Seismic Retrofit of R/C T-Beams with Steel Fiber Polymers under Cyclic Loading Conditions
Round 1
Reviewer 1 Report
The of the article presents results of an experimental study on seismic response of reinforced concrete (RC) T-beams with shear deficiencies strengthened with externally bonded steel fiber reinforced polymer (SFRP) tapes.
From the analysis of the manuscript I found the following:
The order of references, in terms of numbers, is not correct.
There is no mention about how the samples were prepared, what type of cement, aggregates were used, or if a plasticizer was added. Details about the reinforcement (materials specifications) are also missing. The information about technology and curing of fresh concrete should be provided.
The average compressive strength should be provided with the standard deviation.
“The matrix is a typical industrial epoxy resin (Sikadur-330) and the specifications can be provided by the manufacturer.” I would prefer to see some of these specifications mentioned here, or the second part of the sentence should be removed.
“More specifically the anchoring device of TB200P2b was 128 developed and patented by the Laboratory of Strength of Materials and Structures.” I would provide the university, country or city of the lab.
Table 1 – the additional pictures should not cross/overwrite the lines of the table.
Generally, the description of specimens is very long and not easy to read. Materials description is missing.
If I understood it well, the concrete was first 8 days in molds, which means that it was not cured with water during this crucial period? This would not be correct.
From the reading of the first part of the manuscript about materials and strengthening system application, I don’t understand why some systems were selected and what was the main purpose of comparing that many systems.
There is no need to remind “the Laboratory of Strength of Materials and Structures” in every section again.
Figure 3 – it is not obvious what is the experimental setup from this perspective.
The failure mode of the tested specimens should be demonstrated by the pictures as well. The explanation of the results of the test is not well organized and it does not provide a discussion about the obtained results.
Table 3 – Numbers with decimal places contain comma instead of point format.
The obtained results are not discussed nor compared to the existing literature.
Overall, this manuscript is written more like a technical report than a research article. The purpose of this study is not properly defined, the conclusions refer only to a series of technical data without reference to the practical applicability of the results obtained.
On the basis of the specified ones, I consider that the manuscript can only be published after major changes.
Author Response
Initially I would like to thank you in advance for taking the time to review this manuscript. Your comments were very important and by adopting them the manuscript was further improved.
1. The order of references, in terms of numbers, is not correct.
More references were added. I have re-arranged the references with the correct order.
2. There is no mention about how the samples were prepared, what type of cement, aggregates were used, or if a plasticizer was added. Details about the reinforcement (materials specifications) are also missing. The information about technology and curing of fresh concrete should be provided.
Lines 109-131 were added:
“The specimens were cast corresponding to concrete quality C16/20 according to EC2. Ordinary Portland Cement was used with a water to cement (w/c) ratio 0.55 (Mix proportions of the concrete used: concrete IV 32.5 390 kg/m3, water 214.5 kg/m3, sand (0-4 mm) 755 kg/m3, fine aggregate (4-8 mm) 419 kg/m3, coarse aggregate (8-16 mm) 502 kg/m3). Three standard cylindrical specimens (150X300mm) for each T-beam were used to measure the concrete strength, the average 28-day compressive strength was 21.6 MPa whereas the standard deviation SD=2.82 MPa. This corresponds to a concrete with characteristic compressive strength 17 MPa which is very close to the C16/20.
The internal steel reinforcement was made with deformed round bars. The mechanical properties were experimentally determined. More specifically, the yield stress was equal to 530MPa and the ultimate stress equal to 590MPa. The modulus of elasticity was measured with strain gauges equal to 200GPa. Stress-strain curve exhibited an initial elastic region followed the typical hardening behavior of steel. Finally, in order to ensure the adequate development length, all longitudinal reinforcing bars were bent 90oat the ends of the beams. (see figure 1)
Steel fiber strips that were used for the strengthening system are not commercially available. They were fabricated by the manufacturer specifically for this research project. The strips were based on zinc coated steel cords. Each cord consisted of seven mini strands, and each strand was fabricated by twisting three 0.15 mm diameter high strength steel wires. Totally each strip consisted of 28 cords with an equivalent width equal to 100 mm. The total tension capacity was 24.64 kN with an elastic modulus of 210 GPa. The ultimate tensile strain was recorded as equal to 0.009.
