Project on Concrete Mix Design for various grades of Concrete

A Project Report on

Concrete Mix Design for various grades of Concrete

Submitted by:

Abdul Waheed Pedhar

Aim

One of the ultimate aims of studying the various properties of the materials of concrete, plastic concrete and hardened concrete is to enable a concrete technologist to design a concrete mix for a particular strength and durability. The design of a concrete mix is not a simple task on account of widely varying properties of the constituent materials, the condition that prevail at the site of work, in particular the exposure condition, and the condition that are demanded for a particular work for which the mix is designed. Design of concrete mix requires complete knowledge of the various properties of these constituent materials, the implications in case of change on these conditions at the site, the impact of the properties of plastic concrete on the hardened concrete and the complicated inter-relationship between the variables. All these make the task of mix design more complex and difficult. Design of concrete mix needs not only the knowledge of material properties and properties of concrete in plastic condition; it also needs wider knowledge and experience of concreting. Even then the proportion of the materials of concrete found out at the laboratory requires modification and readjustments to suit the field condition.

With better understanding of the properties, the concrete is becoming more and more an exact material than in the past. The structural designer stipulates certain minimum strength; and concrete technologist designs the concrete mix with the knowledge of materials, site exposure conditions and standard of supervision available at site of work to achieve this minimum strength and durability.

Introduction

The process of selecting suitable ingredients of concrete and determining their relative amounts with the objective of producing a concrete of the required, strength, durability, and workability as economically as possible, is termed the concrete mix design. The proportioning of ingredient of concrete is governed by the required performance of concrete in 2 states, namely the plastic and the hardened states. If the plastic concrete is not workable, it cannot be properly placed and compacted. The property of workability, therefore, becomes of vital importance. The compressive strength of hardened concrete which is generally considered to be an index of its other properties, depends upon many factors, e.g. quality and quantity of cement, water and aggregates; batching and mixing; placing, compaction and curing. The cost of concrete is made up of the cost of materials, plant and labour. The variations in the cost of materials arise from the fact that the cement is several times costly than the aggregate, thus the aim is to produce as lean a mix as possible. From technical point of view the rich mixes may lead to high shrinkage and cracking in the structural concrete, and to evolution of high heat of hydration in mass concrete which may cause cracking. The actual cost of concrete is related to the cost of materials required for producing a minimum mean strength called characteristic strength that is specified by the designer of the structure. This depends on the quality control measures, but there is no doubt that the quality control adds to the cost of concrete. The extent of quality control is often an economic compromise, and depends on the size and type of job. The cost of labour depends on the workability of mix, e.g., a concrete mix of inadequate workability may result in a high cost of labour to obtain a degree of compaction with available equipment

It will be worthwhile to recall at this stage the relationship between the aggregate and paste which are two essential ingredients of concrete. Workability of the mass is provided by the lubricating effect of the paste and is influenced by the amount and dilution of paste, since mineral aggregates with rare exceptions, is far stronger than the paste compound. Essentially the permeability of concrete is governed by the quality and continuity of the paste, since little water flows through aggregates either under pressure or by capillary. Further, the predominant contribution to drying shrinkage of concrete is that of paste.

Since, the properties of concrete are governed to a considerable extent by the quality of paste; it is helpful to consider more closely the structure of the paste. The fresh paste is a suspension, not a solution of cement in water.

Requirements of concrete mix design

The requirements which form the basis of selection and proportioning of mix ingredients are:

1. a) The minimum compressive strength required from structural consideration b) The adequate workability necessary for full compaction with the compacting equipment available.
2. c) Maximum water-cement ratio and/or maximum cement content to give adequate durability for the particular site conditions.
3. d) Maximum cement content to avoid shrinkage cracking due to temperature cycle in mass concrete.

Types of Mixes

1. Nominal Mixes

In the past the specifications for concrete prescribed the proportions of cement, fine and coarse aggregates. These mixes of fixed cement-aggregate ratio which ensures adequate strength are termed nominal mixes. These offer simplicity and under normal circumstances, have a margin of strength above that specified. However, due to the variability of mix ingredients the nominal concrete for a given workability varies widely in strength.

