An Explorative Study into the Influence of Different Fibers on the Spalling Resistance and Mechanical Properties of Self-Compacting Concrete after Exposure to Elevated Temperatures

Round 1

Reviewer 1 Report

This manuscript presents systematic experimental investigations on the spalling resistance and residual mechanical properties of self-compacting concrete with steel fibers and/or polypropylene fibers. The research topic is meaningful, especially the suggested fiber cocktail blended with steel fibers and polypropylene fibers is essentially important for the application of different fibers for enhancing the fire resistance of concrete structures. But the following issues should be revised carefully:

 

1) Line 10, there is an extra right bracket after “,,,(Y. L.))”.

2) Lines 217-220: It seems this is not the finding by the authors from their tests. If so, the reference should be cited.

3) Lines 279-282: In recent study on the mechanism of plastic fibers in spalling prevention, microcracks caused by PP fibers are found to be critical for spalling prevention. The author may need to add this to the discussion.

4) Between lines 413 and 414, “Table 5” is missing.

Author Response

Response to Reviewer 1 Comments

Point 1: Line 10, there is an extra right bracket after “,,,(Y. L.))”.

 Response 1: Many thanks for the comment!

As per the expert’s suggestion, the extra right bracket has been deleted. Pls. see the revised manuscript.

 

Point 2: Lines 217-220: It seems this is not the finding by the authors from their tests. If so, the reference should be cited.

Response 2: Many thanks for the comment!

Yes, the finding between line 222 and line 225 in the revised manuscript comes from previous studies. As per the suggestions of the expert, the quotation of the pieces of literature is added as follows:

“This is because the fibers’ contribution to the flexural tensile strength is dominated by the number of fibers crossing the crack, rather than their weight. It should be indicated that the advantageous effect of SF on the tensile strength of the concrete is due to the sum of the tensile strength brought about by each fiber [50].”

50. di Prisco, M.; Plizzari, G.; Vandewalle, L. Fibre reinforced concrete: new design perspectives. Mater. Struct., 2009, 42, 1261-1281.

 

Point 3: Lines 279-282: In recent study on the mechanism of plastic fibers in spalling prevention, microcracks caused by PP fibers are found to be critical for spalling prevention. The author may need to add this to the discussion.

Response 3: Many thanks for the comment!

The mechanism of plastic fibers in spalling prevention is very complicated. Recently, Zhang [1] and Li [2] found that the thermal mismatch between the fibers and the matrix is critical for obtaining an interconnected network of cracks in the matrix even before melting of the polypropylene fibers. This mechanism was also included in section 3.2 of the revised manuscript as follows:

“On the one hand, it is beneficial from the uniform distribution of PF in the concrete matrix and lower melting temperature (165~173℃). When the melting temperature of PF is exceeded, interconnected channels left by the melting PF are favorable for mitigating interior vapor stress, which can help enhance the explosive spalling resistance of the concrete [22]. On the other hand, the thermal mismatch between the polypropylene fibers and the matrix results in an interconnected network of cracks in the matrix even before melting of the polypropylene fibers [51, 52].”

 

22. Liu, X.; Ye, G.; De Schutter, G.; Yuan, Y.; Taerwe, L. On the mechanism of polypropylene fibres in preventing fire spalling in self-compacting and high performance cement paste. Cem. Concr. Res., 2008, 38, 487-499.
23. Zhang, D., Dasari, A., Tan K. H. On the mechanism of prevention of explosive spalling in ultra-high performance concrete with polymer fibers. Cem. Concr. Res., 2018, 113, 169-177.
24. Li, Y., Tan, K. H., Yang E-H. Synergistic effects of hybrid polypropylene and steel fibers on explosive spalling prevention of ultra-high performance concrete at elevated temperature. Cem. Concr. Compos., 2019, 96, 174-181.

Point 4: Between lines 413 and 414, “Table 5” is missing.

Response 4: Many thanks for the comment!

This is our mistake, “Table 5” has been added to the revised manuscript. Pls. see table 5 in the revised manuscript.

 

References:

1. Zhang, D., Dasari, A., Tan K. H. On the mechanism of prevention of explosive spalling in ultra-high performance concrete with polymer fibers. Cem. Concr. Res., 2018, 113, 169-177.
2. Li, Y., Tan, K. H., Yang E-H. Synergistic effects of hybrid polypropylene and steel fibers on explosive spalling prevention of ultra-high performance concrete at elevated temperature. Cem. Concr. Compos., 2019, 96, 174-181.

