The development of monitoring technologies particularly suitable to be used with novel CFRP strengthening techniques has gained great attention in recent years. However, in spite of the high performance of these advanced composite materials in the strengthening and repairing of structures in service, they are usually associated with brittle and sudden failure mainly caused by debonding phenomena, originated either at the CFRP-plate end or at the intermediate areas in the vicinity of flexural cracks in the RC beam. Thus, it is highly recommended for these structures to be monitored in order to ensure their integrity while in service. Specifically, the feasibility of smart sensing technologies such as Fiber Bragg Grating (FBG) sensors and piezo-impedance transducers (PZT) has been studied. To the knowledge of the authors, none serious study has been carried out until now concerned to the topic of damage detection due to debonding in rehabilitated structures with CFRP composites.
SHM
–
Structural Health Monitoring.
CFRP
–
Carbon Fiber Reinforced Polymer.
RC
–
Reinforced Concrete.
FBG
–
Fiber Bragg Grating.
PZT
–
Lead Zirconate Titanate (piezoelectric ceramic material).
EMI
–
Electro-Mechanical Impedance.
RMSD
–
Root Mean Square Deviation
Aerospace, civil and mechanical infrastructures are continuously exposed to deterioration and functional deficiencies for which, regardless of the root cause (aging, accidental damage, weathering of the materials, poor engineering, etc.), a solution is needed in terms of monitoring and maintenance. In order to mitigate this deterioration and efficiently manage maintenance work on civil structures, it has become necessary to develop reliable damage detection strategies, referred to (
Unfortunately, strengthening with CFRP is often associated with brittle and sudden failure caused mainly by some form of debonding of the CFRP from the concrete (
Both kinds of smart sensors are widely recognized as promising technologies in the SHM field, but few efforts have been made so far to incorporate these smart solutions within the SHM procedures that allow the assessment of the health condition of civil infrastructures externally strengthened with advanced composite materials. For that reason, both smart sensor technologies are used in this work in order to test their performance for debonding detection on Reinforced Concrete (RC) structures externally strengthened with CFRP composite materials. The purpose of this paper is, thus, to track damage evolution on experimental beams so that, from the accumulation of damage indications in certain regions of the structure (due mainly to flexural cracks), debonding appearance could be predicted by means of these smart sensors before the beam reaches a critical condition while in service, which could originate a catastrophic failure. Two identical RC beams with the same steel reinforcement and also the same external CFRP strengthening have been used during the experimental campaign carried out for this paper. Furthermore, a different loading sequence has been applied to each beam in order to achieve a more comprehensive experimental test.
Among all fiber optic solutions for sensing technologies, Fiber Bragg Gratings (FBGs) are the most widely applied ones given the number of commercial systems available and their high performance besides other significant advantages compared to other strain measurement techniques, such as their immunity to electromagnetic interference and power fluctuation along the optical path, high precision, durability, low power consumption, absolute strain sensing capability, ease of multiplexing and compatibility with being embedded in a range of structural materials (
These fibers consist of a very small inner core that has a periodic variation of its diffraction index with a periodic lambda (λ), which is generally achieved by exposing the core of photosensitive optical fibers to light from an ultraviolet laser (
In this work, several FBG sensors have been used to monitor the strain variations in reinforced concrete beams externally strengthened with CFRP strips (see Section 5) during different loading programs, as detailed in Section 4. The strains for each experiment were calculated by using the following expression [
where
Lead-zirconate-titanate (PZT) materials have become a very widespread technology for sensing and monitoring solutions in many engineering fields, civil engineering among them, due to their light weight and variety of shapes and sizes (
where Y(ω) is the electrical admittance (inverse of impedance),
Through the application of the EMI method, the integrity of the structure can be assessed by observing the changes experienced by the electromechanical impedance of the structural system between two different stages. In order to obtain a quantitative measure of the damage level present in the structure, it is necessary to use statistical tools that allow one to define different scalar damage metrics that lead to that quantitative assessment of the health condition. In that sense, the root mean square deviation (RMSD) is the most commonly used indicator for the impedance method (
where
Finally, Yang et al (
Two RC beams with the same steel reinforcement and also the same external CFRP strengthening were used during the experimental campaign carried out in this work at the Eduardo Torroja Institute (Madrid-Spain), in order not only to study the behavior of these reinforced concrete beams, but also to test the performance and adequacy of the selected smart sensors for the particular monitoring application evaluated in this paper. The mechanical properties of the tested specimens are presented in
Geometry, loading scheme and sensor location map for the RC beams.
