GUIDE_BS

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 development 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 elements 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 enhancement 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 ductility

S&P Laminate CFK

uni-directional

——— stretched (partly prestressed)

increase in stiffness

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 structural element and are often used in the seismic upgrading of concrete structures. Glass fibres with a modulus of elasticity of 70,000 N/mm2 are 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.

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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/mm2 is required. In the case of prefabricated 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 substrate.

Roughening of the surface by sandblasting or grinding.

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.

Types of fibres for S&P FRP systems Carbon Aramid e

2500

Glas s

2000

Type of fibre

1500

PE S

3.

1000

500

Steel

0

5

10

15

Carbon Aramid Glass Polyester (Steel

Modulus of elasticity GPa

Tensile strength N/mm2

240 - 640 124 65 - 70 12 - 15 210

2,500 - 4,000 3,000 - 4,000 1,700 - 3,000 2,000 - 3,000 250 - 550)

S&P Reinforcement manufactures custom-mode sheets of either a single fibre type or of fibre combinations (hybrids). The advantages and disadvantages of the various fibres are as follows:

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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 nonalkaline 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 combination with a water vapour permeable matrix.

AR-glass:

Alkali-resistant glass is suited to confinement reinforcement for concrete elements (increase in ductility) in combination with an epoxy resin matrix and a water vapour permeable 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: 5% acetic acid 10% hydrochloric acid 10% nitric acid

Chlorides: 10% calcium chloride 10% sodium chloride 10% potassium chloride

Nitrates: 10% calcium nitrate 10% potassium nitrate 10% sodium nitrate 10% ammonium nitrate

Sulphates: 10% calcium sulphate 10% sodium sulphate 10% potassium sulphate

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 manufacture 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 unidirectional 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 Freeze/thaw and de-icing salt resistance.

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4.

S&P FRP glass fibre systems

4.1

Seismic upgrading with S&P G Sheet 90/10 (Types A and B)

Lateral load

2000 mm

Actuator

200/200 mm column

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. Drift Ratio ∆/L (%)

Lateral load

-5

-4

-3

-2

-1

0

1

2

3

4

5

...... Reference column - - - Column wrapped with C Sheet (1kg/mm ) ––– Column wrapped with G Sheet 90/10 (Types A and B) (4kg/mm ) 2

Pull

Push

2

-5

-4

-3

-2

-1

0

1

2

3

4

5 Deflection

4.2 Protection of structures subject to explosion hazards with S&P G Sheet 50/50

Explosion S&P G Sheet AR 2-directional

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 orthogonal directions, can be applied in several layers. S&P offers also prefabricated GFK plates for the same purpose. These are glued on to the structural element with approved epoxy resin. Various test reports are available from S&P Reinforcement. The S&P specialty products have been tested for special protective structural elements for NATO.

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4.3

Repairs to historic buildings with S&P G Sheet 50/50 Combination of adhesives

S&P Sheet

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 microcracks. 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 homogeneous. 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 components 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 extrapolated 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 maximum stresses of less than 87.5% of the short-time tensile strength and amplitudes of up to 1,000 N/mm2, more than 4 · 106 load 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 fc and 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.

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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 rearrangements 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 duration 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 applications. 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.

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Plate type

αT; longit. [10-6K-1]

αT; transv. [10-6K-1]

S&P 150/2000

±0

30

S&P 200/2000

±0

20

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 potential 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 fluctuations 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 standard 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 System approved adhesives

Calculation of the water vapour permeability of the coating Sd = µ H2O · 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 transmission.

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.

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Example A

A total coverage for flexural reinforcement using S&P C Sheets is not recommended. Reinforcement using S&P Laminates CFK, allowing water vapour transmission between the laminates, is a better solution.

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 (anchoring/lap area)

• PU or acrylic adhesive (water vapour permeable area)

Due to the use of different matrices, 50% of the element's surface remain open to water vapour transmission.

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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 2 layers are required

Required 4σx = 1.75 x 3.0 N/mm2 = 5.25 N/mm2

A circular column Ø 300 mm requires its axial compressive stress capacity increased from 4.0 N/mm2 to 7.0 N/mm2 (+3.0 N/mm2), safety factor 1.75:

Example:

Design table: FRP confinement of axially loaded columns

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7.4

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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

Sanding of S&P C Sheet

Mineral-based final coating

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, inexpensive 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.

