0 presentation frcm 20140228 2

Fabric Reinforced Cementitious Matrix forStrengthening/Retrofitting of Reinforced Concrete

Kai Tai WAN, Ph.D, MHKCI, MASCE

Technical Manager of Nano and Advanced Materials Institute LimitedExecutive Board Member of Hong Kong Concrete Institute

28 February 2014

K.T. Wan (NAMI/HKCI) FRCM 28 February 2014 1 / 89

Introduction to NAMI


Introduction to NAMI

NAMI stands for Nano and Advanced MaterialsInstitute Limited.

A non-profit government funded research institutein Hong Kong.

Focus on applied research.

Collaborate with local industry to advancetechnology.


Location of NAMI

HK Science Park HKUST


Market Sectors

Sustainable Energy Solid State Lighting andDisplay

Environmental Technologies Bio and Healthcare


Construction and Building Materials

Team members with 10+years hands-on experienceon cementitiousformulation, process &characterization.

Over 100 publications injournals/conferences onadvancedcomposites/cementitiousmaterials.

We focus on

Advanced cementitiouscomposites

Advanced functionalcoating materials

Thermal insulationSelf-cleanInorganic sealer


Outlines

1 Why do we need structural strengthening/retrofitting?

2 Current Strengthening/Retrofitting Techniques

3 Strengthening Materials of FRCM

4 Strengthening Configurations of FRCM

5 Design Criteria of FRCM

6 Applications of nanotechnology on FRCM


Why do we need structuralstrengthening/retrofitting?


Introduction

Why do we need structural strengthening/retrofitting?

i. Structural degradation after decades of usage.

ii. Extend the structural life.

iii. Change of the use of structures.

iv. After critical events, such as earthquake.

v. Rehabilitation to adopt the upgraded designstandard.


Structural Degradation after Decades of Usage

Reinforced concrete is degraded after environmentalexposure for decades.

For examples, carbonation, water penetration.

Fatigue crack may form.

Under the newly implemented Mandatory BuildingsInspection Scheme, there is great potential demandof strengthening/retrofitting of old buildingstructures.


Extend the Structural Life

Typical design life of reinforced concrete buildingswas 50 years in old day.

It is desirable to extend the service life fromsustainability point of view.

Disturbance is much less for retrofitting rather thanredevelopment.

Less social and political resistance.


Change of the Use of Structures

In some situations, the purpose of structure ischanged after years of use and the required loadingmay be significantly changed.For examples,

Valuable historic building may be revitalised to museum.Residential building is changed to hotel.


After Critical Events

After critical events, the structures may bepartially damaged. It is desirable to restore to thedesign strength.Some examples of critical events are:

EarthquakeTerrorist attackFireHurricane


Rehabilitation to Adopt Upgraded Design Standards

The design standards used for old building may notfit the current design standards.

The design load may change.

The design philosophy may change.

The factors of safety of different materials andloadings may change.

To extend the life of structures, it is necessary toupgrade the structure to fulfil the updated designstandards.


CurrentStrengthening/Retrofitting

Techniques


Common Strengthening Techniques

Conventional methodsRemoval of the deteriorated material and repairing of thestructures with fresh concreteAdding external steel plateExternal steel frameSection enlargement

More advanced methodsFibre reinforced polymerFabric reinforced cementitious matrix


Removal of the deteriorated material and repairing ofthe structures with fresh concrete

Deteriorated and loose materials are being removed.

Surface is properly cleaned.

Check the status of reinforcement and remove all corrosionproduct.

Wire mesh is laid followed by proper guniting on the surface.

Pros:

The procedure is simple.

Cons:

It delays the corrosion activities but does not stop it.It does not enhance the capacity of the structure.The disturbance of existing occupants is significant.


Adding External Steel Plate

Original reinforced concrete beam

External steel plate Bolts

Steel plate is added on the tension side of the beam and isproperly fixed with the help of epoxy or bolts.

The additional steel plate acts as external reinforcement andhence, enhances the flexural capacity of the structure.

Before providing the external reinforcement, all the loose materialsare removed, surface is properly cleaned.

A layer of good quality of epoxy is applied and then, steel plate isfixed.


