lecture 11. concrete reinforcement - aalto · üpre-stressed reinforced concrete üfiber reinforced...

CIV-E1010 Building Materials Technology

Lecture 11. Concrete Reinforcement

17.10.2016

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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy

• What is reinforced concrete?

• Why do we use reinforcement?

• Common types of reinforcementü Conventional steel reinforced concreteü Pre-stressed reinforced concreteü Fiber reinforced concrete

Lecture contents

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1. Knowledge:ü will be able to learn basic theory about main building- and

construction materials:• material composition / Properties• applications in buildings and structures.

2. Skills:ü be able to make right and well-founded choice of materials

3. General competence:ü understand how properties of materials can be related to the

characteristics of the material.

Lecture outcomes

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• SFS-EN 10080- Steel for the reinforcement of concrete.Weldable reinforcing steel. General

• SFS-EN ISO 15630-1:en Steel for the reinforcement andprestressing of concrete. Test methods. Part 1: Reinforcing bars,wire rod and wire (ISO 15630-1:2010)

• SFS-EN ISO 15630-2:en Steel for the reinforcement andprestressing of concrete. Test methods. Part 2: Welded fabric(ISO 15630-2:2010)

• SFS-EN ISO 15630-3:en Steel for the reinforcement andprestressing of concrete. Test methods. Part 3: Prestressing steel(ISO 15630-3:2010)

EN – standards 4 reinforcing steel

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• Concrete - No UsefulTensile Strength

• Reinforcing Steel -Tensile Strengthü Similar Coefficient of

thermal expansionü Chemical Compatibilityü Adhesion Of Concrete To

Steel

• Theory of SteelLocation“Place reinforcing steelwhere the concrete is intension”

What is Concrete Reinforcing?

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Why do we use reinforcement? - video

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Conventional steel• The stresses vary with loads

on the structure.• This does not place any

special requirements on thesteel

Pre-stressed steel• The steel is under

continuous tension.• Any stress relaxation will

reduce the effectiveness ofthe reinforcement.

• Hence, special steels arerequired.

Reinforcing steel

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• Plain bars• Deformed bars• steel wires

• The diameters of plain barsare 6, 8, 10, 12, 14, 16, 18,20 and 22 mm

• The diameters of deformedbars are 6, 8, 10, 12, 14, 16,18, 20, 22, 25, 28, 32, 36, 40and 50 mm

• Steel bars of small diameter(e.g., <6 mm) are also calledsteel wires, whose surfaceis generally smooth.

Conventional steel

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Reinforcement Cage:• Different kinds of reinforcement in structural members can be

strapped or welded into reinforcement cages or wire fabricsbefore being placed in forms.

Conventional steel

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Hooks and bent steel bars:• To prevent a plain bar under tension from slipping in concrete, both ends of

the bar should be hooked.• Sometimes the intermediate segment of reinforcement should be bent due

to the design requirement.

• The detailing of the bentsegments and hooks arelisted in relevant designcodes or acceptancespecifications forconstruction quality

Conventional steel

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1. Black rebarü rebar without anti-rust coatingü lowest price & widely used

2. Epoxy-coated rebarü corrosion-resistant coatingü marine structures

3. Galvanized rebarü normal black rebar with a layer of

zinc coatingà prevent corrosion

4. Stainless steel rebarü long-life cycleü most expansive rebarà

superior cost effectiveness

Common reinforcing steel bars

1 2

3 4

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Classification of reinforcing steel bars

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Classification of reinforcing steel bars

Class Description Yield strengthfyk [MPa]

A500HW Weldable hot-rolled ribbed steel 500A700HW Weldable hot-rolled ribbed steel 700B500K Cold-worked ribbed steel 500B700K Cold-worked ribbed steel 700B600KX Cold-worked stainless ribbed steel 600S235JRG2 Smooth round bar used as a lifting loops 235S355J0 Smooth round bar used as a lifting loops 335

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• Selecting standardreinforcing bars isusually as simple asselecting the size andgrade of thereinforcement bars.

Sizes• Standard Diameters in

Finland are: 6, 8, 10, 12,16, 20, 25, 32 mm ???

