Wiley Encyclopedia of Composites

716 Pages·2012·212.152 MB·English

by  Luigi Nicolais, Assunta Borzacchiello, Stuart M. Lee.

COMPOSITES FOR STRUCTURAL TheFRPmaterialsina‘‘wet-layupsystem’’arewidely
STRENGTHENING used for strengthening since they can ensure the best
compliance to the structural member shape. The sheets
ALBERTOBALSAMO,MARCO can be uniaxial (all the fibers arranged in one direction),
◦ ◦
DILUDOVICO,GIANPIERO biaxial (balanced 0 /90 fiber arrangements), or quadri-
LIGNOLA,ANDREAPROTA, axial (balanced 0◦/90◦/+45◦/−45◦ fiber arrangements).
GAETANOMANFREDI,AND The quadriaxial system is suitable in order to overcome
EDOARDOCOSENZA the anisotropic behavior of uniaxial FRP systems; it can
UniversityofNaplesFederico be used to sustain tensile stresses in different directions
II,Naples,Italy (e.g., in beam–column joints along the principal tensile
stresses).InadditiontotheaforementionedFRPsystems,
hybridFRPsheets,involvingdifferentfibers(e.g.,carbon
INTRODUCTION
andglass)alongthereinforcementdirection,canbeused
intheformofbiaxialandquadriaxialsheets.Inthiscase,
The existing concrete and masonry structures may have
it is possible to optimize the FRP system stiffness and
been damaged by increased external loads, insufficient
the strength as a function of the required strength and
maintenance, and chemical processes caused by harsh
ductilityofthereinforcedmember.Thesamegoalcanbe
environmentalconditions.Evenwhenthesestructuresare
achievedvaryingthenumberofFRPpliesand/orbyusing
not affected by strong signals of deterioration, they often
FRPsystemswithdifferentamountoffibersperunitarea.
needtobeupgradedtomeetnewmandatoryseismicdesign
Inanycase,theuseofcompositematerialsallowsdesign-
requirements or to allow a change of use of the building.
ing an FRP system ‘‘tailor-made’’ according to the design
Applications of fiber-reinforced polymer (FRP) materials
requirements and optimizing the type of matrix, fiber,
forretrofittingandstrengtheningexistingstructureshave
weight, configuration, and installation process. Although
been rapidly grown in past years because of their sev-
the possibility of varying the configuration of the FRP
eraladvantageswithrespecttotraditionalstrengthening
materials makes the system very flexible, special atten-
systems. Their flexibility, ease of handling and applica-
tion should be focused on the material knowledge in the
tion, low installation and maintenance costs as well as
strengtheningdesignphase.Inthecaseofexistingstruc-
durability properties make the use of FRP systems very
tures, the FRP strengthening design process must begin
attractive for a wide range of works. They provide an
withthestageofknowledgeinordertoidentifyindetail:(i)
efficient, noncorroding alternative to externally bonded
structuralsystems(i.e.,one-and/ortwo-wayframes,shear
steelplatesowingtotheirhighdurabilityinenvironmen-
walls,etc.,inthecaseofRCstructures;structuralwallsin
tal conditions that may cause steel corrosion. Further,
thecaseofmasonrystructures);(ii)geometricalcharacter-
FRP strengthening works also provide a sound solution
isticsandpropertiesofstructuralmembers(i.e.,columns,
with low impact on the external appearance, which are
beams, joints, stairs, slabs, and their internal steel rein-
crucial issues especially in the case of heritage build-
forcementintermsofdiameterandpositioninthecaseof
ings.Nowadays,notonlymonumentalmasonrystructures
RC structures; walls, cantonal, arches, vaults, etc. in the
representthearchitecturalheritagebutalsomodernrein-
case of masonry structures); and (iii) mechanical charac-
forced concrete (RC) structures built at the beginning of
teristicsofmaterials(i.e.,concreteandinternalsteelrein-
last century; in addition, in these cases, FRP represents
forcementaswellasstoneandmortarforRCandmasonry
an effective solution to upgrade the structure in compli-
structures,respectively).Inparticular,itisnecessaryto(i)
ance with all requirements related to the preservation
performappropriatedestructiveandnondestructivetests
of original architectural and structural concepts [1]. On
on different structural members; (ii) identify the preser-
the other hand, as most structural materials, FRPs also
vation degree of original mechanical characteristics; (iii)
have few drawbacks mostly related to their capacity to
determine the mechanical properties of existing materi-
resist mainly to tensile forces, to their brittle behavior,
als; (iv) define the members’ substrate state and their
and low performances at high temperatures and under
hydrothermal conditions and surface roughness (in order
directfire.