Reviewer 2 Report

Comments are included in the attached file

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 2 Comments

Point 1: The authors should briefly describe in the introduction the practical applications for testing these specimens to this temperature range.

Response 1: Many thanks for the comment!

As per the suggestions of the expert, the background introduction of fiber reinforced self-compacting concrete (FRSCC) in underground structures, especially in tunnels is presented. Recently, fire incident becomes one of the most frequent and destructive disasters in tunnels. Tunnel fire has characteristics of high peak temperature, rapid heating rate, long duration and nonuniform temperature distribution inside the tunnel, which can result in extensive and complex damage to concrete tunnel linings. Oven-heated specimens were employed to assess the spalling resistance and mechanical properties of FRSCC at target temperatures ranging between 200 ℃ and 1000 ℃. Pls. see the introduction of the revised manuscript.

 

Point 2: Although the duration at the constant target temperature is the same for all levels of target temperatures the total duration is considerably different. That is for target temperature 200C the total duration is approximately 150sec whereas for a target temperature 1000C is approximately 220sec. The regression analysis of the obtained results and the proposed model deals only with the parameter of temperature (T) and not with the exposure time. The authors should discuss this omission.

Response 2: Many thanks for the comment!

In order to make the internal heat of concrete uniform, keeping the target temperature constant for one or two hours was applied by several researchers [1-3]. In our work, specimens were kept in the furnace for 2 hours after reaching the target temperature and then cooled down naturally to room temperature. We guess the reviewer means the total duration of specimens under high temperature of 200 ℃ is “150 min” and “220 min” for specimens under high temperature of 1000 ℃. Due to various target temperatures and the same heating rate of 10 ℃/min, the exposure time of specimens under various target temperatures is different. From the test results in the present work and previous study [1], the mechanical properties were mainly influenced by the target temperatures. Thus, the exposure time was not included in the regression expression of the degradation coefficient. The relationship between the degradation coefficient of residual strength and target temperatures was presented in the present work.

 

Point 3: When the samples are with extensive spalling after being exposed to the target temperature cycles (figures 7 to 10) it should be explained how the compressive and flexural tensile strength values were found from such specimens.

Response 3: Many thanks for the comment!

In our work, three specimens in each group were tested at various temperatures. All the specimens containing polypropylene fibers, whatever specimens with single PF or combined with SF, no spalling is observed. Specimens NC, SF20 and SF40 suffered severe spalling during high temperature tests. While not all the specimens suffered explosive spalling which make mechanical property tests impossible below high temperature of 600 ℃. As for specimens that suffered light spalling and still keep the integrity of the specimen, the mechanical properties were tested and the residual strengths were obtained.

 

Point 4: The proposed model agrees reasonably well with the models proposed by other researchers for predicting the variation of the compressive strength with the temperature increase. However, the various models differs substantially for predicting the variation of the flexural tensile strength with temperature. Moreover, the proposed model does not reflect the influence of the content of the Steel Fibers in the mix although it was shown to be of importance. The authors should provide a discussion on these issues.

Response 4: Many thanks for the comment!

As per the suggestions of the expert, a discussion on the influence of the steel fibers’ content on the proposed model was added to the revised manuscript.

As a novel cementitious composite, fiber reinforced self-compacting concrete (FRSCC) combines the benefits of SCC in the fresh state and those resulting from the addition of fibers in the hardened state. The main difference between FRSCC and traditional fiber reinforced concrete (FRC) is that the fiber content of FRC is mainly determined by the post-cracking behavior of the composite, while the fiber content of FRSCC is mainly restricted by the workability of fresh SCC [4].

In order to satisfy the workability requirements of the fresh SCC, the steel fiber content is restricted and it is usually less than 60 kg/m³ [4]. In this circumstance, the influence of the steel fiber content on the compressive strength of concrete is marginal. As for flexural tensile strength, steel fiber content can affect it greatly in ambient temperature. However, compared with the target temperature, the effect of fiber content on the residual flexural tensile strength after exposure to elevated temperatures is little. For simplicity, in our proposed model, only the effect of target temperatures on the residual strength was considered.

