Lecture 4 - Fundamentals€¦ · Since concrete is weaker in tension than in shear, ... torsion...

HARDENED CONCRETE

M. Al Nasra, PhD, PE

9/25/2008 1Concrete Methods and Principles, (c)

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Hardened Concrete

• Some of the important properties of hardened concrete are as follows:

– Strength

– Elasticity

– Water-tightness

– Resistance to destructive agencies

– Volume changes

– Creep

– Extensibility

– Thermal properties


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Concrete Strength

• Compressive strength

• Tensile strength

• Splitting Strength

• Flexural Strength

• Shear Strength


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Compressive Strength

• Strength is defined as unit force (stress)

required to cause rupture

• Rupture may be caused by:

– Applied tensile stress – failure in cohesion

– Applied shearing – sliding stress

– Compression – crushing stress


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Typical Failure of Concrete in

CompressionTypes of compression failure

There are three modes of failure.

[1] Under axial compression

concrete fails in shear.

[2] the separation of the

specimen into columnar pieces

by what is known as splitting or

columnar fracture.

[3] Combination of shear and

splitting failure.


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Types of compression failure9/25/2008 6Concrete Methods and Principles, (c)

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Stress-Strain CurveUniaxial Stress versus Strain Behavior in Compression

c

Ec

o u

0.45f’c

fc

f’c12”

6”


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Concrete PropertiesThe standard strength test generally uses a cylindrical

sample. It is tested after 28 days to test for strength, fc.

The concrete will continue to harden with time and for a

normal Portland cement will increase with time as follows:

Age Strength

Ratio

Age Strength

Ratio

7 days 0.67 6 months 1.23

14 days 0.86 1 year 1.27

28 days 1.0 2 years 1.31

3 months 1.17 5 years 1.35 8

Concrete Strength increase with age


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Concrete Properties

– Compressive Strength, f’c

• Normally use 28-day strength for design strength

– Poisson’s Ratio,

• ~ 0.15 to 0.20

• Usually use 0.17

c

Ec

o u

0.45f’c

fcf’c


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Concrete Properties– Modulus of Elasticity, Ec

• Corresponds to secant modulus at 0.45 f’c

• ACI 318-99 (Sec. 8.5.1):

where w = unit weight (pcf)

90 pcf < wc <155 pcf

For normal weight concrete

(wc 145 pcf)

)('33)( 5.1 psifwpsiE cc

)('000,57)( psifpsiE cc


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Concrete Properties

– Concrete strain at max. compressive stress, o

• typical curves in compression

•o varies between 0.0015-0.003

• For normal strength concrete, o ~ 0.002

Ec

o u

0.45f’c

fc

f’c


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Concrete Properties

– Maximum useable strain, u

• ACI Code: u = 0.003

• Used for flexural and axial compression

Ec

o u

0.45f’c

fc

f’c


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Concrete PropertiesTypical Concrete Stress-Strain Curves in Compression

9/25/2008 14

Tensile Strength of Concrete

• The Tensile strength

of concrete is roughly

10% of its

compressive strength

or, perhaps more

precisely

)(5 / psifFct


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Flexural Strength

• When concrete is subjected to bending, tensile and compression stresses and in many cases direct shearing stresses are developed. The most common plain-concrete structure subjected to flexure is a highway pavement, and the strength of concrete for pavement is commonly evaluated by means of bending tests on 6X6 inch beam specimens. Flexural strength is expressed in terms of modulus of rupture. Though the modulus of rupture is a fictitious value, it is convenient for purposes of evaluation and is commonly used. It ranges from 60% to 100% higher than the direct tensile strength. The modulus of rupture ranges from 11 to 23 % of the compressive strength.; for concrete of compressive strength of 3,500 to 4,000 psi it is the order of 550 psi, or about 15% of the compressive strength.


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Flexural Strength

Indirect Tensile Strength

– Tensile strength ~ 8% to 15% of f’c

– Modulus of Rupture, fr

• For deflection calculations, use:

– Test:

2

6

bh

M

I

Mcf r

)('5.7 psiff cr ACI Eq. 9-9

P

fr

Mmax = P/2*a

unreinforced

concrete beam


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Splitting Test

Indirect Tensile Test (cont.)

– Splitting Tensile Strength, fct

– Split Cylinder Test

P

Concrete Cylinder

Poisson’s

Effect


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Splitting Test

Cylinder splitting test9/25/2008 19Concrete Methods and Principles, (c)

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Indirect Tensile Test (cont.)

