CH1 - Introduction to Reinforced Concrete

8/18/2019 CH1 - Introduction to Reinforced Concrete

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ReinforcedReinforced

Concrete DesignConcrete DesignCH1 : Introduction to Reinforced ConcreteCH1 : Introduction to Reinforced Concrete ’’

materialmaterial

Dr.Amorn PimanmasDr.Amorn PimanmasSirindhorn InternationalSirindhorn International

Institute of Technology (SIIT)Institute of Technology (SIIT)


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What is Reinforced Concrete ??What is Reinforced Concrete ??

•• Reinforced ConcreteReinforced Concrete sometime shortly called“ RCRC ” is a composite material of steel barsembedded in a hardened concrete matrix.

• Concrete carries

the compressiveforces, while Steel

mainly resists thetensile forces.Casting of RC foundation

RC beam


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Advantages of Reinforced Concrete Advantages of Reinforced Concrete

•• The advantages include the following :The advantages include the following :

• It has considerable compressive strength ascompared to other materials.

• RC has great resistance to action of fire & water.

• RC structures are very rigid.• It is a low-maintenance material.• RC ability to be cast into an extraordinary variety of

shapes.• A lower grade of skilled labor is required for erection

as compared to structural steel.


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Example of RC structuresExample of RC structures

Dome : CMUCMU Waffle slab : Don Muang airportDon Muang airport Hypar Roof : MUMU

Building : Chang buildingChang building Bridge : PridiPridi --DhumrongDhumrong Br.Br.

Monument : Buddha StatueBuddha Statue


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isadvantages of Reinforced Concreteisadvantages of Reinforced Concrete

•• To use concrete successfully, the designer must beTo use concrete successfully, the designer must becompletely familiar with its weak points as well ascompletely familiar with its weak points as well as

its strong ones.its strong ones.• Concrete has a very low tensile strength.

• Forms are required to hold the concrete in placeuntil it hardens sufficiently.

• The low strength/unit weight of concrete leads toheavy members.

• The properties of concrete vary widely due to

variation in its proportioning and mixing.


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Example of RC structural failuresExample of RC structural failures

EarthquakeEarthquake Acid attack Acid attack Fire attackFire attack

Gravity load collapseGravity load collapse

corrosioncorrosion Heavy load VS Bad foundatioHeavy load VS Bad foundatio


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Compatibility of Concrete and SteelCompatibility of Concrete and Steel•• The advantages of each material seem toThe advantages of each material seem to

compensate for the disadvantages of the other.compensate for the disadvantages of the other.

• The shortcoming of concrete is its lack of tensilestrength; but tensile strength is the greatadvantage of steel.

• The two materials bond together very well becauseof chemical adhesion.

• Reinforcing bars are subject to corrosion, but theconcrete surrounding them provides them withexcellent protection.

• Concrete & steel work together in temperaturechanges. ( close Coef. of thermal expansion )


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Design CodesDesign Codes

• The most significant reinforced concrete code in theUS is “ Building Code Requirement for ReinforcedBuilding Code Requirement for ReinforcedConcreteConcrete ” .

• This code available from the A merican Concrete

Institute , generally referred to as ACI318 ACI318 Code.• In this course the design

procedures of the ACI318 ACI318 --08M08M(2008) code is adopted.

• Note : RC Codes in Thailand

are based on ACI318 ACI318 --8989 .


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Design PhilosophiesDesign Philosophies

•• Two philosophies of design have been prevalent.Two philosophies of design have been prevalent.

• The WWorkingorking SS tresstress MMethodethod (WSMWSM ), focusing onconditions at service load was the principal methodused from the 1900s until 1960s.

• Now, this method is called A A lternativelternative DDesignesignMMethodethod ( ADM ADM ).

• Today (2009) the SS trengthtrength DDesignesign MMethodethod (SDMSDM ) is

used, focusing on conditions at loads greater thanservice loads when failure may be imminent.

•• SDMSDM is deemed conceptually more realistic to

establish safety.