 

References:

1. Lau, A.; Anson, M. Effect of high temperatures on high performance steel fibre reinforced concrete. Cem. Concr. Res., 2006, 36, 1698-1707.
2. Kodur, V.; Banerji, S.; Solhmirzaei, R. Effect of temperature on thermal properties of ultrahigh-performance concrete. J. Mater. Civ. Eng., 2020, 32, 04020210.
3. Li, X.; Bao, Y.; Wu, L.; Yan, Q.; Ma, H.; Chen, G.; Zhang, H. Thermal and mechanical properties of high-performance fiber-reinforced cementitious composites after exposure to high temperatures. Constr. Build. Mater., 2017, 157, 829-838.
4. Ding, Y.; You, Z.; Jalali, S. The composite effect of steel fibres and stirrups on the shear behaviour of beams using self-consolidating concrete. Eng. Struct., 2011, 33, 107-117.

Author Response File: Author Response.docx

Reviewer 3 Report

In this paper, the fiber type, fiber content and fire temperature are taken as variables to analyze the properties of self compacting concrete, to evaluate the influence of fibers on the high temperature resistance and mechanical properties of SCC. The article is rich in content, the test process is standardized, and the data analysis and theoretical derivation process are rigorous. It is recommended to be accepted after mineral corrections.

Specific opinions are as follows:

(1) The introduction explains that both single fibers have shortcomings, and the mixed fiber can make up for the disadvantages. However, there is only one group of fiber cocktail SCC. How did you decide the contains of fibers for SPF201? Why not SPF402 ?

(2) It is difficult to draw this conclusion from Figure 8: NC specimens suffered the most severe spalling, while SF specimens suffered less.

(3) In Section 3.2, The spalling of SF and NC and PF were all introduced, however, the spalling condition for specimens of SPF201 was not described. It is difficult to confirm the influence of interaction between fibers. How do you get the conclusion that the SCC’s explosive spalling resistance can be effectively enhanced with the incorporation of fibers? 

(4) In Section 3.4, the mechanism of concrete mechanical property degradation caused by PF is unclear, and the influence of PF on various properties is attributed to PF melting and forming connected pores.

(5) In Figure 13, the comparison between the tested results of PF2 and SPF201 is not obvious, and it is difficult to verify that the addition of SF improves the influence of PF on the degradation of concrete mechanical properties.

(6) The mass loss may contains the loss of water and also the melt of fiber, How could you distinguish the two? And what do you want to explain by the mass loss?

(7) In the conclusion part, it is suggested that the effects of mixed fibers on the properties of SCC should also be summarized.

Author Response

Response to Reviewer 3 Comments

Point 1: The introduction explains that both single fibers have shortcomings, and the mixed fiber can make up for the disadvantages. However, there is only one group of fiber cocktail SCC. How did you decide the contains of fibers for SPF201? Why not SPF402?

Response 1: Many thanks for the comment!

As a novel cementitious composite, fiber reinforced self-compacting concrete (FRSCC) combines the benefits of SCC in the fresh state and those resulting from the addition of fibers in the hardened state. The main difference between FRSCC and traditional fiber reinforced concrete (FRC) is that the fiber content of FRC is mainly determined by the post-cracking behavior of the composite, while the fiber content of FRSCC is mainly restricted by the workability of fresh SCC [1].

In our work, in order to satisfy the workability requirements of fresh SCC, the fiber content is restricted. In the stage of mix design, three hybrid fiber mixtures SPF201, SPF202 and SPF401 were selected to achieve FRSCC. While the content of 2 kg/m³ micro polypropylene fiber has strong negative influence on the workability and is not suitable for reinforcing SCC. Finally, only mixture SPF201 satisfies the workability requirements of the fresh FRSCC. Finally, only mixture SPF201 was utilized in the high temperature and residual strength test.

 

Point 2: It is difficult to draw this conclusion from Figure 8: NC specimens suffered the most severe spalling, while SF specimens suffered less.

Response 2: Many thanks for the comment!

Three specimens in each group were tested in high temperature and residual strength tests. The three NC specimens all suffered explosive spalling at different degrees, while not all the SF specimens suffered explosive spalling during high temperature test. A modified description of Figure 8 is added in the revised manuscript.

 

Point 3: In Section 3.2, The spalling of SF and NC and PF were all introduced, however, the spalling condition for specimens of SPF201 was not described. It is difficult to confirm the influence of interaction between fibers. How do you get the conclusion that the SCC’s explosive spalling resistance can be effectively enhanced with the incorporation of fibers?

Response 3: Many thanks for the comment!