Grades• 500, 600, 700 Steel

Yield Strength (in MPa)

Selecting reinforcing bars

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The principle behind pre-stressedconcrete is that:ü compressive stresses induced by

high-strength steel tendons in aconcrete member before loadsare applied

ü will balance the tensile stressesimposed in the member duringservice

Pre-stressed concrete can be used toproduce:ü beams, floors or bridges with a

longer span than is practical withordinary reinforced concrete.

Pre-stressed Concrete

Video

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1. Steel wireü diameters of the wires are 2.5,

3.0, 4.0, 5.0, 7.0 and 8.0 mm.ü Plain or indented wires

2. Steel Strandsü 3 and 7 wire strands

3. Steel barsü Thread or plain bars

4. Steel tendonsü Group of strands or wiresü Placed in a duct which filled with

grout after the post-tensioning

Steel for pre-stressed concrete

1

2

3

4

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Steel for pre-stressed concrete

Steel wire

Steel strand

Steel bar

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1. Pre-Tensioned Concrete:ü concrete is cast around steel bars or cables under tension.ü The concrete naturally bonds to these “tendons” while it cures.ü Compression by static friction transfers the tension to the concrete once

it is released.ü Subsequently, any tension on the

concrete transfers readily to thetendons.

ü Pre-tensioned concrete elementsare common in beams, lintels,and floor slabs.

How Pre-stressed Concrete Works

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2. Bonded Post-Tensioned Concrete:ü compression is applied in situ during curing.ü A duct of aluminum, plastic, or steel is used in casting and follows the

area where tension would occur in the concrete.ü Tendons are pushed through the duct, then tensioned via hydraulic jack

after hardening.ü Once tendons’ stretching meets design specifications, they are wedged in

place and the duct is grouted.

How Prestressed Concrete Works

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3. Un-bonded Post-Tensioned Concrete:ü the individual tendons retain freedom of movement relative to the

concrete.ü Tendons are prepared with a coating of lithium-based grease, then given

a plastic-based “shell” formed through extrusion.ü The steel cables are tensioned against anchors placed in the slab’s

perimeter.ü This design provides

the ability to de-stressthe embedded tendonsprior to repair.

How Prestressed Concrete Works

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Pre stressed concrete popular in:ü Bridgesü Shopping centersü Parking garagesü Large office complexesü Public buildings including

libraries, schools, andauditoriums.

Applications of pre-stressed concrete

Bridges

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Types of fibersFactors effecting the properties of FRC

Behaviour of FRC in tension & compressionBenefits and problems of FRC

Applications

FIBER REINFORCED CONCRETE

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What is fiber reinforcedconcrete?ü FRC is a Portland cement

reinforced with more orless randomly distributedfibers .

What is fiber?ü fiber is a small piece of

reinforcing materialwhich increasesstructural integrity.

Why fiber ?ü Concrete is weak in

tension and brittlematerial

ü Control crackingü Elimination small cracks

Fiber reinforced concrete

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Common types of fiber used in FRC

Steel Fiber• Diameter Varying 0.3-0.5 mm• Length varying from 35-60 mm• Various shapes of steel fibers

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Polypropylene Fiber• Fibrillated bundles:ü formed by expansion of a

plastic film, which isseparated into strips and thenslit

ü fiber bundles are cut intospecified lengths andfibrillated

• Mono filament ormicrofilament fibers

Fibrillatedbundles

Monofilamentfiber

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Glass Fibers• Alkali resistant glass fibers• The fiber bundles are

meant to stay bundled(about 200 filaments perbundle with 19mm“fibers”).

• Each bundle is bondedtogether so that thefilaments remain groupedduring mixing

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Carbon Fibers Cellulose Fibers Nylon Fibers Natural Fibers

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Type of fiber Tensilestrength

(MPa)

Young’smodulus

(GPa)

Ultimateelongation

(%)Steel 275-2757 200 0.5-35

Polypropylene 551-690 3.45 ~25Glass 1034-3792 ~69 1.5-3.5Nylon 758-827 4.14 16-20

Source: ACI Committee 544, Report 544.IR-82, Concr. Int., Vol. 4, No. 5, p. 11, 1982

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Mix Proportions:• High cement content• Water reducing admixtures

(superplasticizers)• Small maximum size of aggregate• Fibers (1-2% by volume)