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Working Stress MethodWorking Stress Method

• In the WSDWSD , a structural element is so designed thatthe stresses resulting from the action of services

loads.• The stresses are computed by mechanics of elastic

and do not exceed some predesignated allowablevalues.

F


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Strength Design MethodStrength Design Method

In the SDMSDM , the service loads are increased byfactors to obtain the load at which failure is

considered to be “ imminent ” , called “ factored loadfactored load ” .The structural element is then proportioned suchthat strength is reached when the factored load isacting.

Strength reduced by φ >= Factored load

Where φ is reduction factor prescribed by code.


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Material PropertiesMaterial Properties -- CONCRETECONCRETE

Traditionally,Traditionally, ConcreteConcrete has been produced byhas been produced bymixing portland cement, water,mixing portland cement, water,

sand, and crushed stone,sand, and crushed stone,in appropriate proportionsin appropriate proportions

Cement

Graded fine &coarse aggregate Dry mixing

Adding water

+

May beadded

Pozzolanicmaterial

Naturalfibers

Steel fibers

Superplasticizer

Wet mixing

Slump check

Curing


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Concrete structure & HydrationConcrete structure & Hydration

•• HydrationHydration : Chemical process in which the cementpowder reacts with water and sets and hardens

into a solid mass, bonding the aggregates together

Harden Concrete Mixture : MarcoMarcoConcrete phase : MicroMicro

Aggregate Transition Zone Cement paste

CSH Ca(OH) 2 Ettringite


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Failure Mechanism of ConcreteFailure Mechanism of Concrete

Shrinkage MicrocracksShrinkage Microcracks are the initial shrinkagecracks due to carbonation shrinkage, hydration

shrinkage, and drying shrinkage.


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Bond MicrocracksBond Microcracks are extensions of shrinkagemicrocracks, as the compression stress field

increases, the shrinkage microcracks widen but donot propagates into the matrix. Occur at 15-20 %ultimate strength of concrete ( f’c).



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Matrix MicrocracksMatrix Microcracks are microcracks that occur in

the matrix.

The propagate from 20% f’c of concrete. Occur up

to 30-45 % f’c of concrete.

Matrix microcracks start bridge one another at 75%.

Aggregate microcracks

occur just before failure (90%).



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Mechanical properties ofMechanical properties of ConcreteConcrete

• Unit weight• Poisson’s ratio

• Compressive strength• Modulus of elasticity• Tensile strength• Combined Stress• Confined concrete• Shrinkage• Creep• Thermal


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Unit weight of ConcreteUnit weight of Concrete

•• Normal concreteNormal concrete have a density ρ of around2,300 to 2,600 kgf/m. 3.

• For calculating dead loads, the weight ifstructural concrete is often taken to be

24 or 25 kN/m.3

, which includes an allowancefor presence of steel reinforcement.


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Compressive strengthCompressive strengthThe Compressive strength (Compressive strength ( f f ’’cc )) of concrete isdetermined by test to failure 28-days-old 150 mm.by 300 mm. concretecylinder at a specificrate loading.

Compressive failure testStress-strainrelationship


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Rate of loadingRate of loading

• It should be noted that the shape of thestressstress --strain curvestrain curve for various concretes of the

same cylinder strength under various condition ofloading, varies considerably.

Cylinder strengthCylinder s trengthf’c = 21 MPaat 56 days


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Repeated Compressive LoadRepeated Compressive Load

• Repeated high-intensity compressive loadingproduces a pronounced hysteresis effecthysteresis effect in thestress-strain curve.

• From tests indicted that the envelope curve was

almost identicalalmost identical to the curve obtained frommonotonic load application

Monotonic compressive load


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Concrete ClassConcrete Class

• The concrete is classified by compressive strengthas follows:

• Low strength concrete : f’c 40 MPa

In order to reduce the member section andsize of foundation of Baiyoke tower2,High Strength Concrete was selected.


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Compressive Strength in ThailandCompressive Strength in Thailand

4045

3842

3540

3238

3035

28322428

2124

1821

1418

Cylinder : 150x300 mm.Cube : 150x150x150 mm.

