The Influence of Interlock Loss between Rebar and Concrete on Bond Performance of RC Member
Round 1
Reviewer 1 Report
The paper presents an experimental and numerical studies on the effect of the interlock loss between rebar and concrete on bond performance of reinforced concrete elements. The studies are interesting and the paper has a very good quality. The paper cab be improved by considering the following issues:
- The state of the art regarding the modeling of bond behavior between concrete and steel reinforcement can be extended by including several approaches such as the thermodynamic based interface constitutive laws e.g.:
- Engineering Structures 2019;180:762–78.
doi:10.1016/j.engstruct.2018.11.070.
- Cement and Concrete Composites 2021;118:103978.
doi:10.1016/j.cemconcomp.2021.103978. - The description of the used constitutive law for the bond between concrete and steel reinforcement should be improved. Please include more details about the calculation procedure.
- A quick proof reading of the manuscript should be done.
Author Response
The authors appreciate the reviewing. Response to your variable comment are summarized as followings. Same response as follows is also in attached file.
Point 1: The state of the art regarding the modeling of bond behavior between concrete and steel reinforcement can be extended by including several approaches such as the thermodynamic based interface constitutive laws e.g.:
- Engineering Structures 2019;180:762–78.
- Cement and Concrete Composites 2021;118:103978.
Response 1:
The authors also recognize that dynamic analysis can improve the fitting degree for post-fracture pull-out behavior of round bar. Previous researches applied the bond model in this paper into dynamic analysis and succeeded to reproduce fatigue behavior with smooth steel-concrete interface in previous research cited as [29] in the manuscript.
On the other hand, it was clearly showed that interlock effect was greatly superior to chemical adhesion effects and friction effects in previous researches and this research. Focus of analytical investigation was interlock effects before rebar slipping behavior is observed, thus the author judged the current bond model which reproduce chemical and friction effects is enough for discussion for simplifying the static behavior with extremely fine mesh resulting greatly high computation load.
The following description is added in the manuscript in LINE 329.;
"In this pull-out test, displacement measurement around and after bond fracture did not have high reliability. It is because that specimens bounced or split at failure thus affecting measurement of displacement gauge fixed on the concrete cube surfaces. Thus, there was slight difference in softening behavior between experiment and analysis especially after bond fracture for round bar. However, it is enough to evaluate the structural behavior of RC member with interlock, and bond performance until bond fracture which is focus of this study. Dynamic analysis considering local contingency of slipping can improve slip behavior reproduction between smooth surface after bond fracture. [29]"
Point 2: The description of the used constitutive law for the bond between concrete and steel reinforcement should be improved. Please include more details about the calculation procedure.
Response 2:
The following descriptions are modified and added in the manuscript in LINE 259 with modification of Figure 9 showing Constitutive laws.;
"In general, steel-concrete interface shows contact friction behavior in shear with initial adhesive strength [29, 37]. Thus, the author applied Mohr-Coulomb friction law in the shear direction on the contact surface with initial adhesive strength in the both normal and shear direction [38]. Recovery of chemical adhesion is not considered when joint surface contacts again. In other word, after stress in joint element in shear or open direction exceeds its chemical adhesion strength, no normal stress is considered in open direction, and shear stress follows only the Mohr-Coulomb friction law. The friction coefficient μ of 0.4 in Mohr-Coulomb friction law was applied in this study according to the past research focusing smooth interface between steel and concrete [37]."
Author Response File: 
Author Response.docx
Reviewer 2 Report
This manuscript presents the results of experiments performed on the influence of interlock loss on the structural behavior and bond performance of RC member from experimental and analytical approach. The authors conducted both experimental and numerical studies. The manuscript is well written and can be accept in present form.
Author Response
The authors appreciate the reviewing.
Reviewer 3 Report
This research presents an experimental study strengthened by numerical modeling regarding the structural behavior of RC beams. Special attention was given to the effect of bond quality on the overall strength and fracture mechanism of RC beams. The testing setup and series are explained very clearly in the article, and the reviewer found it super interesting. However, the modeling section is quite difficult to follow and unclear. The reviewer can find the comments given below.
Abstract: The final statement in the abstract is not clear; please revise.
Line 57: The authors may consider revising the text as follows: "… RC beams through the experiment and numerical modeling based on Finite Element (FE) Analysis".
Figure 3-4. The legends and figure captions are different?
In section 4, the authors should briefly address early studies (different modeling techniques) regarding the modeling of RC beams and highlight the difference in their study and their contribution. Here are some recent studies about this subject:
Bolander Jr, J. E., and B. D. Le. "Modeling crack development in reinforced concrete structures under service loading." Construction and Building Materials 13.1-2 (1999): 23-31.
