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2016SpringShotcreteEMag

Pool & Recreational Shotcrete Corner As time passed through the 1950s and on until the ’80s, novice (but high-volume) contractors kept entering the field having little or no exposure to the original quality control guidelines enforced by the Cement Gun Co. From the early 1950s on, those rigorous procedures were effectively and increasingly watered down. By the early ’80s, the company’s standards, data, and documentation were little more than a memory among a few old-timers who had managed to stay active. It was not, in sum, a good situation. In addition, a geographical divide soon emerged as some of the “original” gunite contractors on the East Coast became increasingly suspicious of the newcomers out West, where shotcrete was just beginning to gain traction. The perceived need to preserve a competitive edge created an environment in which knowledge sharing and discussion of best practices in the American shotcrete industry effectively ceased. Without standards or guidance, much began to slip: Quality was sorely lacking in many installations, and the former field workers who’d risen in the business and were now owners of their own companies rarely understood what went into “good” shotcrete application. Even today there are contractors who still have not embraced proper practices. As we’ll see in the following, a whole range of substandard methods emerged in these difficult years that threatened the reputation of the shotcrete process as the 20th century entered its last years. To this day, in fact, directly addressing and effectively contradicting poor shotcrete application is one of the primary purposes of both the American Shotcrete Association (ASA) and ACI. Best Practices What are those standards? Why are they so important? Water-Cement Ratio Let’s start with proper mixture design and its key component: the water-cement ratio. In a good mixture design, you’ll typically find a water-cement ratio of 0.35 to 0.45 (0.30 is a good lower boundary for dry-mix shotcrete). The binder in this mixture—that is, cement paste—is portland cement. If a contractor wants to cut costs and carries no claims to pursuing quality, he or she will increase the water content of the mixture while reducing the volume of portland cement. The use of this “water of convenience” and reduction in cement leads to substandard Fig. 2: Nozzleman works in tandem with other trained crewmembers to ensure a successful application results and a cheaper product in more ways than one. Use of Aggregate As another example, let’s consider the ratio of aggregate (sand, gravel) to the cement paste/binder: minimally, there should be four parts of aggregate to one part cement—and, ideally, a three-to-one ratio. Cost-cutting contractors (and sometimes even engineers) will alter that ratio to five to one or even six to one, using lots of aggregate and minimal cement paste. Using this questionable cost-cutting model, the production of rebound and overspray inevitably increases—and these jobs are executed well below the specifier’s original expectation for material composition and strength. Rebound and Overspray Many of these substandard operators also use the rebound and overspray as “filler” in the concrete structure. Rebound and overspray are worthless— binder-free material that has bounced off the receiving substrate. Using this material in any way fundamentally weakens the concrete wherever it is used. Shotcrete • Spring 2016 43


2016SpringShotcreteEMag
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