Compressive strength at 28 Day(MPa)

Cylinder = (approx.) 0.87*Cube(British standard)

(American standard


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Modulus of ElasticityModulus of Elasticity• The figure represent a typical stress-strain curve for

concrete.

• In the figure, the initial modulusinitial modulus (tangent at origin), thetangent modulustangent modulus(at 0.5 f’c), and the

secant modulussecant modulusare noted.

• Usually the secantsecantmodulusmodulus at from25-50% of f’ c is

considered to bethe modulus of elasticity of concrete

Concrete strain, mm./mm.


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Modulus of ElasticityModulus of Elasticity• For many years the modulus of elasticity of

concrete was approximate adequately as 1,000f 1,000f ’’ccby ACI code.

• Recently ACI318 proposed the modulus ofelasticity for normal weight concretenormal weight concrete as

• E c = 4,700(f’ c )0.5 (MPa)

• ACI363 proposed the following equation for HighHighstrength concretesstrength concretes :

• E c = 3,320(f’ c )0.5

+ 6,895 (MPa)


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PoissonPoisson ’’s Ratios Ratio

• The ratio between the transverse strain and thestrain in the direction of applied uniaxial loading,referred to as PoissonPoisson ’’s ratios ratio .

• For concrete, it isusually found to bein the range 0.150.15 to 0.200.20 .

• At high compressive

stresses the transversestrains increase rapidlyincrease rapidly , owing to internal crackingparallel to the direction of loading.


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Tensile StrengthTensile Strength• The tensile strength of

concrete varies from

about 8 to 15%8 to 15% of itscompressive strength, f’c .

• The tensile strength of

concrete doesn’t varyin direct proportion toits compressive strength.

• It does, however, varyapproximately in proportion

to the square rootsquare root of f’c .


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Tensile StrengthTensile Strength• Tensile strength is quite difficult to measure with directdirect

axial tensionaxial tension loads because of problems in gripping,stress concentrations and aligning the loads.

• As a result of these problems, two rather indirect testshave been developed to measure concrete’s tensilestrength.

• These are the modulus of rupturemodulus of rupture and the splitsplit --cylindercylindertesttest .

Direct tensile test Modulus of rupture Splitting Test


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Tensile strengthTensile strength

• Table shows approximate formulas for tensile strengthobtained from three different approaches.

• Based in hundreds oftests, the code providesa modulus of rupturemodulus of rupture

f r = 0.5(f’ c)0.5

0.50(f’ c)0.5Modulus of rupture

0.53(f’ c)0.5Splitting test

0.33(f’ c)0.5Direct test

Normal weight(MPa)

f r = 0.5(f’ c)0.5


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Strength under Combined StressStrength under Combined Stress

• In many structural situations, concrete is subjectedsimultaneously to various stresses acting in variousvariousdirectionsdirections .

• By methods of mechanic of materials, these stressescan be transformed to the principal stressesprincipal stresses , tension

or compression.

M

P 1P 2

w

UniaxialUniaxial BiaxialBiaxial TriaxialTriaxial

C

C

C

C C

T

T

T T

C

C

T CC

C

C

T

T


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Biaxial StressBiaxial Stress

In the picture, f u = f’c

In this case, the stresses act in one plane and the thirdprincipal stress is zero.

Kupfer Kupfer , H. et al (1969), H. et al (1969) concluded that strength ofconcrete subjected to biaxial compressionbiaxial compression may be asmuch as 27% higher than uniaxial strength.

The strength of biaxial tensionbiaxial tensionis approximately equal to theuniaxial tensile strength.

However, the combination of the combination of tensile & compressive loadstensile & compressive loadsreduce both the tensile &

compressive stresses at failure.


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Triaxial compressive stress behavior Triaxial compressive stress behavior

The strength and ductility of concrete are greatlyincreased under conditions of triaxial compression.

RichartRichart , F. E. et al. (1928), F. E. et al. (1928) found the followingrelationship for concrete cylinder loaded axially tofailure while subjected to confining fluid pressure .

f’cc = f’c + 4.1 f l

Where f’cc = Confined compressive strengthf’c = Unconfined compressive strengthf

l= Lateral confining pressure

1


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• The figure shows the axial stress-strain curvesobtained by compression of concrete cylinder concrete cylinder confined by fluid pressurefluid pressure .