Ng, Pui L., Jeffrey YK Lam, and Albert KH Kwan. "Tension stiffening in concrete beams. Part 1: FE analysis." Proceedings of the Institution of Civil Engineers-Structures and Buildings 163.1 (2010): 19-28
Pulatsu, Bora, et al. "Numerical modeling of the tension stiffening in reinforced concrete members via discontinuum models." Computational Particle Mechanics 8.3 (2021): 423-436.
Grassl, Peter, Morgan Johansson, and Joosef Leppänen. "On the numerical modeling of bond for the failure analysis of reinforced concrete." Engineering Fracture Mechanics 189 (2018): 13-26
In Fig 9, the joint element model does not look right. Once the contact is lost, how the capacity increases when the contact is found later during the analysis (in shear).
Line 225: How the reference (commonly referred to as "characteristic") length of finite element is computed? Please explain in the article.
Line 269 and Line 272: "mortar"
It is not clear why the authors choose this approach to model RC Beam. At the end of the day, the smear crack model is used for concrete and steel sections. So, the authors could simply use a well-known homogenization formulation to simulate the concrete section; rather than modeling each constituent of concrete. If the authors would employ some joint elements among the aggregate and mortar, then it will be fine; however, for the current modeling strategy, I do not see any necessity for this detailed approach. Please explain your motivation in the article explicitly. Also, I believe the authors should run just one set of models considering the homogenized concrete section; just to see the difference between detailed FE and homogenized FE models.
In general, the computational modeling part is unclear; in terms of how it is performed and the main motivation and contributions regarding this subject.
Author Response
The authors appreciate the reviewing. Response to your variable comment are summarized as follows. Attached file shows same contents. Please see the attachment.
Point 1: The final statement in the abstract is not clear; please revise.
Response 1:
Abstract describing the summary of this study was modified as follows.
"Through these investigations, it was seen that interlock could work and keep sound bond as long as contact between a lug and concrete was maintained when the rebar lug was flattened due to section loss. Furthermore, under the situation with non-uniform distribution of section loss, pull-out behavior of rebar was prevented by interlock of parts of region in a member even when other regions completely lost interlock due to serious section loss."
Point 2: The authors may consider revising the text as follows: "… RC beams through the experiment and numerical modeling based on Finite Element (FE) Analysis".
Response 2:
According to the comment, description in this part was modified.
Point 3: The legends and figure captions are different?
Response 3:
According to the pointing out, positions of Fig 3 and Fig 4 were modified.
Point 4: The authors should briefly address early studies (different modeling techniques) regarding the modeling of RC beams and highlight the difference in their study and their contribution. Here are some recent studies about this subject:
Bolander Jr, J. E., and B. D. Le. "Modeling crack development in reinforced concrete structures under service loading." Construction and Building Materials 13.1-2 (1999): 23-31.
Ng, Pui L., Jeffrey YK Lam, and Albert KH Kwan. "Tension stiffening in concrete beams. Part 1: FE analysis." Proceedings of the Institution of Civil Engineers-Structures and Buildings 163.1 (2010): 19-28
Pulatsu, Bora, et al. "Numerical modeling of the tension stiffening in reinforced concrete members via discontinuum models." Computational Particle Mechanics 8.3 (2021): 423-436.
Grassl, Peter, Morgan Johansson, and Joosef Leppänen. "On the numerical modeling of bond for the failure analysis of reinforced concrete." Engineering Fracture Mechanics 189 (2018): 13-26
Response 4:
The authors appreciate the comment. Major difference from previous modeling was geometrical reproduction of deformed rebar by solid elements in order to separate out interlock mechanism from other bond factor. Previous model of bond can be mainly divided into two approach, averaged strain-stress relationship by inducing tension stiffening or discrete rebar and bond-slip model between rebar and concrete. The both approaches integrates the multiple bond factors. Thus, the author reproduced interlock effect by solid elements and chemical and friction effects, whose contribution for bond is small, were modeled by simple joint element. In order to improve the reader's understanding about modeling, the following description is added in the manuscript with references in the LINE 193;
"In previous researches, FE analytical model for bond between rebar and concrete can be divided mainly into two approaches, induction of bond-slip behavior between rebar and concrete [22-25] and averaged stress and averaged strain behavior as reinforcement concrete [26-28]. Both approaches had been developed based on pull-out behavior of rebar from concrete, and stress transfer behavior between rebar and concrete was modeled including multiple bond factors; interlock, chemical adhesion, and friction. In this study, the authors reproduced interlock effects explicitly by reproducing the geometry of rebar, not embedded truss or beam rebar model or distributed fiber model for rebar."
Point 5: The joint element model does not look right. Once the contact is lost, how the capacity increases when the contact is found later during the analysis (in shear).
