Project on Concrete Mix Design for various grades of Concrete

Calculate the Concrete Mix Design for M35 grade concrete with fly ash & M50 grade concrete without Fly ash

CONCRETE MIX DESIGN FOR M35 GRADE WITH FLY ASH

AIM: To calculate the mix proportion for the M35 grade of concrete with the inclusion of fly ash.

INTRODUCTION:

Preparing concrete of the desired compressive strength requires a precise step-wise calculation of proportions of the different ingredients of concrete. The ingredients such as cement, sand, aggregate, chemical, and mineral admixtures are to be mixed in a pre-defined ratio with controlled addition of water in accordance with the desired strength and other properties of concrete. The addition of fly ash brings about many advantages into the concrete mix such as increased overall strength and durability, enhanced workability, and so on. Proper mix proportion design of concrete is also necessary from the economical point of view as it projects the exact ratio in which the materials are to be used and hence, minimizing the wastage. The design procedure involves various steps that have to be conducted in a systematic fashion so as to attain the accurate mixed proportion that fulfills the site requirements. A set of data regarding various properties of the materials used in the mix design is also analyzed before going ahead with the design procedure. A detailed mix design of M35 grade of concrete with fly ash as the mineral admixture is discussed in this project.

PROCEDURE:

Stipulations for proportioning

• Grade designation: M35
• Cement type: Ordinary Portland Cement conforming to the specifications as per IS 12269
• Maximum nominal aggregate size: 20mm
• Minimum cement content: 320kg/m^3 (IS-546: 2000)
• Maximum water to cement ratio: 0.45 as per table 5 of IS-456: 2000
• Workability: 100mm slump for pumpable concrete
• Exposure condition: Severe (R.C.C)
• Concreting method: Pumping
• Degree of supervision: Good
• Type of aggregate: Crushed angular aggregates
• Maximum cement content: 450kg/m^3
• Chemical admixture used: Superplasticizer
• Mineral admixture used: Fly Ash (30% of the total cementitious material)

Test Data for Materials

• The specific gravity of cement: 3.15
• The specific gravity of course aggregate: 2.74
• The specific gravity of fine aggregate: 2.74
• The specific gravity of fly ash: 2.2
• Water absorption of course aggregate: 0.5%
• Water absorption of fine aggregate: 1%

Step-1: Calculating the target mean strength of concrete (f'ck)

f'ck = fck + 1.65 x S

where,

f'ck= target mean strength of concrete

fck= desired compressive strength of concrete (concrete grade)

S= standard deviation as per table 1 1of IS-10262: 2009 (5 for concrete grade M30 and above)

Therefore,

f'ck= 35 + 1.65 x 5 = 43.25 N/mm^2

Step-2: Selection of water to cement ratio

water to cement ratio is selected as per table 5 from IS-456: 2000 on the basis of the exposure conditions.

And since we have the severe weather exposure condition;

The maximum water to cement ratio selectable shall be equal to 0.45

But, based on experience, it is always advisable to go for the lower W/C ratio for a better mix

Hence, we adopt the water to cement ratio W/C as 0.40

Step-3: Obtaining the water content

For obtaining the appropriate water content, Table 2 from the IS-10262: 2009 is referred to.

As we have acquired 20mm maximum nominal size of the aggregate, water content for that shall be 186 liters.

But, the specified water content in the table is in accordance with the slump value ranging from 25mm to 50mm. And we have to increase an amount of 3% for every 25mm increase in the slump.

Since, we have a slump value of 100mm for pumpable concrete,

the estimated water content shall be

W = 186 + 6/100 X 186 = 197 litres

But since we're using the superplasticizers as the chemical admixture, it will bring down the water content by up to 30% or more.

Based on experience, a 29% reduction in the water content is considered upon using the superplasticizer.

Hence,

W = 197 X 0.71 = 140 litres

Step-4: Attaining the cement content

As we have the W/C ratio and W (water content) by our side, we can easily calculate the amount of cement content in the mix.

W/C = 0.40

140/C = 0.40

C = 140/0.40 = 350 kg/m^3

And according to table-5 IS-456: 2000, the minimum cement content for severe exposure = 320 kg/m^3

since 350>320

Hence, OK.

Step-5: Fly Ash considerations

Additional 10% of the cement content is to be increased to accommodate for the fly ash mixture.

