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2017SpringShotcreteEMag

the 1.0 yd3 (0.76 m3) pan mixer. Such mixers are commonly used in the precast concrete industry but are also very useful for laboratory studies. Weighed-out quantities of the loose natural and synthetic fibers were added to mixtures WNF and WSF and mixed for a minimum of 5 minutes. The quantities of water-reducing and air-entraining admixtures added were adjusted as necessary to produce the required slump and air content. The mixed shotcrete was discharged into a Putzmeister shotcrete pump, which conveyed the shotcrete in a 2 in. (50 mm) internal diameter rubber hose to the nozzle, where air was introduced from a 185 ft3/min (5.24 m3/min) air compressor to pneumatically project the shotcrete at high impacting velocity onto the receiving surface. The fibers appeared well dispersed in both fiber mixtures and the mixtures shot well with no problems of fiber balling or pump blockages being encountered. Shotcrete application was done by an ACI Certified Shotcrete Nozzleman. The nozzleman shot standard vertical test panels with sloped sides, from which cores were extracted for testing for: a. Compressive strength at 7 and 28 days to ASTM C39; b. Boiled absorption (BA) and volume of permeable voids (VPV) at 28 days to ASTM C642; and c. Rapid chloride penetrability (RCP) at 91 days to ASTM C1202. In addition, two molds were shot horizontally for each of the fiber mixtures for production of test specimens for ASTM C1579, “Standard Test Method for Evaluating Plastic Shrinkage Cracking of Restrained Fiber Reinforced Concrete (Using a Steel Form Insert).” Figure 3 shows the configuration of the test mold. Figure 4 shows shooting into the mold. Plastic Shotcrete Properties The temperature, slump, and air content of the plastic shotcrete was tested at the point of discharge into the shotcrete pump. In addition, the as-shot air content was determined by shooting the shotcrete into a standard ASTM C231 air pressure meter bowl. Test results for all three mixtures are provided in Table 3. The slump for all three mixtures was in the design range of 2 to 3 in. (50 to 75 mm). The slightly higher-than-design air content in the as-batched WNF mixture is attributed to the effect of the higher waterreducing admixture dosage in this mixture. In addition to the standard tests, the plastic shotcretes were assessed for characteristics such as shootability (adhesion and cohesion and resistance to sagging and sloughing) when applied to a vertical surface. Figure 5 shows shooting a beehive (extended thickness mounded in center of shotcreted area) of the plain concrete mixture (WP) on a vertical plywood panel. While all three mixtures shot well and could be built up to a 6 in. (150 mm) thickness on a vertical plywood form in a single pass, some marked differences were found in the shooting characteristics of the mixtures. The plain WP mixture displayed the lowest adhesion and cohesion performance. Shortly after being built up to the full 6 in. (150 mm) thickness, it started to sag, and then sloughed Fig. 2: Prepackaged dry shotcrete materials discharging into pan mixer Fig. 3: Configuration of plastic shrinkage cracking test mold (Note: 1 mm = 0.0394 in.) Fig. 4: Shooting into mold for plastic shrinkage cracking testing 38 Shotcrete | Spring 2017 www.shotcrete.org


2017SpringShotcreteEMag
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