Study on the Compressive Stress–Strain Curve and Performance of Low-Slump Polypropylene Fiber Concrete after High Temperature
Round 1
Reviewer 1 Report
Manuscript ID: applsci-2387520 entitled “Study on the Compressive Stress-Strain Curve and Performance of Low-Slump Polypropylene Fiber Concrete after High Temperature” for journal of “Applied Sciences” has been reviewed.
The manuscript was interesting and well-motivated. However, there are few things need to be corrected and included in the manuscript for better understanding of carried research work to the readers.
+1- The “Abstract“ section is too short. It should be enriched with the data obtained from the study.
+2- The novelty of the study should be further explained (in the introduction section).
+3- More references (different studies) should be added to the introduction. (recent studies, 2021-2023). (More information should be given especially about composite materials.)
+4- It should be explained how the materials used in the study were selected.
+5- More information should be given about the materials used in the experiment. A few photos should be attached.
+6- More information on tensile/pressing testing should be given in the " Experimental Overview" section. (Crosshead speed, humidity, ambient temperature, etc.) The dimensions of the samples should be given in 3D (or tech. drawing). How was the crosshead speed determined? (and ? mm/min)
+7- The highest results of tensile and compression tests should be given as bar graphs.
+8- The resolution of Figure 4, 5, 6, 7, 9, 10, 11, and 12 should be increased. (and magnify) (Since the lines are so thin, they are not distinguishable.)
+9- … Figure 8 shows the relationship between the addition of PPF and compressive strength at room temperature. It can be seen from the figure that the addition of PPF 6 greatly enhances the compressive strength of concrete at room temperature. Under the 7 same fiber content, longer fibers lead to lower compressive strength. When the fiber length 8 is the same, an increase in the fiber content initially increases the compressive strength, 9 but then it decreases, which is consistent with the conclusion obtained in reference [39]. To 320 some extent, an appropriate PPF length and dosage can improve the compressive strength 321 of concrete. However, with the increase of fiber length and dosage, the apparent density 322 decreases, the porosity increases, and the strength decreases. Overly long fibers also tend 323 to become entangled inside, and the distribution uniformity decreases, leading to an in-324 crease in porosity and porosity rate. Compared with the compressive strength of normal 325 concrete at room temperature (41.41 MPa), the compressive strength of fiber-reinforced 326 concrete with PPF lengths of 3 mm and 9 mm reached the maximum at a PPF dosage of 1 327 kg/m3, with an increase of 17.15% and 14.85%, respectively. The compressive strength of 328 fiber-reinforced concrete with PPF lengths of 15 mm and 19 mm reached the maximum at 329 a PPF dosage of 0.5 kg/m3, with an increase of 11.25% and 11.2%, respectively. With the 330 increase of dosage, when the PPF dosage reached 4 kg/m3, the compressive strength of 3 PPF concrete with a length of 19 mm was lower than that of normal concrete, and the 332 compressive strength of PPF concrete with a length of 15 mm was only 1.37% higher than 333 that of normal concrete. When the PPF dosage reached 5 kg/m3, the compressive strength 334 of PPF with a length of 9 mm was 2.67% lower than that of normal concrete, while the 3 compressive strength of PPF concrete with a length of 3 mm was 6.52% higher than that 336 of normal concrete. This indicates that the effect of fiber content on compressive strength 337 is greater than that of fiber length when the fiber content reaches a certain degree, which 338 is related to the larger porosity rate caused by high dosage compared to that caused by 339 long fibers [40]. 340 The influence of PPF dosage on the compressive strength of concrete under high tem-341 perature is shown in Figure 9. After being exposed to temperatures of 600°C and 800°C, 342 fewer cracks were observed on the surface of PPF concrete specimens. This is largely due 343 to the fact that after melting and gasifying at high temperatures, PPF leaves many internal 344 voids, increasing internal connectivity, allowing water vapor and heat to escape more eas-345 ily, reducing vapor pressure, providing more free space, and acting as a thermal shock 346 absorber [41], thereby reducing the damage to the micro-structure of concrete caused by 347 temperature. The strength of ordinary concrete after being exposed to the highest temper-348 ature of 600°C and 800°C was only 68% (28 MPa), % (14.3 MPa), and 22% (9.1 MPa) of 349 that at room temperature. With the increase of temperature, the compressive strength of 0 PPF concrete showed a trend of first increasing and then decreasing with the increase of fiber dosage. Except for the concrete with a fiber length of 9 mm, which had the highest 2 compressive strength at the same dosage, the law of compressive strength reduction fol-3 lowed the increase of length. The reason may be that the pores created by the melting and 4 gasification of PPF at high temperature increase the porosity, reduce the area of the con-5 crete matrix that can withstand loads, and increase the path of micro-crack extension, re-6 sulting in a much lower strength of concrete at high temperature than at room tempera-7 ture. The increase of fiber length leads to high porosity and large capillary pores, which 8 weaken the effective load-bearing cross-sectional area of the concrete. The increase of fiber 9 quantity also leads to an increase in porosity, weakening the effective load-bearing area 360 while providing more paths for the development of micro-cracks. As shown in Figure 9(a), 361 when the fiber dosage was 0.5 kg/m3, the compressive strength of 3 mm, 9 mm, 15 mm, 362 and 19 mm fibers reached the maximum, which was 4.21%, 6.29%, 0.86%, and 0.32% 363 higher than that of ordinary concrete, respectively. When the fiber dosage was 5 kg/m3, 364 the compressive strength decreased by 17.64%, 15.43%, 24%, and 27.04%, respectively. 