Adding External Steel Plate

Pros:

Work is fast.Material added for strengthening i.e. steel is of assured quality (notlike concrete where chances of voids while field application may bethere). Hence, quality of work is better.

Cons:

In case of large span beam, it becomes difficult to handle the steelplate due to its heavy weight.In case of joining of plate is required due to longer span, difficulty isbeing faced in providing butt-welded joint.Corrosion is being experienced to the external provided steel plate.Steel being denser material, it adds up appreciable weight to thestructure.


External Steel Frame

Additional steel struts areadded along the span of beam.

Pros:

Easy to implement.

Cons:

It reduces the open span.It is difficult to control theforce of each strut.


Section Enlargement

Original reinforce concrete column

External steel jacket

New concrete andreinforcement

Strengthening/rehabilitation isdone by addition/ enlargement ofthe section.

Pros:

Design principle is simple andwell understood.Procedure is simple to adopt.

Cons:

Strengthening/rehabilitation forhorizontal portion of thestructure like beam and slab, iseither not possible or verydifficult.Weight of the structure isincreased.


Fibre Reinforced Polymer

Fibre reinforced polymer (FPR) consists of high performanceman-made fibre embedded in polymeric (usually epoxy) matrix.

FRP can provide extra flexural strength, shear strength and axialstrength.

It has been widely used in aerospace industry.

It is relatively new in construction industry.


Common High Performance Man-made Fibre

Carbon

Kevlar

Alkali-resistant (AR) Glass

Basalt

polyparaphenylene benzobisoxale (PBO)


Properties of Common Engineering Materials and HighPerformance Man-made Fibres

Specific Tensile Elastic Ultimategravity strength modulus strain

MPa GPa %Steel 7.8 500 200 -Aluminum 2.7 230 70 -Kevlar (aramid) 1.4 3000 112 2.4Basalt 2.7 4800 89 3.2Carbon 1.6 3000 230 1.3PBO 1.5 5800 270 1.9


Advantages of using FRP for Strengthening

Corrosion free.

Higher specific strength and stiffness and hence lighter in weight.

No transportation problem.Easy application.It can be in the form of roll and hence, adequate length of FRP rollcan be had and there is no requirement of any field joint.

Very good fatigue property.


Disadvantages of using FRP for Strengthening

Lower ductility.

Inconsistent plastic behaviour and ultimate behaviour.

Difficult application on uneven surface.

More expensive

The polymeric matrix is prone to the ultra violet degradation.

Impossible to apply on wet surface.

Impossible to apply at low temperature.

Low glass transition temperature of matrix.

Poor fire/high temperature resistance.

May release toxic gas in high temperature from the matrix.


Methods of FRP Installation

Surface preparation

Before start of the work, it is very much essential to ensure thatsurface should be dry, clean, free from oil and grease and any typeof loose materials.To ensure little roughness, light sand blasting or grinding can bedone followed by proper cleaning of the surface to remove dustparticles.Crack more than 0.3 mm width needs to be properly epoxy groutedto ensure good quality.

There are two methods of FRP applications

(1) Wet lay- up method(2) Pre-fabrication

Surface finish

For further better surface finish, paint (acrylic or urethane coating)can be applied to the repaired surface.


Wet Lay-Up Method

This method is in-situ preparation of FRP.

The steps of installation are:

(1) One coat application of good quality epoxy is required.(2) Then after, saturated fibre in the appropriate resin should be

applied on the surface.(3) Fibre should be properly aligned and slightly stretched to avoid any

bend/twist(4) Number of layers depends on the requirement of thickness.(5) After placement of saturated fibre, the same should be properly

rolled to ensure its proper compression as well as adhesion to thesurface on which the same is applied.

Main advantage is that exact surface profile and corner of thestructures are covered up properly in this process.


Pre-fabrication Method

Appropriate size is properly made for fixing the same to thestructure in the field.

The steps of installation are:

(1) Surface preparation is the same.(2) After application of good layer of epoxy, pre-fabricated FRP

product of the appropriate thickness and size is pasted and surfaceis properly rolled so as to ensure good adherence to the parentsurface.

Quality of this work is better than the wet lay-up process underbetter quality control in factory.

The disadvantage is that it is difficult to be applied on irregularshape and corner.


