Comparative Study on Complex Modulus and Dynamic Modulus of High-Modulus Asphalt Mixture

Round 1

Reviewer 1 Report

This manuscript compared between the results of HMA modulus values obtained from two testing techniques and established some conversion equations between the results of the two testing techniques. Also authors recommended minimum values of the dynamic modulus for the High Modulus Asphalt mixtures. It should be noted that the results obtained are somehow limited to the tested mixtures and cannot be generalized which limits the significance of the study. Following are some issues that should be addressed by the authors.

Show the reference for the definition of the high modulus asphalt mixtures in the first sentence of the introduction.

Add reference to the sentence ending in L54.

Rephrase the sentence starting in L54.

L68, in accordance to what? Please complete the sentence.

L69 “Materials and Experimental Work”

The paper lacks good literature review of relevant studies.

The importance of this research is not well justified or presented.

What are LPC, EME2, CRT stand for? Any acronym should be spelled out the first time it appears.

Please show the allowable gradation limits of the mix aggregate.

What is the abundance coefficient “K”, please explain.

Please show the rest of the aggregate properties, LAA, Specific gravity, absorption, ……….

Please show the setup of the trapezoidal beam complex modulus test.   

Please show the volumetric properties of the tested mixtures.

How many replicates did author do? Please show the standard deviation of the measured values.

In the text authors stated that the complex modulus test was conducted in the frequency domain of 5 to 25 while in Table 6, it shows frequency values of 1 to 25?

Why authors selected high modulus mixtures only in this study? What about other asphalt mixtures, do authors think that the results will hold true.

The manuscript is poorly written with several grammatical errors and many long sentences. Following are few of these language issues:

The whole abstract is only four sentences. Please do not use long sentences and consider divide the abstract into shorter sentences for readability. The first sentence in the introduction is a long one. This is repeated throughout the whole manuscript. There is excessive use of And after each sentence and also excessive and wrong use of “,” to connect sentences. This is very confusing and makes it hard to read through the manuscript.

L30, change to “Mixtures”

L34, change to “MPa”

L36, change to “In France”

Author Response

Special thanks to you for your good comments. We have carefully revised the manuscript, and the paper was also re-scrutinized to improve the English. Our responses are as follows, and we amended the relevant part in revised manuscript and the changes below are noted in red words in the article.

1- Show the reference for the definition of the high modulus asphalt mixtures in the first sentence of the introduction.

Response: In the first sentence of the introduction, we have showed the definition of the high modulus asphalt mixtures. High modulus asphalt mixture refers to an asphalt mixture with a complex modulus ≥14000Mpa under the conditions of 15℃ and 10Hz[1].

2- Add reference to the sentence ending in L54.

Response: The introduction has been revised again, this sentence has been deleted.

3- Rephrase the sentence starting in L54.

Response: The introduction has been revised again, this sentence has been deleted.

4- L68, in accordance to what? Please complete the sentence.

Response: The introduction has been revised again, this sentence has been deleted.

5- L69 “Materials and Experimental Work”

Response: The materials and experimental work was refined in the paper.

6- The paper lacks good literature review of relevant studies.

Response: The introduction has been revised again, the literature review of relevant studies was added in the introduction.

7- The importance of this research is not well justified or presented.

Response: The introduction has been revised again, the importance of this research was added in the introduction.

8- What are LPC, EME2, CRT stand for? Any acronym should be spelled out the first time it appears.

Response: The full name of LCPC is Laboratoire Central des Ponts et Chaussées, but now the LCPC is gone. It merged with the French Institute of Transportation and Security (INRETS) to become the French Institute of Transportation and Land Remediation and Networks. The French name is Institut fran? ais des sciences et technologies des transports, de l‘aménagement et des réseaux (IFSTTAR).

The full name of EME is Enrobés à Module Elevé Class.

The full name of CRT-2PT is cooper technology two point trapezoidal bending test.

The acronym were spelled out the first time it appears in the paper.

In the article, we have made corresponding changes in red words of 1 according to the comments and added the references.

9- Please show the allowable gradation limits of the mix aggregate.