S&P Laminate CFK Thickness of laminate

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 Laminates CFK

S&P Laminate 150/2000

S&P Laminate 200/2000

Modulus of elasticity for design

165,000 N/mm2

205,000 N/mm2

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/mm2 can be custom-made to the specific requirements of a project. However, the application is often not economical because of their low effective 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.

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8.1

Design for flexural strengthening using S&P Laminates CFK

*

* for more details ask for the Expert Opinion by Prof. Rostasy, TU Braunschweig

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 concrete 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 applicability 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. Concrete Concrete

Concrete

Steel

Steel Steel

min. ε (elongation of the laminate)

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max. ε (elongation of the laminate)

Detachment of the laminate

Beam tests have demonstrated that the detachment of the laminate depends on the elastic elongation as well as on the plastic steel elongation. 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 flexural design

S&P Laminate 200/2000 (E = 205,000 N/mm2)

S&P Laminate 150/2000 (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 Laminates CFK.

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8.3

Procedure for design of flexural strength (diagram)

A Reinforcing factor V

η According to German approval η: Strengthening factor Mv: Moment ultimate stress state (after strengthening) Mo: Moment ultimate stress state (before strengthening)

B Design rules

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C Check of spacing

D Development length (anchor length)

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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

• • • •

Moisture content substrate 4% Application up to -20°C Upon request

Standard epoxy Special adhesive for wet substrate Special adhesive for low temperature Special adhesives for underwater applications

Box girder soffit reinforced with S&P Laminates CFK

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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 strengthened 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 program 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 Windows NT operating systems. The program is windows-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 configurations.

9.2. Analytical Basis The program S&P Lamella has been developed for different national code systems as British Standard 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 laminates 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).

Fig. 1 Opening window of the program

Internal forces for the existing system to be strengthened must be determined initially by the user in a hand calculation or with the help, for example, of a separate structural analysis program. The design for unstrengthened and strengthened systems is performed iteratively by finding the internal force equilibrium of the concrete crosssection. Plane sections are assumed to remain plane and the constitutive models for concrete and steel correspond to non-linear stress-strain relationships in accordance with the national 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 following table gives an overview.

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Table 1: Partial safety factors for EC2, BS ACI

Loads Code

materials

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

The program calculates the necessary cross-sectional area of the CFRP strips for the ultimate limit state of the strengthened structure under the following condition MRdf > MSdf

where MRdf indicates the design resistance (internal moment) of the strengthened section and MSdf indicates the ACI 318 1,4 1,7 1 / 0,9 design action or load of the strengthened structure. The serviceability check of the strengthened system is not provided by the program. In every case where it is necessary the serviceability, like the control of crack width and the deflection of structure has to be checked by the user.

9.4. Program Sequence At the beginning of the calculations the user has to enter the sectional area of the concrete structure, the existing reinforcement 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 MSdf the degree of strengthening η. Furthermore the program provides the required crosssectional area for the FRP laminates Af req for 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.

Fig. 2 Load cases before / during and after strengthening the concrete structure

Fig. 3 Superposition of the initial strain and additional strain after strengthening

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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 capacity.

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 material properties are containing some default values. In some special cases default values 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. Fig. 4 Geometry input card file (T-beam)

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.

Fig. 5 Rebars input card file (T-beam)

Fig. 6 Steel input card file

9.5.4 Steel – Material Properties The card file steel contains details of the reinforcement material properties for the tension layer, compression layer and stirrups. 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.

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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 concrete 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. Fig. 7 Concrete input card file

9.5.6 FRP – Material Properties

Fig. 8 FRP input card file

After entering the data of the concrete structure it is necessary to choose the desired 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 elasticity 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 Externally Bonded CFRP laminates: εF max = 5 fsyk/Es εF max = 0.0065 for S&P CFRP Strips 150/2000 εF max = 0.0075 for S&P CFRP Strips 200/2000

9.5.7 Load

Fig. 9 Load input card file

28

Before starting the calculation the design values for load actions have to be specified. First of all the user must check the bending moment at time of strengthening MSk0 (Fig. 1) which is assumed to be a characteristic value. This bending moment is needed to evaluate the initial state of strain in the unstrengthened structure. For the calculation of the necessary cross-sectional area for the laminates the input of the design moment after strengthening MSdf is required. The input value MSdf must include the partial safety factors for combination of action. Next the characteristic moment after strengthening MSkf is needed for the calculation 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.

9.6. Analytical Procedure and Dimensioning

Fig. 10 Dimensioning output window

After entering the required data the calculations 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 laminates 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 strengthening h which indicates the ratio between the resistance of the strengthened section (MSdf) and the resistance of the unstrengthened section (MRd0).