)(')75(

2

psiftof

ld

Pf

cct

ct

(Not given in ACI Code)


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Shear Strength

• Shear is the action of two equal and

opposite parallel forces applied in planes a

short distance apart. Since concrete is

weaker in tension than in shear, failure in

torsion invariably occurs in diagonal

tension.


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Factors Affecting Results of

Strength Tests

• The following factors are most important:

– Size and shape of specimen

– Condition of casting

– Moisture content of specimen

– Temperature of specimen

– Bearing condition

– Rate of loading


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Concrete Properties

Shrinkage and Creep

– Shrinkage: Due to water loss to atmosphere (volume loss).

• Plastic shrinkage occurs while concrete is still “wet” (hot day, flat work, etc.)

• Drying shrinkage occurs after concrete has set

• Most shrinkage occurs in first few months (~80% within one year).

• Cycles of shrinking and swelling may occur as environment changes.

• Reinforcement restrains the development of shrinkage.


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Concrete PropertiesShrinkage of an Unloaded Specimen

• * 80% of shrinkage occurs in first year

Time

Shr.

Strain

T=α


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Concrete Properties

• Shrinkage is a function of

– W/C ratio (high water content reduces

amount of aggregate which restrains

shrinkage)

– Aggregate type & content (modulus of

Elasticity)

– Volume/Surface Ratio


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Concrete Properties

• Shrinkage is a function of

– Type of cement (finely ground…)

– Admixtures

– Relative humidity (largest for relative humidity of

40% or less).

– Typical magnitude of strain: (200 to 600) * 10-6

(200 to 600 microstrain)


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Surface Contraction – Drying Shrinkage

Wants to shrink

Does not want to shrink9/25/2008 27

Free Shrinkage,

causes volume change, but no stresses

before shrinkage After Shrinkage


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Restrained Shrinkage- creates stresses,

which may cause cracking


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Restrained shrinkage cracking

Parallel cracking perpendicularto the direction of shrinkage


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Concrete Properties– Creep

• Deformations (strains) under sustained loads.

• Like shrinkage, creep is not completely reversible.

P

P

L

L, elastic

L, creep

= L/L


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Concrete Properties

• Magnitude of creep strain is a function of all the

above that affect shrinkage, plus

– magnitude of stress

– age at loading


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Concrete Properties

• Creep strain develops over time…

– Absorbed water layers tend to become thinner

between gel particles that are transmitting

compressive stresses

– Bonds form between gel particles in their

deformed position.


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Concrete Properties

– Tri-axial Compression

• Confined Cylinder

• Improved strength and ductility versus uniaxial

compression

• Example: spiral reinforced

where,

F1 = longitudinal stress at failure

F3 = lateral pressure

31 1.4'cf

F1

F1

F3


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Concrete Curing

• Tests show that improper curing can easily cut the strength of even the best concrete mix by 50%. Curing simply means keeping the water in the concrete where it can do its job of chemically combining with the cement and turning it into strong “glue” that will help make strong, durable concrete. Recommended practice calls for at least 7 days of curing.


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Concrete Curing

• All concrete must be cured to get the max.

strength of the concrete mix.

• Start curing as soon as possible after it has

hardened.


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Concrete Curing

Poor curing can cut the strength of

concrete by 50%


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Methods of Curing

• Water spay

• Waterproof papers holds moisture in

concrete by preventing evaporation

• Damp burlap

• Membrane curing compounds seal moisture

in the concrete

• Plastic sheets


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Concrete Curing

• Cure concrete longer when the temperature

is below 70 F

• Good curing results:

– More durable concrete

– More wear-resistance concrete

– Less cracking, crazing and spalling of the

concrete


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Durable Concrete DO’S

• Do specify 5 to 7% entrained air

• Do specify high-strength concrete

• Do specify quality and tested materials

• Do specify good workmanship

• Do specify proper curing

• Do consider of a surface sealer

• Do specify Pozzolith Admixture; improves the workability, placeability, and finishability of concrete; it reduces permeability, absorption and shrinkage cracking, and it increases strength


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Durable Concrete DON’TS

• Don’t assume that air entrainment alone is enough

• Don’t permit addition of “extra” water

• Don’t permit the use of materials of questionable

quality

• Don’t permit overworking of the concrete

• Don’t allow concrete to dry during curing

• Don’t allow application of salt to new concrete


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Cold Weather Concreting

• At low temperature concrete sets slowly and development of strength is delayed. Therefore, job planning should include one or more of the following:

• Heating the water and concrete materials

• Heating the area in which the concrete is placed

• Use additional cement or high early strength cement (Type III)

• Addition of calcium chloride to the mix

• Special provision for curing


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Tips on Cold Weather Concreting

• Temperature of all surfaces to be in contact

with the new concrete should be raised to as

close as practical to the temperature of the

new concrete.

