22CEC31 CONCRETE TECHNOLOGY

By

Dr. S. Vinodhkumar

AP/Civil/KEC

​

MIX-DESIGN

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• The mix design procedure mainly involves determination of the proportions of the constituents of concrete i.e. Cement, Water, Coarse and Fine Aggregates, which results in the production of concrete that would have specified properties both in the fresh and hardened states with maximum overall economy.

FACTORS IN THE CHOICE OF MIX DESIGN

Both IS: 456-2000 as well as IS: 1343 – 1980 envisages the concrete mix

design be based on the following factors:

1. Grade Designation
2. Type of Cement
3. Maximum Nominal Size of Aggregate
4. Minimum Water-Cement Ratio
5. Workability
6. Minimum Cement Content

Grade Designation

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The characteristic strength requirement of the concrete is determined by the grade designation. Depending upon the level of quality control, the concrete is designed for a target mean strength, which is somewhat higher than the characteristic strength.

Type of Cement

Type of cement influences the rate of development of compressive strength of concrete and determines the durability under aggressive environment. The various types of cements that can be used with the approval of Engineer in Charge in concrete constructions are:

• Ordinary or low heat Portland cement conforming to IS : 269-1976
• Rapid hardening Portland cement conforming to IS : 8041-1978
• Portland slag cement conforming to IS : 455-1976
• Portland pozzolana cement conforming to IS : 1489-1976[8]
• High strength ordinary Portland cement conforming to IS : 8112-1976
• Hydrophobic cement conforming to IS : 8043-1978

Workability

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Workability is important for the satisfactory placement and compaction of concrete with respect to size and shape of the section, quantity and spacing of reinforcement and methods of compaction available at site.

Maximum Nominal Size of Aggregate

The maximum nominal size of aggregates to be used in concrete is governed by the size of the section and spacing of the reinforcement. Both IS : 456-2000 and IS : 1343- 1980 specify that the nominal maximum size of coarse aggregate

• Should not be greater than one-fourth of the minimum thickness of the member.
• Should be restricted to 5 mm less than the minimum clear distance between main

bars.

• Should be 5 mm less than the minimum cover to the reinforcement.
• Should be 5 mm less than the spacing between the cables, strands in case of pre- stressed concrete.

Minimum Cement Content

The maximum limit of cement content in the concrete has to be specified as concrete mixes with high cement content may give rise to shrinkage, cracking and creep of concrete also increases with cement paste content.

OUTLINE OF MIX DESIGN PROCEEDURE

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1. Data For Mix Proportioning
1. Grade Designation
2. Type of Cement
3. Maximum nominal Size of Aggregate
4. Minimum Cement Content
5. Maximum Water Cement Ratio
6. Workability
7. Exposure Conditions as per Table 4 and 5 of IS 456 – 2000
8. Maximum Temperature of Concrete at the time of placing
9. Method of transporting and placing
10. Early age strength requirements if required
11. Type of Aggregate
12. Maximum Cement Content
13. Whether an admixture shall or shall not be used and the type of admixture

and the conditions of use.

2 Target Strength For Mix Proportioning

In order that not more than the specified proportions of test results are not likely to fall below the Characteristic Strength, the Concrete Mix has to be proportioned for higher Target Mean Compressive Strength f’_CK. The margin over Characteristic Strength is given by the following relation:

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Standard Deviation

Where Sufficient test results for a particular grade of concrete are not available then the value of Standard Deviation given in Table 1 may be assumed for the proportioning of the mix.

Table 1 Assumed Standard Deviation

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Note:- The above values corresponds to the site control having proper storage of cement; Weigh batching of all materials; Controlled addition of water; Regular checking of all materials -Aggregate grading and moisture content; Periodical checking of workability and strength. Where there is deviation from the above, values given in table shall be increased by 1 N/mm²

3 Selection Of Mix Proportion

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3.1 Selection of Water Cement Ratio

Concretes of Different Compressive Strength for the Same Water Cement Ratio

may be Produced with Different

1. Cements
2. Supplementary Cementitious Materials
3. Aggregates of Different Sizes
4. Grading
5. Surface Texture
6. Shape and other Characteristics
1. Therefore, the relationship between strength and free W/C ratio should

preferably be established for the materials actually to be used.

