Natural Fibers for Out-of-Plane Strengthening Interventions of Unreinforced Masonry Buildings in Aggregate Configuration

Round 1

Reviewer 1 Report

Unfortunately, both the title of the paper and the abstract promise much more than presented in the manuscript.

The authors do not conduct any substantive analysis of the influence of natural fibers reinforcement on the behavior of the structure subjected to out-of-plane interaction. What has been presented here is some kind of simplification - based on completely wrong assumptions - in terms of the behavior of strengthening in the actual construction. The contribution of the strengthening system (parameter “s” in the formula (14)) given here is completely unknown. In this analysis, this is a key parameter that (see description below) is wrong adopted.

The single-lap shear test referred by the authors (literature [10]) are conducted on bricks (any type) that are absolutely not the masonry units (rubble soft stone masonry) from the analyzed building. Thus, the test shows a completely different characteristics of the shear stress-strain than would be observed on the real masonry substrate of this object. Certainly, the adhesion of the composite to the surface of the historical brick is completely different. Thus, the assumption made here is incorrect and cannot be used as a basis for the analysis.

Please take into account that, it is absurd to adopt as effective and efficient a strengthening of wall with dimensions of 8.0 x 3.9 m and a thickness of 0.70 m made of two strips with a width of 60 mm. Thus, the calculation of the parameter "S" (formula 10) is incorrect.

So the case study shown here is wrong and incorrect.

In addition, no numerical analyzes were performed (as written in the abstract), as there is no numerical model that reflects the behavior of actual structure. Everything is based on some theoretical assumptions - not fully defined and incorrect - without any material, model or in-situ tests. The reference to other tests carried out on other masonry elements is incorrect.

 Unfortunately, it is not possible to correct the paper, because the "S" parameter cannot be properly determined - without extensive own research conducted on real building. So, the paper cannot be published.

Author Response

In the revised version of the paper the revisions about comments/suggestions of:

-    reviewer 1 are marked in cyan/yellow;

-    reviewer 2 are marked in cyan/green.

 

Replies to reviewer 1

In the following are reported general comments concerning most of the reviewer’s observations and, in the second part, the detailed replies.

 

General comments:

The second part of the paper is devoted to present a ‘numerical’ study finalized to investigate the contribution provided by FRCM systems made of natural fibers on the ‘global’ out-of-plane response of masonry facades, by taking into account the local bond behavior obtained from tests of literature.

To this end, the Authors employed the common ‘numerical’ analysis method based on the so-called macro-modelling approach by introducing into the equations governing this method the constitutive local bond behavior of reinforcement experimentally deduced in [10]. Regarding the latter, since the type of reinforcing system is composed of a reinforcement embedded into two layers of matrix (one applied on the masonry support and the other covering the reinforcement), the debonding behavior (followed by tensile failure of natural fibers) occurred inside the matrix and not at the level of masonry support. Consequently, the type of masonry support where the accounted strengthening system was applied did not significantly influence its local behavior in terms of strength, stiffness and failure mode. This feature then justifies the assumption made by the Authors in considering the same local behavior experimentally deduced from a support made of clay bricks for the case study composed of a different type of masonry.

As a consequence, the evaluation of the force ‘S’ of the strengthening supposed applied on the façade (the ‘parameter’ S introduced into the equation governing the problem is indeed the force provided by the strengthening during the tensile test or, equivalently, during the progress of the out-of-plane kinematics) is simple evaluated by a geometric proportion in terms of width (the one accounted for the facade and the one accounted for shear lap test). In particular, starting from the number of yarns composing the width accounted in experimental tests (60 mm), the number of yarns in the width assumed for the application on the façade (200 mm) has been evaluated. Then, the cross-section area of reinforcement has been obtained by multiplying the area of a single yarn times the obtained number of yarns. This approach is physically in agreement with experimental evidences where the force obtained during a shear lap test is proportional to the width of strengthening or the number of yarns.