Additional windows allow the control of the strain profiles for the ultimate limit state (ULS) and the service limit state (SLS).

Fig. 11 Strain output window (ULS)

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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 Externally 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 output window. Recommendations for the bonding length are given by the program.

Fig. 12 Bonding output window

9.7.2 Shear Design The shear strength calculation is performed according to the rules of the different national codes. The existing stirrups are taken into account. The magnitude of demand will determine whether or not shear reinforcement 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.

Fig. 13 Shear output window

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.

42

Sampel using S&P design software Cross-section

Moment at time of 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 valid code.

43

S&P Lamella – bending tension reinforcing with CFK Lamella –

44

45

46

10. External shear reinforcement using FRP Often when using CFK laminates for flexural strengthening, it is necessary to increase the shear capacity. 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 crosssection 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.

Slots are cut into the plate in order to allow complete wrapping with S&P C Sheet 640 strips.

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.

47

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.

100 mm

existing stirrups (shear reinforcement)

existing flexural reinforcement

flexural strengthening with S&P laminates CFK shear compatibility with S&P Sheet C 640

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.

48

Technical data of S&P C Sheets 640 (external shear reinforcement) Weight

Theoretical thickness

Elastic modulus of C fibres

Breaking elongation

400 gr/m2

0.190 mm

640´000 N/mm2

0.4%

Width of S&P C Sheet 640

Shear force absorption for design (0.2%), 1 layer application

150 mm

30 · 103 N per side

Potential replacing of ST37 stirrups according to the design software 140 mm2 (beam width 1 m)

300 mm

60 · 103 N 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 concrete. Furthermore, the high tensile strength of the laminate fibres was fully utilized prior to shear failure between laminate and surface.

11.1 Bond tests

Tests:

Comparison of slot-applied and surface-applied CFK laminates by means of bond strength tests.

49

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 surfaceapplied 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 failure 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.

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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.

CFK Laminate

Beam sections

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.

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11.3 Benefits of slot-applied laminates – The improved utilization of the laminate permits higher loads to be applied and laminate cross sections 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 substrate through a stainless steel plate fixed with 4 - 6 steel bolts.

Tests demonstrate that with this type of anchoring the tensile 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 reinforcement of the laminate ends, have been carried out at the University of Lisbon.

Testing arrangement

The test results show that due to the confinement reinforcement of the beam at the ends of the laminates, the bending moment can be increased by approx. 20%.

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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 significant 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 capacity.

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 plastic hinge deterioration, usually limited to shorter regions in the column. It can also happen that the column fails to the debonding 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 present and little confinement is provided.

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• 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 regions increases significantly the failure strain of concrete and therefore the overall ductility. The performance of the S&P G Sheet 90/10 in ductility enhancement 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 composite 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.

54

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.

56

First strengthening concept Regarding masonry wall strengthening is usually done by the application of S&P Laminates CFK, assuming x or + cross styles, depending on each case. Care must be taken in detailing the intersection of the strips. The method was developed at EMPA, Switzerland, to upgrade masonry buildings in seismicly endangered areas using FRP material. CFK laminates are bonded diagonally to the shear walls. Both ends of each laminate must be anchored into the existing concrete slab. In practice a slot is cut into the concrete slab. The slot is filled with a system approved epoxy paste and the laminate is placed into it. At EMPA Switzerland shear walls 3.60 x 2.00 m were subsequently loaded cyclically in the horizontal direction at the upper edge of the walls. The earthquake resistance could be increased with the CFK laminates strengthened walls by a factor of over 4.

Picture and diagram: Dissertation ETH Nr. 10672, G. Schwegler ”Verstärken von Mauerwerken mit Faserverbundwerkstoffen”.

Second strengthening concept The masonry can be strengthened with the S&P G Sheet 50/50. The bi-directional glass woven must be applied to the full surface only on one side of the masonry. If the product is not anchored into the adjoining concrete slabs, the earthquake resistance can only be increased minimally. The best solution is a combination of the first and the second method. Thanks to the S&P G Sheet 50/50 the cracks can be distributed uniformly over the masonry surface. The earthquake resistance of the masonry wall can be increased. Thanks to the release of the CFK laminates and due to the end anchoring into the concrete slabs, the system ductility can be increased.

Design should consider the possibilities of failure in each member as well as in the structure, as a whole, applying appropriate local and global criteria. In any case, strengthening must be considered both for the resistance – shear and flexural (bending and axial interaction) capacities – and ductility considerations. A coefficient considering the non-monolithic behaviour γn = 0,9 must be adopted, and it must be guaranteed that the effects of cyclic loads as well as the formation of plastic hinges are addressed.

