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 is a construction material composed of cement, fine aggregates (sand) and coarse aggregates mixed with water which hardens with time. Portland cement is the commonly used type of cement for production of concrete. Concrete technology deals with study of properties of concrete and its practical applications.

In a building construction, concrete is used for the construction of foundations, columns, beams, slabs and other load bearing elements.

Grade of a concrete defines its compressive strength after 28 days of curing. In M35 grade concrete the 'M' represents the mix and 35 represents the compressive strength gained in 28 days of curing which is expressed in N/mm² .

There are various grades of concrete and each grades are divided in certain classifications such as,

1. Lean Concrete
2. Ordinary grade of concrete
3. Standard grade of concrete
4. High Strength grade of concrete

There are three different types of concrete mix:

a) Nominal mix

b) Standard mix

c) Design mix.

From the above, the design mix is the most scientific approach for a concrete mix because it considers every aspect of the concrete mix and the final result is deduced from trial and error method which can give the desired strength and durability in the given environmental condition and w/c ratio.

There are various cementitious materials which can be used with cement as per the requirement of the structure and fly ash is one of those materials. In this design mix we are using fly ash with cement as the cementitious material.

PROCEDURE:

Step 1: Stipulations for proportioning:

1. Grade designation = M35
2. Type of cement = OPC 43 grade conforming to IS 269
3. Type of mineral admixture = Fly ash conforming to IS 3812 (Part 1)
4. Maximum nominal size of aggregate = 20 mm
5. Minimum cement content = 320 Kg/m³ (From table 5 of IS 456:2000 – Exposure condition)
6. Maximum water-cement ratio = 0.45 (From table 5 of IS 456:2000 – Exposure condition)
7. Workability = 75 mm (Slump value)
8. Exposure condition = Severe (for reinforced concrete)
9. Method of concrete placing = Chute
10. Degree of supervision = Good
11. Type of aggregate = Crushed angular aggregate
12. Maximum cement content = 450 Kg/m³
13. Chemical admixture type = Superplasticizer

Step 2: Test data for materials:

• Cement used = OPC 43 grade confirming to IS 269
• Specific gravity of Cement = 3.15
• Fly ash = confirming to IS 3812 (Part 1)
• Specific gravity of fly ash = 2.2
• Chemical admixture = Superplasticizer confirming to IS 9103

• Specific gravity of:

• Coarse aggregate = 2.74
• Fine Aggregate = 2.65
• Chemical admixture = 1.145

• Water absorption:

• Coarse Aggregate = 0.5%
• Fine Aggregate = 1.0%

• Free Surface moisture:

• Coarse Aggregate = Nil
• Fine Aggregate = Nil

• Sieve Analysis:

• Coarse aggregate: confirming to Table 2 of IS 383
• Fine Aggregate: Confirming to grading Zone 1 of Table 4 of IS 383

Step 3: Target strength for mix proportioning:

f’ck = fck + 1.65 S

Where, f’ck = Target average compressive strength at 28 days

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 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 maximum water-cement ratio for severe exposure (Reinforced concrete) = 0.45

• Also water-cement ratio as per the following graph from IS 10262:2019

The water-cement ratio corresponding to target strength can be assumed as 0.39

Taking the minimum of the above two values as water-cement ratio

i.e., 0.39<0.45, hence ok

w/c = 0.39

Step 5: Selection of water content:

• From Table 4 of IS 10262:2019, maximum water content for 20mm aggregate = 186 kg (for 25mm to 50mm slump range)
• Estimated water content for 75 mm slump = 186 + 3/100 * 186 = 191.58 kg (for every 25mm increase in slump value 3% is increased in water content)
• As superplasticizer is used, the water content can be reduced up to 30% and above. Based on trials water content reduction of 29% is achieved with plasticizer

Hence the arrived water content = 191.58*0.71 = 136 kg = 136 liters

Step 6: Calculation of cement content:

Water-cement ratio = 0.39

Cement content = `136/0.39` = 349 Kg/m³

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

1. The percentage of fly ash needs to be decided based on project requirement and quality of materials.
2. In some cases, increase in cementitious material can be warranted. Increase in cementitious material content and its percentage may be based on experience and trial (This example is with increase of 10% cementitious material content)

Cementitious material content = `349xx1.10` = 384 Kg/m³

Water content = 136 Kg/m³

So, water – Cement ratio = `136/384` = 0.354 Kg/m³

Fly ash @ 30 % total cementitious material content = `384xx30%` = 115 Kg/m³

Cement (OPC) = `384-115` = 269 Kg/m³

Saving of cement while using fly ash = `349-269` = 80 Kg/m³

Fly ash being utilized = 115 Kg/m³

From Table 5 of IS 456:2000, minimum cementitous content for "severe" exposure condition = 320 Kg/m³

`therefore` 384 Kg/m³ >320 Kg/m³ , hence O.K.

