1.
Introduction
The structural design and thus the production of structural elements made of reinforced concrete is based on forces and loads current in codes of the time. However, during the service life of a structure, various circumstances may require that the service loads are changed due to:
•Modification of the structure • Aging of the construction materials
• Deterioration of the concrete caused by reinforcement corrosion • Earthquake design requirements; fire design changes
• Upgrading of building codes relating to load bearing capacity or service loads etc.
The consideration of the actual loads resisted by a structure is a necessary prerequisite for the devel-opment of a comprehensive rehabilitation concept. Basically, the methods available for the structural strengthening of structural elements made of reinforced concrete are as follows:
• Application of cast or sprayed concrete with additional reinforcement • Placing of additional reinforcement in slots cut in the concrete • External post-tensioning
• Installation of supports or additional bearing members • Steel plate bonding to increase shear or flexural capacity.
An alternative to these traditional methods of strengthening is the use of Fibre Reinforced Polymer (FRP) composites.
2.
Selection of the S&P FRP system
The basic fibres used for the FRP composites are imbedded in a matrix and applied as reinforcing ele-ments to an existing structural member. For flexural enhancement, prefabricated FRP composites (S&P Laminates CFK) are used. As prefabricated laminates are not suitable for confinement or shear en-hancement situations, sheets made of different types of basic fibres are offered for manual lamination on the job site.
Different types of FRP composites:
Fibre direction Fibre arrangement Typical application
S&P C Sheet uni-directional ——— stretched increase in stiffness
S&P A Sheet uni-directional ——— stretched special applications
S&P G Sheet bi-directional
~~~~
undulating increase in ductilityS&P Laminate CFK uni-directional ——— stretched increase in stiffness
(partly prestressed)
In both the S&P C Sheets and the S&P Laminates CFK, the fibres are stretched. However, due to manual application, the C Sheet becomes slightly wave like in form. As a consequence, these products are suited for absorbing tensile forces at a minimum elongation. Both sheets and laminates are preferred for use in the increasing of flexural capacity of a structural element. Carbon fibres with a high modulus of elasticity are used in their production. This modulus of elasticity is the decisive parameter when comparing the various types of carbon fibre sheets and CFK laminates.
In the bi-directional S&P sheets the fibres are arranged in the form of waves due to the weaving process used in their production. As a result, the absorption of tensile forces takes place at a higher elongation than with the carbon sheets. Bi-directional sheets are ideally suited for increasing the ductility of a struc-tural element and are often used in the seismic upgrading of concrete structures. Glass fibres with a modulus of elasticity of 70,000 N/mm2are used in these applications. The selection of normal E-glass
or alkali-resistant AR-glass depends on the application. Since strains during load transfer into the contact substrate are relatively high, the stresses in the substrate are distributed by the formation of micro-cracks and are thus locally relieved. Thus, glass sheets are able to be applied to substrates with a low inherent tensile strength.
S&P glass sheets are suited for the strengthening of historic buildings or the increasing of shear capacity of walls made of bricks or masonry. On the other hand, for the application of S&P C Sheets and S&P A Sheets an inherent substrate tensile strength of >1.0 N/mm2is required. In the case of
prefabri-cated S&P Laminates CFK, the load transfer into the substrate is concentrated and thus strengthening with these laminates requires substrates with an inherent tensile strength >1.5 N/mm2.
Demands on the substrate:
Product Inherent tensile strength
S&P C Sheet / S&P A Sheet >1.0 N/mm2
S&P G Sheet approx. 0.2 N/mm2
S&P Laminate CFK >1.5 N/mm2
Testing of the bond (tensile) strength of the bearing Roughening of the surface by sandblasting or grinding. substrate.
In order to ensure the load transfer from the S&P FRP system to the substrate, the surface must be roughened by sandblasting or grinding.
3.
Types of fibres for S&P FRP systems
Type of fibre Modulus Tensile
of elasticity strength GPa N/mm2 Carbon 240 - 640 2,500 - 4,000 Aramid 124 3,000 - 4,000 Glass 65 - 70 1,700 - 3,000 Polyester 12 - 15 2,000 - 3,000 (Steel 210 250 - 550)
S&P Reinforcement manufactures custom-mode sheets of either a single fibre type or of fibre combina-tions (hybrids). The advantages and disadvantages of the various fibres are as follows:
Ca rb o n Gla ss PES 2500 2000 1500 1000 500 0 5 10 15 Ara mid e Steel
E- glass: Uncoated E-glass corrodes in alkaline environments. Since the coating may be subjected to damage near its edges during the application, E-glass should only be applied in non-alkaline areas (reinforcement of asphalt) or where it is completely embedded in the epoxy resin matrix. E-glass sheets should not be used for strengthening of concrete in combina-tion with a water vapour permeable matrix.
AR-glass: Alkali-resistant glass is suited to confinement reinforcement for concrete elements (in-crease in ductility) in combination with an epoxy resin matrix and a water vapour perme-able matrix.
An intensive research program has been carried out with the S&P AR Glass. During 28 days the glass was stored in the substances listed below and then examined:
Saturated solutions of calcium hydroxide:
10% potassium hydroxide 10% sodium hydroxide
Acids: Chlorides:
5% acetic acid 10% calcium chloride 10% hydrochloric acid 10% sodium chloride 10% nitric acid 10% potassium chloride
Nitrates: Sulphates:
10% calcium nitrate 10% calcium sulphate 10% potassium nitrate 10% sodium sulphate 10% sodium nitrate 10% potassium sulphate 10% ammonium nitrate
View of S&P AR Glass in the scanning electron microscope, after storage
The S&P AR Glass was resistant against all acids, salts and alkalis examined in the test. After a storage of 28 days none of the samples showed any weight reduction. No surface modification and thus no attack was observed in the scanning electron microscope. The S&P AR Glass is highly alkali-resistant (resistance against 100% caustic soda).
Aramid: Aramid is a very tough material. Aramid sheets are used, as an example, for the manu-facture of bullet-proof vests. For structural elements this extreme toughness provides benefits for special applications such as the strengthening of rectangular columns. Because of the very high price of the A fibres, the S&P A Sheet (aramid sheet) is often replaced by carbon or glass fibre products.
Upon request, S&P offers the S&P A Sheet 120 (Types A and B). The products are uni-directional fibre sheets of different weights. Several tests have been carried out with aramid sheets. For more information please ask the S&P engineering department.
Carbon fibres: Used as the basic fibres for reinforcement of concrete, carbon provides all the benefits: • High modulus of elasticity (depending on fibre type)
• Minimum coefficient of thermal expansion (approx. 50 times lower than steel) • Excellent fatigue properties
• Excellent resistance to all types of chemical attack • Will not corrode
4.
