Concrete Technology

Unit-V
Mix
Design

Concrete mix design

Concrete mix design is defined as the appropriate selection and proportioning of
constituents to produce a concrete with pre-defined characteristics in the fresh and
hardened states.

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.
 Sampling and Acceptance Criteria (IS 456:2000) :

1. Sampling Criteria
Frequency of Sampling:

Volume of Concrete Number of Samples (Each = 3 Cubes)

1 – 5 m³ 1
6 – 15 m³ 2
16 – 30 m³ 3
31 – 50 m³ 4
> 50 m³ 4 + 1 for every additional 50 m³

Note: One sample consists of three 150 mm cube specimens, tested for compressive strength
typically at 28 days.

2. Acceptance Criteria
Concrete is considered acceptable if the following conditions are met:

Individual test result ≥ fck – 3 N/mm²

Average of four consecutive test results ≥ fck (Characteristic Strength)

If either condition is not met, the concrete is considered not to conform to strength
requirements. In such cases, core testing, non-destructive testing, or load testing
may be carried out as per Clause 17.

Acceptance Criteria Requirement

Individual Test Result ≥ fck – 3 N/mm²
Average of 4 Test Results ≥ fck (Characteristic Strength)
 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.
 Durability of Concrete
Durability is defined as the capability of concrete to resist weathering action, chemical
attack and abrasion while maintaining its desired engineering properties. It normally
refers to the duration or life span of trouble-free performance. Different concretes require
different degrees of durability depending on the exposure environment and properties
desired. For example, concrete exposed to tidal seawater will have different requirements
than indoor concrete.

Factors affecting durability of concrete

1. Cement content
2. Compaction
3. Permeability
4. Freezing and Thawing
5. Curing

1. Cement content:
i. The increase in the cement content will increase the heat of hydration.
ii. As a result, peak temperature, temperature gradient and the temperature difference
between the core and the surface will increase.
iii. Internal cracking, cracking in the surface could be observed due to the temperature
different and due to the higher temperature gradients.
iv. Rise of the core temperature beyond the 70-75 celsius could initiate the delayed
ettringite formation which causes the cracking of concrete.
 v. However, a higher grade of concrete use as it is more durable than the low-grade
concrete in terms of durability. The increase of the heat due to the hydration can
control by different methods such as using fly ash, control the formwork arrangement,
etc.
vi. The reduction of the cement content also becomes an issue due to poor workability.
vii. Further, the quality of the cement used in the mix will also affect the durability of
the concrete.

2. Compaction:
i. Concrete is compacted when it poured for better properties. It directly related to the
strength of the concrete which is directly a function of the durability of the concrete.
ii. It improves the abrasion resistance and durability
iii. Proper compaction decreases permeability and as a result, it helps to minimize the
shrinkage and creep characteristics.
iv. The excess air trap in the concrete is removed by the compaction.
v. Well compacted concrete minimizes the formation of honeycombs that causes cavities
within and the surface of the concrete.
vi. Moisture and air can move through these defects and they can cause durability issues
as discussed previously.
vii. Further, poorly compacted concrete could have more porosity than properly
compacted concrete.
viii. Lack of compaction could lead to water leakages in water retaining structures.