Response 5:
The authors appreciate the comment. Once the chemical adhesion was lost, Mohr-Coulomb friction law was applied, thus shear stress depends on the normal stress. Application of this model for steel-concrete interface was verified in the past researches. Figure 9 was modified to be clear including initial adhesive strength and the following description was modified and added in the manuscript in the LINE 195.:
"In previous researches, FE analytical model for bond between rebar and concrete can be divided mainly into two approaches, induction of bond-slip behavior between rebar and concrete [22-25] and averaged stress and averaged strain behavior as reinforcement concrete [26-28]. Both approaches had been developed based on pull-out behavior of rebar from concrete, and stress transfer behavior between rebar and concrete was modeled including multiple bond factors; interlock, chemical adhesion, and friction. In this study, the authors reproduced interlock effects explicitly by reproducing the geometry of rebar , not embedded truss or beam rebar model or distributed fiber model for rebar."
Point 6: How the reference (commonly referred to as "characteristic") length of finite element is computed? Please explain in the article.
Response 6:
Reference length is applied in smeared crack model because crack is reproduced as strain in a element by considering fracture energy without dependance on mesh size. In this model, reference length is equal to the element dimension in the direction from the geometry and meshing depending on modeling intension. The following discription is added in the manuscript in the LINE 236.:
"In plain concrete, tension softening behavior should be determined by fracture energy and reference length on which the average stress-strain relationship is defined. Stiffening pa-rameter of plain concrete can be given by Eq. (5) [26]. .... Reference length is equal to the element dimension. Parameter c, which reproduces tension softening behavior, has determined on the reference length, thus average stress-strain relationship is dependent on element size."
Point 7: "mortal" It is not clear why the authors choose this approach to model RC Beam. At the end of the day, the smear crack model is used for concrete and steel sections. So, the authors could simply use a well-known homogenization formulation to simulate the concrete section; rather than modeling each constituent of concrete. If the authors would employ some joint elements among the aggregate and mortar, then it will be fine; however, for the current modeling strategy, I do not see any necessity for this detailed approach. Please explain your motivation in the article explicitly. Also, I believe the authors should run just one set of models considering the homogenized concrete section; just to see the difference between detailed FE and homogenized FE models.
In general, the computational modeling part is unclear; in terms of how it is performed and the main motivation and contributions regarding this subject.
Response 7:
The author produced FE model with very fine mesh size (minimum 2mm) in order to focus interlock effects by geometrical reproduction. This size is greatly smaller than aggregate size, thus it is out of assumption as composite behavior of aggregate and paste in smeared crack model primary. In order to make clear modeling in detail, comparison between separated and composite modeling is added in appendix (LINE 573) and the description in the article is modified in the manuscript as follow in the LINE 186 and 271. Figures showing analytical model (Fig. 8 and Fig. 11) were also modified:
LINE 186;
"To more precisely grasp the phenomena caused by differences in interlock conditions, the shapes of the threaded rebar and the anchoring nuts at beam end were reproduced using steel solid elements. Two-dimensional joint element was placed at the boundary between the steel element for main rebar and the concrete element. Chemical adhesion and frictional resistance on smooth surface between steel and concrete were considered in the property of joint element. As the effect of interlock can be directly reproduced by normal stress between concrete element and lug shaped steel element, it was possible to separate out the interlock effects from other bond components, which is reproduced by joint element."
LINE 271;
"In order to clearly reproduce mechanical interlock between rebar lug and concrete, the authors reproduced the shape of threaded bars by hexahedral elements. For achieving that, a small mesh size of 2 mm minimum was used. This mesh size deviated from the applicability of original concrete model that described the averaged behavior of concrete as the composite material of aggregate and cement paste. Thus, the aggregate and mortar parts of the concrete elements were segregated in concrete elements in order to reproduce closer actual condition while there was room for consideration of equivalent softening behavior in homogenized concrete model."
Appendix (LINE 573);
In analytical investigation, aggregate and mortal were separately modeled and randomly placed. The mesh size is greatly smaller than aggregate size in this model, thus it is out of assumption as composite behavior of aggregate and paste in smeared crack model. Figure A1 shows the comparison between homogenized concrete model and separated modeling in the beam analysis of the ST-100 case. Two models had same meshing and material properties in homogenized concrete model were shown in table 3 given from cylinder test. Post-cracking stiffness of homogenized concrete model was lower than experimental result while maximum load and crack distribution in ultimate state were almost same. It seemed to be caused by high resistance of crack progress in local region by aggregate. More accurate averaged strain and averaged stress relationship in extremely fine mesh reproduced by homogenized concrete model by tension stiffening requires more consideration.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
The manuscript can be accepted in the revised form.
Reviewer 3 Report
The authors addressed the concerns raised by the reviewer.