2. Standard mixes

The nominal mixes of fixed cement-aggregate ratio (by volume) vary widely in strength and may result in under- or over-rich mixes. For this reason, the minimum compressive strength has been included in many specifications. These mixes are termed standard mixes. IS 456-2000 has designated the concrete mixes into a number of grades as M10, M15, M20, M25, M30, M35 and M40. In this designation the letter M refers to the mix and the number to the specified 28 day cube strength of mix in N/mm^2.

The mixes of grades M10, M15, M20and M25 correspond approximately to the mix proportions (1:3:6), (1:2:4), (1:1.5:3) and(1:1:2) respectively.

3. Designed Mixes

In these mixes the performance of the concrete is specified by the designer but the mix proportions are determined by the producer of concrete, except that the minimum cement content can be laid down. This is most rational approach to the selection of mix proportions with specific materials in mind possessing more or less unique characteristics. The approach results in the production of concrete with the appropriate properties most economically. However, the designed mix does not serve as a guide since this does not guarantee the correct mix proportions for the prescribed performance. For the concrete with undemanding performance nominal or standard mixes (prescribed in the codes by quantities of dry ingredients per cubic meter and by slump) may be used only for very small jobs, when the 28-day strength of concrete does not exceed 30 N/mm^2. No control testing is necessary reliance being placed on the masses of the ingredients.

Factors affecting the choice of mix proportions

The various factors affecting the mix design are:

1. Compressive strength

It is one of the most important properties of concrete and influences many other describable properties of the hardened concrete. The mean compressive strength required at a specific age, usually 28 days, determines the nominal water-cement ratio of the mix. The other factor affecting the strength of concrete at a given age and cured at a prescribed temperature is the degree of compaction. According to Abraham’s law the strength of fully compacted concrete is inversely proportional to the water-cement ratio.

2. Workability

The degree of workability required depends on three factors. These are the size of the section to be concreted, the amount of reinforcement, and the method of compaction to be used. For the narrow and complicated section with numerous corners or inaccessible parts, the concrete must have a high workability so that full compaction can be achieved with a reasonable amount of effort. This also applies to the embedded steel sections. The desired workability depends on the compacting equipment available at the site.

3. Durability

The durability of concrete is its resistance to the aggressive environmental conditions. High strength concrete is generally more durable than low strength concrete. In the situations when the high strength is not necessary but the conditions of exposure are such that high durability is vital, the durability requirement will determine the water-cement ratio to be used.

4. Maximum nominal size of aggregate

In general, larger the maximum size of aggregate, smaller is the cement requirement for a particular water-cement ratio, because the workability of concrete increases with increase in maximum size of the aggregate. However, the compressive strength tends to increase with the decrease in size of aggregate. IS 456: 2000 and IS 10262 : 2009 recommend that the nominal size of the aggregate should be as large as possible.

5. Grading and type of aggregate

The grading of aggregate influences the mix proportions for a specified workability and water-cement ratio. Coarser the grading lesser will be mix which can be used. Very lean mix is not desirable since it does not contain enough finer material to make the concrete cohesive. The type of aggregate influences strongly the aggregate-cement ratio for the desired workability and stipulated water cement ratio. An important feature of a satisfactory aggregate is the uniformity of the grading which can be achieved by mixing different size fractions.

6. Quality Control

The degree of control can be estimated statistically by the variations in test results. The variation in strength results from the variations in the properties of the mix ingredients and lack of control of accuracy in batching, mixing, placing, curing and testing. The lower the difference between the mean and minimum strengths of the mix lower will be the cement-content required. The factor controlling this difference is termed as quality control.

Mix Proportion designations

The common method of expressing the proportions of ingredients of a concrete mix is in the terms of parts or ratios of cement, fine and coarse aggregates. For e.g., a concrete mix of proportions 1:2:4 means that cement, fine and coarse aggregate are in the ratio 1:2:4 or the mix contains one part of cement, two parts of fine aggregate and four parts of coarse aggregate. The proportions are either by volume or by mass. The water-cement ratio is usually expressed in mass.

