Basics in Building Construction

Basics in Building Construction 4(2+2)

Lesson 14: Plan Cement Concrete

Methods Of Proportionating Concrete Mixes

The aim of evolving different methods of proportinating concrete is to get strongest and the densest mix with least amount of cement. The different methods of proportinating concrete are:

1. Arbitrary Standards:

   In this method the aggregate are so proportioned as to have minimum voids when mixed. This can be achieved when the quantity of fine aggregate is just sufficient to fill the voids of coarse aggregates and the quantity of cement is just sufficient to fill voids in the fine aggregates.

   As a result of experiments and experience it has been possible to fix certain arbitrary ratios in which cement, fine aggregate and coarse aggregate should be mixed to produce concrete suitable for a particular type of work. In general buildings work (Beams, slabs, columns etc) subjected to ordinary stresses, cement, sand and coarse aggregate are taken in the ratio of 1:2:4. For heavily stressed members or for water tight construction the ratios may be 1:1:2, 1:1 ½:3 or 1:2:3 etc.
   The concrete produced by these proportions is termed as ordinary concrete. In proportionating concrete by this method, quantity of cement is measured by weight. The quantity of fine and coarse aggregates may either be measured by volume or by weight.

2. Minimum Void Method:

   This method is based on the principle that to obtain dense concrete the quantity of fine aggregate should be slightly in excess of the voids in the coarse aggregate and the quantity of cement should also be slightly more than the voids in the fine aggregates. In its simplest form the voids in the fine and coarse aggregates are separately found out with the help of graduated cylinder and water. Let V1 be the volume of water in the cylinder. To this a known volume of the specimen aggregate say V2 is added and the volume of the mixture recorded. Let the volume of mixture be V, then percentage voids (x) in the aggragates is given by the equation.

   X= V- V1 / V2× 100

   The concrete mixture is finally decided by using approximately 10% more sand and about 15% more cement than the percentage voids in coarse and fine aggregates respectively.

3. Trial or by Maximum density Method:

   This method is based on the principle, that a concrete mix which is formed by so proportionating its aggregates as to give heaviest weight for same volume, yield the densest concrete. Hence in this method a box is filled with varying proportions of fine and coarse aggregates. The proportion which gives heaviest weight is adopted for the work.

4. Fineness Modulus Method:

   The fineness modulus of an aggregate is the term used to indicate the average size of particles in the aggregates. It is represented in the form of an index number. Thus the coarser the aggregate, the higher will be the value of fineness modules.

   In this method the samples of both coarse and fine aggregates are passed through a set of nine standard sieves and the percentage of the sample retained on each of the said sieves is determined. The total of these persentages divided by 100 gives the fineness modulus of the sample. Certain values of fineness modulus for the mixed aggregates have been accepted as giving the best results. Thus if P is the desired fineness modulus for a mix of fine and coarse aggregate, and p1 and p2 be their respective fineness modulus, then the amount of the aggregate (w) to be mixed with one part of coarse aggregate is given by equation:

   W=P2-P- / P-P1

5. Water Cement Ratio Law:

   The proportions of water to cement in any mixture has the greatest influence on the strength of the concrete. According to water cement ratio law, “for any given condition of test, the strength of well compacted concrete with good workability is dependent only on the water cement ratio”.

   The workability of concrete in addition to other factors like grading and proportionating of aggregates, proportion of cement, the efficiency of mixing etc. also depends upon the quantity of water used in the mixture. According to Abram law the strength of the mixture increases not merely by increasing the quantity of cement in the mix but it is on account of reducing the water cement ratio. The strength of concrete in accordance with the law is given by modified formula.

   P28= 984/ 4x
   Where P28 = cylinder crushing strength of concrete in kg / cm2 after 28 days curing.
   X= water cement ratio by volume.