Biophilic Façades: The Potentiality of Bioreceptive Concrete

Round 1

Reviewer 1 Report (New Reviewer)

Comments and Suggestions for Authors

The author stated how biological colonizations are formed on the bioreceptive materials. Focusing on bioreceptive perspectives, colonization on small-scale materials to large-scale materials is discussed. The overall contents in this review paper are well-organized and easy to follow. The schematic representations are well-established and facilitate the readers to grasp the concepts much better. The perspectives presented in this work are up-to-date and refreshing. Here are some suggestions that the reviewer think might be helpful to improve this manuscript.

1. Please adjust colors of texts and background in Fig. 2 to be more readable, especially the grey zone.
2. Since phototrophs are the first colonizers, allowing other heterotrophs to layered colonize on top of them, is there any chance that those organisms are antagonists. If yes, how this kind of interaction affects the growth of bryophytes. Please discuss.
3. What would happen to the aesthetic properties of biofacade materials when bryophytes reach their death stage? After all organisms’ death, how efficient will it be able to capture CO2 and do other their original functions? Please provide the author’s perspectives on this point.
4. It would be interesting to also see the market share of biofacade in comparison with other types of façade materials. If possible, please provide more information about this.

Author Response

We would like to thank the Referee for reviewing our manuscript and for his/her positive evaluation of the study. The detailed answer to all comments is attached to the document. 

Author Response File: Author Response.docx

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

This manuscript needs a major revision.

The manuscript presents relevant and timely research on the use of biological growth on concrete for applications in biophilic design practice.

1. The ecological interactions (competition, facilitation, nutrient cycling) among these groups (cyanobacteria, green algae, fungi, archaea, and other bacteria) are not deeply explored.
2. There is a gap in explaining how UV radiation, desiccation, and temperature fluctuations specifically shape microbial colonization strategies in terrestrial concrete environments.
3. Specify the biochemical pathways of deterioration (e.g., acid production from fungi, sulfur-oxidizing bacteria in sewers, calcium leaching).
4. Give more details about: physical shielding from UV and weathering; biochemical stabilization of the concrete surface, and competition that prevents colonization by more destructive microbes.
5. Conditioning films are abiotic-biotic hybrid layers, whereas biofilms are organized microbial structures. Give more explanations.
6. How mineral dissolution, organic deposition, and EPS-mediated trapping of dust collectively generate substrates suitable for bryophytes.
7. Specify biofilm growth rates, typical EPS concentrations, acid excretion rates, or penetration depths of fungal hyphae.
8. The threshold conditions that shift a biofilm from protective to destructive are not identified.
9. Highlight ecological succession. This sequential process is absent in lab models.
10. Lack of integration with durability metrics. Introduce resilience metrics.
11. Integrate human perception research. 
12. Consider citing relevant recent articles from Buildings or closely related journals to demonstrate alignment with the journal’s scope.

Addressing these revisions will enhance the manuscript’s clarity, technical depth, and overall contribution to the field.

Author Response

We would like to thank the Referee for reviewing our manuscript and for his/her valuable comments. Please find the detailed answer in the attached document. 

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

Accept in present form.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article is a mixture between a review article (first part) and the presentation of own research on a Layered Living Concrete façade panel system. The review addresses a relevant topic that is attracting gaining interest. It focuses on the multi-stage process of establishing a biofilm on concrete surfaces and test set-ups used by different researchers over the last 10 years. It does not contain all research done in this field but gives a good overview about the methodologies used. The own research results are summarized and illustrated with some photographs.

A similar review was published in 2023 (https://doi.org/10.1016/j.jobe.2023.107201). But as the focus in that article is on the factors affecting the bioreceptivity of concrete the current review focusing on the establishment of biofilms and the limitation of laboratory tests it can be seen as complimentary. The article contains enough recent and relevant publications and reviewing them the author draws coherent conclusions. The figures and the tables are appropriate and easy to understand. However, the labelling within the pictures is too small.

The main weakness of the article is that in the concluding remarks several advantages of biofilms are mentioned (superior thermal, air cleaning and noise-reduction performance) without citation of scientific studies that proof them. Either these citations need to be added or it must be acknowledged that these hypotheses need still to be proofed.

Author Response

We thank the reviewers for their constructive criticism, and the time spent to analyze the manuscript. We have addressed all the points and made most of the changes suggested. All changes in the revised version are highlighted in yellow. Please find attached a detailed answer to the reviewers’ comments. The reviewers’ comments are printed in black, and the responses, and explanations related to their comments are given in blue.

