Development of Sustainable and Innovative Manhole Covers in Fibre-Reinforced Concrete and GFRP Grating

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

 

Summary:

The paper concerns the design of a new type of sewage manhole covers. Covers were designed as composites combining FRC with grids made of glass fiber reinforced polymer (GFRP) (solutions compliant with the classes imposed by BS EN 124:1994 were considered). The covers were to be economically competitive and not have sufficient economic value to justify theft. The goal was to use FRC in a way that eliminates the brittle behavior of concrete under loading conditions (cracking or punching failure).

Reviever general coments:

1.  The paper is typically experimental.

2. The aim of the paper to obtain an FRC composite in a way that eliminates the brittle behavior of concrete was achieved.

3. The paper is written correctly and carefully. The drawings are correct easy to understand and interpret.

4. The reviewer does not understand the idea of using backward analysis to determine the Younge's modulus of the composite. The use of solid elements would probably allow for quite extensive computer analysis, even despite the lack of information on the slippage of FRC and GFRP as well as modeling of damage propagation. Obtaining the Younge modulus of the composite only allows for the analysis of displacements.

Reviever specyfic coments:

1. According to the reviewer, keywords: computer modeling and corrosion problems are not representative enough for this paper.

2. Many publications is older than 5 yers.

3. The dimensions on the figure (diagrams) 12-16 should be larger.

 

Author Response

Reply to General comments:

 

Comments 1 to 3 do not require any explanation from the authors.

Comment 4: The modulus of elasticity of the gratings was not possible to determine experimentally. Therefore, the authors applied a numerical modelling strategy to derive the modulus of elasticity of the GFRP used to manufacture the adopted GFRP gratings. By using the modulus of elasticity, the authors plan in close future to develop optimized manhole covers with HFRC reinforced with GFRP gratings, whose design will be assisted by advanced numerical simulations that considers the relevant properties of the intervenient materials.

In the inverse analysis was used Reissner-Mindlin shell finite elements since the out-of-plane thickness of the components forming the grid is relatively small, being possible to neglect the out-of-plane normal stress component. Due to this relatively small thickness, the option for solid finite elements would require much larger number of finite elements, with a significant increase on the number of degrees of freedom and, consequently, on the computing time, but without any beneficial effect from the modelling accuracy point-of-view.

The stiffness of the GFRP grating is directly proportional to the modulus of elasticity of its material, which is a fundamental requisite for the classification of the manhole cover class according to the BS EN 124:1994, thereby the relevance of determining this parameter.

 

 

Specific comments:

 

Remark #1) According to the reviewer, keywords: computer modeling and corrosion problems are not representative enough for this paper.

Reply: The authors did not propose “Computer modelling” and “corrosion problems” keywords. The authors suppose the Reviewer is referring to “immunity to corrosion”. The authors proposed this keyword to indicate the use of reinforcements not susceptible to corrosion. For better identification of this purpose, the keyword “immunity to corrosion” is replaced by “corrosion-resistant reinforcement”

Action#1: For better identification of this purpose, the keyword “immunity to corrosion” is replaced by “corrosion-resistant reinforcement”. The keywords are now:

“

fibre reinforced concrete; glass fibre reinforced polymer, manhole covers, corrosion-resistant reinforcement

“

 

Remark #2) Many publications is older than 5 yers

Reply/Action: Despite the significant number of publications on the use of FRC and FRP published in the last 5 years, the authors did not find recent publications that can be referred in this manuscript with significant added value for its scientific quality and for the interest of the future readers of this manuscript in case of being accepted for publication in this journal.

 

Remark #3) The dimensions on the figure (diagrams) 12-16 should be larger.

Reply/Action: The width of the figures is almost the width of the page. Furthermore, the char size of the legends is almost equal to the adopted in the text. In any case, the size of some figures was increased a little bit in order to all of them have almost equal size.