57

13. Prestressing System S&P laminate CFK S&P has developed a prestressing system for S&P Laminates CFK and has applied for a patent. The system is exclusively used by a world-wide network of specialized applicators. 햲 Fixed anchoring: (A steel plate is fastened to the concrete suface by means of heavy-duty anchors).

The S&P laminate CFK is glue-applied between the steel plate and the concrete surface.

햳 Mobile anchoring: (The S&P laminate CFK is glued between two steel plates, held together by means of screws.).

After the final setting of the adhesive of plate (3) the mobile anchorage (plate 2) is removed (cut off).

햴 Prestressing system: (The prestressing kit is fastened to a steel plate applied to the concrete surface by means of anchors).

The S&P laminate CFK is applied to the concrete by adhesive, finally setting after the prestressing process.

During a test series carried out at the University of Freiburg (Switzerland) the behaviour of a concrete slab reinforced with untensioned and prestressed S&P Laminates CFK was investigated. The following is a summary of the breaking tests performed on the Laminate Type 80/1.2. For a detailed report please contact S&P or the system applicator.

13.1 Testing Arrangement

Steel longitud. vertical

58

Reference LC1

6 Ø 12

(Ø8 s =150)

LC5

6 Ø 12

(Ø8 s =150)

LC2

6 Ø 12

(Ø8 s =150)

LC4

6 Ø 12

(Ø8 s =150)

FRP

Prestressing 0 /00

Prestressing stress (N/mm2)

Prestressing force (kN)

0

0

0

0

0

0

0

4.0

640

2x61 = 122

6.0

960

2x92 = 184

2 S&P laminate CFK 150/2000, Typ 80/1.2

13.2 Test results Under working load the test specimens which were reinforced with prestressed S&P Laminates CFK exhibited a high reduction in the deflection and the crack widths. Due to the overriding normal force the concrete cross sections remain in the unbroken condition I, up to and above the maximum working loads. The failure condition shows a high increase in the breaking load, at substantially higher deflection levels compared to LC5 specimens. Samples with untensioned CFK laminates show an increase in the breaking resistance of 32%, while prestressed CFK laminates raised the breaking resistance by 82% at a prestress of 4‰ and by 93% at a prestress of 6‰. The breaking elongation measured on the prestressed S&P Laminates CFK was practically increased by a factor corresponding to the respective prestressing force. Research by University of Fribourg, CH

Table: Failure load/Failure moment S&P laminate CFK

Concrete slabs

Failure load (kN)

Failure moment (kNm)

Failure moment (%)

–

LC1 Reference

16.4

82.6

100

2 x 80/1.2

LC5 FRP

24.0

109.4

132

2 x 80/1.2

LP2 FRP 40/00

35.3

150.1

182

2 x 80/1.2

LP4 FRP 6 /00

37.9

159.4

193

0

13.3 Conclusion

35.0 30.0

Load (kN)

Prestressing of the S&P Laminates CFK has a very positive influence on the behaviour of strengthened reinforced concrete structures. Deflection and crack formation under working load are reduced. The breaking moment is substantially increased.

40.0

25.0

/

0 00

0.6 FRP LP4

00 0.4 / FRP LP2 0

FRP LC5

20.0 LC1 RC

15.0 10.0 5.0

With the lightweight S&P prestres0 sing kits it is possible to prestress 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 the S&P Laminates CFK to an elonDeflection (mm) gation of 6‰. The system has been specially developed for the strengthening of reinforced large-span concrete slabs.

160.0

180.0 200.0

There is a high potential of applications in bridge construction: • Post-reinforcement of overloaded elements • External prestressing of bridges with corroded internal prestressing cables • Repairs to connection joints

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14. Outlook for the application of S&P Laminates CFK in timber construction It is a well-known fact that strength and stiffness of glue-laminated timber girders vary considerably. This is due to the different tensile strengths of the individual layers. For this reason, the allowable flexural strength of glue-laminated girders is reduced. Tests were carried out by Wiesbaden Technical University (Germany) to evaluate the reinforcement of glue-laminated girders with S&P Laminates CFK. To this end, three series of glue-laminated girders with different height/length ratios, with and without S&P Laminates CFK, were produced and compared. The S&P Laminate CFK 200/2000 was applied between the support points only, not outside. Height/length ratio and reinforcement level of girder series A to C The objective of the tests was to bridge weak points in the glue-laminated girders, such as dovetail joints and knots, with S&P Laminates CFK. The high modulus of elasticity of the laminate was used to increase the flexural strength of the glue-laminated girder/S&P Laminate composite. Thus, the cross section of the timber girder can be reduced to obtain the identical flexural strength.