S&P FRP glass fibre systems
4.1
Seismic upgrading with S&P G Sheet 90/10 (Types A and B)
Load-bearing column after earthquake in San Francisco (USA)
In the analysis of a structure for seismic upgrading the structure as a whole is considered for ductility. Individual elements may have their ductility increased using S&P G Sheets 90/10 made of either E-glass or AR-glass. The performance of glass in ductility enhancement has been proven by large scale push-pull (reverse cycle) tests. Only system approved epoxy may be used as a primer or for the impregnation of the S&P G Sheet. Please contact S&P for detailed test reports.
4.2 Protection of structures subject to explosion hazards
with S&P G Sheet 50/50
The S&P G Sheet 50/50 has been specially developed for this field of application. The sheet, which has a 50% glass content in two orthogo-nal directions, can be applied in several layers. S&P offers also prefabricated GFK plates for the same purpose. These are glued on to the struc-tural element with approved epoxy resin. Various test reports are available from S&P Reinforce-ment.
The S&P specialty products have been tested for special protective structural elements for NATO.
... Reference column
- - - Column wrapped with C Sheet (1kg/mm
2)
––– Column wrapped with
G Sheet 90/10 (Types A and B) (4kg/mm
2)
-5 -4 -3 -2 -1 0 1 2 3 4 5 -5 -4 -3 -2 -1 0 1 2 3 4 5 Pull Push Drift Ratio ∆/L (%) Lateral load Deflection Explosion S&P G Sheet AR 2-directional Actuator Lateral load 200/200 mm column 2000 mm4.3
Repairs to historic buildings with S&P G Sheet 50/50
The woven S&P G Sheet 50/50 is suited for the reinforcement of historic buildings and for applying to substrates with a low inherent tensile strength (brickwork/natural stone). The high toughness of the S&P G Sheet 50/50 ensures the load transfer from the sheet to the substrate by the formation of micro-cracks. Aspects of building physics in relation to the moisture conditions of the structure should be taken into account in each individual case.
5.
S&P FRP carbon fibre systems
Both uni-directional S&P C Sheets and S&P Laminates CFK can almost be considered as homo-geneous. Since C fibres are linear-elastic and subject to brittle fracture, FRP laminates loaded in the fibre direction are also linear-brittle/elastic. If the fibre volume and the mechanical values of the com-ponents are known, some properties of FRP in the fibre direction can be estimated from a composite calculation; the low contribution of the matrix in practice can be neglected.
5.1
Mechanical long-term properties
Creep strength
The creep strength of CFRP is far superior to that of other fibre composites. This has been proven in a large number of tests. In a creep test conducted on CFRP rods in a saline solution under alternating load, no failure was registered in the rods up to a continuous stress level of 70% of their short-time tensile strength after 10,000 hours. A creep strength of 79% of the short-time tensile strength is extra-polated for CFRP rods for a period of 50 years.
When CFRP is used as reinforcement, the continuous tensile stress under service loading can be expected to be a maximum of 20% of the short-time tensile strength. At this level, no loss of strength of relevance in dimensioning occurs as a result of continuous stress situations.
Fatigue strength
FRP made of C fibres (CFRP) has a very high fatigue strength. In Japanese tests conducted with maxi-mum stresses of less than 87.5% of the short-time tensile strength and amplitudes of up to 1,000 N/mm2, more than 4 · 106load cycles were attained. No fatigue failure and, in the subsequent tensile
test, no reduction in tensile strength was established on CFRP rods embedded in concrete after 4 · 105
load cycles with an oscillation range of 0.05 - 0.5 fcand a frequency of 0.5 Hz.
When CFK plates are used as reinforcement, the maximum stress which can be expected in service will not exceed 20% of the short-time tensile strength and hence it is not the fatigue strength of the CFK material which governs the design, but always the reinforcing or prestressing steel.
Combination
Creep and relaxation
Creep and relaxation are expressions of the viscoelastic behaviour of a material. With fibre composites, it is possible to distinguish between axial creep in the composite cross section and interlaminar creep. Both types of creep are negligible in CFK laminates under continuous stress conditions which occur when used in strengthening applications.
Under continuous loads which exist in the service state, carbon fibres themselves do not display a measurable creep deformation or stress loss as a result of relaxation. In contrast, the epoxy resin matrix is a visco-elastic material and is linear-elastic up to a strain of εM= ±20%. Hence, creep failure will not occur when strains are less than the yield strain of the reinforcing steel (which is not permitted to creep in the service state).
The ratio of the Young’s Moduli Ef/Em= ±30 and the matrix volume component of vM= ±30% means that the tensile force carried by the matrix is only some 0.5% of the total tensile force. Force rearrange-ments of the fibres as a result of matrix creep can thus be neglected. There is seen to be virtually no correlation between time and strength for UD laminates which have continuous fibres and which are loaded in a direction parallel to the fibres.
Relaxation tests conducted on CFRP rods over a period of 3,000 hours with a starting stress level of 70% of the static short-time tensile strength produced relaxation losses of less than 2%. Relaxation losses of 2 to 3% were extrapolated from the logarithmic profile of the creep curve over time for a dura-tion of 50 years and the above stress level.
To sum up, it can be stated that UD-CFRP plates bonded with epoxy resin do not undergo significant creep or relaxation when subjected to the types of low creep generating continuous loading which result from their application to elements which are already subject to dead loads.
5.2
Physical and chemical material properties
Durability
CFRP exhibits very good resistance to all the chemical attacks of relevance to construction applica-tions. This has been proven by a large number of tests. Creep tests on CFRP rods under permanent stress levels of 5 to 75% of the short-time tensile strength and temperatures of 21°C - 80°C, which were exposed to simulated concrete pore liquids at pH = 10 - 13.5 over a period of three months, did not reveal any reduction in interlaminar shear strength.
Component tests on prestressed concrete beams, reinforced with CFRP rods prestressed to 40% of their short-time tensile strength, which were immersed in alkaline solutions for 81 days at pH = 12.5 - 13, did not show any strength reduction. Parallel tests on bare CFRP rods that were exposed to the same solutions at 60°C likewise failed to show any negative influence on the mechanical properties, from the immersion.
Thermal properties
The measured coefficient of linear thermal expansion for the two S&P plate types in the longitudinal and transverse direction are set out in the following table.
Plate type αT; longit. [10-6K-1] α
T; transv. [10
-6K-1]
S&P 150/2000 ±0 30
The coefficient of thermal expansion in the transverse direction is of no relevance for dimensioning. The difference between the coefficient of expansion of the S&P Laminate CFK plates in the longitudinal direction and that of concrete, at αT= ±10 · 10-6K-1, could be expected to have a negative impact on the
bond between the plate and the concrete in the event of major temperature fluctuations.
At EMPA, two series of beams were subjected to a static bending test following 100 freeze/thaw cycles at between -25°C and 20°C and the results were compared to those of identically designed beams that had not been exposed to the freeze/thaw cycles. The beams in one of the series displayed cracks prior to the freeze/thaw cycles on account of prior loading, while the beams in the other series had no cracks. The beams were saturated with water during the freeze cycles, which meant it was possible to study the potential loosening of the composite structure as a result of freezing water, in addition to the poten-tial impact of any thermal incompatibility of the CFRP and the concrete. The results did not reveal any reduction in flexural load bearing capacity, when compared to the reference beams.