As underlined in the paper, the actual state of the art is characterized by studies on strengthening systems made of natural fibers only concerning investigation about the local bond behavior. The majority of tests on this type of strengthening showed failure modes quite different from the ones observed in case of synthetic reinforcements. In particular, the tensile failure of fibers was observed as additional mechanism activating after the beginning of debonding. Then, the contribution provided by the Authors is to examine the feasibility of this type of strengthening systems toward a typical mechanism consisting of the global out-of-plane of building facades in aggregate configuration (a configuration common in Italian historical centers). In addition, differently from other approaches where only the peak force (strength) of the strengthening system is considered, the Authors here account for the whole experimental local response in order to evaluate the contribution of strengthening system on the progress of the global mechanism of the façade (i.e. in terms of strength and displacement).

 

Nevertheless, in agreement with the observation of both Editor and reviewer, the title has been revised in “Natural fibers for out-of-plane strengthening interventions of unreinforced masonry buildings in aggregate configuration”.

 

Detailed replies:

R1 - Unfortunately, both the title of the paper and the abstract promise much more than presented in the manuscript.

In agreement with the title and the contents of the abstract, the paper analyzes the feasibility of strengthening systems made of natural fiber toward out-of-plane mechanisms of masonry facades of buildings in aggregate configuration. Indeed, numerical analyses based on the macro-modeling approach for out-of-plane mechanisms are performed by considering the influence of adjacent structural units (first part of the paper) and the presence of strengthening systems made of natural fiber (second part of the paper). Both aspects are analyzed in detail and translated into the equations governing the problem.

In order to better underline this aspect, the abstract has been opportunely improved.

R2 -The authors do not conduct any substantive analysis of the influence of natural fibers reinforcement on the behavior of the structure subjected to out-of-plane interaction. What has been presented here is some kind of simplification - based on completely wrong assumptions - in terms of the behavior of strengthening in the actual construction. The contribution of the strengthening system (parameter “s” in the formula (14)) given here is completely unknown. In this analysis, this is a key parameter that (see description below) is wrong adopted.

The performed analyses are devoted to evaluate the contribution of natural fiber reinforcing systems toward out-of-plane mechanisms. Then:

• The kinematic analysis approach is employed according to the studies available in literature and the indications contained in current national and international guidelines
• The influence of adjacent units composing the aggregate has been considered and opportunely introduced in the performed analyses
• The contribution of the strengthening system has been directly deduced from experimental tests in order to account for the ‘effective’ local behavior characterizing this ‘new’ type of strengthening system and, then, to correctly introduced it in the performed analyses. In particular, instead of considering the bond strength only (i.e. the peak of the force ‘S’ deduced from test), the Authors have here introduced the full local behavior thanks to the analogy between the behavior of the strengthening system during a shear lap tests and the behavior of the strengthening system during the progress of the mechanism of the facade (this is one the main strength of the paper). The force ‘S’ is indeed generally deduced from a shear-lap test: the formulas available in literature for evaluating the force ‘S’, now available for FRP and FRCM made of synthetic fibers only, are indeed based on the experimental outcomes of shear lap tests. Then, ‘S’ is not an ‘unknown’ parameter but, on the contrary, it represents a ‘known’ input parameter here deduced from experiments since any theoretical formulas for natural fibers are nowadays available. The results deduced from numerical analyses (i.e. the kinematic analysis) have then underlined both the influence of the aggregate configuration and the influence of strengthening systems made of natural fibers toward out-of-plane mechanisms induced by seismic actions.

Additional comments have been added in the revised version of the paper in order to better point out the above aspects.

R3 - The single-lap shear test referred by the authors (literature [10]) are conducted on bricks (any type) that are absolutely not the masonry units (rubble soft stone masonry) from the analyzed building. Thus, the test shows a completely different characteristics of the shear stress-strain than would be observed on the real masonry substrate of this object. Certainly, the adhesion of the composite to the surface of the historical brick is completely different. Thus, the assumption made here is incorrect and cannot be used as a basis for the analysis.

As specified in the general comments, since the debonding behavior for the analyzed reinforcement occurs inside the matrix, the type of support does not significantly influence neither the local bond behavior nor the local failure mechanism. Consequently, the results obtained from the accounted experimental shear lap tests on a masonry support made of clay bricks can be used also for other types of supports. Indeed, this is a further main difference between FRP, where debonding generally involves the masonry support, and FRCM where debonding generally does not involve the masonry support (the support of reinforcement is indeed in this case the lower layer of matrix).

