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CE303Lecture

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Title: CE303Lecture

1
CE-303-Lecture 1

Fundamentals of Concrete

2
Objectives

To explain the basic concepts of concrete
To explain briefly the properties of freshly
mixed concrete
To explain briefly the properties of hardened
concrete

3
Concrete

A mixture of two components
Paste (25-40)
Aggregates (60-75) and

4
Paste

Comprises of
a binder (cement)
Water
Entrapped air or purposely entrained air

5
Aggregates

Comprises of
Sand (fine aggregates)
Gravel or crushed stone (coarse aggregates)

When paste and aggregates are mixed together the
particles of aggregates get coated with the
paste.
Because of the chemical reaction of the cement
and water the paste hardens and binds the
aggregates together to form a rocklike mass
called Concrete

7
Components of Concrete
8
Concrete
9
Volume Distribution of Materials in Concrete
10
Quality of Concrete

Aggregate Requirement
Paste Requirement

11
Aggregate Requirement

Aggregates should consist of particles with
adequate strength against compression, impact,
and abrasion.
Aggregate particles should have resistance
against aggressive exposure conditions such as
highly fluctuating temperature and humidity

12
Aggregate Requirement

Aggregates should not contain deleterious
materials such as chloride, sulfate, etc. that
will cause deterioration of concrete.
The size, shape and particle size distribution
(gradation), of the aggregates should be such
that the least amount of paste is required to
fill the spaces between aggregate particles.

13
Paste Requirements

Quality of paste greatly affects the quality of
the concrete
Paste should be sufficient enough to completely
coat each particle of aggregate and to fill
spaces between aggregate particles completely.

14
Paste Requirements

The quality of hardened concrete is determined by
the amount of water used in relation to the
amount of cement
(W/C ratio of the paste)

15
Cement-paste cylinders
16
Advantages of reducing the W/C ratio of the paste

Increase compressive strength and flexural
strength.
Lower permeability, thus increased
water-tightness and lower absorption.
Increased resistance to weathering.
Better bond between successive layers and between
concrete and reinforcement.

17
Advantages of reducing the W/C ratio of the paste

Less volume change from wetting and drying
Reduced shrinkage cracking tendencies.

18
Effect of W/C ratio on Compressive Strength of
Concrete
19
Uses of Admixtures to Improve the Quality of
Concrete

Adjusting setting time or hardening in abnormal
conditions of concreting such as cold and hot.
Reducing water demand
Increasing workability in case of a very low w/c
ratio
Intentionally entrained air in concrete

20
Mixing Requirements

All the ingredients of concrete should be mixed
properly so as to get a homogeneous mixture.
The sequence of charging ingredients into the
mixer plays an important part in the uniformity
of the finished product.
Size of the batch in relation to the size of the
mixer

21
Mixing Requirements

The elapsed time between batching and mixing.
Design, configuration, and condition of the mixer
drum and blades.

22
Consolidation Requirements

Consolidation of the freshly placed concrete
removes voids in concrete and improves the
quality of concrete on account of increased
density.
For a highly workable concrete the consolidation
should be done by hand rodding.

23
Consolidation Requirements

For stiffer mixtures having low w/c ratio the
consolidation should be done by vibration.
Vibration sets into motion the particles in
freshly mixed concrete, reducing friction between
them and thereby increasing the mobility of
concrete which helps in removing the voids even
at low w/c ratio.

24
Consolidation Requirements

Thus consolidation by vibration permits the lower
w/c ratio which in turn results in improved
quality.
Vibration of highly workable concrete should be
avoided as it can cause segregation of the
concrete.

25
Effect of Consolidation
26
Curing Requirements

Curing of a placed and finished concrete is done
for maintaining a satisfactory moisture and
temperature in concrete for some defined period
after placing and finishing, to allow the
complete hydration of the cement.

27
Curing Requirements