A further concrete beam reinforced with a CFRP plate was cooled to -60°C without the plate becoming detached from the concrete or any fibre buckling resulting from the induced compressive stresses. The adhesive bond between a CFK plate and an aluminium substrate did not reveal any damage to the bond up to the minimum temperature of -133°C. The problem of thermal incompatibility is more pronounced with CFRP/aluminium composites than with CFRP/concrete composites on account of the aluminium’s high coefficient of expansion (αT,AL= 23.4 10
-6K-1). Even though it was not possible to study
potential bond failure in this test, the test nonetheless showed that, under the temperature conditions of relevance for construction purposes, no fibre or plate buckling has to be expected as a result of forced compressive stresses with standard-modulus CFRP plates.
According to the current state of knowledge, the dissimilar thermal expansion behaviour of CFRP plates and concrete in no way impairs the load carrying capacity of structural elements strengthened with CFRP plates. Nonetheless, in order to make provision for any potential uncertainty and knowledge gaps, it is recommended (in agreement with design guidelines contained in an existing CFRP plate approval), that the design value for the bond failure stress be reduced by 10% when major temperature fluctua-tions have to be taken into account. This recommended reduction is based on approximate estimates.
Fire behaviour
Carbon fibres have high heat resistance. The glass transition point of the matrix resin is TGM= 100 -130°C. The load carrying capacity of the composite system is determined by the adhesive which has a glass transition point of TGK= ±47°C. This temperature will be attained after just a few minutes of a stan-dard fire.
With reinforcement levels of η ≤ γv ≤ 1.75 below the safety level for the structural element in the reinforced state, element safety will not fall below γresidual = 1.0 in the event of plate failure. The fire resistance period will then be conditioned by the reinforced concrete component, i.e. by the cover on the concrete. The fire resistance period can be increased by a fire protection cladding.
With reinforcement levels ηB, of γv< η ≤2, the structural element safety in the event of plate failure will
fall to γresidual= γv/η<1.0. The fire protection cladding must thus prevent the layer of adhesive from heating up to TGKover the desired fire resistance period. This should be verified in each individual case.
6.
Manual lamination at the job site
with S&P C Sheet
UD carbon sheets are used as confinement reinforcement for load bearing elements or as external shear reinforcement for beams. As the influence of the matrix is negligible in terms of strength, only the fibre properties (not the properties of the composite) and the theoretical fibre cross section are used for design purposes.
The theoretical fibre thickness for design of a UD carbon sheet layer is determined as follows:
Due to the application process, the site lamination does not always produce an optimum fibre ar-rangement. There is also a risk of damage to the fibres while they are being rolled on to the surface. It is therefore recommended that the E-modulus used for the fibres be reduced by a safety factor [S].
Recommended safety factor [S]:
1.2 UD stretched products
1.5 woven products
The value of strain used in design should not be greater than 50% of the ultimate strain and it also depends on the individual case at the strengthening (Shear/Confinement/Flexural)
Recommended design values for strain of C fibre sheets:
• Shear reinforcement Limiting design strain = 0.2-0.3%
• Confinement reinforcement
of load-bearing structures Limiting design strain = 0.4-0.6%
• Flexural reinforcement Limiting design strain = 0.6-0.8%
The selection of the ideal carbon fibre for the production of a sheet is based on the following criteria:
Carbon fibre type: Modulus of elasticity 640,000 N/mm2(Safety factor S = 1.2)
Ultimate strain 0.4% Limiting design strain 0.2%
Field of application: Shear reinforcement S&P C Sheet 640
S&P C-Sheet
Theoretical fibre
thickness for design =
weight of C fibre
density of C fibre
Properties of S&P C Sheet 640 for external shear strengthening
Weight Theoretical Modulus Ultimate
thickness of elasticity elongation
400 gr./m2 0.190 mm 640,000 N/mm2 0.4%
Width of S&P C Sheet Shear force (0.2%) for design (1 layer)
150 mm 30 · 103N each side
300 mm 60 · 103N each side
Carbon fibre type: Modulus of elasticity 240,000 N/mm2(safety factor S = 1.2)
Ultimate strain 1.55%
Limiting design strain 0.4-0.6%
Field of application: Confinement
reinforcement of load-bearing structures S&P C Sheet 240
(Replacing of corroded stirrups)
Properties of S&P C Sheet 240 for confinement reinforcement
Weight Theoretical thickness Modulus Ultimate
(C fibres only) of elasticity strain
200 gr./m2 0.117 mm 240,000 N/mm2 1.55%
300 gr./m2 0.176 mm 240,000 N/mm2 1.55%
Width of carbon (1 layer of sheet) Tensile force (0.6%) for design (1 layer) 200 gr./m2weight 300 gr./m2weight
300 mm 42 · 103N 63 · 103N
6.1
Aspects of building physics to consider in site application
A STRUCTURE MUST BE PERMEABLE TO WATER VAPOUR FROM THE INSIDE
TO THE OUTSIDE.
VITRUVIUS De-bonding of a coating Roman architect and engineer, approx. 50 B.C. due to vapour pressure.
When total surface wrapping of concrete is intended, aspects of building physics must be considered. 30-50% of the surface of the element should remain water vapour permeable. A total surface coverage with an epoxy matrix is therefore not suitable. In such cases, the element is only partially wrapped. As an alternative, the matrix can be varied as follows:
Anchoring area of the sheet: Epoxy matrix => System approved
Water vapour permeable area of the sheet: PU or acrylic matrix adhesives
Calculation of the water vapour permeability of the coating
Sd = µH
2O · layer thickness [m] < 4 m
Sd: Resistance of the coating against water vapour transmission
µH2O: Water vapour transmission rate of the coating
As is the case with thin coatings, aspects of building physics must be checked when intending to fully wrap with fibre composites. The type of matrix used is the limiting factor relating to water vapour trans-mission.
FRP matrix µH2O Shear modulus
Epoxy matrix 1,000,000 >3,000 N/mm2
PU or acrylic matrix 400 - 3,000 50 - 500 N/mm2
In areas where the fibre composites are subject to load transfer (anchoring and lap zones), it is always necessary to apply an epoxy matrix with a high shear modulus. In the water vapour permeable zones of the fibre composite, however, a PU or acrylic matrix is used to protect the C fibres.
When using FRP for reinforcement, it is essential that aspects of building physics be considered in each individual case.
Example A
Example B
A total surface coverage using C sheets and an epoxy matrix to the underside of the box girder soffit is possible. Water vapour permeability into the inside of the box girder is assured.
Example C
• Epoxy adhesive • PU or acrylic adhesive (anchoring/lap (water vapour
area) permeable area)
Due to the use of different matrices, 50% of the element's surface remain open to water vapour transmission.