Response: the allowable gradation limits of the mix aggregate was shown in Table 4.

10- What is the abundance coefficient “K”, please explain.

Response: The abundance coefficient K is a ratio of the conventional thickness of the asphalt film attached to the surface of the aggregate with an asphalt binder[27]. K has nothing to do with the density of gravel, according to LPC Bituminous Mixtures Design Guide, the amount of asphalt is calculated and estimated by the abundance coefficient K, which satisfies K≥3.4. The define of the abundance coefficient K was added in 2.2.

In the article, we have made corresponding changes in red words of 2.2 according to the comments and added the references.

11- Please show the rest of the aggregate properties, LAA, Specific gravity, absorption, ……….

Response: The aggregate properties, such as apparent relative density, water absorption rate, crushing value and so on were added shown in Table1.

12- Please show the setup of the trapezoidal beam complex modulus test.  

Response: The setup of the trapezoidal beam complex modulus test is as follow, and Corresponding modification was made in the revised manuscript in 2.3.2.

At the beginning of the test, the trapezoidal beam specimen was vertically installed and fixed in the temperature control box, and the top of the specimen was subjected to a constant sine wave load and reciprocated in the horizontal direction. At the same time, shear stress and bending moment were generated inside the specimen. The force state of the specimen was shown in Figure 1, close to the force state of the actual pavement. During the test, the displacement and force changes of the specimen during the horizontal reciprocating movement were recorded by the displacement sensor and the stress sensor, which simulates the bending and tensile deformation of the bottom of the asphalt layer on the road surface to obtain the complex modulus of the mixture.

13- Please show the volumetric properties of the tested mixtures.

Response: This article refers to the French asphalt mixture design guidelines for high modulus asphalt mixture design. The French high-modulus asphalt mixture design concept based on performance verification and mechanical indicators is different from the Superpave design concept and the Marshall design concept (using the volume method for grading design), but a special asphalt mixture design method. The performance of the asphalt mixture is designed as the ultimate goal, and the performance test of level 1 to level 4 is carried out according to the actual different needs of the road.

Performance test verification is carried out from the simplest level 1 to the most comprehensive and complex level 4, and the high design level should be carried out after the low design level requirements are met.

In the basic level 1, for the volume parameter requirements of the mixture, only the mixture must meet the threshold index requirements of the porosity and water sensitivity of the rotary compaction test.

The performance indicators of the mixture level 1 to level 4 was carried out in the article, shown in Table8.

14-In the text authors stated that the complex modulus test was conducted in the frequency domain of 5 to 25 while in Table 6, it shows frequency values of 1 to 25?

Response: It is our negligence and we are sorry about this. The description was corrected in 2.3.2 and Table 7, the complex modulus test was conducted in the frequency domain of 1 to 25.

15-Why authors selected high modulus mixtures only in this study? What about other asphalt mixtures, do authors think that the results will hold true.

Response: Thank you for your question which is important to our research.

In China, high modulus asphalt mixtures do not have a unified index and standard for modulus parameters. At present, there are mainly two evaluation methods: one is based on the complex modulus index of 2PT trapezoidal beams, and the other is based on the dynamic modulus of SPT uniaxial compression. In order to find out the appropriate modulus value of high modulus asphalt mixture suitable for China's modulus parameter conditions to better carry out the reasonable design and evaluation of high modulus asphalt mixture in China, the high modulus asphalt mixture was selected for research. Moreover, EME-14 and AC-20 are the two types of typical high modulus asphalt mixture gradations which are used often in France and China, suitable for the middle and lower layers, so that the EME-14 and AC-20 were selected in the study.

The modulus indexes of high modulus asphalt mixtures only based on two test methods was studied in our artice, and the relationship between the two was established. Whether the results are applicable to other mixtures requires further verification.

