PROPORTIONING CONCRETE
The process of selection of relative proportions of cement , sand , coarse aggregate and water,so as to obtain a concrete of desired quality is known as the proportioning concrete. It is observed that if a vessel, as shown in fig. 16-1, is taken and filled with stones of equal size, the voids to the extent of about 45 percent are formed. This result is independent of the size of stones. It is interesting to note that if sand is taken in place of stones, the same result will be obtained. The result can be verified by pouring water in the vessel, till it is just full . The volume of water added in the vessel represents the amount of voids.
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| PROPORTIONING CONCRETE |
The theory of formation of concrete is based on this phenomena of formation of voids. When coarse aggregate is placed, such voids are formed. When fine aggregate i.e. sand is added, it occupies these voids. Further, when finely powdered cement is added, it occupies the voids of sand particles. Finally, when water is added, it occupies very fine voids between the cement particles. During the process of setting, a chemical reaction takes place between water and cement. This results in an absolutely solid substance, known as the concrete.
In general , the proportions of coarse aggregate, fine aggregate, cement and water should be such that the resulting has the following properties:
(i) When the concrete is fresh, it should have enough workability so that it can be placed in the formwork economically.
(ii) The concrete must possess maximum density or in other words, it should be the strongest and most watertight.
(iii) The cost of material and labour, required to form the concrete, should be minimum.
These are various methods for determining the volumetric proportions of various components of concrete and their detailed discussion can be studied from any standard text-book on the design of R.C.C. structure.
Methods of proportioning concrete
(i) Arbitrary method
(ii) Fineness modulus method
(iii) Minimum voids method
(iv) water-cement ratio method
(v) Maximum density method
Each of the above method of proportioning concrete will now be briefly described.
(1) Arbitrary method
In the method of arbitrary volumetric proportions, the proportions of cement , sand and coarse aggregate are fixed arbitrarily such as 1:2;:4 or 1:3:6 etc, depending on the nature of work for which concrete is required . Usually the fine coarse ratio is 1:2 on the assumption that most of the coarse aggregates have voids to the extent of about 50 percent. Other ratios may be used. Thus the general expression for the proportions of cement, sand and coarse aggregate is 1:n:2n by volume. The recommended proportions are :
1:1:2 and 1:1:2:2:4 for very high strength concrete
1:1 1/2:3 and 1:2:4 for normal work
1:3:6 and 1:4:8 for foundations and mass concrete work
In this method, there is no rigid control over the strength of the concrete mix. However this method is widely used for all works of small magnitude because of its simplicity in the design.The recommended mixes of concrete for various types of construction are given in table 16.2. The maximum size of aggregates is also mentioned in the tables. The proportions are by volume.
The concrete as per BIS 456:1978 is designated in seven grades, namely, M 10, M 15, M 20, M 25, M 30, M 35, and M 40. The letter M refers to the mix and the number indicates the specified compressive strength of that mix at 28 days expressed in N/mm2 . For lean concrete bases and simple foundations for masonry walls, M 5 and M 7.5 grades of concrete may be used . These mixes need not be designed . The grades of concrete lower than M 15 are not to be used in R.C.C. work. For general guidance, the normal mixes correspond approximately to the different grades as follows:
M 5 - 1:5:10 M 15 - 1:2:4
M 7.5 - 1:4:8 M 20 - 1:1:1/2:3
M 10 - 1:3:6 M 25 - 1:1:2
(2) Fineness modulous method
In the fineness modulous method , the following modules of sand and aggregates is determined by the standards tests. The term fineness modules is used to indicate an index number which is roughly proportional to the average size of the particle in the entire quantity of aggregates. The standards tests for determining size of the particles in the entire quantity of aggregates. The standards tests for determining fineness modules are carried out with the help of a set of ten BIS sieves (Metric sieves ) and dividing the sum y 100. The number of the BIS sieves used are from 80 mm to number 15. The sieves adopted for all in aggregates, coarse aggregates and fine aggregates are as follows:
For all in aggregates : 80 mm,40 mm,20 mm, 10 mm, nos. 480,240,120,60,30, and 15
For coarse aggregates : 80 mm,40 mm,20 mm, 10 mm, and no. 480
For fine aggregates ; Nos. 480, 240, 120 ,60 30 and 15.
It is found from various experiences that certain values of fineness modules for fine and coarse aggregates and mixed aggregates give better workability with less quantity of cement . Hence the fineness modules of aggregates in a given sample of concrete is studied with respect to the desirable value of fineness modulus of aggregate and suitable adjustment is then recommended. The aggregates are mixed in such a proportion that the recommended fineness modules of combined aggregates is obtained.
The corresponding ten sieves specified by BIS are 80 mm, 40 mm, 20 mm, 10 mm, 4.75 mm,2.36 mm,1.18 mm, 600 micron, 300 micron and 150 micron respectively .
Let p be the desired fineness modules for a concrete mix of fine and coarse aggregates.
Then , R = ( p2 - p)/(p - p1) X 100
where R = proportion of fine aggregates to the combined aggregates by weight
p1 = fineness modules of fine aggregates
p2 = fineness modules of coarse aggregates.
(3) Minimum voids method
In this method , the voids of coarse aggregates and fine aggregates are determined separately and to get the dense concrete, it is so arranged that ;
(i) the quantity of fine aggregates completely fills the voids of the coarse aggregates;
(ii) the quantity of cement completely fills the voids of the fine aggregate ; and
(iii) sufficient water is added to the mix of cement, fine aggregate and coarse aggregates to make the mix workable.
In actual practice , the quantity of fine aggregates used in the mix is about 10% more than the voids in the coarse aggregate and the quantity of cement is kept as about 15% more than the voids in the aggregates.
The method of minimum voids does not give satisfactory result because of the following fact:
(i) The presence of cement, fine aggregates and water separates the constituents of the coarse aggregates and the coarse aggregates determined previously in the absence of fine aggregates and cement are increased.
(ii) The voids of the fine aggregates and cement are increased due to the additionof cement and water.
(iii) The grading of aggregates is not done to achieve the least amount of water i.e., the water-cement ratio.
(4) Maximum Density Method
This method is based on the principle that the densest concrete is achieved by proportioning its aggregates in such a manner that the heaviest weight of concrete for same volume is obtained . A box filled with varying proportion of fine and coarse aggregates. The proportion which gives the heaviest weight is then adopted.
The method of maximum density is not very popular may because of the following two reasons:
(i) The grading cannot be accurately achieved.
(ii) These is no control over the strength of concrete.
(5) Water- cement ratio method
According to the water- cement ratio law given by Abram as a result experiments, the strength of well compacted concrete with good workability is dependent only on the water- cement ratio.
It is to be noted that to app;y this law, the concrete is assumed to be fully compacted. The lower water content produces stiff paste having greater binding property and hence the lowering of water-cement ratio within certain limits results in the increased strength.
Similarly the higher water content increase the workability . But it is not useful for the chemical action. The excess water evaporates leaving pores in the concrete. thus the increased water-cement ratio lowers the strength of concrete.
The optimum water-cement ratio for the concrete of required compressive strength is decided from graphs and expression developed from various experiments.
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