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Placement of concrete in inaccessible areas has necessitated the use of pumps in today’s construction. Especially with the growth of ready mixed concrete across India, the need for pumping has increased manifold. While the ease of pumping depends on the type of pump available and the distance over which concrete is to be pumped, the properties of the concrete also plays an important role in concrete pumping.

Ordinary concrete doesn’t lend itself to being pumped. Standard concrete mixes resist being moved under pressure. The mortar gets squeezed through the coarse aggregate, allowing large stone particles to come together and block the line. Pressure can also force the water out of the mortar itself, leaving some portions excessively dry and others excessively fluid. Another difficulty in pumping standard concrete is the internal friction caused by fine sand particles which cannot move easily to the surface. Higher pumping pressures are frequently required to overcome the problem.  

To overcome the pumping problems with standard mixes it is always necessary to modify the concrete mix to make it pumpable. These modifications involved compromises in sand/aggregate ratios, and in water content. But today many specifiers will not allow alterations to mix proportions merely to accommodate concrete pumping because of possible undesirable side effects on the physical properties of the concrete. Developing combinations of materials that will pump successfully and consistently can lead to many pitfalls.  

Merely changing the ratio of components in a standard mix is obviously not the best answer for improving pumpability. Either the cost is increased or the physical properties of the finished product are adversely affected. A more successful approach involves the use of chemical admixtures.  

With the use of chemical admixtures in the concrete mix, it can now be pumped under a much wider range of job conditions. It can be placed on the hottest summer day or the coldest winter night. It can be pumped farther, faster, and higher than ever before. Admixtures make concrete more plastic and homogeneous; at the same time, they enhance the physical properties of the hardened concrete.  

Air-entraining admixtures incorporate a large number of very small bubbles in the concrete. The main reason for putting entrained air in the mix is to improve resistance to deterioration caused by freezing and thawing. However, up to a point, the air also increases pumpability because of improved plasticity, less bleeding, and less segregation. Especially with crushed aggregates, too little air increases line friction and makes startups after pumping delays more difficult. About 3 to 5 percent air by volume of the concrete is the best amount for pumping purpose. Too much air, in excess of 7 to 8 percent, can decrease pumping efficiency by absorbing some of the pump stroke energy as the air compresses.  

Water- reducing and water- reducing retarding admixtures, commonly-known as dispersing agents, when introduced into a concrete mix disperses the cement grains and increases 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. Also, as the name denotes, less water is required for a given workability. When these materials are used as mentioned before, excessive water does not increase the pumpability of concrete. On the contrary, it is generally a detriment to pumping. A good water-reducing admixture may reduce the water requirement of a given mix by as much as 15 percent while maintaining the required slump.

Retarding admixtures, particularly those that reduce the water requirement, have also been found to be beneficial to concrete that is pumped. The water reduction is obtained by the same mechanism as mentioned for the regular water- reducers, but in addition to this retarders also provide a means for delaying the set of concrete. This is important in hot weather pumping. The concrete moving through a pipeline will be warmed by the friction

developed. This increase, augmented by the temperature of the pipe exposed to the heat of the sun, can be considerable. It is well known that higher concrete temperatures mean faster setting times. Under such conditions, an extended pump shut down due to inadequate concrete scheduling or placing problems could cause a set-up in the pump line. Retarders make the concrete sets more slowly. They help the pumping operation under hot weather conditions, when very long pipelines are used or when the concrete placing rate is very slow.  

Pumping aids represent a new development in the role played by admixtures in the pumpability of concrete. 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.  

One of the major advantages of pumping, aids is the fact that it is not necessary to increase the sand factor or cement content to provide lubrication. This not only results in a cost reduction, but also minimizes shrinkage due to higher water and cement contents.