Bond Behavior of WAAM Reinforcements in Comparison to Conventional Steel Reinforcements

Round 1

Reviewer 1 Report

Dear authors,
Thank you for submitting your manuscript, below are my comments

1. Quantified data and numerical data should be presented in abstract.
2. Presentation of the chemical composition of individual tested products should be considered (especially for GTR, which is not as obvious as the others).
3. The parameters of the welding process should be extended by the average arc voltage and current (Table 1). What does the welding speed (m/min) mean for "spot" welding? It is worth extending the manuscript with a sketch of the movement of the welding gun.
4. The WAAM product has the lowest mechanical properties. Isn't it worth saying on this basis that it is the weakest of the materials, especially since the B500B is a standard solution. Unfortunately, this undermines the validity of subsequent detailed studies. I understand that the advantage of WAAM lies in the possibility of its arbitrary shaping, but in the case of building structures, B500B steel bars are successfully bent.

Author Response

Dear Reviewer 1,

Thank you for your feedback and valuable input on our paper. We have implemented your comments and suggestions as follows:

• Unfortunately, another review noted an abstract that was too long, so we already had to shorten it and therefore could not include quantified data and numerical data in the abstract.
• In line 240, the standard has been added with regard to the requirements for the threaded rods. A chemical analysis does not seem to us to be useful in the context of the present investigations since the bond behavior is primarily influenced by the surface properties of the material.
• The parameters welding current and voltage have been added in Table 1. In lines 262-264 additional explanation for the welding speed has been included. The specimens were produced with continuous welding and not spot welding.
• The fact that WAAM has the lowest mechanical properties compared to the other reinforcements studied here was added in lines 414-415. This was supplemented by the addition that the bond zone, however, is the weak point in the entire system. The objective of this research was to investigate the bond behavior of an additively manufactured reinforcement. Even though reinforcing steel can be sucessfully bent in different spatial directions, it is not suitable for additive manufacturing and integration into SPI.

 

Best regards,

Stefan Rappl

Reviewer 2 Report

Dear authors, 

Your idea of research is very interesting. The scope of your research is almost complete, but from the scientific point of view, there are some crucial issues that disqualify this paper from being published. I listed all my comments below: 

1. Please use proper terminology related to ISO/ASTM 52900:2021 standard. Before introducing a WAAM term, you need to assign it to a standardized group of technologies. 

2. The abstract part is too long. Please read the instruction for authors and fit the abstract to those requirements. In the abstract part, there should be some information about your most important outcomes with the quantified values provided. 

3. Line 52 - as far as I know WAAM is a Wire Arc Additive Manufacturing - not Wire And Arc Additive Manufacturing.

4. I can imagine the combination of WAAM and SPI technologies but figure 1 is totally unreadable. It seems like you want to weld the wire which is a part of the existing structure. Please make it clear to be easier to understand. 

5. In the whole introduction part you forgot about the need of using postprocessing after AM process which is related to surface treatment (mostly sandblasting) and stress release with the use of proper heat treatment. Such kind of approach is very important, especially from your application point of view. 

6. Table 2 - based on those results it seems that the success probability of using WAAM for such kind of application is 20% which is unacceptable. Additionally, the idea to put only one data from the research as a reliable result is unacceptable. You need to provide more test results - I suggest using proper postprocessing which I mentioned above - it would lead you to obtain better and more reliable results of WAAM technology. 

7. There is not any information about the surface roughness measurement method. 

8. To determine the quality of the WAAM method I suggest making a cutout of a part of the material - then mount it, grind, and analyze with the use of a microscope. You can also etch it to reveal the microstructure. You can also analyze a fractured cross-section of the bar.

9. I think chapter 3.2 should be moved into the experimental description part. 

10. I cannot find the relationship between the scanning method and the WAAM production of rods. You need to provide the data in the proper order and better describe the design method because now I can see only some measurements and a photo (fig.4) at the begging of the manuscript - and those two parts are not connected at all. 

11. Based on only one succeeded test part in the case of tensile tests you cannot state that the "use of WAAM reinforcements in civil engineering is a promising area of research". You need to provide more test results with a proper statistical approach. 

 

To summarize, in its present form, this manuscript should not be published. 

Author Response

Dear Reviewer 2,

Thank you for your feedback and valuable input on our paper. We have implemented your comments and suggestions as follows:

1. In line 73, the standard has been added with regard to the term of WAAM.
2. Thank you for pointing out an abstract that is too long. This has been shortened according to the MDPI specifications.
3. Thank you for this hint. You are right according to DIN EN ISO 17024-2 it is Wire Arc Additive Manufacturing.
4. Thank you for pointing this out. Figure 1 was adapted to make it clearer.
5. These commonly used techniques of postprocessing are not useable for the process combination of WAAM and SPI. Through the simultaneous printing of SPI and WAAM sandblasting cannot be used and neither can heat treatment. Both postprocessing techniques would damage the concrete properties. As we can’t use postprocessing, we decided for clarity reasons only to discuss the steps which are relevant to the combined process.
6. As a result, a second series was manufactured under the same conditions. The results are also given in Table 2. The result of the first series shows a good agreement of the mechanical properties with those of the second series.
7. The operating principle of the LLS developed at TUM can be found in: Osterminski, K.; Gehlen, C. Development of a laser-based line scan measurement system for the surface characterization of reinforcing steel. Materials Testing 2019, Volume 61, Issue 11, pp. 1051-1055. DOI https://doi.org/10.3139/120.111418. The LLS detects the surface with a very high accuracy (step width = 0.01885 µm). The surface roughness was calculated on the basis of the surface coordinates resulting from the digitization.
8. Thank you for this detailed description. We decided to use the WAAM bars as-built for tensile testing, because postprocessing (e.g., machining or grinding) is not possible in the real combined process. Further investigations on the microstructure are beyond the scope of this paper as the focus of this paper is on the bond behavior.
9. Since chapter 3.2 already contains results of the concrete characterization, we have included it in the chapter "Results and Discussion".
10. The WAAM specimens were additively manufactured in a first step. Subsequently, only the surface was digitally recorded with the LLS-system (laser-based line scan system) in order to analyze the surface characteristics.
11. The second series of tests confirmed the results of the first.

