lecture 11. concrete reinforcement - aalto · üpre-stressed reinforced concrete üfiber reinforced...
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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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
• 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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
• 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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
• 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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
Common types of fiber used in FRC
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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
Common types of fiber used in FRC
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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Common types of fiber used in FRC
Carbon Fibers Cellulose Fibers Nylon Fibers Natural Fibers
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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
Common types of fiber used in FRC
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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
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.
Factors effecting the properties of FRC
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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
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
Factors effecting the properties of FRC
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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
Behaviour in compression & tension
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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
Behaviour in compression & tension
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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
• 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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
• Greater reduction ofworkability.
• High cost of materials.• Difficulty in finishing and
working with it• Difficulty in repairs
Disadvantages of FRC
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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
• 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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
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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CIV-E1010 Building Materials TechnologyFahim Al-Neshawy
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