Review Reports - Critical Stress Conditions for Foam Glass Aggregate Insulation in a Flexible Pavement Layered System

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The topic presented in the paper is valid and important. In the reviewed paper, the Authors presented the problem of critical stress conditions for foam glass aggregates insulation in the flexible pavement layered system. The Authors in this study investigated the critical stress conditions that FGA layers can tolerate within flexible pavement systems to ensure long-term structural integrity. Laboratory cyclic triaxial tests and fullscale accelerated pavement testing using a heavy vehicle simulator were conducted to evaluate the resilient modulus and permanent deformation behavior of FGA. Results show that FGA exhibits stress-dependent elastoplastic behavior, with resilient modulus values ranging from 70 to 200 MPa. Most samples exhibited plastic creep or incremental collapse behavior, underscoring the importance of careful stress management. A strainhardening model was calibrated using both laboratory and full-scale data, incorporating a reliability level of 95%. The study identifies critical deviatoric stress thresholds to maintain stable deformation behavior under realistic confining pressures. These findings support the development of mechanistic design criteria for FGA insulation layers, ensuring their durability and optimal performance in cold-climate pavements. In my opinion, the paper can be considered for publication, after taking into account the following remarks:

in the Keywords section, it would be good to add "road infrastructure" keywords, in order to strenghteen the connection between the paper topic and a journal scope,
at the end of the abstract section, one sentence should be added about the further usefulness of the obtained results presented in the paper,
at the end of the Introduction section, the Authors wrote about the main aim of the paper, presenting at the same time the research questions. It is very well, but also a short information what was included in each paper section should be added, at the end of the Introduction section,
there is a lack of a solid "Literature review" section. There is a second sectio called "2. Background" with presentation of topic bacground. The solid literature review dedicated to tprevious research in this area should also be added,
there is a lack of explanation in the paper text for some of the used acronyms. The Authors are kindly asking to check, if all given acronyms have given their full meanings,
in the section called "3. Methods" the step-by-step research chart presenting the research steps should be presented,
a discussion in which a comparison of obtained research results with results of other researchers in this area should be presented,
in the Conclusions section, some more detailed conclusions dedicated to obtained research results should be included.

Thank you very much.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This paper examines the application of FGA as a recycled material in asphalt pavements. Please address the following comments:

The abstract should focus on the study's main contributions and the needs that motivated the authors to conduct it.
Follow the MDPI citation style!
More references to back up the study and address the research gap should be included in the introduction section.
Line 62 should state that temperature is a major contributor to rutting distress. Please emphasize that.
Increase the resolution of the figures!
Overall, the introduction section requires more work on the structure and flow of ideas from one paragraph to the next.
Where is the research gap in lines 138–140?
Figure 3 shows the passing or retained percentages. This could be part of the materials properties section, which should be followed by the methods section.
No need for Figure 4. Please remove.
In the methods section, include a flow chart that depicts the testing program steps.
More in-depth statistical analysis should come before the conclusion section.
The conclusion section repeats the results; instead, try to highlight the key findings numerically.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This study makes a significant contribution to cold-region pavement engineering by establishing scientifically supported design limits for Foam Glass Aggregate (FGA) insulation layers. However, before the paper can be considered for publication, the authors need to address the following key questions and clarify certain aspects of the research.

1.Given the premature termination of permanent deformation tests (only 2/5 stages completed), how do you justify the model's validity for high-stress conditions (q/σ₃ >3)? Were alternative compaction methods (e.g., kneading) considered to improve sample integrity?

2.Equation 3 infers σ₃ in full-scale tests using lab-derived Er. Given the erratic σ₃ values (53–80 kPa, Table 4) and lack of direct field measurement, how was this approach validated? Could 3D stress redistribution under moving tires invalidate the K-θ model?

3. Why was surface temperature maintained at 10°C instead of sub-zero conditions? Does this not overlook frost-related mechanical degradation (e.g., ice lensing), a core motivation for FGA use?

4. At identical σ₃=45 kPa, FGA shifts from Range A (q/σ₃=1.3) to Range B (q/σ₃=1.8) (Table 3). What microstructural mechanisms drive this abrupt transition? Is particle crushing a factor?

