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2017SpringShotcreteEMag

poor shootability and would require higher dosages of set accelerating admixtures to ensure proper built-up onto the receiving surface particularly for overhead zones. MINIMIZING THE SET ACCELERATOR DOSAGE AND AVOIDING OVERDOSING The detrimental effect of overdosing set accelerators on shotcrete durability has been clearly demonstrated in literature (Jolin, Melo, Bissonnette, Power, and Demmard, 2015). In practice, the risk of overdosing is generally higher in the wet-mix process since the volume of accelerator is field controlled. In the dry-mix process, the set accelerator (when used) is pre-dosed/blended in dry form into the dry mixture before being used on site. In the dry-mix process, the plastic consistency needed for the desired material build-up is adjusted in real-time simply by adjusting the water content at the nozzle immediately before discharge. In contrast, in a highly plasticized wet-mix shotcrete, the material build-up during shotcreting is generally produced by the addition of set accelerator at the nozzle to achieve the desired stiffening/ hardening effect. In wet-mix application, the risk of overdosing set accelerator is increased particularly when shooting overhead zones that require a higher degree of stiffening/hardening for build-up while reducing the risk of fall-out. The use of supplementary cementitious materials such as silica fume and/or fly ash and rheology modifier additives help to improve the material build-up process, and therefore reduce the set accelerator demand. Fundamentally, the most efficient way to reduce set accelerator demand in wet-mix shotcrete, while ensuring both excellent pumpability and shootability, is the concept of temporary high initial air content, as illustrated in Fig. 4 (Jolin and Ginouse, 2012) developed over 20 years ago (Beaupré, 1994). As explained in Fig. 4, the high initial air content (10 to 20%) produces excellent pumpable material while ensuring good sag resistance and build-up once the material hits the surface. The air content typically drops to 4 to 6% during impact, which produces a “slump-killing effect” ensuring material consolidation without sagging or fall-out (Jolin and Beaupré, 2003). This concept significantly reduces the set accelerator demand to build thicker shotcrete layers per pass, particularly for overhead surfaces. In practice, accurate control and monitoring on-site of the set accelerator dosage being added to the nozzle is critical for wet-mix shotcrete to limit the risk of overdosing, and the detrimental effect it can have on later-age performance and durability. In comparison, the monitoring of set accelerator dosage is well controlled in the dry-mix process since the additives are pre-dosed/blended into the dry mixture before use on site. Mobile Shotcrete Production Unit For specific wet-mix projects, it is not always possible to order and receive wet-mix shotcrete from a ready-mix plant. Following is a partial list of potential situations when this is not possible: Fig. 5: Mobile self-loading mixer with bulk bag lifter • Projects in remote areas; • Small volumes; • Challenging schedules (difficult ground conditions, night shift, lane closures…); • Local ready-mix producers do not have the knowledge and/or experience to produce high-quality specialty shotcrete mixtures; • Limited availability of raw materials; • Shotcrete mixture needs to be mixed on-site due to a short pot life; and • Excessive dosage of hydration retarder and thus excessive accelerator dosage at the nozzle. For these types of projects, it is currently possible to produce shotcrete on-demand, on-site using new types of equipment and dry pre-blended materials. Dry preblended material is usually produced in manufacturing facilities ensuring consistent raw materials, proven batching records and strict quality control. Dry preblended wet-mix shotcrete material can be stored on-site in bulk tote bags, which are then available to be mixed on-demand using equipment such as the new mobile bag-lifting mixer (Fig. 5). A production ticket is printed and provided for each batch, allowing for the control and tracking of every batch produced. This innovative system helps to reduce material waste and facilitate logistics in remote areas with difficult access for standard ready-mix trucks. This type of system allows for the on-demand, on-site production of shotcrete minimizing the use of retardant and other admixtures traditionally used to offer logistical/delivery flexibility but also higher dosages of accelerator for overhead shotcreting and rapid strength gain. It is also possible to use a mobile bulk dry-mix shotcrete sprayer. Using a bulk dry sprayer allows the user to take advantage of the robustness, flexibility and durability provided by the dry-mix process shotcrete while guaranteeing www.shotcrete.org Spring 2017 | Shotcrete 21


2017SpringShotcreteEMag
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