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2016SpringShotcreteEMag

Nozzleman Knowledge Cold-Weather Shotcreting By Raymond Schallom III The dry-shotcrete process has a long history with placing concrete pneumatically for over 105 years. Additionally, the wet-shotcrete process has been in use for over 50 years and has expanded to all 50 states and 120 countries. The shotcrete process has a wide variety of application uses, such as new construction, tunneling, mining linings, soil (geotechnical), and repair. These proven process methods have shown great versatility and efficiency in concrete placement. Shotcrete placement has been proven to save time and money over conventional form-and-pour methods. As the industry community begins to use these processes more widely, the one constant that hasn’t changed is the challenges faced during placement in cold and hot weather. This article will talk about cold weather shotcrete (concrete) practices. This author has performed and consulted on shotcrete work in temperatures down to –45°F (–43°C), which required close attention to heat protection of the mixtures during transport, placement, finishing, curing, and protection, as well as appropriate thermal clothing for the workers. Work has a tendency to slow down in the colder climate, and there are more safety issues to contend with, that require careful consideration of production while providing full protection of the placed material. These costs are usually worked out with the owner or general contractor well in advance of the cold weather to see if it is feasible to continue work or better to shut down during the winter months. The shotcrete industry and the American Concrete Institute (ACI) have made great strides to convince the engineering community that shotcrete is simply a placement method for concrete. Therefore, concrete—wet or dry—placed by the shotcrete process, is the same as the concrete covered in ACI 306R-10, “Guide to Cold Weather Concreting,” and ACI 306.1-90, “Standard Specification for Cold Weather Concreting.” ACI 306R-10 defines cold weather as: “Cold weather exists when the air temperature has fallen to, or is expected to fall below 40°F (4°C) during the protection period. The protection period is defined as the time required to prevent concrete from being affected by exposure to cold weather.” Concrete placed during cold weather will develop sufficient strength and durability to satisfy the intended service requirements when it is properly produced, placed, and protected. The necessary degree of protection increases as the ambient temperatures decrease. Because the shotcrete placement is often slower than form-and-pour operations and usually has one surface exposed to ambient temperatures, controlling internal and surface material heat becomes a critical task. One must protect the concrete from early-age freezing during the initial curing process until the compressive strengths reach at least 500 psi (3.5 MPa). At 50°F (10°C), most well-proportioned concrete mixtures reach this strength in a few hours or up to 48 hours in some cases. With shotcrete mixtures generally having a higher paste content and a lower water-cementitious materials ratio (w/cm), one can expect to reach the 500 psi (3.5 MPa) stage within a shorter time span— usually within 24 hours. ACI 506.2-13, “Specification for Shotcrete,” and ACI 506R-05, “Guide to Shotcrete,” both mention that while actively shooting shotcrete, the ambient temperature needs to be 50°F (10°C) or above. At 40°F (4.4°C), it can take concrete up to 16 hours to reach initial set, while the material temperature needs to be at least 40°F (4.4°C) to start the hydration process. Most DOT work uses the minimum of 50°F (10°C) for ambient, material, and surface temperature ranges unless completely protected and heated. This author has experienced many temperature variances over the years. The one that stands out that has been a challenge on a few projects is the rapid temperature changes within the repaired areas, particularly before the con 50 Shotcrete • Spring 2016


2016SpringShotcreteEMag
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