METHOD OF DESIGN MIX

Parameters:

1. Characteristic strength of concrete ( grade of concrete )
2. Degree of quality control expected to be exercised at construction site
3. Type and max. size of aggregate
4. Degree of workability desired (slump or compaction factor )
5. Type of Exposure -mild, moderate, etc.
6. Standard deviation of compressive strength of concrete.

Test Data on materials required :

1. Sp. gravity of cement
2. Test data on cement - testing ( 28 days comp. strength if available )
3. Sp. gravity and water absorption of coarse and fine aggregate
4. Grading of coarse and fine aggregates

Steps of mix design :

1. Ascertain characteristic strength.
2. Select workability of concrete . ex-for pumpable slump required is 100 mm(80-120)or for manual 25 to 40 mm.
3. From the known degree of control and the required characteristic strength, determine the standard deviation(SD) (clause 9.2.4.2-IS 456) from table -8 IS-456.
4. Fix the target strength from the equation : Target strength=fck + 1.65*SD
5. Choose the brand of cement
6. Collect the curve of w/c ratio VS target mean strength from cement manufacturer
7. Determine free w/c ratio (from curve of cement manufacturer)
8. Determine Maximum free w/c ratio(IS-456 table -5) . If maximum w/c ratio is greater than w/c ratio chosen from curve then take value(from curve) as ok otherwise take maximum value as per code(lower of the two values).
9. Select and adjust water content - "maximum water content (table -6 of IS -10262) in accordance to use of super plasticizer or any admixture"
10. Select type and maximum size of aggregate
11. Select and adjust fine aggregate percentage -take 33% as per general thumb rule (i.e., fine aggregate-sp 23 table 36 and table 6 of IS -10262)
12. Calculate cement content-having known the quantity of water we go back to step -8 and taking that water-cement ratio , determine the weight of cement from the equation cement in kg=(water in kg /water –cement ratio ).

On going through the above steps , we have obtained the following values :

Cement in kg per Cu m of concrete
Water in kg per Cu m of concrete
The percentage of sand (fine aggregate) in the total aggregate.

13.Check minimum cement content (IS -456 table 5 ) and compare it with that obtained above and apply correction if required, adopt the greater of the two values .

14.Estimation of entrapped air (if required) .This correction is for the entrapped air in concrete. The percentage of entrapped air depends upon the degree of compaction. Even after good compaction, there will be some entrapped air in concrete. (IS – 10262 table -3).

15.Calculate aggregate content: the sand and coarse aggregate content are calculated by absolute volume method using the following equations :-

Equation 1 - V= [W+(c/Sc) + (1/p)*(fa/Sfa)]*(1/1000)

Equation 2- V= [W+(c/Sc) + (1/(1-p))*(Ca/Sca)]*(1/1000)

Where ,

V=absolute volume of fresh concrete, which is equal to gross volume in m3 minus the volume of entrapped air,(being 1% for 40 mm max size of agg, 2% for 20 mm max size of agg and 3 % for 10 mm max size of agg)

W=mass of water kg per m3 of concrete

C=mass of cement kg per m3 of concrete

Sc =specific gravity of cement .

p=ratio of fine aggregate to total aggregate by absolute volume,

fa ,Ca =total masses of sand and coarse aggregate, kg per m3 of concrete respectively

Sfa,Sca=specific gravities of saturated surface dry sand and coarse aggregate respectively.

16.Corrections are to be made in water and aggregate contents for the moisture . which is already there in the coarse, and fine aggregates as supplied at site . if not known, as first approximation one can assume it from experience . Generally, the coarse aggregates carries 0.5% and sand (Zone II to 2%)(sp 23 table 36 )

17.Select type and dosage of admixture-Calculate admixture content.(as per the dosage prescribed by the manufacturer for the type of cement being used in the site)

18.Blending of coarse aggregates-we have to take different sizes of aggregate in such a ratio so it should fall within the limits as per IS -383 table 2 and 5.

19.Tabulate initial trial mix .

20.Conduct the trial mix and check slump(initial, after ½ hr, after 1 hr) cast 9 nos of cubes for testing compressive strength of concrete- 3 cubes for 3 days , 3 cubes for 7 days , 3 cubes for 28 days.

21.Additional trial varying (water +/- 5% , cement +/- 5 %,admixture +/- 10 %) check slump in each of the six cases.

22.Choose the final concrete mix.

Assumptions : (if not given)

1. Specific gravity of fine(2.6) and coarse(2.67) aggregate .
2. Water absorption of fine(1%) and coarse(0.5%) aggregate.
3. Free surface moisture of fine(2%) and coarse(nil) aggregate.
4. Entrapped air(2%).
5. Degree of quality control(GOOD).