Strengthening Materials ofFabric Reinforced

Cementitious Matrix


Introduction to FRCM

Fabric reinforced cementitious matrix (FRCM) is similar to fibrereinforced polymer (FRP).

The major difference in material between FRCM and FRP is toreplace the polymeric matrix of FRP to cement-based matrix ofFRCM.

FRCM composite consists of one or more layers of cement-basedmatrix reinforced by fibre fabric.

The fibre fabric is an open mesh of strands made of highperformance manmade fibre.


Advantages of using FRCM for Strengthening

It possesses most of the advantages of FRP.

The thermal/fire resistance of the cementitious matrix is superiorcompared with epoxy resin.

The cementitious matrix allows vapour permeability.

It can be applied on wet surfaces.


Fibre Fabric

There are three common techniques to fabricate the open mesh.

Weft Direction

Warp Direction

Plain weave Knitting Stitching


Requirements of FRCM

There is an acceptance criterion AC434 Masonry and ConcreteStrengthening Using Fiber-reinforced Cementitious Matrix(FRCM) Composite Systems updated in US in February 2013.

AC434 provides guidelines for demonstrating compliance withperformance features.

FRCM material properties.Axial, flexural and shear capacities of FRCM system.Performance of FRCM system under environmental exposures.Performance under exposure to fire conditions.Structural design procedures.


Properties of FRCM Required

Cementitious Matrix(1) The minimum cylindrical compression strengths at the

7th and 28th day are 17.0 and 24.0 MPa, respectively.(2) The dry shrinkage should be measured and the additional

stress induced by shrinkage should be addressed duringthe design stage.

(3) Void content


Properties of FRCM Required

FRCM Composite(1) Tensile strength(2) Elongation(3) Elastic modulus(4) Interlaminate shear strength(5) Bond strength with old concrete substrate. The minimum

bond strength between FRCM and concrete substrate is1.38 MPa.


Durability Tests of FRCM Required

After measuring the mechanical properties of FRCM, thedurability of FRCM should be tested with different environmentalconditions.

After all durability tests, there is no visible crack observed under a5X magnification for all specimens.

The tensile and shear properties should be at least 85% of thecontrol specimens for

(1) those durability tests with 1000 hours(2) freezing/thawing cycles(3) fuel resistance

The tensile and shear properties should maintain at least 80% ofthose of the control specimens

(1) For those durability tests with 3000 hours.


Durability Tests for FRCM

Tests Conditions Duration

Freezing/thawingcycles

4 hours at -18C followedby 12 hours at 37.7C

20 cycles

Water 100% RH at 37.7C 1000 and 3000 hours

Salt water Immersed in seawater(ASTM D1141) at 23C

1000 and 3000 hours

Alkali pH 9.5 or higher at 23C 1000 and 3000 hours

Fuel Immersed in diesel fuelreagent

4 hours


Failure Mode of FRCM under Unidirectional TensileForce

Strain

Stress

Fibre

CompositeMatrix

FRP

Strain

Stress

Fibre

CompositeMatrix

Ultimate failureMatrix cracks

FRCM


4-stage Cracking of FRCM under Unidirectional TensileForce

There are 4 stages of cracking of FRCM underunidirectional tensile force before ultimate failure.1st stage

Both the fibre and the matrix are elastic.No mismatch in strain between the fibre andcement-based matrix exists.There is therefore no shear stress between the two phases.



2nd stageThe initiation of matrix cracking results in the transfer ofthe load to the fibre at the crack plane.The stress in the fibre right after cracking at the level ofmatrix crack is increased.



3rd stageInitiation of multiple cracks.The stress in both matrix and fibre oscillates between themaximum and the minimum levels such that the totalforce remains constant throughout the sample length.Multiple cracking up to crack saturation point cancontinue under steady-state conditions or increasing loadlevels.The cracking process continues in a composite until afailure criteria for one or all of the stress measures areexhausted.



3rd stageIt is also possible for frictional sliding to occur.

The stress distribution along the interface isnonlinear.The debonding mode would inherently slow down thelevel of matrix cracking toward a saturation levelwhere no more matrix cracks could occur.



4th stageFully debonded interface is achieved.Only the frictional shear between the fibre and matrixcan transfer the load to the matrix.Fibre slipping dominates the response, and the crackingremains saturated and evenly distributed.Because no more cracking takes place, no more force canbe transferred into the matrix through the interface.