Factors to be considered for mix design

• The grade designation giving the characteristic strength requirement of concrete.
• The type of cement influences the rate of development of compressive strength of concrete.
• Maximum nominal size of aggregates to be used in concrete may be as large as possible within the limits prescribed by IS 456:2000.
• The cement content is to be limited from shrinkage, cracking and creep.
• The workability of concrete for satisfactory placing and compaction is related to the size and shape of section, quantity and spacing of reinforcement and technique used for transportation, placing and compaction.

Guidelines used for mix design

There are some guidelines available for making a design mix concrete such as,

1982. IS method: Concrete mix proportioning guidelines (Bureau of Indian standards- IS 10262-2009) and recommended guidelines for concrete mix design – IS 10262-1982.
1983. BS method: (British standard – BS EN 206-1 and its complementary standard BS 8500 part 1&2)
1984. ACI method: (American standard – ACI 211, 211-91, reapproved 2002)

Mix design is the most intellectual and rational approach for the selection of specific materials with their proportion and more or less for their unique characteristics. Design mix contains weight batching and makes concrete of desired strength and also cost efficient.

Procedure

1. Determine the mean target strength Ft from the specified characteristic compressive strength at28-day fck and the level of quality control. Ft = fck + 1.65*S where S is the standard deviation obtained from the Table 8 of IS 456: 2000 of approximate contents given after the design mix.
2. Obtain the water cement ratio for the desired mean target using the empirical relationship between compressive strength and water cement ratio so chosen is checked against the limiting water cement ratio. The water cement ratio so chosen is checked against the limiting water cement ratio for the requirements of durability given in table-5 of IS 456: 2000 and adopts the lower of the two values.
3. Estimate the amount of entrapped air for maximum nominal size of the aggregate from the table.
4. Select the water content, for the required workability and maximum size of aggregates (for aggregates in saturated surface dry condition) from table-2 of IS 10262:2009.
5. Determine the percentage of fine aggregate in total aggregate by absolute volume from Table-2 of IS 10262: 2009 (selected depending upon the aggregate zone) for the concrete using crushed coarse aggregate.
6. Adjust the values of water content and percentage of sand as provided in the table for any difference in workability, water cement ratio, grading of fine aggregate and for rounded aggregate the values are given in table.
7. Calculate the cement content form the water-cement ratio and the final water content as arrived after adjustment. Check the cement against the minimum cement content from the requirements of the durability, and greater of the two values is adopted.
8. From the quantities of water and cement per unit volume of concrete and the percentage of sand already determined in steps 6 and 7 above, calculate the content of coarse and fine aggregates per unit volume of concrete from the following relations:

V = [w + c/sc +1/p*fa/Sfa] * 1/1000

V = [w+ c/sc + 1/1-p * c/Sca] * 1/1000

Where, V = absolute volume of concrete= gross volume (1m^3) minus the volume of entrapped air.

Sc = specific gravity of cement

W = Mass of water per cubic meter of concrete, kg

C = mass of cement per cubic meter of concrete, kg

p = ratio of fine aggregate to total aggregate by absolute volume

fa, Ca = total masses of fine and coarse aggregates, per cubic meter of concrete, respectively, kg, and

Sfa, Sca = specific gravities of saturated surface dry fine and coarse aggregates, respectively

9. Determine the concrete mix proportions for the first trial mix.
10. Prepare the concrete using the calculated proportions and cast three cubes of 150 mm size and test them wet after 28-days moist curing and check for the strength.
11. Prepare trial mixes with suitable adjustments till the final mix proportions are arrived.

