lecture 3b - concrete (2012)

8/17/2019 Lecture 3b - Concrete (2012)

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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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lecture 3b - concrete (2012)

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