Is Cross-Laminated Timber (CLT) a Wood Panel, a Building, or a Construction System? A Systematic Review on Its Functions, Characteristics, Performances, and Applications

Round 1

Reviewer 1 Report

Comments for author File: Comments.pdf

Author Response

Recommendation: Minor revision

1. Line 66-69, reference needed.

R: Thank you for the observation. But, there are no references, as the statements were studied. You can check that we wrote an additional phrase to specify it: “These statements were analyzed using a complete review engendered by multiple sources and discussions.”

2. Line 148-149, please revise the sentence.

R: We really appreciated your observation. We revised all items in this sentence as you can check: “About the categories, it is worth mentioning the use of the following considerations: • Due to numerous authors identified, graphical representations were designed to demonstrate only those most popular ones, that is, those authors with three or more papers in any of the databases under consideration; •By the same reason considered in the previous item, the same way was followed in the designs of graphics for the volumes of countries and conference papers;”. As you can verify, all authors were regarded in the study (Table 2), but only those with three or more papers were named in the Figure 2, evincing the most popular ones.

3. Line 148-149, why does the author eliminate the authors that have less than 3 articles? How about selecting the articles based on other parameters, such as citation scores, or journal impact factors?

R: As mentioned in the previous suggestion, we revised the item. Considering that two different databases were regarded, the obtained data allowed for greater accuracy with regard to the number of articles per author. For that reason, we selected this alternative.

4. Based on the summaries from literature review, the title should be different. The findings are more related to the development of CLT in different aspects. How the academy name CLT is not the main focus although it worth mentioning at the beginning of the findings.

R: Good suggestion. You can confirm that we satisfied this point when we mentioned in the title that it is “A systematic review on its functions, characteristics, performances, and applications”. But it is the further approach. But the main approach is the systematic review and, above all, thes findings about the incorrect mentions about CLT, which justified this question. Comparing to traditional reviews, they usually address different views on the subject, but this discovery on the incorrect mention of technical terms is somewhat rare. Due to the importance of findings related to correct designations, we decided to be emphatic in this main topic. As you can check, our study verified that dozens of relevant papers published in regarded journals mentioned the CLT, which is correctly understood as a product/panel/element, as other function, for example, building, system, construction system…

5. Conclusion is too long.

R: We really understand your point, but it is complex to reduce the observations in a complete approach that addressed different views in the same subject. However, you can check that we reduced some phrases and paragraphs to the point that this section takes up less than a page.

Reviewer 2 Report

Please explain the description of columns FR and CR in table 10. The index is 2 for both columns. What is probably labeled as fire reaction should, in my opinion, be properly Reaction to fire class.

Author Response

Please explain the description of columns FR and CR in table 10. The index is 2 for both columns. What is probably labeled as fire reaction should, in my opinion, be properly Reaction to fire class.

R: Thank you. We rewrote this part to clarify better. You can check that legend was adjusted as well as the sentence about the table. Furthermore, the classes were detailed in the table too.

Reviewer 3 Report

The Author propose an interesting and systematic review about cross-laminated timber (CLT) products and their applications in the construction field from single-family residences to low-rise buildings. A particular aspect of the review is the systematic investigation of papers existing in the literature about different features of the CLT. The Authors summarize their approach pointing out that “All discussions are presented by the identification of the function of cross-laminated timber, explanation of this product and basic definitions, main raw materials, production process, structural performance, fire resistance, applications, residential uses and their advantages and obstacles, modularization of residences, strategies to proliferate modular houses based on cross-laminated timber and, lastly, the conclusions”.

The part on the structural aspects of CLT products is well presented and the paper is well organized and written. Unfortunately, my great concern is that the Authors did not carry out a proper review about very important topics, such as the moisture effects, the related dimensional stability and the durability against fungal decay in CLT. These topics cannot be just mentioned without clarifications (as it is done in the current version of the paper). Therefore, I have provided several detailed comments and suggestions, and some additional references. Please check all my comments in the revised pdf file. In addition to this, I encourage the Authors to do a better review on these topics and eventually add more references and modify some of the presented Tables including the new references. Consequently, the manuscript should be slightly modified. Also the title of the paper could be improved, see my comments in the revised pdf file.

Some additional references

·      Bobadilha, G., Stokes, C., Kirker, G., Ahmed, S., Ohno, K., and Verly Lopes, D. (2020). Effect of exterior wood coatings on the durability of cross-laminated timber against mold and decay fungi. BioRes. 15(4), 8420-8433.   

·        Fragiacomo, M., Fortino, S., Tononi, D., Usardi, I., Toratti, T. (2011) Moisture-induced stresses perpendicular to grainin cross-sections of timber members exposed to different climates. Engineering Structures 33 (11), 3071–3078.

·        Lukacevic, M. , Autengruber, M., Raimer, T., Eberhardsteiner, J., Füssl, J. (2021). Effect of cast-in-place concrete application on moisture distribution in timber-concrete composite floors with notched connections, investigated via finite element simulations. Journal of Building Engineering 42,103005.

