Effects of Freeze–Thaw Cycles on Performance and Microstructure of Cold Recycled Mixtures with Asphalt Emulsion

Round 1

Reviewer 1 Report

Author Response

Response to Reviewer 1 Comments

1: It is necessary to try to connect well the specific chemical compositions of the binder and the mechanical characteristics of the asphalt concrete after different cycles of freezing and thawing.

Thank you for the reviewer. The purpose of the specific chemical compositions analysis of the binder is to prove the falling off of asphalt. So the correlation between the specific chemical composition and mechanical properties of asphalt concrete is same to microstructure analysis. (line 400-411)

2: The conclusion is written very weakly. We recommend rewriting it again, expanding it somewhat. Here it is also necessary to give established relationships between the chemical compositions of the binder and the mechanical characteristics of the asphalt concrete after different cycles of freezing and thawing.

As recommended, the conclusion has been rewritten in revised manuscript. The relationships between the chemical compositions of the binder and the mechanical characteristics of the asphalt concrete have been established after different cycles of freezing and thawing.

(2) The fracture surface of CRME is changing by the SEM test. The asphalt strips from the surface of hydration products and the hydration products become little, short and fine gradually as freeze-thaw cycles increasing, which illustrate freeze-thaw cycles has destroy the CRME microstructure. (line 446-449)

(3) The chemical composition of CRME is changing by the energy spectrum analysis. The element contents increasing of Si, Ca and the element content decreasing of C verify the falling off of asphalt, which also illustrate that the mortar develop into the cement materials gradually. (line 450-453)

(4) According to the analysis of the fracture surface and chemical composition, the asphalt strips from the surface of hydration products which caused the increasing of air voids (included formation of new air voids, coalescing of two separated air voids and expansion of existing individual voids), the decreasing of cohesive. Therefore, the high-temperature stability and moisture susceptibility of CRME decrease. The composite structure mainly consists of hydration products as freeze-thaw cycles increasing result in the decreasing of the low-temperature cracking resistance of CRME, and the freeze-thaw cycles have a negative effect on the CRME performance and microstructure. (line 454-462)

3: The Abstract section in the paper should be supplemented with the new results re-included in the Conclusions section.

Thank you for the reviewer. “the asphalt strips from the surface of hydration products, and the composite structure mainly consists of hydration products as freeze-thaw cycles increasing; the microstructure of CRME is destroyed.” has been added in the Conclusions section. (line 21-23)

Author Response File: Author Response.pdf

Reviewer 2 Report

1: The description of results lacks deep discussion of these results. This discussion should at least include a comparison of the results obtained by earlier study.

Many thanks to the reviewer. Authors have read the earlier studies carefully, and earlier studies have been quoted in revised manuscript (line 79-92, line 97-98). The earlier studies mainly force on hot-mixed asphalt mixtures. However, the material composition and void structure of hot-mixed asphalt mixtures are different from CRME. The comparison of the results obtained by earlier studies is tried to write.

Due to the large void ratio and structural characteristics of CRME, the increasing speed of air voids was faster than hot mix asphalt mixture under freeze-thaw cycles. (line 314-317)

Compared with hot mix asphalt mixture, the decreasing of high-temperature was significant under freeze-thaw cycles, which were results of the rapid increasing of air voids and the loss of shear strength. (line 333-345)

The binder of CRME was mainly made up of hydration products as freeze-thaw cycles increasing, which was different from hot mix asphalt mixture. The CRME was damaged at low temperature condition easily. So the low-temperature cracking resistance was worse than hot mix asphalt mixture. (line 369-372)

Due to the large void ratio and the rapid increasing of air voids, water could enter the internal mixture easily and reduce the cohesive between asphalt and aggregate, which caused the significant declined of moisture susceptibility in comparison to hot mix asphalt mixture under freeze-thaw cycles in completely water-saturation condition. (line 387-391)

Author Response File: Author Response.pdf

Reviewer 3 Report

1: On the basis of which norms / regulations is the maximum grain size defined can the grain size be related to the maximum thickness of the designed layer? Please include in the text.

Thank you for the reviewer. “The nominal maximum aggregate size of CRME is 19 mm according to Chinese Technical Specifications for Highway Asphalt Pavement Recycling (JTG T5521-2019)” has been added in revised manuscript. ( line 187-188)

2: Has the resistance of the unit to freezing been tested and by which method? Is the resistance class / absorption class of the aggregate defined? Please include in the text.

Many thanks to the reviewer.