365 Figure 9 (b) shows that the residual strength of PPF concrete at 800°C is consistent with 366 that at 600°C. The influence of PPF type on strength shows less variability and the trend 367 of the curve is closer, indicating that the effect of PPF type on compressive strength of 368 concrete decreases continuously at higher temperatures. The main reasons for the de-369 crease in strength are the development of macro-cracks caused by micro-cracks and ma-370 terial calcification damage. When the fiber content is 0.5 kg/m3, the compressive strength 371 of 3 mm, 9 mm, 15 mm, and 19 mm reaches its maximum, increasing by 9.65%, 11.33%, 372 7.90%, and 2.87%, respectively, compared to ordinary concrete. When the fiber content is 373 5 kg/m3, the compressive strength decreases by 17.83%, 17.27%, 22.59%, and 23.92%. Ac-374 cording to reference [42], the residual strength of ordinary concrete without PPF is the 375 highest under high temperature. In contrast, reference [37] found that the fluctuation of 376 residual compressive strength affected by PPF type variation was not significant. In addi-377 tion, references [43, 44] indicate that the compressive strength of ordinary concrete is 378 higher than that of PPF concrete at a dosage of 3.6 kg·m-3. This conclusion is different from 379 the research results of this article, which indicates that different slump values have incon-380 sistent effects on the performance of PPF concrete. Therefore, the research on low slump 381 values in this article has practical significance. In this experiment, when the dosage of PPF 382 concrete is greater than 0.5 kg·m-3, the residual compressive strength of concrete at any 383 length significantly decreases, and the rate of strength reduction increases significantly 384 with the increase of length. At a dosage of 0.5 kg·m-3 and a length of 9 mm, the residual 385 compressive strength reaches the maximum value, which is very different from the re-386 search results of PPF concrete at room temperature. Overall, the research in this article 387 shows that PPF has a positive effect on concrete under high temperature conditions. How-388 ever, the rate of strength reduction of PPF concrete at a high dosage is much higher than 389 the effect of PPF length … <<< Please explain this part in more detail. >>> (Please write again if possible.) +10- The sections where the formulas are found should be explained step by step and rewritten. Also, it should be checked. +11- The “Conclusion” section should be enriched with clear results and comparisons. +12- References should be rearranged according to the journal “Instructions”. +13- More literature studies should be added to the introduction and other sections (DOIs given below). DOI-1 https://doi.org/10.1515/mt-2020-0024 (info about composites) DOI-2 https://doi.org/10.1515/mt-2021-2038 (different studies) DOI-3 https://doi.org/10.1007/s13369-021-06243-w (info about crosshead speed, humidity, ambient temperature, etc.) DOI-4 https://doi.org/10.26701/ems.989945 (info about crosshead speed, humidity, ambient temperature, etc.)
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**After revision, I would like to review the article again.
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Congratulations to the authors.
I wish the authors success in their future academic studies.
Kind regards.
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Author Response
Please refer to the attachment
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The article "Study on the Compressive Stress-Strain Curve and Performance of Low-Slump Polypropylene Fiber Concrete after High Temperature" by Zheng et al. investigated the mechanical properties of low-slump polypropylene fiber. The paper has a major flaw, since the data presented lacks standard deviation from multiple sample analysis. The citation authors names are mixed up in terms of first and last names format presentation. It is recommended to follow a uniform format. Some parts are unclear as in line 82 where “The above study “refers to the previous study based on literature cited? If so there is no definitive explanation of the motivation and objective behind the current study. Figure legends lack proper description and presentation. The paper needs to be rewritten with significant data and could not be accepted in the present form.
needs to be improved for clarity
Author Response
Please refer to the attachment
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Manuscript ID: applsci-2387520 entitled “Study on the Compressive Stress-Strain Curve and Performance of Low-Slump Polypropylene Fiber Concrete after High Temperature” for journal of “Applied Sciences” has been reviewed.
