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2017SpringShotcreteEMag

Fig. 10: Synthetic fiber mixture WSF finished with a magnesium trowel and appeared to act as a finishing aid. This resulted in relatively smoother-textured surface finishes for all the finishing tools used, particularly when compared against the mixture with synthetic fiber. Virtually no fibers were present in the finished surface of the WNF mixture finished with steel and magnesium trowels. Figure 11 shows an example of the WNF mixture finished with a magnesium trowel. Very few fibers were drawn to the surface, or were evident in the panels finished with wood, rubber, or sponge floats. The greater ease of finishing for mixtures incorporating the natural fiber has very positive implications for shotcrete productivity. Hardened Shotcrete Tests Tests were conducted on cores extracted from hardened shotcrete test panels for determination of: a. Compressive strength to ASTM C39; b. Boiled absorption (BA) and volume of permeable voids (VPV) to ASTM C642; and c. Rapid chloride penetration to ASTM C1202. Test results for the hardened shotcrete properties are provided in Table 4. With respect to compressive strength, all three mixtures readily meet the CSA A23.1 minimum design strength of 5100 psi (35 MPa) at 28 days for concrete with a Class C1 exposure. Clearly, natural fiber addition does not have any Fig. 11: Natural fiber mixture (WNF) finished with a magnesium trowel detrimental effect on compressive strength. The mixture with natural fibers displayed the highest compressive strength at both 7 and 28 days of all three mixtures tested. With respect to tests for BA and VPV, all three mixtures had values well below the typical maximum of 8% for BA and 17% for VPV for quality shotcrete with nonporous aggregate with normal cement factors as recommended in ACI 506R-16, “Guide to Shotcrete.” Natural fiber addition does not have any detrimental effect on permeability. The mixture with natural fiber displayed the lowest BA and VPV values of all three mixtures tested. With respect to the tests for rapid chloride penetration (RCP), CSA A23.1 requires a maximum allowable value of 1500 Coulombs at 91 days for a concrete with Class C1 exposure. With test results for all three mixtures in the 898 to 1037 range, ASTM C1202 and CSA A23.1 would rate such shotcretes as having “Low” chloride penetrability. The addition of natural fiber to the mixture is not detrimental to the chloride penetrability of the shotcrete. FIELD EVALUATION Mixture Designs and Shotcrete Batching, Mixing, and Supply The second phase in the evaluation of the use of the natural hemp-based fiber in wet-mix shotcrete was to assess its Table 4: Hardened shotcrete properties Mixtures Fiber content Compressive strength Boiled absorption Volume of permeable voids RCP results at 91 days kg/m3 lb/yd3 % volume 7 days, MPa 7 days, psi 28 days, MPa 28 days, psi % % Coulombs WP 0 0 0 35 5075 47.9 6950 5.3 11.9 966 WNF 2 3.4 0.15 43.7 6340 57.6 8355 5.1 11.5 1037 WSF 1.35 2.3 0.15 36.6 5310 51.7 7500 5.4 12 898 Design/specified value: 30 4350 40 5800 max. 8.0% max. 17% 1500 www.shotcrete.org Spring 2017 | Shotcrete 41


2017SpringShotcreteEMag
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