.
Aggregates of different sizes, grading, surface texture
shape and other characteristics and the quality & qty of cement & water used will produce concretes of different strengths. A judicious choice of these elements in a particular proportion is necessary to ensure the characteristic strength and durability of concrete.
Parameters involved:
Standard deviation of compressive strength of concrete.
Cement content
water cement ratio --workability
proportion of fine and coarse aggregates
Grading of aggregates
Size, surface texture and shape of aggregates
vibration
The standard Deviation
According to IS 456, the characteristic strength is defined as that value below which not more than 5% (1 in 20) results are expected to fall.
At site depending on the degree of control, the strength of each cube varies. Some far below and some far above and many scattered around the average strength. The degree of scatter is measured by standard deviation. The greater the value of standard deviation,
the greater is the scatter and less the control at site.
It is usually decided at site based on the test strength of samples. (IS 456)When sufficient test results for a particular grade of concrete is not available, the value can be assumed.
Grade of concrete Assumed standard deviation
M10, M15 3.50
M20, M25 4.00
M30, M35, M40 M45, M50 5.00
These values apply when the concrete is produced in fully controlled condition-with weigh batching, control on w/c ratio & workability, proper aggregate grading &mixing.
Where there is a deviation from eh above, increase the values by 1
1.Target Strength of Concrete (at 28 days) as per IS 456,
If, Ft is the target strength, Fck the characteristic strengths the standard deviation,
Ft = Fck+1.65X S
S can be taken from the table above.
For M20, Ft = 28.25 N/mm2 (Like this you may find out for any grade)
2. Selection W/C ratio
a. From table 5 of IS 456, the maximum w/c ratio can be ascertained
b. From fig.2 of IS 10262 -1982, find the limiting w/c ratio for a compaction factor. Chose the lower of the 2
For M20, a gives a value of .55 and b gives 50. Hence fix it as >50
3. Cement content
From Is 456, Table 5 the cement required is 300Kg.
(You may have to increase this weight if the required strength is not obtained)
It can become 310 or 320 but not more generally)
4. Water in Kg = Cement in Kg X W/C ratio
Hence Qty.of water = 300x.5= 150 liters
5. Determination of fine and coarse aggregates:
The coarse aggregates should confirm to IS 380-1970
and fine aggregate either Zone two or three
The % of sand (from IS 10262) for 20mm aggregate is 0.35
The specific gravity of coarse aggregate is 3.15
The specific gravity of fine aggregate is 2.6
From table 3of IS 10262, for 20 mm aggregate, the amount of entrapped air in wet concrete is 2%. Taking this into account, apply the formula,
V= X 1/1000
Where, V=Volume of concrete
W=weight of water
C=weight of cement
Sc=specific gravity of cement
P=% of fine aggregate
Fa=weight of fine aggregate
Sfa=specific gravity of fine aggregate
Thus, 0.98= <150 + (300/3.15) + 1/.35XFa/2.6>X1/1000
From this, find out the value of Fa (around 650Kg)
Now taking up equation 2 from IS 10262,
V= X1/1000
(Where Ca is the weight of coarse aggregate & Sca its specific gravity.)
From this equation, Sca works out to around 1200 kg.
It is important to decide the specific gravity of:
Sand
Coarse aggregate
and the quality of water going to be used for construction.
Also, it very important to fix the grading of the aggregates to avoid voids. The lower the size of the coarse aggregate, the higher will be air entrapping nature. Hence a judicious mix is to be decided.
By adding a suitable admixture, the w/c ratio can be reduced and correspondingly the cement content reduced.
Published: January 05, 2008
More summaries by varghese
More