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the shotcrete process uses a blow pipe to continuously clean the top of wall and the waterproofing as placement progresses. Using the blow pipe is much easier when shotcreting because there is no formwork protruding above the pour line. This would also make cleaning off the waterproofing membrane virtually impossible when using form-and-pour. • Another urban shotcrete myth is that cold joints are created from multiple lift placements. Refer to a previous Shotcrete magazine article, “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints,” by Charles Hanskat (www.shotcrete.org/media/Archive/2014Spr_Technical Tip.pdf), which dispels this myth and explains how proper shotcrete lifts are achieved. Form-and-pour is also placed in lifts but the risk of a cold joint is certainly greater and happens when concrete placement is delayed by pump or crane malfunctions, or just simply a late concrete truck, which happens often in a busy city. Once the vibrator cannot penetrate the previously placed lift of cast concrete, a cold joint occurs. This is a blindside waterproofing issue that does not happen with properly placed structural shotcrete. • Liquid head pressure within formwork can reach 600 to 800 lb/ft2 (2900 to 3900 kg/m2), exerting extremely high lateral pressure on the waterproof membrane, and forcing it against uneven shoring substrates and the sharp anchor points. This often damages the waterproofing. Structural shotcrete exerts only 15 to 25 lb/ft2 (73 to 120 kg/m2), helping the waterproofing to bridge irregularities and not warping the smoothing materials added behind the waterproofing in extreme conditions. • One-sided forms require form tie anchorage to resist form pressure created by each placement lift (refer to Fig. 3). Many projects don’t use “A-frame” one-sided forms due to availability, cost, height, shoring raker/strut projections, or complex shapes. The only alternative is to install numerous formwork tie anchor points attached to the shoring substrate, creating many penetrations in the waterproofing. This requires a lot of attention to detail for successful performance, and many opportunities for the penetration to leak. The structural shotcrete process requires far less anchorage to the substrate. Shotcrete placement is comparable to safely restrained reinforcing bar curtains used with “A-frame” formwork. These reinforcing bar anchorage points are at much greater spacing than form tie anchorage. Having a greatly reduced number of penetrations in the waterproofing substantially reduces the chances of leakage in the system. SITE CONDITIONS AND BUILDING DYNAMICS Many different site conditions can affect the performance between the cast-in-place concrete blindside walls and the waterproofing system, depending on Fig. 3: Form-and-pour requires far more form tie anchor penetrations in the blindside waterproofing than reinforcing bar stabilization for shotcrete which type of waterproofing is used. The following should be taken into consideration when selecting a blindside waterproofing system: • Shoring subsidence, especially with wood lagging, where the hand-packed fill material settles behind the lagging or is washed out by groundwater flowing down the backside of the lagging, carrying the fines away, and creating a loss of density, resulting in movement that decreases the confining pressure between the concrete wall and the shoring substructure. This can be an issue for some types of waterproofing that rely on confining pressure. • Differential settlement can occur between the building and the shoring system, caused by the building compressing the ground beneath and the anchored shoring system staying in place. This shearing movement can negatively affect the blindside waterproofing if not planned for. • Substrate smoothness is always a factor and can affect the performance of any blindside waterproofing, except for integral waterproofing admixtures. BLINDSIDE WATERPROOFING TYPES A brief and simple explanation of blindside waterproofing can be broken down into five types of membranes, either sheet- or liquid-applied: 1. Unbonded hydrophilic sheet membranes, such as bentonite, swell when exposed to water and permanently rely on the confining pressure between the exterior concrete wall face and the shoring substrate. 2. Mechanically bonded systems rely on a fabric or hairlike structure to bond with the concrete wall during initial set. This in turn is attached to various waterproofing materials, including bentonite sheeting. www.shotcrete.org Summer 2017 | Shotcrete 49


2017SummerShotcreteEMag
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