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2014FallShotcreteEMag

Shotcrete FAQs As a service to our readers, each issue of Shotcrete will include selected questions and provide answers by the American Shotcrete Association (ASA). Questions can be submitted to info@shotcrete.org. Selected FAQs can also be found on the ASA website, http://shotcrete.org/pages/products-services/technical-questions.htm. Question: We are replacing an undersized box culvert carrying Answer: The answer is no; shotcrete will bond well to sand- a creek under a road with a vehicular bridge. To reduce blasted reinforcing bar on overhead or vertical applications. excavation limits, we are using top-down caisson wall construction Shotcrete, like conventionally placed concrete, can be placed with shotcrete facing between caissons for abutments and over uncoated black bar or bar that is coated with rust inhibitors. wingwalls. The shotcrete will be placed in lifts as soil is In repair areas where there is heavy scale on the reinforcing excavated between abutment/wingwall caissons. bar and spalling of the concrete, the repair can sometimes The architectural pattern for the face of the abutments and include some sort of reinforcing bar treatment or inclusion of wingwalls is a rectangular pattern of an indented, V-shaped a rust inhibitor in the shotcrete mixture. It depends on the notch. The notches have a maximum depth of 2 in. (51 mm). situation and the assessment of the design engineer as to what The structural portion of the shotcrete wall will be 12 in. (305 mm) is necessary. thick with steel reinforcement. Can this horizontal and vertical V-notch pattern be formed or stamped into the face of the Question: Is shotcrete applied to hardened cast-in-place concrete structural wall (with additional thickness as required for considered monolithic by the American Concrete Institute pattern) in one wall placement? Or does the pattern have to (ACI)? Is shotcrete-to-shotcrete considered monolithic by ACI? be a separate placement after the structural wall is cured? Can either of these connections be made watertight? Or at least If this is done in two placements, I assume that we would as watertight as the concrete? Is shotcrete without admixtures need reinforcing bars from the structural portion of the wall truly watertight or waterproof? (My definition of “watertight” into the architectural placement and reinforcement within the would be a measurable amount or more than leaching of architectural placement to lock it in place. What is the minimum moisture and calcium.) required thickness of the architectural layer to account for reinforcing bar embedded from the structural layer and the Answer: Shotcrete properly applied to a well-prepared existing required reinforcing bar in the architectural layer? concrete surface will create an excellent bond and structurally act as a monolithic system without joints or layers. Pulloff Answer: There are many ways to approach this situation. It testing of shotcrete applied to concrete will often fail in the would be difficult, but not impossible, to install all of the work underlying concrete substrate, and not at the bond interface or in a top-down sequence and end up with an architecturally within the shotcrete section. uniform surface. Shotcrete is a method of placing concrete and should have Approach 1: Install a minimal initial layer top-down with similar watertightness characteristics. These characteristics can either fibers or welded wire reinforcement. Install dowels from be enhanced with admixtures and supplementary cementitious the caissons into the structural facing layer. Install the facing materials such as silica fume. Please refer to the images of from the bottom up with preplaced V-strips to make the pattern. cores from shotcrete applied to existing concrete. Finish to the outermost face of the detail strips. Alternately tool the details, but likely more like 1 in. (25 mm) instead of 2 in. (51 mm). Approach 2: Install the structural wall top-down, encapsulating the outer reinforcing steel to a plane at the depth of the detail strips. Prepare the surface by sandblasting or water blasting to create a favorable bonding surface. Install detail strips to the face of the roughened wall. Place and finish the finish layer to the depth of the detail strips. If the base layer is properly prepared, the bond should be very good and adding dowels would be redundant. There is nothing wrong with redundancy and if so, the minimum layer thickness would be 2 to 3 in. (51 to 76 mm). The nature of this work will mandate the use of a highly qualified shotcrete subcontractor who has experience in installing similar-quality architecturally significant walls. Question: Do we need to coat reinforcing steel after sand- blasting and prior to placement of product? 62 Shotcrete • Fall 2014


2014FallShotcreteEMag
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