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2016SummerShotcreteEMag

Shotcrete Corner Convert Peanuts into Gold with the Right Pump or Gunite Machine By Jim Farrell A concrete pump and a gunite machine are work tools that can help a contractor turn a nickel’s worth of concrete material into a dime, a quarter, or even a dollar using the wet- or dry-mix shotcrete process. The odds of being successful are greatly enhanced with the selection of the correct tool and the correct material, combined with the knowledge and experience to properly use both. There are several types of pumps and gunite machines that can be used for the shotcrete process. Shotcrete is defined by ACI CT-13, “Concrete Terminology,” as “concrete placed by a high-velocity pneumatic projection from a nozzle.” Concrete material can also be air-placed using a lower velocity of compressed air combined with a smaller orifice nozzle. This technique is not recognized as shotcrete by the American Concrete Institute (ACI) or the American Shotcrete Association (ASA). In Europe, this low-velocity process has been referred to as “spray-up”; however, in the United States, spray- up is used in many concrete applications. The four types of pumps for the wet-mix shotcrete process include: • Hydraulic swing tube; • Hydraulic peristaltic or squeeze; • Rotor stator; and • Hydraulic ball valve or ball seat pumps. The most popular type of gunite machine for the dry-mix process is a rotary gun. Hydraulic Swing Tube Pump The swing tube pump (refer to Fig. 1) is a combination of two hydraulic cylinders that act as differential cylinders that are connected to and shift two material-pumping cylinders that come in direct contact with the material. A single “S” tube shifts from one material cylinder to the other, activated by a hydraulic cylinder, so that each cylinder that is full of material can be discharged through the “S” tube and into a delivery line that is connected to the outlet of the pump. Each differential cylinder includes a proximity switch that sends an electrical signal to the hydraulic cylinder, which shifts the “S” tube. This pumping process is controlled by a sealed electrical control box and is synchronized. Swing tube pumps recommended for shotcrete come in sizes with 3 and 4 in. (75 and 100 mm) outlets. The 3 in. (75 mm) pump would be recommended for jobs from 2 to 8 yd3/h (1.5 to 6 m3/h), and the 4 in. (100 mm) pump for jobs that require 9 to 20 yd3/h (7 to 15 m3/h). Swing tube pumps with outlets larger than 4 in. (100 mm) will result in significant surging at the nozzle because the speed of the pump must be reduced so that the nozzleman will not be overwhelmed with material, which is unacceptable. The output or material pressure at pump discharge from a swing tube pump can vary from 750 to 2100 psi (5.2 to 14 MPa). The size of the delivery line is typically reduced down to 2 in. (50 mm) or even 1.5 in. (38 mm) at the nozzle. Trying to handle a delivery line in excess of 2 in. (50 mm) is almost impossible for a nozzleman because of the weight of the hose and nozzle. The higher pumping pressure capabilities of the pump typically results in the ability of the pump to convey harsh, lower-slump materials and to pump longer distances vertically and horizontally. The swing-out receiving hopper is very helpful in cleaning and servicing the pump. Fig. 1: The swing tube pump is a combination of two hydraulic cylinders that act as differential cylinders that are connected to and shift two material-pumping cylinders that come in direct contact with the material 56 Shotcrete • Summer 2016


2016SummerShotcreteEMag
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