Concrete is pumped on almost every sizable concrete placement these days. This delivery method, that in its early days was reserved for especially challenging circumstances, now routinely provides convenient and quick concrete placement for flatwork and vertical elements, as well as foundations, mass concrete, and other applications. Today on many projects the question has gone from “should we pump this job?” to “what's the best way to pump this job?”
Dave Alexander, senior vice president with Chicago-based James McHugh Construction, says that up until 10 or 12 years ago, the company didn't pump anything. Today, most of the concrete they place is pumped. The company has built many of the high-rise concrete buildings that make up Chicago's skyline, including the recently topped-out Trump Tower and the Aqua Building—both made economically feasible by the high production rates offered via pumping. “I don't know who first brought concrete pumping into our company,” Alexander says. “But I wish I knew who to give the credit.”
Developed in the second half of the 20th century, concrete pumping technology today is reliable and widely available. Many contractors own their own pumps and specialty contractors offer concrete pumping services all across the country.
Boom pumps, which provide concrete pumping with high visibility, are identified by their reach, usually included in the model name and typically given in meters. Their mobility offers flexibility in positioning, allowing boom pumps to cover large areas relatively easily. Delivery capacities as high as 300 cubic yards/hour are available.
The stationary option
Truck-mounted boom pumps are well-suited to placing slabs on grade, belowgrade pours, and the first few floors of a building. But for high-rise projects, once the concrete work reaches several floors above ground it may be time to switch to a stationary pump.
Also called line pumps, these machines are designed to stay in place and feed into a piping system, which is where it gets tricky. Line pumps range in capacity from relatively small units to some of the most powerful concrete pumps available, and are offered in trailer-mounted and truck-mounted configurations. Trailer-mounted pumps generally have their own diesel engines, which power the hydraulic system that moves the concrete. Most truck-mounted pumps are powered by the truck engine through a power takeoff; some are equipped with a separate engine for the pump.
Although a truck-mounted boom pump has a specific length of piping and is matched to a truck engine appropriate to the power needs of the pump and the boom, a stationary pump must be chosen to meet the maximum demands of the project. The pump must provide enough power to overcome the friction in the piping system and the head pressure that results from pumping vertically, and do so at an acceptable rate.
“Three things determine pump selection,” says Tom O'Malley, director of marketing, national accounts, and product development for Schwing America, St. Paul, Minn. “You have to look at the hydraulic horsepower, the maximum output you want, and the maximum pressure you need.”
A pump's power is expressed in hp or kW, but Schwing also uses the technical identification number (TK), or its English equivalent, the power factor (PF), to simplify comparing available power to project requirements. A pump's TK value is found by multiplying the delivery rate in cubic meters per hour by the pumping pressure in bar. Finding the PF is the same but uses cubic yards per hour and pounds per square inch. To convert a TK value to the corresponding PF, multiply by the conversion factor 18.966.
Multiply the maximum delivery rate required times the maximum pressure required yields the project PF based on field performance requirements. A pump whose PF is greater than the project PF should have sufficient power to do the job. However, its maximum pressure and output capacities cannot be exceeded.
Pump manufacturers provide nomographs to help in selecting the appropriate equipment. Based on years of field experience, these design aids include pump-specific data in one quadrant and generally provide results accurate to within 10% of actual field conditions.
A sample nomograph for a Schwing trailer pump shows in its four quadrants the working relationship between several factors. To use it, one first selects the desired output (Q on the vertical axis in the center). To place 100 cubic yards/hour, for example, assume that pumping occurs 45 minutes out of every hour. That would mean pumping at the rate of 100/0.75=133 cubic yards/hour.
Selecting 133 on the vertical axis, one then draws a horizontal line to the right to where it meets the line for the planned pipe size, say 5 inches. From there, one turns a right angle and draws a straight line down, again turning a right angle at the line for the appropriate “proportional value of pipeline,” which represents the effective length of the system.
The effective length of a piping system (or proportional value) is different—longer—than its actual length because it represents a system's frictional resistance to concrete flow. One foot of slickline is taken as the base unit. A long radius elbow has a proportional length equivalent to three times its radius and a short radius elbow four times its radius. Hose resists concrete flow three times more than slickline, so for each foot of hose in the system, one would add 3 feet to its proportional length.