1. The article is a mixture between a review article (first part) and the presentation of own research on a Layered Living Concrete façade panel system. The review addresses a relevant topic that is attracting gaining interest. It focuses on the multi-stage process of establishing a biofilm on concrete surfaces and test set-ups used by different researchers over the last 10 years. It does not contain all research done in this field but gives a good overview about the methodologies used. The own research results are summarized and illustrated with some photographs.

We would like to thank the Referee for reviewing our manuscript and for his/her positive evaluation of the study.

2. A similar review was published in 2023 (https://doi.org/10.1016/j.jobe.2023.107201). But as the focus in that article is on the factors affecting the bioreceptivity of concrete the current review focusing on the establishment of biofilms and the limitation of laboratory tests it can be seen as complimentary. The article contains enough recent and relevant publications and reviewing them the author draws coherent conclusions. The figures and the tables are appropriate and easy to understand. However, the labelling within the pictures is too small.

As suggested, we increased the labeling in the figures.

3. The main weakness of the article is that in the concluding remarks several advantages of biofilms are mentioned (superior thermal, air cleaning and noise-reduction performance) without citation of scientific studies that proof them. Either these citations need to be added or it must be acknowledged that these hypotheses need still to be proofed.

We agree, that some conclusions were not supported by the results of the study. Therefore we focused on the findings of the current study, avoiding vague, or unsupported statements.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

This study analyzes biological growth on concrete and assesses its biosensitivity. The relationship between time and biological growth is also indicated in the study and LLC panels are evaluated along with aesthetic appearance. The following comments are made.

·         The study is an article based on observations and existing studies.

·         The negative effects of biological growth on the structural behavior of concrete are not mentioned.

·         The concrete presented in Figure 2 and Figure 3 is load-bearing concrete. Biological growth on this concrete will have negative effects.

·         There is no mention of the negative properties of bio-growth such as corrosion effect on the reinforcement in the concrete.

·         I am of the opinion that the bio-growth process can only be used in pavements over the bearing concrete or in secondary non-bearing elements (such as pavements).

·         It is not yet clear what the long term effects might be.

Author Response

We thank the reviewers for their constructive criticism, and the time spent to analyze the manuscript. We have addressed all the points and made most of the changes suggested. All corrections are highlighted in yellow in the revised version. Please find attached a detailed answer to the reviewers’ comments. The reviewers’ comments are printed in black, and the responses, and explanations related to their comments are given in blue.

This study analyzes biological growth on concrete and assesses its biosensitivity. The relationship between time and biological growth is also indicated in the study and LLC panels are evaluated along with aesthetic appearance. The following comments are made.

 We would like to thank the Referee for reviewing our manuscript and for his/her evaluation of the study.

1. The study is an article based on observations and existing studies.
2. The negative effects of biological growth on the structural behavior of concrete are not mentioned.

We thank the Referee for the important comments. Following the recommendation, we have discussed the possible negative effects, caused by microorganisms in Section 2.2.

3. The concrete presented in Figure 2 and Figure 3 is load-bearing concrete. Biological growth on this concrete will have negative effects.

We agree that biological growth on concrete may cause negative effects. The possible bio-deteriorative and bio-protective mechanisms were discussed in Section 2.2.

4. There is no mention of the negative properties of bio-growth such as corrosion effect on the reinforcement in the concrete.

As suggested, we add the discussion about the negative effects of bio growth on concrete (Section 2.2), including the possible effect on the steel reinforcement.

5. I am of the opinion that the bio-growth process can only be used in pavements over the bearing concrete or in secondary non-bearing elements (such as pavements).

Biological growth on concrete structures may cause severe damage, especially for structures in sulfate- or ammonia-rich environments. In terrestrial environments, biological deterioration is a much slower process, which, in some cases, may even have a bio-protective effect.  A broader discussion on a possible biological attack in different environmental conditions is presented in Section 2.2.

6. It is not yet clear what the long term effects might be.

We agree with this point. The long-term effect of biological colonization on concrete durability is a subject of ongoing investigation. We are performing long-term field tests observing any structural damage caused by biological colonization. To minimize the risks of durability issues, we use a structural high-performance synthetic fiber reinforced concrete (HPSFRC) layer, in our LLC panels.  The layer of HPSFRC has low porosity, low water/cement ratio and high compressive strength (110 MPa). This layer must ensure durability, whereas a porous light-weight concrete layer serves as a mineral substrate for biological growth. Possible biological damage for this layer will not have any effect on the structural integrity of the panels.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

- The author doesn´t address the following:

The open debate and current state of the art, which shows a shift towards a more positive view of biofilms, is not clear. For example, there is no reference to the problem of biodegradation processes or the open debate about the destructive potential of biofilms and how to avoid them, which is mentioned in some of the literature in the bibliography. The article doesn't explain the problems that this biofilm can cause on the concrete, nor is it clear whether the concrete is 'cast reinforced concrete', as in the walls shown in one picture, or 'porous lightweight concrete slab', as in another. It is not clear how the degree of porosity is an important feature in this study. If the study is about presenting greenery outdoors (which is not well explained), what happens to the interior when the optional material is a glass façade?

