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2015SummerShotcreteEMag

Nozzleman Knowledge Hot-Weather Shotcreting By Andrea Scott Tback over 100 years. It has a wide variety of can negatively affect your finished product. The(38°C) with humidity levels of less than 10%.This “dry heat” can add to the list of things thathe shotcrete process has a long history, withthe pneumatic application of concrete going applications that include new structures, tunnel rapid moisture loss caused by this environment and mine lining, soil, and repair. Shotcrete is a is much greater than when working in similar versatile and efficient method of placing concrete temperatures with high humidity. These harsh that can save clients a great deal of time and conditions are inescapable, as projects need to money. As its use in the construction industry be completed despite the extreme working con- continues to grow, new innovations are constantly ditions. The factors that influence the quality and expanding the realm of what can be done with durability of the finished product are many, in this process. One thing that hasn’t changed are this article they will be discussed as they pertain the challenges faced with shotcrete placement of to the wet-mix shotcrete process. Let’s start with concrete in hot weather. high ambient temperatures. As shotcrete is simply pneumatically placed Mixed concrete is unforgiving at high tem- concrete, many of the factors that apply to the peratures. A concrete mixture that has a set time placement of quality shotcrete will be the same as of two hours at 70°F (21°C) will set up in one those discussed in the American Concrete Insti- hour at 80°F (27°C). A good rule of thumb is that tute’s (ACI) document ACI 305.1-14, “Specifica- for every 10°F (5.6°C) increase, your set time will tion for Hot Weather Concreting.” The ACI decrease by 50%. It is often recommended, but 305.1-14 specification defines hot weather as: seldom practical to provide shade or cooling “one or a combination of the following misters to large areas of in place work. The forms conditions that tends to impair the quality and grade where the concrete is to be placed of freshly mixed or hardened concrete by should be moistened and kept cool to give the accelerating the rate of moisture loss and benefits of evaporative cooling, as well as not rate of cement hydration, or otherwise adding or subtracting moisture from the concrete. causing detrimental results: high ambient Some of the steps that can be taken to lower temperature, high concrete temperature, low ambient temperature can be as simple as watching relative humidity, and high wind speed.” the weather report and choosing a cooler day of Because shotcrete placement can be slower the week for placement of shotcrete, or more than cast-in-place and has one face of the fresh commonly, by adjusting the start time of the work shotcrete exposed, controlling temperature shift. With projects underway during the hottest takes on heightened importance. The shotcrete weeks of the warm summer months, our crews contractor also has to contend with moisture often work at night to take advantage of the lower loss from the nozzle spray. Some of the problems temperatures (Fig. 1). With temperatures falling when dealing with plastic concrete under as much as 20°F (11.1°C), the difference in pro- extreme weather conditions may include duction is clearly noticeable. High ambient tem- increased rate of slump loss, concrete tempera- perature does have a profound effect on employees. ture, and water demand, as well as additional In high-heat conditions, the productivity of the plastic shrinkage cracks and crazing. When you workers drops. By changing the start times to the add in a shortened setting time, the stakes just coolest part of the day, you can minimize crew get higher. fatigue and increase productivity. After a certain Working with shotcrete in the arid American point on the thermometer, the rate of production Southwest during the brutal summer months can actually seems to be inversely proportional to the be daunting. It is not unusual for crews to expe- temperature. This is particularly true the first few rience daytime temperatures in excess of 100°F weeks after a sudden increase in temperatures. 32 Shotcrete • Summer 2015


2015SummerShotcreteEMag
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