A total coverage for flexural reinforcement using S&P C Sheets is not recommended. Reinforce-ment using S&P Laminates CFK, allowing water vapour transmission between the laminates, is a better solution.
6.2
Special adhesives for site application of FRP sheet systems
Standard adhesives System approved standard adhesives may only be applied to substrates with a moisture content of <4%. In addition, the dew point must be measured during the application; the temperature must be at least 3°C above the dew point.
Special adhesives System approved special adhesives are required for substrates with a high moisture content. In order to ensure an optimum load transfer from the FRP to the substrate, a system approved primer must be brushed into the sub-strate.
PU or acrylic matrix These adhesives are not suitable for load transfer areas and must not be applied to lap areas of C sheets. The vapour permeable adhesives serve as a protection of the C fibre.
Note: Special adhesives for underwater applications of S&P Sheets (off-shore works) are available from S&P.
The application guidelines of the system approved primer and the impregnation epoxy must be fol-lowed in each case. These guidelines must be ordered from the supplier of the reinforcing system. Recommended Health and Safety measures for the use of epoxies must be observed. The S&P FRP reinforcing systems may only be applied in combination with system approved adhesives. In the case of non-compliance, all product liabilities are declined.
7.
Strengthening of load bearing elements with
S&P C Sheet 240
7.1
Replacing of corroded reinforcement by using FRP
In the case of traditional concrete repair, also the corroded steel reinforcement is replaced by FRP. Thus, the original safety factor of the constructional element is guaranteed after the repair.
Concept:
• Remove all spalled concrete.
• If the chloride content of the concrete exceeds the approved limit, apply a corrosion inhibitor. • Repair with cementitious repair mortar.
• New: The corroded reinforcement may be replaced with FRP.
Replacement of corroded flexural reinforcement with
S&P Laminates CFK bonded into slots.
S&P Laminates CFK (type 10 mm/1.4 mm) are epoxy bonded into slots cut into the column to prevent buckling.
The corroded reinforcing stirrups are replaced by confinement
reinforcement with S&P C Sheets 240.
In order to verify the reinforcing effect of the FRP confinement against compressive loading, various tests were conducted by internationally recognized testing institutes on behalf of S&P Reinforcement.
Slot-applied S&P Laminate CFK Slot-applied S&P Laminate CFK Epoxy adhesive
7.2
Large scale tests on FRP reinforced circular columns
At the Technical University of Gent (Belgium) large scale tests were carried out on circular columns of height 2.0m and diameter 400mm, to which different FRP systems had been applied. FRP • S&P C Sheet 240 (stretched fibres) systems: • S&P C Sheet 640 (stretched fibres)
• Carbon/glass hybrid (undulating fibres) • Glass sheet (undulating fibres)
The increase in compressive strength obtained from the FRP confinement was measured.
In order to achieve an identical increase in compressive strength, the following fibre quantities were required in the confinement direction:
• S&P C Sheet 240 (stretched fibres) 1.0 kg of C fibres • S&P C Sheet 640 (stretched fibres) 1.6 kg of C fibres • Carbon/glass hybrid (undulating fibres) 2 - 3 kg of fibres
• Glass sheet (undulating fibres) ≈ 4.0 kg of G fibres (3.6 kg confinement, 0.4 kg vertical)
Since the consumption of epoxy adhesive increases in proportion to the fibre volume, the use of cheap glass fibres for confinement reinforcement is not economical.
The cost/benefit comparison shows that
the confinement reinforcement of the load bearing column
with the S&P C Sheet 240 is the most economical solution.
In an additional test series the columns were wrapped with S&P C Sheet. The approved adhesive was applied to the anchoring/lap areas only, and the remaining surface of the columns was left without adhesive (water vapour permeable). The increase in compressive strength of the columns with partial and with full surface adhesive application was more or less identical. The reinforcing effect of the water vapour permeable FRP confinement could thus be established.
Large scale test on a column with confinement reinforcement
of 5 layers of S&P C Sheet 240
Identical tests were carried
out on circular and rectangular
cross sections.
increase of the compressive strength 100% Rounding of edges R = 1 cm 60% R = 3 cm 80% In order to guarantee two confined cores, a line of horizontal bars, firmly attached to the external concrete sur-face on its both ends, should cross the section of the column. The verti-cal spacement between these bars shall not exceed the diameter of the cores to be confined.7.3
Design concept for the confinement of axially
loaded columns
While the design of structures subject to bending moments is generally based on capacity design prin-ciples, where a high level of ductility is required of an element when it passes into the non-linear range, the strengthening of axially loaded columns by means of external confinement reinforcement produces a triaxial stress situation due to the fact that strains at right angles to the direction of loading are im-peded, thus allowing a substantial increase in the load carrying capacity of the column. Since this increase is proportional to the increase in transverse compressive strength, confining materials should therefore exhibit a low ductility. Carbon fibres are very well suited for this application as they offer a high modulus of elasticity and exhibit linear-elastic behaviour until failure, without yielding. Thus, what appears to be a negative feature in the case of flexural strengthening, can be considered an advantage for confinement.
The design concept is based on a circular column completely wrapped with S&P C Sheet. If an axial load N is applied to this column, a vertical stress σzis created. In addition, due to the fact that the strain at right angles to the longitudinal axis is impeded by the confinement reinforcement, a horizontal com-pressive stress σxis created which acts evenly from all sides (Fig. 1). The highest stress σxthe C sheet can carry, is calculated as follows:
fFRP: Tensile strength of the C fibre sheet [N/mm2]
tFRP: Thickness of the C fibre sheet [mm] r: Radius of the column [mm]
Due to the confinement reinforcement, the con-crete of the column is exposed to a triaxial com-pressive stress condition. If the cross elongation of steel reinforced concrete columns is impeded by means of steel clamps, the concrete com-pressive strength can be increased to a maxi-mum of:
f
C FRP= f
c+ 4 ·
σ
xFig. 1: Longitudinal and cross section of a con-crete column exposed to a vertical load and completely wrapped with a C sheet.
The related longitudinal compressive strain εccof the column can be expressed as:
Non-regular situations, especially those regarding columns with some of its longitudinal bars not strongly tied by the internal stirrups should be carefully analysed, regarding, for example, what is estab-lished at the FIB Bulletin nr. 1. In this situation the S&P A Sheet 120 can be an interesting solution.
f
FRP· t
FRPσ
x=
r
fC FRP: Concrete compressive strength after FRP confinement [N/mm2]
fc: Uniaxial concrete compressive strength [N/mm2]
σx: Horizontal concrete compressive stress resulting from confinement reinforcement with C fibres [N/mm2]
ε
cc=
ε
co·
[
1+5
{
f
C FRP-1
}]
f
Cε
co=
f
c60
Design table: FRP confinement of axially loaded columns
Example:
A cir
cular column Ø 300 mm r
e
quir
es its axial compr
essive str
ess capacity incr
eased fr
om
4.0 N/mm
2to 7.0 N/mm
2(+3.0 N/mm
2), safety factor 1.75:
Requir
ed 4
σ
x= 1.75 x 3.0 N/mm
2= 5.25 N/mm
21 layer of S&P C Sheet 240 (200 gr
./m
2) = 4 N/mm
2=>
2 layers ar
e r
equir
e
d
7.4
FRP confinement of axially loaded columns:
Application guidelines
Slot-application of S&P Laminate CFK Rounding of edges R =1-3 cm
Checking of flatness (max. deviation 1 mm per 30 cm) Application of S&P C Sheet 240
8.