5. Field deformation was 67% of lab predictions. Does this imply pavement confinement enhances FGA stability? Should your model include a "confinement efficiency factor" dependent on overlaying layer properties?

6. What caused the poor model fit for Stage 2 (HSAL=4000 kg)? Could sensor misalignment, non-uniform loading, or material heterogeneity explain this anomaly?

7. Typical tire contact stresses exceed 500 kPa. How can FGA layers realistically tolerate only 15–25 kPa deviatoric stress without excessive overdesign? Does this necessitate impractical layer thicknesses or burial depths?

8. Can α=0.67 apply to pavements with differing structures (e.g., thinner asphalt layers)? Have you tested its sensitivity to overlaying layer stiffness/thickness?

9.If FGA requires thick protective layers to meet stress limits, does its environmental benefit (recycled glass) outweigh added resource consumption? Provide a life-cycle analysis comparison vs. polystyrene insulation.

10. While FGA insulates, does Range B/C deformation (e.g., rutting) compromise frost protection by altering layer geometry/thermal continuity?

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The article is interesting and of a high academic standard. However, it has not been prepared in accordance with the journal's guidelines, particularly with regard to referencing. Numbering should be used rather than (name, year), etc. Detailed comments are provided below. (1) In my opinion, the Introduction and Background sections should be combined. The purpose of the study is stated twice. This, in my opinion, somewhat distorts the reception of the article and introduces confusion. I consider the literature review to be sufficient. (2) What is the research hypothesis? (3) Have the authors introduced an innovative element to the research methodology? Please highlight the innovative element of the research. The research results are presented correctly. (4) A discussion of the results should also be included. Please compare the results with those presented in the literature. (5) The conclusions were developed on the basis of the research results. I consider them to be correct. In summary, the article raises an interesting issue but requires a few changes.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 5 Report

Comments and Suggestions for Authors

This study focuses on the critical stress conditions of foam glass aggregate (FGA) insulation layers in flexible pavements within cold regions. The mechanical properties of FGA are evaluated through laboratory cyclic triaxial tests and full-scale accelerated pavement testing using a heavy vehicle simulator. Comments are as following:

The term "silicium carbide" used in the document is non-standard in English; the correct terminology should be "silicon carbide," which may cause confusion among readers familiar with technical vocabulary in English.
In the introduction, the relationship between "ice lens formation" as a manifestation of frost heave and the three essential conditions for frost heave (frost-susceptible soil, water availability, and freezing temperatures) is insufficiently developed, thereby weakening the causal link between the phenomena and their underlying causes.
The description of axial strain measurement in triaxial tests is ambiguous: "axial strain, which was inferred from the central 200 mm, based on the average of resilient and permanent displacement" fails to clarify that the 200 mm refers to the middle segment of the 300 mm-high sample, potentially leading to misinterpretation of the measurement scope.
The explanation regarding FGA's sensitivity to permanent deformation in triaxial tests is overly simplistic, attributing it solely to stress states (e.g., increased deviatoric stress and decreased confining pressure) without considering its material properties (e.g., high porosity up to 90% and a fragile granular structure), thus leaving the intrinsic mechanisms (e.g., particle breakage or pore collapse) unexplored.
The discrepancy between full-scale and laboratory test results (with full-scale deformation being approximately 67% of the laboratory prediction) is explained only qualitatively as "pavement structural confinement" and "stress distribution under moving wheels," lacking a quantitative analysis of how lateral constraints from surrounding layers or non-axisymmetric stress fields specifically influence FGA deformation.
The critical deviatoric stress thresholds (15–25 kPa) identified for stable Range A behavior are not linked to FGA's inherent mechanical properties (e.g., compressive strength of 0.5–1.5 MPa), failing to justify why these thresholds prevent excessive deformation or structural failure.
Table 2 (Permanent deformation test sequence) lacks clear labeling of the relationship between "Test sequence 1/2" and the corresponding confining stress (σ₃) and deviatoric stress (q), making it challenging to discern the stress stages applied in the tests.
The applicability of the strain-hardening model to FGA is asserted based on fitting results (average R²=0.84) but lacks a comparison with its performance for other granular materials, and the physical significance of model parameters (a, b) is not elucidated in the context of FGA’s unique characteristics (e.g., low elasticity and high plasticity).