3. Permeability:
i. Permeability directly affects the durability as it allows the movement of moisture.
ii. Movement of moisture with reactive chemicals in an aggressive environment affects
the durability.
iii. Carbonation of the concrete could occur due to the porosity of the concrete. Higher
permeability means it has more space to the air to travel. Carbon dioxide could travel and
with the presence of water and carbon dioxide, carbonation takes place creating a
situation for reinforcement corrosion.
iv. Similarly, corrosion could also affect the durability of concrete with low permeability.
v. Absorption of the water and passing through the concrete sections due to the low
permeability creates serviceability issues such as discoloring of paints, formations of
different bacteria, etc.
 4. Resistance to Freezing and Thawing
i. Freezing and Thawing occurs due to the seasonal variation in the world. Water
molecules in the concrete freeze and it increase the volume by 9%. As a result, tensile
stresses are developed creating the cracks in the concrete.
ii. Freezing and thawing effect can be controlled by controlling the entrained air in the
concrete.
 5. Curing
i. It is a process of controlling the moisture in the surface of the concrete.
ii. Creates dense microstructure and Improve the permeability.
iii. Prolong curing enhance the durability.
iv. Proper curing improve the surface hardness and concrete can withstand surface
wear and abrasion.
v. Improvement of the permeability due to the adequate curing avoids the entering
water-borne chemicals. Increase the durability and life span of the concrete structure.
 Mix Design:
Mix design can be defined as the process of selecting suitable ingredients of concrete
and determining their relative proportions with the object of producing concrete of
certain minimum strength and durability as economically as possible.
The purpose of designing to achieve the stipulated minimum strength and durability and
to make the concrete in the most economical manner.
Cost wise all concretes depend primarily on two factors;
I. Cost of material:
The cost of cement is many times more than the cost of other ingredients;
attention is mainly directed to the use of as little cement as possible consistent
with strength and durability.
II. Cost of labor:
Labor cost, by way of formworks, batching, mixing, transporting and curing is
nearly same for good concrete and bad concrete.

Variables in Proportioning:

With the given materials, the four variable factors to be considered in connection with
specifying a concrete mix are:

1. Water-Cement ratio
2. Cement content or cement-aggregate ratio
3. Gradation of the aggregates
4. Consistency.

Various Methods of Proportioning:

1. Arbitrary proportion
2. Fineness modulus method
3. Maximum density method
4. Surface area method
5. Indian Road Congress, IRC 44 method
6. High strength concrete mix design
7. Mix design based on flexural strength
8. Road note No. 4 (Grading Curve method)
9. Mix design for pumpable concrete
10. ACI Committee 211 method
11. DOE method
12. Indian standard Recommended method IS 10262-8
 Out of the above methods, some of them are not very widely used these days because
of some difficulties or drawbacks in the procedures for arriving at the satisfactory
proportions. The ACI Committee 211 method, the DOE method and Indian standard
recommended Methods are commonly used.

BIS Method:

Procedure

1. Target mean strength for mix design:

Determine the mean target strength ft from the specified characteristic
compressive strength at 28-day fck and the level of quality control.
ft = fck + 1.65 S
Where S is the standard deviation obtained from the Table 8 IS 456:2000 of
approximate contents given after the design mix.
2. Selection of Water/Cement ratio:
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 and adopts the lower of the two values. ( From Table 5 IS456)
3. Estimation of Entrapped Air:
Estimate the amount of entrapped air for maximum nominal size of the
aggregate from the table.

4. Selection of Water Content and Fine to Total Aggregate ratio:

Select the water content, for the required workability and maximum size of
aggregates (for aggregates in saturated surface dry condition) from table 2 IS 10262.
5. Determine the percentage of fine aggregate in total aggregate by absolute volume
from table 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 from 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:

Where V = absolute volume of concrete

= Gross volume (1m3) minus the volume of entrapped air
Sc = specific gravity of cement
W = Mass of water per cubic metre of concrete, kg
C = mass of cement per cubic metre 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 metre 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
at.
 Special Concrete
Introduction:
Concrete is a composite material consisting of cement, fine aggregates, coarse aggregates addition of
water in suitable proportion

Concrete has established as a universal building material because of its high compressive strength, its
adoptability to take any form and shape and good resistance to fire and corrosion.

Frequently, CONCRETE may be used for some special purpose for which special properties are
more important than normal concrete.

In order to achieve a special concrete, suitable proportions of chemical and mineral admixtures are
used. This concrete is called as special concrete.

Uses of special concrete:

 Special concrete is used in extreme weather.
 It has been used in large structures
 Good cohesiveness or sticky in mixes with very high binder content
 Comparable flexural strength and elastic modulus
 Better drying shrinkage and significantly lower creep
 Good protection to steel reinforcement in high chloride environment
 Excellent durability in aggressive sulphate environments
 Lower heat characteristics
 PC pipes with good resistance to chemical attack.

