Performance Evolution of Recycled Aggregate Concrete under the Coupled Effect of Freeze–Thaw Cycles and Sulfate Attack

Round 1

Reviewer 1 Report

Relevant topic and adequate set of experimental data. The paper requires some modifications before a final decision is made.

 

The authors are required to indicate where would concrete in practice be subjected to both sulphate attack and freeze thaw. Indicate which part of the world this exposure is likely to occur For example in the Gulf area concrete may be subjected to both sulphate and chloride.

 

The equations used to calculate the various parameter in the results and discussion should normally be part of the experimental part

Discussion can be improved

The conclusions can be improved by adding some values about the tend (e.g. percentage trend)

Author Response

Comment 1: Relevant topic and adequate set of experimental data. The paper requires some modifications before a final decision is made.

 Response: Thanks for your suggestions. In the revised manuscript, we have made a detailed discussion of the experimental results. Discussion in relation to the previous research work in literature is also added. In addition, the novelty and knowledge gaps filled by the current research are stated in the revised abstract and conclusions.

 

Comment 2: The authors are required to indicate where would concrete in practice be subjected to both sulphate attack and freeze thaw. Indicate which part of the world this exposure is likely to occur. For example in the Gulf area concrete may be subjected to both sulphate and chloride.

Response: Thanks for the comments. The regions where both sulphate attack and freeze-thaw cycles of concrete occur have been indicated in the revised manuscript as follows:

“The area of saline-alkali land in China ranks third in the world. These saline-alkali lands are mainly located in the northwest part of China, such as Xinjiang, Ningxia and Inner Mongolia, and northeast of China [25]. Most of these regions are also characterized by high altitude/latitude and low winter temperature.  Consequently, the combined damage is more relevant to in-service conditions than the classical FT cycles and sulfate immersion tests.”

 

Comment 3: The equations used to calculate the various parameter in the results and discussion should normally be part of the experimental part

Response: In the revised manuscript, we have repositioned the equations.

 

Comment 4: Discussion can be improved

Response: Thank you for pointing this out. In the revised manuscript, we have given the detailed discussion on the results. For example:

“For water- and sulfate-exposed samples subjected to 180 FT cycles, the residual compressive strength is about 71% and 78% of the original strength, respectively. The residual strength of W180 is about 92% of that of S180. This phenomenon is also confirmed by Jiang [42] through studying the durability of high-performance concrete, which was subjected to the same exposed conditions.

Also, for both RAC in this study and high-performance concrete (w/c=0.56) in  [42]  subjected to FT cycles, their compressive strengths are in a pronounced linear relationship with FT cycles. Moreover, as a consequence of high porosity, the RCA concrete has more loss in compressive strength compared with ordinary concrete [42], despite adopting a lower w/b ratio.

However, for the splitting tensile strength of RAC tested in this study, slight differences were noticed between different exposed conditions. For both water- and sulfate-exposed samples subjected to 180 FT cycles, the residual splitting tensile strength is about 65% of the original strength. For RAC subjected to sulfate-FT damage, except for 180 FT cycles, the loss in splitting tensile strength was smaller compared to the ones subjected to water-FT damage.”

 

Comment 5: The conclusions can be improved by adding some values about the tend (e.g. percentage trend)

Response: Thank you for your suggestions. We have improved this in the revised manuscript as follows:

“(2) The water-exposed FT cycles would result in severer deterioration in mass loss, elastic modulus and compressive strength. While for the sulfate-exposed FT cycles, the splitting tensile strength and fracture energy have more serious degradation. Compared with compressive strength, deterioration in splitting tensile strength is more severe. The maximum loss in compressive and splitting tensile strength is 28.7% and 35%, respectively.

(3) The fracture energy has showed an increasing trend to 60 FT cycles, followed by an overall decrease until 180 FT cycles. It has indicated a maximum increment of about 45% and 39% for water- and sulfate-exposed samples respectively after being subjected to 60 FT cycles. The analysis of failure modes of coarse aggregate reveals that FT damage results in a significant deterioration in the binding force of mortar. After being subjected to 180 FT cycles, the area percentage of pulled-out failure was increased from 7.3% to larger than 17.3%.”

Author Response File: Author Response.pdf

Reviewer 2 Report

1.       Define RAC, FT, and other abbreviations in the abstract in their first appearance

2.       Results are not well written in the abstract. Rewrite.

3.       To what extent do the damage caused by freezing of sulfate solution was lower? Also, to what extent do the deterioration of RAC was occurred.