Factoring of the Effectiveness of FRCM

1. Bond between single fibre filament and matrix

2. Matrix penetration into rovings and capacity of wetting singlefilaments

3. Bond between external fibres in a fibre bundle directly incontact with the matrix and internal fibers

4. Nonuniformity of tensile stress in fibres of a roving

5. Contribution of joints between longitudinal and transversefibers

6. Cracking of cementitous matrix

7. Bond strength at FRCM/old concrete substrate interface

8. Failure of the old concrete surface


Matrix Penetration into Rovings and Capacity ofWetting Single Filaments


Contribution of joints between longitudinal andtransverse fibres

The cross-section of fibre strands is elliptic and consists of manycircular fibre filaments.


Contribution of joints between longitudinal andtransverse fibers

Along Warp direction Along weft direction

Fibre along the warp direction isstraightened and the cross-section of thestrands is more circular.

Fibre along the weft direction becomes morewavy and the cross-section is flattened.


StrengtheningConfigurations of FRCM


Flexural Strengthening by FRCM

Reinforced concrete beams

FRCMSide view Front viewFabricOne or more layers of highperformance man-made fabric areattached at the tension side ofbeam.

Number of layers depend on theadditional flexural strengthrequired.


Additional Concerns of Flexural Strengthening byFRCM

Flexural strengthening may change the failurebehaviour from ductile flexural failure to brittleshear failure.

Additional shear reinforcement is required toprevent shear failure occurring.


Flexural and Shear Strengthening by FRCM

Reinforced concrete beams

FRCMSide view Front viewFabric

Fabric is in U-shape to provideboth flexural and shearstrengthening.

It can be either along the wholebeam or at region with excessiveshear force.


Common Failure Modes of Beams strengthened byFRCM

The ultimate flexural strength depends on the failure mode.

There are 4 common failure modes of flexural strengthened RCbeam by FRCM.



The first one is intermediate debonding.The strengthening material detaches from the beam with fracturesurface within the old concrete cover.The debonding starts from the flexural cracks near the region withmaximum bending momentThe failure is sudden and brittle.



The second failure mode is slippage between fibreand matrix of FRCM near the region of maximumbending moment for flexural strengthening ormaximum shear region for shear strengthening.

The force carried by fibres is gradually reduced during slippingFibres may be ruptured by excessive frictional force at thefibre/matrix interfaceThis failure mode exhibits significant ductility compared withthe first failure mode.



The third failure mode is the detachment of thestrengthening material from the old concrete beamnear the region of maximum bending momentwithout involving the old concrete

It is common to flexural strengthening only configurationThis failure mode is very brittle.



The forth failure mode is the delamination of thefibre fabric from the matrix near the region ofmaximum bending moment before slippage at thefibre/matrix interface.

The ductility of this failure mode is in between of the second andthird failure mode.


Typical Load-Displacement Curves of BeamsStrengthened by FRCM

Deflection

Load

Normal3RC3beam3under3bendingRC3beam3strengthened3by3FRCM3under3bending3(Failure3mode34)

RC3beam3strengthened3by3FRCM3under3bending3(Failure3modes31,33)

RC3beam3strengthened3by3FRCM3under3bending3(Failure3mode32)


Column Strengthening by FRCM

Reinforced concrete columns

FRCM

Side view Top viewFabric

Fabric is wrapped around column.

The fabric provides confinementand hence compressive strength.


Typical Load-Displacement Curves of ColumnsStrengthened by FRCM

Deflection

Load

Normal RC column under compression

RC column strengthened by FRCM under compression


Design Criteria of FRCM


General Design Criteria of FRCM

The value of all material properties (tensile strength, elasticmodulus, ultimate strain) is defined as the average valueminus 3 standard deviation.

The structural performance is validated and evaluated bycorresponding tests (flexural, shear or axial).

For experimental validation, at least 3 samples should betested and none of the test exceeds 15% from the averagedvalue.

The design capability at ultimate limit state must not exceed5% of the averaged value from experimental validation.

The cross-section corners must be rounded to a radius, r, notless than 20 mm, before placing FRCM to avoid stressconcentration.


