Challenge

Project on Concrete Mix Design for various grades of Concrete

VVaradana Hari Chandana9 Mar 2021

Introduction :

CONCRETE: It is a binder material which binds the materials together.It is a compacted mass made by mixing cement,fine aggregate(sand/crushed stone),coarse aggregate,water together.

Concrete Mix Design:

*Concrete mix design is the process of choosing economical, relative proportion of various ingredients from the available material that produces concrete of required strength,durability&workability etc.

*Concrete mix design is a factor for compressive strength of the concrete.

*Concrete mix is of two types

1.Nominal mix :

*In nominal mix the proportions of the ingredients of concrete are specified by Indian standard are choosen from IS456:2000 specified by Indian standard to obtain concrete of adequate strength.

*It is adopted for concrete grade ranging from M5 to M30.

*There is no need to calculate the ingredients proportion ratio ,it can be adopted from IS456:2000 as a result, time for calculation is saved.

*It is adopted for small scale construction works like homes,commercial buildings of small size etc, where the applied load is less than 30N/mm2.

2.Design mix:

*The design mix is adopted for concrete grade greater than M30 .

*The proportions of the ingredients has to be calculated depending upon the type of the structure,its condition etc.

*It is designed by the structural engineer, the design ratio is not constant ,it may vary based on the exposure conditions etc.

*It is adopted for the structures that carry heavy load exceeding 30N/mm2.

Ex:Industrial buildings,bridges etc.

~Nowadays some portion of the cement in concrete is being replaced by fly ash as it is ecofriendly and also to reduce the expenditure on cement.Inorder to calculate the mix design, the fly ash has to be considered in cementitious material.

AIM: To prepare the concrete mix design for M50 grade without fly ash.

PROCEDURE:

*Inorder to calculate the mix design data is required,it is taken as follows:

1.Grade designation of concrete:M35

2.Type of cement :OPC 43 grade conforming toIS8112

3.Maximum nominal size of aggregate:20mm

4.Minimum Cement content:360kg/m3(from table 5 of IS 10262:2009)

5.Maximum water cement ratio:0.40(based on exposure condition from table 5 of IS 10262:2009)

6.Workability:100mm

7.Exposure condition:Extreme

8.Method of placing concrete:pumping

9.Degree of supervision:Good

10.Type of aggregate:Angular

11.Maximum cement content:450Kg/m3

12.Chemical admixture type:Super plasticizer

Test data for materials:

1.Cement used:OPC 43 grade conforming to IS8112

2. Specific gravity of cement:3.15

3.Chemical admixture :super plasticizer conforming to IS9103

4.Specific gravity of fine aggregate:2.74

5.Specific gravity of coarse aggregate:2.74

6.water absorption by coarse aggregate:0.5%

7.Water absorption by fine aggregate:1%

8.Free surface moisture for coarse aggregate:NIL

9.Free surface moisture for fine aggregate:NIL

10.Specific gravity of flyash=2.2

11.Specific gravity of admixture=1.145

*Sieve analysis for coarse aggregate has to be followed as per table 2 of IS383,for fine aggregate table 4 of IS383 has to be taken as reference.

CALCULATION:

1.TARGET MEAN STRENGTH: It is calculated for concrete by the formula

fct=fck+1.65*S

where, fct=target mean strength of concrete,fck=compessive strength of concrete obtained in 28 days,S is the standard deviation obtained from table 1 of IS10262:2009.

*For M35 grade concrete S=5N/mm2.

fct=35+1.65*5=43.25N/mm2

The target strength for M35 grade is 43.25N/mm2

2.SELECTION OF WATER CEMENT RATIO: It is calculated based on the exposure condition ,from the table 5 of IS456:2000.

*Here the given exposure condition is extreme ,for extreme condition water cement ratio is 0.40

3.SELECTION OF WATER CONTENT: It is obtained from table3 of IS456:2000,for 20mm aggregate maximum water content to be used is 186kg when the slump is between 25mmto50mm.For every additional slump of 25mm,3% of water has to be added.

*For 100mm slump,6%has to be added,therefore water content=186+0.06*186=197.16kg.

*As superplasticizer is used, water can be reduced upto 20%and above ,here based on trials 29% is achieved

100-29=71%

Arrived water content=71%197.16=139.9kg.

4.CALCULATION OF CEMENT CONTENT:

*It can be calculated by knowing the water cement ratio and water content as

water cement ratio=weight of water in kg/weight of cement in kg.

*Cementitious material= cement +flyash

*weight of cementitious material in kg=weight of water in kg/water cement ratio=139.9/0.40=349.75kg

obtained value is less than 360kg/m3, therefore it cannot be be taken,360kg/m3 has to be choosen.

