PROPORTIONING OF CONCRETE

PROPORTIONING OF CONCRETE

         

          This is a process of determining the proportion of cement of fine aggregates and coarse aggregates for concrete mix is called proportioning of concrete.
           The principal of proportioning of aggregates is that the smaller particles will fill up the voids between large particles. A well proportioned concrete mix requires minimum amount of cement per unit volume.

factors Those Effects

• Strength
• Workability
• Types Of Aggregate
• Grading Of Aggregates

Strength

           The mix should be designed for a considerably mean strength required for structural use.

Workability

           The term workability indicate the ease of difficulty with which the concrete is handled, transported and placed. The amount of water present in the concrete should be in proper ratio. If water is much more than required, it will result in non-uniform composition of concrete. If water is less than required it will result in difficulty to handle and place it in position.

Types of Aggregates

         The shape of aggregates should be irregular and surface of aggregates should have rough texture.

Grading of Aggregates

      The grading of aggregates is the art of dong gradation (particle size distribution) of an aggregates as determined by sieve analysis.
       Fine and coarse aggregates to be used in a concrete mix, should be well graded because grading of an aggregate has considerable effect on workability, uniformity and finishing qualities of plastic concrete.

 METHOD Of PROPORTIONING

•  Arbitrary method 
• Trial mixtures method
• Minimum voids method

Arbitrary method

         In This method one part of cement, M part of fine aggregate and 2M parts of coarse aggregate are taken as the basis.

       The quantity of water required for mixing is determined by according to desired workability.

        By this method such mixes as 1:11/2:3 (one part cement, 11/2 parts fine aggregate, 3 parts coarse aggregate) 1:2:4, 1:3:6, 1:4:8, are arbitrarily chose by volume.

        These days the ratio of of fine to coarse aggregate is not taken as 1:2 as experiments have shows that the quantity of fine aggregate needed is more than 50% of the coarse aggregate; thus the ratio of coarse to fine aggregate; may be varied from 11/2 to 31/2.

 Limitations of Arbitrary method

1.  The lack of proper control over the quantity of water that goes into a batch of concrete tends to result in a concrete of varying strength and denseness.
2. The fixed specified proportioning of cement, fine aggregate and coarse aggregate do not permit taking advantage of favourable types and grading of aggregates to secure a workable mix at the least possible cost of materials, and without harm to the strength of the concrete.

             However, this method is simple and if carefully considered gives fairly satisfactory results.

Trial mixtures method

         A certain weight of cement, fine aggregate, coarse aggregate, and water are put into a graduated cylindrical vessel and tamped.
       The vessel is emptied after noting the height of the upper surface.
       The vessel is then carefully and the method repeated for several trial mixtures in varying proportions.
        The mixture that gives the smallest volume for the same weight i.e. one which is the densest is adopted.
        This method is quite suitable for impermeable construction.

Minimum voids method

       This method is based on the principle that the concrete which has the minimum voids is the densest and strongest. Amount of fine aggregate should be such as to fill the voids.