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2016FallShotcreteEMag

Technical Tip Steel Pipe: More Volume, Less Cost By Andy Kultgen Difficult-to-access points-of-placement are common in concrete pumping and are a trademark of shotcrete. A major driving factor behind the development of concrete pumping was the requirement of placing concrete in areas that are out of reach of a ready-mix truck, and may not even be accessible by buggy or bucket. These difficult-to-access jobsites often mean that you must locate your pump far away from where you are placing the concrete, sometimes several hundred feet away. On occasion, typically in mines and tunnels or on bridges, you may need to pump thousands of feet. For many shotcrete and small-line contractors, the go-to pumping delivery line is a rubber hose. It is easy to see why—if you need a flexible line to allow the nozzleman to move around the jobsite and place the concrete around obstructions, why not just get more hoses to cover that ground between the pump and the nozzleman? However, on jobs where you need to pump longer distances or a lot of volume, steel pipeline can be a useful option to decrease the load on your pump, decrease the wear on your hoses, and provide a steady flow of concrete to your nozzleman, all at a lower cost. Those things add up to a better finished product and a better bottom line for your company. Throughout the last several decades, concrete pumps have increased in power and maximum pressure rating, constantly raising the bar on how far concrete can be pumped. In recent years, highperformance concrete has been pumped well over a thousand feet (300 m) vertically in high-rise construction, and specialized mixtures have been pumped several miles horizontally. Those are the unique and specialized jobs, while the bulk of line concrete pumping is within several hundred feet. It is not uncommon to explain the wet-mix shotcrete process to a layman and have them remark “You can do that?!” to several aspects of the job. They are surprised that you can pump concrete at all, and equally surprised that you can propel that concrete with compressed air onto a surface and it stays there. When concrete is pumped, it does not behave like a liquid does when traveling down the line. Concrete moves as a relatively uniform, undisturbed ‘slug’ through the line, sliding along on a very thin film of cement, water, and fines. Studies have found that this lubricating paste layer is only about 40 mils (1 mm) thick. When this slug is moving down a straight, uniform pipe, it moves relatively easily. However, bends, reducers and any changes or breaks in the inner surface of the line can cause increased resistance to that slug of concrete moving along because the slug needs to change size or shape, aggregate is forced through the lubricating layer, or the lubricating layer is stripped away. This is the reason why a longer reducer and larger radius bends generally result in easier pumping. The concrete slug has more time to deform; that is, the aggregate rearranges, and less aggregate is forced through the lubricating layer against the pipe wall as it passes through the reducer or around a bend. Aggregate rearranging as more paste moves out to the surface of the slug is the reason you usually experience rock jams in reducers or bends, and rarely in straight sections of pipe. This slug flow with a thin lubricating layer is the basis for the advantages of steel pipeline. A length of steel pipeline has a smooth, straight inner surface interrupted only by the joint and coupling to the next length of pipe. By comparison, the inner surface of a rubber hose can vary greatly and is not perfectly straight. The inner surface of a hose is generally rougher than a steel pipe, and there are interruptions both where the hose couples to another hose as well as where the steel hose barb starts and ends. It is because of these differences that a rubber hose is generally considered to have three to five times more resistance to pumping than an equal length of steel pipe. Additional benefits of steel pipe over rubber hose are increased wear life, ease of wear life monitoring, and lower cost. An Example Consider this situation to illustrate these differences between hose and pipe. A job will require 250 ft (76 m) of line between the pump and the area of placement. The concrete is a typical mixture with a 2.5 in. (64 mm) slump and you expect to pump at a rate of approximately 15 yd3 (11 m3) 32 Shotcrete • Fall 2016


2016FallShotcreteEMag
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