Study on Mechanical Properties and Mesoscopic Numerical Simulation of Recycled Concrete

Round 1

Reviewer 1 Report

The manuscript presents an experimental and numerical study of recycled concrete aggregate (RCA), and the impact of this type of aggregate on the strength of concrete. The study includes a competently executed numerical study that is, unfortunately, let down by the experimental component.

To the reviewer, there are numerous problems with (1) the methods used for experimental testing and (2) the presentation of the methods; the second can be corrected with editing, the former cannot. The reviewer hopes that by correcting the manuscript methods and clarifying details, the authors can clear up what - in the manuscript as submitted - are hopefully poorly conveyed methods, rather than experimental method errors. Most specifically, there is no evidence of the use of Standards-based methods for testing (ASTM, CN, EU or other Standards body), and some potentially problematic test method problems (in particular, L:d ratio of specimens, and no replicate specimens reported for mechanical testing).

The following are specific comments identified in the manuscript:

Line 34: Recycled concrete aggregate (RCA)

Line 35: "certain process" vague and grammar

Line 37: "certain" is a vague term and should be replaced

Line 37: grammar

Line 40: "certain" vague

Line 41: "large discreteness" not sure what is meant by this.

Lines 44-45, 46-47: repetition

Line 55: "researches" > research

Line 57: compressive and tensile strengths (much better grammar)

Line 85: Water absorption is an ASTM standard test. Why was the standard test not used?

Line 95: absorbance

Lines 96 - 102: repetition from literature review

Line 106: grading data for sand and coarse aggregate?

Line 109: There are serious problems with using a L:d ratio < 2  for mechanical testing of concrete. There are also well-established standard test methods for mechanical testing and aggregate characterisation. Why were the Standard test methods not used? The reviewer is not very familiar with CN standards (implied by the locality of the authors) by the China Building Material Industry Standards, but equally if the authors were not intending to use CN standards, ASTM concrete testing standards (or EU, GB or Aus, each very similar) are applied very often for laboratory experimental tests.

Line 112: There is a consistent grammatical issue through the presentation of results; the authors have use the present tense ('is') when the reporting of results should use the past tense when discussing the results and analysing them ('was', 'were'). These mistakes have not been noted individually. The authors should carefully check their grammatical language throughout the manuscript.

Line 114: No information on 1) mixing/fabrication including compaction or actual mixing 2) curing method

Line 116: Any statistical variation - it is impossible to tell if only one specimen was tested at each replacement rate at each age, or multiple due to wording and figure.

Line 119-120: this is true of all concrete

Line 120-121: Not strictly true, according to the data shown

Line 127: Were there replicates to provide validity to the results? It is impossible to draw any conclusions for concrete from single results - testing standards all agree a minimum of 3 replicates to reduce spurious results (possibly shown at 100% replacement rate @28 days).

Lines 131 - 132: It is established in the literature that it is best practice for RCA to be pre-wetted to avoid excess water absorption of the RCA during the mixing process.

Line 133 - 134: This contradicts the authors' statement on lines 120-121.

Line 142: Again, there is no error bars or other indication of statistical variation of the data (multiple samples tested). This must occur especially for indirect tensile strength testing - it is further unclear what method was used for the testing.

Line 166: 'coagulation rate' - this term is unclear and confusing. Please elaborate on what is meant by this term.

Line 174: At what concrete age? There is no test information.

Line 179: The authors have presented only a very small range of strain rates, and have not normalised for the DIF (dynamic increase factor) for the concrete mixes.

Line 286: 'motor' is used several times in the table; this should be 'mortar'

Line 368: 'coalescated' > 'coalesced'

 

Accordingly, due to the problems observed in the experimental component, this reviewer recommends that the authors perform major revisions to their manuscript.

Author Response

We appreciate the comment. Please refer to the attachment for modification instructions.

Reviewer 2 Report

In this paper, the mechanical properties of recycled concrete with different replacement rates of RCA were studied experimentally and numerically. The uniaxial compressive strength, uniaxial splitting tensile strength and dynamic compressive strength of recycled concrete were obtained, and the failure processes were simulated by ABAQUS. The research work has significance for investigating the mechanical properties of recycled concrete, and the results have significant contributions to the advancement of the subject. The reviewer recommends its publication with the following suggestions for the authors.

(1) In FIG. 9, the numerical calculation results of recycled concrete are compared with the test results in reference 30, but reference 30 is the research results of ordinary concrete. It should be compared with the test results of recycled concrete.

(2) In finite element analysis of the meso level of concrete, the mesh should be as small as possible and the accuracy should be as high as possible, usually not exceeding a maximum of 2 mm. However, in FIG. 9, the calculation results of the 3mm and 2mm and 1mm grids are basically the same. Is it OK to use 3mm mesh for microscopic analysis of concrete? Please explain and discuss clearly.