Prefabricated FRP laminates
Prefabricated laminates are produced by the extrusion method. In a continuous process, the carbon fibres are soaked in epoxy resin and hardened through heating. For technical reasons, the extrusion method limits a maximum fibre content to approx. 70%. The elastic properties of a uni-directional layer can be calculated from the performance of the fibres and of the matrix. Since the modulus of elasticity and the tensile strength of the matrix can be neglected for the calculation of the laminate properties, the values are approx. 70% of the values of the carbon fibre. The S&P laminates are manufactured by a new method. For the production of S&P hybrid laminates various types of carbon fibres with different moduli of elasticity and different tensile strengths are used. Normally, the modulus of elasticity of a hybrid laminate does not have a linear progression. Carbon fibres with a high modulus of elasticity and a low ultimate elongation will break sooner than fibres with a low modulus of elasticity. In the hybrid, however, the C fibres with a higher elongation are prestressed during the production procedure. This produces a linear progressive modulus of elasticity in the laminate. Due to this hybrid technology, inex-pensive C fibres with a low modulus of elasticity can be used.
While the design for manual on-site lamination is based on the theoretical fibre cross section and the parameters of the fibres only, the design for application of prefabricated CFK laminates is based on the cross section and the parameters of the composite.
The production of the S&P Laminates CFK is subject to a strict quality control procedure in accordance with ISO 9001. Thus, the properties of the laminates are guaranteed by the manufacturer.
Properties
S&P Laminate 150/2000
S&P Laminate 200/2000
S&P Laminates CFK
Modulus of elasticity
165,000 N/mm2 205,000 N/mm2
for design
Ultimate tensile strength 2,700 - 3,000 N/mm2 2,400 - 2,600 N/mm2
Special laminates with a modulus of elasticity of 300,000 N/mm2can be custom-made to the specific
requirements of a project. However, the application is often not economical because of their low effec-tive tensile strength (~1,200 N/mm2). For the laminates with a modulus of elasticity equivalent to steel,
calculation rules for steel plate bonding are used. Due to the higher utilization of the tensile strength, this type of laminate is normally the most economical alternative.
S&P Laminate CFK
Thickness
of laminate
Detachment of the laminate Concrete Steel Concrete Steel min.
ε
(elongation of the laminate)
Concrete Steel
max.
ε
(elongation of the laminate)
8.1
Design for flexural strengthening using
S&P Laminates CFK
The bonding of an additional external S&P Laminate CFK on to the tensile stress zone of a structural element subject to bending is carried out with a system approved epoxy resin. Thus, a reinforced con-crete structure is produced with an elastic-plastic (steel reinforcement) and a perfectly elastic tensile element (S&P Laminate CFK). Models for the calculation of the load bearing capacity of the composite structure and of the anchor lengths were found by means of bond tests.
Related bond failure forces of all bond tests on CFK laminates, depending on the anchor length (TU Braunschweig, Germany)
The existing design model for the bond strength of reinforcements glued on to concrete is based on the non-linear mechanics of brittle fracture and is suited for any type of elastic laminate material. The appli-cability of the existing models was established during the bond tests carried out to obtain the approval for S&P Laminates CFK in several countries.
8.2
Approved elongation limit for design
Cracks in the exposed area of the concrete are necessary for the steel reinforcement to absorb the bending stresses. A high allowable elongation limit of the laminate permits large depth cracks. With a low allowable laminate elongation the crack depth remains minimal. Large crack depths lead to a detachment of the CFK laminates due to shear forces caused by displacements.
*
for mor
e details ask for the Expert Opinion by
Pr
of. Rostasy
, TU Braunschweig
Beam tests have demonstrated that the detach-ment of the laminate depends on the elastic elongation as well as on the plastic steel elon-gation. Failure of the internal reinforcement was initiated at an elongation of the CFK laminate of approx. 0.65% which corresponds to 5.7 times the yielding point of the internal steel. Failure of the structure occurred at an elongation of the S&P laminate of approx. 1.3%.
Measured plate tensile forces from local strains:
Theoretical and measured plate tensile forces in slab
The approved maximum elongation limit for design
of S&P Laminates should therefore be restricted to 0.6 - 0.8%.
Recommended tensile strength of the S&P Laminate for design:
Elongation limit for S&P Laminate 200/2000 S&P Laminate 150/2000
flexural design (E = 205,000 N/mm2) (E = 165,000 N/mm2)
0.6% 1200 N/mm2 1000 N/mm2
0.8% 1600 N/mm2 1300 N/mm2
In addition, it must be checked that the internal reinforcement of the structural element will not undergo plastic deformation in the service state. In practice, the tensile strength for design of the S&P Laminate CFK will often be restricted by the elongation limit of the internal reinforcement (DIN 0.5%, Euro Standard 1.0%).
The following diagram shows the procedure for dimensioning of the flexural strength with S&P Lami-nates CFK.
8.3
Procedure for design of flexural strength (diagram)
A Reinforcing factor V
B Design rules
η
η: Strengthening factorMv: Moment ultimate stress state (after strengthening) Mo: Moment ultimate stress state (before strengthening)
According to German approval
C Check of spacing
8.4
Flexural strengthening with S&P Laminates CFK:
Application guidelines
Delivery of S&P Laminates CFK Cutting of S&P Laminates CFK
Application of adhesive to laminate Pressing on of laminate
• Standard epoxy Moisture content substrate <4%
• Special adhesive for wet substrate Moisture content substrate >4%
• Special adhesive for low temperature Application up to -20°C
• Special adhesives for underwater applications Upon request
9.
Design software “flexural/shear”
for S&P FRP systems
Peter Onken, Wiebke vom Berg, Dirk Matzdorff; bow ingenieure, Braunschweig
9.1. General Introduction
S&P Lamella is an analysis program for reinforced concrete slabs and beams that have to be
strengt-hened with S&P FRP laminates to resist uniaxial bending. The program can be used both as a design tool for strengthening beams and slabs and as an analysis tool for checking existing designs. The pro-gram provides the user with the appropriate FRP laminates, internal forces and performs the necessary calculations for proving that a given design satisfies the requirements of a specific job.
The program was developed in Microsoft Visual Basic 6.0 for Windows '95, Windows '98 and Windo-ws NT operating systems. The program is windoWindo-ws-based both for input and output. The analysis results can be printed directly by a standard windows printer. Routines for automatic installation and deinstallation will be provided on the compact disc. The program is adjustable to different country con-figurations.