Types of Special concrete:

Depending upon the special properties of concrete, it can be classified into different types as follows;

1. Lightweight concrete
2. High strength concrete
3. Fibre reinforced concrete
4. Polymer concrete
5. High performance concrete
6. Self compacting concrete
7. Aerated concrete
8. No- fines concrete
9. High Density Concrete
 1. Lightweight concrete :
 Lightweight concrete is a concrete whose density varies from 300 to 1850 kg/m3.
 This type of concrete mainly used to reduce the dead weight of the concrete with same load carrying
capacity like normal concrete.
 It is achieved by using light weight aggregate or by introducing air bubbles in mortar or by omitting
fines (fine aggregate). [Density of normal concrete varies from 2200 to 2600 kg/m3]

Type of lightweight concrete:

a. Lightweight aggregate concrete

b. Aerator or Cellular or Foamed concrete
c. No-fine lightweight concrete

b. Aerator or Cellular or Foamed concrete:

Aerated concrete is made by introducing air or gas into slurry composed of Portland cement
or lime and finely crushed siliceous filler so that when the mix sets and hardens, a uniformly cellular
 structure is formed. Though it is called aerated concrete it is really not a concrete in the correct sense
of the word.
As described above, it is a mixture of water, cement and finely crushed sand. Aerated
concrete is also referred to as gas concrete, foam concrete, cellular concrete.
In India we have at present a few factories manufacturing aerated concrete. A common
product of aerated concrete in India is Siporex.

There are several ways in which aerated concrete can be manufactured.

a. By the formation of gas by chemical reaction within the mass during liquid or plastic State.
b. By mixing preformed stable foam with the slurry.
c. By using finely powdered metal (usually aluminum powder) with the slurry and made to
react with the calcium hydroxide liberated during the hydration process, to give out large
quantity of hydrogen gas. This hydrogen gas when contained in the slurry mix, gives the
cellular structure.

d. No-fine lightweight concrete:

The third method of producing light concrete is to omit the fines from conventional concrete.
No-fines concrete as the term implies, is a kind of concrete from which the fine aggregate fraction
has been omitted.
This concrete is made up of only coarse aggregate, cement and water. Very often only single
sized coarse aggregate, of size passing through 20 mm retained on 10 mm is used.
No-fines concrete is becoming popular because of some of the advantages it possesses over
the conventional concrete. The single sized aggregates make a good no-fines concrete, which in
addition to having large voids and hence light in weight, also offers architecturally attractive look.

2. High Strength concrete:

 High strength concrete can be defined by compressive strength of concrete at 28 days of curing.
 When the grade of concrete exceeds M35, then the concrete may be called as High strength
concrete.
  In general, producing of HSC is difficult with the use of conventional materials like cement,
aggregate and water alone and it can be achieved by using of chemical and mineral admixtures or
any one of the following methods.
a. Seeding
b. Re vibration
c. High speed slurry mixing
d. Use of admixture
e. Use of cementitious aggregates

3. Fibre reinforced concrete:

Fiber Reinforced Concrete can be defined as a composite material consisting of mixtures of cement,
aggregate and uniformly dispersed fibers.
 Fiber is a small piece of reinforcing material possessing certain characteristics properties. The fiber
is often described by a convenient parameter called “aspect ratio”.
 The aspect ratio of the fiber is the ratio of its length to its diameter.
 Typical aspect ratio ranges from 30 to 150.
 Types of fibre:

Following are the different type of fibers generally used in the construction industries.
 Steel Fiber
 Polypropylene Fiber
 GFRC Glass Fiber
 Asbestos Fibers
 Carbon Fibers
 Organic Fibers
 Natural fibre (Coir fibre, Cotton fibre, Sisal fibre, Jute fibre and Wool fibre)

Necessity of Fiber Reinforced Concrete:

 It increases the tensile strength of the concrete.
 It reduces the air voids and water voids the inherent porosity of gel.
 It increases the durability of the concrete.
 Fibres such as graphite and glass have excellent resistance to creep.
 Deferential deformation is minimized.
 It has been recognized that the addition of small, closely spaced and uniformly dispersed fibers to
concrete would act as crack arrester.
 It substantially improves its static and dynamic properties.