Conclusions

Based on the above comments, I would suggest that the paper could be considered for publication in MDPI Forests journal after a minor revision.

Kind regards,

The Reviewer  

Comments for author File: Comments.pdf

Author Response

The Author propose an interesting and systematic review about cross-laminated timber (CLT) products and their applications in the construction field from single-family residences to low-rise buildings. A particular aspect of the review is the systematic investigation of papers existing in the literature about different features of the CLT. The Authors summarize their approach pointing out that “All discussions are presented by the identification of the function of cross-laminated timber, explanation of this product and basic definitions, main raw materials, production process, structural performance, fire resistance, applications, residential uses and their advantages and obstacles, modularization of residences, strategies to proliferate modular houses based on cross-laminated timber and, lastly, the conclusions”.

R: Of course you described the paper approaches, but they were the secondary components in the document development, since the main goal was evinced by the identification (occurred after the readings of the literature prospected) since inaccurate mentions were observed about CLT designations, especially as a building, construction system or system, although it is only a construction input in the category of structural panel, also classified as an engineered wood product, being utilized in the form of elements for prefabricated and modular buildings.

The part on the structural aspects of CLT products is well presented and the paper is well organized and written. Unfortunately, my great concern is that the Authors did not carry out a proper review about very important topics, such as the moisture effects, the related dimensional stability and the durability against fungal decay in CLT. These topics cannot be just mentioned without clarifications (as it is done in the current version of the paper). Therefore, I have provided several detailed comments and suggestions, and some additional references. Please check all my comments in the revised pdf file. In addition to this, I encourage the Authors to do a better review on these topics and eventually add more references and modify some of the presented Tables including the new references. Consequently, the manuscript should be slightly modified. Also the title of the paper could be improved, see my comments in the revised pdf file. Some additional references

• Bobadilha, G., Stokes, C., Kirker, G., Ahmed, S., Ohno, K., and Verly Lopes, D. (2020). Effect of exterior wood coatings on the durability of cross-laminated timber against mold and decay fungi. BioRes. 15(4), 8420-8433.
• Fragiacomo, M., Fortino, S., Tononi, D., Usardi, I., Toratti, T. (2011) Moisture-induced stresses perpendicular to grainin cross-sections of timber members exposed to different climates. Engineering Structures 33 (11), 3071–3078.
• Lukacevic, M. , Autengruber, M., Raimer, T., Eberhardsteiner, J., Füssl, J. (2021). Effect of cast-in-place concrete application on moisture distribution in timber-concrete composite floors with notched connections, investigated via finite element simulations. Journal of Building Engineering 42,103005.

R: we really appreciated your comments about the organization and writing, reasons why the paper offers a proper review. We understand your opinion about further topics to be inserted, but there are different topics approached such as product definition and accurate/inaccurate designations, main raw materials (including wood species and resins), production process of panels, structural performance and fire resistance of panels, panel applications, residential uses and their positive and negative points, further approaches to explain production technologies to build any CLT-based construction to explain prefabrication and modularization, strategies to proliferate modular houses based on cross-laminated timber. Moreover, our study is designed to be different and raise some forgotten topics (and other unprecedented too) in relation to the available reviews on CLT panels. These former reviews are usually dedicated to address panel features and physical-mechanical properties, not including these topics also important for the timber industry and construction markets. Although moisture effects, dimensional stability and durability are very important topics too, they would be only secondary points in this approach, being more important to those reviews dedicated to specifically evaluate and discuss properties of CLT panels. Nevertheless, we decided to partially follow some of your suggestions, since we inserted shrinkage-swelling behavior percentages in two conditions (in-plane and out-of-plane) in Table 9 to support observations and citations about importance and influence of moisture content. Such modifications are clearly inserted in the position before the item fire resistance as suggested. Therefore, we modified some tables to insert new information. Lastly, information on durability against fungal decay and pest attacks were inserted too.

Conclusions

Based on the above comments, I would suggest that the paper could be considered for publication in MDPI Forests journal after a minor revision.

Kind regards,The Reviewer 

R: Thank you for the suggestions.

Line 1: I would suggest to delete or modify this part of the title because it creates confusion. The CLT is a typology of engineered wood product used for wooden components in buildings (such as the glued laminated timber, LVL,  etc.). I would avoid to create alternative definitions as the ones you mention, for instance to define the CLT as "a building" looks quite unusual... Then, you can be more specific in the abstract and manuscript.

R: Unfortunately, this title change would “delete” the most relevant outcome identified by our contribution. In review studies, the citations are mentioned to build dialogues, but they are findings of other authors. Our review exceeded the normal expectations of any conventional discussion of what has already been published. Inaccurate mentions about the function of a cross-laminated timber were verified, as they were different from the standard designations mentioned by leading studies and technical standards about this topic. We led a thorough analysis detailed in the first topic of discussions to evince errors and confusions said by many academics, which mentioned CLT as a building, a construction system, a system, a house, etc. Based on numerous the main European technical standards and several papers published by notable researchers, we identified that CLT is only a construction input of the structural panel category, also classified as an engineered wood product, which is utilized in elements for prefabricated and modular buildings. Therefore, we do not create alternative definitions, we pointed out that there are many alternative wrong definitions stated by many studies, which have erroneously mentioned the CLT products as buildings, construction systems, systems, etc. Thus, we were really specific in the title, detailing our main goals as you can verify.