9.2. Analytical Basis
The program S&P Lamella has been developed for different national code systems as British
Stan-dard BS 8110, American Concrete Institute ACI 318 and Eurocode 2. For calculations following the
ACI code system, the program supports the European metric system as well as the American imperial system (psi). Additionally the program is based on the analytical approach of the German General
Approval for the Strengthening of Reinforced Concrete Members by Externally Bonded CFRP lamina-tes and also on the guidelines in the Rules for the Strengthening of Reinforced Concrete Members via External Bonding of Unidirectional Carbon Fiber Reinforced Polymer laminates, Draft Sept. 1998.
The prevailing national code can be chosen by the user at the start of the program as shown in the opening window (fig. 1).
Internal forces for the existing system to be strengthened must be determined initially by the user in a hand calculati-on or with the help, for example, of a separate structural analysis program. The design for unstrengthened and strengthened systems is performed ite-ratively by finding the internal force equilibrium of the concrete cross-section. Plane sections are assumed to remain plane and the constitutive models for concrete and steel corre-spond to non-linear stress-strain rela-tionships in accordance with the na-tional codes. Concrete is assumed to have no tensile strength.
9.3. Safety Concept
The program is following the safety concept of the different national standards according to the code chosen in the opening window. The safety concept of the provided national codes is based on partial safety factors for ultimate limit states, i.e. on one side the partial safety factors for actions or loads on building structures and on the other side the partial safety factors for materials. In the special case of the ACI 318 the partial safety factor on the material side is expressed by a reduction factor. The follo-wing table gives an overview.
Table 1: Partial safety factors for EC2, BS ACI
Loads materials
Code dead loads live loads concrete reinforcement
γG γQ γC γS
Eurocode 2 1,35 1,5 1,5 1,15
BS 8110 1,4 1,6 1,5 1,15
ACI 318 1,4 1,7 1 / 0,9
9.4. Program Sequence
At the beginning of the calculations the user has to enter the sectional area of the concrete structure, the existing reinfor-cement and the material properties of the concrete structure. Furthermore it is necessary to choose the desired type of CFRP strip (high or low modulus of elasticity) before starting the calculations.
The user then specifies the characteristic bending moment present in the reinforced concrete section at the time just prior to strengthening, i.e. what moment, MSk0, acts on the section at the time when the lamellas are bonded in place. Commonly this will be the dead load moment. This input defines the initial state of strain in the section. Next, the expected moment demand after strengthening MSdL must be
specified, which is corresponding to design loads. This procedure is shown schematically in figure 2.
The program calculates the design resistance of the unstrengthened section MRd0 and in dependence of the expected design moment MSdfthe degree of strengthening η. Furthermore the program provides the required cross-sectional area for the FRP laminates Af reqfor the strengthened section. Figure 3 depicts the superposition of the initial strain and additional demand in the strengthened state. The strains are assumed to have linear distribution.
After dimensioning the cross-sectional area of the external bonded FRP laminates additional windows provide the user with the ability to control the strain profiles for the ultimate limit state (ULS) and service limit state (SLS). Furthermore in additional proofs the program performs a bonding check for the anchorage of the FRP laminates and shear strength check of the strengthened structure. If necessary, the level of required externally bonded FRP sheets will be determined to increase the shear capa-city.
Fig. 2 Load cases before / during and after strengthening the concrete structure
The program calculates the necessary cross-sectional area of the CFRP strips for the ultimate limit state of the strengthened structure under the follo-wing condition
MRdf > MSdf
where MRdf indicates the design resi-stance (internal moment) of the strengt-hened section and MSdf indicates the design action or load of the streng-thened structure.
The serviceability check of the strengthened system is not provided by the program. In every case where it is neces-sary the serviceability, like the control of crack width and the deflection of structure has to be checked by the user.
Fig. 3 Superposition of the initial strain and addi-tional strain after strengthening
9.5. Input data
9.5.1 General
To enter the input data for the calculations the program supports the user by a special card file system. The card files are sorted in different blocks like geometry, reinforcement, material properties and load actions as depicted in figure 1. According to the different national codes the input fields for the materi-al properties are containing some default vmateri-alues. In some specimateri-al cases default vmateri-alues can only be changed by the provided enable key-button in the menu. These values may not be conform to the regulations of the national codes. For quick changes the program allows to enter input data in any card file in spite of the sequence of the card files. Special graphic elements in every card file will support the user to identify the required input data.
9.5.2 Geometry
The program supports the calculation of 3 different section types: slabs, rectangular beams and T-beams. After entering the dimensions of the concrete section it has to be considered if the calculations will perform for positive and negative moments, i.e. a moment of span or a moment at support. The input card file for the geometry of the concrete section is depicted in figure 4.
9.5.3 Rebars
The next card file contains the parameters of reinforcement. The program allows the input of 2 layers of rebars on the tension side and also includes possibility of compression rebars. The input card file for reinforcement is depicted in figure 5.
9.5.4 Steel –
Material Properties
The card file steel contains details of the reinforcement material properties for the tension layer, compression layer and stir-rups. According to the different national codes the input data fields for the material properties are containing default values. The reinforcement steel is assumed to be elastic-perfectly plastic as depicted in figure 6. The modulus of elasticity, the maximum steel strain and the partial safety factor for reinforcement can be changed if necessary by a special enable key-button.
Fig. 6 Steel input card file
Fig. 5 Rebars input card file (T-beam)
9.5.5 Concrete – Material Properties
In accordance to the CEB Model Code 90 concrete is assumed to have parabolic stress-strain characteristics up to a point where it is assumed to behave perfectly plastic, as shown in figure 7. The maximum strain for concrete subjected to uniaxial compression is limited as stated in the prevailing national codes. Also the concre-te classes and the partial safety factor for concrete are classified to these codes. The special enable button can be used to change the default values.
9.5.6 FRP – Material Properties
After entering the data of the concrete structure it is necessary to choose the de-sired type of FRP laminates. There are two types of FRP laminates available: S&P 150/2000 and S&P 200/2000. The FRP laminates are assumed to behave linear elastically (figure 8). The modulus of elasti-city will be chosen according to type of laminates. The strain limit states for the laminates are set according to the German General Approval for the Strengthening of Reinforced Concrete Members by Exter-nally Bonded CFRP laminates:
Fig. 8 FRP input card file εFmax = 5 fsyk/Es
εFmax = 0.0065 for S&P CFRP Strips 150/2000
εFmax = 0.0075 for S&P CFRP Strips 200/2000
9.5.7 Load
Before starting the calculation the design values for load actions have to be spe-cified. First of all the user must check the bending moment at time of strengthening MSk0(Fig. 1) which is assumed to be a cha-racteristic value. This bending moment is needed to evaluate the initial state of strain in the unstrengthened structure. For the calculation of the necessary cross-sectio-nal area for the laminates the input of the design moment after strengthening MSdf is required. The input value MSdfmust include the partial safety factors for combination of action.