Attention should be paid to the first review. There are parts that have not been made. It should be checked by the author. (must be revised)
There are few things need to be corrected and included in the manuscript for better understanding of carried research work to the readers.
+1- The “Abstract“ section is too short. It should be enriched with the data obtained from the study.
+2- The novelty of the study should be further explained (in the introduction section).
+3- More references (different studies) should be added to the introduction. (recent studies, 2021-2023). (More information should be given especially about composite materials.)
+4- It should be explained how the materials used in the study were selected.
+5- More information should be given about the materials used in the experiment. A few photos should be attached.
+6- More information on tensile/pressing testing should be given in the " Experimental Overview" section. (Crosshead speed, humidity, ambient temperature, etc.) The dimensions of the samples should be given in 3D (or tech. drawing). How was the crosshead speed determined? (and ? mm/min)
+7- The highest results of tensile and compression tests should be given as bar graphs.
+8- The resolution of Figure 4, 5, 6, 7, 9, 10, 11, and 12 should be increased. (and magnify) (Since the lines are so thin, they are not distinguishable.)
+9- … Figure 8 shows the relationship between the addition of PPF and compressive strength at room temperature. It can be seen from the figure that the addition of PPF 6 greatly enhances the compressive strength of concrete at room temperature. Under the 7 same fiber content, longer fibers lead to lower compressive strength. When the fiber length 8 is the same, an increase in the fiber content initially increases the compressive strength, 9 but then it decreases, which is consistent with the conclusion obtained in reference [39]. To 320 some extent, an appropriate PPF length and dosage can improve the compressive strength 321 of concrete. However, with the increase of fiber length and dosage, the apparent density 322 decreases, the porosity increases, and the strength decreases. Overly long fibers also tend 323 to become entangled inside, and the distribution uniformity decreases, leading to an in-324 crease in porosity and porosity rate. Compared with the compressive strength of normal 325 concrete at room temperature (41.41 MPa), the compressive strength of fiber-reinforced 326 concrete with PPF lengths of 3 mm and 9 mm reached the maximum at a PPF dosage of 1 327 kg/m3, with an increase of 17.15% and 14.85%, respectively. The compressive strength of 328 fiber-reinforced concrete with PPF lengths of 15 mm and 19 mm reached the maximum at 329 a PPF dosage of 0.5 kg/m3, with an increase of 11.25% and 11.2%, respectively. With the 330 increase of dosage, when the PPF dosage reached 4 kg/m3, the compressive strength of 3 PPF concrete with a length of 19 mm was lower than that of normal concrete, and the 332 compressive strength of PPF concrete with a length of 15 mm was only 1.37% higher than 333 that of normal concrete. When the PPF dosage reached 5 kg/m3, the compressive strength 334 of PPF with a length of 9 mm was 2.67% lower than that of normal concrete, while the 3 compressive strength of PPF concrete with a length of 3 mm was 6.52% higher than that 336 of normal concrete. This indicates that the effect of fiber content on compressive strength 337 is greater than that of fiber length when the fiber content reaches a certain degree, which 338 is related to the larger porosity rate caused by high dosage compared to that caused by 339 long fibers [40]. 340 The influence of PPF dosage on the compressive strength of concrete under high tem-341 perature is shown in Figure 9. After being exposed to temperatures of 600°C and 800°C, 342 fewer cracks were observed on the surface of PPF concrete specimens. This is largely due 343 to the fact that after melting and gasifying at high temperatures, PPF leaves many internal 344 voids, increasing internal connectivity, allowing water vapor and heat to escape more eas-345 ily, reducing vapor pressure, providing more free space, and acting as a thermal shock 346 absorber [41], thereby reducing the damage to the micro-structure of concrete caused by 347 temperature. The strength of ordinary concrete … <<< Please explain this part in more detail. >>> (Please write again if possible.)
+10- The sections where the formulas are found should be explained step by step and rewritten. Also, it should be checked.
+11- The “Conclusion” section should be enriched with clear results and comparisons. +12- References should be rearranged according to the journal “Instructions”.
+13- More literature studies should be added to the introduction and other sections (DOIs given below). DOI-1 https://doi.org/10.1515/mt-2020-0024 (info about composites) DOI-2 https://doi.org/10.1515/mt-2021-2038 (different studies) DOI-3 https://doi.org/10.1007/s13369-021-06243-w (info about crosshead speed, humidity, ambient temperature, etc.) DOI-4 https://doi.org/10.26701/ems.989945 (info about crosshead speed, humidity, ambient temperature, etc.)
