Gate Handwritten Class Note For Civil Engineer

Gate Handwritten Class Note For Civil Engineer

civilengghub 02:47
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Mix Design of Concrete And Types Of Mixes

civilengghub 09:48

Mix Design of Concrete:
The process of selecting suitable ingredients of concrete and determining their relative amounts with the objective of producing a concrete of the required, strength, durability and workability as economically as possible, is termed the concrete mix design.
Ø Types of Mixes:
1. Nominal Mixes
In the past specifications for concrete prescribed the proportions of cement, fine and coarse aggregates. These mixes of fixed cement aggregate ratio which ensures adequate strength are termed nominal mixes.
2. Standard Mixes
The nominal mixes of fixed cement –aggregates ratio (by volume) vary widely in strength and may result in under or over rich mixes. For this reason, the minimum compressive strength has been included in many specifications. These mixes are termed as standard mixes.
IS:456-2000 has designated the concrete mixes into a number of grades as M10,M15,M20,M25,M30,M35 & M40.In this designation the letter ‘ M ‘ refers to the mix and the number to the specified 28 days cube strength of mix in N/mm .
3.Designed Mix
In these mixes the performance of the concrete is specified by the designer but the mix proportions are determined by the producer of concrete, except that the minimum cement content can be laid down. This is most rational approach to the selection of mix proportions with specific materials in mind possessing more or less unique characteristics. The approach results in the production of concrete with the appropriate properties most economically.

Ø Design of M30 concrete mix as per IS: 10262-2009, Concrete mix proportioning-guidelines:

a)    Grade designation      :    M30


b)    Type of cement           :     OPC 43 grade confirming to IS 8112


c)     Maximum nominal size of aggregates : 20 mm


d)    Minimum cement content         :    320 kg/m


e)    Maximum water cement ratio  :   0.50


f)      Exposure condition                     :   Moderate


g)    Maximum cement content       :    450 kg/m

·       TEST DATA FOR MATERIALS
i) Cement used     :    OPC 43 grade confirming to IS 8112
ii) Specific gravity of cement    : 3.15
iii) Specific gravity of
                      Coarse aggregate  : 2.68
                      Fine aggregate       : 2.65
iv) Water absorption
                          Coarse aggregate  : 0.6 percent
                           Fine aggregate      : 1.0 percent
v) Free (surface) moisture
                        Coarse aggregate    : Nil (absorbed moisture full)
                         Fine aggregate        :  Nil
vi) Sieve analysis
                        Coarse aggregate     : Conforming to Table 2 of IS: 383
                        Fine aggregate         : Conforming to Zone I of IS: 383

·       TARGET STRENGTH FOR MIX PROPORTIONING
f’ck = fck + 1.65 s
Where
f’ck = Target average compressive strength at 28 days,
fck = Characteristic compressive strength at 28 days,
s= Standard deviation
From Table 1 standard deviation, s = 5 N/mm2
Therefore target strength = 30 + 1.65 x 5 = 38.25 N/mm

·       SELECTION OF WATER CEMENT RATIO

From Table 5 of IS: 456-2000, maximum water cement ratio = 0.50 (Moderate exposure)

·       SELECTION OF WATER CONTENT

From Table-2, maximum water content = 186 liters (for 25mm – 50mm slump range and for
20 mm aggregates)
Estimated water content for 25-50 mm slump = 186 liters

·       CALCULATION OF CEMENT CONTENT

Water cement ratio = 0.45
Cement content = 186/0.45 = 413 kg/m3 >320 kg/m3(given)
From Table 5 of IS: 456, minimum cement content for moderate exposure condition = 300 kg/m3
Hence OK

·       MIX CALCULATIONS

The mix calculations per unit volume of concrete shall be as follows

i)Volume of concrete    =  1 m3




ii) Volume of cement                       

                                                         =   




                                                           = [413/3.15] x [1/1000]
                                                           = 0.131 m3

iii)Volume of water                        = [186/1] x [1/1000]
                                                          = 0.186 m3

iv) Volume of all in aggregates (e)    =a – (b + c)
                                                        = 1 – (0.131 + 0.186)
                                                        = 0.683 m3

v) Volume of coarse aggregates    = e x Volume of CA x specific gravity of CA
                                                       = 0.683 x 0.61 x 2.68 x 1000
                                                       = 1117 kg

l) Volume of fine aggregates          = e x Volume of FA x specific gravity of FA

                                                       = 0.683 x 0.39 x 2.65 x 1000
                                                       = 706 kg
·       MIX PROPORTIONS FOR TRIAL NUMBER
Cement = 413 kg/m3

Water = 186 kg/m3

Fine aggregate = 706 kg/m3

Coarse aggregates = 1117 kg/m3

Water cement ratio = 0.45