Unit 2 : concrete mix design

UNIT 2

CONCRETE MIX DESIGN

Mix design:

It is defined as the process of selecting a suitable ingredients of concrete and determining their relative proportions with the objective of producing concrete of certain minimum strength and durability as economical as possible.

Purpose / objective:

To achieve stipulated minimum strength To make concrete in most economical manner.

Methods of calculating mix design:

IS method ACL committee method Grading C curve method (road note No.4 method )IRC 44 method Arbitrary proportion Maximum density method Fineness modulus method Surface area methodDesign based on flexural strength High strength concrete mix design

Variables in proportioning mix design:

w/c ratio cement content /cement aggregate ratiogradation of aggregate

1. IS method:

High strength & medium strength concrete.

Target strength for mix design:

Target average mean compressive strength @ 28 days is given by,

F ck =FCK + ts

F ck target avg. compressive strength@ 28 days

FCK characteristic compressive strength @ 28 days

S standard deviation for each grade of concrete (table 1) of IS 456 : 1978

T statistical value depending on the expected proportion of low result ( risk factor) table : 2

F ck =FCK +1.65 S

2.Selection of w/c ratio:

w/c ratio selected is checked with limiting value .(fig 1& 2)

w/c ratio for durability requirement (table)

Lower value is adapted

3.Estimation of entrapped air :

From table 3 for normal max size of aggregate used .

4.Selection of water content & fine aggregate to total aggregate ratio :

Water content & %of sand of total aggregate by absolute volume are determined from table :4 for medium ( below M35) & high ( above M35 )

Adjustment of values in water content & sand % for other conditions :

Calculation of cement content :

Calculated from w/c ratio & quantity of water per unit volume of concrete values can be checked with minimum cement content for requirement of durability

Grater of two values to be adopted calculations of Aggregate Content :

V = [w+Ca/Sc+1/f* Fa/sFa]* 1/1000

Ca 1-p/p *Fa * sCa/sFa

V= absolute volume of fresh concrete (equal to gross volume .cm3 ) minus volume of entrapped air

W = mass of water (kg/cm3 ) of concrete

Ca = mass of cement (kg/m3 )

Sc = sp. Gravity of cement

P = ratio of Fa to total agg. By absolute volume

Fa, Ca = total mass of FA to CA (kg/m3 )of concrete

sFa , sCa = sp. Gravity of FA & CA

Design m20 grade concrete :

Design stipulation

Characteristics compressive strength required is field @ 28 days =200 kg/ cm2 Maximum size of aggregate =20mm ( angular) =20 N/mm2Degree of workability = 0.9 compacting factor Degree of quantity control = good Type of exposure

Test data

Spe . gravity of cement =3.15Compressive strength of cement @ T days to be satisfied by IS 269 1976 requirement Specific gravity of course agg. =2.6Specific gravity of fine agg =2.6 Water absorption :

Course agg = 0.5%

Fine agg =1.0%

Free surface moisture

CA = nil , fine agg = 2.0%

Solution :

Step 1: target strength for mix design :

Fck =FCK +1.65 S

= 20+1.654.6

= 27.59 N/mm2

Step 2: selection of w/c ratio:

Form fig (1).

w/c ratio is 0.50

std value w/c ratio : 0.65

w/c ratio :0.50

Step 3:estimation of air content table 3 exten :2%

Step 4: selection of water and sand content :

from table (4)

for 20mm size : water content : sand content

186 kg 35%

Step 5: adjustment of values ;

Required sand content as % of total agg by absolute volume (from table 4)

=35 - 4.5 = 30.5%

Required water content :186+ [1863/100]

=191.6 l/m3

Step 6: determination of cement content:

Water cement ratio = 0.50

Water =191.6 l

Cement =[196.6/0.5] = 383.2 kg/m3

Step 7: determination of coarse & fine aggregate :

V = [w+c/sc+1/pFa /sFa]1/100

V = 1-[2/100] = 0.98

W = 191.6 l/m3

C = 383.2 kg/m3

Sc = 3.15

P = 30.5% = 0.305

Fa = ?

sFa = 2.60

0.98= [191.6+(383.2/3.15)+(1/0.305)fa/2.60]1/1000

0.98 = [316.425fa/2.60]1/1000

Fa = 548 kg/m3

To find c:

V = [w+(c/Sc)+(1/1-p)Ca/SCa]1/1000

0.98 = [191.6+(383.2/3.15)+(1/1-0.305)Ca/2.6]1/1000

Ca = 1204.9 kg/m3

Mix proportion :

Design M25 grade concrete:

Max size = 40mm Degree of workability = 0.95 compacting factorDegree of quality control good mildType of exposureSp. Of F.A 2.65Sp.of C.A = 2.7Zone iv

Step 1:

Target strength for mix design :

Fck = FCK + 1.65 S

Fck = 25 +1.65 5.3

Fck = 33.745 N/mm2

Step2:

Selection of w/c ratio:

From fig :

w/c ratio 0.45

std value of w/c ratio = 0.65

w/c ratio 0.45

Step 3:

Estimation of air content :

From the table 3:

For 40 mm size , the air content = 1%

Step 4:

Selection of water and sand content :

From table(4)

Step 5:

Adjustment of values :

Absolute volume = 30- 7 =23

Required water content= 165+(1654.5/100) = 172.425 l/m3

Step 6:

Determination of cement content :

w/c ratio = 0.45

water = 172.425

cement = 172.425/0.45 = 383.16 kg/m3

Step 7:

Determination of course & fine aggregate

V = [w+(c/Sc)+(1/p)+(Fa/sFa)]1/1000

0.99 = [172.425+(383.16/3.15)+(1/23)(Fa/2.05)]1/1000

990 = [172.425+121.63+(1/0.23)Fa/2.65]

695.945 = 1/0.6095 Fa

695.945 = 1.640 Fa

Fa = 424.356

V= [w+(c/Sc)+(1/1-po)Ca/sCa]1/1000

0.99 = [172.425+(383.16/3.15)+(1/0.77Ca/2.7)]1/1000

990 = [294.06+(0.481 Ca)]

695.94 = 0.481 Ca

1446.86 = Ca