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Outline
Properties of Hardened Concrete
CE 231 Construction Materials
July 19th, 2011
Withit PANSUK
Department of Civil Engineering
Faculty of Engineering
Chulalongkorn University
Outline
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Introduction
Practical Criteria of Strength
g of Concrete
Factors in Strength
Development of Strength
Tensile and Compressive Strengths
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Introduction
Fatigue Strength
Abrasion Resistance
Bond to Reinforcement
Elasticity
Creep and Relaxation
Permeability
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Hardened concrete
(After Final set)
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Introduction
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Introduction
• Properties of hardened cement paste,
depend on the physical structure of
hydration more than chemical composition
in cement paste
• In many practical cases the durability,
permeability and volume stability of
concrete are the most important properties
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• Flaws, microcracking, discontinuities and
pores are significance in concrete durability
but they are very difficult to quantify in a
useful manner
• Thus, ‘Strength of concrete’ is considered to
be the most valuable property of hardened
concrete
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Introduction
Introduction
• Strength of Concrete depends on
– Strength of cement paste
– Strength of aggregate
– Interface between cement paste and
aggregates
• In many practical cases, the aggregates
always have the higher strength than
cement paste
• For the factor which effect on the strength
of concrete, we will consider only the
strength of cement paste and the Interface
between cement paste and aggregates
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Practical Criteria of Strength
Practical Criteria of Strength :
– Porosity
– Total void in concrete
– Pore size distribution
– Microcracking and Stress-Strain
Relation
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Porosity
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Porosity
Pores in hydrated cement paste
• The hydrated cement paste contains
yp of p
pores which have an
several types
important influence on its properties
– Gel pores (Interlayer space in C-S-H)
– Capillary pores
– Air voids
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Practical Criteria of Strength
• The most important practical factor is the
W/C, but the underlying parameter is the
number and size of pores in the hardened
cement paste
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Gel Pores :
• The gel pores are very small (about 2 nm
i di
in
diameter)
t ) and
d th
the volume
l
off gell water
t iis
about 28% of the cement gel
• The pore size is too small to have an
adverse effect on the strength and
permeability of the hydrated cement paste
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Porosity
Porosity
• ‘Gel Water’ can be held by hydrogen
bonding, and its removal under certain
conditions mayy contribute to drying
y g
shrinkage and creep
• In addition to gel water, there exists
‘Combined Water’, which is combined
chemically or physically with the product of
hydration, and is thus held very firmly
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Porosity
• The quantity of combined water can be
determined as the non-evaporable water
content and in fully hydrated cement
content,
represents about 23% of the mass of dry
cement
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Porosity
Diagrammatic
representation of the
volumetric
proportions:
(a) before hydration
(b) during hydration
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Porosity
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Porosity
• The mix contained more water than
necessary for full hydration, the capillary
pores will excess 18.5% and these are full
of water
• The W/C is the main influencing factor on
porosity
• The porosity will decrease if cement paste
increase the degree of hydration
• For fully hydrated cement with no excess
water above the required for hydration,
capillary
p
yp
pores is about 18.5%
% of the
original volume of dry cement
• Capillary pores can be empty or full of
water, depending on the amount of water
in the mix
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Capillary pores :
• Capillary pores represent the space not
fill d b
filled
by th
the solid
lid components
t off th
the
hydrated cement paste
• Capillary pores are much larger than gel
pores (diameter about 1mm)
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Porosity
Porosity
Influence of W/C and
degree on hydration on
capillary and total
porosities of cement paste
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Porosity
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Porosity
Relation between
compressive
i
strength and
logarithm of
porosity of
cement paste
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Porosity
Air Voids :
• Air voids are generally spherical
• A small amount of air usually gets trapped
in the cement paste during concrete
mixing
• Admixture may be added to concrete to
entrain purposely tiny air voids
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Porosity
• Both entrapped and entrained air voids in
hydrated cement paste are much bigger
than capillary
y voids and are capable of
adversely affecting the strength
• The total amount of voids in concrete can
be calculated by the same concept as
cement paste associated with the mix
proportion
• 2 Types of Air Voids
– Entrapped air voids :
May be as large as 3 mm.
– Entrained air voids :
Usually range from 50 – 200 μm
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• There is a corresponding relation between
porosity and strength, and this is
independent of whether the capillary pores
are full of water or empty
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Porosity
Total Voids in Concrete
Volumetric proportions
of concrete of mix
proportions 1:2:4 by
mass (W/C = 0.55 &
entrapped air =2.3 %)
(a)before hydration
(b)when the degree of
hydration is h=0.7
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Pore Size Distribution
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Pore Size Distribution
• Capillary pore are much larger than gel
pores, and there is a whole range of pore
sizes throughout the hardened cement
paste
• When cement is partly hydrated, the paste
contains an interconnected system of
capillary pores
• The effect of this is a lower strength and,
through increased permeability, a higher
vulnerability to freezing and thawing and to
chemical attack
• This vulnerability depends also on W/C
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Pore Size Distribution
Pore Size Distribution
• These problems are avoided if the degree
of hydration is sufficiently high for the
capillary
p
yp
pore system
y
to become
segmented through partial blocking by
newly developed cement gel
• If so, the capillary pores are
interconnected only by the much smaller
gel pores, which are impermeable.