FBG sensors (top) and PZT sensors (bottom).
Basic FBG strain sensing principle.
Specimen preparation's procedure: (a) tied stirrups and longitudinal reinforcement; (b) steel reinforcement inside the formwork; (c) concrete casting; (d) beams’ bottom surfaces after sand blasting operations; (e) CFRP reinforcement; (f) instrumentation of the beams; (g) simple supports and loads preparation
Experimental setup for the RC beams externally strengthened with CFRP.
Material properties
Property | Concrete 1 | Concrete 2 | Steel | Adhesive | CFRP |
---|---|---|---|---|---|
|
25.90 | 24.86 | 210 | - | 150 |
|
- | - | - | 4300 | - |
|
27.30 | 24.64 | 510 | - | - |
|
2350 | 2350 | 7850 | - | - |
|
0.2 | 0.2 | 0.3 | - | 0.35 |
|
- | - | - | 3.45 | 1.4 |
In each test program performed, the corresponding strengthened beam was subjected to a series of increasing quasi-static load tests (detailed in
Static loading test points for each beam (see
Beam | Load F (kN) | Damage | Beam | Load F (kN) | Damage | ||
---|---|---|---|---|---|---|---|
1. | 13 | D11 | 1. | 26 | D21 | ||
2. | 20 | D12 | 2. | 32.5 | D22 | ||
3. | 32.5 | D13 | 3. | 40 | D23 | ||
1 | 4. | 50 | D14 | 2 | 4. | 50 | D24 |
5. | 69.5 | D15 | 5. | 56.5 | D25 | ||
6. | 60 | D26 | |||||
7. | 66.5 | D27 |
During the application of each loading step, the microstrains were constantly measured by using identical os3200 Non-metallic Optical Strain Gages supplied by Micron Optics, as indicated in
After each quasi-static test, the impedance was then measured by using, in both specimens, eleven identical P-876 Dura Act Patch Transducers of 0.5 mm thickness supplied by Piceramics, which were externally bonded with an epoxy adhesive along the CFRP strip with a constant spacing ratio of 12 cm (
Specimen number 1 failed when the last loading step reached F = 69.5 kN (139 kN in total), while specimen number 2 failed with F = 66.5 kN (133 kN in total). In both cases, the dominant failure mode was the one expected for these kinds of strengthened beams: by intermediate crack debonding of the external CFRP reinforcement.
As detailed in Section 4, an optical sensing interrogator was used to collect measurements from all sensors at once, with the support of a channel multiplexer, monitoring the structure not only during the load application, but also before and after, so the evolution of the strains in the external surface of the CFRP strip would be obtained during the entire loading procedure at different locations along the beam. These results are shown in
Microstrains measured for RC-Beam #1.
In
Cracking map of the RC-Beam #1 after the last loading stage.
Damages with concrete cover separation between sensors 2 and 3 (left) and sensors 6 and 7 (right) for the RC-Beam #1.
Analogue strain measurements were taken for beam #2, whose results are shown in
Microstrains measured for RC-Beam #2.
From this Figure it is easy to see how FBG number 3 is the one collecting the highest strain increments among all the sensors deployed along the beam. This induces one to consider that this sensor is in the vicinity of an area with a high concentration of cracks, which may lead to debonding failure, and due to the remarkable difference between the strain increments measured by this sensor and the rest, it actually seems reasonable to conclude that the concentration of flexural cracks in this location incite the debonding appearance (
Cracking map of the RC-Beam #2 after the last loading stage.
Damages with concrete cover separation between sensors 5 and 6 (left) and sensors 3 and 4 (right) for the RC-Beam #2.
