Concrete Materials and Properties Guide

1.4 CONCRETE
** these slides contain referenced materials

1.4.1 GENERAL
What is Concrete?
 Concrete is one of the most commonly
used building materials.
 Concrete is a composite material made
from several readily available constituents
(aggregates, sand, cement, water).
 Concrete is a versatile material that can
easily be mixed to meet a variety of
special needs and formed to virtually any
shape.

Advantages
 Ability to be cast- ability to be molded or cast into almost any desired
shape.
 Economical- when ingredients are readily available.
 Durable- relatively low maintenance requirements
 Fire resistant
 Energy efficient
 On-site fabrication
 It is not as likely to rot, corrode, or decay as other building materials.
 It is a non-combustible material which makes it fire-safe and able
withstand high temperatures.
 It is resistant to wind, water, rodents, and insects. Hence, concrete is
often used for storm shelters.
 It has high compressive strength, resistance to weathering, impact and
abrasion
 Building of the molds and casting can occur on the work-site which
reduces costs.

Disadvantages
 Low tensile strength
 Low ductility
 Volume instability
 Low strength to weight ratio
 High cost of cement, steel and formwork ( in developing countries).
 Difficult quality control on building sites, with the risk of cracking and
gradual deterioration, if wrongly mixed, placed and insufficiently cured
with water.

Cement
Water
Fine Agg.
Coarse Agg.
Admixtures
Constituents

Aggregates
 Aggregates generally occupy 65-
80% of the volume of concrete.
Hence due consideration should be
given in their selection and
proportioning.
 Earlier, aggregates were considered
as chemically inert materials but now
it has been recognised that their
physical, thermal and at times
chemical properties influence those
of the concrete.

Basically aggregate serves
the following purposes:
 Form the inert mineral filler material which the cement
paste binds together.
 Reduce the volume changes resulting from the setting
and hardening process and from moisture changes in the
paste.
 Provides better durability than hydrated cement paste
alone.
 Economical advantages.

Aggregate production: Quarry in Kality

In choosing aggregate for use in
particular concrete attention should be
given to three important requirements:
 Workability when fresh for which the size and
gradation of the aggregate should be such that
undue labour in mixing and placing will not be
required.
 Strength and durability when hardened for which
the aggregate should:
 be stronger than the required concrete strength
 contain no impurities which adversely affect strength
and durability
 not go into undesirable reaction with the cement
 be resistant to weathering action

 Economy of the mixture: the aggregate should be
 available from local and easily accessible
deposit or quarry
 well graded in order to minimize paste, hence
cement requirement.

Classification of
aggregates
 Aggregates can be classified based on
their source, mode of formation,
weight, size, and chemical
composition.

Aggregates
Based on source
Natural
Artificial
Recycled
Based on mode of
formation
Igneous
Sedimentary
Metamorphic
Based on
weight
Light
weight
Normal weight
Heavy
weight
Based on
size
Fine
Coarse
Based on chemical
composition
Argillaceous
Calcareous
Siliceous
13

Classification of aggregates
based on source
Natural aggregates are taken from natural
deposits without change in their nature during
production, with the exception of crushing, sizing,
grading, or during production. In this group
crushed stone, gravel, and sand are the most
common.
Manufactured aggregates include blast furnace
slag and lightweight aggregates.
Recycled Aggregate – e.g. crushed concrete,
clay bricks

Classification of aggregates
based on size
• Fine aggregate: < 4.75 (No.4 sieve)
• Coarse aggregate: predominantly retained
on the No.4 (4.75mm) sieve.
 Coarse aggregate > 5 mm (10 mm, 20 mm,
40mm)

Classification based on
Condition
• Crushed
From quarry - sharp, angular
particles, rough surface, good
bond strength, low workability
• uncrushed
From river - round shapes,
smooth surface, low bonding
properties, high workability

Aggregate Terms and
Types
 The terms used to describe aggregates are many
and varied. These descriptive terms are based on
source, size, shape, type, use and other properties.
 Some typical terms used in describing
aggregates are:
1. Fine aggregate- aggregate particles passing the
No. 4 (4.75mm) sieve and retained on the No. 200
(75µm) sieve.