Mix example:• Cement : 325 to 550 kg/m3• W/C Ratio : 0.4 to 0.6• % of sand to total aggregate : 50

to 100%• Maximum Aggregate Size : 10 mm• Air-content : 6 to 9%• Fiber content : 1 to 2% by vol of

mix

Mixing FRC

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Volume of fibers:• Low volume fraction (<1%):ü Used in slabs and pavement

that have large exposedsurface leading to shrinkagecracking

• Moderate volume fraction(between 1 and 2%):ü Used in construction method

such as shotcrete• High volume fraction (> 2%):ü Used in making high

performance FRC

Aspect ratio of fiber:= fiber length / fiber diameter

Factors effecting the properties of FRC

Reference:Concrete Technology: Theory and Practice: M.S. Shetty

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Orientation of fibers:• Aligned inü direction of loadü perpendicular to load

• Randomly distribution offibers

Relative fiber matrix:• Fiber should be significantly

stiffer than matrix• Low modulus of fibers (e.g.

polypropylene) providemore energy absorption

• High modulus of fibers (e.g.steel, glass and carbonfibers) provide strength andstiffness.


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Workability and compactionof concrete:• Usage of steel fibers , higher

aspect ratio and non-uniformdistribution of fibers will reduceworkability

• These workability can beimproved byü increasing water/cement ratio

orü by using water reducing

admixtures

Mixing:• Mixing of FRC needs careful

precautions to avoid segregationand bleeding

• Mixing difficulties:ü Increasing the aspect ratioü Increasing the volume

percentage of fibersü Increasing the size of coarse

aggregateà Restricted to 10 mm


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Behaviour in compression & tension

0.5% fiber

3

1.0% fiber

4

0.25% fiber

2

0% fiber

1

Fiber effectà improvement of compressive strength (0 to 15 %)

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The presence of 3 % fiber by volumeà increase the splitting tensile strength ofmortar about 2.5 times that of the unreinforced one

0% fiber 0.25% fiber 0.50% fiber 1% fiber

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Reference: Cement & Concrete Institutehttp://www.cnci.org.za

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FRC vs NRC

Fiber Reinforced Concrete Normal Reinforced concrete• High Durability • Lower Durability• Protect steel from Corrosion • Steel potential to corrosion• Lighter materials • Heavier material• More expensive • Economical• With the same volume, the

strength is greater• With the same volume, the

strength is less

• Less workability • High workability ascompared to FRC.

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• Main role of fibers is tobridge the cracks thatdevelop in concrete andincrease the ductility ofconcrete elements.

• Improvement on Post-Cracking behavior ofconcrete

• Provides more resistance toImpact load

• Controls plastic and dryingshrinkage cracking

• Easily placed, Cast, Sprayed• Higher flexural strength,

depending on addition rate.• Can be made into thin

sheets or irregular shapes.• Lowers the permeability of

concrete matrix and thusreduce the bleeding ofwater

Benefits of FRC

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• Greater reduction ofworkability.

• High cost of materials.• Difficulty in finishing and

working with it• Difficulty in repairs

Disadvantages of FRC

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• Runway, Aircraft Parking,and Pavements

• Tunnel Lining and SlopeStabilization

• Dams and HydraulicStructure

• Thin Shell, Walls, Pipes, andManholesà shotcrete

• Agricultureà used inanimal storage structures,walls, silos, paving, etc.

• Precast Concrete andProducts

• Warehouse / Industrialàin light to heavy dutyloaded floors.

• Residentialà in driveways,sidewalks, poolconstruction, basements,colored concrete,foundations, drainage, etc.

Applications of FRC

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Applications of FRC

Source: Prof. Savan Maniya

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Applications of FRC

Source: Prof. Savan Maniya

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Applications of FRC

Footbridge inFredrikstad,Norway

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Applications of FRC

SFRC used at TehriDam, India

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Concrete reinforcement:ü Reinforced concrete: what

and why?ü Common types of

reinforcement(Conventional steel, pre-stressed steel, fibers)v Typesv Mechanismsv Application

Next Lecture:• Non-conventional and

advanced building materials

Summary

lecture 11. concrete reinforcement - aalto · üpre-stressed reinforced concrete üfiber reinforced...

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