A specific comment has been introduced in the paper to better emphasize this aspect.

R4 - Please take into account that, it is absurd to adopt as effective and efficient a strengthening of wall with dimensions of 8.0 x 3.9 m and a thickness of 0.70 m made of two strips with a width of 60 mm. Thus, the calculation of the parameter "S" (formula 10) is incorrect. So the case study shown here is wrong and incorrect.

As underlined in the general comments, the evaluation of the force ‘S’ of the strengthening supposed applied on the façade (the ‘parameter’ S introduced into the equation governing the problem is indeed the force provided by the strengthening during the tensile test or, equivalently, during the progress of the out-of-plane kinematics) is simple evaluated by a geometric proportion in terms of width (the one accounted for the facade and the one accounted for shear lap test). In particular, starting from the number of yarns composing the width accounted in experimental tests (60mm), the number of yarns in the width assumed for the application on the façade (200mm) has been evaluated. Then, the cross-section area of reinforcement has been obtained by multiplying the area of a single yarn times the obtained number of yarns. This approach is physically in agreement with experimental evidences where the force obtained during a shear lap test is proportional to the width of strengthening or the number of yarns.

The corresponding text in the revised version of the paper has been improved.

R5 - In addition, no numerical analyzes were performed (as written in the abstract), as there is no numerical model that reflects the behavior of actual structure. Everything is based on some theoretical assumptions - not fully defined and incorrect - without any material, model or in-situ tests. The reference to other tests carried out on other masonry elements is incorrect.

The analyses performed in the paper have been carried out considering the common and well-known approach for out-of-plane mechanisms based on the kinematics of rigid bodies. This is a ‘numerical’ approach used for ‘numerically’ studying the behavior of masonry elements toward out-of-plane mechanisms. The results presented in the paper in terms of capacity curves are indeed obtained from ‘calculations’ by using this approach (instead of a Finite Element Model which indeed is generally adopted for analyzing the in-plane behavior). The use of this approach allows to derive simple equations governing the problem, i.e. describing the kinematic of the macro-model toward the account mechanism. Then, the introduction of the reinforcement or the aggregate effect is equivalent to modify the numerical macro-model by equipping it of additional features.

The theoretical assumptions concerning the macro-model are the ones at the basis of the kinematic analysis approach (also suggested by documents of literature and standard guidelines) and not ‘incorrectly’ here introduced by the Authors. Also, the assumptions concerning the aggregate effect and the reinforcement behavior are in agreement with the theory and experimental outcomes (see the previous comments).

R6 -Unfortunately, it is not possible to correct the paper, because the "S" parameter cannot be properly determined - without extensive own research conducted on real building. So, the paper cannot be published.

See the previous comments concerning this aspect. In our opinion, they clarify the meaning and the derivation of this ‘parameter’.

Author Response File: Author Response.pdf

Reviewer 2 Report

Natural fibers for out-of-plane strengthening interventions of URM buildings in aggregate configuration

1.       It is not a good idea to acronym in the title, particularly when they are not very common. Masonry buildings are usually unreinforced and are commonly known as masonry building, so why you are using the term unreinforced here.

 

2.       The introduction of the paper failed to set a clear agenda for the research. There are many points/ contents which do not connect with each other.

 

3.       The abstract is reported in a confused manner. First sentence is not clear – try to revise to make the meaning clear. the research method is somehow clear but need some adjustment to reflect the approach with more clarity. You also need to report the findings, limitations, and future direction of the research.

 

4.       There are too many figures and equations which make the paper significantly difficult to follow. Several basic figures and common equations can be easily removed without any loss. If these are important to report, you need to report in the appendix.

 

5.       There is no content which help to understand the research approach adopted.

 

6.       If the study includes any case study, then this should be reported in the abstract and also in the research approach.

 

7.       The Discussion and final remarks section need to be refined making sure the contents are easily followed by non-specialized readers. make sure you report the limitations of the research and outline the areas for further research.