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(a) High permeability - capillary pores interconnected by
large passages
(b) Low permeability - capillary pores segmented and
only partly connected.
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Pore Size Distribution
Pore Size Distribution
Pore Size
Distribution
in hydrated
cement paste
(Vary in W/C)
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Microcracking and Stress-Strain Relation
• Such microcracking occurs as a result of
differential volume changes between the
cement paste and the aggregate
• These cracks remains stable and do not
grow under stress up to 30% of the
ultimate strength of concrete
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Microcracking and Stress-Strain Relation
• At stress higher that 30% of ultimate
strength, the microcracks begin to
increase in length
length, width and number
• In consequence the strain increases at a
faster rate than stress = ‘slow propagation
of microcracking’
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Microcracking and Stress-Strain Relation
• If the lateral strain is observed, it was found
that, the ratio of lateral strain to axial strain
(Poisson’s ratio) is constant for stresses
below approximately 30% of the ultimate
strength
• Beyond this point, Poisson’s ratio increases
slowly, and at 70-90% it increases rapidly
due to the formation of mainly vertical
unstable cracks
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Pore Size
Distribution in
hydrated
cement paste
(vary in age)
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Microcracking and Stress-Strain Relation
• At this stage, the specimen is no longer a
continuous body as shown by volumetric
strain curve
• There is a change from slow contraction in
volume to a rapid increase in volume
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Microcracking and Stress-Strain Relation
Microcracking and Stress-Strain Relation
• At 70-90% of ultimate strength, cracks open
through the matrix and thus bridge the bond
crack so that a continuous crack pattern is
f
formed
d (fast
(f t propagation
ti off cracks)
k )
• If the load is sustained, failure will probably
occur with the passage of time
• If the load is increased, rapid failure will
take place at the nominal ultimate strength
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Microcracking and Stress-Strain Relation
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Microcracking and Stress-Strain Relation
Stress-strain
relations for
cement paste,
aggregate, and
concrete
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Stress-strain
relation for
concretes
tested at a
constant rate
of strain
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Factors in Strength of Concrete
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Factors in Strength of Concrete
• Although porosity is a primary factor
influencing strength, it is a property difficult
to measure or even to calculate
• Similarly, the influence of aggregate on
microcracking is not easily quantified
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• For these reasons, the main factors on
strength are taken in practice as
– Water – Cement Ratio
– Degree of compaction
– Age and temperature
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Factors in Strength of Concrete
Factors in Strength of Concrete
• However, there are also other factors such
as
– Aggregate/cement ratio,
– Quality of aggregate
– The maximum size of aggregate
• These factors are considered secondary
factors when usual aggregates up to a
maximum size of 40 mm are used
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Factors in Strength of Concrete
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Factors in Strength of Concrete
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Influence of the
aggregate/cement
t /
t
ratio on strength
of concrete
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Development of Strength
• To obtain good quality concrete, ‘Curing’
during the early stage of hardening must be
done
• Curing = the procedures used for promoting
the hydration of cement
• The curing procedures being control of the
temperature and of the moisture movement
from and into concrete
Effect of max.
