Page 16

2015FallShotcreteEMag

material back to the original consistency (moist Wet-Mix Nozzle Setups sand and cement). By predampening the mixture, it made the static electricity disappear and also made adjustments to the mixing water at the nozzle easier to control. It wasn’t until later in my career that I was introduced to an actual contin- uous auger-fed predampening machine. This eliminated shoveling the mixed material into the gun (a back saver). Figure 8 is an electric combi- nation predampener/rotary barrel gun (GM-060 for production) and a peristaltic accelerator pump for use on civil and mining work, where early set Fig. 11: Wet-mix shotcrete nozzle setups have a times were required. The electric version allows minimum of 3/4 in. (20 mm) air line running into for consistent material flow from the mixing to them with 8 to 16 small 1/4 in. (6 mm) air holes the delivery. Air-driven machines fluctuate with in the air ring for maximum material velocity. the air demand from other equipment on the same Accelerator is introduced into the air stream at air source. the nozzle and is driven into the material by the From the original Micon mixing rig design, the air flow (much like the dry-process water ring). rig pictured in Fig. 9 was developed as the first The nozzle setup at the far right has an inner mixing and predampening rig available in the mid material ring and outer air ring. This nozzle to late 1960s. In the picture, an N-gun was needs the air volume to swirl the material coming mounted under the material holding hopper, out of the nozzle tip. With the separate acceler- allowing the gun operator to continually feed the ator port, the air volume is crucial to mix it into top chamber of the N-gun while the bottom the material as it swirls out the tip. If the air chamber still stayed pressurized. With the spray flow is low, only the outside of the swirled bar located in the mixer, it allowed the batchman material will have the accelerator in it (this to predampen the dry sand or dry bagged material nozzle is a newer version of the early technology) back to the desired 3 to 5% moisture. Once the batch was mixed, the batchman would open the swing tube to switch quickly between cylinders, mixer door, allowing the material to travel up the reducing the delay between strokes and cutting conveyor into the holding hopper above the gun. down on the line surge at the nozzle. Some contractors have modified their rigs to rotary For medium- and large-volume projects, con- barrel-type guns for continuous feed and simplicity sider using a piston-type pump with outputs of (Fig. 10). The art of running an N-gun (pressure 20 to 60 yd3/h (15 to 45 m3/h). A remixer in the vessel) is now down to well-established contrac- hopper helps agitate and push low-slump mixtures tors that have several of the N-guns in their fleet. toward the cylinders. Grates with vibrators on the grate help with low-slump mixtures or mixtures Wet Concrete Pumps with fibers (special grates are available that have Wet mix was introduced back in 1955 in North vibrators attached and smaller openings to help America and was later called the wet-mix shot- keep larger rocks out of the mixture while letting crete process by ACI. Like the dry process, the stiff mixtures or mixtures with fibers pass through evolution of the wet-mix concrete pumps have it). An accelerator dosing system can be connected come a long way from the mid-1950s. Pictures of to the concrete pump, which can be designed to the old pumps can be found in earlier versions of inject the proper accelerator dosage per cylinder the ACI 5062-6 documents. stroke or as a standalone unit with flow control The concrete pump shown in Fig. 12 is a devices into the wet nozzle setup. ACI 506R-054 medium-to-low volume swing tube pump that is Chapter 3, Fig 3.1, and Section 3.4 on air require- capable of pumping through a 1-1/2 to 4 in. (38 to ments will help you establish required air needs. 100 mm) diameter hose depending on the project. For the wet-mix pumps, the fibers are part of the This size of pump is an ideal shotcrete pump that aggregate in the concrete mixture and have an can pump concrete up to 20 yd3/h (15 m3/h). It has impact on how much cementitious material is a mixer on the back end to remix concrete mixtures required in the mixture to bind it together during from a truck or site mix bagged material. This pumping. How far from the actual work area will wet-mix pump is designed to handle lower-volume the material be conveyed? How many times will output for repair and grouting applications as well. the crew have to stop/start or move throughout The pump should be capable of reducing the mate- the day? These are just a few items to consider in rial outlet to a 1.5 in. (38 mm) diameter hose for planning the setup of your equipment. repair and have the option to do low-pressure grout There are several factors that dictate the proper work. The electronic setup in the pump allows the selection of a concrete pump: type of project; 14 Shotcrete • Fall 2015


2015FallShotcreteEMag
To see the actual publication please follow the link above