to assess the debonding mechanisms in case of external
FRP systems offer several possible strengthening
reinforcement by using composites); (v) identify existing
solutions by using different typologies and amounts
damages and cracks; (vi) evaluate dead and live loads
of material. Indeed, the designer, according to the
(static and dynamic ones) depending on the future build-
performance requirements, can (i) use different types
ing use; (vii) assess the environmental conditions which
of matrices (e.g., thermosetting, cementitious, etc.); (ii)
can significantly affect the effectiveness of FRP rein-
selectdifferenttypesoffibers(e.g.,carbon,glass,aramid,
forcement. Furthermore, it is necessary to determine the
basalt, steel, or natural fibers); and (iii) choice dry or
performancetargetsrequiredbytheretrofit/strengthening
preimpregnated sheets with different amounts of weight
solution (strength and/or ductility target). At this stage,
in warp and weft direction and/or rigid elements (plates,
the FRP systems and materials can be optimized. The
rods, etc.) obtained by extrusion in an industrial process
selectionofmaterialtypecanbeachievedbyadoptingthe
(pultrusion).
followingcriteria:
WileyEncyclopediaofComposites,SecondEdition.EditedbyLuigiNicolaisandAssuntaBorzacchiello.
©2012JohnWiley&Sons,Inc.Published2012byJohnWiley&Sons,Inc.
1
2 COMPOSITESFORSTRUCTURALSTRENGTHENING
fibers are preferred in the case of external applica-
tionwhileglassonesforinternalapplication).
Choice ofStrengtheningTechnique withFiber Impreg-
nating System. The so-called ‘‘wet-layup system’’ is
used in the case of particular shape and flexibility
requirements since the reinforcement can be easily
applied on the member regardless of its geometry
(e.g., U shapes and wrapping). Typical applications
arebeamand/orcolumnshearstrengthening,beam
flexural strengthening, column confinement, and
beam–column joint strengthening. To simplify the
application cycle by avoiding twice application of
mediumviscosityepoxyadhesiveforfibers’impreg-
nationascommon(Fig.1),asingleapplicationstep
isthesheetimpregnationbymanualimmersionina
Figure1. FRP‘‘wet-layup’’onmasonryvaults. superfluidepoxyresininaspecialbox(Fig.2a)(for
smallapplications)orbytheuseofsuitableimpreg-
natingmachine (for extensive works) (Fig.2b); this
ReinforcementExposureConditions.Thisinfluencesthe allows operators to perform both saturation and
choiceoffibertypeonthebasisofdurabilityagainst dripping. This simplified application requires the
thepotentialenvironmentalaggression(e.g.,carbon use of appropriate equipment; a typical example of
(a) (b)
Figure2. (a)Sheetimpregnationbymanualimmersioninasuperfluidepoxy;and(b)theuseofimpregnatingmachine.
Figure3. Field cases for FRP
‘‘wet-layup’’ application: (a) FRP
strengtheningonanRCframewith
onlypartiallydemolishedinfillsand
(b)FRPstrengtheninginthecaseof
spacelimitation. (a) (b)
COMPOSITESFORSTRUCTURALSTRENGTHENING 3
typicalexampleisrelativetourgentsecurityappli-
cationsinemergencyconditions(Fig.4).