(1) The uniaxial penetration test, the indirect tensile strength test and the freeze-thaw spit test are carried out after 0, 5, 10, 15, and 20 freeze-thaw cycles with different water-saturation conditions.

The high-temperature stability of CRME is evaluated by the uniaxial penetration test based on Chinese Specifications for Design of Highway Asphalt Pavement (JTG D50-2017). (line 256-259)

The low-temperature cracking resistance of CRME is assessed with the indirect tensile strength test according to Chinese Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011). (line 269-271)

The moisture damage of CRME is evaluated using the freeze-thaw spit test Chinese Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011). (line 281-283)

(2) The resistance class / absorption class of the aggregate is not required in CRME. The characteristics of aggregate are listed in Table 2 and Table 3 in revised manuscript. (line 182-183)

3: Are there regulations that define the minimum required conditions that an aggregate must meet in a mixture? Please specify.

Thank you for the reviewer.

(1) The RAP should meet the maximum particle size and sand content (the size of RAP need less than 4.75mm) according to Chinese Technical Specifications for Highway Asphalt Pavement Recycling (JTG T5521-2019). “The sand content of RAP is 65 more than the requirement of 50 (line 170).The biggest size of RAP is less than the nominal maximum aggregate size of 19.0 mm (line 174).” has been added in revised manuscript.

(2) The requirements of aggregate are shown in Table 2 and Table 3 according to Chinese Technical Specifications for Construction of Highway Asphalt Pavements (JTG F40-2004). (line 182-183)

Table 2. Characteristics of coarse aggregate

Table 3. Characteristics of fine aggregate

4: Are there guidelines according to which the choice of binder in the mixture is defined? Please specify.

Thank you for the reviewer. The binder is required using the cationic slow-cracking asphalt emulsion in CRMR according to Chinese Technical Specifications for Highway Asphalt Pavement Recycling (JTG T5521-2019). (line 163-164)

5: According to what are the conditions for freezing the samples listed in the text prescribed?

Experience shows that it is very difficult to define test conditions for the durability properties of materials. Were the samples on all sides equally loaded during the test? Is there a photograph of the samples before testing? It would be nice to include it in the text.

Many thanks to the reviewer.

(1) The conditions for freezing the samples are defined by the meteorological data in Shenyang, Liaoning Province. The annual average lowest temperature is −17.0°C, and the annual average highest temperature is 16.8°C in freeze-thaw months. Considering present F-T test, a modified F-T test was proposed according to the practical climate conditions of Shenyang in earlier study. At the same time, the research results in the following literature are also referred.

Duojie C, Si W, Ma B, Hu Y, Wang X. Assessment of freeze-thaw cycles impact on flexural tensile characteristics of asphalt mixture in cold regions. Math. Probl. Eng. 2021, 2021, 1-8, doi.org/10.1155/2021/6697693.

Fakhri M, Siyadati S A, Aliha M R M. Impact of freeze–thaw cycles on low temperature mixed mode I/II cracking properties of water saturated hot mix asphalt: An experimental study. Constr. Build. Mater. 2020, 261: 119939, doi.org/10.1016/j.conbuildmat.2020.119939.

Wu H, Li P, Nian T, et al. Evaluation of asphalt and asphalt mixtures’ water stability method under multiple freeze-thaw cycles. Constr. Build. Mater. 2019, 228: 117089, doi.org/10.1016/j.conbuildmat.2019.117089.

(2) The samples are not on all sides equally loaded during the test.

The uniaxial penetration test is based on Chinese Specifications for Design of Highway Asphalt Pavement (JTG D50-2017). (line 258-259)

The indirect tensile strength test is based on Chinese Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011). (line 270-271)

The freeze-thaw spit test is based on Chinese Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011). (line 281-283)

(3) The photos of the samples before testing are shown in Figure 2. (line 233)

Figure 2 The photos of the samples before testing

6: Is the equivalence coefficient calculated to calculate the structural number (SN)? Please specify.

Thank you for the reviewer. The structural number (SN) is not studied in the article. Your idea is very good. We will try to calculate the equivalence coefficient to gain the structural number (SN) in the next research.

7: Are there any other tested / declared properties of the aggregate on the basis of which you have determined that it is suitable for installation? (crushing, absorption, resistance to Mg SO4)?

Thank you for the reviewer. We just start to study the effects of freeze-thaw cycles on performance and microstructure of CRME. The other tested / declared properties of crushing, absorption, resistance to MgSO4 that provide a new and important research direction by reviewer. Therefore, they have been considered mainly in the future. Thank you again.

Author Response File: Author Response.pdf