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This lecture is one of important lectures in Civil Engineering. Keywords in this lecture are: Concrete Operations, Storing of Concrete Materials, Object to Storing, Storage of Cement, Method of Strong Cement, Storing of Aggregates, Precautions, Storing of Water, Batching, Volume Batching
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The operations which are followed in actual practice in the making of concrete and in improving and maintaining the quality of concrete are known as concreting operations. The following operations are involved in concrete making :
Storing of materials
Batching of materials
Mixing of various ingredients
Transportation of concrete mix
Placing of concrete
Compaction of concrete
Finishing of concrete surface
Curing of concrete
Joints in concrete.
Storage of cement Cement is a finely ground material. It is
highly hygroscopic. It absorbs moisture which may be in the form of free water. An absorption of 1 to 2% of water has no effect ,but further amount of absorption reduce the strength of cement. If the absorption exceeds 5% the cement is, for all ordinary purposes ruined. During the storing and transporting of cement, care is always taken to keep it away from moisture.
removed on the principle of first in and first out. For this purpose, each
Storing of aggregates should be done in such a way as to:-
►Maintain the uniformity of grading
►Prevent segregation
►Maintain uniform surface water
condition
►Avoid mixing of harmful material in
the aggregate.
► During stock piling successive consignments should not be dropped on the same place to form a pyramid. It will lead to segregation i.e. the coarser material rolls down the sides of piles and fine material will concentrate in the centre.
► The aggregates should be placed in layer not thicker than one truck dumped at same place.
► The aggregates should not fall from a height while being transported and dumped.
► The pile should be as near as possible to the mixer to avoid greater distance of transport.
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►The area occupied by each pile should be as large as possible. The height of pile should be from 1.25m to 1.75m.
►The pile should be allowed to stand for 24 hours at least before being used so that the moisture has time to settle.
►In the case of sand, the bottom layer (last 300mm) should not be used as the bottom layer becomes saturated with water.
STORING OF WATER Water is stored at site in a masonry tank built for the purpose or in other clean containers. The walls of the tank should be somewhat higher than the surrounding ground. Sufficient quantity of water should be stored in advance to ensure the continuity of concreting operations. If the water obtained from a source contains dust etc. it should be collected a day in advance to allow such suspended impurities to settle down before use.
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(c) Batching of water :-It is practice in the field to add water by tin cans or buckets. It is not an accurate method. It results in variable strength of concrete. Some of mixers are equipped with calibrated water tank attached permanently to the mixers. For the mixers not provided with water tank calibrated syphon system can be easily got installed, such as the one used in cisterns of water closets. If, however, there are no such automatic devices, water should be measured in calibrated cans very accurately and then only should be added in the mixer.
MIXING OF CONCRETE
The process of mixing of various ingredients of concrete in specified proportions is termed as mixing of concrete. Methods of mixing :- There are two methods of mixing :
(a) Hand mixing
(b) Machine mixing
► A platform of bricks ,lean concrete or iron sheets is constructed. The size of the platform depends upon the quantity of concrete to be mixed at a time. ► Spread out a measured quantity of sand evenly on the mixing platform. ► Spread the cement uniformly on this sand and mix it till the colour of mixture is uniform. ► Spread this mixture evenly again on platform. ► Spread the measured coarse aggregate evenly on the mixing platform. ► Mix the material dry..
► Make a hollow in the centre of the mixed material. After this 75% of the required quantity of water based on water-cement ratio is added and then start remixing taking care that no water escapes the mixture.
► The remaining water is added with the continuation of mixing process.
► Normally mixing time should not exceed 3 min.
► The platform should be cleaned at the end of day's work so that it is ready for use on the next day.
