Abstract
Currently, the effect of the concrete substrate on the load-bearing capacity of externally bonded carbon fiber-reinforced polymer (CFRP) strips is determined in the guidelines solely on the basis of the concrete strength. In addition to strength, surface preparations can improve the bond resistance by exposing the aggregates of the substrate. However, the interlocking also depends on the parameters that derive from the inner concrete structure such as the aggregate grain dimension and shape, as well as the air void distribution. Therefore, the influence of the concrete inner structure on the bond is investigated with double shear tests by comparing specimens with equivalent concrete strength and varying grain shape and dimension. Bond parameters are calculated using a novel procedure based on optical measurements. Furthermore, crack paths are analyzed by means of computed tomography (CT) scans, which prove to be an essential inspecting technique for characterizing the inner concrete structure. The fracture energy is approximately 30% higher for concrete with a maximum grain size of 8 mm compared to an equivalent mortar with 4 mm maximum grain size.
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Acknowledgments
The research is funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) Research Training Group (project number 255042459).
Notation
The following symbols are used in this paper:
- Asp
- Specific area of the aggregates;
- Ef
- Elastic modulus CFRP strip (N/mm2);
- FACT
- Force at the actual time step (N/mm2);
- FDEB
- Debonding force (N/mm2);
- fck
- Characteristic value of compressive strength;
- fcm,cube
- Mean value of compressive strength (MPa);
- fctm,split
- Mean value of tensile strength from splitting tests (MPa);
- fctm,surf
- Mean value of the surface tensile strength (MPa);
- ff
- Maximum stress value for CFRP (MPa);
- ft,surf
- Surface tensile strength (MPa);
- GF
- Fracture energy N/mm;
- GF,fib
- Fracture energy according to fib (2001) (N/mm);
- GF,mean
- Mean value of fracture energy (N/mm);
- j
- Aggregate fraction considered in the excess paste thickness calculation;
- L
- Glued length (mm);
- Ra
- Profile mean height of a texture line;
- rsp,j
- Specific area ratio (area to volume) (1/mm);
- Sa
- Arithmetic mean height of a texture surface (μm);
- scalc
- Displacement in the approximation procedure (mm);
- si
- Slip at iteration step i (mm);
- sL1
- Slip at maximum shear stress according to Holzenkämpfer (1994) (mm);
- sL0
- Slip value at shear stress zero according to Holzenkämpfer (1994) (mm);
- smeas
- Displacement difference between the adherent CFRP strip and the concrete substrate (mm);
- tE,PASTE
- Excess paste thickness (μm);
- VA,COMP
- Volume of air after compaction (mm3);
- VAGG
- Aggregate volume (mm3);
- VE,PASTE
- Excess paste volume (mm3);
- VPASTE
- Volume of paste (mm3);
- ɛcalc
- Strain in the approximation procedure (N/mm2);
- ɛf
- Ultimate strain for CFRP (N/mm2);
- ɛi
- Strain value at iteration step i (N/mm2);
- ϕ
- Packing density;
- σ2
- Variance;
- τcalc
- Tangential stress value in the approximation procedure;
- τi
- Tangential stress at iteration step i (N/mm2); and
- τL1
- Maximum tangential stress (N/mm2).
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Published In
Journal of Composites for Construction
Volume 26 • Issue 5 • October 2022
Copyright
© 2022 American Society of Civil Engineers.
History
Received: Jan 30, 2022
Accepted: May 10, 2022
Published online: Jul 21, 2022
Published in print: Oct 1, 2022
Discussion open until: Dec 21, 2022
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