In mixtures with a high slump, the aggregate will segregate from the mortar and paste and cause choking. The mixing water requirements vary for different maximum sizes and type of aggregates.
The approximate quantity of water for a slump of 85 mm and mm is given in Table 1. In such cases, water reducing admixtures are useful. In the addition to this type of admixtures at normal dosage levels, to obtain higher workability for a given concrete mix, there is no necessity to make any alteration to the mix design from that produced for the concrete of the initial lower slump.
There is generally no loss of cohesion or excess bleeding even when the hydroxylic acid-based materials are used. A loss of slump during pumping is normal and should be taken into consideration when proportioning the concrete mixes.
A slump loss of 25 mm per meters of pipeline length is not unusual, the amount depending upon ambient temperature, length of the line, the pressure used to move the concrete, moisture content of aggregates at the time of mixing, truck-haulage distance, whether the mix is kept agitated during haulage etc.
The loss is greater for hose than for pipe and is sometimes as high as 20 mm per 30 meter. Aggregates The maximum size of crushed aggregate is limited to one-third of the smallest inside diameter of the hose or pipe based on simple geometry of cubical shape aggregates. The shape of the coarse aggregate, whether crushed or uncrushed has an influence on the mix proportions, although both shapes can be pumped satisfactorily. The crushed pieces have a larger surface area per unit volume as compared to uncrushed pieces and thus require relatively more mortar to coat the surface.
Coarse aggregate of a very bad particle shape should be avoided. The gradation of coarse aggregate plays an important part. Difficulties with pump have often been experienced when too large a proportion of coarse aggregate is used in an attempt to achieve economy by reducing the amount of cement; such mixes are also harsh and costly to finish.
The grading of coarse aggregate should be as per IS: If they are nominal single sized then 10 mm and 20 mm shall be combined in the ratio of to get a 20 mm graded coarse aggregate. In the same way, 10 mm, 20 mm and 40 mm aggregates shall be combined in the ratio of A fine aggregate of grading as given in Table 2 is best for pumped concrete. The guidance supersedes old code of practice which was first released in Other references :BS Code of practice for the safe use of concrete pumps.
Membership Membership Benefits of Membership Join here. Overview Qualifications Events calendar. Magazine Concrete. Labels: concrete. Newer Post Older Post Home. Air-entraining admixture enhances concrete mixture cohesion and improves concrete pumpability. Nonetheless, if the cement content of the mixture is high, providing an air-entraining agent offers no benefits. Retarder admixture is considered advantageous in hot environments and situations where unanticipated delays come up.
It can increase concrete workability, and hence providing more time for concrete placement. Pumpable concrete is considerably affected by errors in batching, the quantity of mixing water, and other possible errors that may happen during material batching. Therefore, it is required to conduct concrete batching accurately.
For material mixing, pumpable concrete should be mixed adequately, and machines used for this purpose should be employed based on the manufacturer's recommendation. Lastly, it is advised to remix ready-mix concrete before loading it into the pump. The factors that oblige concrete pumps are time and labor savings, enhanced scheduling, decreased site preparation, high-quality control, and the versatility of placing the concrete easily.
Major problems in concrete pumping work: how avoid them? Concrete pumps- types and selection. R eciprocating Pump-components, working and uses. Join Join TheConstructor to ask questions, answer questions, write articles, and connect with other people.
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