(3) Is it possible to compare and analyze the results of numerical simulation of splitting tensile strength with the test results?

  (4) How is the load applied in the numerical simulation of the Hopkins test? Is it consistent with the actual Hopkins test？

(5) In the introduction of the paper, some references of numerical simulation analysis methods on the recycled concrete should be added and commented, such as the following papers:

[1] Yao Wang, Yijiang Peng, Mahmoud M.A. Kamel, Linqi Gong. Modeling interfacial transition zone of RAC based on a degenerate element of BFEM. Construction and Building Materials, 2020, 252 (119063) : 1–15.

[2] Yao Wang, Yijiang Peng, Mahmoud M.A. Kamel, Liping Ying. Base force element method based on complementary energy principle for damage analysis of recycled aggregate concrete. International Journal for Numerical Methods in Engineering, 2020, 121(7): 1484–1506.

[3] Liping Ying, Yijiang Peng, Hongming Yang. Meso-analysis of dynamic compressive behavior of recycled aggregate concrete using BFEM. International Journal of Computational Methods, 2020, 17(6), 1950013: 1-22.

Author Response

We appreciate the comment. Please refer to the attachment for modification instructions.

Author Response File: Author Response.docx

Reviewer 3 Report

All the comments are as follows:

Not suitable for publication unless extensively edited.

-This paper has shown a very high similarity which is more than 30% can be detected by running the Turnitin software. 

-The abstract of the manuscript does not reflect the main findings of the work. I suggest presenting some results quantitatively.

-The author is required to provide a sieve analysis of the coarse aggregate. 

 

-It would be good if the authors can add a future scope section with a proposed application after the conclusion section.

Author Response

We appreciate the comment. Please refer to the attachment for modification instructions.

Author Response File: Author Response.docx

Reviewer 4 Report

The manuscript presents an interesting experimental and numerical investigation concerning the mechanical properties of recycled concrete. The topic is relevant and the reviewer believes that the manuscript's subject is within the journal's scope However, some aspects described below must be better clarified and discussed. Also, the revision and improvement of the english language shall be carried out. Based on the following list of observations/suggestions, I recommend a major revision of the manuscript.

 

Abstract

·        Please revise lines 17-22. The idea is not pretty straightforward and the language needs deep revision.

·        The conclusions of the work are not well presented.

 

Section 1

Other strategies for reusing existing structural elements (previously damaged or not) may be replacing the reinforcement that can be a sustainable strategy instead of re-constructing the structural system. Please discuss about that and consider the following research examples (e.g. Doi:         10.1016/j.istruc.2015.03.001; 10.1016/j.engstruct.2017.05.021)

·        The novelty of this work needs to be well presented.

 

Section 2

·        Please include dimensions of aggregates in the figure (i.e. reference scale);

·        Equation 1 is not relevant;

·        Please present some images of the concrete samples indicating their dimensions;

 

 

Section 3

 

·        The results are very interesting and promising. Please provide more details concerning the deterioration of the strength properties with the increment of the replacement rates;

 

Section 4

 

·        The input material properties adopted are not well justified. Please justify it.

·        More details are needed concerning the modelling approach adopted.

 

Section 5

 

·        Please include future works and identify the existing gaps in the literature

Author Response

We appreciate the comment. Please refer to the attachment for modification instructions.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have made significant improvements to the manuscript per reviewers' comments. Comments are below:

Line 28 - 'RC' - authors should introduce acronyms first, thus: 'Recycled aggregate (RC)'

Line 116 - 'rearing'; the reviewer believes that this should be 'curing'

Line 176 - 177: the reviewer does not follow the logic that 'due to the high WAR of the old mortar, the hydration reaction rate of PC was obviously slower than that of RC'; this would require more explanation, as it is not borne out by the mechanical results (unless the authors are attributing that relative deterioration solely to poor interfacial bond), and as the concretes were saturated water cured, this high WAR should not be playing a major role in hydration. Moreover, [21] pertains to geopolymer recycled concrete (with different reactions and reaction rate), thus reference to a Portland cement-based RC study would be advised. The asserted slower reaction rate of the PC does not follow from the high WAR of the old mortar.

Author Response

Thank you for your valuable comments. Please see the attachment for the modification instructions.

 

Author Response File: Author Response.docx

Reviewer 3 Report

All the comments have been revised accordingly. Therefore, this manuscript paper can be accepted for publication. 

Author Response

Thank you for your valuable advice.

Reviewer 4 Report

The authors revised the manuscript according to the reviewer's suggestions. The manuscript quality was improved and it is now suitable for publication.

Author Response

Thank you for your valuable advice.