14.1 Flexural tests / Arrangement of beams

The tests were carried out using a 1,000 kN bending test machine. To avoid tilting, the beams were laterally fixed to the support and the loading points. Static system and loading device:

Loading cycle This testing procedure basically consisted of a bearing system which was subjected to a defined loading pattern during a defined period of time. The deformations measured at given intervals were continuously registered and recorded for subsequent evaluation.

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Arrangement of measuring points The deflections on the loading points and at mid-span of the beam were measured by means of loaddeformation detectors. The strain at mid-span of the beam across the entire cross section of the beam and the CFK laminate was measured using strain gauges.

Test results at breaking load FAILURE Girder type

PERIPHERAL STRAIN in the timber section

Force (kN)

Deflection (mm)

Pressure (mm/m)

Tension (mm/m)

A series B

50

without CFK

168.7

56.4

-3

3.1

BC

50

with CFK

169.3

49.1

-2.55

2.4

0.4

12.9

0.2

3.8

Difference (%) B series B

30

without CFK

56.2

68.8

not measured

2.6

BC

30

with CFK

95.1

112.6

not measured

4.8

69.2

63.7

n/a

84.6

Difference (%) C series B

36

without CFK

38.3

119.3

-2.2

2.35

BC

36

with CFK

62.6

183.1

-3.82

3.4

63.4

53.5

73.6

44.7

Difference (%)

Summary The examination of the loading of flexural beams up to rupture provided information on the load bearing capacity and the performance under loading of the composite structure in the plastic state. It was found that the increase of the load bearing capacity was substantial. In one of the tests the breaking load of the fibre reinforced girder could be raised by up to 93% compared to conventional gluelaminated girders. With a reinforcement level of 0.47%, this is a considerable increase.

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A stress/strain diagram was made based on the strain measurement in the timber section and the laminate. The measurement of the strains in the composite cross section should indicate from which stress level the timber girder changes into the plastic state and also if the CFK laminate is utilized in its ultimate tensile strength range. In a second part of the investigation of this system, design concepts are defined and applied. The concepts should basically be designed to ensure that in the event of a failure of the CFK reinforcement the remaining cross section is able to absorb the assumed load with a safety factor of 1.1. Stress/strain diagram: Girder cross section, dimensions w/h

Strain line in the failure state, girder series B and C

Stresses in the timber section and the laminate

Both girder series with a higher height/length ratio (B and C series) displayed a very elastic deformation behaviour in the compression zone. In the case of the A series with a lower height/length ratio, flexural failure of the timber could likewise be observed on the reference sample without fibre reinforcement. The fibre reinforced samples, by contrast, showed a different failure mechanism. As a result of the high load application, the girder cross section failed due to shear displacements: first in the support area and then over the entire length of the girder. This means that the change in the failure mechanism was caused by the reinforcement of the girder. In this case, it could be possible to further increase the breaking load by means of external steel jackets. Note: Currently, a further testing programme is in progress: In order to prevent shear displacements in the support area, additional S&P Laminates CFK are applied into slots on the outside of the glue-laminated girder.

14.2 Bond tests FRP application to timber The BOKU University in Vienna has conducted tensile tests to examine the strengths of S&P Laminates CFK applied to timber structures. Samples for testing of the mean shear strengths

S&P Laminates CFK were glued in parallel to the fibres onto one face of the timber beams, and the shear strengths [tm] of the applied CFK laminates were measured. The following parameters were varied: adhesive, laminate surface (rough, smooth), application areas of the adhesive.

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Adhesive (kN)

Fm [kN]

tm (N/mm2)

Type of failure

Epoxy resin adhesive - type A

6,71

5,59

Failure in timber, scarcely any adhesive failure

Epoxy resin adhesive - type E

6,75

5,63

Failure in timber, scarcely any adhesive failure

Resorcinol-phenolic adhesive type C

5,00

4,12

Adhesive failure

Resorcinol-phenolic adhesive type D

3,56

2,97

Adhesive failure

Application of adhesive: • Ambient temperature: 20°C • Relative humidity of air: 30% • Moisture content equilibrium of timber: approx. 7% The suitability of the system approved epoxy adhesive on dried timber was established during these tests.