As mentioned earlier, an impedance analyzer applying a sinusoidal-sweep voltage of 1 volt was used to capture the electromechanical impedance signatures of the structure, monitoring it from 10 kHz to 100 kHz after each static load test (
Impedance signatures for the initial stage (D10) of RC-Beam #1.
The differences encountered in
Stage D10 vs Stage D11 comparison for sensors #1 and #3.
In order to obtain a quantitative evaluation of the damage from the impedance signatures, the statistical indicator RMSD is now used, as explained in Section 3. Since RMSD is computed for one stage by using the previous stage as baseline, only the appearance of new damage with respect to the baseline stage should be reflected in the RMSD value. Following with the example of the analysis of damage stage D11,
RMSD values for the comparison between Stage D10 and Stage D11.
It is clear, from the figure, that sensor 3 is the most affected by the damage, even when no visual cracks were detected during and after the loading process. This indicates that, around the position of sensor 3, there is a potential debonding location. By evaluating the RMSD values for the rest of the sensors, it is easy to see how sensor 2 is the second more affected by the presence of damage, while sensor 4 is practically not affected at all. From this fact, it can be concluded that there is a potential damage by debonding emerging between sensors 2 and 3, which actually came to be confirmed at the end of the test program as shown in
After increasing the loads, as could be expected, all sensors experienced an increment of their respective RMSD values in every damage case, but only the sensing regions close to sensors numbers 2, 7 and 9 could be considered as potentially affected by debonding failure, according to the specially high RMSD values found for them after some loading steps.
In the case of sensor number 7, the RMSD after D12 was 4.32% at 50–60 kHz, and it kept above 2.5% for the high frequency ranges until the failure of the beam, which supposes a noticeable and constant weakness of the interface between FRP and concrete in this region. A similar behavior is found for sensor number 2 after D12 (4.5% at 70–80 kHz) and after D13 (4.47% at 60–70 kHz), and again with values around 2.5% for the high frequency ranges until the failure of the beam. Therefore, these two regions are likely to suffer debonding effects, as it is actually well demonstrated in
In the same way as was done with the other specimen, this beam was subjected to the corresponding static loads specified in
Impedance signatures for the initial stage (D20) of RC-Beam #2.
Differences encountered in
In
As in the previous example, once again, the RMSD value is used in order to assess the integrity of the structure.
RMSD values for the comparison between Stage D25 and Stage D26.
Damage stage D26, which is actually very close to the failure condition of the beam, is analyzed this time in
Although sensor number 7 did not show such remarkable RMSD values in previous stages, the ones collected here at lower frequency ranges than the rest of the sensors (sensor 6 in particular), made us think that this sensor also detected the weakness of the interface around sensor 6 as a further damage at this stage. Finally, it is also clear that the region between sensors #3 and #4 is also likely to be debonded during the last loading step, given their high indications.
As pointed out in the previous paragraph, these predictions came to be confirmed after the following and last loading step of the test program, since the only debonding failures found in the beam appeared just in the location mentioned in this analysis (
Two identical RC beams with the same steel reinforcement and also the same external CFRP strengthening were tested during the experimental campaign carried out for this paper, with the purpose of applying smart sensors such as FBGs or PZTs in order to track damage evolution, thus successfully predicting debonding appearance at the FRP-concrete interface. From the direct analysis of the strain curves for each experimental test, it is not possible to conclude the presence of debonding in the strengthened beam, since FBG sensors equally respond to both kinds of damage presence in the beam: concrete cracks and interfacial debonding, taking into account that this interfacial debonding is due to a noticeable and considerably high concentration of cracks in a small region. For this reason, further treatment of these data is needed in order to obtain accurate conclusions. A model updating procedure might be used with this purpose. However, quite more precise are the conclusions obtained from the impedance-based direct analysis of the data collected by the PZT sensors, from which the location of the several debonding origins can be estimated, matching reasonably well with the real position of the debonding at the end of the test program.
The authors acknowledge the support for the work reported in this paper from the Spanish Ministry of Science and Innovation (project BIA2013-46944-C2-1-P). Financial support for the FPI research fellowship given to Enrique Sevillano and CSC research fellowship given to Rui Sun is also acknowledged.