Construction Cost Estimating

      

Detailed Procedure for Concrete Mix Design

Concrete is the prima donna material in most structures. As such, figuring out the right ratio of the elements in the concrete mixture is extremely important before you start building. The process of determining the right proportions of cement, sand and aggregates for concrete is called concrete mix design. This is done in order to achieve target strength of concrete.

Designing the right kind of concrete mix is very important since it gives you the advantage of making the concrete use economy in achieving required strength of structural members. The construction industry uses huge amounts of concrete; so, it is obviously needed to make an economical concrete mix design.

Using the following calculation procedure, you can design many types of concrete, like M10, M20, M30 etc. Let us see how to calculate concrete mix design.

Concrete Mix Design As Per IS 10262

Data Required for Mix Design of Concrete

1. Concrete Mix Design Data

(a) Characteristic compressive strength of concrete required at end of 28 days = M 25
(b) Nominal maximum size of aggregate used = 20 mm
(c) Shape of Coarse Aggregate = Angular
(d) Required workability at site = 50-75 mm (slump Value)
(e) Quality control is done by as per IS: 456
(f) Type of exposure Condition of concrete (as defined in IS: 456) = Mild
(g) Type of cement used = PSC conforming IS: 456 – 2000
(h) Method of placing Concrete on Site = pumpable concrete

2. Material testing data

(a) Specific gravity of cement = 3.15
(b) Specific gravity of FA = 2.64
(c) Specific gravity of CA = 2.84
(d) Aggregates are assumed to be having surface dry conditions
(e) Fine aggregates are confirm to Zone II of IS – 383

Procedure for M25 Concrete Mix Design

Step 1: Determining the Target Strength of concrete

Himsworth constant for 5% risk factor is 1.65
Standard deviation is taken from IS: 456 2000 for M25 Grade of concrete is 4.0
Therefore ftarget = fck + 1.65 x S
= 25 + 1.65 x 4.0 = 31.6 N/mm2
Where,
S = standard deviation (N/mm2) = 4 (Refer: IS 10262- 2009 table -1)

Step 2: Determining water / cement ratio

Maximum water-cement ratio taken for Mild exposure condition = 0.55
Assume water-cement ratio as 0.50 for experiment base
0.5 < 0.55, hence it is OK.

Step 3: Selection of Water Content for mix

Maximum water content taken = 186 Kg (maximum size of aggregate = 20 mm)
Correction in water content is
Estimated water content for mix = 186+ (3/100) x 186 = 191.6 kg /m3

Step 4: Selection of Cement Content for mix

Water-cement ratio = 0.50
Corrected water content for mix = 191.6 kg /m3
Cement content =
Minimum cement Content required for mild exposure condition = 300 kg/m3
383.2 kg/m3 > 300 kg/m3, hence, OK.
This value is required to be checked for durability requirement from IS: 456 – 2000
[In this example for mild exposure and for the reinforced concrete the minimum cement content is 300 kg/m3 which are less than 383.2 kg/m3. Hence cement content adopted = 383.2 kg/m3]
Maximum cement content for mix = 450 kg/m3.

Step 5: Estimation of Coarse Aggregate proportion for mix

For Nominal max. size of aggregate for mix = 20 mm,
Zone of fine aggregate = Zone II
And For w/c = 0.5
Volume of coarse aggregate per unit volume Concrete = 0.62
Hence,
Volume of coarse aggregate per unit volume of total Volume of concrete
= 0.62 x 90% = 0.558
Volume of fine aggregate in mix
= 1 – 0.558 = 0.442

Step 6: Estimation of the mix Materials

a) Volume of concrete is taken = 1 m3
b) Volume of cement = (Weight of cement / Specific gravity of cement) x (1/100)
= (383.2/3.15) x (1/1000) = 0.122 m3
c) Volume of water = (Weight of water / Specific gravity of water) x (1/1000)
= (191.6/1) x (1/1000) = 0.1916 m3
d) Volume of aggregates
= a – (b + c ) = 1 – (0.122 + 0.1916) = 0.6864 m3
e) Weight of coarse aggregates
= 0.6864 x 0.558 x 2.84 x 1000 = 1087.75 kg/m3
f) Weight of fine aggregates
= 0.6864 x 0.442 x 2.64 x 1000 = 800.94 kg/m3

Concrete Mix proportions for Concrete Trial Mix 1

Cement = 383.2 kg/m3
Water = 191.6 kg/m3
Fine aggregates = 800.94 kg/m3
Coarse aggregate = 1087.75 kg/m3
W/c = 0.5

For making trial -1, mass of ingredients required will be calculated for 4 no’s cube assuming 25% wastage.

Volume of concrete required for Making 4 cubes = 4 x (0.153 x1.25) = 0.016878 m3

Step 7: Correction for absorption / moisture of aggregate

As we assumed the aggregate is saturated surface dry condition, therefore no correction is required.

Step 8: Concrete Trial Mix 1

The mix proportion determined in Step 6 Make trial mix1 & with this proportion, concrete is manufactured and tested for fresh concrete properties requirements like workability, bleeding and finishing qualities. In this Trial Mix,

Slump value = 25 mm
Compaction Factor = 0.844

From the Mix Slump test, the concrete is workable and has a true slump of about 25 mm and it is free from segregation and bleeding.

therefore, required slump = 50-75 mm

So modifications and changes are needed in trial mix 1 to get the desired workability.

Detailed Procedure for Concrete Mix Design