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15. Fire protection measures for FRP strengthened elements When strengthening with laminates made of steel or CFK, one must consider that the heat resistance of epoxy based adhesives is limited to temperatures between 60° and 80°C (140° and 180°F). In the event of a fire, this leads to a premature failure of the laminate. Therefore, measures must be taken to protect S&P Laminates CFK against premature failure.

In the event of a failure of the CFK laminate, the residual safety factor S is decisive for the necessary measures for fire protection. The S&P design software automatically outputs the residual safety factor in the case of failure of the S&P Laminate CFK. Residual safety in the event of laminate failure

S < 1.0

S > 1.0

Fire resistance ensured by internal reinforcement

Conventional fire protection for internal reinforcement only

Example: Fire protection plates

Additional protective measures for S&P Laminates CFK are required!

Concrete ceiling

A

A

D

S&P Laminate CFK

Fire resistance

A = 100 mm (D)

A = 200 mm (D)

F 30

2 x 20 mm

2 x 20 mm

F 60

2 x 40 mm

2 x 30 mm

F 90

> 110 mm

2 x 40 mm

F 120

> 110 mm

> 110 mm

Fire protectio n

plates

Please contact the S&P Technical Service for detailed documentation. Fire protection plates

64

16. Texts for specifications 16.1 Flexural strengthening with S&P Laminates CFK Item

Text

1

Installation Transport (to and from) and supply of necessary appliances and materials for the reinforcing job. Additional site visits.

2

3

Unit

Scaffolding Installation and supply of scaffolding, incl. required measures for dust protection during the construction phase.

4.1 5

6

6.1

6.2

P

0000 Pcs.

0000

P

0000 0000 0000 0000

m m m m

0000 0000

h kg

0000

m2

0000 0000 0000 0000

m m m m

Substrate preparation Removal of cement skin by sandblasting, incl. cleaning with brush or vacuum cleaner. Theoretical application surface + 20% excess: Laminate width 50 mm Laminate width 80 mm Laminate width 100 mm Laminate width 120 mm

4

0000

Levelling of substrate Removal of local excess profile and reprofiling of cavities and unevennesses in the application area with system approved levelling mortar. Bill of quantities, based on actual consumption Labour System approved levelling mortar If the substrate is poor, priming of the concrete surface is recommended. Priming with system approved primer. S&P Laminates CFK Delivery and application of S&P Laminates CFK with system approved epoxy adhesive according to the application guidelines of the system supplier: Elastic modulus CFK laminates: Tensile strength at 0.8% elongation: Elongation at break CFK laminates: Product names:

>200,000 N/mm2 >1,600-1,700 N/mm2 150 GPa

2500 N/mm2

> 200 GPa

2500 N/mm2

Width/thickness mm/mm 50/1.2

Tensile force at elongation of 0.6/0.8 % 58/77 x 103 N

Width/thickness mm/mm

Tensile force at elongation of 0.6/0.8 %

50/1.4

84/112 x 103 N

80/1.4

134/179 x 103 N

100/1.4

168/224 x 103 N

120/1.4

201/269 x 103 N

50/1.4

67/90 x 103 N

80/1.2

92/123 x 103 N

80/1.4

108/143 x 103 N

100/1.2

115/154 x 103 N

100/1.4

134/179 x 103 N

* special dimensions upon request Delivery in rolls of 150 m or ready-to-use (unroll set available upon request)

Levelling mortar:

Adhesive:

Only approved systems

Only approved systems

S&P G Sheet 90/10 S&P C Sheet 240 S&P C Sheet 640 S&P A Sheet 120 or G Sheet 50/50 2-directional

uni-directional

uni-directional

uni-directional

Modulus of elasticity (N/mm2)

> 65’000

240’000

640’000

120’000

Tensile strength (N/mm2)

> 1’700

3’800

2’650

2’900

Fibre density (kg/cm3)

2.6

1.7

2.1

1.45

Ultimate elongation (%)

2,8

1,55

0,4

2,5

different types available

200 300

400

290 420

Weight (gr./m2)

Delivery in rolls

Width 680 mm Lenght 50 m

Width 300 mm Width 300 mm Lenght 150 m or more Lenght 50 m or more

Adhesives in the overlapping/anchoring zone:

Upon request:

