Page 24

2013FallShotcreteEMag

Technical Tip material Velocity at the Nozzle By Nicolas Ginouse and Dr. Marc Jolin Stechnical challenges of the industry due to rubber “convergent” nozzle (Fig. 2) were used.the wet-mix shotcrete, an Allentown Powercreter10 pump and a 2 in. (50 mm) hose with a short-ince the early days of shotcrete, the reduction of rebound has been one of the major its obvious impact on material and labor costs In both cases, conventional dry and wet shotcrete and, although less recognized, its detrimental mixtures supplied by King Packaged Materials effect on material properties. Although today, were shot. rebound mechanisms are still poorly understood A pressure gauge and an electronic airflow from a physical and mathematical point of view, meter were used to measure the input air pressure it has been shown that the impact velocity of and the volume of airflow, respectively. The input particles plays an essential role on shotcrete air pressure was kept constant and equal to rebound (Armelin et al. 1999). It is, therefore, 100 psi (700 kPa) for all shotcreting tests. For essential to have a precise idea of the impact the velocity measurements, a high-speed velocities generated by a given shotcreting con- imaging system with a 1250 frames-per-second figuration (for example, nozzle type, process, capacity was used to film the shotcrete spray. As distance, and angle) in order to allow optimization illustrated in Fig. 3, this high-speed camera was of rebound. In practice, shotcrete material velocity placed perpendicular to the horizontal nozzle is adjusted by changing the input airflow at the axis to visually capture the entire stream of gunning machine in dry-mix shotcrete or at the particles. Note that for these measurements, the nozzle in wet-mix shotcrete. These adjustments nozzle was kept motionless because the goal was are currently based on the nozzleman and machine to study the material as it exited the nozzle. operator’s experience. One of the most impressive portions of the This “Technical Tip” presents the experimental setup was the software for the image analysis. The setup developed in the Shotcrete Laboratory at software tracked, frame by frame, the recorded Laval University in Québec, Canada, to measure particle’s position as it exited the nozzle (refer to particle velocities at the nozzle outlet. Velocity Fig. 4) to deduce nozzle velocity. A second values obtained for dry- and wet-mix shotcrete analysis system included an in-house Matlab® will also be discussed. In order to reproduce a program to correct optical errors induced by the realistic shotcrete spray, shotcreting equipment camera lens and positioning. and mixtures common in the industry were used. The experimental measurements show that the An Aliva 246.5 with a 0.95 gal. (3.6 L) electric particle velocities are not uniformly distributed rotor and a 1.5 in. (37 mm) double-bubble nozzle around the (horizontal) spray axis. Indeed, in both (Fig. 1) were used for the dry-mix process. For configurations (dry and wet), the maximum Fig. 1: Double-bubble dry-mix nozzle Fig. 2: Rubber wet-mix nozzle (Bolduc 2009) 22 Shotcrete • Fall 2013


2013FallShotcreteEMag
To see the actual publication please follow the link above