A commercial two component epoxy adhesive was selected to be utilized as a binding material for the application of the SFRP strengthening system. The mechanical characteristics after the curing of the mix at 23°C and 50% relative humidity, according to the manufacturer, are having a tensile strength of 30MPa, elongation at break equal to 1,5% and flexural modulus of elasticity 3,8GPa”
3. The average compressive strength should be provided with the standard deviation.
Lines 112-115 were added:
“Three standard cylindrical specimens (150X300mm) for each T-beam were used to measure the concrete strength, the average 28-day compressive strength was 21.6 MPa whereas the standard deviation SD=2.82 MPa. This corresponds to a concrete with characteristic compressive strength 17 MPa which is very close to the C16/20.”
4. “The matrix is a typical industrial epoxy resin (Sikadur-330) and the specifications can be provided by the manufacturer.” I would prefer to see some of these specifications mentioned here, or the second part of the sentence should be removed.
Lines 128-131 were added:
“A commercial two component epoxy adhesive was selected to be utilized as a binding material for the application of the SFRP strengthening system. The mechanical characteristics after the curing of the mix at 23°C and 50% relative humidity, according to the manufacturer, are having a tensile strength of 30MPa, elongation at break equal to 1,5% and flexural modulus of elasticity 3,8GPa”
5. “More specifically the anchoring device of TB200P2b was 128 developed and patented by the Laboratory of Strength of Materials and Structures.” I would provide the university, country or city of the lab.
The suggestion was adopted through the whole manuscript
6. Table 1 – the additional pictures should not cross/overwrite the lines of the table.
Corrected
7. Generally, the description of specimens is very long and not easy to read. Materials description is missing.
The description was improved. Materials are described similarly with comment 2
8. If I understood it well, the concrete was first 8 days in molds, which means that it was not cured with water during this crucial period? This would not be correct.
In the manuscript it was not explained clearly the RC specimen’s preparation and curing. This is why lines 178-192 were added:
“ The application of the strengthening system took place at least 40 days after casting. Before the application of the strengthening system the surface of the beam was grinded with an angle grinder to remove all laitance and dirt from the surface. The concrete surface was then cleaned with a steel wire brush and finally compressed air was used to remove concrete dust and dirt that had settled on the beam. This ensured that the concrete surface was clean of contaminants and derbis. In addition, the bottom corners were rounded. A first layer of resin was applied on the surface and the SFRP strip was positioned on the resin. Using grooved rollers, it was ensured that no air voids were left in the resin. It should be noted that the specific SFRP strips are not very stiff, as a result there was no need of pre-bending them to their final U- shape. The specific SFRP strips were calculated and fabricated by the industry for the present study having geometrical and strength similarities to an equivalent CFRP strip. This is the reason why plastic black cords with negligible stiffness and strength (see photos in table 3) were placed in between each steel cord. Comparing the application of this specific SFRP with an equivalent CFRP strip, it could be said that no difficulties were observed. On the other hand, it is well known from the literature that SFRP strips are much stiffer compared to CFRP, this is why they need to be pre-bent.”
9. From the reading of the first part of the manuscript about materials and strengthening system application, I don’t understand why some systems were selected and what was the main purpose of comparing that many systems.
Lines 149-154 were added:
“The investigated anchoring devices have been chosen as representative mechanical systems among the most commonly used for strengthening applications. In addition, through the present study the construction difficulties during the application of these systems is investigated together with their modes of failure that might occur on the anchoring device itself. Finally, the construction detailing and application is realistic for all the investigated anchoring systems due to the existence of the RC slab which makes the wrapping of the strip non-feasible.”
10. There is no need to remind “the Laboratory of Strength of Materials and Structures” in every section again.
The manuscript was revised.
11. Figure 3 – it is not obvious what is the experimental setup from this perspective.
Figure 3 was slightly revised and lines 216-223 were added:
“At this specific point, the beam’s free end deflection was measured using LVDT, together with both uplifts to capture the rotation of the joint. Four additional LVDTs were place diagonally to measure the openings of the diagonal cracks. LVDT’s were also positioned perpendicular to the longitudinal axis of the beam to ensure that the beam did not deflect out of plain. Totally 9 displacement recordings were obtained during each test. In addition, SFRP strains were also measured with strain gauges. Strain gauges were attached on 8 consecutive SFRP strips as depicted in figure 3 and 4. A strain gauge was attached to the mid-height of each SFRP strip along the fiber direction. Finally, the applied load together with 9 LVDT measurements and 8 strains were recorded.”
12. The failure mode of the tested specimens should be demonstrated by the pictures as well. The explanation of the results of the test is not well organized and it does not provide a discussion about the obtained results.
A new Table 3 has been added in the manuscript. “Results and discussions” have been revised. The following comments have been added:
Lines 248-261
Lines 290-295
Lines 300-306
Lines 329-351
Lines 364-369
13. Table 3 – Numbers with decimal places contain comma instead of point format.
Table 3 became Table 4 which is revised.