2. Coarse aggregate- aggregate predominantly
retained on the No.4 (4.75mm) sieve.
3. Crushed gravel (gravel and sand)- that has
been put through a crusher either to break many of
the rounded gravel particles to a smaller size or to
produce rough surfaces.

4. Crushed rock- aggregate from the crushing of
rock. All particles are angular, not rounded as in
gravel.
5. Screenings- the chips and dust or powder that are
produced in the crushing of rock for aggregates.
6. All-in-aggregate- aggregate composed of both
fine and coarse aggregate.
7. Concrete sand- sand that has been washed
(usually) to remove dust & fines.
8. Fines- silty-clay or dust particles smaller than 75
micro m (No.200 sieve) usually undesirable
impurities in aggregates.

Properties of Aggregates
Important properties of aggregates include:
 Gradation (grain size distribution)
 Shape and surface texture
 Specific gravity (relative density)
 Absorption
 Hardness (resistance to abrasion or wear)
 Durability (resistance to weathering)
 Crushing strength
 Cleanliness (deleterious substances)
 Chemical stability

Grading: is the distribution of
particles of angular materials
among various sizes

The gradation of
aggregates influences:
the amount of paste required
the workability of the concrete
the strength and
water tightness of the finished product
In general, it is desirable that the size
increase uniformly from fine sand to the
maximum allowed for a given job.
Most specifications for concrete require a
grain size distribution that will provide a
dense and strong mixture.

Types of gradation
Aggregates may be:
 Dense
 Well graded
 Gap-graded
 Uniform
 Open-graded
24
Well-graded
Poorly graded

25
Grading of aggregates
The range of sizes
is approximately in
equal amounts
Well graded Uniform graded Gap graded
Most particles
are of large or
small size
Most particles
are of the
same size

Well graded aggregates:
 Improve workability of the concrete and
economy of the cement.
 Such aggregate has a decreased amount
of voids between the particles and
consequently requires less cement paste.
 Produces a stronger concrete than a poorly
graded one (less water is required to give
suitable workability).

SIEVE ANALYSIS
 The grading or particle size
distribution of aggregate is determined
by sieve analysis.

Special Use Gap-Graded
aggregates
 When certain particle sizes are intentionally
omitted. Ex., for an aggregate of 19 mm maximum
size, the 4.75 mm to 9.5 mm particles can be
omitted without making the concrete harsh subject
to segregation.
 Gap-graded mixes are used in architectural
concrete to obtain uniform textures in exposed –
aggregate finishes.

Shape and Surface
Texture of Aggregates

Aggregate Shapes
Rounded
Rounded and angular

 Rough-textured, angular, and
elongated particles require more
water to produce workable concrete
than smooth, rounded compact
aggregate. Consequently, the cement
content must also be increased to
maintain the water-cement ratio.
 Flat, slivery pieces make concrete
more difficult to finish

 Aggregate should be free of flat or
elongated particles. Because they
require an increase in mixing water
and thus may affect the strength of
concrete particularly in flexure.
 Generally, flat and elongated particles
are avoided or are limited to about 15
percent by weight of the total
aggregate.

Chemical Stability
 Aggregates need to be chemically
stable so that they will neither react
chemically with cement nor be
affected chemically by outside
influences.
 In some cases aggregates with certain
chemical constituents react with alkalis
in cement. This reaction may cause
abnormal expansion and resultant
cracking of concrete.

38
Effects of Alkali-silica reaction (ASR)
Pop outs
Crack

Handling and Stockpiling of Aggregates
 The purpose of appropriate handling and stock piling of
aggregates is to avoid breakage, segregation,
contamination, and degradation.
Precautions:
 Storing on hard and dry ground or on platforms of
planks, sheets, lean concrete
 Storing separately each aggregate size in compartments
 Avoiding segregation of aggregates resulting from free
fall
 Damping consignments at different places.
 Proper collection and mixing of test batches is important
to ensure that test samples accurately represent the
aggregate in the entire stockpile.

