Page 36

2013FallShotcreteEMag

Sustainability experimental Procedure a load cell to measure rebound. The material was The raw materials were dried, weighed, also shot into two standard ASTM C1604/ blended dry, and packaged in the laboratory using C1604M14 wood panels measuring 24 x 24 x 3.5 in. ovens and the sun for drying, bench-top scales for (610 x 610 x 90 mm) to extract cores for compres- weighing, a concrete mixer for blending dry sive strength, boiled absorption, volume of per- ingredients together, and plastic bags for pack- meable voids, chloride ion permeability, and aging. Roughly 530 lb (240 kg) of each dry density testing. mixture was prepared at the King Packaged Materials Facility in Blainville, QC, and shipped experimental results to Laval University in Québec City, QC, for Table 2 presents the results achieved during testing. The world-class shotcrete testing facility the testing phase. at Laval University features mass balance moni- toring for all inputs, including material weight, Discussion water, and air, in addition to an accurate rebound The water-cementitious material ratio (w/cm) measurement system and a fully equipped concrete of the fresh shotcrete for the control mixture was testing lab. A certified nozzleman placed the 0.33 compared to 0.52 for the sustainable mixture. samples. An Aliva 246 dry-mix shotcrete machine Both of these were determined by taking the was used for testing, using 66 ft (20 m) of a 1-1/2 initial mass and then drying freshly shot shotcrete in. (38 mm) inside diameter, 2 in. (50 mm) outside in a microwave until a constant mass is achieved. diameter hose, with a double-bubble type nozzle The ratio is then calculated using the mixture’s and a water ring immediately at the end of the in-place cementitious content, measured by hose, right before the nozzle. A hydromix washout over an 80 mm sieve immediately after assembly was not available during the test period, shooting. The in-place cementitious content was but would have offered longer and better mixing. higher for the control mixture, as a result of the The machine, the operator, and the material were higher rebound. This method does not distinguish all located on a large scale to weigh the outgoing between the water consumed to hydrate cement shotcrete material. The material was shot against and the water absorbed by the aggregates. Blast- a steel panel with a beveled edge that hangs from furnace slag aggregates absorb between 6 to 9% water by mass, as compared to 0.6% for natural sand.12 It follows that a significant part of this water content can be attributed to absorption by the slag aggregates and that the actual w/cm is less than 0.52 for the sustainable mixture. The air flow for the control mixture, at 196 ft3/ min (5.55 m3/min), was higher and closer to the normal air flow rate for standard shotcrete mix- tures with the selected equipment than the sustain- able mixture, which was 136 ft3/min (3.85 m3/ min) or 30.7% less. The reduced air flow rate for the sustainable mixture was a result of unfamil- iarity of the shooting performance of the mixture. The all-slag mixture was found to be stable at a higher w/cm, masking the requirement for a higher air-flow rate to produce a denser and stronger hardened shotcrete. Because the slag particles are smooth and more slippery within the cement matrix than natural sand, they should decrease the water demand to achieve a proper fresh shotcrete consistency. Yet, because the particles are porous, Fig. 2: Aliva 246, material and operator on scale (top left), double bubble they tend to absorb more water than natural con- nozzle (top middle), short hydromix nozzle assembly (top right), shotcrete crete sand, offsetting the more fluid consistency. panels (middle left), rebound panel (middle middle) and shooting area The mixing time was also shortened, due to the overview (middle right), fresh control mixture on rebound panel (bottom unavailability of a long hydromix nozzle left), measuring buildup (bottom middle), shooting control mixture panel assembly, resulting in a less than ideal mixing (bottom right) time for either mixture. 34 Shotcrete • Fall 2013


2013FallShotcreteEMag
To see the actual publication please follow the link above