Design of Flexural Strengthening

The basic assumptions of the calculation of flexuralstrength after FRCM strengthening are:(1) plane sections remain plane after loading(2) the bond between the FRCM and the substrate remains

effective(3) the maximum usable compressive strain in the concrete is

0.003(4) FRCM has a linear elastic behaviour to failure



Failure modes of flexural strengthening in thedesign are(1) Crushing of the concrete in compression before yielding of

the reinforcing steel.(2) Yielding of the steel in tension followed by concrete

crushing.(3) Shear/tension delamination of the concrete cover (cover

delamination).(4) Debonding of the FRCM from the concrete substrate

(FRCM debonding).(5) Tensile rupture of FRCM material.



Design criteria of flexural strengthening

fd = 0.7fu 0.012ffe = 0.85Effe, fe fdMd = mMn = (Ms +Mf )

fd = maximum allowable strain in FRCM

fu = ultimate strain in FRCM by unidirectional tensile test

ffe = effective tensile force in FRCM

Md = design flexural strength after strengthening by FRCM

Mn = nominal flexural strength

Ms = contribution by steel reinforcement

Mf = contribution by FRCM

m = strength reduction factor



Strength Reduction Factor for Flexural Strengthening

m =

0.90, for t 0.0050.65 +

0.25(tsy)0.005sy , for sy < t < 0.005

0.65, for t sy

t = tensile strain of steel reinforcement

sy = yield strain of steel reinforcement



Other considerations:(i) To limit the total force per unit width tranferred to

existing concrete, the increment in flexural strengthprovided by FRCM must be 50% of the capacitywithout strengthening.

(ii) Tensile stress in the steel reinforcement under service loadis limited to 80% of steel yield strength.

(iii) To prevent creep rupture and fatigue of FRCM, thetensile stress level in FRCM reinforcement under serviceload is limited from its tensile strength fu as the followingtable.

AR Glass Aramid Carbon PBO0.20 fu 0.30 fu 0.55 fu 0.30 fu


Design of Shear Strengthening

Shear strengthening using external FRCM may beprovided at locations of expected plastic hinges orstress reversal.

It can be used for enhancing post-yield flexuralbehavior of members in moment frames resistingseismic loads

Only continuous FRCM U-wraps or continuouscomplete wraps should be considered.



Design Criteria of Shear Strengthening 1

fV = KV fu 0.004ffV = 0.85EffV

fV = design tensile strain of FRCM shear reinforcement

fu = ultimate tensile strain of FRCM under unidirectionaltensile test

KV = bond reduction coefficient = 0.4

ffV = design tensile strength of FRCM shear reinforcement



Design Criteria of Shear Strengthening 2

Vf = nAfffV d

Vd = V Vn = V (Vc + Vs + Vf )

n = the number of layers of reinforcement

V = strength reduction factor = 0.75

Af = the area of reinforcement by unit width effective in shear

d = the distance from extreme compression fibre to centroid oftension reinforcement

Vd = design shear strength

Vc, Vs and Vf = contribution by concrete, steel reinforcement andFRCM, respectively.



Other limitations:To limit the total force per unit width transferred to theconcrete, the increment in shear strength provided by theFRCM reinforcement 50% of the original capacity.The total shear strength provided by FRCM and steelreinforcement is also limited.

Vs + Vf 0.66f cbwd

f c is the cylindrical compressive strength ofconbcrete

bw is the width of the beam


Design of Axial Strengthening

fc =

{Ecc (EcE2)

2

4f c(c)

2, 0 c < tf c + E2c, t c ccu

E2 =f cc f cccu

c = compressive strain of concrete

t = transition strain in the stress-strain curveccu = ultimate compressive strain of confined concrete

fc = compressive stress in concrete

f c = specified compressive strength of concretef cc = maximum compressive strength of confined concreteEc = elastic modulus of concrete



The ultimate compressive strength of confined concrete

f cc = fc + 3.3faf`

f is strength reduction factor = 0.95.

a is the efficiency factor.

There is minimum confinement ratio that

f`f c 0.08



For circular section,

f` =2nAfEffe

D

For rectangular section,

f` =2nAfEffe

b2 + h2

The effective tensile stress in FRCM is limited by fe = 0.55fu

Af is the area of reinforcement by unit width.