ChoiceofStrengtheningTechniquewithPreformedSys-
tems.Fibersarepreimpregnatedwithresin,andthe
system is obtained by extrusion in an industrial
process(pultrusion).Inthiscase,theapplicationpro-
cess consists of gluing preformed laminates on the
member.Atypicalexampleistheflexuralstrength-
ening of RC beams or slabs or of woods beams
with laminates located along the member longitu-
dinal axis in order to sustain tensile stresses. The
applicationprocedureisparticularlyeasy,fast,and
cheap;furthermore,highlyspecializedworkersand
equipments are not necessary. Recently, consider-
able research has been directed to characterize the
useofFRPbarsandstripsasnearsurface-mounted
(NSM) reinforcement. It consists of pultruded com-
posite bars or strips fixed by using epoxy adhesive
intoprecutgroovesontheconcretecoverofthemem-
berstobestrengthened.NSMtechniquecouldallow
overcoming some typical drawbacks of externally
bonded reinforcing (EBR) technique since FRP is
not directly exposed to weathering conditions, neg-
ativeinfluencesoffreeze-thawcycles,andeffectsof
high and low temperatures as well as to fire and
acts of vandalism. Furthermore, the main benefits
providedbythissystemareasfollows:surfaceprepa-
ration other than grooving is no longer required;
the reinforcement can be better anchored to pre-
vent debonding; the aesthetic of the strengthened
Figure4. FRP ‘‘wet-layup’’ for applications in emergency con- structureisunchanged[2].
dition(e.g.,intheearthquakeaftermath)onhistoricalmasonry Potentials of Innovative Systems Based on New Fibers
building. and Matrices. The use of basalt fibers allows to
reduce the costs of the strengthening work, still
guarantyingtheperformanceofglassfibers[3].The
field application is the case of columns’ wrapping higher alkali resistance of basalt fibers allows the
with space limitations (Fig. 3a,b) that necessarily use of a cementitious matrix instead of organic,
requires to stretch preimpregnated strips. Another resin-based one [4]. The use of cementitious grout
(a) (b)
Figure5. DamagedconcreteremovalonanRC(a)beamand(b)column.
4 COMPOSITESFORSTRUCTURALSTRENGTHENING
(a) (b)
Figure6. Steelbarscleaningon(a)beamsand(b)columns.
Figure7. (a)Cracksrepair;(b)concrete
surfacefinishingbymeansofthixotropic
bicomponent controlled shrinkage mor-
tar. (a) (b)
Figure8. RoundingofRCmembercornersbeforeFRPapplica- Figure9. Surfacerepairandpreparationfortheapplicationof
tion. FRPsheets.
COMPOSITESFORSTRUCTURALSTRENGTHENING 5
(a) (b)
Figure10. Applicationoffiber-reinforcedgrout.
FRPSTRENGTHENINGUNDERGRAVITYLOADS
In this section, several possible FRP strengthening
solutions to increase the capacity of structural members
againstgravityloadsareoutlined.
InthecaseofRCstructures,FRPcompositestrengthen-
ingsystemsmaybeusedtoimprovethememberflexural
capacity, shear capacity, axial capacity, or any combina-
tionofthem.Onmasonrystructures,theFRPsystemsmay
betypicallyusedtoresistthehorizontalforcescausedby
thrustingstructures(e.g.,vaultroofs,arches,andvaults),
while direct strengtheningof the masonry panels is typi-
callyrelatedtoseismicstrengthening.
Ineachcase,apreliminaryassessmentofthemember
existingloadcarryingcapacity,deficiencies,andsubstrate
condition may be strictly required by both theoretical
calculationsandfieldinvestigations(i.e.,memberdimen-
Figure11. FRPflexuralstrengtheningwithuniaxiallaminates. sions,existingreinforcingsteel,concretequality,masonry
type,presenceofcracks,etc.).TheeffectivenessoftheFRP
strengthening is highly dependent on a sound substrate
anditsproperpreparation.
