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The tests to be conducted on trial mixes are:
(i) slump and/or Vee-Bee test
(ii) determination of unit weight, kg/m^3 , of fresh concrete, and
(iii) making and curing of test cubes for the compression test.
Based on the results of the tests some adjustments are then required to be made to the mix
proportions. For workability, an experienced technologist can adjust the water content by
inspection if the workability falls outside the specified range. Initially, it would be prudent to
without a small quantity of the mixing water until the technologist has visually satisfied
himself that more water is definitely required to obtain the desired workability. If, however, at
the designed water content the workability of the trial mix seems lower than that required,
additional water should be added to achieve the required workability. When the mixing and
workability measurements are completed a change of water content may still be required,
either for use in the field mix or for a further trial mix which may be estimated from various
procedures prescribed in different mix design methods. When measuring workability, the
concrete should also be subjectively assessed for its other rheological properties, which may
require a change in the relative aggregate properties.
The density of fresh concrete should be measured and the resultant value compared with the
density value, kg/m^3 , employed in mix design. If there is an appreciable difference, the
constituent contents in kg/m^3 and the unit proportions of the trial mix will differ from those
given in the initial mix design. The initial design values should then be corrected by the ratio
of measured density to assumed density so as to obtain the actual weights in kg/m^3 of the trial
mix.
The strength test results should become available for comparison with the design strength
and an adjustment made, if required, to the water-cement ratio obtained from the curves given
in Fig 11.2. Minor adjustments may be made to the mix proportion for use in field mixes
without preparing further trial mixes. Nevertheless if large adjustments have to be made to the
water-cement ratio it would be advisable to prepare a second trial mix adopting the revised
proportions and recalculated batch weights based on the updated value for density of the mix.
With a view to avoid delay in making a second trial mix it may be more expedient to make two
or more trial mixes initially using the same water content but with different water-cement
ratio. This has been explained earlier.
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This method, now widely used in the country, is recommended for designing mixes for general
types of construction, using the ingredients of concrete normally available. The design is
carried out for a specified compressive strength and workability of concrete using continuously
graded aggregates. The method can be used for both, reinforced and prestressed concretes. The
method is not to be used for the design of mixes for flexural strength, or when gap graded
(not continuously graded) aggregates, or when puzzolana, or admixtures are used.
The basic assumption made in the IS method is that the compressive strength of concrete is
based on the water-cement ratio of the concrete mix. Further, for a given type, shape, size and
grading of aggregates, the amount of water determines the workability for normal concretes.