aggregate
size
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Factors in Strength of Concrete
Influence of age on
compressive strength of
OPC concrete at
different W/C
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Relation
between
strength
t
th and
d
water/cement
ratio of
concrete
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Development of Strength
Development of Strength
• The latter affects not only strength but also
durability
• The object of curing is to keep concrete
saturated, until the originally water-filled
space in the fresh cement paste has been
occupied to the desired extent by the
products of hydration
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Development of Strength
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Influence of curing
conditions on
strength of test
cylinders
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Development of Strength
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Development of Strength
• The period of curing can not be prescribed
in a simple way
p
is above 10oC,, ACI lays
y
• If the temperature
down a minimum of 3 days for Portland
cement type III, 7 days for type I, and 14
days for type IV
• The temperature also affects the length of
the required period of curing
Influence of
moist curing on
the strength of
concrete with a
W/C of 0.50
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Development of Strength
• If, however, curing proceed until the
capillaries in the hydrated cement have
become segmented
segmented, then concrete will
impermeable and this is vital for good
durability
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• The necessity for curing arise from the fact
that hydration of cement can take place
only in water
water-filled
filled capillaries (loss of water
by evaporation from concrete must be
prevented)
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Development of Strength
Development of Strength
Minimum period of protection required for different
cements and curing conditions, (by BS 8110: Part 1: 1985)
Minimum period of protection required for different
cements and curing conditions, (by BS 8110: Part 1: 1985)
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Influence of Temperature
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Influence of Temperature
• The higher the temperature of the
concrete at placement the greater the
initial rate of strength development,
development but the
lower long-term strength
• This is why important to reduce the
temperature of fresh concrete when
concreting in hot climate
• The explanation is that a rapid initial
hydration causes a non-uniform
distribution of the cement gel with a poorer
physical structure, which is probably more
porous than the structure developed at
normal temperatures
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Influence of Temperature
Influence of Temperature
• As a result, a concentration of hydration
products is built up in the vicinity of the
hydrating
y
g cement g
grains, a process which
retards subsequent hydration and the
development of longer-term strength
• The influence of the curing temperature on
strength indicated a higher initial strength
development, but lower 28 days strength
• With a high initial temperature, there is
insufficient time available for the products
of hydration to diffuse away from the
cement grains and for a uniform
precipitation in the interstitial space
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Influence of Temperature
Influence of Temperature
Relation between
compressive
strength and curing
time of neat cement
paste compacts at
different curing
temperatures
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Influence of Temperature
Relation between
compressive strength
and curing time of neat
cement paste compacts
at different curing
temperatures
(W/C = 0.14; OPC)
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Tensile and Compressive Strengths
• The theoretical compressive strength was
stated to be 8 times larger than tensile
strength
• In fact, The ratio of the two strengths
depends on the general level of strength of
the concrete
• The ratio of tensile/compressive strengths
is lower the higher compressive strength
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Tensile and Compressive Strengths
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Fatigue Strength
• Two type of failure in fatigue can take
place in concrete
– 1st , failure occurs under a sustained load (or
slowly increased load). This is known as static
fatigue or creep rupture.
– 2nd, type occurs under cyclic or repeated load,
and is known simply as fatigue.
Relation between tensile and compressive strengths of concrete
made with normal weight and lightweight aggregates
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Fatigue Strength
Fatigue Strength
• In both instances, a time-dependent failure
occurs only at stress which are greater
than a certain threshold value but smaller
than the short-term static strength
• At rapid rates of loading, concrete appears
more brittle in nature than under lower
rates of loading when creep and
microcracking increase the strain capacity
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Fatigue Strength
• Under low rates of loading, static fatigue
occurs when the stress exceeds about 70
to 80 per cent of the short-term strength
• This level represents the onset of rapid
development of microcracks
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Fatigue Strength
• A similar phenomenon takes place under a
sustained load, a certain load is applied fairly
quickly and then held constant
• Above
Ab
th
the same stress
t
level
l
l off 70 - 80 % off
the short-term strength, the sustained load
will eventually result in failure
• At the stress level lower that the threshold,
failure will not occur and the concrete will
continue to creep
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Fatigue Strength
Influence of
test duration
(or rate of
loading) on
strength and
on strain
capacity in
compression
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Impact Strength
Influence of
sustained
stress on
strength
t
th and
d
on strain
capacity of
concrete in
compression
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• Impact strength is generally of important in
driving concrete piles, foundations for
machines exerting impulsive loading,
loading and
when accidental impact is possible
• There is no unique relation between
impact strength and compressive strength.
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Impact Strength
Impact Strength
• For a given type of aggregate, the higher
the compressive strength of the concrete
the lower the energy absorbed per blow
before cracking
cracking, but the greater the No
No. of
blows to reach the state of ‘no-rebound’
• The impact strength and total energy
absorbed by concrete increase with its
static compressive strength and therefore
with age at a progressive increasing rate
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• The relation between impact strength and
compressive strength depends also upon
the type
y of coarse aggregate
gg g
but the
relation depends also on the storage
condition of the concrete
• The impact strength of water-stored
concrete is lower than when concrete is
dry
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Impact Strength
Relation between
compressive strength and
No. of blows to ‘no rebound’ for concretes
made different aggregates
and OPC, stored in water
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Abrasion Resistance
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Abrasion Resistance
• Concrete surface can be subjected to
various types of abrasive wear
• Sliding
Slidi or scraping
i can cause attrition.
