Outlook for the application of Laminates CFK in timber

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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 flex-
     ural 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 be-
     tween 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 lami-
     nate was used to increase the flexural strength
     of the glue-laminated girder/S&P Laminate com-
     Thus, the cross section of the timber girder can
     be reduced to obtain the identical flexural

     14.1 Flexural tests / Arrangement of beams

                                                                  The tests were carried out using a 225 kips
                                                                  (1,000 kN) bending test machine. To avoid til-
                                                                  ting, 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 deforma-
     tions measured at given intervals were continuously
     registered and recorded for subsequent evaluation.

Arrangement of measuring points

The deflections on the loading points and at mid-span of the beam were measured by means of load-
deformation 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 – metric table

                                           FAILURE                     PERIPHERAL STRAIN
                                                                       in the timber section
  Girder type                      Force          Deflection          Pressure          Tension
                                    (kN)            (mm)               (mm/m)           (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
  Difference (%)                    0.4              12.9                0.2               3.8
  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
  Difference (%)                    69.2             63.7                n/a              84.6
  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
  Difference (%)                    63.4             53.5                73.6             44.7

The examination of the loading of flexural beams up to rupture provided information on the load bear-
ing 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 fiber reinforced girder could be raised by up to 93% compared to conventional glue-
laminated girders. With a reinforcement level of 0.47%, this is a considerable increase.

     A stress/strain diagram was made based on the strain measurement in the timber section and the lami-
     nate. 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,       Strain line in the failure state,               Stresses in the timber section
     dimensions w/h              girder series B and C                           and the laminate

     Both girder series with a higher height/length ratio (B and C series) displayed a very elastic deformation
     behavior 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 fiber reinforcement.
     The fiber 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.

     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

     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 fibers onto one
                                              face of the timber beams, and the shear strengths [tm] of the
                                              applied CFK laminates were measured. The following parame-
                                              ters were varied: adhesive, laminate surface (rough, smooth),
                                              application areas of the adhesive.

 Adhesive                           Fm                 tm            Type of failure
 (kN)                               kips (kN)          ksi (N/mm2)
                                                                     Failure in timber,
 Epoxy resin adhesive - type A       1.5 (6,71)        0.8 (5,59)
                                                                     scarcely any adhesive failure
                                                                     Failure in timber,
 Epoxy resin adhesive - type E       1.5 (6,75)        0.8 (5,63)
                                                                     scarcely any adhesive failure
 Resorcinol-phenolic adhesive      1.12 (5,00)         0.6 (4,12)    Adhesive failure
 type C
 Resorcinol-phenolic adhesive        0.8 (3,56)    0.43 (2,97)       Adhesive failure
 type D

Application of adhesive:

• Ambient temperature: 68˚F (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


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