42
Gathering
Ingredients Mixing
Transporting
Placing
Compaction
&
finishing
Curing
Hardened
Concrete
Proportioning

 The aim of mixing is to blend all of
the ingredients of the concrete to
form a uniform mass and to coat the
surface of aggregates with cement
paste.

 Ready-Mix concrete: In this type
ingredients are introduced into a
mixer truck and mixed during
transportation to the site.
• Wet – Water added before
transportation
• Dry – Water added at site
 Mixing at the site
• Hand mixed

Mixing time should be sufficient to
produce a uniform concrete. The
time of mixing depends on the
type of mixer and also to some
properties of fresh concrete.
 Undermixing → non-homogeneity
 Overmixing → danger of water loss,
breakage of aggregate particles

Hand Mixing
Adopted for small works and quantity of
concrete used is small
Procedure:
a. Sand + cement  dry mix
b. Spread the sand -cement mix on a flat
platform
c. Spread the measured quantity of
coarse aggregate on the cement-sand
mix

d. Mix the cement + sand + c.agg. At least three times by
shoveling from center to the side and then back to the
center and again to the side
e. Make a hallow in the middle of the mixed pile and pour
slowly into it half to three-quarter of the total quantity of
water required
f. Add the remainder of the water slowly, turning the
mixture over and again until the color and consistency are
uniform throughout the pile
Note: 1. Time of mixing should not exceed 3 minutes
2. Mixing platform is cleaned at the end of the days
work, so that it is ready for use the next day

Machine mixing
 Used in case of a
large quantity of
concrete is to be
produced
 Concrete can be
produced at a faster
rate at a lesser cost
and of better quality

Transporting Concrete
1. Pans
- When quantity is small
- When access to work is restricted
- Method is tedious, slow and
costly

2. Wheel barrows
 Moderate distance and
medium quantities

Transporting Concrete
3. Truck mixer
- When place of
deposit of concrete is at a
very long distance from the
mixer such that the
concrete cannot be
transported and placed
in the forms within 30
minutes
- Happens in case of
ready-mixed concrete
- Drum containing the
concrete rotates
continuously to prevent the
concrete from being stiff
and to prevent
segregation

4. Belt conveyors
- When the concrete is to be transported continuously and to a
higher level
- Installed in an inclined position
- Concrete should be stiff consistency having a slump not more than
50 mm
5. Chutes
- When concrete is to be placed below ground level, the mixer may be
placed on an upper level and concrete discharged to the lower level
through a chute of corrugated iron or timber

7. Pumps
- When large quantity of
concrete is to be
transported
continuously to
congested sites where
mixing plant can not be
installed
- To a maximum of 300
m horizontally and 40m
vertically

Placing of concrete
- Concrete should be placed and
compacted before setting commences
- Method of placing should be in such a
way as to prevent segregation ( should
not be dropped from a height more than
about 1m)

 An elevation
column of
h=3.71m is being
casted with out a
window at
h=1.50m (one of
the reasons for
segregation).

 Good construction methodology, in
casting columns from convenient
height by providing a window.

Formwork
Material
i. Timber  Most commonly
used
ii. Plywood  Bounded with
water proof synthetic resin
adhesives
iii. Hard board  Manufactured
from wood fibers, usually
impregnated with drying oils
and factory applied plastic
coatings
iv. Metal forms  very common
nowadays

Compaction of Concrete
When first placed in the form, normal
concrete excluding those with very low or
very high slumps will contain between 5%
and 20% by volume of entrapped air.
Compaction is the process which expels
entrapped air from freshly placed concrete
and packs the aggregate particles together
so as to increase the density of concrete.

Proper compaction
 Increase significantly the ultimate strength of concrete
and Enhances the bond with reinforcement.
 Increases the abrasion resistance and general durability
of the concrete,
 Decreases the permeability and helps to minimize its
shrinkage-and-creep characteristics.
 Also ensures that the formwork is completely filled – i.e.
there are no pockets of honeycombed material – and
that the required finish is obtained on vertical surfaces.

