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2016SpringShotcreteEMag

Technical Tip Reinforcing Bar Shadowing and What You Can Do By Joey Bell Nearly all shotcrete projects I come across in my travels consist of some form of steel reinforcement, such as reinforcing bars or welded wire reinforcement. It is sometimes overlooked how important reinforcement encapsulation can be on the job. Be it aesthetic, structural, or even safety reasons, preventing voids behind steel reinforcement is important on every job. If reinforcement is not properly encased, there are many negative effects on the structure and the potential for corrosion is increased. When shotcrete is sprayed against reinforcing bar, a “shadow” area occurs behind the steel because the bar blocks the shotcrete material stream. The size of the shadow increases with an increase in the diameter of the bar. The shotcrete in the shadow area behind the bar is not compacted because the stream of material does not directly impact the shadow area. This shadow area is filled when using good shotcrete shooting technique to provide full encapsulation of the bar. This requires the nozzleman to keep sufficient velocity of the material stream to wrap fresh paste around the bar and fill the shadow areas. With poor velocity (too far away from the bar or too low an air flow), too dry or stiff a material, or inattention to proper shooting techniques, voids can develop. Nozzle Angle and Slump If a mixture is too stiff (wet-mix), too dry (drymix), or does not have sufficient impact velocity, the material will build up on the face of the reinforcing bar, creating a larger obstacle for the shotcrete spray to wrap around, thus forming a void behind the reinforcing bar. The stiff mixture, to some degree, can be overcome by using a highimpact velocity to cause the material to flow around the bar. Reducing the distance to the receiving surface will increase the velocity. In most applications, a distance of 2 to 4 ft (0.6 to 1.2 m) is advisable for adequate velocity. Also, adding more air at the gun (dry-mix) or nozzle (wet-mix) can increase impact velocity. Wet-mix shotcrete should have a slump of at least 2 to 3 in. (50 to 75 mm) to properly encase reinforcing bar. The key to good encasement is a good paste content with proper plasticity. When encasing bars that are larger than No. 6 (No. 19M), a higher slump is required. It requires less impact velocity to force flow of material around to the backside of a bar if a higher plasticity mixture with a 3 to 4 in. (75 to 100 mm) slump is used. On larger bars, the shadow can be reduced or eliminated by directing the shotcrete spray at a slight angle from both sides to force the material behind the bar. Sometimes, the volume and nozzle stream should be directed to permit the material to hit more directly behind the reinforcement to reduce the size of the shadow area. It may be necessary to reduce the air volume so the nozzle can be held Fig. 1: Improperly compacted concrete behind steel closer to the receiving surface. 40 Shotcrete • Spring 2016


2016SpringShotcreteEMag
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