Thank you again for your detailed and helpful review.

Best regards,

Stefan Rappl

Reviewer 3 Report

The authors present a comparative experimental study covering the tensile and bond behaviours, along with surface characterization of the novel Wire and arc additive manufacturing (WAAM) reinforcement compared to conventionally manufactured reinforcement options (reinforcing bars, galvanized threaded rods, plain bars).

The motivation and State of the Art provided by the authors is well written and includes relevant references to justify the purpose of the research presented in the manuscript. The WAAM and selective paste intrusion (SPI) technologies are very clearly explained, along with the background related to the underlying bond mechanisms for reinforced concrete.    

 

The novel WAAM reinforcements show comparable bond behaviour to conventional reinforcing steel bars (B500B), which is definitely one of the positive outcomes from this initial study. The experimental study is clear and reveals interesting results, but it is thought that the methodology and result discussion could be improved to further enhance the findings of this study. The conclusions and next steps for future research and improvements are distinctly presented. 

Specific comments:

Lines 296-297: It is suggested to write this instead: “For each specimen, the load was applied until reaching bond failure.”

Section 2:  It is suggested to include a subsection for “2.3 Tensile tests” to provide an account of the standards, parameters, machine etc. used for tensile testing.

Section 2.3.4 (Lines 299-303): How is the concrete broken open? Also, it could be worth explaining what is of interest to inspect visually. Which features are focused on when looking at the bond zone post testing.

Lines 396-397: Can the authors specify if the same clamping pressure was used for all specimens or if there was an adjustment made after getting the invalid results? If the clamping force needs to be e.g., lower for the WAAM reinforcement, what does this indicate regarding this reinforcement compared to the other tested options?

Section 3.1: Even if there is only one result for WAAM reinforcements, are these obtained values within an expected range compared to previous studies/literature? Is there a way to improve the production to further increase the performance so that it is even more comparable to the other reinforcements? It could be worth noting this in the text. Are the authors planning on testing more WAAM reinforcement bars in the future to get a statistically sound tensile strength characterization? It could be worth noting this in the text.

Line 361: Please specify the purpose of the epoxy glue. Was it to adhere the displacement transducer? 

Line 371 and Figure 7 caption: It is not clear what is meant by “concrete consoles”. Are the authors trying to describe the remaining concrete deposits on the reinforcements? Please try to clarify this. 

Lines 402-405:  It is not clear as to why the authors “suddenly” include these example results from reference [39].  They could be comparable, but it seems like even the type of concrete used in [39] differs from that in the presented study in this manuscript, thus making the comparison less relevant. These example results do not enhance the quality of the discussion and the authors should focus on discussing the generated results instead. 

Figure 2: The arrow for “Plain round steel” needs to be adjusted.

Figure 5: It would worth improving the quality of this image for the sake of clarity/visibility/repeatability.  

Figure 9: It is not clear which data points in the graph are related to S355 natural surface vs surface roughened artificially by sandblasting, as the same type of point is used for all of “S355 [39]”. Please rectify this or remove it (see above comment). 

Table 5: Is the load capacity calculated based on the results presented in Table 2? Please refer to this in the text above the table if it is the case.

Author Response

Dear Reviewer 3,

Thank you for your feedback and valuable input on our paper. We have implemented your comments and suggestions as follows:

• The proposed amendment was adopted as it stands. Line 343-344.
• We have introduced a subsection on the subject of tensile tests, in which the test parameters have also been included. Lines 296-303.
• Additions to this were given in lines 325, 328-329.
• As a result, a second series was manufactured under the same conditions. The results are also given in Table 2. The result of the first series shows a good agreement of the mechanical properties with those of the second series. Due to the triaxial stress condition in the clamping area, there is a unfavorable load distribution there.
• Thank you for your comment. As a result, a second series was manufactured under the same conditions. The results are also given in Table 2. The result of the first series shows a good agreement of the mechanical properties with those of the second series. In addition the results are compared to experiments in the literature (line 359-365).
• Further information were given in line 397. 
• The explanation is given in lines 166-167.
• The other results round bars S355 with different surface roughness were added to show that these results can also be incorporated into the relationship established in this paper. This was also further explained in lines 441-446. To work out the influence of different concrete strengths, the bond stress was divided by the splitting tensile strength of the respective concrete series.
• The figure was revised accordingly.
• A detailed view of the attachment of the displacement transducer has been included in Figure 5.
• This has already been revised by above point.
• The reference to Table 2 has been inserted in line 414.

Thank you for your detailed and helpful feedback.

Best regards,

Stefan Rappl

 

Round 2

Reviewer 2 Report

I am impressed by the improvements that the authors made. It is a pity that there is not any micorstructural analysis that I mentioned previously.