Building Materials, Third Edition

QQR f2w—

—

to combine the fraction I and II in the proportion of, say 40:60 to see whether the combined
grading is obtained. This is given below:

s   2   5 p—™2sD p—™2ssD g˜ h   

  — IHE2wƒeD PHE2wƒeD — —2PHE

RH22™ TH22™ wƒeD2—  22sƒXQVQ

RHE-- RH TH IHH IHH

PSE-- RH TH IHH IHH

PHE-- RH SSP WSP WS22IHH

IPSE-- RH IS SS '

IHE-- QSP VR RQT PS22SS

RUSE-- RV IV ST H22IH

PQTE-- IP ' IP '

It can be observed that if the fractions I and II are combined in the ratio of 40:60 the desired
combined grading as recommended in IS: 383–1970 is obtained. The mix proportions for the
trial mix will now be

Cement water sand coarse fraction I aggregate fraction II

10 mm MSA 20 mm MSA

410.43 183.57 577.4 485.09 727.63

OR, 1 0.447 1.406 1.182 1.773

Example 11.4: Design a concrete mix to be used in structural elements by IS method for
following requirements.
(i) Characteristic compressive strength at 28 days, fck 30 N/mm^2
(ii) Maximum size of the available aggregate 20 mm
(iii) Shape of coarse aggregate angular
(iv) Degree of workability desired, compacting factor 0.85
(v) Degree of quality control good
(vi) Type of exposure moderate
Test data for Concrete Making Materials
Specific gravity of cement 3.15
Specific gravity of coarse aggregate 2.72
Specific gravity of fine aggregate 2.66
Water absorption (air dry to saturated surface dry)
in coarse aggregate, per cent 0.5
Surface moisture
Coarse aggregate nil
Fine aggregate, per cent 2