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**After revision, I would like to review the article again.
Minor editing of English language required.
Author Response
Point 1: The “Abstract“ section is too short. It should be enriched with the data obtained from the study.
Response 1: Added the content of the abstract section.
The results of compressive test show that PPF can significantly improve the mechanical properties of concrete after high temperature when the fiber content is small, and the compressive strength of low collapse polypropylene fiber concrete after high temperature showed a tendency to rise and then fall at the same temperature with the increase of fiber admixture.When the fiber content is 0.5 kg/m3, the compressive strength of 3 mm, 9 mm, 15 mm and 19 mm reaches the maximum, which is 9.65%, 11.33%, 7.90% and 2.87% higher than that of ordinary concrete, respectively, with the increase of fiber length, the effect of PPF on the compressive strength of concrete is not obvious. PPF at high admixture further increases the pore and air content in the concrete, which decreases the compactness of the concrete, thus leading to a decrease in the compressive strength of the concrete. When the temperature is 800°C and the fiber admixture is 5.0kg/m3, the compressive strength of PPF concrete with different lengths is reduced by 17.83%, 17.27%, 22.59% and 23.92% respectively compared to normal concrete.In addition, according to the results, the optimal combinations of strength at room temperature and after high temperature were 3 mm fiber length and 1.0 kg/m3 dosing and 9 mm fiber length and 0.5 kg/m3 dosing, respectively, which increased the compressive and tensile strengths by 17.15% and 25.72% at room temperature and at least 6% and 20% after high temperature compared to the concrete without fiber dosing.
Point 2: The novelty of the study should be further explained (in the introduction section).
Response 2: The innovative nature of the study is further elaborated in the introduction section.
The modified content is as follows：In summary, research on the effect of high temperature on PPF concrete mainly focuses on the inhibitory effect of PPF on the bursting phenomenon of different types of concrete (including ultra-high performance concrete (UHPC), Self-compacting concrete (SCC), lightweight concrete, etc.), the performance of PPF applied in actual structural components (such as reinforced concrete beam-slab structures, concrete shield tunnel lining structures, etc.), and the mixed mechanical properties of PPF with other fibers. No studies have been found on the three-factor coupling effect of PPF length, PPF dosage, and temperature on multiple samples. This study conducts mechanical research on PPF concrete with eight dosages and four lengths after high temperature, which can more fully understand the working mechanism of PPF in concrete and fill the gap in related research. Regarding research on stress-strain curves of PPF concrete, studies related to PPF concrete subjected to high temperatures mainly demonstrate the characteristic changes in compressive strength and modulus of elasticity of PPF concrete after exposure to high temperatures. However, experimental data show a large dispersion, and the conclusions of related studies differ significantly, indicating a need for further research [38-41]. On actual construction sites, the slump test is a simpler and more direct performance indicator that can quickly provide a general understanding of the concrete mix. Modern concrete buildings are typically constructed using concrete batching plants for batching, concrete mixers for mixing and transportation, and concrete pumps for pouring to the specified location. This also re-quires testing for the concrete's slump. Studies have shown [42] that the degree of slump has a correlation with the mechanical properties of concrete. The determination of the slump degree is measured according to the relevant provisions of the specification GB/T50080-2002 Standard for Test Methods for the Properties of Ordinary Concrete Mixes [43]. According to the provisions of GB50164-92 "Concrete Quality Control Standard" [44], the slump degree grade is divided into four levels, and low-slump concrete refers to concrete with a slump degree range of 10 mm-40 mm. In actual production practice, concrete segregation and segregation should be avoided as much as possible, and the working properties of low-slump concrete happen to meet the requirements. As the stability and integrity of low-slump concrete are better than normal concrete, higher strength can still be obtained with reduced cement dosage [45]. In environments such as pavement construction and bridge construction, low-slump concrete is used in large quantities due to its advantages [46]. Most of the concrete studied in the literature has a slump greater than 50 mm and is generally concentrated above 100 mm, resulting in few studies on low-slump concrete and almost a void in the study of low-slump PPF concrete. In actual manufacturing, transportation, and pumping of low-slump concrete, adequate vibration and the addition of fibers are needed to ensure that the low-slump concrete meets the commonly used acceptance standards for slump concrete. With increasing attention to fire hazards, the mechanical properties of low-slump PPF concrete after high temperature need to be urgently addressed, and the principal structure relationship of low-slump PPF concrete after high temperature has more value for practical engineering applications and theoretical analysis. The strength test of low-slump polypropylene fiber (PPF) concrete at room temperature and after high temperature with different admixtures and lengths was carried out, and the physical and mechanical properties of PPF concrete were compared and analyzed based on the test results. Finally, the principal structure relationship of low-slump PPF concrete after high temperature was concluded based on the test data.