D is the diameter of circular member.

b and h are the short and long side dimensions of rectangularmember.



The ultimate compressive strain of confined concrete

ccu = c

(1.5 + 12b

f`f c

(fec

)0.45) 0.01

c is compressive strength of unconfined concrete.b is the efficiency factor.

f cc should be limited by ccu 0.01.



For circular cross-section,

a = 1.0

b = 1.0

For rectangular withb/h 2,

a =AeAc

(b

h

)2b =

AeAc

(h

b

)0.5

Ac is the net cross-sectional area of the compression member.

Ae is the area of the effectively confined concrete.



AeAc

=1 (b/h)(h2r)2+(h/b)(b2r)23Ag g

1 g

Ag is the gross cross-sectional area of the compression member.

g is the ratio of the area of longitudinal steel reinforcementAs, to the gross cross-sectional area of the compressionmember.


Applications ofNanotechnology on FRCM


Why do we need nanotechnology on the cement-basedmatrix for FRCM?

By using nanotechnology, the performance ofFRCM can be enhanced.Three kinds of nanotechnology will be introduced.They are:

Nano-silicateCarbon nanocubeNanoclay


Nano-silicate

The overall performance of FRCM depends notonly on the FRCM, but also the quality of existingconcrete.

However, concrete near surface is usually degradedafter decades of exposure in environment.

Inorganic primer may be applied on existingconcrete surface prior to installation of FRCM.

The primer consists of alkali silicate with silicatenano-particle.


Nano-silicate

The alkali silicatepenetrates and carriessilicate nano-particle intoconcrete cover.

The alkali silicate leachesthe Portlandite out intothe pore solution andreacts with both alkalisilicate and nano-silicateto form C-S-H.

It can heal and seal thecracks in concrete cover.


Nano-silicate

By tuning the concentration of silicatenano-particle and drying rate of alkali silicate, thefresh cementitious matrix is applied before theinorganic primer dries.

Part of alkali silicate and silicate nano-particle canpenetrate the fresh cementitious matrix as well.

The bonding between the fresh cementitious matrixand the existing concrete substrate can besignificantly improved.


Carbon nanotube

By adding small amountof multiwall carbonnanotube (MWCNT)into cement mortar(0.048 wt% to cement forlong MWCNT or 0.08%short MWCNT), it canimprove both the macroand micro structures ofhardened cement paste.


Carbon nanotube

Macroscopically, MWCNTimproves the fractureproperties significantly sothat flexural strength ofcement mortar can beincreased by 25% whileautogeneous shrinkage canbe reduced by half.


Carbon Nanotube

Microscopically, theMWCNT reinforced thecementitious matrix atnanoscale by increasing theamount of high stiffnesscalcium silicate hydrateand reducing the nano-pore(porosity).


Nanoclay

Nanoclay is thinsheet in nanoscale.

Nanoclay canincrease the viscosityof cement mortar infresh state.


Nanoclay

It can be used to tunethe thixotropy of thecementitious matrix.

It can increase thestiffness of cementitiousmatrix in fresh state afterrelease from shearing sothat it can hold its shapeafter application on thebottom surface of beamsor vertical surface ofcolumns.


Nanoclay

Matrix

Silicate layer

The sheet-like structureof nanoclay increases thepenetration path of wateras well as corrosive agent(such as chloride ion).

By adding 1 wt% ofcement nanoclay, thepermeability of chlorideion can be significantlyreduced.


Conclusions Remarks

FRCM is an innovative alternative solution forstrengthening/retrofitting/rehabilitation of reinforcedconcrete structures.

The basic requirements of the FRCM materials areintroduced.

Different configurations for flexural, shear and axialstrengthening by FRCM are discussed.

The basic design philosophy and method of using FRCM forflexural, shear and axial strengthening by FRCM issummarised.

The possibility of using nanotechnology to enhance theperformance of FRCM is elaborated.


Why do we need structural strengthening/retrofitting?Current Strengthening/Retrofitting TechniquesStrengthening Materials of FRCMStrengthening Configurations of FRCMDesign Criteria of FRCMApplications of nanotechnology on FRCM

0 presentation frcm 20140228 2

Documents

wan namihkci frcm

structural degradation

examples of critical

years of use

residential building

use of structuresin

research institutein

building materialsteam