• In the absence of such data, the W/C Ratio given in Table 5 of IS 456 – 2000

for respective environmental exposure conditions may be used

NOTE: - The supplementary cementitious material, that is mineral admixtures shall also be considered in W/C Ratio calculations in accordance with Table 5 of IS 456 – 2000

3.2 Selection of Water Content

The Water content of concrete is influenced by a number of factors such as:

1. Aggregate Size
2. Aggregate Shape
3. Aggregate Texture
4. Workability
5. Water-Cement Ratio
6. Type and Content of Cement and other supplementary Cementitious Materials
7. Chemical Admixtures
8. Environmental Conditions

Water Demand is Reduced with

1. An Increase in Aggregate Size
2. A Reduction in W/C Ratio and Slump
3. Use of Rounded Aggregate
4. Use of Water Reducing Admixtures

On the other hand Water Demand is Increased Due to Increase in

• Temperature
• Cement Content
• Slump and W/C Ratio
• Aggregate Angularity
• Decrease in the Proportion of CA to FA

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• The Quantity of Maximum Water per Unit Volume of Concrete may be determined from Table 2 of IS 10262 – 2009
• The Water Content is for
1. Angular Aggregate
2. Slump Range of 25 to 50 mm
• The Water Estimate in Table 2 can be Reduced to produce same Workability

by Approximately

• 10 Kg for Sub-Angular Aggregate
• 20 Kg for Gravel with some Crushed Particles
• 25 Kg for Rounded Gravel
• For the desired Workability (other than 25 to 50 mm Slump Range) the

desired Water Content may be established

• by an Increase in Water content by about 3 Percent for every additional

25 mm Slump

• Alternatively by use of Chemical Admixtures Conforming to IS 9103
• Water Reducing Admixtures or Superplasticizing Admixtures usually Decreases Water Content by 5 to 10% and 20% and above respectively at Appropriate Dosages

Table 2

Maximum Water Content per Cubic Metre of Concrete

for Nominal Maximum Size of Aggregate

3.3 Calculation of Cementitious Material Content

• The Cement and other Supplementary Cementitious Material Content per

Unit Volume Of Concrete may be calculated from

• Free – Water Cement Ratio ( see 3.1)
• Quantity of Water per Unit Volume of Concrete
• The Cementitious Material Content so Calculated shall be checked against the Minimum Content for the Requirements of Durability and greater of the two values shall be adopted
• The Maximum Cement Content shall be in Accordance with IS 456.

3.4 Estimation of Coarse Aggregate Proportions

• Approximate Volume of Coarse Aggregate per unit Volume of Total Aggregate is given in Table 3 for W/C Ratio of 0.5 , which may be suitably adjusted for other W/C ratios.
• For equal workability, the volume of Coarse Aggregate is dependent only on
• Its Nominal Maximum Size
• Grading Zone of Fine Aggregate

Table 3

Volume of Coarse Aggregate per Unit Volume of Total

Aggregate for Different Zones of Fine Aggregate

3.5 Combination of Different Coarse Aggregate Fractions

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• The Coarse Aggregate Used Shall confirm to IS 383
• Coarse Aggregate of Different Sizes may be Combined in Suitable Proportions so

as to Result in an overall Grading Conforming to Table 2 of

IS 383 for particular Nominal Maximum Size of Aggregate

3.6 Estimation of Fine Aggregate Proportions

• With the completion of procedure given in 3.4 all the Ingredients have been

estimated except the Coarse and Fine Aggregate content.