Reviewer 2 Report

Comments and Suggestions for Authors

This paper presents a new manhole cover is developed using fibre reinforced cementitious (FRC) materials and glass fibre reinforced polymer (GFRP) gratings. An experimental program with manhole cover specimens made by two types of FRC and two types of GFRP gratings was executed by investigating the strength, stiffness and post-cracking tensile capacity of the FRCs and the stiffness and flexural capacity of the two GFRP gratings. This study could help understanding and applications of FRC manhole covers. The topic is interesting, and the topic is within scope of this journal. The authors can enhance the paper by considering the following comments.

1. The English language should be checked so that the language errors can be avoided. 

2. The abstract section can be shorten as the current section contains too much information of background.

3. The stiffness and cracking strength for the manhole covers are significant. Using high strength concrete or UHPC with FRP can solve this issue as the normal concrete is weak in elastic modulus, as well as FRP. Examples include using FRP reinforcement and high strength concrete has been well documented in an existing paper of Shear behavior of FRP-UHPC tubular beams, this solves the issue of weak stiffness of FRP reinforced concrete by using the high modulus of UHPC.

4. The authors mentioned that beside the higher price, the structural performance of these manhole covers is dependent on the glass transition temperature (Tg) of the polymer component of the composite material. More information of the working temperature of manholes and glass transition temperature of FRP composites should be provided to support this background.

5. As has been mentioned in line 95, punching failure may happen for thin FRC plates. The existing study and design against punching shear of FRP-UHPC plates has been well documented, such information could benefit the paper as the loading and failure of specimens are alike to those in the above study.

6. Please provide details of physical and mechanical properties of the GFRP grids.

7. Finite element analysis: more information could help better understanding the FE model.

8. How was the bond between concrete and GFRP grid during loading? A reliable bond between FRP and concrete is significant for a better performance of FRC plates. Please clarify.

9. The authors may suggest a simple design procedure or a design model for the proposed manhole covers.

Comments on the Quality of English Language

 Minor editing of English language required to avoid language errors.

Author Response

Remark #1) The English language should be checked so that the language errors can be avoided.

Reply/Action: The manuscript was reviewed from the English grammar quality and style, and the typos found were eliminated.

 

Remark #2) The abstract section can be shorten as the current section contains too much information of background.

Reply: The abstract was shortened by eliminating the information considered of smaller importance for the purpose of an abstract.

Action: The abstract reads now:

“

(…) In several countries, the manhole covers made by steel are being stolen, with significant economic losses for private and public entities, and even causing accidents. In this work, a new manhole cover is developed using fibre reinforced cementitious (FRC) materials and glass fibre reinforced polymer (GFRP) gratings. Since the GFRP gratings are immune to corrosion, and FRC is a relatively low-cost material, manhole covers in FRC reinforced with GFRP gratings are durable and not so appealing to be stolen like those in steel. An experimental program with manhole cover specimens made by two types of FRC and two types of GFRP gratings was executed by investigating the strength, stiffness and post-cracking tensile capacity of the FRCs and the stiffness and flexural capacity of the two GFRP gratings. It was demonstrated that the developed manhole cover concept can be of class A15 up to D400 according to the recommendations of BS EN 124:1994.

“

 

Remark #3) The stiffness and cracking strength for the manhole covers are significant. Using high strength concrete or UHPC with FRP can solve this issue as the normal concrete is weak in elastic modulus, as well as FRP. Examples include using FRP reinforcement and high strength concrete has been well documented in an existing paper of Shear behavior of FRP-UHPC tubular beams, this solves the issue of weak stiffness of FRP reinforced concrete by using the high modulus of UHPC.