Triaxial compressive stress behavior Triaxial compressive stress behavior


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Concrete confined by reinforcementConcrete confined by reinforcement• In practice, concrete may be confined by transverse

reinforcement, commonly in the form of closely spacedsteel spiralsspirals or hoopshoops .

• At low levels of stresslow levels of stress in the concrete, the transversereinforcement is hardly stress; hence the concrete isconcrete isunconfinedunconfined .

• The concrete becomes confinedconfined when at stressesstressesapproaching the uniaxial strengthapproaching the uniaxial strength .

Reinforced concreteReinforced concreteColumn confined byColumn confined by

various techniquesvarious techniques


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•• Iyengar Iyengar et al. (1970)et al. (1970) tested three sets of concretecylinder confined by circular spirals, each set was fora different unconfined compressive strength.

Concrete confined by reinforcementConcrete confined by reinforcement

The increase in strengthand ducti lity with content

of confining steel is significant1 2 3


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• Tests have demonstrated that circular spiralsconfine concrete much more effectively thanrectangular or square hoops.


Tie ColumnTie Column Spiral ColumnSpiral Column


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The reason for the difference between the confinement byspirals & hoops is illustrated by Fig.

Circular spirals, because of their shape, are in axial hooptension and provide a continuous confining pressurecontinuous confining pressure

around the circumference.In contrast, square hoops can apply only confiningonly confiningreactions near the cornersreactions near the corners of the hoops tends to bend the

sides outwards.



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Confined Compressive strengthConfined Compressive strength• In a recent study by KapposKappos , A. J. (1991), A. J. (1991) the

confined compressive strength can be obtained bymultiply KK.

•• KK = 1 + ( ωw)b f’cc = KKf’c

• where ωw = ρwf yw /f’c

• a = 0.55, b = 0.75 for

• a = 1.00, b = 1.00 for

• a = 1.25, b = 1.00 for

f c

ε c

f’c

f’cc


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Shrinkage of ConcreteShrinkage of Concrete

Unless kept under water or in air at 100% relativehumidity, concrete loses moisture with time anddecrease in volume, a process known as shrinkageshrinkage .

The amount of shrinkagedepends strongly upon thecomposition of the concrete,with the total amount oftotal amount of

water in mixwater in mix being especiallyimportant.

125 150 175 200 225 251.2

1.0

0.8

0.6

0.4

0.2

250300 350 400

lb of water per yd 3 of concrete

kgf./m. 3

S h r i n

k a g e s

t r a

i n x

1 0 3

125 150 175 200 225 251.2

1.0

0.8

0.6

0.4

0.2

250300 350 400

lb of water per yd 3 of concrete

kgf./m. 3

S h r i n

k a g e s

t r a

i n x

1 0 3

Influence of amount

of water on shrinkage

h k f


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Shrinkage of ConcreteShrinkage of Concrete

• In lieu of specific information on the shrinkageproperties of the concrete, the followingapproximate expressionsapproximate expressions for shrinkage can beused.

sh = [t/(35+t)]( sh )u

• t = times (days) after moist curing• (esh)u = ultimate shrinkage strain

= 0.000415 – 0.00100 mm./mm.Branson, D. E. (1977) recommended about 0.0008

fC f


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Creep of concreteCreep of concrete

•• CreepCreep is the properties of concrete by which itcontinues de deform with time under sustained loadswithin elastic rangewithin elastic range (say below 0.5 f’c).

• Frequently creep is associated with shrinkage.

• In general, “ true elastic straintrue elastic strain ” decreases since themodulus of elasticity is a function of concretestrength whichincreaseswith time.

C f CC f C


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Creep of ConcreteCreep of Concrete

• Accurate prediction of creep is complicatedbecause of the variables involved.

•• Branson, D. E. (1977)Branson, D. E. (1977) gives a standard creepcoefficient equation.

