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2014FallShotcreteEMag

type of equipment used, the water-binder contact Based on work previously conducted by Lam- comes only a fraction of a second before placement. beret (2005), numerous mixture formulations were This particularity enables this process to bypass tested. The stability was evaluated using standard workability issues usually associated with the mixing 2 in. (50 mm) cubes incorporating only fine aggre- and pumping of regular concrete. Figure 1 shows a gate (sand). Each mixture had the same proportions schematic of a typical dry-mix shotcrete setup. and the same water-cementitious material ratio Since materials are introduced in the dry state (w/cm). To ensure sufficient workability for casting, in the hose, production of dry-mix shotcrete usu- a polycarboxylate high-range water-reducer-super— ally takes advantage of pre-bagged material. This plastisizer was used. The admixture dosage varied enables easier quality control of the material as based on the formulation as the efficiency was the production of such bags is done in a controlled influenced by the major component of the binder environment. (OPC or CAC). Table 2 presents the mixture design proportions (by weight) used in Phase 1. Comparison of Both Processes The difference between each mixture was the Each process has its own advantages and dis- proportion of OPC, CAC, and CS of the paste. All advantages. Wet-mix shotcrete is usually used cubes were cured in a 100% relative humidity only when high volumes of shotcrete are needed room at 82.4 ± 1.8°F (21 ± 1°C). Forms were due to the important mobilization needs. On the removed as soon as possible and depending of the other hand, while the dry-mix process cannot setting of the mixture could be as soon as 2 hours. reach the production rates of the wet-mix process, its mobilization costs, preparation time, and flex- Phase 2 ibility are key factors in its selection. The main objective of the project consisted in Table 1 presents the major differences between realizing a conventional dry-mix shooting session both processes. of a special mixture based on a ternary binder using standard dry-mix equipment and techniques (ACI Development of New Shotcrete 2005, 2009). To ensure consistent shotcreting and The development of a new and alternative adequate quality control, the shotcreting took place binder to Portland cement has opened the field of at Laval University’s shotcrete laboratory. The cementitious binders to a whole new variety of laboratory is equipped with full-scale shotcreting binder. One of those binders exhibiting very equipment in an indoor environment that is tem- promising properties is a binder composed of perature controlled. The shotcrete was sprayed using OPC, CAC, and CS (Lamberet 2005). This type a rotating barrel ALIVA 246 with a 1.5 in. (38.1 mm) of mixture shows very rapid hardening behavior interior diameter hose with the water ring placed but is unfortunately often accompanied with 10 ft (3 m) upstream from the outlet of the nozzle— important workability issues. With a pot life of also known as the hydromix nozzle (Fig. 2). less than 10 minutes, regular cast-in place appli- The mixture was shot indoors at an average cation of this type of concrete is impossible temperature of 70°F (21°C). The shotcrete labora- without the use of numerous chemical admixtures tory’s equipment includes an electronic air flow to control set and fluidity. A means to avoid this meter, a water flow meter, and a set of electronic type of problem was sought and this is where scales; the data acquisition system records the dry-mix shotcrete becomes a very interesting airflow, the water flow, and the material used process since water is added only a fraction of a during the spraying operations (Fig. 3). The tar- second before impact onto the receiving surface, geted airflow is 180 ft3/min (4.25 m3/min) at a avoiding workability issues. The validity of such working air pressure of 100 psi (700 kPa). The a hypothesis was part of a research project at Laval water flow was adjusted (by the operator holding University (Lemay 2013). the nozzle) to what is referred to in the industry as the wettest stable consistency. The evolution Research Program of the water and material flows are used as a The research program was divided in two quality control point. Irregular or non-uniform phases. The first part focused on the mixture flows lead to the rejection of a mixture. composition and the second on an actual life-size Prior to the shotcreting operation, 1200 lb equipment testing. (780 kg) of dry shotcrete material was pre-weighted and pre-blended to allow for the production of the Phase 1 various panels and samples needed for testing. Table Ternary binder made of OPC, CAC, and CS 3 shows the dry proportion of the pre-blended material. can present expansion if the mixture design is not Table 2: Mixture design for Phase 1 mixture properly done. The expansion can be such as to Water, % Paste, % Aggregate, % ruin the concrete (or shotcrete) and so, must be correctly assessed. 7.7 17.5 74.4 16 Shotcrete • Fall 2014


2014FallShotcreteEMag
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