• The temperature of freshly placed concrete

in cold weather should be at least 50F and

not more than 90F


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Cold weather Concreting (cont.)

• Use calcium chloride or high early strength

cement. Calcium chloride up to 2% by

weight of cement is often recommended.


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Cold Weather Concreting (cont.)

• Protect from wind and rapid moisture loss; provide heated

enclosure if necessary.

• Provide insulation or heated enclosure to maintain concrete

temperature for min. periods as shown

70 F 50F

Plain concrete 3 days 7 days

PC - calcium Chloride 2 days 3 days

Type III Cement 2 days 3 days9/25/2008 46Concrete Methods and Principles, (c)

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Cold Weather concreting (cont.)

• Curing and protection from start to finish

should be continuous.


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Hot Weather Concreting

• Before placing

– Concrete sets faster in hot weather, use retarder such as Pozzolith Retarder.

– Periodically spay the forms, reinforcing steel and subgrade with water.

– Erect sun shields and wind barriers to protect the fresh concrete from stiffening or crusting and to help minimize cracking, crazing, plastic shrinkage and rubber set


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• During placing and finishing

– Don’t let ready mix trucks stand in the sun

– Promptly notify ready mixed concrete producer

of any delay in placing.

– Vibrate or screed without delay

– Protect test specimens by covering, and

maintaining at 60F to 80F


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• After Finishing

– Start curing immediately

– Periodically spray water on the outside of forms

for effective cooling and curing.


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Materials Unfriendly to Hardened Concrete- How to Prevent Damage, From ACI

Chemical

Substance

Effect in Concrete How to minimize

the effect

Protective coating

suggested

Carbon dioxide Harmless to

mature concrete,

may dissolve in

water to produce

carbonic acid

Be sure

combustion

heaters are

properly vented

when placing

concrete in a

heated enclosure

Surface hardeners

and various

coatings, per ACI

515

Carbonic acid Highly corrosive

to lean, permeable

concrete, causes

slow disintegration

of better concrete

Use dense,

impermeable

concrete with high

cement content

Epoxy, neoprene,

vinyl and other

coating per ACI

515

Garbage Disintegrate

concrete slowly

Use good quality

concrete of low

permeability

Regular scraping

with metal blade is

necessary 51


Chemical

Substance


the effect

Protective coating

suggested

Gasoline Not harmful to

hardened concrete

Lactic acid Causes slow

disintegration

Antibacterial

cements. Dry mix,

w/c <0.44

Protective coating

Milk Fresh, not harmful,

sour see lactic acid


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Chemical

Substance


the effect

Protective coating

suggested

Nitric acid Continued contact

with strong

solution destroy

concrete. Weak

solution attack

slowly

Be sure

combustion

heaters are

properly vented

when placing

concrete in a

heated enclosure

Protective coating,

per ACI 515

Sea water Disintegrate

concrete of

inadequate sulfate

resistance and

attacks

reinforcement

Provide a high

quality air-

entrained mix with

3-inch cover over

reinforcement

Protective coating

Vinegar, 5% acetic

acid

Disintegrate

concrete slowly

Heavy duty

concrete

Protective coating

53, (c) Al Nasra


Chemical

Substance


the effect

Protective coating

suggested

Sewage Usually not

harmful to good

concrete. If

hydrogen sulfide

gas is present and

exposed to air,

sulfuric acid may

form and attack

concrete

Use carbonate

aggregates.

Prevent the gas

conversion

process. For

industrial waste,

use cement with <

8% C3A

Bituminous,

epoxy, vinyl

coating. Brick or

tile covering

Sugar and sugar

products

Dry, no effect on

hardened concrete.

In solution: attacks

concrete

Cure concrete

thoroughly, then

allow to dry >28

days.

Fluo-silicate

treatment or

bituminous, epoxy,

neoprene, vinyl

and other coating

per ACI 51554

Lecture 4 - Fundamentals€¦ · Since concrete is weaker in tension than in shear, ... torsion...

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Transcript of Lecture 4 - Fundamentals€¦ · Since concrete is weaker in tension than in shear, ... torsion...