If pumping through a boom system, the proportional length of the piping system attached to the boom must be included in the system length. Also, the maximum height of the boom would have to be considered in determining the total required pump pressure.
Assume a 100-foot horizontal pipe runs from the pump to where it turns upward and goes 300 feet vertically. At the top, it connects to 60 feet of pipe and a 40-foot hose. The proportional length is then 100+3x3 (assuming 3-foot radius elbows) +300+3x3+60+40x3 (for the hose)=598 feet, which is rounded to 600.
On the nomograph, the turn would then be at the 600-foot line. Draw a horizontal line to the concrete slump in the lower left quadrant—assume 5 inches. From there draw a vertical line up to the horizontal axis at the base of the upper left-hand quadrant. That value, 875 psi in this example, represents the concrete pumping pressure (not the hydraulic pressure) needed to overcome friction in the system. However the additional pressure due to the vertical pumping distance also must be added. To do that, the vertical rise is multiplied by 1.105 psi, in this case 1.105x300=332 psi. The total pressure required would be 875+332=1207 psi.
Three additional pieces of information in the upper left quadrant allow you to determine pump suitability. The curved line represents the pump's power curve. Any point along this line shows what the pump output would be at that pressure.
The two rectangles, one solid and the other dashed, show the maximum pressures and capacities of the pump. The two are different because the hydraulic system that powers the pump pistons can be plumbed to exert pressure on either the piston side or the rod side. The piston side has a larger area, which allows it to develop a larger force and therefore exert more pressure on the concrete. However, because it requires more fluid, this arrangement also means fewer strokes per minute and a lower rate of concrete delivery. If it is plumbed to the rod side—which has a smaller surface area due to the rod—the maximum pressure is less but the maximum concrete delivery rate is higher.
Going back to our example, we find the point representing the combination of (133 cubic yards/hour) and the1207 psi falls below the pump curve so the pump is adequate. This can be confirmed by multiplying those values to come up with a project power factor of 160,531, which is less than the pump power factor (198,000). The nomograph also shows that the hydraulic power must be plumbed to the rod side because the required delivery rate, Q, is greater than piston-side operation can offer.
Proper pump size is important, but other things also must be considered, such as concrete mix design. “We use mixes we know are pumpable,” says McHugh's Alexander. That means working with the concrete producer to control aggregate size and ensure sufficient fluidity.
Pump location also is important. “It's best to find some place where you can put the pump and leave it there,” Alexander says, because it's expensive to move. On the Aqua project, McHugh located the pump 300 feet from the vertical portion of the slick-line, which Alexander says was unusually far away. However, that provided the necessary access and allowed the pump to remain in one place throughout the project.
Site logistics is another key consideration. There should be room at the pump hopper for two ready-mix trucks, and space to maneuver them into position. If the pump is in an enclosed area, as McHugh's pump was on the Trump Tower, provisions must be made to provide fresh air and vent engine exhaust.
Wayne Bylsma, president of Hampton, Ga.-based Cherokee Pumping Inc., says overhead clearance is important. “You also have to provide water for washout,” he says, “especially when using a stationary pump.” Bylsma says as much as a ½ yard of concrete can end up back at the pump as a result of cleaning the slickline. “You need to have some place to put it,” he says, “and you need the capability to clean up the area at the end of the day.”
Alternatives should be in place to avoid midpour problems. McHugh installed a second slickline in the Trump Tower in case the main line clogged. On the Aqua project, the company embedded the slickline in the shear wall as it was placed. Strategically located penetrations were left in each floor slab to allow for installing another slickline in case it was needed. On critical pours, such as for large mat slabs, McHugh will stage an additional pump onsite as a backup. However, Alexander says they also have relationships with local pumping contractors they can call on if something breaks down during concrete placement.
Schwing's O'Malley says in recent years advances in concrete pumping technology at the placement end have been driving development. Improvements in placing booms, as well as in the ways they can be mounted, have led to a need to deliver higher pressures and greater output quantities. Although pump manufacturers continue to rise to the challenge, he observes that achieving production efficiency still requires good planning. That, in turn, is based on a combination of experience, being willing to continue to push the envelope, communicating early on in the project, and, of course, making good choices in equipment.
Tom Klemens is a freelance writer based in Palatine, Ill. He can be reached at email@example.com.