In the present work, the spalling condition for specimen SPF201 was discussed together with specimens with mono PF in paragraph one of Section 3.2 in the revised manuscript. From the explosive spalling test results of specimens at elevated temperatures, we found that in specimens incorporated with PF, whatever single or blended with SF, no spalling was observed. While regarding specimens with relatively high steel fiber content (such as 50 kg/m³), good explosive spalling resistance was observed. Thus, we give the conclusion that the SCC’s explosive spalling resistance can be effectively enhanced with the incorporation of fibers.

 

Point 4: In Section 3.4, the mechanism of concrete mechanical property degradation caused by PF is unclear, and the influence of PF on various properties is attributed to PF melting and forming connected pores.

Response 4: Many thanks for the comment!

From Figure 13 and Table 5 in the revised manuscript, compared with plain SCC, no significant enhancement in residual flexural tensile strength of specimens containing PF was observed after exposure to various target temperatures. While the detrimental effect of PF on the residual strength was observed. On the one hand, this may be attributed to PF melting and forming connected pores [2]. On the other hand, the thermal mismatch between the polypropylene fibers and the matrix results in an interconnected network of cracks in the matrix even before melting of the polypropylene fibers[3, 4]. The two mechanisms were added to explain the influence of PF on the residual flexural strength after exposure to different target temperatures. Pls. see paragraph four of  Section 3.2 in the revised manuscript.

 

Point 5: In Figure 13, the comparison between the tested results of PF2 and SPF201 is not obvious, and it is difficult to verify that the addition of SF improves the influence of PF on the degradation of concrete mechanical properties.

Response 5: Many thanks for the comment!

The inclusion of 2 kg/m³ micro PF has strong negative influence on the workability of fresh SCC. From Table 3 in the revised manuscript, we can see that mixture SPF201 has better workability than that of mixture PF2. From Figure 13 in the revised manuscript, a fluctuation of residual flexural tensile strength between specimens SPF201 and PF2 was observed. While from Figure 11 in the revised manuscript, the residual compressive strength of specimen SPF201 shows less degradation than that of specimen PF2 after exposure to high temperature above 400 ℃. The addition of SF can improve the influence of PF on the degradation of concrete mechanical properties.

 

Point 6: The mass loss may contains the loss of water and also the melt of fiber, How could you distinguish the two? And what do you want to explain by the mass loss?

Response 6: Many thanks for the comment!

Yes, the mass loss should contain the loss of water and also the melting of the polypropylene fibers. While in our work, the mass loss illustrates the total loss of specimens after exposure to various target temperatures. From Figure 15 in the revised manuscript, we can see that almost all the specimens show the same trend of mass loss according to the target temperatures. It is illustrated that target temperatures influence mass loss more greatly than that of different fibers in the case of relatively low fiber content.

 

Point 7: In the conclusion part, it is suggested that the effects of mixed fibers on the properties of SCC should also be summarized.

Response 7: Many thanks for the comment!

As per the suggestions of the expert, the effects of mixed fibers on the properties of SCC are added to article 5 of the conclusion. It was rewritten as follows:

“5. The incorporation of cocktail fibers blended with 20 kg/m³ steel fiber and 1 kg/m³ polypropylene fiber in SCC satisfies well with the workability requirements of the fresh SCC. No explosive spalling and less degradation of mechanical properties are observed in specimen SPF201 after exposure to various high temperatures. The mixed fibers exhibit a promising way to improve the fire resistance feature of SCC.”

 

References:

1. Ding, Y.; You, Z.; Jalali, S. The composite effect of steel fibres and stirrups on the shear behaviour of beams using self-consolidating concrete. Eng. Struct., 2011, 33, 107-117.
2. Liu, X.; Ye, G.; De Schutter, G.; Yuan, Y.; Taerwe, L. On the mechanism of polypropylene fibres in preventing fire spalling in self-compacting and high performance cement paste. Cem. Concr. Res., 2008, 38, 487-499.
3. Zhang, D.; Dasari, A.; Tan K. H. On the mechanism of prevention of explosive spalling in ultra-high performance concrete with polymer fibers. Cem. Concr. Res., 2018, 113, 169-177.
4. Li, Y.; Tan, K. H.; Yang E-H. Synergistic effects of hybrid polypropylene and steel fibers on explosive spalling prevention of ultra-high performance concrete at elevated temperature. Cem. Concr. Compos., 2019, 96, 174-181.

Author Response File: Author Response.docx