Point 3: More references (different studies) should be added to the introduction. (recent studies, 2021-2023). (More information should be given especially about composite materials.)
Response 3: More recent references to different types of research have been added to the introduction as follows :
Roy et al. [21] discovered that adding polypropylene fibers can significantly reduce the loss of tensile strength in high-strength concrete, and the mass loss rate is proportional to the fiber content. Chen et al.'s research [22] showed that adding polypropylene and steel fibers can improve not only the bonding performance but also the mechanical properties of an ultra-high-performance concrete (UHPC) repaired cementitious composite system at high temperatures. It can also inhibit the high-temperature bursting and spalling of concrete. Meanwhile, Qin et al.'s results [23] demonstrated that adding a 0.2% volume fraction of polypropylene fibers to beam specimens of ultra-high-performance concrete cured at room temperature can effectively reduce the bursting of the beam surface under fire conditions. Zhang and Tan [24] suggested that the aspect ratio of polypropylene fibers is the critical factor that affects the spalling effect of UHPC concrete. Thinner and longer polypropylene fibers are better at preventing spalling. When the fiber content is below 3 kg/m3, UHPC can be prevented from peeling, and the critical value of the aspect ratio of peeling is not less than 300. Self-compacting concrete (SCC) is a current research hotspot. Ning et al.'s research [25] showed that a small amount of polypropylene fiber can effectively inhibit the explosive spalling of self-compacting concrete. Moosaei et al. [26] found that compared with steel and glass fibers, polypropylene fibers can more effectively reduce the slump of concrete, and the mechanical properties of polypropylene fiber concrete with a volume fraction of 0.5% are optimal after being exposed to 800°C. Xu et al. [27] found that when the mixing ratio of polypropylene and steel fibers is 1:0.2, the dynamic splitting performance and tensile toughness of concrete at high temperatures can be significantly improved. Kencanawati et al. [28] showed that when the amount of added polypropylene fiber does not exceed 2.5 kg/m3, the alkaline components in the concrete decrease with the increase in temperature, and the degree of decrease increases with the increase in polypropylene fiber amount. Zheng [29] pointed out that when the addition of polypropylene fiber is 0.25% volume fraction, the impact strength of foamed concrete increases by 300% compared to the blank control group. To reduce the bursting and peeling of concrete shield tunnel segments under fire and improve durability, Zhang et al. [30-35] investigated the fire performance of concrete shield tunnel segments with pol-ypropylene fibers and their mixtures. Krishna et al. [36] observed that the hybrid mixture of polypropylene and micro steel fibers exhibited good impact resistance at high temperatures. Meena and Ramana [37] stated that adding 0.5% PPF significantly improved various mechanical properties of concrete at high temperatures.
Point 4: It should be explained how the materials used in the study were selected.
Response 4: Added information about the materials used in the study.
Point 5: More information should be given about the materials used in the experiment. A few photos should be attached.
Response 5: More information about the materials used in the experiments has been attached with more photos.
Point 6: More information on tensile/pressing testing should be given in the " Experimental Overview" section. (Crosshead speed, humidity, ambient temperature, etc.) The dimensions of the samples should be given in 3D (or tech. drawing). How was the crosshead speed determined? (and ? mm/min)
Response 6: More information about the tensile/stamping tests is provided in the "Experimental Overview" section. The dimensions of the samples are also given as 3D drawings.
The test should be performed at an air temperature of 28±3°C and a humidity of 78±5%. Before the strength test, the central axis of the concrete test piece should be aligned with the compressive central axis of the press, and the upper surface of the concrete test block should be parallel to the upper pressing plane of the press to prevent eccentric damage. The compressive test's loading rate is set at 0.5 MPa/s, and the splitting tensile test is set to 0.05 MPa/s, in accordance with the relevant provisions of the Standard for Test Methods of Properties of Plain Concrete Mixes (GB/T 50080-2002) [43] and the Standard for Test Methods of Mechanical Properties of Plain Concrete (GB/T 50081-2002) [47], with reference to the methods used in Zhao's test [48]. To obtain more precise test results and control test error, a force sensor is included between the test block and the indenter plane of the test machine in the uniaxial compressive test, as shown in Figure 7.
Point 7: The highest results of tensile and compression tests should be given as bar graphs.
Response 7: The highest results of the tensile and compression tests have been given in the form of bar graphs.
Point 8: The resolution of Figure 4, 5, 6, 7, 9, 10, 11, and 12 should be increased. (and magnify) (Since the lines are so thin, they are not distinguishable.)