The modulus under the two evaluation systems through the two-point bending complex modulus test of CRT-2PT trapezoidal beam and the SPT uniaxial compression dynamic modulus test was analyzed in this paper. The conversion relationship model between the complex modulus of the trapezoidal beam and the uniaxial compression dynamic modulus was established, and the correlation coefficients of the fitted linear equations were all above 0.99, showing a good correlation between the modulus obtained by these two test methods. What’s more, 5 kinds of high modulus asphalt mixtures used in actual road engineering, were carried out CRT-2PT trapezoidal beam two-point bending complex modulus and SPT uniaxial compression dynamic modulus test. The test results were consistent with the recommended dynamic modulus evaluation values calculated by the conversion model.

16-The manuscript is poorly written with several grammatical errors and many long sentences. Following are few of these language issues:

Response: Special thanks to you for your good comments. We have carefully revised the manuscript, and the paper was also re-scrutinized to improve the English.

17-The whole abstract is only four sentences. Please do not use long sentences and consider divide the abstract into shorter sentences for readability. The first sentence in the introduction is a long one. This is repeated throughout the whole manuscript. There is excessive use of And after each sentence and also excessive and wrong use of “,” to connect sentences. This is very confusing and makes it hard to read through the manuscript.

L30, change to “Mixtures”

L34, change to “MPa”

L36, change to “In France”

Response: Special thanks to you for your good comments. We have carefully revised the whole abstract, the error has been corrected. 

Reviewer 2 Report

• The abstract is unreadable. Your abstract does not tell the story. It looks like a bunch of sentences put together. Improve the flow. Start with a short introduction, then an objective, and some major conclusions.
• Your introduction needs more information about performance. I missing some general information regarding the performance of asphalt mixtures such as doi.org/10.1080/14680629.2017.1283353 and doi.org/10.1080/14680629.2021.1908408.
• State clearly your objectives.
• Provide more mix design data, especially volumetrics.
• Conclusions should be more concise. Don`t just repeat the discution.

Author Response

I am very grateful to your comments for the manuscript. According with your advice, we amended the relevant part in manuscript. Your questions were answered below, and the changes are noted in red words in the article.

1- The abstract is unreadable. Your abstract does not tell the story. It looks like a bunch of sentences put together. Improve the flow. Start with a short introduction, then an objective, and some major conclusions.

Response: Thank you for your suggestion. We have carefully revised the abstract according to your suggestion and the paper was also re-scrutinized to improve the English.

2-Your introduction needs more information about performance. I missing some general information regarding the performance of asphalt mixtures such as doi.org/10.1080/14680629.2017.1283353 and doi.org/10.1080/14680629.2021.1908408.

Response: Thank you for your suggestion. In the article, more information about performance was added in introduction, and we have made corresponding changes according to the comments and added the references.

3-State clearly your objectives.

Response: We are very sorry for our unclear report in stating clearly your objectives.

According to LPC (Laboratoire Central des Ponts et Chaussées) Bituminous Mixtures Design Guide, the judgment of high modulus asphalt mixture and the selection of modulus index should refer to the complex modulus of trapezoidal beam under 15°C and 10Hz conditions[15]. However, In China, engineers and researchers believe that the dynamic modulus can better characterize the performance of asphalt mixtures [16-19], and use the dynamic modulus of the mixture as the main reference parameter. In the engineering application of high modulus asphalt mixture, the selection of the stiffness modulus index of the mixture is more the dynamic modulus under the conditions of 15°C, 10Hz, 20°C, 10Hz and 45°C, 10Hz[20-23]. CRT-2PT (COOPER TECHNOLOGY TWO POINT TRAPEZOIDAL BENDING TEST) complex modulus test and SPT (Simple Performance Tester) dynamic modulus test are two different stiffness modulus tests, where SPT uniaxial compression dynamic modulus test is compression mode, while CRT-2PT trapezoidal beam complex modulus test is bending-tension mode. Different loading methods will cause big differences in test results. In China, if the high modulus asphalt mixture is designed based on the complex modulus index value, the design of the pavement structure layer thickness will be too large due to the small modulus value, and the construction cost will be greatly increased. Moreover, the complex modulus test equipment for trapezoidal beams is expensive, which is difficult to popularize in China.