Stages of Compaction
Compaction of concrete is a two-
stage process.
First the aggregate particles are
set in motion and slump to fill
the form giving a level top
surface.
In the second stage, entrapped
air is expelled.

Inadequate compaction
Inadequate consolidation
can result in:
– Honeycomb
– Excessive amount of
entrapped air voids
(bugholes)
– Sand streaks
– Placement lines (Cold joints)

VIBRATION OF CONCRETE
 The process of compacting concrete
consists essentially of the elimination of
entrapped air. This can be achieved by:
– Tamping or rodding the concrete
– Use of vibrators

VIBRATORS
 Internal vibrator: The poker is immersed
into concrete to compact it. The poker is
easily removed from point to point.
 External vibrators: External vibrators
clamp direct to the formwork requiring
strong, rigid forms.

Internal Vibration
d
1½ R
Vibrator
Radius of Action

Internal Vibrators
Diameter
of head,
(mm)
Recommended
frequency,
(vib./min.)
Approximate
radius of
action, (mm)
Rate of
placement,
(m3/h)
Application
20-40 9000-15,000 80-150 0.8-4
Plastic and flowing
concrete in thin
members. Also used for
lab test specimens.
30-60 8500-12,500 130-250 2.3-8
Plastic concrete in
thin walls, columns,
beams, precast piles,
thin slabs, and along
construction joints.
50-90 8000-12,000 180-360 4.6-15
Stiff plastic concrete
(less than 80-mm
slump) in general
construction .
Adapted from ACI 309

Systematic Vibration
CORRECT
Vertical penetration a few inches
into previous lift (which should not
yet be rigid) of systematic
regular intervals will give
adequate consolidation
INCORRECT
Haphazard random penetration
of the vibrator at all angles and
spacings without sufficient
depth will not assure intimate
combination of the two layers

 To aid in the removal of trapped air the
vibrator head should be rapidly plunged into
the mix and slowly moved up and down.
Internal Vibrators
 The actual completion
of vibration is judged
by the appearance of
the concrete surface
which must be neither
rough nor contain
excess cement paste.

External Vibrators
 Form vibrators
 Vibrating tables (Lab)
 Surface vibrators
– Vibratory screeds
– Plate vibrators
– Vibratory roller
screeds
– Vibratory hand floats
or trowels

 External vibrators are rigidly clamped to the
formwork so that both the form & concrete are
subjected to vibration.
 A considerable amount of work is needed to
vibrate forms.
 Forms must be strong and tied enough to
prevent distortion and leakage of the grout.
External Vibrators

 Vibrating Table:
used for small
amounts of
concrete
(laboratory and
some precast
elements)
External Vibrators

Finishing concrete
 Concrete that will be
visible, such as slabs like
driveways, highways, or
patios, often needs
finishing. Concrete slabs
can be finished in many
ways, depending on the
intended service use.

Options include various colors and textures, such as
exposed aggregate or a patterned-stamped surface.
Some surfaces may require only strike off and screeding
to proper contour and elevation, while for other surfaces
a broomed, floated, or troweled finish may be specified.
In slab construction, screeding or strike off is the process
of cutting off excess concrete to bring the top surface of
the slab to proper grade. A straight edge is moved across
the concrete with a sawing motion and advanced forward
a short distance with each movement.

Curing Concrete
Curing is the process which controls the loss of
moisture from concrete either after it has been
placed in position (or during the manufacture of
concrete products), thereby providing time for the
hydration of the cement to occur.
Since the hydration of cement does take time –
days, and even weeks rather than hours – curing
must be undertaken for a reasonable period of
time if the concrete is to achieve its potential
strength and durability.

CURING OF CONCRETE
 Properties of concrete can improve with age as
long as conditions are favorable for the
continued hydration of cement. These
improvements are rapid at early ages and
continues slowly for an indefinite period of
time.
 Curing is the procedures used for promoting
the hydration of cement and consists of a
control of temperature and the moisture
movement from and into the concrete.

