Building Materials, Third Edition

g™

2w2h QPQ

reasonable number of lows the engineer can hold the contractor to a standard which is possible
to attain. It may be mentioned that the engineer-in-charge may permit a higher percentage of
lows for mass concrete, foundation concrete, and for concrete used in hydraulic structures.
The savings in cement is made possible by quality control. However, this should not be
regarded as the sole advantage, or even the main advantage, of using quality control methods.
A well-designed and controlled concrete will be of uniform workability; this should result in
the avoidance of porous honeycombed patches and weak patches of laitance. In consequence
there will be no need to ‘make-good’ or to hack out and repair such patches. A good uniform
concrete, moreover, should be less permeable and more durable than one that contains weak
sections, and this should make for reduced maintenance costs.

IIS €‚y€y‚„syxsxq2„ri2sxq‚ihsix„ƒ

Some of the important methods of concrete mix design are the Indian Standard Method, the
Road Research Laboratory (RRL) method, the Department of Environment (DOE) method, the
American Concrete Institute (ACI) method, the Surface Index method, the Trial Mix method,
the Maximum Density method, the Minimum Voids method, and the Fineness Modulus method.
The use of any one of these methods, involves making preliminary and trial mixes, conducting
tests on them and making adjustments in the proportions based on the test results. The scope
of the book permits elaboration of IS method of mix design only.

€—2w



The object of any mix design method is to arrive at the optimum mix proportions of the various
ingredients that will impart to the concrete the properties of workability, strength and durability
as near to that as specified. To start with, a preliminary mix is prepared of about 12 kg, using
the mix proportions given in any mix design method. The purpose of the preliminary mix is to
judge whether the desired properties of the fresh concrete, such as workability, cohesiveness,
compactability, finishability, etc. can be obtained with the proportions selected. This is judged
by the mix designer through visual observation of the preliminary mix and certain thumb rules
based on past experience.
If the mix is too sloppy the water content should be reduced and the cement and aggregate
contents reworked, if the mix is too harsh and segregates, the fine aggregate should be increased
and the coarse aggregate reduced in the same proportion, keeping the total aggregate-cement
ratio constant. Depending on the experience of the designer two to three preliminary mixes
might be necessary to arrive at the optimum mix proportions which will most likely meet the
design parameters for the fresh concrete.

„—2w



The mix proportions arrived at from the preliminary mixes are then used to make a ‘trial mix’.
Generally one trial mix is sufficient but to ensure the designed strength of concrete at 7 and 28
days more than one trial mix is prepared in such a way that the water-cement ratio is increased
or decreased from that obtained from the trial mix by 10 per cent. Necessary adjustments have,
of course, to be made in the cement content and quantities of coarse and fine aggregates
keeping the total quantity of water per m^3 of concrete constant in all the so obtained three
mixes. These additional trial mixes are prepared to avoid delay in remaking the trial mixes in
case the first one does not meet the strength requirements at 7 and 28 days.