In the case of RC structures, some preliminary steps
is highly relevant as it could overcome the issue of before applying FRP may be strictly necessary in the
fire resistance and further reduce the cost of the case of cracked, damaged, or carbonated concrete as well
strengtheningsystem[5].Inthis,matrixcanbealso as corroded internal steel reinforcement: plaster and
embedded dry or preimpregnated fibers as well as damaged concrete removal by means of not punching
high strength steel chords having similar stiffness machines (Fig. 5); removal of all residuals of previous
ascarbonfibersbuthighershearstrength.Further- steps; steel bars cleaning by means of sandblasting or
more, steel fibers can be easily inserted as spikes differentmechanicalcleaningandbarsprotectionthrough
to increase the flexural capacity at member ends. anticorrosive coatings (Fig. 6); cracks repair performed
Steel fibers are also used in flexural reinforcement through injected or drained epoxy resin with proper
application with polymeric resins (steel-reinforced viscosity and fluidity, and concrete surface finishing
polymer,SRP)[6].Recently,naturalfibers(e.g.,flax, (Fig. 7a,b); and corners rounding of RC members in the
hemp, agave, silk) are also investigated in exter- zoneswhereFRPshouldbeapplied(Fig.8).
nallybondedapplicationsmainlywithcementitious In the case of masonry structures, proper preparation
matrix given their good mechanical properties and and profiling of the masonry surface is required; other-
lowenvironmentalimpactandproductioncosts[7]; wise, debonding or delamination of the FRP system can
however, the main critical issue is related to the occurbeforeachievingthedesignloadtransfer.Ingeneral,
durabilityofthesefibers. specificguidelinesregardingproceduresforsurfacerepair
6 COMPOSITESFORSTRUCTURALSTRENGTHENING
(a) (b)
(c)
Figure12. FRPflexuralstrengtheningwithpultrudedplates:(a)surfacepreparationand(b,c)plates’application.
or preparation (Fig. 9) for each FRP system should be most of the critical bond issues. It is not only noncom-
obtained from the system manufacturer; in fact, the pro- bustible but also protects the embedded fibers against
cedures for installing FRP systems often differ between directfireexposure.
systems and can vary within a system, depending on the
typeandconditionofthestructure. FRPFlexuralStrengthening
Differentmasonrysubstratesandmaterialsexistthat Existing RC members may be deficient in terms of
can strongly influence the bond properties of the FRP flexural capacity because of several reasons: corrosion
composite system. Temperature, relative humidity, and of internal reinforcing steel with corresponding nominal
surface moisture at the time of installation can affect steel area reduction; soundness of concrete cover and
the performance of the FRP system, also in the case of contemporary presence of active corrosion; change of the
concretestructures,butespeciallyhumiditycanbemuch use of building that induces a higher flexural demand
with respect to the original design; and common errors
morecriticalinmasonrystructures.Resinsshouldnotbe
in design or building execution phases. To increase the
appliedtowetsurfacesunlesstheyhavebeenspecifically
flexural capacity of an existing member, FRP axially
formulated. Furthermore, the transmission of moisture
oriented laminates can be installed along the longitu-
vaporfromamasonrysurfacethroughtheuncuredresin
dinal axis of the member. In the case of beams, they
materials typically appears as surface bubbles and can
are bonded at bottom face to increase their positive
compromise the bond between the FRP system and the
moment capacity and at the top for negative moment
substrate. To avoid this pitfall, the resin matrix can be
capacity increase. The external FRP reinforcement is
replaced by an inorganic matrix, based on cementitious an additional strengthening in tension; it can replace
orlimeandpozzolan-basedmortarandthereinforcement the traditional solutions based on the installation of
hastheformofgrids(Fig.10).Theadvantagesarethecom- steel plates (beton plaque´). Several FRP systems can be
patibilitywiththemasonrysubstrateintermsofbonding, used to achieve the flexural strengthening: FRP uniaxial
moisture permeability, and thermal properties, avoiding laminates installed by using ‘‘dry or wet-layup’’ method
COMPOSITESFORSTRUCTURALSTRENGTHENING 7
(a) (b)
(c) (d)
Figure13. FRP flexural strengthening of PC girders by FRP sheets: (a) Bridge A5657, South
of Dixon, Route 28 over Gasconed River, Missouri; (b) PC girder surface preparation; (c) FRP
longitudinalreinforcementinstallation;and(d)anchoringU-wrapdevicesmadeofFRPsheets.
Figure15. FRPstripsappliedattheintradossurfaces.