Line 70: At the end of the Introduction, you should shortly discuss the important topics of moisture variation and durability related to fungal decay in CLT. Please check my detailed comments in Section 4.4, add the suggested references and also eventual other references on these topics. You can add a specific Section on these topics before Section 4.6.

R: As I explained previously, panel properties are secondary topics in a discussion exclusively dedicated to be different of traditional reviews, which are dedicated to approach properties of CLTs. It should be noted that our contribution addressed uncommon topics of the CLT subject, specifically about construction, production, technologies, raw materials in use, applications, market potential for residences, and, after the literature reading, the unprecedented findings supported by strong and representative discussions about correct/incorrect designations. The introduction was designed using a “funneling” strategy, starting from more generalist points (timber, timber forest products, use of wood products in construction, new ways provided by engineered wood products, EWP examples, application in residences, and some points to reinforce the potential uses in residences. As you can verify, no property was specifically stated in this part, since panel properties are less usual in discussions focused on construction aspects, designations of nomenclatures and technical terms, production, technologies, applications, raw materials, and potentials of the residential uses. Despite its importance, the properties are only secondary topics and their insertions in this initial part (which are usually dedicated to explore the leading ways of paper goals) sound very strange and, therefore, unnecessary in this initial part. We remember that this review is being designed to satisfy several gaps forgotten by the current review papers, whose contributions are also important because they are really focused on the discussions of panel properties, detailing their correlations and specificities. We checked your suggestions and partially accepted them. Therefore, we inserted some clarifications about moisture content as well as shrinkage and swelling behavior properties (Table 9), both in-plane and out-of-plane. They were inserted in the point suggested, however, together with physical properties due to the relation to moisture content and other physical properties. We inserted information about durability and preventions against fungal decay and pest attacks as well.

Line 236: Before this I would add "moisture effects, dimensional stability and durability against fungal decay"

R: They were inserted in the point suggested (before fire resistance topic), as they presented a relation to the moisture effects and other information mentioned in the section of physical properties.

Line 383: This is the first time that you mention the "moisture content" in the manuscript. Before this sentence, i.e. in the Introduction, you should clarify that CLT, being wood a hygroscopic material, is sensitive to moisture variations as the other engineered wood products. The difference of moisture contents between the surface of the wood product and the internal locations under variable climates (in indoor or outdoor conditions) creates moisture gradients and, consequently, moisture induced stresses that can be responsible of failures such as cracks. In a specific section, before Section 4.6 below, you can clarify that this phenomenon has been investigated by several numerical studies, as Fragiacomo et al. 2011 and Lukacevik et al. 2021 (see the full references in the Reviewer report). In addition, you introduce the concept of fungal attack without specifying anything about durability of CLT... Actually the durability of CLT has not been explored so much at present.  I would suggest to read and cite the paper by  Bobadilha et al. 2020 where the state of art of CLT durability against mold and decay fungi is discusses (see the full reference in the Reviewer report). It would be very important to point out that the wood durability plays an important role for the performance of the wooden components under various climate exposures. To estimate this performance, material resistance models able to predict the decay rates, such as the one proposed in (Brischke et al. 2021), currently used for several types of wooden components, could be also adopted for the CLT,  for structural integrity purposes.

R: We appreciated your suggestions, but they were inserted in the final part of the section about physical properties, since they complete better the discussions raised in that part. The line 383 is in the part of raw materials, while the property part is specific to CLT performance.

Line 418: You should clarify that this if the "initial moisture content" after production. During the service life of the product, the moisture in CLT will vary depending on the environment, see the previous Reviewer's comments.

R: Of course you are right. Considering that we explained a topic about raw materials, which are used to produce a CLT panel, this information is related to production. It was adjusted.

Line 424: Do you mean the swelling and shrinkage due to moisture variations? Please explain and relate this to the other parts of the paper in which you should better discuss the concepts of moisture effects in CLT.

R: We explained in detail after the table to satisfy your expectations and possible doubts.

Line 455: Before this section you should add another one regarding the moisture effects, dimensional stability and durability of CLT against fungal decay, see my other comments.

R: We inserted the information as you constantly required us in the previous suggestions. All these points were considered and discussed in accordance with your other comments.

Line 575: Once again, you should relate the concept of dimensional stability to the wood response to moisture effects, please check the other comments and your cited paper by Brandner et al. 2016. Please note that the dimensional stability is important during the service life of the product.

R: Thank you. We inserted a Brandner’s remark to complete the discussion.

Line 727: At the end of the Conclusions, you could add that, for future research and development, it would be important to investigate more the durability of CLT against fungal decay in order to optimize the structural integrity of the product (see again the papers by Bobadilha et al 2020 and Brischke et al 2021).

R: we inserted these points and other relevant suggestions as well. Thank you. :)