- There is a drawing dealing with the hypothetical behaviour of organisms in horizontal ... although the study is for vertical surfaces/elements. 

- The structure of the article could be clearer.

- The methodology used is not clear. 

- The text on page 10 would be easier to understand if it were presented in a table.

- The comparison between building materials doesn't make sense or isn't well justified. The article presents concrete walls as an alternative to glass facades and, at the very end of the article, aluminium facades. This comparison is not clear and does not make sense, at least the way it is presented, because these materials have many different qualities: concrete is heavy, opaque and can be part of the building structure (note that it is not a thermal insulation material itself), glass allows natural light and visual connection to the outside (which can be an important biophilic tool), it is not commonly a building structure material, aluminium is opaque but light... and so on.

Many questions arise such as: In a humid climate zone is it desirable to have a heavy and not breathing wall? doesn´t it make more sense to have a breathing light weight wall?

- The author makes wrong/incorrect statements about what biophilic design is, and there is no relevant literature on the subject in the bibliography.

- The author quotes people who were interviewed about their beauty preferences, but there is no information about why their criteria are relevant or good enough and how this was conducted. There is no information on: number of participants, who they are (gender, age, cultural background, nationality (biophilic responses depend on many factors, this is not considered at all). It is not just about the greening of facades or the visual connection, it is about memory and personal experience. It's about beauty, but not about the current debate about grey concrete as a material that people don't like. Please note that 'biophilic design' is a keyword, although it is poorly developed.

- The author refers to what people find beautiful in a little superficial way, although this is one of the main ideas behind the study, and there is no relevant literature in the bibliography on "beauty and people's perception of beauty".

- The author argues important questions that are not supported by the literature and seems to respond more to his/her own opinion than to facts.

- The author supports part of the study based on some data that he doesn't share, so what he/she argues is not properly supported.

Author Response

We thank the reviewers for their constructive criticism, and the time spent to analyze the manuscript. We have addressed all the points and made most of the changes suggested. All corrections are highlighted in yellow in the revised version. Please find attached a detailed answer to the reviewers’ comments. The reviewers’ comments are printed in black, and the responses, and explanations related to their comments are given in blue.

1. The open debate and current state of the art, which shows a shift towards a more positive view of biofilms, is not clear. For example, there is no reference to the problem of biodegradation processes or the open debate about the destructive potential of biofilms and how to avoid them, which is mentioned in some of the literature in the bibliography. The article doesn't explain the problems that this biofilm can cause on the concrete, nor is it clear whether the concrete is 'cast reinforced concrete', as in the walls shown in one picture, or 'porous lightweight concrete slab', as in another.

We thank the referee for raising this issue. Following the recommendation, we have discussed the possible negative effects, caused by microorganisms in Section 2.2.  The monolithic concrete walls shown in Figures 2 and 3, represent an example of irregular biological colonization on existing structures. Our proposed system, composed of a layer of porous concrete, a layer of high-strength concrete, and a biological booster is presented in Figures 4-6. We rewrote the Section 4, making the discussion more consistent, and avoiding the confusing formulation.  

2. It is not clear how the degree of porosity is an important feature in this study.

The proposed LLC panel system combines three materials: a structural high-performance synthetic fiber reinforced concrete (HPSFRC) layer, porous light-weight concrete, and a biological booster. The layer of HPSFRC features low porosity, low water/cement ratio and high compressive strength (110 MPa). This layer must ensure durability, whereas a porous light-weight concrete layer serves as a mineral substrate for biological growth. The porosity of this layer ensures the uniform flow and irrigation water, and has a significant effect on a water retention capacity. We added an explanation regarding the importance of  porous lightweight concrete layer in the first paragraph of page 11.

3. If the study is about presenting greenery outdoors (which is not well explained), what happens to the interior when the optional material is a glass façade?

As was noted by the reviewer, this article reviews the studies on concrete bioreceptivity and proposes a panel system for the building’s façade.  We agree that the presentation was not clear enough. We rewrote the key points in the introduction (Page 1) and discussion (Page 13), suggesting to use the bioreceptive concrete walls in facades with the optimal window-to-wall ratio, rather than “replacing” fully glazed facades.

4. There is a drawing dealing with the hypothetical behaviour of organisms in horizontal ... although the study is for vertical surfaces/elements.

