Concrete Mix Proportioning

 Concrete Mix Proportioning

Concrete Mix Proportioning

Concrete Mix Proportioning : The determine of the proportions of cement, aggregates and water to attain the desired strength and properties such as workability, durability etc., is called as concrete mix proportioning.

The design of concrete mix is classified into following two types by IS 456:2000 (Clause 9) :

(1)       Nominal mix concrete

(2)       Design mix concrete.

  Nominal Mix Concrete

A concrete mix in which the proportions of cement, aggregate and water are adopted is called as nominal mix. It is not necessary that such a mix will give the desired strength and properties. For example, nominal mix proportions of M15 is 1:2:4. Nominal mix is not used for grades higher than M20. It is used for ordinary concrete works only.

As per IS 456:2000 (Clause 9.3.1). The proportional of materials for nominal mix concrete shall be in accordance with the following Table 1.5.

TABLE 1.5. Proportions for Nominal Mix Concrete

Grade of concrete Total quantity of dry aggregate by mass per 50 kg of cement to be taken as the sum of the individual masses of fine and coarse aggregates Proportion of fine aggregate to coarse aggregate (by mass) Maximum quantity of water per 50 kg of cement (l)
M5 800 Generally 1:2 but may very from 1:1 \[\frac{1}{2}\] to 1:2 \[\frac{1}{2}\] 60
M7.5 625 45
M10 480 34
M15 330 32
M20 250 30

  Design Mix Concrete

The concrete obtained by properly determining the proportions of the ingredients of concrete to get the concrete of desired strength and properties is called as Design Mix Concrete. It is seen that ‘mix design’ not only produce concrete of desired strength but also an economical one. A design mix concrete is preferred over a nominal mix concrete.

IS 10262:2009 is the code for ‘concrete mix design’ which gives recommendations for design of concrete mixes. Following information data is required for the design of a concrete mix.

(i)        Grade of concrete

(ii)       Standard deviation

(iii)     Type of cement

(iv)      Maximum water cement ratio by weight

(v)       Type of aggregate

(vi)      Size of aggregate

(vii)    Minimum cement content (kg/m3)

(viii)   Degree of workability and expose condition

(ix)      Type of admixture to be used.

The step wise procedure for design of concrete mix is as follows :

(i)        Calculate the mean target strength \[(f_{cm}) \]from the desired characteristic strength \[(f_{ck})\]

\[f_{cm}=f_{ck}+1.65\sigma\]

where s is the standard deviation which depends upon the quality control. In the absence of any specific data, Table 8 of code IS 456:2000 or Table 1.6 may be used for values of \[sigma\] .

TABLE 1.6. Assumed Standard Deviation

Grade of conrete Assumed standard deviation
M10 3.50N/mm2
M15
M20 4.00N/mm2
M25
M30 5.00N/mm2
M35
M40
M45
M50

(ii)       Determine the water cement ratio using appropriate charts from IS 20262:2009.

This ratio should not exceed the values given in the Table 5 of IS 456.

(iii)     Determine the water content (Vw) based on workability requirements and nominal maximum size of aggregate from Table 1.8 (Table 2 of IS 10262) and select the ratio of fine aggregate to coarse aggregate (by mass), based on the type and grading of aggregate from Table 3 of IS 10262.

TABLE 1.7. Minimum Cement Content, Maximum Water-Cement Ratio and Minimum Grade of Concrete for Different Exposures with Normal Weight Aggregates of 20 mm Nominal Maximum Size

Sl. No. Exposure Plain Concrete Reinforced Concrete
Minimum Cement Content kg/m3 Maximum Free Water- Cement Ratio Minimum Grade of Concrete Minimum Cement Content kg/m3 Maximum Free Water- Cement Ratio Minimum Grade of Concrete
(1) (2) (3) (4) (5) (6) (7) (8)
(i) Mild 220 0.60 300 0.55 M 20
(ii) Moderate 240 0.60 M 15 300 0.50 M 25
(iii) Severe 250 0.50 M 20 320 0.45 M 30
(iv) Very severe 260 0.45 M 20 340 0.45 M 35
(v) Extreme 280 0.40 M 25 360 0.40 M 40

 

Notes :

  1. Cement content prescribed in this table is irrespective of the grades of cement. The additions such as fly ash or ground granulated blast furnace slag may be taken into account in the concrete composition with respect to the cement conent and water-cement ratio if the suitability is established and as long as the maximum amounts taken into account do not exceed the limit of pozzolona and slag specified in IS 1489 (Part I) and IS 455 respectively.
  2. Minimum grade for plain concrete under mild exposure condition is not specified.

(iv)      Calculate cement content in kg/m3 by dividing the water content with water cement ratio and this cement content should not be less than cement content specified in (IS code Table 5) or Table 1.7 given here.

TABLE 1.8 Maximum Water Content Per Cubic Metre of Concrete for Nominal Maximum Size of Aggregate

S.No. Nominal Maximum Size of Aggregate Maximum Water Content kg
(i) 10 208
(ii) 20 186
(iii) 40 165

 

(v)       Determine the volume of coarse aggregate and fine aggregate.

(vi)      Determine the proportions i.e., volume of cement, water and mass.

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