Project on Concrete Mix Design for various grades of Concrete

AIM

To calculate the concrete mix design for M35 grade concrete with fly ash.

INTRODUCTION

Concrete mix design is a procedure of selecting the suitable ingredients of concrete and their relative proportions with an objective to prepare concrete of certain minimum strength, desired workability, and durability as economically (value engineered) as possible.

Flyash is the most economical and easily available mineral admixture that can partially replace cement in a concrete mix, the resultant concrete will have excellent fresh, hardened, and durable properties.

The advantages of fly ash concrete over the corresponding plain cement concrete are:

• Improved workability.
• Lower heat of hydration.
• The increase in creep with fly ash content up to 15% is negligible.
• Increases the modulus of elasticity of concrete when concretes of the same strength with and without fly ash are compared.
• Superior resistance to freezing and thawing.
• Improved sulphate resistance.
• Lower water and air permeability.
• Lower leaching of lime liberated during hydrated cement.
• Reduced alkali-aggregate reactions.
• Greater resistance to attack of aggressive waters.

Since a huge quantity of cement is used in concrete in mass concrete construction and the cost of fly ash is negligible as compared to that of cement, the use of fly ash concrete brings about a substantial saving in cement consumption and overall construction cost.

Fly ash concrete may be used in general RCC structures including high-strength concrete without any risk of steel corrosion. Researchers have proved that concrete with approved quality fly ash does not induce corrosion of reinforcing steel even in marine and industrial aggressive environments. With proper mix design, the 7 and 28-days strength of fly ash concrete may be equal to or even more than plain concrete. The 90 days strength of fly ash concrete may be more than 140% of plain concrete. The cost of fly ash is negligible. Therefore, the use of fly ash in structural concrete may bring a substantial saving in cement consumption and the overall cost of concrete production. Fly ash is an industrial waste and a great hazard to our environment. The designers of concrete structures therefore must incorporate the use of fly ash in their structural concrete.

M35 grade stands for a mix of concrete with characteristic compressive strength of 35 N/mm². The mix design procedure for M35 grade concrete as per IS Code is discussed.

PROCEDURE IN DETAIL

PRELIMINARY DATA REQUIRED FOR MIX DESIGN

SPECIFICATIONS FOR PROPORTIONING

1. Grade designation: M35
2. Type of cement: OPC 53 Grade confirming to IS 8112
3. Type of mineral admixture: Fly ash conforming to IS 38I2 (Part 1)
4. Maximum nominal size of aggregate: 20mm
5. Minimum cement content: 340 kg/m³ (Table 5 of IS 456:2000)
6. Maximum water-cement ratio: 0.45 (Table 5 of IS 456:2000)
7. Workability: 100mm slump
8. Exposure condition: Severe (For Reinforced Concrete)
9. Method of concrete placing: Pumping
10. Degree of supervision: Good
11. Type of aggregate: Crushed Angular Aggregates
12. Maximum cement content: 450 kg/m³ (clause 8.2.4.2 of IS 456:2000)
13. Chemical admixture type: Superplasticizer conforming to IS 9103

TEST DATA FOR MATERIALS

1. Cement used: OPC 53 Grade confirming to IS 8112
2. Specific gravity of cement: 3.15
3. Fly ash: conforming to IS 38I2 (Part 1)
4. Specific gravity of fly ash: 2.2
5. Chemical admixture: Super Plasticizer conforming to IS 9103
6. Specific gravity of
• Coarse aggregate: 2.74
• Fine aggregate: 2.74
7. Water absorption
• Coarse aggregate 0.5 %
• Fine aggregate 1 %
8. Free (surface) moisture:
• Coarse aggregate: Nil
• Fine aggregate: Nil
9. Sieve analysis:
• Coarse aggregate: Conforming to all in aggregates of Table 2 of IS 383
• Fine aggregate: Conforming to Grading Zone II of Table 4 of IS 383

TARGET STRENGTH FOR MIX PROPORTIONING

f'_ck =f_ck + 1.65s

where

f’_ck = target average compressive strength at 28 days,

f_ck = characteristics compressive strength at 28 days, and

s = standard deviation.

From Table 1 of IS 10262:2009,

Standard Deviation, s = 5 N/mm².

Therefore,

Target strength = 35 + 1.65 x 5 = 43.25 N/mm²

SELECTION OF WATER-CEMENT RATIO

From Table 5 of IS 456 for severe exposure maximum water-cement Ratio is 0.45

Adopted w/c ratio= 0.40

0.40 < 0.45 (Hence OK)

SELECTION OF WATER CONTENT

From Table 2 of IS 10262:2009, maximum water content for 20 mm aggregate = 186 litres (for 25 to 50 mm slump range)

However, we need water content for a 100mm slump. Clause 4.2 suggests increasing 3% of water content to get an extra 25mm slump from a 50mm slump

Therefore, as per clause 4.2 for aggregate size 20mm, and for slump 100mm

Water-content for 100mm Slump = 186 + (0.06*186)