14. The obtained results are not discussed nor compared to the existing literature.
Lines 334-351 were added:
“The results obtained in the study are further discussed with equivalent results from the literature. Anil et al [7] and Tanarslan et al. [32] have conducted similar experimental investigation with the difference of strengthening material. Both studies from the literature have investigated CFRP strips whereas we utilized SFRP strips. For all studies the employed anchoring devices are mechanical with slight differences concerning the final geometric arrangement. All studies have demonstrated that when an efficient anchoring device is used, the preliminary debonding mode of failure is avoided. In addition, when CFRP strips are combined with anchoring devices, the mode of failure is driven to the CFRP strip, close to beam corners, or near the anchoring device where there is stress concentration. On the other hand, when SFRP strip are utilized with efficient anchoring devices, this premature mode of failure is avoided. The authors believe that the lateral stiffness of SFRP strips compared to CFRP strips is bigger, a parameter that could explain this phenomenon. Furthermore, when SFRP are used, the mode of failure is driven to the anchoring device, a more complicated mode of failure with extensive diversities. By comparing the results obtained in this study together with the results from the literature, it is demonstrated that the overall, behavior of the developed and patented anchoring device that was utilized together with SFRP strips was efficient. Finally, the present study is the first in the literature that investigates SFRP strips for strengthening T-section RC beams under cyclic loading conditions, when different anchoring devices are combined with applied SFRP strips.”
Lines 334-351 were added:
“The aforementioned observations are valid in the literature [7, 32] as well. The increase of the shear capacity together with the maximum imposed deflection was observed for both studies. The percentage of increase for both cases is comparable but slightly smaller. More specifically Beam-3, Beam-4 and Beam-5 [7] have a shear increase capacity at the range of 120% combined with an average increase of the maximum imposed displacement equal to 300%. Similarly, Spec-3 to Spec-6 [32] obtained percentages smaller than Anil et al.”
15. Overall, this manuscript is written more like a technical report than a research article. The purpose of this study is not properly defined, the conclusions refer only to a series of technical data without reference to the practical applicability of the results obtained.
The manuscript was revised extensively, the authors believe that the purpose is clearer now. The following phrase has been added in the 6thsection – Conclusion:
“A strengthening system that combines high strength steel fibers together with organic epoxy is experimentally investigated under cyclic loading conditions. This system was proven equivalent to the existing systems when it is combined with an efficient anchoring device. The following comments summarize the most important conclusions of the present study”
Reviewer 2 Report
This paper presents outcome of experiments carried out to examine the feasibility of retrofitting RC T-beams with Steel Fiber Polymers in simulated cyclic loading. This papers present novel results and of merit to the community but needs to be slightly improved in order for it to be considered for publication in buildings journal.
Major Questions, Comments and Concerns
1) Overall quality of English needs to be further improved. Extra attention is needed for to avoid use of unnecessary articles and typos. For instance,
a. Line 43/44, please include the references into the sentence “.. economical solution [6,9-10]”.
b. In Fig. 3, “RIGID WALL” is written upside down at the top of the figure.
c. Sample TB150B1b shown in Fig. 5 is not shown in Table 1.
d. Move the References section to line 415.
2) Please describe the ease of bending and installing SFRPs as oppose to traditional FRP sheets.
3) Does drilling through SFRP led to any localized damage/disintegrates the sample?
4) Why the authors refer to the strengthening system as “SFRP tape”? it seems that the SFRP was bonded to the specimen using conventional adhesive. Does the SFRP comes with a pre-built tape?
5) In these tests, a number of specimens failed through debonding. Was this anticipated before hand as in where these specimens designed to fail in this manner or this happed unexpectedly?
6) From the researchers experience, was the debonding capacity of SFRP higher than that of traditional FRP systems?
7) Similar to the above point, please comment on some tests observations and recommendation to delay or mitigate debonding of SFRP.
8) The introduction section could use some modern references on the wide variety of FRP systems such as the following:
a. Hawileh, R., et al. ACI Spec. Publ 301 (2015): 1-18.
b. Minnaugh, Patrick L., and Kent A. Harries. Materials and Structures 42.2 (2009): 271-278.
Author Response
Initially I would like to thank you in advance for taking the time to review this manuscript. Your comments were very important and by adopting them the manuscript was further improved.
1) Overall quality of English needs to be further improved. Extra attention is needed for to avoid use of unnecessary articles and typos.
Major revisions have taken place.
For instance,
a. Line 43/44, please include the references into the sentence “.. economical solution [6,9-10]”.
Corrected
b. In Fig. 3, “RIGID WALL” is written upside down at the top of the figure.
