M-25 CONCRETE MIX DESIGN
DESIGN MIX OF CONCRETE:
The strength is mainly influenced by water cement ratio, and is almost independent of the other parameters the properties of concrete with a compressive strength of 25MPa,are influenced by the properties of aggregate in addition to that of water cement ratio. To obtain good strength, it is necessary to use the lowest possible w/c ratio which affects the workability of the mix. In the present state of art, concrete which has a desired 28days compressive strength of minimum 25 MPa, can be made by suitable proportion of the ingredients using normal methods for compacting the mixes.
MIX DESIGN PROCEDURE:
- The strength of the cement as available in the country today has greatly improved since 1982. The 28-day strength of A, B, C, D, E, F. Category of cement is to be reviewed.
- The graph connecting, different strength of cements and W/C is to be reestablished.
- The graph connecting 28-day compressive strength of concrete and W/C ratio is to be extended up to 80Mpa, if this graph is to be cater for high strength concrete.
- As per the revision of 456-2000, the degree of workability is expressed in terms of slump instead of compacting factor. This results in change of values in estimating approximate sand and water contents for normal concrete up to 35Mpa and high strength concrete above 35Mpa. The table giving adjustment of values in water content and sand % for other than standard conditions, requires appropriate changes and modifications.
- In the view of the above and other changes made in the revision of IS456-2000, the mix design procedure as recommended in IS 10262-82 is required to be modified to the extent considered necessary and examples of mix design is worked out.
WITH EXAMPLE
a) Design stipulations
Characteristic compressive strength required in the field at 28
days : 25Mpa
Maximum size
of aggregate : 20mm
i.
Degree of quality control : Good
ii.
Type of exposure : Mild
b) Tested data for materials
Specific gravity of cement : 3.14
Comp Strength of cement at 7 day = Satisfies there quirement
IS:269-1989
IS:269-1989
Specific gravity of Coarse aggregates :
2.63
Specific gravity of Fine aggregates :
2.75
Water absorption of coarse aggregate : 1%
Free moisture in CA & FA : Nil
c) Target mean strength of concrete
c) Target mean strength of concrete
The target mean strength for specified characteristic cube strength is
f’ck =fck + 1.65 s
25+1.65*4=31.6N/mm2
Where
f’ck= Target mean
compressive strength at 28 day’s in
fck= characteristic compressive strength at 28 day's in N/mm2, and
d) Selection of water - cement ratio
The free w/c ratio required for the target mean strength of 31.6 N/mm2 is 0.55
The
maximum free water-cement ratio for mild exposure is 0.55
The free w/c ratio is taken as the minimum of
the above two values, i.e.
W/c
ratio = 0.47
e) Estimation of air content:
For maximum size of aggregate of 20mm, the air content is taken as 2.0%
f) Selection of water and sand content
From IS method for 10mm max size of aggregate, Sand conforming to grading Zone II. For change in value in W/C ratio, compacting factor and sand belonging Zone II,
W/c ratio adjustments.
Change in condition
|
adjustment required
|
|
Water content %
|
Percentage sand in total aggregate
|
|
For decrease in water-cement ratio(0.60-0.50) that is 0.10
Therefore, 0.10/0.05×1=2.0
|
0%
|
-2.0
|
For increase in compacting factor (0.9-0.8) =0.1
Therefore, 0.1/0.1×3=3.0
|
+3
|
0
|
Total
|
+3
|
-2.0
|
Water content per cubic meter of concrete = 186kg and sand content % of total aggregate by absolute volume = 35%.
Water = 186kg/m3 of
concrete
Sand = 35%
of total aggregate by absolute volume
following
adjustment required. Therefore, required water content = 186+ (186×3)/100
= 186+5.58
=191.6 lit/m3
Therefore, required
sand content as percentage of total aggregate by absolute volume,
P=35-2.0 =33%
f) Determination of cement content
W/C ratio =0.50
Water = 191.6 lit
Cement = 191.6/0.474 =405kg/m3
g) Determination of Coarse and Fine aggregate
contents
Consider volume of concrete=1 m3 but
Entrapped air in wet
concrete=2%
Therefore, absolute volume of fresh concrete,V=1-2/100=1-0.02=0.98m3
Therefore, absolute volume of fresh concrete,V=1-2/100=1-0.02=0.98m3
Taking into account and
applying in equations.
Formula for fine aggregate:-
V= [W+C/Sc+1/P×fa/Sfa]×1/1000
0.98 = [191.6+
(405/3.14) + {fa/ (0.33×2.5)}] ×10-3
fa = 515kg fa= Content of Fine aggregate
Formula for coarse aggregate:-
V= [W+C/Sc+1/ (1-P) ×Ca/Sca]
×1/1000
0.98= [191.6+(405/3.14)+(1/
(1-0.33))×Ca/2.63]×10-3
Ca= 1210 kg Ca=
Content of Coarse aggregate
The mix proportion becomes
Water cement Fine agg. Coarse agg.
|
191.6 lit 405kg 515kg 1210kg
|
0.50 : 1 : 1.274 : 2.99
|
Hence the Mix is 1:1.274:2.99 (Designed for M25)
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