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2017SummerShotcreteEMag

on lift pumps to discharge into the city sewer mains. This will become more common as cities grow because existing storm or combined storm/sanitary sewers are not designed for the increasing density and are becoming overloaded and nearly impossible to upgrade. The solution is to “bathtub” waterproof the below-grade structure, meaning not just the blindside walls but also the basement floor. This bathtub, or essentially a concrete tank, resisting the outside groundwater with no hydrostatic drainage relief, increases the need for more competent blindside wall waterproofing. I see using structural shotcrete in combination with a proper blindside waterproofing system as a solution over traditional form-and-pour to address these new challenges by creating watertight and dry, below-grade concrete structures in our growing cities. There is a wealth of information available on blindside waterproofing for both form-and-pour and structural shotcrete. Unfortunately, much of this information is written based on a long history of using form-and-pour and with a lack of knowledge of the structural shotcrete process. As a result, the information on the finished product typically relates to traditional form-and-pour. Also, most information and articles have been written from the waterproofing side, with at times the obvious lack of knowledge of concrete construction methods regardless of form-and-pour or structural shotcrete methods. The goal of this article is to address and clarify the similarities and differences of both these methods of placing concrete in reinforced concrete structures while focusing on blindside walls below-grade to dispel the myths and show the advantages of using structural shotcrete in building construction. To compare and show the advantages of structural shotcrete, the following points need to be addressed: • Shotcrete is concrete; • Form-and-pour versus structural shotcrete; • Site conditions and building dynamics; and • Types of blindside waterproofing. SHOTCRETE IS CONCRETE Many times, the structural shotcrete process is referred to as a product, as though we are substituting an alternative concrete mixture for ready mixed concrete. Builders and consultants need to realize that the concrete mixture used in wet-mix shotcreting is very similar. Whether for a small-diameter line pump or in a heavily reinforced, high-strength core wall, both require reduced aggregate size and a higher cement content. The only difference is that when they use these mixtures in form-and-pour placements, they need to have higher slumps, as compared to shotcrete placement that needs a lower slump (with the benefit of a lower watercementitious materials ratio w/cm) depending on the stack rate, reinforcement density, and wall thickness. Given that shotcrete is concrete, the next typical question is, “What are the differences in construction joints, details, and sequencing?” The answer is, “none.” Construction joints, control joints, and placing sequences are not altered and use the same construction details as traditional form-and-pour. Structural shotcrete experiences drying shrinkage as it matures and gains strength, the same as form-and-pour. This rate of concrete material shrinkage varies based on many factors. The major influences are aggregate size, w/cm, and curing. There are several ways to design for this shrinkage, including admixtures, reinforcement, and control joints. In the end, concrete cracks in different ways, sizes, and spacing, and at times induced by the overall structure settlement. So, comparing the slight difference between concrete mixtures placed by form-and-pour and structural shotcrete to see whether the cracking occurs at 12 ft (3.7 m) on center or 14 ft (4.3 m) on center doesn’t really matter when comparing blindside waterproofing options. Stopping water flowing from any crack regardless of spacing is the goal, not by what method the concrete was placed. When making critical blindside waterproofing choices, it is important for the waterproofing and building construction industries to address concrete crack control by making sure that control joints are properly detailed and spaced with adequate reinforcement. Another factor I see that is rarely taken into consideration when choosing a blindside waterproofing system is linear shrinkage of the overall structure below grade. Proof of this often-overlooked movement is that buildings over a certain length commonly have delay strips or gaps designed into the structure to allow for contraction caused by shrinkage and are only filled in 28 days later. Common sense says that this shrinkage occurs in both horizontal directions regardless of the length, and pulls inward toward the center point of the structure, not just one way from the delay strip. This inward shrinkage, mostly restrained by the floor slabs, decreases the confining pressure against the excavation shoring substrate, which in turn can create issues for blindside waterproofing systems requiring confining pressure to work. STRUCTURAL SHOTCRETE VERSUS FORM-AND-POUR I have found when introducing the relatively new shotcrete process for placing concrete in blindside walls that the builders, engineers, and especially the waterproofing industry really don’t fully understand how the process works, nor the finished product. Because of this lack of overall knowledge and myths about the shotcrete process, and at times even the process of concrete placement for form-and-pour, incorrect statements and decisions are made when considering blindside waterproofing. The number-one factor and myth to dispel is that it’s all just about the nozzlemen. Over my 40 years of experience in the form-and-pour industry, it’s also been all about the vibrator man. The skill and necessity of these designated skilled persons is the same for either placement method, and the first major step to achieving high-quality cast-in-place concrete blindside walls. Proper compaction is essential. www.shotcrete.org Summer 2017 | Shotcrete 47


2017SummerShotcreteEMag
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