Corrected
c. Sample TB150B1b shown in Fig. 5 is not shown in Table 1.
Corrected
d. Move the References section to line 415.
Corrected
2) Please describe the ease of bending and installing SFRPs as oppose to traditional FRP sheets.
In the manuscript it was not explained clearly the RC specimen’s preparation and curing. This is why lines 178-192 were added:
“ The application of the strengthening system took place at least 40 days after casting. Before the application of the strengthening system the surface of the beam was grinded with an angle grinder to remove all laitance and dirt from the surface. The concrete surface was then cleaned with a steel wire brush and finally compressed air was used to remove concrete dust and dirt that had settled on the beam. This ensured that the concrete surface was clean of contaminants and derbis. In addition, the bottom corners were rounded. A first layer of resin was applied on the surface and the SFRP strip was positioned on the resin. Using grooved rollers, it was ensured that no air voids were left in the resin. It should be noted that the specific SFRP strips are not very stiff, as a result there was no need of pre-bending them to their final U- shape. The specific SFRP strips were calculated and fabricated by the industry for the present study having geometrical and strength similarities to an equivalent CFRP strip. This is the reason why plastic black cords with negligible stiffness and strength (see photos in table 3) were placed in between each steel cord. Comparing the application of this specific SFRP with an equivalent CFRP strip, it could be said that no difficulties were observed. On the other hand, it is well known from the literature that SFRP strips are much stiffer compared to CFRP, this is why they need to be pre-bent.”
3) Does drilling through SFRP led to any localized damage/disintegrates the sample? No the cords could be very easily overpassed (another advantage of SFRP)
Lines 195-198 were revised:
“Using a drill bit a hole was drilled in the slab. The SFRP strip was applied using resin and subsequently the steel plate was positioned on top of the hole. Finally, the bolt was screwed in the concrete hole through the steel plate and the SFRP, by overpassing the high strength steel cords in order not to damage them.”
4) Why the authors refer to the strengthening system as “SFRP tape”? it seems that the SFRP was bonded to the specimen using conventional adhesive. Does the SFRP comes with a pre-built tape?
The strengthening scheme combines steel fibers together with a conventional adhesive. This is why the authors have replaced the word “tapes” with the word “strips”, throughout the manuscript.
5) In these tests, a number of specimens failed through debonding. Was this anticipated before hand as in where these specimens designed to fail in this manner or this happed unexpectedly?
Lines 248-261 were added:
“The ultimate bearing capacity of TB150 is 73.2kN whereas TB200 failed for a maximum load equal to 60kN. The bedonding mode of failure prevailed for both strengthened beams with no anchoring device. As expected, the spacing of the SRP strips plays an important role for increasing the ultimate shear capacity under cyclic loading. The present study did not focus on the delamination of SFRP strips. The non-anchored specimens were utilized for comparison purposes with the anchored strengthened T-beams in order to prove their efficiency under cyclic loadings. In general, the delamination of composite strips from concrete substrate has been extensively investigated and quantified in the past. The debonding mechanism of SFRP strips is similar with CFRP. In the literature Anil et al. [7] and Tanarslan et al [32] have conducted similar tests without anchoring devices using CFRP. By comparing their results with the present study, it could be said that the delamination of SFRP occurred slightly after the delamination of CFRP strips. The developed strains on SFRP strip are at the rang of 4500με. Nevertheless, this is only an indication since it is more than well known that the critical aspects of the debonding mode of failure are the surface preparation and the strength of the concrete”
6) From the researchers experience, was the debonding capacity of SFRP higher than that of traditional FRP systems?
Comments are included in the text Lines 255-261
“The debonding mechanism of SFRP strips is similar with CFRP. In the literature Anil et al. [7] and Tanarslan et al [32] have conducted similar tests without anchoring devices using CFRP. By comparing their results with the present study, it could be said that the delamination of SFRP occurred slightly after the delamination of CFRP strips. The developed strains on SFRP strip are at the rang of 4500με. Nevertheless, this is only an indication since it is more than well known that the critical aspects of the debonding mode of failure are the surface preparation and the strength of the concrete”
7) Similar to the above point, please comment on some tests observations and recommendation to delay or mitigate debonding of SFRP.
Comments were included in the text Lines 248-261
8) The introduction section could use some modern references on the wide variety of FRP systems such as the following:
More references were added. I have re-arranged the references with the correct order.
Round 2
Reviewer 1 Report
I would like to thank the Authors for all applied modifications. I believe that their (Author's) effort led to a significant improvement of the previous version of the manuscript. In my opinion, this version can be accepted and published in present form after some minor English changes.
Reviewer 2 Report
Thank you for your efforts.