Response 8: Increased the resolution of all figures in the text and enlarged them to the appropriate size.
Point 9: Figure 8 shows the relationship between the addition of PPF and compressive strength at room temperature. It can be seen from the figure that the addition of PPF 6 greatly enhances the compressive strength of concrete at room temperature. Under the 7 same fiber content, longer fibers lead to lower compressive strength. When the fiber length 8 is the same, an increase in the fiber content initially increases the compressive strength, 9 but then it decreases, which is consistent with the conclusion obtained in …<<< Please explain this part in more detail. >>> (Please write again if possible.)
Response 9: A more detailed explanation of “Test Results and Analysis of Compressive Strength”was provided.
Point 10: The sections where the formulas are found should be explained step by step and rewritten. Also, it should be checked.
Response 10: The derivation of the final stress-strain equation is further explained and rewritten.
Nonlinear curve fitting was achieved by the fitting function in OriginPro software developed by OriginLab. The fitted α and β values could be obtained by the custom function, and the fitted R2 values were both close to 1, indicating that the curve proposed by Zhenhai Guo [69] could simulate the stress-strain curve of low slump PPF concrete with different lengths and admixtures very well. To make the equations more concise, the α and β values derived from fitting at different temperatures were used for secondary fitting to obtain the α, β, and temperature-dependent parametric equations. The R2 of the secondary fitting was close to 1, and the product of the R2 values of the two fits was also close to 1. The fitting process cannot be listed in detail due to the space limitation of the article.
Point 11: The “Conclusion” section should be enriched with clear results and comparisons.
Response 11: The "Conclusions" section is enriched with clear results and comparisons.
2.The changes in several parameters of Poisson's ratio, peak strain, peak stress, and modulus of elasticity after high temperature are related to the amount and length of PPF admixture and depend on the change in porosity, release of vapor pressure, and the degree of material deterioration. The PL15C0.5 group had the maximum peak strain after 600°C-800°C, with an increase of 36.66% in the average peak strain com-pared to the fiber-free concrete group, while the minimum was found in the PL3C0.5 group with a 7.17% decrease in average peak strain compared to the fiber-free concrete group. This indicates that longer PPF can have a toughening effect on the concrete at low admixture levels, and the peak strains of PPF concrete all increase to varying degrees after high temperatures. The largest increase in peak stress after 600°C was in the PL9C0.5 group, which was 6.70% higher than the fiber-free concrete group, and the largest decrease was in the PL9C1.5 group, which was 29.41% lower. Overall, the presence of PPF reduces the peak stress of concrete, and the weakening effect brought about by the increase in content is more obvious than the weakening effect produced by the increase in length. A significant weakening of the modulus of elasticity occurred at 400°C. In contrast, the addition of fibers was able to increase the modulus of elasticity of the concrete to some extent, while the degree of reduction and increase depend-ed on the change in porosity due to PPF. The Poisson's ratio increased by 5.8% and decreased by 8.68%, respectively, compared to the fiber-free concrete.
3.The compressive strength and splitting tensile strength of PPF concrete at room temperature were significantly influenced by both the length of fibres and the amount of admixture. The strength exhibited a decreasing trend with increasing PPF length, at a admixture amount of 5.0 kg/m3, the fiber specimens with a length of 19 mm showed the most significant drop, with a compressive strength drop of 8.2% compared to normal concrete.while the effect of admixture on strength demonstrated an initial increase followed by a decrease with higher admixture levels. The maximum compressive strength and splitting tensile strength were achieved at 48.5 MPa and 3.9 MPa, respectively, with a PPF fibre length of 3 mm and an admixture amount of 1.0 kg/m3. This resulted in a strength increase of 17.15% and 25.72% compared to concrete without PPF.
4.The change in compressive and splitting tensile strength of PPF concrete under high temperature follows the same pattern as that at room temperature. Under high temperature conditions, PPF with smaller content can significantly improve the mechanical properties of concrete after high temperature, the compressive strength of low collapse polypropylene fiber concrete after high temperature shows a trend of rising and then falling at the same temperature with the increase of fiber admixture, when the temperature is 800℃ and the fiber admixture is 0.5kg/m3, the compressive strength of 3mm, 9mm, 15mm and 19mm reaches the maximum 9.65%, 11.33%, 7.90% and 2.87% than ordinary concrete, and the effect of PPF on the compressive strength of concrete is not obvious with the increase of fiber length, and the rate of strength reduction in PPF concrete with a large admixture is much higher than that caused by the length of PPF. The maximum compressive and tensile strengths were achieved with a PPF admixture of 0.5 kg/m3 and a length of 9 mm, resulting in a 6.29% increase in compressive strength and a 28.57% increase in tensile strength at 600°C and an 11.33% increase in compressive strength and a 20% increase in tensile strength at 800°C compared to normal concrete.