Reply/Action: The authors adopted two types of FRC to demonstrate the relevance of the compressive strength, modulus of elasticity and post-peak tensile capacity to increase the load carrying capacity and stiffness of these type of construction systems. Ultra-high performance concrete (UHPC) brings benefits in these aspects, as long as the modulus of elasticity increases proportionally with the compressive strength, which can be assured, but with significant costs that will compromise the cost competitiveness of the solution. In this paper it is demonstrated to be possible to obtain manhole covers up to D400 class at competitive prices. Some of the authors have developed research on UHPC and are aware of their potentialities, but also their debilities, mainly the costs, so their use should be limited to special applications, which is not the case in the understanding of the authors. For the main reinforcement of the developed manhole covers was adopted GFRP gratings in an attempt of getting a good balance in terms of flexural performance and costs. Using gratings or other forms of FRP reinforcements of more performance fibres (like carbon, CFRP) will enhance the behaviour of these elements regarding the serviceability and ultimate design conditions, but their prices are much higher, without the possibility of being economically competitive for manhole cover applications. So, the authors do not see any interest for the readers the inclusion of references dealing with UHPC and CFRP that cannot be used for the manufacture of this type of manholes due to cost competitiveness reasons.

 

Remark #4) The authors mentioned that beside the higher price, the structural performance of these manhole covers is dependent on the glass transition temperature (Tg) of the polymer component of the composite material. More information of the working temperature of manholes and glass transition temperature of FRP composites should be provided to support this background.

Reply: Manhole covers are intended to be applied in roads and motorways, where temperatures can attain values exceeding 70 degrees [6]. The glass transition temperature of the polymeric matrix used in the major part of GFRP applied in construction systems is in between 65 and 150 [7], therefore the mechanical properties of these GFRP are detrimentally affected when subjected to temperatures possible to be attained in pavements.

Action: Just after line 49 the following text was added:

“

In fact, temperature exceeding 70 Celsius degrees have been recorded in pavements [6]. The glass transition temperature of the polymeric matrix used in the major part of the GFRP applied in construction systems is in between 65 and 150 Celsius degrees [7], therefore the mechanical properties of these GFRP are detrimentally affected when subjected to maximum temperatures possible to be attained in pavements.

“

 

Remark #5) As has been mentioned in line 95, punching failure may happen for thin FRC plates. The existing study and design against punching shear of FRP-UHPC plates has been well documented, such information could benefit the paper as the loading and failure of specimens are alike to those in the above study.

Reply/Action: Following the recommendations of the editor, the authors removed from the references a publication that includes a mini state-of-the-art on the use of FRC for structural systems subjected to load and support conditions susceptible of inducing punching failure mode. One of the authors has intensive experimental, analytical and numerical research on the use of short fibres and FRP for punching domain. The number of references (4) indicated in the manuscript in this domain seem to the authors sufficient. Furthermore, it is indicated a fib bulletin (reference 23) that includes experimental evidence of the benefits of fibres for punching, analytical models and even a design example.

 

Remark #6) Please provide details of physical and mechanical properties of the GFRP grids.

Reply/Action: In the manuscript it is indicated the geometric information that defines with rigor the adopted GFRP gratings, as well as the properties provided the supplier and determined experimentally and numerically. In fact, Figure 3 details the geometry of the adopted GFRP gratings, Figure 7 presents the force-deflection obtained in representative specimens of GFRP gratings adopted in the manufacture of the manhole covers, and in section 2.4.2 is described the methodology followed to determine the modulus of elasticity of these GFRP gratings. The authors do not have nothing more to present, since this information seem to the authors the fundamental for the research conducted in this manuscript.

 

Remark #7) Finite element analysis: more information could help better understanding the FE model.

Reply: The number of finite elements, and the number of nodes of the type of finite element adopted were added. Furthermore, it is indicated the theory adopted in the numerical simulations, and a reference is added where this theory is described in detail.

Action: The two sentences after line 281 reads now:

“

For this purpose, the G_type1 and G_type2 specimens were modelled using 112 plane shell finite elements of 8 nodes and 2x2 integration points for the evaluation of the membrane, bending and shear stiffness components, and assuming linear-elastic behaviour for the GFRP (Error! Reference source not found.). The formulation is based on the Reissner-Mindlin theory, whose implementation in FEMIX is described elsewhere [60].