In order to find out the appropriate modulus value of high modulus asphalt mixture suitable for China's modulus parameter conditions to better carry out the reasonable design and evaluation of high modulus asphalt mixture in China, the modulus under the two evaluation systems through the two-point bending complex modulus test of CRT-2PT trapezoidal beam and the SPT uniaxial compression dynamic modulus test was analyzed in this paper. Under the premise of satisfying the complex modulus ≥ 14000Mpa under the conditions of 15℃ and 10Hz, the conversion relationship model between the complex modulus of the trapezoidal beam and the uniaxial compression dynamic modulus was established. Based on the conversion model, a range of values suitable for the design and evaluation of the dynamic modulus of high modulus asphalt mixtures in China was recommended

4-Provide more mix design data, especially volumetrics.

Response: This article refers to the French asphalt mixture design guidelines for high modulus asphalt mixture design. The French high-modulus asphalt mixture design concept based on performance verification and mechanical indicators is different from the Superpave design concept and the Marshall design concept (using the volume method for grading design), but a special asphalt mixture design method. The performance of the asphalt mixture is designed as the ultimate goal, and the performance test of level 1 to level 4 is carried out according to the actual different needs of the road.

Performance test verification is carried out from the simplest level 1 to the most comprehensive and complex level 4, and the high design level should be carried out after the low design level requirements are met.

In the basic level 1, for the volume parameter requirements of the mixture, only the mixture must meet the threshold index requirements of the porosity and water sensitivity of the rotary compaction test.

The performance indicators of the mixture level 1 to level 4 was carried out in the article, shown in Table8.

In the paper, the gradation limits of the mix aggregate was added in Table 4.

5-Conclusions should be more concise. Don`t just repeat the discution.

Response: Thank you for your suggestion. The conclusions was refined to be more concise.

Reviewer 3 Report

1. This statement is not all right High Modulus Asphalt Mixture will have low fatigue resistance performance  "In recent years, High Modulus Asphalt Mixture have become research hotspots due to their excellent high-temperature stability, resistance to water damage, fatigue resistance, higher modulus and excellent load transfer and dispersion ability"  
2. In Table 6, include the test temperature in the table. 
3. Wrong Figure number: "It can be seen from Figures 1 to 4: the modulus master curve and phase angle master curve obtained by the two test modes are quite different[25-27], indicating that difference." 
4. Noting included it about sample preparation.
5. What is the target Air Voids? It is really important to report. This is what properly affects the results. 

Author Response

Special thanks to you for your good comments. We have carefully revised the manuscript, and the paper was also re-scrutinized to improve the English. Our responses are as follows, and we amended the relevant part in revised manuscript and the changes below are noted in red words in the article.

1-This statement is not all right High Modulus Asphalt Mixture will have low fatigue resistance performance  "In recent years, High Modulus Asphalt Mixture have become research hotspots due to their excellent high-temperature stability, resistance to water damage, fatigue resistance, higher modulus and excellent load transfer and dispersion ability"

Response: Thank you for your suggestion. This statement has been corrected as follow: High modulus asphalt mixture has become a research hotspot due to its excellent high temperature stability, strong resistance to water damage and good fatigue resistance, which has great advantages in reducing the thickness of pavement and saving resource.

2-In Table 6, include the test temperature in the table.

Response: Thank you for your suggestion. Table 6 showed the Complex modulus results of different types of low-mark high modulus asphalt mixtures at 15°C. Corresponding modification was made in the revised manuscript noted in 3.2.

3-Wrong Figure number: "It can be seen from Figures 1 to 4: the modulus master curve and phase angle master curve obtained by the two test modes are quite different[25-27], indicating that difference."

Response: It is our negligence and we are sorry about this. The error has been corrected in the paper.

4-Noting included it about sample preparation.

Response: Thank you for your suggestion. The sample preparation was added in 2.3.

5-What is the target Air Voids? It is really important to report. This is what properly affects the results.

Response: Thank you for your suggestion. The target Air Voids were added in 3.2 to Table 9 and Table 10.

Round 2

Reviewer 1 Report

Accepted

Reviewer 2 Report

.