4.       In the keywords, it is recommended to use recycled aggregate concrete instead of recycled concrete.

5.       Grading curves of sand, coarse RCA, and fly ash need to be provided.

6.       Check the unit in Table 1. It should be kg.m^-3

7.       What is NA in Table1

8.       Why do you take 20% fly ash as replacement of cement? Why not 30, 15, …

9.       Properties of the SP need to be supplied.

1.   Fig. 1a is not clear. Enhance the resolution and quality of all figures throughout the manuscript.

1.   Specimen sizes, testing standards should be provided for compression, flexural, and splitting strength tests.

1.   Complementary discussion in comparison to literature is needed, to validate the study, show its novelty and gaps which have been filled with this research.

Author Response

Comment 1: Define RAC, FT, and other abbreviations in the abstract in their first appearance

Response: These have been defined in the revised manuscript accordingly.

 

Comment 2: Results are not well written in the abstract. Rewrite.

Response: Thank you for pointing this out. We have rewritten the results in the revised abstract as follows:

“Results showed that the water-exposed FT cycles would result in severer deterioration in mass loss, elastic modulus and compressive strength. While for the sulfate-exposed FT cycles, the splitting tensile strength and fracture energy have more significant degradation. Moreover, compared with compressive strength, deterioration in splitting tensile strength is more severe. The maximum loss in compressive and splitting tensile strength was 28.7% and 35%, respectively. The fracture energy showed an increasing trend to 60 FT cycles, followed by an overall decrease until 180 FT cycles. The fracture energy showed a maximum increment of about 45% and 39% for water- and sulfate-exposed samples respectively after being subjected to 60 FT cycles. The analysis of failure modes of coarse aggregate has revealed that FT damage results in a significant deterioration in the binding force of mortar. After being subjected to 180 FT cycles, the area percentage of pulled-out failure was increased from 7.3% to larger than 17.3%.”

 

Comment 3: To what extent do the damage caused by freezing of sulfate solution was lower? Also, to what extent do the deterioration of RAC was occurred.

Response: Thank you for pointing this out.

The results of this study have shown that the water-exposed FT cycles result in severer deterioration in mass loss, elastic modulus and compressive strength. For the sulfate-exposed sample subjected to 180 FT cycles, its loss in oven-dried mass, dynamic elastic modulus and compression strength are about 66.2%, 87.4% and 92.0% of the ones of the water-exposed specimen (W180), respectively. While for the sulfate-exposed FT cycles, the fracture energy has more significant degradation. Residual fracture energy of S180 is about 92.7% of the one of W180.

For specimens subjected to sulfate-exposed 180 FT cycles, the loss in elastic modulus, compressive strength, splitting tensile strength and fracture energy are 21.5%, 22.5%, 35% and 15.6%, respectively.

 

Comment 4: In the keywords, it is recommended to use recycled aggregate concrete instead of recycled concrete.

Response: This has been changed in the revised manuscript.

 

Comment 5: Grading curves of sand, coarse RCA, and fly ash need to be provided.

Response: Thank you for your suggestions. We have provided the grading curves of fly ash, fine aggregate, natural coarse aggregate (NCA) and recycled coarse aggregate (RCA), as shown in newly added Fig. 1. In the revised manuscript, these figures are marked with blue borders.

 

Fig. 1 The gradation curves for fly ash, fine aggregate, NCA and RCA.

Comment 6:  Check the unit in Table 1. It should be kg.m^-3

Response: Thank you for pointing this out. It has been changed in the revised manuscript.

 

Comment 7:  What is NA in Table1

Response: Thank you for pointing this out. We have used NCA to represent natural coarse aggregate in the revised manuscript.

 

Comment 8: Why do you take 20% fly ash as replacement of cement? Why not 30, 15, …

Response:

According to Chen et al. [1], the concrete with a higher w/c ratio (or w/b ratio) is more likely to suffer FT damage. Although a w/b ratio of 0.45 is adopted in the literature [2], it still has a significant better resistance to FT damage than the E-0.47 (w/c=w/b=0.47) [1], by utilizing additional 20% fly ash.

Hence 20% cement (by mass) is replaced by fly ash to improve the resistance of RCA to FT damage in this study.