Next the characteristic moment after strengthening MSkfis needed for the
calcu-lation of strain profile under service state conditions (SLS). This value can be entered exactly by the user or calculated from the program with an average load safety factor. The default value may be changed by the user. The input card file for the load is depicted in figure 9.
Fig. 9 Load input card file
9.6. Analytical Procedure and Dimensioning
After entering the required data the calcu-lations will be started by the calc button. The very exact iterative solution procedure, the use of non-linear material properties for both concrete and steel and the possibility to include the effects of compression rebars enable an especially economical allocation of FRP laminates. The necessary cross-sectional area for the strips will be determined by satisfying the equilibrium conditions for the internal forces (ΣH = 0,
ΣM = 0) through varying the state of the internal strain distribution.
An opening window (dimensioning card file as shown in figure 10) provides the user with the ability to choose the cross-section of FRP laminates and the number of lami-nates or especially for slabs the distance between the laminates. If the provided cross-sectional area of the laminates exceeds the required value the program conducts the proof: MRdf > MSdf. and the following card files will be enabled. Further the program is giving the degree of streng-thening h which indicates the ratio between the resistance of the strengthened section (MSdf) and the resistance of the
unstreng-thened section (MRd0).
Additional windows allow the control of the strain profiles for the ultimate limit state (ULS) and the service limit state (SLS). Fig. 10 Dimensioning output window
9.7. Additional Proofs
9.7.1 Bonding Check
The proof of bonding strength is performed according to regulations of the German General Approval for the Strengthening of Reinforced Concrete Members by Extern-ally Bonded CFRP laminates as a function of the substrate strength and the end point of the laminates. Therefore the beginning of the loaded portion of laminates (in tension) can be pinpointed depending on local moment distribution and also depending on staggered reinforcement. The proof that the bond strength is greater than the bond tensile force in the theoretical ultimate moment will be performed in a specific out-put window. Recommendations for the bonding length are given by the program.
9.7.2 Shear Design
The shear strength calculation is performed according to the rules of the different natio-nal codes. The existing stirrups are taken into account. The magnitude of demand will determine whether or not shear reinfor-cement is required. If necessary, the level of required externally bonded strap binder is determined either for steel material or by using S&P C Sheets 640. The program is given recommendations for the anchoring of the required strap binder.
9.8. Other Features
In addition to purely technical functions, the program offers the possibility to enter project and position numbers as well as descriptions. Data blocks can be loaded or saved as needed. The program also enables the user to input a letterhead that then appears at the top of every page of output. Input data, analysis results and strain profiles can be printed by a standard windows printer.
Fig. 12 Bonding output window
Sampel using S&P design software
Moment at time of
Cross-section
FRP strengthening
Moment due to service
(after FRP strengthening)
Moment due to design loads
(after FRP strengthening)
The code for the access to
the S&P Design Software is
replaced every year. Please
ask S&P or our regional
representative for the new
S&P Lamella
10. External shear reinforcement using FRP
Often when using CFK laminates for flexural strengthening, it is necessary to increase the shear capa-city. This can be achieved by using S&P C Sheets 640.
Shear force (V) must be resisted by the concrete (Vc), by the internal stirrups (Vs) and by the glued-on
S&P C sheet 640 (Vf). V = Vc+ Vs+ Vf
Usually in the International Codes Shear, V is controlled by a limit called Vr, that depends on the cross-section area. Assuming that V < Vr, two situations may be distinguished:
Situation 1: The existing internal shear reinforcement (plus concrete) is not sufficient to absorb the
shear force: Vf = V – Vc–Vs
In this case, the S&P C Sheets 640 must be anchored in the compression zone.
The S&P C Sheet 640 is anchored in the web, 150 mm above the neutral axis. According to some International Standards, encapsulation of the stirrups in the compression zone is required. Slots are cut into the plate in order to allow
Situation 2: The existing internal shear stirrups (plus concrete) are sufficient to absorb the shear force.
V < Vs+ Vc
In this case, the S&P C Sheet 640 must be dimensioned to transfer the shear force from below (from the flexural strengthening) up to the stirrups, in order to guarantee the complete compliance to the truss model (strut and tie).
So, the S&P C Sheet 640 doesn’t need to go up to the compression zone, but only to have a length that guarantees the overlapping with the stirrups beyond the centroid of the flexural reinforcement.
The S&P design software automatically calculates the cross section of stirrups made of steel or S&P C Sheet 640. These additional external shear stirrups can be replaced by external shear reinforcement with S&P C Sheets 640.
The maximum strain when S&P C Sheet 640
is used in shear applications shall not exceed 0.2-0.3 %,
in order to be compatible with elongation of
the internal stirrups.
100 mm
existing stirrups (shear reinforcement)
existing flexural reinforcement
flexural strengthening with S&P laminates CFK shear compatibility with S&P Sheet C 640
Technical data of S&P C Sheets 640 (external shear reinforcement)
Weight Theoretical Elastic modulus Breaking elongation
thickness of C fibres
400 gr/m2 0.190 mm 640´000 N/mm2 0.4%
Width of S&P C Sheet 640 Shear force absorption for design Potential replacing of (0.2%), 1 layer application ST37 stirrups according
to the design software
150 mm 30 · 103N per side 140 mm2(beam width 1 m)
300 mm 60 · 103N per side 280 mm2(beam width 1 m)
During a test series at Potsdam Technical University, the strengthening effect of FRP strips used to increase the resistance to shear forces of reinforced concrete structures was established.
11. Slot application of S&P Laminates CFK
The S&P Laminate CFK 10/1.4 with a width of 10 mm and a thickness of 1.4 mm is specially designed to be bonded into slots in concrete or timber structures. A concrete saw is used to cut slots approx. 3 mm wide and 10-15 mm deep into the substrate. The slots are filled with the system approved epoxy adhesive, and the S&P Laminates are pressed on edge into the adhesive.
The performance of slot-applied laminates has been tested at the Technical University in Munich. The test results positively proved that a good and uniform bond exists between the laminate and the con-crete. Furthermore, the high tensile strength of the laminate fibres was fully utilized prior to shear fail-ure between laminate and surface.
11.1 Bond tests
Tests: Comparison of slot-applied and surface-applied CFK laminates by means of bond strength tests.
The illustration shows the relative displacement in the static tensile strength test.
Strength-displacement ratios of CFK laminates bonded into slots versus CFK laminates bonded on to the surface (anchor length 25 cm)
The illustration clearly shows the considerably more ductile behaviour of the slot-applied laminate. The bond performance of the S&P Laminate corresponds to that of deformed reinforcing steel encased in concrete. However, the stiffness of the bond in the lower load range is higher than with surface-applied CFK.
At an anchor length of 25 cm, the slot-applied laminate
provided three times the tensile strength of the surface-applied
laminate with the same cross section.