C t = creep strain/initial elastic strain= [t 0.60 /(10+t 0.60 )]Cu

• t is duration of load (days)• C u is the ultimate creep coefficient = 2.35

Eff f T ChEff f T Ch


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Effect of Temperature ChangeEffect of Temperature ChangeLike most material, concrete expands with increasingexpands with increasingtemperaturetemperature and contracts with decreasing temperaturecontracts with decreasing temperature .

The effects of such volumechanges are similar to thosecaused by shrinkage.

• The coefficient of thermalThe coefficient of thermalexpansion (expansion ( )) and contractionvaries somewhat, dependingupon the type of aggregate and richness of the mix.

It is generally within 10x10 -6 /C o .

40

30

20

10

00 0.01 0.02 0.03 0.04 0.05

,

f ’ c

( . /

. 2

)

, ε c ( ./ .)

T = 70 ° C

T = 400 ° C

T = 600 ° CT = 200 ° C

T = 800 ° C

C o m p r e s s

i v e s

t r e n g

t h ( M

P a

)

Stain,c

(mm./mm.)

M h i l i fM h i l ti f S lSt l


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Mechanical properties ofMechanical properties of SteelSteel

• Steel size available in Thailand• Modulus of elasticity

• Tensile strength• Reversed loading• Repeated loading

Deformed barsDeformed bars

Various types of ribVarious types of rib

M t i l P tiM t i l P ti STEELSTEEL


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Material PropertiesMaterial Properties -- STEELSTEEL

• Generally, two types of reinforcing bar have beenused :

(1) Round Bars (RB)Round Bars (RB): Grade SR24 = f y = 240 MPa(2) Deformed Bars (DB)Deformed Bars (DB)

: Grade SD30 = f y = 300 MPaSD40 = f y = 400 MPaSD50 = f y = 500 MPa

Bars in ThailandBars in Thailand

Si f bSize of bars (I Th il d) RB(In Thailand) : RB


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Size of barsSize of bars (In Thailand) : RB(In Thailand) : RB

4913.8578.5725RB252842.2359.7119RB19

1771.3947.1415RB15

1130.8937.7112RB12

63.60.5028.299RB9

28.30.2218.866RB6

Area(mm. 2)

Mass(kgf/m)

Perimeter (mm.)

Diameter (mm.)

Code

Size of barsSize of bars (In Thailand) : DB(In Thailand) : DB


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Size of barsSize of bars (In Thailand) : DB(In Thailand) : DB

8046.31100.632DB32

6164.8388.0028DB284913.8578.5725DB25

3142.4762.9020DB20

2011.5850.2916DB16

1130.8937.7112DB12

780.6231.4010DB10

Area(mm. 2)

Mass(kgf/m)

Perimeter (mm.)

Diameter (mm.)

Code

Modulus of Elasticity and strengthModulus of Elasticity and strength


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Modulus of Elasticity and strengthModulus of Elasticity and strength• The modulus of elasticity of the steel is given by the

slope of the linear elastic portion of the curve.

• The modulus of Elasticity (E s ) for steel is 2.0x102.0x1055

MPaMPa

Yield strengthYield strength Tensile strengthTensile strength

Tensi le test of steel by UTMTensi le test of steel by UTM

Repeated Stress behaviorRepeated Stress behavior


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Repeated Stress behavior Repeated Stress behavior

If the load is released before failure, the specimen willrecover along a stressrecover along a stress --strain pathstrain path that parallel to the

original elastic portion of the curve.If loaded again, the specimenwill follow the same path upto the original curve withperhaps a small hysteresis

and/or strain-hardening effect.

Reversed Stress BehaviorReversed Stress Behavior


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Reversed Stress Behavior Reversed Stress Behavior If reversed axial loading isapplied to a steel specimenin the yield range, a stress-strainof the curve of the type arepresented.

The figure shows the Bauschinger Bauschinger effecteffect , in which underreversed loading the stress-strain curved becomes

nonlinear at a stress much lower than the initial yieldingstrength

Fig (b) show ElasticElastic --Perfectly Plastic IdealizationPerfectly Plastic Idealization for

reversed loading.


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CH1 - Introduction to Reinforced Concrete

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