Pumped concrete can be used for most structural concrete construction but is most useful where space for construction equipment is limited or access is difficult. Concrete pumps can be either truck- or trailer mounted and can range from smaller units, with pressures ranging from 250 to 300 psi and outputs of 15 to 30 yd.3/hr, for larger units, a pressure of 1,000 psi and 150 yd . output up to3, The effective capacity of a pump depends not only on the pump itself but also on the system as a whole. Many factors affect the effective working efficiency of a concrete pump, including line length, number of bends in the line, type of line, line size, height at which concrete is being pumped, and concrete mix. is an excellent reference ACI 304.2R,
1. pump lines
Pump lines are usually a combination of rigid pipe and heavy-duty flexible hose. Acceptable rigid pipe can be made of steel or plastic and is available in sizes from 3 to 8 inches in diameter. Aluminum alloy pipe should not be used as a pump line. Flexible hose is made of rubber, spiral wound flexible metal and plastic. It is useful in curves, difficult placement areas, and as a connection to moving cranes, but exhibits greater line resistance to movement of concrete than rigid pipe and may have a tendency to kink. In order to obtain the least line resistance, the pipeline should be primarily composed of rigid pipe with flexible ducting where necessary. If possible, the pipeline should be of one size and should be laid in such a way that it has the least number of turns.
2. mixing ratio
The concrete mix ratios of pumpable mixes are essentially the same as those to be maintained by other methods, except that more emphasis should be placed on the grading of fine aggregates. Pumped concrete must be cohesive. Harsh mixtures do not pump well. The pressure exerted by the pump can drive the mortar away from the coarse aggregate causing line blockage if the mixture is not properly proportioned. The cement content for pumped mixes will generally be somewhat higher than for mixes laid by conventional methods. Higher finer composite content will have higher water demand, which in turn will require higher cement content. However, additional cement should not be used to correct pumping deficiencies resulting from poorly graded aggregates. It is usually better to correct deficiencies in fine aggregates by blending in additional fine aggregates or pozzolana than by adding cement.
3. coarse aggregate
The nominal maximum size of coarse aggregates is limited to one-third of the smallest inner diameter of the pump line for crushed aggregates or 40 percent of the smallest inner diameter of the pump line for well-rounded aggregates. Large sized particles should be eliminated. A higher mortar content will be required to effectively pump concrete containing crushed aggregates than concrete containing rounded aggregates. Depending on the type and size of coarse aggregates, it may be necessary to reduce the content of coarse aggregates by 5 to 10 percent compared to mixtures maintained by conventional methods.
4. fine aggregate
The ratio of pumpable mixtures to the properties of fine aggregates is more important than properties of coarse aggregates. Together with cement and water, the finer aggregates form the mortar that transports the coarser aggregates into suspension via a pump line. The fine aggregate should conform to the requirements given in ASTM C 33 For the right set. In addition, there are 6-inch lines for the pump system. In diameter and smaller, 15 to 30 percent of fine aggregates should pass through a 300-micron (No. 50) sieve and 5 to 10 percent should pass through a 150-micron (No. 100) sieve. Fine aggregates lacking in either of these two sizes must be mixed with the selected fine aggregates to produce the desired grading. The pumpability of concrete usually improves with a decrease in the fineness modulus. Fine aggregates with a fineness modulus between 2.40 and 3.00 are generally satisfactory provided that the percentages passing through the 300- and 150-micron (No. 50 and No. 100) sieves meet the guidelines described earlier. Fineness modulus value alone cannot produce satisfactory results without conditions on finer size. Both manufactured fine aggregates and natural sand can be used in pumped mixtures, provided their grading is appropriate; However, natural sands are preferred because of their rounded shape.
The need for water to establish optimum deceleration and to maintain control of that deceleration during pumping placement are both extremely important factors. Concrete with a slope of less than 2 inches is difficult to pump when transported to the pump. Concrete with a slope of more than 6 inches can cause blockage in the pump line and may require pump assistance to increase the cohesion of the concrete to prevent it from separating from the mortar during pumping.
It is more important to achieve a cohesive concrete through proper proportioning than trying to correct deficiencies by adding extra water. In fact, using excess water creates more problems than it solves.
Materials that improve workability, such as reducing water, high-grade water reducing, and air-permeable mixtures, as well as pozzolana, generally improve pumpability. It is common to experience a decrease in air volume during pumping. The specified air content required for durability must be obtained at the point of placement in the structure. Therefore, it may be necessary to introduce high air content into the concrete mix prior to pumping. Pumping aids are mixtures that can reduce friction, reduce bleeding, and increase cohesion, all of which make pumping concrete easier.
7. pumpability test
There is no standard laboratory test method available to accurately test the pumpability of a concrete mix. Testing a concrete mixture for pumpability involves mimicking anticipated job conditions from start to finish. A full-scale field test for pumpability should be considered to evaluate both the mixing ratio and the pumping equipment. Prior use of mixing and pumping equipment on another job may provide evidence of pumpability if the job conditions are repetitive.
Proper planning of the entire pumping operation including pump location, line layout, placement sequence and concrete supply will save time and expense. The pump should be as close to the placement area as possible. The concrete delivery system should have easy access to the pump. The lines from the pump to the placement area should be primarily made of rigid pipe and have a minimum number of turns. For large placement areas, alternate lines should be laid for faster connection when required, and standby power and pumping equipment should be readily available to replace the starting equipment if a breakdown occurs.
When pumping down 50 feet or more, an air release valve in the middle of the top turn will prevent vacuum or air buildup. When pumping upstream, a shutoff valve near the pump will prevent reverse flow of concrete during fitting of cleaning equipment or while working on the pump. Direct communication must be maintained between the placing crew and the pump operator. Good communication between the pump operator and the concrete batch plant is also important. It is desirable that the delivery of concrete be such that the pumping can proceed continuously. When there is a delay, it can be difficult to re-line the concrete, especially if the delay is long enough. This critical delay time will depend on factors such as the concrete mix, temperature, length of the pipeline and the type of pump. It may be necessary to clear the line and start again as the delay increases. Whenever pumping is started from clean lines, grout or mortar should be used to lubricate the pipeline, but it should not be pumped into forms.
A high level of quality control must be maintained to provide assurance that the concrete is of the desired quality. Concrete should be sampled at both ends of the pumpline to determine what, if any, changes in deceleration, air content and other concrete properties occur during pumping. However, the quality of the concrete being laid in the structure can be measured only at the end of the location of the pumpline.