Curing Methods
1. Methods which supply additional water to the surface
of concrete during early hardening stages.
– Using wet covers
– Sprinkling
– Ponding

Curing Methods
2. Methods that prevent loss of moisture
from concrete by sealing the surface.
– Water proof plastics
– Use liquid membrane-forming compounds
– Forms left in place

3. Methods that accelerate strength gain by supplying
heat & moisture to the concrete.
– By using live steam (steam curing)
– Heating coils.
Curing Methods

PROPERTIES OF FRESH
CONCRETE
 Workability
 Consistency
 Segregation
 Bleeding
 Setting Time
 Unit Weight
 Uniformity

WORKABILITY
It is desirable that freshly mixed concrete
be relatively easy to transport, place,
compact and finish without harmful
segregation.
A concrete mix satisfying these
conditions is said to be workable.

Factors Affecting Workability
 Method and duration of transportation
 Quantity and characteristics of cementing
materials
 Aggregate grading, shape and surface texture
 Quantity and characteristics of chemical
admixtures
 Amount of water
 Amount of entrained air
 Concrete & ambient air temperature

WORKABILITY
 Workability is the most
important property of
freshly mixed concrete.
 There is no single test
method that can
simultaneously measure all
the properties involved in
workability.
 It is determined to a large
extent by measuring the
“consistency” of the mix.

 Consistency is the fluidity or degree of
wetness of concrete.
 It is a major factor in indicating the workability
of freshly mixed concrete.
Test methods for measuring consistency are:
 Flow test → measures the amount of flow
 Kelly-Ball test → measures the amount of
penetration
 Slump test (Most widely used test!)
CONSISTENCY

 Slump Test is related with the ease with
which concrete flows during placement

10 cm
20 cm
30 cm
The slump cone is filled in 3 layers. Every
layer is evenly rodded 25 times.
Measure the slump by determining the vertical difference
between the top of the mold and the displaced original center
of the top surface of the specimen.

 Segregation refers to a separation of the components of
fresh concrete, resulting in a non-uniform mix
 The primary causes of segregation are differences in
specific gravity and size of constituents of concrete.
Moreover, improper mixing, improper placing and
improper consolidation also lead to segregation.
SEGREGATION

Some of the factors affecting segregation:
– Larger maximum particle size (25mm) and
proportion of the larger particles.
– High specific gravity of coarse aggregate.
– Decrease in the amount of fine particles.
– Particle shape and texture.
– Water/cement ratio.
SEGREGATION

 Bleeding is the tendency of water to rise to
the surface of freshly placed concrete.
BLEEDING
 It is caused by the
inability of solid
constituents of the
mix to hold all of
the mixing water
as they settle
down.
 A special case of
segregation.

Undesirable effects of bleeding are:
• With the movement of water towards the top, the top
portion becomes weak & porous (high w/c). Thus
the resistance of concrete to freezing-thawing decreases.
• Water rising to the surface carry fine particles of
cement -This portion is not resistant to abrasion.
• Water may accumulate under the coarse agg. and
reinforcement. These large voids under the particles may
lead to weak zones and reduce the bond between
paste and agg. or paste and reinforcement.
BLEEDING

The tendency of concrete to bleeding
depends largely on properties of cement.
It is decreased by:
 Increasing the fineness of cement
 Increasing the rate of hydration
 Adding pozzolans
 Reducing water content
BLEEDING

Hot Weather Concrete
 Rapid hydration  early setting  rapid loss of
workability
 Extra problems due to
– Low humidity
– Wind, excessive evaporation
– Direct sunlight
Solutions
– Windbreaks
– Cooled Concrete Ingredients
– Reflective coatings/coverings

Cold Weather Concrete
 Keep concrete temperature above 5 °C to
minimize danger of freezing
Solutions
– Heated enclosures, insulation
– Rely on heat of hydration for larger sections
– Heated ingredients --- concrete hot when placed
– High early strength cement

UNIFORMITY OF CONCRETE
 Concrete uniformity is
checked by conducting
tests on fresh and
hardened concretes.
Slump, unit weight, air
content tests
Strength tests

UNIFORMITY OF CONCRETE
 Due to heteregeneous nature of concrete,
there will always be some variations. These
variations are grouped as:
– Within-Batch Variations : inadequate mixing,
non-homogeneous nature
– Batch-to-Batch Variations : type of materials
used, changes in gradation of aggregates,
changes in moisture content of aggregates

PROPERTIES OF
HARDENED CONCRETE
 The principal properties of hardened
concrete which are of practical importance
can be listed as:
1. Strength
2. Permeability & durability
3. Shrinkage & creep deformations
4. Response to temperature variations
Of these compressive strength is the most
important property of concrete.