Data required for concrete mix design

• Grade of concrete
• Maximum Size of aggregate
• Minimum cement content
• Maximum water cement ratio
• Workability in terms of slump
• Exposure condition
• Maximum temperature at the pouring point
• Early age strength
• Grading zone of fine aggregates
• Type of aggregate
• Maximum cement content.
• Admixture used
• Specific gravity of all the materials used
• Moisture content

To determine the mix design for M35 concrete with fly ash consisting of the following properties:

Grade designation: M35

Type of cement: OPC 43 grade confirming IS 12269

Type of mineral admixture: Fly ash

Maximum nominal size of aggregate: 20

Minimum water-cement ratio: 0.45 (Table 5 of IS 456:2000)

Workability: 100-120mm Slump

Exposure condition: moderate

Method of concrete placing: Pumping

Degree of supervision: Good

Type of aggregate: Crushed Angular aggregate

Maximum cement content: 360 kg/cum

Chemical admixture type: Super Plasticizer

Data obtained:

Cement used: OPC 43 grade (IS 12269)

Specific gravity of cement: 3.15

Fly ash – specific gravity: 2.2

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%

Coarse aggregate- Sieve analysis as per Table 2 of IS 383

Fine aggregate- Sieve analysis as per Grading zone I of Table 4 of IS 383

Procedure:

Target Strength for mix proportion is obtained from

f’ck = fck + 1.65s (Table1: IS 10262:2009)

Considering s = 5N/sq.mm.

Thus f’ck = 35+1.65(5) = 43.25 N/sq.mm.

Water-cement Ratio:

Considering maximum water-cement ratio = 0.4 (Table 5: IS 456:2000)

Water content

Maximum water content for 20 mm aggregate = 186 litres (Table 2 : IS 10262:2009)

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

However, if super plasticizer is used, the water content can be reduced up to 20% to 30%.

Thus the water content obtained is 197-(30% of 197) = 132 litres.

Adopt 140 liters.

Cement Content

Adopted w/c ratio = 0.4

Cement content = 140/0.4 = 350 kg/cum.

Minimum cement content for moderate exposure = 320 kg/cum. (Table5: IS 456:2000)

350 kg/ cum > 320kg/ cum.

Hence OK

Fly ash content:

Considering 10% increase in cementitious material

Thus, Cementitious content = 350x1.1 = 385 kg/cum.

Fly ash = 30% of total cementitious content = 385 x 0.3 = 115 kg/cum.

Proportion of volume of coarse aggregate and fine aggregate:

Volume of coarse aggregate and fine aggregate for w/c ratio 0.5 = 0.6 (Table 3: IS 10262:2009)

In this case, w/c ratio is 0.4. Therefore, volume of coarse aggregate is required to be increased to decrease the fine aggregate content. As the w/c ratio is reduced by 0.1, 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).

Thus, the corrected proportion of volume of coarse aggregates for W/c ratio of 0.4 = 0.62.

For pumpable concrete, it can be reduced up to 10%. Hence, volume of coarse aggregate = 0.62x0.9 = 0.56.

Volume of fine aggregate = 1-0.56 = 0.44.

MIX CALCULATIONS

The following mix calculations are carried out:

Volume of concrete = 1 cum.

Volume of cement = Mass of cement / (Sp. gravity x 1000) = 270/ (3.15x1000) = 0.086 cum.

Volume of Fly ash = Mass of fly ash / (Sp. gravity x 1000) = 115/ (2.2x1000) = 0.052 cum.

Volume of water = mass of water / (Sp. gravity x 1000) = 140/ (1x1000) = 0.14 cum.

Volume of chemical admixture = Mass of chemical admixture / (Sp. gravity x 1000) = 7.6/(1.145x1000) = 0.006cum.

Volume of all in aggregate = [1-(0.086+0.052+0.14+0.006)] = 0.716 cum.

Mass of coarse aggregate = Vol. of all aggregates x Vol. of coarse aggregate x Sp. gravity of coarse aggregates x 1000 = 0.716x0.56x2.74x1000 = 1098 kg/cum.

Mass of fine aggregates = Vol. of all aggregates x Vol. of fine aggregate x Sp. gravity of fine aggregates x 1000 = 0.716x0.44x2.74x1000 = 863 kg/cum.

RESULT:

Mix Proportion Obtained:

Cement = 270 kg/cum.

Fly ash = 115 kg/cum

Water = 140 litres

Fine aggregates = 863 kg/cum.

Coarse aggregate = 1098 kg/cum.

Chemical admixture = 7kg/cum.