= 197 litres

As a superplasticizer is used, the water content can be reduced by up to 30%

Based on trials with superplasticizer water content reduction of 25% has been achieved Hence the arrived water content = 197*0.75 = 147.75 litres

Adopted water content = 148 litres

CALCULATION OF CEMENT AND FLY ASH CONTENT

Adopted w/c ratio= 0.40

Cementitious material (Cement + fly ash) required = water content / w/c ratio

= 148/0.40

=370 kg/m³ > 340 kg/m³ (Hence OK)

Now, to proportion a mix containing fly ash the following steps are suggested:

1. Decide the percentage of fly ash to be used based on project requirement and quality of materials
2. In certain situations increase in cementitious-material content may be warranted, The decision on the increase in cementitious material content and its percentage may be based on experience and trial

Since fly ash is not as active as cement. It is usual to increase the cementitious material by some percentage. In this example, an increase of 10% in cementitious material is considered.

Hence, cementitious material (Cement + fly ash) content = 370 x 1.10

= 407 kg/m³

Water content = 148 litres

w/c ratio = 148/407

= 0.364

Let us assume the percentage of fly ash is 30%

Fly ash content = 407 x 0.30

= 122.0 kg/m³

Cement content = 407 – 122

= 285 kg/m³

Therefore, saving of cement while using fly ash

= 370 – 285 = 85 kg/m³

Fly ash being utilized = 122 kg/m³

PROPORTION OF VOLUME OF COARSE AGGREGATE AND FINE AGGREGATE CONTENT

From Table 3 of (IS 10262:2009) volume of coarse aggregate corresponding to 20 mm size aggregate and fine aggregate (Zone II) for the water-cement ratio of 0.50 =0.62

In the present case, the water-cement ratio is 0.40

Therefore, the volume of coarse aggregate is required to be increased to decrease the fine aggregate content. As the water-cement ratio is lower by 0.05. The proportion of the volume of coarse aggregate is increased by 0.01 (at the rate of -/+ 0.01 for every ± 0.05 change in water-cement ratio).

Therefore, the corrected proportion of the volume of coarse aggregate for the water-cement ratio of 0.4 = 0.64

For pumpable concrete, these values should be reduced up to 10%

Therefore,

Volume of coarse aggregate =0.64 x 0.9 =0.576

Volume of fine aggregate content = 1 – 0.576= 0.424

MIX CALCULATIONS

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

1. Volume of concrete = 1 m³
2. Volume of cement = [Mass of cement] / {[Specific Gravity of Cement] x 1000} = 285/ (3.51*1000) = 0.081 m³
3. Volume of fly ash = [Mass of fly ash] / {[Specific Gravity of fly ash] x 1000} = 122/ (2.2*1000) =0.055 m³
4. Volume of water = [Mass of water] / {[Specific Gravity of water] x 1000} = 148/ {1 x 1000} = 0.148m³
5. Volume of chemical admixture (superplasticizer) @2.0% by mass of cementitious material = 407 × 2% = 8.14 kg/m³
6. [Mass of superplasticizer] / {[Specific Gravity of superplasticizer] x 1000} =8.14/ (1.145*1000) =0.007 m³
7. Volume of total aggregates = a – (b + c + d + e) = 1- (0.081 + 0.055 + 0.148 + 0.007) =0.845 m³
8. Mass of coarse aggregates = f × Volume of coarse aggregate × Specific gravity of coarse aggregate × 1000 =0.845 x 0.576 × 2.74 × 1000 =1333.61 kg/m³
9. Mass of fine aggregates = f x volume of fine aggregate × Specific gravity of fine aggregate × 1000 = 0.845 × 0.424 × 2.74 × 1000 =981.68 kg/m³

MIX PROPORTIONS

Cement = 285 kg/m³

Fly ash = 122 kg/m³

Water = 158 l/m³

Fine aggregate = 981.68 kg/m³

Coarse aggregate = 1333.61kg/m³

Chemical admixture = 8.14 kg/m³

Water-cement ratio = 0.40

RESULT

The desired proportion by weight of M35 grade of concrete with fly ash is 1:3.44:4.68

AIM

To calculate the concrete mix design for M50 grade concrete without fly ash.

INTRODUCTION

M50 grade stands for a mix of concrete with characteristic compressive strength of 50 N/mm². The mix design procedure for M50 grade concrete as per IS Code is discussed.

M50 Grade concrete is most suitably in heavily reinforced structures to withstand dynamic loading.

