lecture 3b - concrete (2012)
TRANSCRIPT
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LECTURE 3B
HARDENED CONCRETE
Presented by: Mr. Milton McIntyre
University of Technology, Jamaica
Sep. 2015
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CONCRETE 2
Properties of hardened concrete
Factors affecting hardened concrete properties
Corrosion of reinforcement in concrete
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PROPERTIES OF HARDENED CONCRETE
The main properties of hardened concrete are:
Strength
Deformation under loading
Shrinkage
Permeation
Durability
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STRENGTH
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The strength of concrete refers to the maximum load
(stress) it can withstand.
Concretes strength is observed in two main load
applications:
Compression
Tension
Due to the fact that concrete is a brittle material it has a
low tensile strength but a very high compressive strength.
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time
in air entire time
moist cured entire time
in air after 3 days
in air after 7 daysStrength
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100%
STRENGTH AND CURING
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DEFORMATION
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This include the manner in which concrete deforms
under load. There are two main forms of
deformation:
a) Load dependent (Elastic )
b) Time-dependent (creep)
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ELASTIC DEFORMATION 8
Elastic deformation is the change in shape to which
concrete undergoes when subjected to a continuously
increasing load.
The deformation relationship is non-linear. When theapplied load is released, the concrete does not
recover its original shape, unlike metals.
Its resistance to load is dependent on the factors that
also affect strength.
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MODULUS OF ELASTICITY (E)
This is the ratio of load per unit area (stress) to the
elastic deformation per unit length (strain).
It is used when estimating the deformation, deflection
or stresses under normal working loads.
For concrete, E increases as strength increases.
POISSON RATIO
This is referred to as the ratio of the lateral strain to theassociated axial strain and varies from 0.1 to 0.3 fornormal working stress.
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CREEP DEFORMATION 10
Creep deformation is the change in shape to which concrete
undergoes when load is continuously applied after elastic
deformation has occurred and continues to deform with time.
The term creep is used to describe the increase in strain withtime and creep recovery is the term used to describe the
gradual decrease in strain over a period of time after the
load is sustained removed.
Creep strain is a very important factor in structural design asconcrete in service is subject to sustained loads for long
periods of time and creep strains normally exceeds elastic
strains.
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CREEP DEFORMATION
Creep strain is influenced by the type of concrete (curing
history, strength, cement type, age & environmental
conditions) and the magnitude of the load with respect to
concrete strength.
For a given concrete, creep strain depends on the stress-
strength ratio.
For practical purposes, concrete creep strain may be
assumed to be directly proportional to the elasticdeformation up to a stress-strength ratio of about two-
thirds.
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SHRINKAGE
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This is a type of deformation that occurs in concrete
independent of loads.
Its is caused by:
settlement of solids and the loss of free water from the plasticconcrete (plastic shrinkage)
chemical combination of cement with water (autogenous
shrinkage)
the drying of concrete (drying shrinkage)
Shrinkage may cause cracking in concrete when movement
is restricted by producing tensile stresses within the
concrete.
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PLASTIC SHRINKAGE 13
Takes place before concrete sets. Caused by the rapid loss of free water, with or without settlement
of solids.
Common in slabs, identified by the appearance of surface cracks.
Prevented by methods of reducing water loss.
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Plastic Shrinkage
To minimize plastice shrinkage:
Start curing the concrete as soon as possible.
Spray the surface with liquid membrane curing
compound or cover the surface with wet burlap and
keep it continuously moist for a minimum of 3 days.
Consider using synthetic fibre to resist plastic
shrinkage cracking.
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AUTOGENOUS SHRINKAGE 15
This is produced by thehydration of cementwithin the concrete.
Common in mass concretestructures.
Influenced by chemicalcomposition of cement,
initial water content,temperature and time.
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Autogenous Shrinkage
Autogenous shrinkage is an important phenomenon in youngconcrete.
At low water/cement ratios, less than about 0.42, all the wateris rapidly drawn into the hydration process and the demand
for more water creates very fine capillaries.The surface tension within the capillaries causes autogenousshrinkage which can lead to cracking.
This can be largely avoided by keeping the surface of theconcrete continuously wet; conventional curing by sealing the
surface to prevent evaporation is not enough and water curingis essential.
With wet curing, water is drawn into the capillaries and theshrinkage does not occur.