Therefore,

Cementituous content = 350 x 1.10 = 385kg/m^3

Now,

W/C = 140/385 = 0.364

30% of the cementitious materials to be replaced by fly ash as the mineral admixture

Therefore,

fly ash content = 30% x 385 = 115 kg/m^3

Cement content after using 30% fly ash = 385 - 115 = 270 kg/m^3

The overall amount of cement saved by using fly ash = 350 - 270 = 80 kg/m^3

Step-6: Determination of the volume of coarse and fine aggregate

According to table 3 of IS-10262: 2009, the volume of coarse aggregate per unit volume of the total aggregate for the maximum nominal aggregate size of 20mm conforming to zone-1 as per the water to cement ratio of 0.50 = 0.60

But we have adopted water to cement ratio of 0.40

And, a change of +/-0.01 is to be made for every 0.05 increase or decrease in the water to cement ratio.

According to our condition, the W/C ratio is lower by 0.10 hence, we will have to increase the volume of coarse aggregate by 0.02

Hence,

The volume of coarse aggregate = 0.62

But, as the method of placing concrete is by using pumps, the pumpable concrete reduces this volume by 10%

Therefore,

Volume of coarse aggregate per unit volume of total aggregate = 0.62 x 0.90 = 0.56

Hence, the volume of fine aggregate = 0.44

Step-7: Mix calculation

Mix calculations of different ingredients per unit volume of concrete shall be as follows:

a) Volume of concrete = 1 m^3

b) Volume of cement = (cement mass/ cement specific gravity) x (1/1000) = (270/3.15)x(1/1000) = 0.08 m^3

c) Volume of fly ash = (fly ash mass/ fly ash specific gravity) x (1/1000) = (115/2.2)x(1/1000) = 0.05 m^3

d) Volume of water = (water mass/water specific gravity) x (1/1000) = (140/1)x(1/1000) = 0.14 m^3

e) Volume of chemical admixture: (chemical admixture mass/chemical admixture specific gravity)x(1/1000)=(7.6/1.145)x(1/1000) = 0.006 m^3

f) Volume of fine and coarse aggregate = (a - (b+c+d+e)) = 0.724 m ^3

g) Mass of coarse aggregate = f x volume of coarse aggregate x specific gravity of coarse aggregate x 1000

= 0.724 x 0.56 x 2.74 x 1000 = 1111 kg/m ^3

h) Mass of fine aggregate = f x volume of fine aggregate x specific gravity of coarse aggregate x 1000

= 0.724 x 0.44 x 2.74 x 1000 = 873 kg/m ^3

Step-8: Obtaining the mixed proportion

The attained masses of all the ingredients of concrete are as follows:

• Cement = 270 kg/m ^3
• Fly-Ash = 115 kg/m ^3
• Water = 140 kg
• Fine aggregate = 873 kg/m ^3
• Coarse aggregate = 1111 kg/m ^3
• Chemical admixture = 7.6 kg/m ^3
• Water to cement ratio = 0.40

Hence, according to the obtained values, the mix design ratio shall be as

Mix proportion = 1: 3.23: 4.11

Results

The obtained mix design ratio for the M35 grade of concrete using fly ash is 1: 3.23: 4.11. However, according to the specific circumstances at the site, two or more trials with 10% variation in water content shall be conducted before finalizing the mix proportion of concrete for the desired compressive strength and workability.

CONCRETE MIX DESIGN FOR M50 GRADE WITHOUT FLY ASH

AIM: To calculate the mix proportion for the M50 grade of concrete without fly ash

INTRODUCTION:

Although using fly ash brings many benefits to the concrete mix design, but fly ash has its limitations and can't be utilized under certain conditions. For instance, under extreme cold weather, fly ash is said to extend the setting time of concrete leading to delay in construction. Hence, it is advisable to design concrete without fly ash in such situations. A detailed mix design for M50 grade of concrete without using fly ash is covered in this part of the project.

Stipulations for proportioning

• Grade designation: M50
• Type of cement: Standard OPC 53 grade cement conforming to IS8112
• Maximum nominal aggregate size: 20mm
• Exposure condition: Very severe (R.C.C)
• Minimum cement content: 340 kg/m^3 (Table 5 IS-456: 2000)
• Maximum W/C ratio: 0.45 (Table 5 IS-456: 2000)
• Workability: Slump value of 100mm for pumpable concrete
• Method of concreting: Pumping
• Degree of supervision: Good
• Aggregate type: Crushed angular aggregate
• Maximum cement content: 450 kg/m^3
• Chemical admixture used: Superplasticizer

Test data for materials

• Cement specific gravity: 3.15
• Coarse aggregate specific gravity: 2.74
• Fine aggregate specific gravity: 2.74
• Water absorption of coarse aggregate: 0.5%
• Water absorption of fine aggregate: 1%

Step-1: Target mean strength

f'ck = fck + 1.65 x S

where,

f'ck= target mean strength of concrete

fck= desired compressive strength of concrete (concrete grade)

S= standard deviation as per table 1 1of IS-10262: 2009 (5 for concrete grade M50 and above)

Therefore,

f'ck= 50 + 1.65 x 5 = 58.25 N/mm^2

Step-2: Determination of water to cement ratio

water to cement ratio is selected as per table 5 from IS-456: 2000 on the basis of the exposure conditions.