• These Quantities are Determined by Finding out Absolute Volume of
1. Cementitious Material
2. Water
3. Chemical Admixture
4. By Dividing their Mass by their Respective Specific Gravity
5. Multiplying by 1/1000
6. And Subtracting the result of their summation from Unit Volume
• The Values so obtained are divided into Coarse and Fine Aggregate Fractions by Volume in Accordance with Coarse Aggregate Proportions determined in 3.4
• The Coarse and Fine Aggregate Contents are then determined by multiplying by

their respective Specific gravities and then by 1000

AIMS OF MIX DESIGN

• Determination of most APPROPRIATE PROPORTIONS of constituents to meet the needs of construction
• The Design Concrete Should
• Comply with structural Strength Stipulations i.e. Compressive Strength
• Have Satisfactory DURABILITY in the Environment in which Structure is used
• Have Satisfactory APPEARANCE where Exposed to View
• Be of Suitable WORKABILITY
• Be as ECONOMICAL as possible
• Choose Economic Proportions to Obtain a Cohesive Concrete of Desired Strength, Workability and Durability from Available Materials

• ECONOMY
• For Specified Strength mixes requiring LEAST WATER also REQUIRE LEAST CEMENT CONTENT
• Following Conditions Contribute to Economy in General:
• Use Largest Mean Size Aggregate (MSA) permissible, consistent with Handling & Placement facilities available
• Use Aggr. With MOST FAVOURABLE PARTICLE SHAPE, Consistent with AVAILABILITY at

REASONABLE COST

• Use COARSEST GRADING & LOWEST CONTENT of F.A consistent with STABILITY &

FINISHABILITY

• Use LOWEST WORKABILITY consistent with ADEQUATE PLACEMENT & COMPACTION

with AVAILABLE FACILITIES

METHODS OF CONCRETE MIX DESIGN

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• The mix design methods which are used in different countries are mostly based on empirical relationships, charts and graphs developed from extensive experimental investigations. Most of them follow the same basic principles and only minor differences exist in different mix-design methods for selecting the most appropriate mix proportion. Some of the mix design procedures are:
• IS : 10262 – 1982
• IRC: 44 – 1976 Method
• Road Note No. 4 Method
• ACI – 211.1 – 1976
• USBR Method – 1968
• High Strength Concrete Mixes [ACI Method]
• Murdoch method
• Arbitrary Proportions
• Maxm. Density Method
• Fineness Modulus Method
• Surface Area Method

• The ACI method gives mix design for normal and heavy weight concrete in the
• Workability range of 25 to 100 mm slump,
• 28-day cylinder compressive strength of 45 N/mm².
• There is another method for mix design of
• No slump concrete (slump being 0 to 25 mm) and
• Maximum 28-day cylinder compressive strength of 47.5 N/mm².
• The British method outlines a procedure for design of normal concrete mixes having 28-day cube compressive strength as high as 75 N/mm² The workability is given in terms of slump value.
• In the USBR method mix proportioning is done only for air-entrained concrete for maximum 28-day cylinder compressive strength of 45.5 N/mm², when water reducing admixtures are added.

In all the above four methods, the W/C ratio is chosen for the target mean strength from empirical strength – w/c ratio relationships and water content is chosen for the required workability for aggregates in saturated surface (SSD) condition.

• The Indian Standard recommended guidelines for mix design includes design of

normal concrete mixes both for medium and high strength concrete.

• In this method of mix design,
• The water content and proportion of fine aggregate corresponding to a

maximum size of aggregate are first determined for reference values of

• workability,
• w/c ratio and
• grading of fine aggregate.
• The water content and proportion of fine aggregate are then adjusted for any difference in
• workability,
• w/c ratio and
• grading of fine aggregate in any particular case from the reference values.
• The batch weight of materials per unit volume of concrete is finally calculated

by the absolute volume method.

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• The specific relationships (tables and figures) that are used in this method are

arrived at by exhaustive tests at the ‘Cement Research Institute of India

as well as on the basis of data generated in the country for the design of

concrete..