4. Polymer concrete :

Polymer concrete is impregnations of monomer into the pores of harden concrete and then getting it
polymerized by thermal process is called polymer concrete. By this polymerization, the strength of
the concrete is much improved.

Types of polymer concrete:

 Polymer Impregnated concrete

 Polymer cement concrete

 Polymer concrete

 Partially impregnated and surface coated polymer concrete

Advantages of polymer concrete:
 To reduce the porosity of the concrete.

 It has high impact resistance and high compressive strength.

 Polymer concrete is highly resistant to freezing and thawing.

 Highly resistant to chemical attack and abrasion.

 Permeability is lower than other conventional concrete.

Application of polymer concrete:

 Nuclear power plants.
 Prefabricated structural element.
 Precast slabs for bridge decks & Roads.
 Marine Works.
 Pre stressed concrete.
 Irrigation works and Sewage works.
 Sewage works.
 Waterproofing of buildings

5. High Performance Concrete:

High performance concrete is a type of special concrete and it has high workability, high strength,
and high modulus of elasticity, high density, low permeability and resistance to chemical attack.

This concrete is achieved by the following materials;

 Cement
 Fine Aggregate
  Coarse aggregate
 Water
 Chemical admixture (Superplasticizer)
 Mineral Admixture (Fly ash and Silica fume)
 Fibre etc…

Characteristic or properties of high performance concrete:

 High strength, High early strength

 High modulus of elasticity , High abrasion resistance

 High durability and long life in severe environments

 Low permeability and diffusion

 Resistance to chemical attack

 High resistance to frost and deicer scaling damage

 Toughness and impact resistance

 Volume stability

 Ease of placement

 Compaction without segregation

 Inhibition of bacterial and mold growth

6. Self compacting concrete(SCC)

Self compacting concrete (SCC) can be defined as fresh concrete that flows under its own weight and
does not require external vibration to undergo compaction. It is used in the construction where it is hard to
use vibrators for consolidation of concrete. Filling and passing ability, segregation resistance is the
properties of self compacting concrete.

The main ingredients used in design of self compacting concrete are:

i. Cement
ii. Aggregates
iii. Water
iv. Mineral Admixtures
v. Chemical Admixtures

Advantages of Self Compacting Concrete

The main advantages of self compacting concrete are:

 The permeability of the concrete structure is decreased

 SCC enables freedom in designing concrete structures
 The SCC construction is faster
 The problems associated with vibration is eliminated
 The concrete is placed with ease, which results in large cost saving
 The quality of the construction is increase
 The durability and reliability of the concrete structure is high compared to normal concrete structures
 Noise from vibration is reduced. This also reduce the hand arm vibration syndrome issues

Applications of Self Compacting Concrete:

The major applications of self compacting concrete are:

 Construction of structures with complicated reinforcement

 SCC is used for repairs, restoration and renewal construction
 Highly stable and durable retaining walls are constructed with the help of SCC
 SCC is employed in the construction of raft and pile foundations
7. High Density Concrete:
Density of normal concrete is in the order of about 2400 kg. per cubic metre.

The density of light-weight concrete will be less than about density 1900 kg per cubic metre.
To call the concrete, as high density concrete, it must have unit weight ranging from about 3360 kg
per cubic metre to 3840 kg per cubic metre, which is about 50% higher than the unit weight of
conventional concrete.
They can, however be produced with the densities up to about 5280 kg per cubic metre using
iron as both fine and coarse aggregate. The high density concrete is used in the construction of
radiation shield