W/c ratio = 0.4

Mix proportion by weight = 1:2.24:2.85

CONCLUSION:

The slump should be measured and the water content and admixture can be adjusted to obtain the required slump. The mix proportion can be adjusted and reworked as per requirement.

To determine the mix design for M50 concrete as per the following properties:

Grade designation: M50

Type of cement: OPC 43 grade confirming IS 12269

Maximum nominal size of aggregate: 20

Minimum water-cement ratio: 0.45 (Table 5 of IS 456:2000)

Workability: 100-120mm Slump

Exposure condition: moderate

Method of concrete placing: Pumping

Degree of supervision: Good

Type of aggregate: Crushed Angular aggregate

Maximum cement content: 360 kg/cum

Chemical admixture type: Super Plasticizer

Data obtained:

Cement used: OPC 43 grade (IS 12269)

Specific gravity of cement: 3.15

Chemical admixture – specific gravity: 1.145

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%

Coarse aggregate- Sieve analysis as per Table 2 of IS 383

Fine aggregate- Sieve analysis as per Grading zone I of Table 4 of IS 383

Procedure:

Target Strength for mix proportion is obtained from

f’ck = fck + 1.65s (Table1: IS 10262:2009)

Considering s = 5N/sq.-mm.

Thus f’ck = 50+1.65(5) = 58.25 N/sq.-mm.

Water-cement Ratio:

Considering maximum water-cement ratio = 0.4 (Table 5: IS 456:2000)

Water content

Maximum water content for 20 mm aggregate = 186 litres (Table2: IS 10262:2009)

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

However, if super plasticizer is used, the water content can be reduced up to 20% to 30%.

Thus the water content obtained is 197-(30% of 197) = 132 litres.

Adopt 140 litres.

Cement Content

Adopted w/c ratio = 0.4

Cement content = 140/0.4 = 350 kg/cum.

Minimum cement content for moderate exposure = 320 kg/cum. (Table5: IS 456:2000)

350 kg/cum > 320 kg/cum.

Hence OK

Proportion of volume of coarse aggregate and fine aggregate:

Volume of coarse aggregate and fine aggregate for w/c ratio 0.5 = 0.6 (Table3: IS 10262:2009)

In this case, w/c ratio is 0.4. Therefore, volume of coarse aggregate is required to be increased to decrease the fine aggregate content. As the w/c ratio is reduced by 0.1, 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).

Thus, the corrected proportion of volume of coarse aggregate for w/c ratio of 0.4 = 0.62.

For pumpable concrete, it can be reduced up to 10%. Hence, volume of coarse aggregate = 0.62x0.9 = 0.56.

Volume of fine aggregate = 1-0.56 = 0.44.

MIX CALCULATIONS

The following mix calculations are carried out:

Volume of concrete = 1 cum.

Volume of cement = Mass of cement / (Sp. gravity x 1000) = 350/(3.15x1000) = 0.111 cum.

Volume of water = mass of water /(Sp. gravity x 1000) = 140/(1x1000)= 0.14 cum.

Volume of chemical admixture = Mass of chemical admixture /(Sp. gravity x 1000) = 7.6/(1.145x1000) = 0.006cum.

Volume of all in aggregate = [1-(0.111+0.14+0.006)] = 0.743 cum.

Mass of coarse aggregate = Vol. of all aggregates x Vol. of coarse aggregate x Sp. gravity of coarse aggregates x 1000 = 0.743x0.56x2.74x1000 = 1140 kg/cum.

Mass of fine aggregates = Vol. of all aggregates x Vol. of fine aggregate x Sp. gravity of fine aggregates x 1000 = 0.743x0.44x2.74x1000 = 896 kg/cum.

RESULT:

Mix Proportion Obtained:

Cement = 350 kg/cum.

Water = 140 litres

Fine aggregates = 896 kg/cum.

Coarse aggregate = 1140 kg/cum.

Chemical admixture = 7kg/cum.

W/c ratio = 0.4

Mix proportion by weight = 1:2.56:3.26

CONCLUSION:

The slump should be measured and the water content and admixture can be adjusted to obtain the required slump. The mix proportion can be adjusted and reworked as per requirement.