PROPERTIES OF
HARDENED CONCRETE
Of the abovementioned hardened
properties compressive strength is one
of the most important property that is
often required, simply because;
1. Concrete is used for compressive loads
2. Compressive strength is easily obtained
3. It is a good measure of all the other
properties.

What Affects Concrete Strength
What
Doesn’t?

Factors Affecting Strength
 Effect of materials and mix proportions
 Production methods
 Testing parameters

STRENGTH OF CONCRETE
 The strength of a concrete specimen
prepared, cured and tested under
specified conditions at a given age depends
on:
1. w/c ratio
2. Degree of compaction

COMPRESSIVE STRENGTH
 Compressive Strength is determined by
loading properly prepared and cured
cubic, cylindrical or prismatic specimens
under compression.

 Cubic: 15x15x15 cm
Cubic specimens are crushed after rotating
them 90° to decrease the amount of friction
caused by the rough finishing.
 Cylinder: h/D=2 with h=15
To decrease the amount of friction,
capping of the rough casting surface is
performed.

Cubic specimens
without capping
Cylindrical specimens
with capping

Bonded sulphur capping Unbonded neoprene pads

Leaching & Efflorescence
 When water penetrates into concrete, it
dissolves the non-hydraulic CH (and
various salts, sulfates and carbonates of
Na, K, Ca)
 Remember C-S-H and CH is produced
upon hydration of C3S and C2S
 These salts are taken outside of concrete
by water and leave a salt deposit.

Sulfate Attack
 Ground water in clayey soils containing alkali
sulfates may affect concrete.
 These solutions attack CH to produce gypsum.
Later, gypsum and calcium alumina sulfates
together with water react to form “ettringite”.
 Formation of ettringite is hardened cement
paste or concrete leads to volume expansion
thus cracking.
 Moreover, Magnesium sulfate may lead to the
decomposition of the C-S-H gel.

 Seawater contains some amount of Na and Mg
Sulfates. However, these sulfates do not cause
severe deleterious expansion/cracking because
both gypsum and ettringite are soluble in
solutions containing the Cl ion. However, problem
with seawater is the frequent wetting/drying and
corrosion of reinforcing steel in concrete.
 To reduce the sulfate attack
1. Use low w/c ratio→ reduced permeability & porosity
2. Use proper cement → reduced C3A and C3S
3. Use pozzolans → they use up some of the CH to
produce C-S-H
Sulfate Attack

Acid Attack
 Concrete is pretty resistant to acids. But in
high concentrations:
 Causes leaching of the CH
 Causes disintegration of the C-S-H gel.

Carbonation
 Ca(OH)2 + CO2 → CaCO3 + H2O
 Accompanied by shrinkage → carbonation
shrinkage
 Makes the steel vulnerable to corrosion
(due to reduced alkalinity)

Alkali-Agg. Reactions
 Alkalies of cement + Reactive Silica of Aggs
→ Alkali-Silica Gel
 Expansions in volume
 Slow process
 Don’t use aggs with reactive silica or use
cements with less alkalies.

Corrosion
 Electrochemical reactions in the steel rebars
of a R/C structure results in corrosion
products which have larger volumes than
original steel.
 Thus this volume expansion causes cracks in
R/C. In fact, steel is protected by a thin film
provided by concrete against corrosion.
However, that shield is broken by CO2 of air
or the Cl- ions.

Freezing and Thawing
 Water when freezes expands in volume.
This will cause internal hydraulic pressure
and cracks the concrete.
 To prevent the
concrete from this
distress air-entraining
admixtures are used
to produce air-
entrained concrete.

Concrete Materials and Properties Guide

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