 

References:

[1] Chen S, Song X, Liu X. Compressive strength degradation and evolution of failure surfaces in compressively preloaded concrete under freeze-thaw cycles[J]. Materials Research Innovations, 2015, 19(sup5): S5-433-S5-437.

[2] Jiang L, Niu D, Yuan L, et al. Durability of concrete under sulfate attack exposed to freeze-thaw cycles[J]. Cold Regions Science and Technology, 2015, 112: 112–117.

 

Comment 9: Properties of the SP need to be supplied.

Response: Thank you for your suggestion. Properties of the SP have been supplied in the revised manuscript (Table 2).

Table 2

Properties of the polycarboxylic superplasticizer.

Appearance	Water reducing ratio	Moisture content	Bulk density/kg.m-3
Grey powder	≥25%	≤5.0%	500~700
PH value (20℃, 10% aqueous solutions)	Chloride ions content	Sodium sulfate content	Alkali content
6~8	≤0.03%	≤3.0%	≤0.5%

 

Comment 10: Fig. 1a is not clear. Enhance the resolution and quality of all figures throughout the manuscript.

Response: Thank you for your suggestions. In the revised manuscript, we have replaced Fig. 1a with a high-resolution one.

 

Comment 11: Specimen sizes, testing standards should be provided for compression, flexural, and splitting strength tests.

Response: The specimen sizes and testing standards have been provided in the revised manuscript as follows:

“Three-point bending tests (TPBT), referring to the RILEM Technical Committee 50-FMC[35], were conducted using a 100 kN electronic universal testing machine (SHIMADZU). The beam specimen has a dimension of 100 mm × 100 mm × 400 mm. The notched specimens were loaded until failure at a crosshead displacement rate of 0.1  . A 2000 kN electro-hydraulic testing machine was used for compression and splitting tests. According to GB/T 50081-2002[36], the stress rate was set to 0.5 and 0.05   in compression and splitting tests, respectively. The cubic specimens were cut to a size of 100×100×100 mm from the fractured prism specimens, which is to ensure that the corresponding compression and splitting tensile results were obtained from the concrete with the same properties [37].”

 

Comment 12: Complementary discussion in comparison to literature is needed, to validate the study, show its novelty and gaps which have been filled with this research.

Response: Thank you for your suggestions. We have added discussion in comparison to literature as follows:

“The severer deterioration in elastic modulus is also seen in the specimen exposed to deicing salt solution, which shows a difference in liquid pressure caused by water and salt solution [21,24]. Moreover, the study conducted by Zeng et al. [21] shows that the sodium chloride in solution tends to decrease the ice formation and as a result, the maximum liquid pressure decreases.

For water- and sulfate-exposed samples subjected to 180 FT cycles, the residual compressive strength is about 71% and 78% of the original strength respectively. The residual strength of W180 is about 92% of that of S180. This phenomenon is also confirmed by Jiang [42] through studying the durability of high-performance concrete, which was subjected to the same exposed conditions.

Also, for both RAC in this study and high-performance concrete (w/c=0.56) in [42] subjected to FT cycles, their compressive strengths are in a pronounced linear relationship with FT cycles. Moreover, as a consequence of high porosity, the RCA concrete has more loss in compressive strength compared with ordinary concrete in [42], despite adopting a lower w/b ratio.

Kazberuk et al. [43] reported a similar variation trend in the study of the effects of internal frost damage on fracture energy of aerated and non-aerated concrete.”

 

The novelty and gaps filled in this study are rewritten in the abstract of the revised manuscript as follows:

“Results showed that the water-exposed FT cycles would result in severer deterioration in mass loss, elastic modulus and compressive strength. While for the sulfate-exposed FT cycles, the splitting tensile strength and fracture energy have more significant degradation. Moreover, compared with compressive strength, deterioration in splitting tensile strength is more severe. The maximum loss in compressive and splitting tensile strength is 28.7% and 35%, respectively. Thefracture energy shows an increasing trend to 60 FT cycles, followed by an overall decrease until 180 FT cycles. The fracture energy exhibits a maximum increment of about 45% and 39% for water- and sulfate-exposed samples respectively after being subjected to 60 FT cycles. The analysis of failure modes of coarse aggregate reveals that FT damage results in a significant deterioration in the binding force of mortar. After being subjected to 180 FT cycles, the area percentage of pulled-out failure was increased from 7.3% to larger than 17.3%.”

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments addressed accordingly.