 

8.       The writing need improvement. Try to use short sentence to avoid derailing the message you want to deliver. A thorough proofread is recommended.

 

9.       Remove all the personal citations where possible.

Comments for author File: Comments.pdf

Author Response

In the revised version of the paper the revisions about comments/suggestions of:

-    reviewer 1 are marked in cyan/yellow;

-    reviewer 2 are marked in cyan/green.

 

Replies to reviewer 2

 

R1 - It is not a good idea to acronym in the title, particularly when they are not very common. Masonry buildings are usually unreinforced and are commonly known as masonry building, so why you are using the term unreinforced here.

The acronym URM is widely used in current literature for distinguish common masonry constructions from ones equipped with traditional or innovative (composite materials) reinforcements.

Nevertheless, in agreement with the observation of both Editor and reviewer, the title has been revised in “Natural fibers for out-of-plane strengthening interventions of unreinforced masonry buildings in aggregate configuration”.

 

R2 - The introduction of the paper failed to set a clear agenda for the research. There are many points/ contents which do not connect with each other.

Thank you for the comment. The Introduction section has been reviewed with the aim to better explain the research agenda and the adopting methodology.

 

R3 - The abstract is reported in a confused manner. First sentence is not clear – try to revise to make the meaning clear. the research method is somehow clear but need some adjustment to reflect the approach with more clarity. You also need to report the findings, limitations, and future direction of the research.

Thank you for the suggestions. The abstract and conclusions have been revised. In particular the conclusions have been appropriately improved by emphasizing that one of the limitations of the proposed approach is the knowledge of the experimental constitutive bond behavior of reinforcement. Nevertheless, the future development of formulas for deriving simplified constitutive laws for this type of strengthening, will allow to use the proposed approach as a practical design tool.

 

R4 - There are too many figures and equations which make the paper significantly difficult to follow. Several basic figures and common equations can be easily removed without any loss. If these are important to report, you need to report in the appendix.

The Authors understand the observation of the reviewer, therefore in the revised version of the paper some equations and a figure have been removed.

 

R5 - There is no content which help to understand the research approach adopted.

Specific comments have been introduced in the revised version of the paper in order to better explain the adopted research approach.

 

R6 - If the study includes any case study, then this should be reported in the abstract and also in the research approach.

Thank you for the observation. Reference to the case study has been added in the abstract and in the research approach.

 

R7 - The Discussion and final remarks section need to be refined making sure the contents are easily followed by non-specialized readers. make sure you report the limitations of the research and outline the areas for further research.

Additional comments have been added in the revised version of the paper in order to make the contents easily followed by non-specialized readers. In particular, it has been emphasized that one of the limitations of the proposed approach is the knowledge of the experimental constitutive bond behavior of reinforcement. Nevertheless, the future development of formulas for deriving simplified constitutive laws for this type of strengthening, will allow to use the proposed approach as a practical design tool.

 

R8 - The writing need improvement. Try to use short sentence to avoid derailing the message you want to deliver. A thorough proofread is recommended.

The paper has been carefully revised in order to improve the quality of the text.

 

R9 - Remove all the personal citations where possible.

The paper here presented is part of a research activity still in progress. The personal citations represent important contributions to the development of the present study.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

I thank the authors for the explanations provided.

I agree with the kinematic analysis performed here and the introduction of an additional term to the gain equations. This approach is generally correct.

Unfortunately, I completely disagree with the adopted approach of the authors: “the contribution of the strengthening system has been directly deduced from experimental tests in order to account for the ‘effective’ local behavior characterizing this ‘new’ type of strengthening system and, then, to correctly introduced it in the performed analyses”. This assumption is completely wrong. On the basis of the laboratory tests quoted here, it is not possible to determine the effective contribution of strengthening because this strengthening is unrealistic in the practice and completely inappropriate to the analyzed issue.

In the case of such a structure, it is incorrect to assume that such a minimum strengthening will bring anything, so it makes no sense to analyze it at all. It doesn't matter what kind of failure occurs, because the mode of strengthening is wrong. The authors' conviction about the mode of failure is also interesting, because it is not known - and the failure presented in other research samples (completely different than the analyzed strengthening) cannot be considered here as a basis for assuming that the damage will occur in the matrix, and not as debonding. So, the point describing the case study is incorrect here.

