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2017SummerShotcreteEMag

| SHOTCRETE CORNER The next most important step required for quality blindside walls is using a placement-specific mixture design with properly spaced reinforcement, maintaining required reinforcing bar clearance from formwork and to the finished surface. The formwork must also be constructed to create proper access for concrete placement and consolidation. The only way to achieve all these steps to quality is by using a qualified concrete contractor who uses properly trained people, the correct placement equipment, and plans ahead with the on-site team to make sure the concrete is placed correctly. Everything stated in the previous three paragraphs applies completely to both form-and-pour and structural shotcrete placement. The needs for both processes are the same, and the quality of concrete placement with blindside waterproofing depends on the experience of the concrete contractor, whether it is shotcrete or form-and-pour. The opportunity and risk to place poor-quality concrete is equal between both methods (refer to Fig. 2(a) and (b)). The ability to achieve the required concrete quality for blindside waterproofing is also equal with both placement methods. Just as there are similarities, there are also differences between both placement methods: • Form-and-pour subjects the waterproofing to puncturing formwork industry slang for this is “vibrator burn,” which commonly occurs when a vibrator head snakes between the exterior reinforcing bar curtain and the form plywood, gouging out multiple layers of veneer from the plywood— this also happens to the blindside waterproofing on the outside of the wall 50% of the time! a) b) Fig. 2(a) and (b): Honeycomb and voids in form-and-pour (left) and dense sharp-edged structural shotcrete (right) 48 or splitting open seams from internal vibration. The • Most blindside form-and-pour is not placed using an elephant trunk or tremie drop pipe; instead, the concrete is dropped full height. This creates two major issues: 1. Impact energy commonly dislodges the waterproofing attachments, and the horizontal impact force from the “big blob” effect can tear open seams, especially at deviations in the shoring substrate. Additionally, segregation of aggregate and paste from dropping concrete through reinforcing bar (pinball effect) occurs on form-and-pour projects always! This commonly creates unconsolidated pockets of gravel at the bottom of wall joint to the floor slab, a critical location when it comes to blindside waterproofing and a watertight below-grade structure. Properly placed structural shotcrete employs the use of a blow pipe along this initial bottom of the wall pass, which basically eliminates the common form-and-pour rock pockets. Many cores and saw-cut testing of floor-wall joints have proven the superior performance of this method; and 2. Walls over 8 ft (2.4 m) tall are poured at a rate of usually 4 ft (1.2 m) high per hour to control form pressure, meaning the taller a wall, the more time to complete the pour. This delayed pour process creates a dried cement paste buildup on the blindside waterproofing, which can affect certain waterproofing systems’ bonding performance. I have been recently surprised to hear from major waterproofing suppliers, consultants, and installation contractors that they were unaware of this phenomenon! Properly installed structural shotcrete does not create this paste buildup issue. During lift placement and at the top of the wall, Shotcrete | Summer 2017 www.shotcrete.org


2017SummerShotcreteEMag
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