5.Based on Zhenhai Guo's stress-strain model for concrete, we have developed full-curve principal structural equations for the stress-strain of PPF concrete at both room temperature and high temperature. Our results show that the stress-strain curve model proposed by Zhenhai Per can be used to fit the stress-strain curve of low-slump PPF concrete after exposure to high temperatures. The fitting method takes into ac-count the effects of temperature, PPF admixture, and PPF length on the two important curve parameters α and β. Finally, the normalized fitting produces equations and tables that relate α and β to temperature, PPF content, and PPF length, which can be used to obtain accurate stress-strain curves for PPF concrete at specific temperatures, PPF contents, and lengths.
Point 12: References should be rearranged according to the journal “Instructions”.
Response 12: References have been rearranged according to the journal " Instructions ".
Point 13: More literature studies should be added to the introduction and other sections (DOIs given below).
Response 13: More literature studies have been added to the introduction and other sections to validate their ideas.
Author Response File: Author Response.pdf
Reviewer 2 Report
The revised version of the manuscript does not address proper data representation.
1. Scale bars are not visible in Figure 1
2. Scale bars are missing in Figure 2
3. For Figure 8-11, the authors did not perform additional experiments providing a standard deviation from multiple sample analysis for each combination of PPF concrete
4. The tensile strength bar graphs lack proper data description in the legend. It is not clear what the numbers in each bar refer to.
5. The citation author names are still mixed up in terms of first and last names format presentation. (line 56, reference 14)
Overall the paper cannot be accepted in its present form.
Moderate editing of English language suggested
Author Response
Point 1: Scale bars are not visible in Figure 1
Response 1: The original image size captured using a microscope cannot be scaled to scale.
Point 2: Scale bars are missing in Figure 2
Response 2: At present, only the approximate differences in size of PPF fibers can be displayed, and size display cannot be performed.
Point 3: For Figure 8-11, the authors did not perform additional experiments providing a standard deviation from multiple sample analysis for each combination of PPF concrete
Response 3: Analysis of variance (ANOVA) was performed on the basis of the available data as well as the addition of a portion of the experimental data.
Point 4: The tensile strength bar graphs lack proper data description in the legend. It is not clear what the numbers in each bar refer to.
Response 4:The splitting tensile strength results for plain concrete and PPF concrete at room temperature are shown in Figure 10, and the numbers in the bars represent the maxi-mum splitting tensile strength that the specimens can reach.
Point 5: The citation author names are still mixed up in terms of first and last names format presentation. (line 56, reference 14)
Response 5:The formatting of the authors' names in the citations has been carefully checked and corrected.(Shirsath and Yaragal [14] found that PPF concrete could improve surface cracks of specimens...)
Author Response File: Author Response.pdf
Round 3
Reviewer 1 Report
Manuscript ID: applsci-2387520 entitled “Study on the Compressive Stress-Strain Curve and Performance of Low-Slump Polypropylene Fiber Concrete after High Temperature” for journal of “Applied Sciences” has been reviewed.
-Abstract clearly presents objects methods and results.
“This study aims to investigate the effect of high temperature on the mechanical properties of low-slump polypropylene fiber (PPF) concrete, and tests the tensile and compressive properties of 204 groups of low-slump PPF concrete with eight different dosages and four different lengths at normal temperature and after high temperature. The experimental results show that the addition of polypropylene fibers increases the Poisson's ratio, peak strain, peak stress, and elastic modulus of low-slump concrete at normal temperature. The mechanical properties of low-slump PPF concrete after high temperature are related to the dosage and length of fibers, depending on the changes in porosity, release of vapor pressure, and degree of deterioration of materials. In addition, the tensile and compressive strengths of low-slump PPF concrete at normal temperature and after high temperature first increase and then decrease with the increase of fiber dosage, and decrease with the increase of fiber length. The optimal combinations are fiber length of 3 mm and dosage of 1.0 kg/m3, and fiber length of 9 mm and dosage of 0.5 kg/m3, respectively. Moreover, the stress-strain constitutive equations of PPF concrete at normal temperature and after high temperature were established, which can be used for finite element simulation and related mechanical analysis of PPF after high temperature.”
-Scientific methods are adequately used.
-Terminology is adequate.
-Results are clearly presented.
-Conclusions are logically derived from the data presented.