And since we have very severe weather exposure conditions;

The maximum water to cement ratio selectable shall be equal to 0.45

But, based on experience, it is always advisable to go for the lower W/C ratio for a better mix

Hence, we adopt the water to cement ratio W/C as 0.40

And since, 0.40<0.45 Hence ok.

Step-3: Water content selection

Referring to table 2 of the IS-10262: 2009, the maximum water content for 20mm nominal maximum aggregate size is 186 liters.

But, this value is in correlation with the 25-50mm slump of concrete. And since we're aiming for 100mm slump workability, we will have to increase this amount by 6%

Therefore,

W = 186 + 6% X 186 = 197 liters

But, usage of superplasticizer as the chemical admixtures bring this value down by up to 30%

Hence, we are considering a water content reduction of 25% because of the superplasticizer.

Therefore,

W = 197 X 0.75 = 148 liters

Step-4: Obtaining cement content

As we have already determined W/C ratio and water content (W), we can obtain the cement content as below:

W/C = 0.40

148/C = 0.40

C = 148/0.40 = 370 kg/m^3

And according to table 5 IS-456: 2000, minimum cement content for very severe exposure condition = 340 kg/m^3

370>340, Hence ok.

Step-5: Determination of the volume of coarse and fine aggregate per unit volume of total aggregate

According to table 3 of IS-10262: 2009,

for the maximum nominal aggregate size of 20mm, the volume of coarse aggregate for zone-1 as per the W/C ratio of 0.50 = 0.60

But, since we're using a water to cement ratio of 0.40 which is 0.10 lesser than specified, we will have to increase the coarse aggregate volume of 0.02 to accommodate for the chosen water to cement ratio because, for every +/-0.05, increase or decrease of 0.01 has to be made.

Therefore,

Volume of coarse aggregate = 0.62

However, placing concrete by pumping can reduce this value by 10%

So,

Volume of coarse aggregate = 0.62 x 0.90 = 0.56

Volume of fine aggregate = 1 - 0.56 = 0.44

Step-6: Mix calculations

Volumes of all the ingredients per unit volume of concrete are as follows:

a) Volume of concrete = 1 m^3

b) Volume of cement = (cement mass/ cement specific gravity) x (1/1000) = (370/3.15)x(1/1000) = 0.117 m^3

c) Volume of water = (water mass/water specific gravity) x (1/1000) = (148/1)x(1/1000) = 0.148 m^3

d) Volume of chemical admixture: (chemical admixture mass/chemical admixture specific gravity)x(1/1000)= (7.6/1.145)x(1/1000) = 0.006 m^3

e) Volume of fine and coarse aggregate = (a - (b+c+d)) = 0.73 m ^3

f) Mass of coarse aggregate = e x volume of coarse aggregate x specific gravity of coarse aggregate x 1000

= 0.73 x 0.56 x 2.74 x 1000 = 1120 kg/m ^3

h) Mass of fine aggregate = f x volume of fine aggregate x specific gravity of coarse aggregate x 1000

= 0.73 x 0.44 x 2.74 x 1000 = 880 kg/m ^3

Step-7: Mix ratio

The attained masses of all the ingredients of concrete are as follows:

• Cement = 370 kg/m ^3
• Water = 148 kg
• Fine aggregate = 880 kg/m ^3
• Coarse aggregate = 1120 kg/m ^3
• Chemical admixture = 7.6 kg/m ^3
• Water to cement ratio = 0.40

Hence, according to the obtained values, the mix design ratio shall be as

Mix proportion = 1: 2.38: 3

Results:

After carrying out the step-wise calculations, the mix proportion for the M50 grade of concrete for R.C.C under very severe exposure condition is obtained as 1: 2.38: 3. However, two or more trial calculations with 10% variation in water content shall be conducted so as to come down to the proportion that is perfectly suited for the particular site conditions.