These test results confirm those numerous investigations which have shown that the bond performance of reinforcement glued on to the surface is very brittle. The potential relative displacements up to fail-ure in the bonding area between the CFK laminate and the concrete are within a range of below 0.3 mm, while the potential displacements of deformed bars encased in concrete are in the range of 1 mm. Thus, the bond of encased reinforcement is clearly more ductile. This leads to a redistribution of forces between the surface-applied CFK laminate and the encased deformed steel. Therefore, this type of bond provides only a relatively low load transfer into the laminate. In addition, this load transfer is highly dependent on the concrete quality and, particularly, on the substrate bond strength.
The bond provided by slot-bonded CFK laminates has a
considerably higher load canying capacity. Within the range of
working loads, the bond performance is higher and, within the
range of failure loads, far more ductile compared to
surface-applied laminates.
11.2 Flexural tests on beams
Three point load tests with a span of 2.5 m were carried out on various reinforced concrete beams.
Each sample was either reinforced by a surface-applied CFK laminate 50/1.2 or by two slot-applied CFK laminates 25/1.2.
– On test beams A1 and B1 failure occurred due to debonding of the CFK laminate. – On test beam A2 failure occurred due to tensile fracture of the slot-applied laminate.
– On test beam B2, with a low shear reinforcement made of steel, shear failure occurred in the concrete.
The load-deflection curves of test beams A1 and A2 are shown in the illustration:
Interpretation of results from test beams A
At equal stiffnesses, the breaking load was more than doubled using the slot-applied laminate. This is due to the high utilization of the tensile strength of the CFK laminate.
The load-deflection curves of test beams B1 and B2 are shown in the illustration:
Interpretation of results from test beams B
The load-deflection curves are almost identical except that the slot-applied CFK laminate exhibited a substantially higher breaking load.
CFK Laminate
11.3 Benefits of slot-applied laminates
– The improved utilization of the laminate permits higher loads to be applied and laminate cross sec-tions to be reduced.
– The quality of the substrate (tensile strength of the surface) is less important. Slot-applied laminates can also transfer loads into substrates with a low bearing capacity (brickwork, masonry).
– The slot application is more economical than levelling and roughening required for surface-applied laminates.
– The slot-applied laminate is protected against mechanical damage. Better performance is achieved in the event of a fire, thus reducing the cost of fire protection measures.
11.4 Anchoring at the ends of the laminates
The S&P design software automatically calculates the required anchor length. In case of anchoring the laminates in a compression zone, no mechanical aid is needed. In the case of steep moment curves, under tension zone, the anchor length of the S&P laminate as indicated by the S&P design software is often insufficient. In this situation, two alternatives for anchoring at the laminate ends are available.
A) Strengthening of slabs
The forces are transferred from the laminate into the sub-strate through a stainless steel plate fixed with 4 - 6 steel bolts.
Tests demonstrate that with this type of anchoring the ten-sile force transferred from the laminate into the substrate can be doubled.
B) Strengthening of beams
The ends of the S&P Laminate CFK are wrapped with the S&P C Sheet 640. Thus, the forces are transferred into the web of the beam. Tests on bending beams, with and without confinement rein-forcement of the laminate ends, have been carried out at the University of Lisbon.
Testing arrangement
The test results show that due to the confine-ment reinforceconfine-ment of the beam at the ends of the laminates, the bending moment can be increased by approx. 20%.
12. Seismic retrofitting
Strengthening is often a necessary measure to overcome an unsatisfactory deficient situation or where a new code requires the structure – or a part of it – to be modified to achieve new requirements. For a structure to remain elastic under seismic action, typically associated with a 10% exceedance in 50 years, it has to be designed for lateral forces with magnitude in the order of 50% or more of its weight. As it will cost a lot of money, seismic design codes allow, as design philosophy, the development of sig-nificant inelastic response since the inherent deformations do not endanger the integrity of the individual members and of the structure as a whole.
Recent earthquakes in urban areas have repeatedly demonstrated the vulnerability of older structures to seismic deformation, not only the traditional masonry ones but also those made with reinforced concrete, particularly regarding the column - beam connection, showing deficient shear strength, low flexural ductility, insufficient lap splice length of the longitudinal bars and, very often, inadequate seismic detailing, as well as, furthermore, in many cases, very bad original design, with insufficient flexural ca-pacity.
Failure modes
For a good design of the seismic strengthening of an existing structure it is necessary to understand the seismic actions and the typical failure modes that can be observed under its load / deformation input. Regarding reinforced concrete frames, these modes could be summerized as follows:
• The most critical mode of failure is the column shear failure – see picture – where inclined cracking leads to the concrete cover spalling and to the rupture – or opening – of the stirrups. To prevent this brittle failure, the columns need to have guaranteed shear capacity both in its ends – potential plastic hinge regions, where concrete shear capacity can degrade with increasing ductility demands – and in the column center portion, between flexural plastic and/or existing built-in column hinge.
• Another mode consists of a confinement failure of the plastic hinge region, where subsequent to flexural cracking, cover concrete crushing and spalling, buckling of the longitudinal reinforcement or compression failure of concrete initiates pla-stic hinge deterioration, usually limited to shorter regions in the column. It can also happen that the column fails to the debon-ding of the lap splices of the longitudinal reinforcement. The associated flexural capacity degradation can occur rapidly at low flexural ductilities in cases where short lap splices are pre-sent and little confinement is provided.
• Regarding the horizontal elements, failure can also occur due to shear, near the plastic hinge regions, but also as a consequence of deficient flexural capacity, both on the top and bottom of the beams.
Methodology of strengthening
Many researchers have shown that a better confinement of concrete on the potential plastic hinge re-gions increases significantly the failure strain of concrete and therefore the overall ductility. The perform-ance of the S&P G Sheet 90/10 in ductility enhperform-ancement has been proven by push-pull tests (chap. 4.1). For this reason, retrofit methods typically utilise schemes for increasing the confining forces either in the potential plastic hinge regions or over the entire columns or beams span.
Advanced composites have shown to be technically just as effective as, and more economical than, conventional steel jacketing.
As a result of the confinement provided by the composite belts – either glass and carbon fibre epoxy impregnated sheets - wrap, concrete will fall at higher strains. The lateral pressure exerted by the com-posite will increase the compressive strength of concrete in both the core and shell regions, resulting in higher axial as well as lateral load carrying capacity. The lateral confinement guaranteed by the wrap will also provide additional support against buckling of the longitudinal bars.
S&P G Sheet 90/10 A
Round column D = 100 – 300 mm
1 – 3 layers of S&P G Sheet 90/10 A are required to achieve the maximum displacement level.
Square column D = 100 – 300 mm
(The edges must be rounded)
S&P G Sheet 90/10 B
Round column D = 300 – 600 mm
2 – 3 layers of S&P G Sheet 90/10 B are required to achieve the maximum displacement level.
Square column D = 300 – 600 mm
(The edges must be rounded)
The S&P engineering team will assist you with more information on static design.
Strengthening of masonry
Masonry structures or the masonry pieces of some constructions typically show cross cracking between overtures or rigid elements as a consequence of seismic actions.