“1. In general, the addition of PPF enhanced the Poisson's ratio, peak strain, peak 612 stress, and elastic modulus of concrete at room temperature. In comparison to regular 613 low-slump concrete, the most considerable enhancement in Poisson's ratio was observed 614 in the PL3C0.5 group, whereas the most substantial improvements in peak strain, peak 615 stress, and elastic modulus were seen in the PL3C1.5 group, with enhancements of 11.13%, 616 15.6%, 19.6%, and 21.65%, respectively. 617 2.The changes in several parameters of Poisson's ratio, peak strain, peak stress, and 618 modulus of elasticity after high temperature are related to the amount and length of PPF 619 admixture and depend on the change in porosity, release of vapor pressure, and the de-620 gree of material deterioration. The PL15C0.5 group had the maximum peak strain after 621 600°C-800°C, with an increase of 36.66% in the average peak strain compared to the fiber-622 free concrete group, while the minimum was found in the PL3C0.5 group with a 7.17% 623 decrease in average peak strain compared to the fiber-free concrete group. This indicates 624 that longer PPF can have a toughening effect on the concrete at low admixture levels, and 625 the peak strains of PPF concrete all increase to varying degrees after high temperatures. 626 The largest increase in peak stress after 600°C was in the PL9C0.5 group, which was 6.70% 627 higher than the fiber-free concrete group, and the largest decrease was in the PL9C1.5 628 group, which was 29.41% lower. Overall, the presence of PPF reduces the peak stress of 629 concrete, and the weakening effect brought about by the increase in content is more obvi-630 ous than the weakening effect produced by the increase in length. A significant weakening 631 of the modulus of elasticity occurred at 400°C. In contrast, the addition of fibers was able 632 to increase the modulus of elasticity of the concrete to some extent, while the degree of 633 reduction and increase depended on the change in porosity due to PPF. The Poisson's ratio increased by 5.8% and decreased by 8.68%, respectively, compared to the fiber-free con-635 crete. 636 3.The compressive strength and splitting tensile strength of PPF concrete at room 637 temperature were significantly influenced by both the length of fibres and the amount of 638 admixture. The strength exhibited a decreasing trend with increasing PPF length, at a ad-639 mixture amount of 5.0 kg/m3, the fiber specimens with a length of 19 mm showed the most 640 significant drop, with a compressive strength drop of 8.2% compared to normal con-641 crete.while the effect of admixture on strength demonstrated an initial increase followed 642 by a decrease with higher admixture levels. The maximum compressive strength and 643 splitting tensile strength were achieved at 48.5 MPa and 3.9 MPa, respectively, with a PPF 644 fibre length of 3 mm and an admixture amount of 1.0 kg/m3. This resulted in a strength 645 increase of 17.15% and 25.72% compared to concrete without PPF. 646 4.The change in compressive and splitting tensile strength of PPF concrete under high 647 temperature follows the same pattern as that at room temperature. Under high tempera-648 ture conditions, PPF with smaller content can significantly improve the mechanical prop-649 erties of concrete after high temperature, the compressive strength of low collapse poly-650 propylene fiber concrete after high temperature shows a trend of rising and then falling 651 at the same temperature with the increase of fiber admixture, when the temperature is 800652 ℃ and the fiber admixture is 0.5kg/m3, the compressive strength of 3mm, 9mm, 15mm 653 and 19mm reaches the maximum 9.65%, 11.33%, 7.90% and 2.87% than ordinary concrete, 654 and the effect of PPF on the compressive strength of concrete is not obvious with the in-655 crease of fiber length, and the rate of strength reduction in PPF concrete with a large ad-656 mixture is much higher than that caused by the length of PPF. The maximum compressive 657 and tensile strengths were achieved with a PPF admixture of 0.5 kg/m3 and a length of 9 658 mm, resulting in a 6.29% increase in compressive strength and a 28.57% increase in tensile 659 strength at 600°C and an 11.33% increase in compressive strength and a 20% increase in 660 tensile strength at 800°C compared to normal concrete. 661 5.Based on Zhenhai Guo's stress-strain model for concrete, we have developed full-662 curve principal structural equations for the stress-strain of PPF concrete at both room tem-663 perature and high temperature. Our results show that the stress-strain curve model pro-664 posed by Zhenhai Per can be used to fit the stress-strain curve of low-slump PPF concrete 665 after exposure to high temperatures. The fitting method takes into account the effects of 666 temperature, PPF admixture, and PPF length on the two important curve parameters α 667 and β. Finally, the normalized fitting produces equations and tables that relate α and β to 668 temperature, PPF content, and PPF length, which can be used to obtain accurate stress-669 strain curves for PPF concrete at specific temperatures, PPF contents, and lengths.”
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The authors have revised the manuscript carefully and the revised version could be published in the journal.
Moderate editing of English language required
Reviewer 2 Report
The authors addressed the raised concerns. Manuscript can be accepted.