CONCRETE MIX-DESIGN TO IS : 10262 – 2019

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(1)

STIPULATIONS for PROPORTIONING

k) Chemical Admixture Used

= Superplasticizer

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Conforming to

2 TEST DATA FOR MATERIALS

1. Cement Used =

​

• Specific gravity of Cement=
• Chemical Admixtures =

OPC 43 Grade

IS 8112

3.15

Superplasticizer

conforming to

IS 9103 (If Required)

d) Specific Gravity of:

1. Coarse Aggregate
2. Fine Aggregate

= 2.74

= 2.74

e) Water Absorption

1. Coarse Aggregate
2. Fine Aggregate

= 0.5 Percent

= 1.0 Percent

f) Free (Surface) Moisture

1. Coarse Aggregate
2. Fine Aggregate

= Nil

= Nil

Sieve Analysis

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Coarse Aggregate

Fine Aggregate : Conforming to Grading zone II of Table IV of IS:383-1970

SOLUTION

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3 TARGET MEAN STRENGTH

In order that not more than the specified proportion of test results are likely to fall below the CHARACTERISTIC STRENGTH, the concrete mix has to be designed at somewhat HIGHER TARGET AVERAGE COMPRESSIVE STRENGTH σ_t

​

Target Mean strength is Given By

(1)

Where,

From Table 1 Standard Deviation , S = 5 N/mm²

Therefore

Target Strength = 30 + 1.65 x 5 = 38.25 N/mm²

​

NOTE: According to IS:456-1978 & 1343-1980 the Characteristic Strength is defined as that value below which not more than 5% (1 in 20) results are expected to fall.

f’_CK = f_CK + 1.65 S

4 SELECTION OF WATER-CEMENT RATIO

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• From Table 5 IS 456 – 2000, W/C Ratio = 0.45 [ for M 30 and Severe Exposure Conditions]

​

5 SELECTION OF WATER CONTENT

• For 100 mm Slump [Workability]
• From Table 2 of IS 10262 – 2009;
• Maximum Water Content = 186 Litres

[ For 25 to 50 mm Slump

Range and 20 mm MSA]

Therefore

• Estimated Water Content for 100 mm Slump =

= 197 Litres

​

Note: Increase water by 3% for every additional 25 mm Slump

5 MINM. Cement Content, MAXM. W/C Ratio and Minm Grade of Concrete with different exposures with Normal Weight Aggregate of 20 mm Nominal Maxm Size[As per IS 456-2000]

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TABLE :-

TABLE :- 6

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Adjustments to Minimum Cement Contents for

Aggregates Other Than 20 mm Nominal maximum Size [As per IS 456-2000]

6 CALCULATION OF CEMENT CONTENT

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• Water Cement Ratio W/C = 0.45

​

• Cement Content =

​

= 438 Kg/m³

Note: From Table 5 of IS 456 – 2000 Minimum Cement Content for Severe

Exposure Condition = 320 Kg/m³

438 Kg/m³ > 320 Kg/m³ Hence OK

​

7 PROPORTION OF VOLUME of COARSE AGGREGATE AND

FINE AGGREGATE CONTENT

From Table 3 For W/C Ratio = Volume of CA =

0.5 and Zone II of FA

0.62

Decrease in W/C Ratio = 0.5 – 0.45 = 0.05

• In the present case there is a Decrease in W/C Ratio and hence
• CA is required to be Increased to Decrease the Fine Aggregate Content
• The Rate of increase = —/+ 0.01 for every ± 0.05 Change in W/C

Ratio Therefore,

Corrected Proportion of Coarse Aggregate

Proportion of Fine Aggregate = 1 – 0.63

CA = 0.63

FA = 0.37

8 MIX CALCULATION

• The Mix Calculation Per Unit Volume of Concrete Shall be as Follows
1. Volume of Concrete = 1 m³
2. Volume of Cement =

=

= 0.139 m³

(c) Volume of Cement

30

=

= 0.197 m³

=

​

4. Volume of Admixture =

​

• Volume of all in Aggregate =

[a − (b + c + d)]

[1 − ( 0.139 + 0.197 + 0)]

0.664 m³

= e × Volume of CA × Specific

CA × 1000

0.664 × 0.63 × 2.74 × 1000

1146 Kg

= e × Volume of FA × Specific FA × 1000

=

=

6. Mass of Coarse Aggregate

Gravity of

=

=

7. Mass of Fine Aggregate Gravity of

=

=

0.664 × 0.37 × 2.74 × 1000

673 Kg

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MIX PROPORTIONS FOR TRIAL No. 1