Challenge

Project on Concrete Mix Design for various grades of Concrete

PParvathi Venugopal26 Jun 2021

Mix design for M35 grade Concrete with fly ash

Aim: Mix Designing of M35 grade concrete with concrete with fly ash

Introduction: The proportion of ingredients of M35 concrete mix using fly ash as mineral admixture is expressed in the ratios of cement, fine aggregate, and coarse aggregate.

Procedure:

Step 1: Stipulation for proportioning

Grade of designation: M35 grade
Type of cement: OPC 43 grade conforming to IS 8112
Type of mineral admixture: Fly ash conforming to IS 3812 (Part 1)
Maximum nominal size of aggregate: 20mm
Minimum cement content: 320 kg/m³ (From Table 5 of IS 456: 2000)
Maximum water-cement ratio: 0.45 (From Table 5 of IS 456:2000)
Workability: 100 mm-120 mm (Slump value for pumpable concrete)
Exposure condition: Severe (For Reinforced concrete)
Method of concrete placing: Pumping
Degree of supervision: Good
Type of aggregate: Crushed angular aggregates
Maximum cement content: 360 kg/m³
Chemical admixture type: Super plasticizer

Step 2: Test data for materials

Cement used: OPC 43 grade conforming IS 8112
Specific gravity of cement: 3.15
Fly ash: Conforming to IS 3812 (Part 1)
Specific gravity of fly ash: 2.2
Chemical admixture: Super plasticizer
Specific gravity of,

Coarse aggregate: 2.74
Fine aggregate: 2.74

Water absorption of,

Coarse aggregate: 0.5%
Fine aggregate: 1%

Free surface moisture:

Coarse aggregate: Nil
Fine aggregate: Nil

Sieve analysis:

Coarse aggregate: Conforming to Table 2 of IS 383
Fine aggregate: Conforming to grading zone 1 of Table 4 of IS 383

Step 3: Target strength of mix proportioning

Target mean compressive strength, f’ck = fck + 1.65 s

Where, fck = Characteristic compressive strength at 28 days

S = Standard deviation

From Table 1 of IS 10262:2009, Standard deviation, s = 5 N/mm²

Therefore, Target Mean compressive strength, f’ck = 35 + (1.65 * 5) = 43.25 N/mm²

Step 4: Selection of Water-Cement ratio

From Table 5 of IS 456:2000, for severe exposure maximum water-cement ratio is 0.45.

But based on the experience, adopted water-cement ratio is 0.40.

0.40 < 0.45; Hence OK.

Step 5: Selection of Water content

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

Estimated water content for 100 mm slump = 186 +( 6*186 /100) = 197 litre.

As Super plasticizer is used, the water content can be reduced up to 30%. Based on trials, the water-content reduction of 29% is achieved with plasticizer.

Hence the arrived water content = 197 * (1-0.29) = 140 litre.

Step 6: Calculation of cement content

Adopted w/c ratio = 0.40

Cementitious material (Cement + Fly ash) content = 140/0.40 = 350 kg/m³

From Table 5 of IS 456:2000, Minimum cement content for Severe expose condition = 320 kg/m³

320 kg/m³< 350 kg/m³; Hence OK

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

The percentage of fly ash needs to be decided based on project requirement and quality of materials.

In some cases, increase in cementitious material can be warranted. Increase in cementitious content and its percentage may be based on the experience and trial.

Here, this mix designing is done with increase of 10% cementitious material content.

Cementitious material content = 350 * 1.10 = 385 kg/m³

Water content = 140 kg/m³

Therefore, w/c ratio = 140/385 = 0.365

Fly ash at 30% total cementitious material content = 385 * 30% = 115 kg/m³

Cement (OPC) = 385- 115 = 270 kg/m³

Saving of cement while using fly ash = 350 – 270 = 80 kg/m³

Fly ash being utilized = 115 kg/m³

Step 7: Proportion of Volume of Coarse aggregate and Fine aggregate

From Table 3 of IS 10262:2009, the volume of coarse aggregate corresponding to 20 mm aggregate and Fine aggregate (Zone 1) for w/c ratio of 0.50 = 0.60

Here, the w/c ratio is 0.40. Therefore, the volume of coarse aggregate needs to be increased to decrease the fine aggregate content. As the w/c ratio is lower by 0.10, the proportion of volume of coarse aggregate is increased by 0.02 (at the rate of -/+ 0.01 for every ± 0.05 change in w/c ratio).

Therefore, corrected volume of coarse aggregate for the w/c ratio of 0.40 = 0.62

Note: If coarse aggregate is not angular type, then also the volume of coarse aggregate may be required to be increased suitably based on experience and site conditions.

For pumpable concrete, these values are reduced by 10%

i.e; The volume of coarse aggregate = 0.62 * (100% - 10%) = 0.56

The volume of fine aggregate = 1 – 0.56 = 0.44

Step 8: Mix Calculation

The mix calculation per unit volume of concrete shall be as follows.

Volume of concrete = 1 m³
Volume of cement = (Mass of cement) / (Specific Gravity of cement * 1000) = 270 / (3.15*1000) = 0.086 m³
Volume of fly ash = (Mass of fly ash) / (Specific Gravity of fly ash * 1000) = 115 / (2.2*1000) = 0.052 m³
Volume of water = (Mass of water) / (Specific Gravity of water * 1000) = 140 / (1*1000) = 0.140 m³
Volume of chemical admixture = (Mass of chemical admixture) / (Specific Gravity of chemical admixture * 1000) = 7.6 / (1.145*1000) = 0.006 m³
Volume of Aggregate (Coarse aggregate + Fine aggregate) = (a – [b + c + d +e]) = (1 – [0.086 + 0.052 + 0.140 + 0.006]) = 0.716 m³
Mass of coarse aggregate = f * Volume of coarse aggregate * Specific gravity of Coarse aggregate * 1000 = 0.716 * 0.56 * 2.74 * 1000 = 1098 kg/m³
Mass of fine aggregate = f * Volume of fine aggregate * Specific gravity of fine aggregate * 1000 = 0.716 * 0.44 * 2.74 * 1000 = 863 kg/m³

Step 9: Mix proportions

Cement = 270 kg/m³
Water = 140 litre/m³
Fine aggregate = 1098 kg/m³
Coarse aggregate = 863 kg/m³
Fly ash = 115 kg/m³
Chemical admixture = 7 kg/m³ (0.4% by weight of cement)
Water-cement ratio = 0.40

Mix proportion by weight = 1 : 2.24 : 2.85

Step 10: The slump shall be measured, and the water content and dosage of admixture shall be adjusted for achieving the required slump based on trial, if required. The mix proportions shall be worked for the actual water content and checked for durability requirements.

Step 11: Two more trials having variations of ±10 percent of water-cement ratio in Step 10 shall be carried out and a graph between three water-cement ratios and their corresponding strength shall be plotted to work out the mix proportions for the given target strength for field trials. However, durability requirements shall be met.

Mix design for M50 grade Concrete without fly ash

Aim: Mix Designing of M50 grade concrete with concrete with fly ash

Introduction: The proportion of ingredients of M50 concrete mix without using fly ash as mineral admixture is expressed in the ratios of cement, fine aggregate, and coarse aggregate.