“

 

Remark #8) How was the bond between concrete and GFRP grid during loading? A reliable bond between FRP and concrete is significant for a better performance of FRC plates. Please clarify.

Reply/Action: The numerical simulations presented in Section 2.4.2 are exclusively dedicated to predict the modulus of elasticity of the adopted GFRP gratings by inverse analysis, so FRC is not considered and consequently bond model is not in the context.

 

Remark #9) The authors may suggest a simple design procedure or a design model for the proposed manhole covers.

Reply/Action: The development of an analytical model is planned for close future, but it is out of the objectives of the present phase of this research program. It should be noted that for developing a model capable of simulating with enough rigour the deflection at serviceability limit state (SLS) conditions and the load carrying capacity at ultimate limit state (ULS) conditions, the model will have some sophistication. In fact, for predicting the deflection it will be necessary to evaluate the flexural stiffness of the composite system formed by the FRC and GFRP grating, eventually including the decrease of this stiffness for considering the influence of deformability component due to shear of the GFRP grating. The consideration of being not possible to assure perfect bond between GFRP and FRC might be also necessary for an adequate estimation of the deflection for SLS.

Regarding the prediction of the maximum load, it will be necessary to determine the maximum effective stress supported by the GFRP grating considering its anisotropic nature (see Figure 8).

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Authors,
thank you for your paper focused on a new type of manhole covers made from Fiber Reinforced Concrete and GFRP Grating. In the paper, the experimental and partially numerical results are presented. A total of 7 samples were experimentally verified.
My comments are:
- chapter 2.2 - it is presented here that 7 types of samples were prepared - how many samples were made and tested of each type? It is not entirely clear from the text, it seems as if only one sample of each type was made. Just one sample each? Isn't that enough? Statistics cannot be made from this and the results cannot be generalized. I would expect 7 series of at least three samples each so that they can be statistically evaluated.
- in chapter 2.4.2 (Fig. 7) it is stated that only 2 samples of type 1 and 2 samples of type 2 were tested from GFRP gratins - this is also not much even if the results from the two samples correspond completely,
- chapter 3.1 and Table 6 - the maximum forces at failure of individual samples for classes A15 to D400 are presented here, while the forces achieved at failure Pmax are greater than they should be for individual categories A15 (15kN) to D400 (400kN) - failure was also monitored, cracks and deformations even at the forces given for individual categories A15 to D400 (breakage of samples at corresponding forces of 15, 125, 250 and 400kN for individual samples)? Were the samples not so broken that they no longer served their purpose?
- chapter 3 - experimental measurements on 7 samples are presented, but they are static tests - are there no plans to do dynamic tests? Aren't dynamic stresses more decisive than static ones? After all, it is a construction/element (manhole covers) that will be stressed dynamically rather than statically?
Best regards.

Comments for author File: Comments.pdf

Author Response

Remark #1) chapter 2.2 - it is presented here that 7 types of samples were prepared - how many samples were made and tested of each type? It is not entirely clear from the text, it seems as if only one sample of each type was made. Just one sample each? Isn't that enough? Statistics cannot be made from this and the results cannot be generalized. I would expect 7 series of at least three samples each so that they can be statistically evaluated

Reply: As indicated in the beginning of section 2.2 and Table 2, seven manhole cover specimens were tested, one specimen for each type in order to investigate the influence of the following parameters the authors consider critical for the performance of this construction system in serviceability and ultimate limit state conditions (SLS and ULS, respectively): thickness of the manhole, type of FRC, type of GFRP grating and cover thickness of the GFRP grating. The authors recognize the possibility of some scatter on the test results due to the scale of the two material systems forming the specimens, FRC and GFRP grating, as well as the influence of the fibre distribution and orientation for this scatter. However, the dimensions of the specimens are identical to the real prototypes, avoiding to consider the influence of the size effect on the relevant results. Consequently, the authors gave preference on testing 7 different solutions of almost real scale instead of testing higher number of smaller specimens of 2 or more equal twins. In close future it is planned to execute a more extensive experimental program of 3 classes of manhole cover prototypes for deriving information for the design model.