Based on the existing literature, Figure 1 summarizes the possible mechanisms involved in the biological colonization of concrete surfaces. This schematic representation is applicable for any orientation of concrete.

5. The structure of the article could be clearer. The methodology used is not clear.

We agree that the presentation was not clear enough. We rewrote the manuscript, extending section 2, adding section 2.2, and avoiding confusing formulations regarding the use of bioreceptive concrete in facades (introduction and discussion sections).

6. The text on page 10 would be easier to understand if it were presented in a table.

Due to specific Table format requirements, we left the text in bullet points.

7. The comparison between building materials doesn't make sense or isn't well justified. The article presents concrete walls as an alternative to glass facades and, at the very end of the article, aluminium facades. This comparison is not clear and does not make sense, at least the way it is presented, because these materials have many different qualities: concrete is heavy, opaque and can be part of the building structure (note that it is not a thermal insulation material itself), glass allows natural light and visual connection to the outside (which can be an important biophilic tool), it is not commonly a building structure material, aluminium is opaque but light... and so on.

We agree that the comparison in the previous version was confusing. As suggested we avoided the direct comparison of glass façade with the bioreceptive concrete façade. Instead, we compared façade with the optimal window-to-wall ratio, with the fully glazed facades. When properly designed (optimizing window placement for daylight, views, and ventilation) facades with the optimal window-to-wall ratio may still offer a visual connection with the environment while maintaining the low energy demands for heating and cooling.

8. Many questions arise such as: In a humid climate zone is it desirable to have a heavy and not breathing wall? doesn´t it make more sense to have a breathing light weight wall?

The structural solution of bioreceptive concrete panels is applicable for usual ventilated façade systems, as an alternative to stone, fiber cement, or aluminum cladding. The relatively thin panels (the structural layer of HPSFRC is 20 mm thick)  may be attached to the usual double-railing profile systems. For clarity, we added the statement regarding the application of the LLC panels (top of the page 14).

9. The author makes wrong/incorrect statements about what biophilic design is, and there is no relevant literature on the subject in the bibliography.

We agree that the statements in the previous version could be incorrect. After studying the literature, we corrected the statements regarding the biophilic design (abstract, introduction, and discussion section om page 13). We added two new references to the list:

7. Wijesooriya, N., & Brambilla, A. (2021). Bridging biophilic design and environmentally sustainable design: A critical review. Journal of Cleaner Production, 283, 124591.
8. Kellert, S., & Calabrese, E. (2015). The practice of biophilic design. London: Terrapin Bright LLC, 3(21).

10. The author quotes people who were interviewed about their beauty preferences, but there is no information about why their criteria are relevant or good enough and how this was conducted. There is no information on: number of participants, who they are (gender, age, cultural background, nationality (biophilic responses depend on many factors, this is not considered at all). It is not just about the greening of facades or the visual connection, it is about memory and personal experience. It's about beauty, but not about the current debate about grey concrete as a material that people don't like. Please note that 'biophilic design' is a keyword, although it is poorly developed.

Following the recommendation we added details regarding the survey on page 9.

11. The author refers to what people find beautiful in a little superficial way, although this is one of the main ideas behind the study, and there is no relevant literature in the bibliography on "beauty and people's perception of beauty".

As suggested, we studied and added new references on the perception of beauty in architecture. Indeed, the perception of beauty is a far more complex issue than the practical questions discussed in the current study regarding the bioreceptivity of concrete. Therefore, the in-depth discussion about the perception of beauty is out of the scope of the present study.

83. Goodrum, W. M., Zhai, Z. J., & Robles, M. (2023). Impacts of architectural beauty to building energy performance. Architecture, Structures and Construction, 3(1), 87-111.
84. Kozamernik, J., Rakuša, M., & Nikšič, M. (2020). How green facades affect the perception of urban ambiences: Comparing Slovenia and the Netherlands. Urbani izziv, 31(2), 88-100.

12. The author argues important questions that are not supported by the literature and seems to respond more to his/her own opinion than to facts.

We agree, that the formulation of conclusions was not supported by the results of the study. Therefore we focused on the findings of the current study, avoiding vague, or unsupported statements.

13. The author supports part of the study based on some data that he doesn't share, so what he/she argues is not properly supported.

The present study is mainly a review article, critically analyzing the existing studies on concrete bioreceptivity. The interim results, shown in pictures at the end of the study have an illustrative purpose, supporting the possible trends in the use of bioreceptive concrete for the building envelope. We leave the in-depth methodological analysis of biological growth on concrete in the field conditions for a separate dedicated study.

Author Response File: Author Response.docx