Primer and impregnation adhesive: only approved systems

Special adhesive Special adhesive Special adhesive

Width 300 mm Length 150 or more

for wet substrates for low temperatures for underwater applications

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18. Literature • Versuche zur Bestimmung der Werkstoffeigenschaften, der Verbundtragfähigkeit, des Entkoppelungsverhaltens von CFK-Lamellen sowie Biegeschubversuche an mit CFK-Lamellen verstärkten Platten, Untersuchungsbericht Nr. 8524/5247 des Institutes für Baustoffe, Massivbau und Brandschutz der TU Braunschweig vom 20.05.1998. • Dolan. C.W., Leu, B.L., Hundley, A.: Creep rupture of fiber reinforced plastics in a concrete environment. Proceedings of the Third International Symposium on Non-Metallic (FRP) reinforcement for Concrete Structures, Sapporo, October 1997, 2, 187 - 194. • Ando, N., Matsukawa, H., Hattori, A., Mashima, M.: Experimental studies on the long – term tensile properties of FRP-tendons. Proceedings of the Third International Symposium on Non-Metallic (FRP) reinforcement for Concrete Structures, Sapporo, October 1997, 2, 203 - 210. • Uomoto, T., Nishimura, T., Ohga, H.: Static and fatigue strength of FRP rods for concrete reinforcement. Non-Metallic (FRP) reinforcement for Concrete Structures, Proceedings of the Second International RILEM Symposium (FRPCS-2), Gent, August 1995, 100-1070. • Adini, R., Rahman, H., Benmokrane, B., Kobayashi, K: Effect of temperature and loading frequency on the fatigue life of a CFRP-bar in concrete. Fiber Composites in Infrastructure, Proceedings of the Second International Conference on Composites in Infrastructure, ICCCI 98, Tucson, Jan. 1998, 2, 203 – 210. • Sheard, P., Clarke, J., Dill, M., Hamemrsley, G., Richardson, D.: Eurocrete – Taking account of durability for design of FRP reinforced concrete structures. Proceedings of the Third International Symposium on Non-Metallic (FRP) reinforcement for Concrete Structures, Sapporo, October 1997, 2, 75 – 82. • Porter, M.L., Mehus, J., Young, K.A., O’Neil, E.F., Barnes, B.A.: Aging for fiber reinforcement in concrete. Proceedings of the Third International Symposium on Non-Metallic (FRP) reinforcement for Concrete Structures, Sapporo, October 1997, 2, 59 - 66. • Kaiser, H.: Bewehren von Stahlbeton mit kohlenstofffaserverstärkten Epoxidharzen. Dissertation Nr. 8918, ETH Zürich, 1998. • Terrasi, G.P., Kaiser, H.: CFK-Lamellen verstärkte Querschnitte bei Temperatursprüngen, nachträgliches Verstärken von Bauwerken mit CFK-Lamellen, Referate und Beiträge zur EMPA/SIAStudientagung vom 21. September 1995 in Zürich, Dokumentation SIA D 0128 • Terrasi, G.P.: Aluminium/CFK Hybride unter thermischer Beanspruchung. Diplomarbeit an der ETH Zürich, Abt. für Werkstoffe. • Vielhaber, Joh.: Versuchsreihe Verstärkung der Querkrafttragfähigkeit von Stahlbetonkonstruktionen, Fachhochschule Potsdam. • Holzenkämpfer, P.: Ingenieurmodell des Verbunds geklebter Bewehrung für Betonbauteile. Dissertation TU Braunschweig, 1994. • Pichler, D.: Die Wirkung von Anpressdrücken auf die Verankerung von Klebelamellen. Dissertation, Universität Innsbruck Institut für Betonbau, 1993. • Rostasy, F.S., Holzenkämpfer, P., Hankers, Ch.: Geklebte Bewehrung für die Verstärkung von Betonbauteilen. Betonkalender 1996, Teil II, S. 547 – 638, Berlin: W. Ernst & Sohn, 1996. • Verbundversuche an Doppellaschenkörpern mit CFK-Lamellen und Biegeversuche an mit CFKLamellen verstärkten Platten, Untersuchungsbericht Nr. 8511/8511 -Neu- des Institutes für Baustoffe, Massivbau und Brandschutz Braunschweig vom 05.11.1996. • Täljsten, B.: Plate Bonding, Strengthening of existing concrete structures with epoxy bonded plates of steel or fibre reinforced plastics. Dissertation, Lulea University of Technology, Lulea, 1994. • Fracture mechanics of Concrete Structures, From Theory to applications, Report of the Technical Committee 90-FMA Fracture Mechanics to Concrete – Applications, Capman and Hall, London, New York, 1989, 188 – 190. • Rostasy, F.S., Ranisch, E.-H.: Sanierung von Betontragwerken durch Ankleben von Faserverbundwerkstoffen. Forschungsbericht, Institut für Baustoffe, Massivbau und Brandschutz der Technischen Universität Braunschweig, Dezember 1994. • Statische Belastungsversuche an mit CFK-Lamellen verstärkten Plattenbalken “Instandsetzung Fürstenlandbrücke”, EMPA-Bericht Nr. 165'432 vom 28.04.1997.

