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2014FallShotcreteEMag

The following article originally appeared in Deep Mining 2014, published by the Australian Centre for Geomechanics, www.acg.uwa.edu.au. Ultra Rapid Strength Development in Dry-Mix Shotcrete for Ultra Rapid Support in Challenging Mining Conditions By Jean-Daniel Lemay, Marc Jolin, and Richard Gagné Rtraditional binder systems. Engineers acceptance. First presented to the construction-crete placement process developed at the beginning of the last century that has gained globalecent developments in the field of cementingmaterials have brought forward many non- involved in the fields of emergency repairs and industry in 1910 by Carl Akeley (American Con- rapid ground support have been on the lookout crete Institute ACI 2005), shotcrete is now for materials that allow rapid production, place- present everywhere, particularly in the mining ment, and, most of all, very rapid strength devel- and tunneling businesses. opment kinetics. One binder system that fits the Shotcrete (also known as sprayed concrete in description is composed of ordinary portland many parts of the world) is a placement method cement (OPC), calcium aluminate cement (CAC), for concrete. Shotcrete is defined as a mortar or and calcium sulfate (CS). However, this type of concrete pneumatically projected at high velocity binder also sometimes exhibits difficult work- onto a surface (ACI 2005). The high velocity is ability that severely limits its use in regular cast essential to the process as it ensures an adequate in-place concrete. This limitation is overcome compaction or consolidation and allows the mate- when using dry-mix shotcrete as a placement rial to stick to the sprayed surface. Without the method. Since the contact between water and proper compaction, quality shotcrete cannot be cement occurs in the nozzle immediately before produced (ACI 2009). placement, workability problems are avoided. As with all placement methods, different As a part of a graduate project at Laval University, advantages of shotcrete make it more appropriate 49 different mixtures, including simple, binary, and in certain situations. With its capacity to stick to (mainly) ternary blends, were tested. Two major vertical and overhead surfaces, shotcrete need parameters were studied, the development of com- only minimum, if any, formworks. To take full pressive strength and the volumetric stability. The advantage of this specificity, shotcrete is best used numerous binder compositions tested allowed the in ground supports (tunnel, mine, slope stabiliza- selection of a stable optimized formulation in regard tion, etc.), overhead civil work reparation (bridge to early compressive strength and volumetric sta- deck and parking lower surface, etc.), and where bility. Finally, the selected formulation was success- complex formworks make cast-in place concrete fully tested with industrial dry-mix shotcrete very expensive (arc type bridge, circular column equipment to verify the large-scale placement fea- repairs, etc.). sibility of such a product. Shotcrete can be produced using two different processes, the wet-mix and the dry-mix. The Introduction major differences between these processes are the Transport and placement methods for concrete conveying method of the material through the have evolved tremendously throughout history. hose and the location where the water is added to Although cast-in place concrete is still the most the mixture. common placement method, other processes have been developed; let us simply think of the first Wet-Mix Shotcrete concrete pump, self-leveling concrete, or roller In the wet-mix process, all the ingredients are compacter concretes. There is also another con- mixed together before being pumped through the 14 Shotcrete • Fall 2014


2014FallShotcreteEMag
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