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DRYING SHRINKAGE 17
Occur after the initial curing
phase of the concrete, when
concrete is allowed to dry.
Influenced by the type,
content & proportion of the
constituent materials, size &
shape of structure, amount &
distribution of reinforcementand relative humidity.
Accommodated by control
joint in slabs
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Drying Shrinkage
Drying shrinkage can be reduced by using the
maximum practical amount of aggregate in the mix.
The greater the amount of aggregate is, the smaller is
the amount of shrinkage. The higher the stiffness of the aggregate is, the more
effective it is in reducing the shrinkage of the concrete.
The lowest water-to-cement ratio is important to
avoid this type of shrinkage.
The higher the water content is, the greater is the
amount of shrinkage from drying.
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PERMEATION
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This can be defined as the ease to which fluids can move intoor out of concrete.
Fluid may move through concrete in three ways:
Absorption Permeability Diffusion
Each method leads to concrete deterioration.
Concrete is a semi-permeable material that permitaggressive fluids from the environment to pass through causingphysical and chemical damage to its structure orreinforcement.
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ABSORPTION
This is defined by the process
by which a fluid passes
through concrete by capillaryaction.
The rate of absorption is
dependent on the size and
interconnection of the
capillary pores, also the
gradient of moisture from the
surface.
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DIFFUSION
This is defined as the process by
which vapour, gas or an ion can
pass into concrete by aconcentrated gradient.
The rate of diffusion is
dependent on the concentration
gradient from the concrete
surface, the type of agent andany reaction with the hydrating
cement paste.
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PERMEABILITY
This is defined as the ease of
which a fluid passes through
by a pressure differentialaction.
Like absorption, it depends on
the size and interconnection of
the pores and also thepressure gradient.
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DURABILITY
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The durability of concrete can be defined as its resistance to
deterioration, which may occur as a result of the interaction
with its environment (external) or between the materials or
their reaction with other agents present (internal).
Concrete deterioration is as a result of either steel
reinforcement corrosion, physical or chemical attacks (which
may occur within or at the surface of the concrete).
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CORROSION OF REINFORCEMENT
Corrosion of concrete
embedded steel
reinforcements may be as a
result of Carbonation or
Chloride ingress in the
concrete.
Normally concrete provides
very good protection for itsembedded steel due to it
physical and chemical
properties.
These are:
Concrete cover and binder
– bind & immobilize
ingressing agents withoutexpansion.
High alkalinity pH >12.5
(CaOH and alkalis in
cement) High electrical resistivity
limits corrosion currents –
influenced by moisture
content & materials.
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CARBONATION
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This refers to the process by which carbon dioxide (in air)enters into concrete causing a chemical reaction with the
components to cause the steel to corrode.
CO2 enters the concrete by virtue of diffusion and reacts
with the alkali Ca(OH)2 produced from the hydration ofcement to form CaCO3.
This reduces the alkalinity which breakdown the passive
protective environment causing the steel to corrode over
time.
It is influenced by environmental conditions (re. humidity,
temp., and CO2 concentration), w/c and cement content
and curing conditions.
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http://www.cement.org/tech/cct_dur_corrosion.asp
Carbonation >75% humidity
http://www.cement.org/tech/cct_dur_corrosion.asphttp://www.cement.org/tech/cct_dur_corrosion.asp
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Exposure of reinforced concrete to chloride ions is theprimary cause of premature corrosion of steel
reinforcement.
Chloride ions present in deicing salts or seawater, may
enter into reinforced concrete causing steel corrosion if
oxygen and moisture are also available to sustain the
reaction.
Chlorides dissolved in water can permeate through soundconcrete either by absorption (dry) or diffusion (wet)
through cracks or interconnecting pores.
Admixtures used in concrete that contain chloride can also
cause corrosion.
CHLORIDE
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Chlorides may be present in
concrete in three forms:
Free in pore fluids Physically absorbed within
the pore walls
Chemically bound within
cement hydrates
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The risk of corrosion increases as the chloride content ofconcrete increases. When the chloride content at the surface
of the steel exceeds a certain limit, called the threshold
value, corrosion will occur if water and oxygen are also
available. However, only water-soluble chlorides promotecorrosion.
The primary rate-controlling factors are the availability ofoxygen, the electrical resistivity, relative humidity of the
concrete (wetting & drying), the pH and temperature and
chloride concentration.
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Corrosion of steel in concrete
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Questions
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