*The percentage of fly ash is decided based on the project requirement and quality of materials.

*In some cases increase in cementitious material can be warranted and its percentage is based on experience and trial,here we consider 10%increase in the cementitious material content

cementitious material=360*1.10=396kg/m3

*The water cement ratio for increased cementitious content=139.9/396=0.353

Fly ash to be added=30% increased cementitious material=0.30*396=118.8kg/m3.

cement content= increased cementitious material-fly ash=396-118.8=277.2kg/m3.

Amount of cement saved by using fly ash=349.75-277.2=72.55kg/m3

5.PROPORTION OF VOLUME OF COARSE AGGREGATE AND FINE AGGREGATE:

*Volume of coarse aggregate corresponding to 20mm size,fine aggregate corresponding to grading zone 1 is 0.60 with a water cement ratio of 0.50(from table 3 of IS10262:2009)

*But obtained water cement ratio is 0.40 therefore to increase the water cement ratio , the coarse aggregate size has to be increased inorder to decrease the fine aggregate size.

For every 0.50 change in the water cement ratio ,+/- 0.01 is added to the volume of coarse aggregate.As there is a change of +0.10 in the water cement ratio, we have to add +0.02 to the volume of coarse aggregate.

*Therefore corrected volume of coarse aggregate=0.60+0.02=0.62m3 .

(Here if the coarse aggregate is not of angular type then we have to increase the volume of coarse aggregate based on experience)

*For pumpable concrete the volume of coarse aggregate is reduced by 10%

Arrived volume of coarse aggregate=0.62*90%=0.62*0.90=0.558m3

Volume of fine aggregate=1-volume of coarse aggregate=1-0.558=0.442m3

6.MIX CALCULATION:

*It is calculated per unit volume of concrete

a.Volume of concrete=1m3

b.Volume of cement=(mass of cement in kg/specific gravity of cement)/1000=(277.2/3.15)/1000=0.088m3

c.Volume of water=(mass of water in kg/specific gravity of water)/1000=(139.9/1)/1000=0.1399m3

d.Volume of admixture=(mass of admixture /specific gravity of admixture)/1000=(7.2/1.145)/1000=0.006m3.

here mass of admixture=2%cementitious material=0.02*360=7.2kg

e.Voluume of fly ash=(mass of fly ash/specific gravity of fly ash)/1000=(118.8/2.2)/1000=0.054m3.

f.Volume of coarse and fine aggregate=f=(a-(b+c+d+e))=(1-(0.088+0.1399+0.006+0.054))=0.7121m3

g.Mass of coarse aggregate=f*volume of coarse aggregate*specific gravity of coarse aggregate*1000

=0.7121*0.558*2.74*1000=1088.74kg

g.Mass of fine aggregate=f*volume of fine aggregate*specific gravity of fine aggregate*1000

=0.7121*0.442*2.74*1000=862.41kg.

h.Mix proportion for trial 1:

Cement=277.2kg/m3

water=139.9kg/m3

fly ash=118.8kg/m3

fineaggregate=862.41kg/m3

coarse aggregate=1088.74kg/m3

admixture=7.2 kg/m3

water cement ratio=0.40

RESULT:Trial mix ratio= cement:fine aggregate: coarse aggregate=277.2 :862.41 :1088.74=1 :3.11 : 3.92

*It is the design mix ratio ratio obtained forM35grade concrete with flyash under above specified conditions.

REPORT: The same procedure has to be repeated for some more trials by adjusting the water content ,changing water cement ratio by 10%,admixture content etc and corresponding strength of concrete is noted , the mix design meeting the desired workability and strength of concrete is choosen , addition of fly ash reduced the content of cement as a result expenditure on the cement is saved, it is much adaptable as it is ecofriendly and the strength of concrete doesn't vary .

AIM: To prepare the concrete mix design for M50 grade without fly ash.

PROCEDURE:

*Inorder to calculate the mix design data is required,it is taken as follows:

1.Grade designation of concrete:M50

2.Type of cement :OPC 43 grade conforming to IS8112

3.Maximum nominal size of aggregate:20mm

4.Minimum Cement content:360kg/m3(from table 5 of IS456:2000)

5.Maximum water cement ratio:0.40(from table 5 of IS456:2000)

6.Workability:100mm

7.Exposure condition:Extreme

8.Method of placing concrete:Pumping

9.Degree of supervision:Good

10.Type of aggregate:Angular

11.Maximum cement content:450kg/m3

12.Chemical admixture type:Super plasticizer

Test data for materials:

1.Cement used:OPC 43grade conforming to IS8112

2. Specific gravity of cement:3.15

3.Chemical admixture :Super plasticizer conforming to IS8112