Quality Requirements of Pumped Concrete
Many factors like length, number of bends in the line, type of pipe, size of a line, height to which the concrete is being pumped, and the concrete mixture influence the working capacity of a concrete pump.
1) Pump Lines
Pipelines used in concrete pumps are generally a combination of rigid pipe and heavy-duty flexible pipe. The acceptable rigid pipe can be made of steel or plastic and is available in sizes from 3 to 8 inches in diameter.
The aluminum alloy pipe should not be used as a pump line. The flexible hose is made of rubber, spiral wound flexible metal, and plastics. If possible, the pipeline should be of one size and laid out to contain a minimum number of bends.
2) Mixture Proportions
Proportions of concrete ingredients of pumpable mixtures are essentially the same as those to be placed by other methods. Concretes which are pumped must be cohesive.
Harsh mixtures do not pump well. The 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 content of cement will generally be slightly higher for pumped mixes than those of mixtures placed by conventional methods.
3) Coarse Aggregate
The nominal maximum size of the coarse aggregate is restricted to one-third of the inside diameter of the pump for crushed aggregates or 40 percent of the smallest inside diameter of the pump for well-rounded aggregates.
Oversize particles should be eliminated. Excessive mortar content will be necessary to effectively pump a concrete containing crushed aggregates than for concrete containing rounded aggregates.
4) Fine Aggregate
The properties of fine aggregates as an ingredient in concrete are more critical in proportioning pumpable mixtures than are the properties of the coarse aggregates.
Combined with the cement and water, the fine aggregates compose the mortar which transfers the coarse aggregates in suspension through the pump line.
Both types of aggregate manufactured fine aggregates and natural sands can be used in pumped mixtures.
The necessity of water for optimum slump and to maintain control of that slump throughout a pumping placement are both extremely important factors.
Concretes having slumps less than 2 inches when delivered to the pump are difficult to pump. Concretes having slumps over 6 inches can segregate causing a blockage in the pump line and may require a pumping aid to increase the cohesiveness of the concrete mix.
Materials which enhance workability, such as water-reducing agent, high-range water-reducer, and air-entraining admixtures, as well as pozzolanas, usually improve pumpability.
It is usual to experience a decrease in air content during pumping. The specified air contents essential for the durability of concrete should be obtained at the point of placement on the site.
Therefore, it may be necessary to entrain a higher air content into the concrete mixture before pumping.
Proper arrangement of the whole pumping operation including pump location, line layout, placing sequence, and concrete supply will result in savings of time and expense.
The pump should be feasible as near the placement area as possible. Concrete conveying systems should have easy approaches to the pump.
Pipelines from the pump to the placement area should be made up of rigid pipe and contain a minimum number of bends.