Action: In Section 2.2 the following information was added:

“

It was given preference to test seven different solutions of almost real scale manhole cover prototypes (despite it is recognized the possibility of occurring some dispersion on the experimental results) instead of testing higher number of smaller specimens of two or more twins and then deal with the ambiguity of transforming these results to the ones expected to be obtained in real dimension prototypes for considering the size effect.

“

 

Remark #2) in chapter 2.4.2 (Fig. 7) it is stated that only 2 samples of type 1 and 2 samples of type 2 were tested from GFRP gratins - this is also not much even if the results from the two samples correspond completely

Reply/Action: Please see the answer provided to the previous remark.

 

Remark #3) chapter 3.1 and Table 6 - the maximum forces at failure of individual samples for classes A15 to D400 are presented here, while the forces achieved at failure Pmax are greater than they should be for individual categories A15 (15kN) to D400 (400kN) - failure was also monitored, cracks and deformations even at the forces given for individual categories A15 to D400 (breakage of samples at corresponding forces of 15, 125, 250 and 400kN for individual samples)? Were the samples not so broken that they no longer served their purpose?

Reply/Action: Table 6 presents the relevant results obtained from the experimental tests in the 7 specimens, namely: , which is the maximum load; that is the deflection at  and  that is the initial tangent stiffness. The other two parameters,  and  are considered in order to determine the class of the tested manhole cover specimens according to the recommendations of BS EN 124:1994. The meaning and procedure to determine  and  are explained before introducing Table 6.

 

Remark #4) chapter 3 - experimental measurements on 7 samples are presented, but they are static tests - are there no plans to do dynamic tests? Aren't dynamic stresses more decisive than static ones? After all, it is a construction/element (manhole covers) that will be stressed dynamically rather than statically?

Reply/Action: For the determination of the class of manhole covers, BS EN 124:1994 recommends the tests carried out in the scope of the research resumed in this manuscript. Assessing the behaviour of this type of specimens under fatigue loading conditions seems interesting, and it will be a possible task for future developments in this domain.

Reviewer 4 Report

Comments and Suggestions for Authors

The scientific wor should be better explined in terms of innovation and additional contribution compared with the same tematics already showed in the existing licterature.

Comments on the Quality of English Language

A fully revision of englis is raccomaded

Author Response

Remark #1) The scientific wor should be better explined in terms of innovation and additional contribution compared with the same tematics already showed in the existing licterature.

Reply/Action: In the introduction section the authors presented the benefits of using FRC and GFRP for the development of innovative systems in order to justify the decision of combining two different types of FRC and two types of GFRP grating for the development of manhole cover prototypes in the context of a portfolio of manhole cover from A15 to D400 according to BS EN 124:1994. This mini state-of-the-art includes representative references of notable pieces of research on combining concrete, FRC and FRP on the development of several innovative construction systems. The authors did not find papers of public domain in the critical subject covered in this manuscript with scientific quality to constitute references for this journal, but the authors are open to receive recommendations for the references that this Reviewer can suggest. However, the authors inform that they prefer to have this manuscript not published in this journal then be constrained to add references to papers the authors consider do not have significant contribution for the quality of the paper and mainly for the interest of the readers.

 

Remark #2) A fully revision of englis is raccomaded

Reply/Action: The manuscript was reviewed from the English grammar quality and style, and the typos found were eliminated.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The paper can be accepted in the current form as the authors have made sufficient revisions.

Comments on the Quality of English Language

Minor editing of English language required

Author Response

Dear Esteemed Reviewer,

We are deeply grateful for your thorough review and constructive comments. Your input is invaluable to us.