic characteristics of the synthetic fiber. Some architects and engineers find this to be an annoyance and ask the contractor to burn off protruding synthetic fibers using a torch. Figure 10 shows an example of the WSF mixture finished with a magnesium trowel. Protruding synthetic fibers were present in the finished surface. By contrast, the mixture with natural fiber displayed superior finishing characteristics to both the plain shotcrete mixture and the synthetic fiber mixture. The treated natural fiber provided the applied shotcrete with greater cohesion 40 Shotcrete | Spring 2017 www.shotcrete.org


2017SpringShotcreteEMag
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