Figure14. FRPstripsappliedovertheextradossurfaces.
no additional transverse reinforcement is required in
the anchorage zones. In the case of FRP laminates or
(Fig.11);FRP-pultrudedplates(Fig.12);FRPbars/strips pultruded plates, for which anchoring problems may
with NSM method; and SRP composites. If adequate occur, an additional anchorage system can be provided
anchorage is provided to the strengthening system, by laminates glued transversally to the strengthening
8 COMPOSITESFORSTRUCTURALSTRENGTHENING
(a) (b)
Figure16. FRPflexuralstrengtheningoftimberslabsbymeansof(a)pultrudedlaminatesor(b)NSMbars.
(a) (b)
(c) (d)
Figure17. Beams shear strengthening by means of FRP laminates: (a) fully wrapping; (b,c)
U-wrapconfigurationatbeamend;and(d)discreteU-wrapsalongthebeam.
COMPOSITESFORSTRUCTURALSTRENGTHENING 9
(a) (b)
Figure18. Columns’shearstrengtheningbymeansofFRPlaminates:(a)fullywrappingofsquareand(b)rectangularcolumns.
due to collision on PC girders by overheight vehicles or
construction equipment; inadequacy to new and heavier
loads;degradation,typicallyduetofatigueandcorrosion;
and oldness and obsolescence. The use of anchoring sys-
tems (e.g., FRP strips U-wrapped around the bulb of the
girderoverthelongitudinallaminates)topreventordelay
thelaminatesdebondingisstrictlyrequiredtoexploitthe
full capacity of the FRP system (Fig. 13). In such a case,
FRP could represent a sound alternative to traditional
methods from a constructability standpoint (financially
competitive,easyandfastinstallation,andnodisruption
oftrafficlanes)[8].
In the case of masonry structures, the most common
solutionistheincreaseofthecapacityofvaultsandcurved
structures, in general, used typically as floors or roofs.
Flexuralstrengtheningofslendermasonrypanelsloaded
by horizontal forces generated by thrusting structures
(e.g., vaults roofs, arches, and vaults) represents other
frequentapplications,evenifinthelattercasetheappli-
cation of FRP ties in reducing thrust forces in thrusting
structuresseemspreferable.
FRPstripscanbeappliedovertheentireextradossur-
faces or only on critical areas to strengthen the curved
structures. In most frequent cases, the coverage of the
Figure19. Columnconfinement:circularcolumnswithcontinu-
ouslyappliedstrips. entire surface is about 35–45% (Fig. 14), thus leading to
good moisture transmission. The application of cemen-
titious mortar-based composites, FRG (fiber-reinforced
direction (i.e., U-shaped fiber sheets) or by using power grout), guarantees adequately permeable surfaces. The
actuatedfasteners. conservation of thermohygrometric conditions is usually
FRP reinforcement can also be effective to restore the crucial, especially in monumental buildings or historic
originalultimateflexuralcapacityofprestressedconcrete surfaces: when paintings are on the intrados of curved
(PC) girders, which often require repairing or strength- structures,theapplicationattheintrados(Fig.15)isnot
ening measures due to several issues: damage, typically possible.
10 COMPOSITESFORSTRUCTURALSTRENGTHENING
(a) (b) (c)
Figure20. Typicalcolumnshearfailureduetoseismicactions:(a)circularcolumn;(b)shortcolumn;and(c)RCwall.
(a) (b)
Figure21. Bucklingoflongitudinalsteelbarsincompression:(a)circularcolumnand(b)rectangularcolumn.
Inmasonrystructures,besidescurvedstructures,floors making some slits parallel to the beam longitudinal axis
are usually made of timber or mixed steel beams and (Fig.16b).
concrete slabs that can also be strengthened with FRP
composites. Typical strengthening solutions regard beam FRPShearStrengthening
elements(eithermadeofconcreteortimberorsteel)com-
Shear strengthening of RC members using FRP may be
plying roughly to the principles of RC beams both in
provided by bonding the external reinforcement with the
shear and flexure (Fig. 16). FRP-preformed laminates principal fiber direction transverse to the axis of the
are usually applied to the tension side of the beams to member or perpendicular to potential shear cracks. Typ-
be strengthened; FRP-pultruded plates (Fig. 16a) can be ically,shearstrengtheningisprovidedbyFRPlaminates
easily replaced by FRP bars/strips with NSM method, wrapped around the existing member (i.e., full wrap or