How Pump Lines and Concrete Mixture affect the Concrete Pump Operation
Placement of concrete in inaccessible areas has necessitated the use of pumps in today's construction. Especially with the growth of ready mixed concrete, the need for pumping has increased manifold. While pumping is very efficient and reliable means of placing concrete, a number of finer aspects can affect the pumping operation. Factors including pump line length, bends in the line, type of line, size of line, and line layout; all related to pump lines and the concrete mixture parameters affect the effective working of a concrete pump.
Pump Lines :
Pump lines are usually a combination of rigid pipe and heavy-duty flexible hose. Acceptable rigid pipe are made of steel or plastic and is available in sizes from 3 to 8 in. in diameter. Aluminium pipes are avoided, as the aluminium reacts with alkalis in the cement, and leads to the evolution of hydrogen gas. These gases tend to introduce voids in the concrete, which reduce the efficiency of pumping. Flexible hose are generally made up 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 is made up primarily of rigid pipe with flexible hose only where necessary.
In order to optimize the concrete pumping operation, the most efficient configuration i.e. the capacity and pressure requirements of the system must be determined. A specific line pressure must be determined to move concrete of a given slump at a specified flow rate through a pipeline of a particular length and diameter. Line pressure requirements also depend on pipeline layout. The length of straight and horizontal pipeline, the vertical rise, the number and severity of bends, the amount of flexible hose used in the line—each has an effect on pressure needed to move the concrete.
Pump line length directly affects the line pressure requirement due to the friction between the pumped concrete and the pipeline internal wall. The longer the pipeline length, the more rigid pipe, with its smooth interior surface, becomes desirable. While a larger inside pipeline diameter cuts down on the pressure needed, it also has one enormous drawback: the larger the line, the more labour, blocking and bracing it will require. As a rule of thumb, the pipe diameter should be at least 3 times the maximum aggregate size.
Pipeline layout also affects the pressure required to pump concrete. When a change in direction occurs in a pipeline, there is increased resistance to movement of the concrete. For the least line resistance, the pipeline should contain a minimum number of bends. In addition, the bends should not be sharp. Each 10o bend is equivalent to an extra length of pipe of 1 m. Line resistance also increases where there is a decrease in cross-sectional area anywhere in the pumping system, so the same diameter line should be used throughout a pumping system whenever possible. Where it is absolutely necessary to reduce line size, reducers should be as long as possible to facilitate reshaping of the concrete to the smaller dimension and thus reduce pumping pressure.
Concrete Mixture :
Pumpable concrete mixture must be capable of being pushed under pressure through a pipeline as a cylinder, separated from the pipe wall by a lubricating layer of mortar (water, cement, and sand). A concrete mix must be such that the concrete can pass through reducers in the pipeline system, and can go around bends in the line. In order to obtain this type of pumpability, the mix must be dense, cohesive, and have a sufficient paste and mortar fraction to minimise voidage. The mortar volume required depends on the line size, efficiency of concrete pump, and pressure available for pumping the concrete.
Concrete mixture contains cement, water, fine aggregate or sand and coarse aggregate, usually gravel or crushed stone. Admixtures such as air-entraining agents, fly ash or water-reducing agents may also be added. Properties and proportions of all these content in the mixture affect the pumpability of concrete. The concrete mix design must be correctly proportioned and consistence so that the concrete will flow easily and uniformly through the pipeline. This improves the concrete pumpability and helps ensure that the pipeline will not become blocked.
Concrete mixture should not be too harsh as hash mixtures do not pump well. Harsh concrete mixes do not have sufficient mortar to adequately coat the surface of the aggregate and to fill the voids between the aggregate. Because of this they lack cohesion. They are more likely to segregate and, are more difficult to pump than mixes with sufficient mortar.
Concrete mixes that bleed excessively are difficult to pump. Bleeding is movement of water to the top surface of concrete as heavier materials settle. This is caused by poorly graded sand which does not properly fill all the voids, and allows the water to "bleed" through the very small open channels.
Concrete mixes that have too many fines may also be difficult to pump. With too high amount of fines in the mixture the friction between the concrete and the line may be so great that pump pressure isn't high enough to move the concrete. This type of pumping problem is more common with high strength concretes or with concretes containing a high proportion of very fine materials such as rock dust. These concretes are sticky and additional pressure is needed to overcome adhesion between the mortar and the pipe walls. Increasing the amount of well-graded coarse aggregate in these mixes will help to reduce the fines content and improve pumpability. Use of coarser sand is also recommended.
The important properties of coarse aggregates that affect pumpability are maximum size, shape and surface texture, and most importantly, gradation of particle sizes present. If the porosity of the aggregate is exceptionally high, water absorption can also affect pumpability. The nominal maximum size of the coarse aggregate should be limited to one-third of the smallest inside diameter of the pump line. Shape and surface texture of coarse aggregate have an effect on mix proportions although concretes with angular or rounded and rough or smooth particles can be pumped satisfactorily. Concretes made with angular, rough particles usually have to have a higher mortar content to be pumpable.
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.
Admixtures and pumping aids can also be used in the concrete mix to improve the concrete pumpability. Admixtures which improve workability, such as water-reducing, high-range water-reducing and air-entraining admixtures, as well as pozzolanas, usually improve pumpability. The small, stable bubbles developed by an air-entraining admixture act as flexible ball bearing in the mix. These enable the sand particles to move by each other more freely thus increasing the plasticity of the mix without the need of extra water. The bubbles also supplement the sand grading which is very important in concrete mixes that are to be pumped. Water-reducing agents and water reducing retarding admixtures when added to concrete mix disperses the cement grains and increase the plasticity of the mix by minimizing agglomeration. Thus, for a given amount of cement a better coated aggregate is provided for the pump to handle.
Fly ash is a fine material which can be added to concrete either as an admixture or as a partial cement replacement. The additional fines reduce the void content of the solid materials and make the mix more pumpable. Because of their smooth surface and rounded shape, fly ash particles also reduce bleeding and internal friction without increasing the water required to keep the slump constant.
Pumping aids represent a new development in the role played by admixtures in the pumpability of concrete, one which promises to be a significant advance in concrete pumping technology. A pumping aid is a lubricant that improves the ease with which a given mix can be pumped through a line. This means lower pump pressures, reduced pressure on the mortar phase of the concrete, less chance of segregation and a reduction in internal friction.