Step 7: Proportion of volume of CA & FA content:

From Table 3 of IS 10262:2009, volume of coarse aggregate corresponding to 20mm size aggregate and fine aggregate (Zone 1) for water-cement ratio (0.5) = 0.6

In the present case the water cementitious ratio is 0.354

• Therefore, the volume of coarse aggregate needs to be increased to decrease the fine aggregate content.
• As water-cement ratio is lower by 0.146, the proportion of volume of coarse aggregate is increased by 0.029 (at the rate of + or – 0.01 for every + or – 0.05 change in water-cement ratio)

Therefore, corrected volume of coarse aggregate for the water-cement ratio of 0.354 = 0.629 `~~`0.63 m³

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

Volume of fine aggregate content = `1-0.63 = 0.37`m³

Step 8: Mix Calculations:

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

a) Volume of concrete = 1 m³

b) Volume of cement = (mass of cement/specific gravity of cement)*(1/1000)

= `(269/3.15)xx(1/1000)` = 0.085 m³

c) Volume of Fly Ash = (mass of fly ash/specific gravity of fly ash)*(1/1000)

= `(115/2.2)xx(1/1000)` = 0.052 m³

d) Volume of water = (mass of water/specific gravity of water)*(1/1000)

= `(136/1)xx(1/1000)` = 0.136 m³

e) Volume of chemical admixture (superplasticizer) @ 2% by mass of cementitious material

= (mass of admixture/specific gravity of admixtrue)*(1/1000)

= `(7.68/1.145)xx(1/1000)` = 0.007 m³

f) Volume of aggregate (FA & CA) = `{a-(b+c+d+e)}`

= `{1-(0.085+0.052+0.136+0.007)}` = 0.72 m³

g) Mass of coarse aggregate = f * Volume of coarse aggregate * Specific gravity of coarse aggregate * 1000

= `0.72xx0.63xx2.74xx1000 = 1242.86 ~~1243`Kg/m³

h) Mass of fine aggregate = f * Volume of fine aggregate * Specific gravity of fine aggregate * 1000

= `0.72xx0.37xx2.65xx1000 = 705.96 ~~ 706`Kg/m³

Step 9: Mix Proportion for Trial Number 1:

Cement = 269 Kg/ m³

Fly ash = 115 Kg/ m³

Water = 136 Kg/ m³

Fine aggregate = 706 Kg/ m³

Coarse aggregate = 1243 Kg/ m³

Chemical admixture = 7.68 Kg/ m³

Water – Cement ratio = 0.354

Trial Mix Ratio = 1 : 2.62 : 4.62

• The slump shall be measured, and the water content and dosage of admixture shall be adjusted for achieving the required slump on trial. The mix proportion shall be reworked for the actual water content and checked for durability requirements.

• Two more trials having variation of + or - 10% of water-cement ratio in Step 9 shall be carried out and a graph between these water-cement ratios and corresponding strengths shall be plotted to work out the mix proportions for the given target strength for field trials. However, durability shall be met.

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

INTRODUCTION :

Concrete is a construction material composed of cement, fine aggregates (sand) and coarse aggregates mixed with water which hardens with time. Portland cement is the commonly used type of cement for production of concrete. Concrete technology deals with study of properties of concrete and its practical applications.

In a building construction, concrete is used for the construction of foundations, columns, beams, slabs and other load bearing elements.

Grade of a concrete defines its compressive strength after 28 days of curing. In M50 grade concrete the 'M' represents the mix and 50 represents the compressive strength gained in 28 days of curing which is expressed in N/mm² .

There are various grades of concrete and each grades are divided in certain classifications such as,

1. Lean Concrete
2. Ordinary grade of concrete
3. Standard grade of concrete - The M50 comes under this grade which are used for construction of beam, slabs, columns, footings etc.,
4. High Strength grade of concrete

There are three different types of concrete mix:

a) Nominal mix

b) Standard mix

c) Design mix.

From the above, the design mix is the most scientific approach for a concrete mix because it considers every aspect of the concrete mix and the final result is deduced from trial and error method which can give the desired strength and durability in the given environmental condition and w/c ratio.

PROCEDURE:

Step 1: Stipulations for proportioning:

1. Grade designation = M50
2. Type of cement = OPC 43 grade conforming to IS 269
3. Maximum nominal size of aggregate = 20 mm
4. Minimum cement content = 320 Kg/m³ (From table 5 of IS 456:2000 – Exposure condition)
5. Maximum water-cement ratio = 0.45 (From table 5 of IS 456:2000 – Exposure condition)
6. Workability = 75 mm (Slump value)
7. Exposure condition = Severe (for reinforced concrete)
8. Method of concrete placing = Chute
9. Degree of supervision = Good
10. Type of aggregate = Crushed angular aggregate
11. Maximum cement content = 450 Kg/m³
12. Chemical admixture type = Superplasticizer

Step 2: Test data for materials:

• Cement used = OPC 43 grade confirming to IS 269
• Specific gravity of Cement = 3.15
• Chemical admixture = Superplasticizer confirming to IS 9103

• Specific gravity of:

• Coarse aggregate = 2.74
• Fine Aggregate = 2.65
• Chemical admixture = 1.145

• Water absorption:

• Coarse Aggregate = 0.5%
• Fine Aggregate = 1.0%

• Free Surface moisture:

• Coarse Aggregate = Nil
• Fine Aggregate = Nil

• Sieve Analysis:

• Coarse aggregate: confirming to Table 2 of IS 383
• Fine Aggregate: Confirming to grading Zone 1 of Table 4 of IS 383

Step 3: Target strength for mix proportioning:

f’ck = fck + 1.65 S

Where, f’ck = Target average compressive strength at 28 days

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 strength (f'ck) = 50 + (1.65*5) = 58.25 N/mm²

Step 4: Selection of water-cement ratio:

• From Table 5 of IS 456: 2000 maximum water-cement ratio for severe exposure (Reinforced concrete) = 0.45

• Also water-cement ratio as per the following graph from IS 10262:2019

The water-cement ratio corresponding to target strength can be assumed as 0.34

Taking the minimum of the above two values as water-cement ratio

i.e., 0.34<0.45, hence ok

w/c = 0.34

Step 5: Selection of water content:

• From Table 4 of IS 10262:2019, maximum water content for 20mm aggregate = 186 kg (for 25mm to 50mm slump range)
• Estimated water content for 75 mm slump = 186 + 3/100 * 186 = 191.58 kg (for every 25mm increase in slump value 3% is increased in water content)
• As superplasticizer is used, the water content can be reduced up to 20% and above. Based on trials water content reduction of 29% is achieved with plasticizer

Hence the arrived water content = 191.58*0.71 = 136 kg = 136 liters

Step 6: Calculation of cement content:

Water-cement ratio = 0.34

Cement content = `136/0.34` = 400 Kg/m³

From Table 5 of IS 456:2000, minimum cement content for "severe" exposure condition = 320 Kg/m³

`therefore` 400 Kg/m³ >320 Kg/m³ , hence O.K.

Step 7: Proportion of volume of CA & FA content:

From Table 3 of IS 10262:2009, volume of coarse aggregate corresponding to 20mm size aggregate and fine aggregate (Zone 1) for water-cement ratio (0.5) = 0.6

In the present case the water cementitious ratio is 0.34

• Therefore, the volume of coarse aggregate needs to be increased to decrease the fine aggregate content.
• As water-cement ratio is lower by 0.16, the proportion of volume of coarse aggregate is increased by 0.032 (at the rate of + or – 0.01 for every + or – 0.05 change in water-cement ratio)

Therefore, corrected volume of coarse aggregate for the water-cement ratio of 0.34 = 0.632 `~~`0.63 m³

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

Volume of fine aggregate content = `1-0.63 = 0.37`m³

Step 8: Mix Calculations:

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

a) Volume of concrete = 1 m³

b) Volume of cement = (mass of cement/specific gravity of cement)*(1/1000)

= `(400/3.15)xx(1/1000)` = 0.126 m³`

c) Volume of water = (mass of water/specific gravity of water)*(1/1000)

= `(136/1)xx(1/1000)` = 0.136 m³

d) Volume of chemical admixture (superplasticizer) @ 1 % by mass of cementitious material

= (mass of admixture/specific gravity of admixtrue)*(1/1000)

= `(4/1.145)xx(1/1000)` = 0.003 m³

e) Volume of aggregate (FA & CA) = `{a-(b+c+d)}`

= `{1-(0.126+0.136+0.003)}` = 0.735 m³

f) Mass of coarse aggregate = f * Volume of coarse aggregate * Specific gravity of coarse aggregate * 1000

= `0.735xx0.63xx2.74xx1000 = 1268.76 ~~1269`Kg/m³

g) Mass of fine aggregate = f * Volume of fine aggregate * Specific gravity of fine aggregate * 1000

= `0.735xx0.37xx2.65xx1000 = 720.66 ~~ 720`Kg/m³

Step 9: Mix Proportion for Trial Number 1:

Cement = 400 Kg/ m³

Water = 136 Kg/ m³

Fine aggregate = 720 Kg/ m³

Coarse aggregate = 1269 Kg/ m³

Chemical admixture = 4 Kg/ m³

Water – Cement ratio = 0.34

Trial Mix Ratio = 1 : 1.8 : 3.17

• The slump shall be measured, and the water content and dosage of admixture shall be adjusted for achieving the required slump on trial. The mix proportion shall be reworked for the actual water content and checked for durability requirements.

• Two more trials having variation of + or - 10% of water-cement ratio in Step 9 shall be carried out and a graph between these water-cement ratios and corresponding strengths shall be plotted to work out the mix proportions for the given target strength for field trials. However, durability shall be met.