Author Response

Reviewer 1

I thank the authors for the explanations provided.

I agree with the kinematic analysis performed here and the introduction of an additional term to the gain equations. This approach is generally correct.

Unfortunately, I completely disagree with the adopted approach of the authors: “the contribution of the strengthening system has been directly deduced from experimental tests in order to account for the ‘effective’ local behavior characterizing this ‘new’ type of strengthening system and, then, to correctly introduced it in the performed analyses”. This assumption is completely wrong. On the basis of the laboratory tests quoted here, it is not possible to determine the effective contribution of strengthening because this strengthening is unrealistic in the practice and completely inappropriate to the analyzed issue.

In the case of such a structure, it is incorrect to assume that such a minimum strengthening will bring anything, so it makes no sense to analyze it at all. It doesn't matter what kind of failure occurs, because the mode of strengthening is wrong. The authors' conviction about the mode of failure is also interesting, because it is not known - and the failure presented in other research samples (completely different than the analyzed strengthening) cannot be considered here as a basis for assuming that the damage will occur in the matrix, and not as debonding. So, the point describing the case study is incorrect here.

Reply to reviewer 1

The authors thank the reviewer for his observations which gave them the opportunity to better explain the approach with it main issues and limits.

The approach adopted in the paper is based on the fact that the ‘effective’ behavior in terms of the local bond behavior of fiber reinforced systems is commonly deduced from shear lap bond tests. These tests, indeed, provide specific information about the local bond behavior in terms of strength, slip, failure modes. In addition, during these tests, the key parameter is the bond length which guarantees the attainment of the maximum value of the bond strength (this length is called “optimal bond length”). The width of the strip is not important. Indeed, the obtained peak of the force attained during shear-lap tests is normalized with respect to the width in order to obtain a ‘constitutive’ normal stress-slip law to be adopted for practical design application where larger strips are used. It is the local bond behavior obtained from shear-lap tests which allows for subsequently studying the contribution of the strengthening system on the global response of structures.

The goal of the present paper has been to propose a methodology aimed at accounting for both the friction and reinforcement contributions on the evolution of the kinematics of the façade. To this end, it has been supposed that the local bond behavior of a FRCM system made of natural fibers deduced from shear lap tests of literature on a different type of masonry support, is the same for the masonry material composing the accounted façade. Since the literature tests underlined failure modes occurring inside the matrix, it seems reasonable to assume that the accounted law for the development of numerical analyses on the façade is not particularly influenced by the type of masonry support. Nevertheless, it is also important to underline that, before applying the selected reinforcement system on the specific type of masonry composing the façade, it is necessary to perform specific tests finalized to assess the effective bond behavior of the selected type of the reinforcement system. Indeed, in the particular case of historical masonry supports, incompatibilities between the material composing the matrix and one composing the masonry could lead to inefficient solutions characterized by mechanisms occurring at the interface between the masonry and the matrix instead of inside the matrix, with a consequent different local bond behavior.

Additional comments have been added in the re-revised version of the paper in order to better point out the above aspects (see the Section 4. Discussion and final remarks).

In the re-revised version of the paper the revisions about comments/suggestions of:

-    reviewer 1 are marked in yellow;

-    reviewer 2 are marked in green.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors have revised the paper but all the previous comments are not fully addressed. I am therefore, would like to give another change to authors to consider my previous comments completely. There are still many figures and equations which make the paper difficult for follow. The research approach is required to be presented with more clarity. 

Author Response

Reviewer 2

The authors have revised the paper but all the previous comments are not fully addressed. I am therefore, would like to give another change to authors to consider my previous comments completely. There are still many figures and equations which make the paper difficult for follow. The research approach is required to be presented with more clarity. 

Reply to reviewer 2

The paper has been re-revised by better explaining the research approach and by reducing the number of figures and equations.

In the re-revised version of the paper the revisions about comments/suggestions of:

-    reviewer 1 are marked in yellow;

-    reviewer 2 are marked in green.