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• Biege- und Schubtragverhalten eines mit geklebten CFK-Lamellen und Stahllaschenbügeln verstärkten Stahlbetonträgers, Untersuchungsbericht Nr. 8516/8516 Neu- des Instituts für Baustoffe, Massivbau und Brandschutz der Technischen Universität Braunschweig vom 13.05.1996. • Tagung Kreative Ingenieurleistungen TU Darmstadt, BOKU Wien, Referate: Meier Urs, EMPA CH: Unidirektionale CFK Profile im konstruktiven Ingenieurbau Scherer Josef, S&P Reinforcement: Bewehrungsalternativen aus CFK in der Bauwerkverstärkung Luggin Wilhelm, BOKU Wien: Verstärkung von Holzbauteilen mit geklebten CFK Lamellen Zilch Konrad, Blaschko Michael, TU München: Verstärkung mit eingeschlitzten CFK Lamellen • Fachveranstaltung “Zusätzliche Beanspruchung für bestehende Bauwerke: Verstärken mit Lamellen und durch Querschnittsvergrösserung”, TFB Wildegg/CH: Scherer Josef, S&P Reinforcement: Anwendungsmöglichkeiten von Faserverbundwerkstoffen in der Bauwerkverstärkung Bronner J., Tenax Fibers: Herstellverfahren / Technische Eigenschaften der Kohlefaser Hankers Ch. Dr., Torkret: Bemessungsbeispiel “Schubverstärkung mittels Stahlbügeln resp.Carbontücher” Ladner M. Prof., Hochschule Technik + Architektur: Bemessungsansätze für die Nachverstärkung von Druckelementen mittels Faserverbundwerkstoffen • Suter René, Hértier Christof: Rapport Nr. 6C-1999-02, Ecole d’ingénieurs et d’architectes de Fribourg • Forschungsergebnisse faserverstärktes Leimholz FH Wiesbaden, BOKU Wien • ACI Spring Convention March 14 – 19, 1999, Chicago: Scherer Josef, S&P Reinforcement: Fiber reinforced polymer FRP systems for external strengthening of concrete structures • Taerwe L., Matthys S., Universiteit Gent: Inrijgen van Betonkolomnen met vezelcomposietlaminaten. • Allgemeine bauaufsichtliche Zulassung Deutschland-236.12-54 für S&P Lamellen CFK • Deutsches Institut für Bautechnik: Gutachten Nr. 98/0322 Ingenieurbüro Prof.Dr.Ing.Dr.Ing.E.h.F.S. Rostasy, Braunschweig • Bemessungsdiskette BOW Ingenieure, Braunschweig • Procédé “Carbone CFK”, Dossier SOTEL No EX 1443 • T. Pauly and M.J.N. Priestly: Seismic design of reinforced concrete and masonry buildings • M.J.N. Priestly, F. Seible and G.M. Calvi: Seismic design and retrofit of bridges • Procédé “Carbone CFK”, Dossier SOTEC No. Ex 1443 • G. Schwegler: Dissertation ETH Zürich CH 10672 • T. Ripper, J. Scherer: "AVALIAÇÃO do DESEMPENHO de PLÁSTICOS ARMADOS com FOLHAS UNIDIRECCIONAIS de FIBRAS de CARBONO como ELEMENTO de REFORÇO de VIGAS de BETÃO ARMADO", IBRACON 41st Congress, 1999, Salvador, Bahia, Brasil.

Patents: S&P Reinforcement Company has applied world-wide for various patents in the field of FRP reinforcement: • Application of water vapour permeable FRP systems using a combination of adhesives for the reinforcement of constructional elements. • Use of AR glass with FRP systems, for the reinforcement of constructional elements. • Application of fast-setting adhesives in combination with CFK laminates, for the post-reinforcement of load-relieved constructional elements. • Application of special adhesives in combination with FRP systems onto wet substrates. • Application of APS adhesives (up to -20˚C) in combination with CFK laminates. • Production of prestressed CFK hybrids and application of these hybrids for the reinforcement of constructional elements. • Prestressing system for CFK laminates. • Anchoring elements.

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