Quality Requirements of Pumped Concrete
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 range from small units, exerting pressures from 250 to 300 psi and outputs of 15 to 30 yd3/hr, to large units, exerting pressures of 1,000 psi and outputs up to 150 yd3. The effective capacity of a pump depends not only on the pump itself but also on the complete system. Several factors including line length, number of bends in the line, type of line, size of line, height to which the concrete is being pumped, and the concrete mixture affect the effective working capacity of a concrete pump. An excellent reference is 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 in. in diameter. Aluminum alloy pipe should not be used as pump line. Flexible hose is made of rubber, spiral wound flexible metal, and plastics. It is useful in curves, difficult placement areas, and as connections to moving cranes but exhibits greater line resistance to the movement of concrete than rigid pipe and may have a tendency to kink. To obtain the least line resistance, the pipeline should be made up primarily of rigid pipe with flexible hose only where necessary. If possible, the pipeline should be of one size and laid out so as to contain a minimum number of bends.
2. Mixture proportions
Concrete mixture proportions of pumpable mixtures are essentially the same as those to be placed by other methods, except that more emphasis should be placed on the grading of the fine aggregates. Concretes which are pumped must be cohesive. Harsh mixtures do not pump well. Pressure exerted by the pump can force the mortar away from the coarse aggregate causing a blockage in the line if the mixture is not proportioned properly. The cement content will generally be somewhat higher for pumped mixtures than those of mixtures placed by conventional methods. The higher fine aggregate content will have a higher water demand, which in turn will require higher cement content. However, extra cement should not be used to correct pumping deficiencies resulting from poorly graded aggregates. It is usually more preferable to correct deficiencies in the fine aggregates by blending in additional fine aggregates or pozzolana than by adding cement.
3. Coarse aggregates
The nominal maximum size of the coarse aggregate is limited to one-third of the smallest inside diameter of the pump line for crushed aggregates or 40 percent of the smallest inside diameter of the pump line for well-rounded aggregates. Oversize particles should be eliminated. Higher mortar content will be necessary to effectively pump a concrete containing crushed aggregates than for a concrete containing rounded aggregates. Depending upon the type and size of the coarse aggregate, it may be necessary to reduce the coarse aggregate content from 5 to 10 percent as compared to mixtures placed by conventional methods.
4. Fine aggregate
The properties of fine aggregates are more critical in proportioning pumpable mixtures than are the properties of the coarse aggregates. Together with the cement and water, the fine aggregates constitute the mortar which conveys the coarse aggregates in suspension through the pump line. Fine aggregates should conform to the requirements given in ASTM C 33 for fine aggregates. In addition, for pump systems having lines 6-in. in diameter and smaller, 15 to 30 percent of fine aggregate should pass the 300-μm (No. 50) sieve and 5 to 10 percent should pass the 150-μm (No. 100) sieve. Fine aggregates that are deficient in either of these two sizes should be blended with selected finer aggregates to produce the desired grading. Pumpability of concrete is generally improved with a decrease in the fineness modulus. Fine aggregates having a fineness modulus between 2.40 and 3.00 are generally satisfactory provided that the percentages passing the 300- and 150-μm (No. 50 and No. 100) sieves meet the previously stated guidelines. Fineness modulus values alone without stipulations on the finer sizes may not produce satisfactory results. Both manufactured fine aggregates and natural sands can be used in pumped mixtures provided their gradings are appropriate; however, natural sands are preferred due to their rounded shape.
The water requirements to establish the optimum slump and to maintain control of that slump throughout the course of a pumping placement are both extremely important factors. Concretes having slumps less than 2 in. when delivered to the pump are difficult to pump. Concretes having slumps over 6 in. can segregate causing a blockage in the pump line and may require a pump aid to increase the cohesiveness of the concrete to prevent the aggregate from separating from the mortar during pumping.
It is much more important to obtain a cohesive concrete through proper proportioning than to try to overcome deficiencies by adding extra water. In fact, the use of excess water creates more problems than it solves.
Materials which improve workability, such as water-reducing, high-range water-reducing, and air-entraining admixtures, as well as pozzolanas, usually improve pumpability. It is common to experience a decrease in air content during pumping. The specified air contents required for durability should be obtained at the point of placement in the structure. Therefore, it may be necessary to entrain a higher air content into the concrete mixture prior to pumping. Pumping aids are admixtures which can reduce friction, reduce bleeding, and increase cohesiveness, all of which make concretes pump easier.
7. Pumpability tests
There is no standard laboratory test method available to accurately test the pumpability of a concrete mixture. Testing a concrete mixture for pumpability involves duplicating anticipated job conditions from beginning to end. A full-scale field test for pumpability should be considered to evaluate both the mixture proportions and pumping equipment. Prior use of a mixture and pumping equipment on another job may furnish evidence of pumpability if job conditions are duplicated.
Proper planning of the entire pumping operation including pump location, line layout, placing sequence, and concrete supply will result in savings of time and expense. The pump should be as near the placement area as possible. Concrete delivery systems should have easy access to the pump. Lines from the pump to the placement area should be made up primarily of rigid pipe and contain a minimum number of bends. For large placement areas, alternate lines should be laid for rapid connection when required, and standby power and pumping equipment should be readily available to replace an initial piece of equipment should a breakdown occur.
When pumping downward 50 ft or more, an air release valve at the middle of the top bend will prevent vacuum or air buildup. When pumping upward, a shutoff valve near the pump will prevent the reverse flow of concrete during the fitting of cleanup equipment or when working on the pump. Direct communication should 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 to have the concrete delivery such that the pumping can proceed continuously. When a delay occurs, it may be difficult to start the concrete moving in the line again, especially if the delay has been for a considerable length of time. This critical delay time will depend upon such factors as the concrete mixture, temperature, length of pipeline, and type of pump. It may be necessary to clean the line and start again if the delay becomes extended. A grout or mortar should be used to lubricate the pipeline anytime pumping is started with clean lines, but it should not be pumped into the 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 the slump, air content, and other concrete properties occur during pumping. However, the quality of the concrete being placed in the structure can only be measured at the placement end of the pumpline.