tt iti
IIn
the case of hydraulic structures, the action
of abrasive solid carried by water leads to
erosion of concrete
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Impact Strength
• For the same compressive strength,
impact strength is greater for concrete
made with coarse aggregates of greater
angularity and surface roughness
• Thus, impact strength of concrete is more
closer related to its flexural strength than
compressive strength
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Abrasion Resistance
apparatus
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Abrasion Resistance
Bond to Reinforcement
Influence of the W/C of the
mix on the abrasion loss
of concrete for different tests
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Bond to Reinforcement
• The strength of bond between
reinforcement and concrete arises primarily
f
from
f i ti and
friction
d adhesion
dh i
• Bond is affected by the properties both of
steel and concrete, and by relative
movement due to volume change
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Bond to Reinforcement
• In general terms, bond strength is
approximately proportional to the
compressive strength of concrete up to 20
MPa
• For higher strength, the increase in bond
strength becomes smaller and eventually
negligible
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Bond to Reinforcement
Influence of the
strength of concrete
on bond determined
by pull-out
test
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Elasticity
ASTM C 234
Standard Test Method
for Comparing
Concrete on the Basis
of the Bond Developed
with Reinforcing Steel
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• The moisture condition of the specimen is
a factor, a wet specimen as a higher
modulus
• The properties of aggregate also influence
the modulus of elasticity
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Elasticity
Elasticity
• The influence of the aggregate arises from
the value of the modulus of the aggregate
and its volumetric proportion, the higher the
modulus of aggregates the higher the
modulus of concrete
• The relation between the modulus of
elasticity of concrete and strength depends
also on age, the modulus increases more
rapidly than strength
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Elasticity
T i l stress-strain
Typical
t
t i curve
for concrete
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Elasticity
Typical range of values of 28-day static modulus of
elasticity for normal weight concrete, according to BS
8110:Part 2:1985
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Creep and Relaxation
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Creep and Relaxation
• Creep is defined as the increase in strain
under a sustained constant stress after
taking into account other time-dependent
deformations
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Definition of creep
under a constant
stress s0; E is the
secant modulus of
elasticity at age t0
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Creep and Relaxation
Creep and Relaxation
• Creep effects may also be viewed from
another standpoint
• If a loaded
l d d concrete
t specimen
i
iis
restrained so that it is subjected to a
constant strain, creep will manifest as a
progressive decrease in stress with time
• This phenomenon is termed relaxation
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Definition of relaxation for
concrete subjected initially
to stress s0 and then kept
at a constant strain; E is
the secant modulus of
elasticity at age t0
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Permeability
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Permeability
•
•
•
•
• There are no prescribed test methods for
permeability, it can be expressed as
coefficient of permeability,
permeability k,
k given by
Darcy’s equation
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Permeability
• There is no unique relation between air
and water permeabilities for any concrete,
although they are both mainly dependent
on W/C and the age of concrete
dq/dt = the rate of flow of water
A = cross-sectional area of sample
Dh = drop in hydraulic head
L = thickness of the sample
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Permeability
• Permeability is the ease with which liquids
or gases can travel through concrete
• This
Thi property
t is
i off interest
i t
t in
i relation
l ti to
t the
th
water-tightness of liquid-retaining
structures and to chemical attack
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Permeability
Permeability
Reduction in
permeability of
cement paste
with the
progress of
hydration; W/C
ratio = 0.7
Relation between
permeability
p
y and
water/cement ratio
for mature cement
pastes (93 % of
cement hydrated)
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• Boonchai Stitmannaithum, "Advance Concrete Technology
(Lecture Note)" Chulalongkorn University 2005 : Chapter III
Properties of Hardened Concrete
• D. M. ROY and G. R. GOUDA, Porosity
y - strength
g relation in
cementitious materials with very high strengths, J. Amer.
Ceramic Soc., 53, No. 10, pp. 549-50 (1973).
• P. T. WANG, S. P. SHAN, and A. E. NAAMAN, Stressstrain curves of normal and lightweight concrete in
compression, J. Amer. Concr. Inst., 75, pp. 603-11 (Nov.
1978)
Concrete Water Permeability Apparatus
(Photo from CPAC Concrete testing Lab)
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References
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References
• B. G. SINGH, Specific surface of aggregates related to
compressive and flexure strength of concrete, J. Amer.
Concr. Inst., 54 , pp. 897-907 (April 1958).
• P.KLIEGER, Early
y high
g strength
g concrete for p
prestressing,
g
Proc. of World Conference on Prestressed Concrete, pp.
A5-1 - A5-14 (San Francisco, July 1957).
• W. H. PRICE, Factors influencing concrete strength, J.
Amer. Concr. Inst., 47, pp. 417-32 (Feb. 1951).
• CEMENT AND CONCRETE ASSOCIATION, Research and
development on materials, Annual Report, pp. 14-19
(Slough 1976).
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References
Permeability
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• CPAC Concrete Academy: The Concrete Product and
Aggregate co.,ltd; http://www.cpacacademy.com
• Portland Cement Association (PCA). Cement & Concrete
Technology; http://www.cement.org/
http://www cement org/
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Next : Improvement of
Concrete Quality
CE 231 Construction Materials
July 26th, 2011
Withit PANSUK
Withit.P@chula.ac.th
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