PROCEDURE IN DETAIL

STIPULATIONS FOR PROPORTIONING

1. Grade designation: M50
2. Type of cement: OPC 53 Grade confirming to IS 8112
3. Maximum nominal size of aggregate: 20mm
4. Minimum cement content: 360 kg/m³
5. Maximum water-cement ratio: 0.35 ( 1 IS 10262:2019)
6. Workability: 100mm slump
7. Exposure condition: Severe (For Reinforced Concrete)
8. Method of concrete placing: Pumping
9. Degree of supervision: Good
10. Type of aggregate: Crushed Angular Aggregates
11. Maximum cement content: 450 kg/m³ (clause 8.2.4.2 of IS 456:2000)
12. Chemical admixture type: Superplasticizer conforming to IS 9103

TEST DATA FOR MATERIALS

1. Cement used: OPC 53 Grade confirming to IS 8112
2. Specific gravity of cement: 3.15
3. Chemical admixture: Super Plasticizer conforming to IS 9103
4. Specific gravity of
   • Coarse aggregate: 74
   • Fine aggregate: 74
5. Water absorption
   • Coarse aggregate 5 %
   • Fine aggregate 1 %
6. Free (surface) moisture:
   • Coarse aggregate: Nil
   • Fine aggregate: Nil
7. Sieve analysis:
   • Coarse aggregate: Conforming to all in aggregates of Table 2 of IS 383
   • Fine aggregate: Conforming to Grading Zone II of Table 4 of IS 383

TARGET STRENGTH FOR MIX PROPORTIONING

f'_ck =f_ck + 1.65s

where

f’_ck = target average compressive strength at 28 days,

f_ck = characteristics compressive strength at 28 days, and

s = standard deviation.

From Table 1 of IS 10262:2009,

Standard Deviation, s = 5 N/mm².

Therefore,

Target strength = 50 + 1.65 x 5 = 58.25 N/mm²

SELECTION OF WATER-CEMENT RATIO

From Fig. 1 IS 10262:2019 maximum water-cement Ratio is 0.35

Adopted w/c ratio= 0.35

SELECTION OF WATER CONTENT

From Table 2 of IS 10262:2009, maximum water content for 20 mm aggregate = 186 litres (for 25 to 50 mm slump range)

However, we need water content for a 100mm slump. Clause 4.2 suggests increasing 3% of water content to get an extra 25mm slump from a 50mm slump

Therefore, as per clause 4.2 for aggregate size 20mm, and for slump 100mm

Water-content for 100mm Slump = 186 + (0.06*186)

= 197 litres

As a superplasticizer is used, the water content can be reduced up to 20% and above

Based on trials with superplasticizer water content reduction of 20% has been achieved Hence the arrived water content = 197*0.8 = 158 litres

CALCULATION OF CEMENT CONTENT

Adopted w/c ratio= 0.35

Cement required = water content / w/c ratio

= 158/0.35

=451.42 kg/m³ > 360 kg/m³ (Hence OK)

Adopted cement content = 450 kg/m³

PROPORTION OF VOLUME OF COARSE AGGREGATE AND FINE AGGREGATE CONTENT

From Table 3 of (IS 10262:2009) volume of coarse aggregate corresponding to 20 mm size aggregate and fine aggregate (Zone II) for the water-cement ratio of 0.50 =0.62

In the present case, the water-cement ratio is 0.35

Therefore, the volume of coarse aggregate is required to be increased to decrease the fine aggregate content. As the water-cement ratio is lower by 0.05. The proportion of the volume of coarse aggregate is increased by 0.01 (at the rate of -/+ 0.01 for every ± 0.05 change in water-cement ratio).

Therefore, the corrected proportion of the volume of coarse aggregate for the water-cement ratio of 0.35 = 0.65

For pumpable concrete, these values should be reduced up to 10%

Therefore,

Volume of coarse aggregate =0.65 x 0.9 =0.585

Volume of fine aggregate content = 1 – 0.585= 0.415

MIX CALCULATIONS

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

1. Volume of concrete = 1 m³
2. Volume of cement = [Mass of cement] / {[Specific Gravity of Cement] x 1000} = 450/ (3.51*1000) = 0.128 m³
3. Volume of water = [Mass of water] / {[Specific Gravity of water] x 1000} = 158/ {1 x 1000} = 0.158m³
4. Volume of chemical admixture (superplasticizer) @2.0% by mass of cementitious material = 450 × 2% = 9 kg/m³
5. [Mass of superplasticizer] / {[Specific Gravity of superplasticizer] x 1000} = 9/ (1.145*1000) =0.007 m³
6. Volume of total aggregates = a – (b + c + d) = 1-(0.128 + 0.158 + 0.007) =0.707 m³
7. Mass of coarse aggregates = e × Volume of coarse aggregate × Specific gravity of coarse aggregate × 1000 =0.707 x 0.585 × 2.74 × 1000 =1133.25 kg/m³
8. Mass of fine aggregates = e x volume of fine aggregate × Specific gravity of fine aggregate × 1000 = 0.707 × 0.415 × 2.74 × 1000 =803.9 kg/m³

MIX PROPORTIONS

Cement = 450 kg/m³

Water = 158 l/m³

Fine aggregate = 803.9 kg/m³

Coarse aggregate = 1133.25 kg/m³

Chemical admixture = 9 kg/m³

Water-cement ratio = 0.35

RESULT

The desired proportion by weight of M50 grade of concrete is 1:1.78:2.52