1 Introduction
2 Experimental Program
Specimen | Bond length, L
b
(mm) | Strengthening technique | Instrumentation |
---|---|---|---|
SS-EBR-L50 | 50 | EBR | 2 LVDTa, 3 SGb, 1 DCc |
SS-EBR-L100a | 100 | EBR | 2 LVDTa, 4 SGb, 1 DCc |
SS-EBR-L100b | 100 | EBR | 2 LVDTa, 3 SGb, 1 DCc |
SS-EBR-L160 | 160 | EBR | 2 LVDTa, 5 SGb, 1 DCc |
SS-EBR-L240 | 240 | EBR | 2 LVDTa, 7 SGb, 1 DCc |
SS-EBR-L300 | 300 | EBR | 2 LVDTa, 8 SGb, 1 DCc |
SS-EBR-L400 | 400 | EBR | 2 LVDTa, 11 SGb, 1 DCc |
SS-EBR-L560 | 560 | EBR | 2 LVDTa, 15 SGb, 1 DCc |
SS-EBR-L640 | 640 | EBR | 2 LVDTa, 16 SGb, 1 DCc |
SS-EBR-L800 | 800 | EBR | 2 LVDTa, 21 SGb, 1 DCc |
SS-NSM-L35 | 35 | NSM | 2 LVDTa, 1 DCc |
SS-NSM-L50 | 50 | NSM | 2 LVDTa, 1 DCc |
SS-NSM-L75 | 75 | NSM | 2 LVDTa, 1 DCc |
SS-NSM-L100 | 100 | NSM | 2 LVDTa, 1 DCc |
SS-NSM-L200 | 200 | NSM | 2 LVDTa, 1 DCc |
SS-NSM-L300 | 300 | NSM | 2 LVDTa, 1 DCc |
2.1 Mechanical Properties of the Materials
Material | Section type | Yield stress, f
y,m
(MPa) | Ultimate stress, f
u,m
(MPa) | Ultimate strain, ε
u,m
(MPa) | Young modulus, E
m
(GPa) |
---|---|---|---|---|---|
Steel B 500 SD | ϕ6 | 538 | 634 | 7.5 | 199 |
ϕ8 | 573 | 675 | 6.5 | 212 | |
ϕ12 | 530 | 637 | 11.4 | 211 | |
Stainless steel EN 1.4404 | 20 × 5 | 260 | 618 | 27.2 | 192 |
Stainless steel EN 1.4301 | ϕ8 | 1008 | – | 9.7 | 195 |
2.2 Geometry and Preparation of the Specimens
2.3 Measurements and Procedures Followed During the Tests
3 Failure Modes and Rupture Loads
Specimen | Bond length, L
b
(mm) | Rupture loads, F
rup
(kN) | Failure mode |
---|---|---|---|
SS-EBR-L50 | 50 | 6.3 | Type I |
SS-EBR-L100a | 100 | 12.8 | Type II |
SS-EBR-L100b | 100 | 12.4 | Type I |
SS-EBR-L160 | 160 | 14.5 | Type I |
SS-EBR-L240 | 240 | 15.9 | Type II |
SS-EBR-L300 | 300 | 21.9 | Type II |
SS-EBR-L400 | 400 | 18.6 | Type III |
SS-EBR-L560 | 560 | 14.6 | Type III |
SS-EBR-L640 | 640 | 18.5 | Type II |
SS-EBR-L800 | 800 | 14.8 | Type III |
SS-NSM-L35 | 35 | 13.8 | Type IV |
SS-NSM-L50 | 50 | 26.2 | Type IV |
SS-NSM-L75 | 75 | 35.6 | Type IV |
SS-NSM-L100 | 100 | 40.1 | Type V |
SS-NSM-L200 | 200 | 48.9 | Type V |
SS-NSM-L300 | 300 | 47.8 | Type V |
4 Accuracy of the DIC Technique
4.1 DIC vs. Strain Gauge-Based Measurements
4.2 Load–Slip Response
4.3 Slips Developed Within the Interface
4.4 Axial Stresses and Strains Developed in the Stainless Steel
5 Data Interpretation
5.1 Definition of the Effective Bond Length
5.2 Interfacial Bond–Slip Relationship of the EBR System
5.3 Interfacial Bond–Slip Relationship of the NSM System
5.4 Interfacial Bond–Slip Relationships: EBR System Versus NSM System
6 Conclusions
-
The use of ribbed SS rods showed that it is possible to obtain the rupture of the rod if an appropriate bonded length is used. In the present experimental work, it was found that for 200 mm the rupture of the SS rod is reached. Thereby, the premature debonding phenomenon of the SS rod is avoided and the mechanical properties of the SS rod are fully used;
-
the EBR samples performed poorly when compared to the NSM samples. In all the tests carried out, the premature debonding of the SS strip was observed at a strain somewhat lower than its rupture value. In the EBR samples with a short bond length, i.e. with a bonded length shorter than the effective bond length, the rupture occurred within the SS-to-adhesive interface, which means that the resin has poor properties for bonding SS strips. However, when the bond length of the SS-to-concrete interface increases, a mixed failure mode was observed with the separation of a thin layer of concrete from the substrate with 2–3 mm of depth and, at the same time, with an adhesive rupture within the SS-to-adhesive interface;
-
the DIC technique can be used, although carefully, to evaluate the bond transfer between the SS and concrete. The displacements measured with the DIC technique and the slips calculated from these results were reasonably well estimated. Mainly when those values were greater than one tenth of a millimetre, the DIC proved to be capable of predicting the results fairly well. However, the noisy signal obtained for the slips make it difficult to determine the strains and bond stresses due to its higher order, i.e. due to the first and second derivatives of the slips with respect to x (axis parallel to the bond length) for the calculation, respectively. Still, the methodologies followed permitted the yielding of the SS rods in the NSM samples to be identified and allowed us to get a fair perspective of the strain distribution in the SS strip in the EBR samples;
-
the DIC technique also allowed the load–slip distribution to be captured accurately. This weighs heavily in the evaluation of the bond between two materials because, based on the load–slip response, the interfacial behaviour can be predicted. Thus, depending on the load–slip response until failure, the different stages that characterize the bond–slip relationship can be estimated. For instance, an initial linear load–slip response means that the interfacial bond–slip relationship has a linear and elastic stage as well. Afterwards, the nonlinear load–slip response observed from the samples means that the interfacial bond–slip relationship has a softening stage. This transition between the linear and the nonlinear load–slip response corresponds to a maximum bond stress value of the bond–slip relationship;
-
The effective bond length of the EBR samples was 235 mm, whereas the NSM samples had an effective bond length of 168 mm, which represents 71.5% of the value obtained for the EBR samples;
-
the bond–slip relationships obtained for the two types of samples studied here are different. In the EBR samples, a power function was able to describe a mid positioning of the experimental bond stresses (i.e. the corresponding mid-range values between the maximum and the minimum experimental bond stresses) obtained along the slips within the interface at the SS loaded end. However, in the NSM samples, a trapezoidal shape to describe the bond–slip relationship was proposed to approximate the experimental findings. Comparing the limit points of both bond–slip relationships, it can be concluded that the mid-range maximum bond stress found for the NSM samples reached 1.8 times of that found for the EBR samples. In term of slips, the NSM samples had higher values with the mid-range value of the ultimate slip developed within the interface of the NSM samples being approximately 0.5 mm, whilst an ultimate slip of 0.4 mm was never exceeded in the EBR samples.