S&P laminate CFK Concrete slabs Failure load Failure moment Failure moment
(kN) (kNm) (%) – 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 60/ 00 37.9 159.4 193
13.3 Conclusion
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 substanti- ally increased.
With the lightweight S&P prestres- sing kits it is possible to prestress the S&P Laminates CFK to an elon- gation of 6‰. The system has been
specially developed for the strengthening of reinforced large-span concrete slabs.
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
40.0 35.0 30.0 25.0 20.0 15.0 10.0 5.0 0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 Deflection (mm) Load (kN) LP4 FR P 0.6 0/00 LP2 FRP 0.40/00 LC5 FR P LC1 RC
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- posite.
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 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
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
Summary
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 fibre 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 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 parame- ters were varied: adhesive, laminate surface (rough, smooth), application areas of the adhesive.
Adhesive Fm tm Type of failure
(kN) [kN] (N/mm2)
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 5,00 4,12 Adhesive failure
type C
Resorcinol-phenolic adhesive 3,56 2,97 Adhesive failure type D
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.
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.