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applied sciences Article Eects of Parameters of Air-Avid Structure on the Salt-Frost Durability of Hardened Concrete 1 , 2 1 , 2 2 3 Hui Zhang , Peiwei Gao *, Zhixiang Zhang , Youqiang Pan and Weiguang Zhang College of Civilaviation, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China; zh@sinoroad.com Jiangsu SinoRoad Engineering Research Institute, Nanjing 211806, China; zzx@sinoroad.com (Z.Z.); pyq@sinoroad.com (Y.P.) School of Transportation, Southeast University, Nanjing 211189, China; wgzhang@seu.edu.cn * Correspondence: gpw1963@163.com Received: 25 November 2019; Accepted: 9 January 2020; Published: 16 January 2020 Abstract: Through laboratory testing, this research studied the connection between air-void structures of hardened concrete and fresh concrete and discussed the eects of the air-void structure on the salt-frost durability of the concrete. The results demonstrate that, in comparison with fresh concrete, the air-void spacing factor shows a close correlation with hardened concrete air-content and decreases in the form of a power function as the air-content increases. When the fresh concrete air-content is more than 6% and the hardened concrete air-void spacing factor is less than 0.18 mm, the influence of parameters of air-void structure on the salt-frost resistance of the concrete reduces. The air-void spacing factor more significantly aects the salt-frost resistance of the concrete compared with air content and the correlation reaches 0.93. Therefore, air-content and air-void spacing factor are recommended for dual control. Keywords: air-content; air-void structure; air-void spacing factor; salt-frost scaling 1. Introduction Deicing salt inducing salt-frost damage to road and bridge concrete in cold regions has been an important adverse factor for the durability of concrete structures. Earlier studies have pointed out that micro-air-void structures in concrete exert important eects on durability [1]. The osmotic hypothesis [2] proposed by Powers and Helmuth in 1953 further shows confirmation of how an air-void structure aects the frost resistance durability of concrete. Air entrainment has been proved to be an eective measure for improving the frost resistance and the salt-frost resistance of concrete. After air entrainment in the concrete, air voids are able to eectively buer diverse stress generated in cement mortars, thus taking on the function as a buer airbag [3]. The research of G. Fagerlund [4] shows that too high air content causes a less stable air-void structure and forms continuous air voids, raising the absorption amount of water during freezing and thawing, which is disadvantage to frost resistance. At present, air-void testing technology [5] has been gradually developed from a single index (i.e., air content of fresh concrete in the early stages into multi-index, such as air-void spacing factor and air-void specific surface area of hardened concrete currently). This also leads to in-depth researches on the salt-frost resistance of concrete. A number of studies [6–8] have found that the necessary condition to protect concrete from frost damage is not the air content, but, rather, a good air-void structure in hardened cement pastes, especially the air-void spacing factor. It is generally believed that concrete has good frost resistance when the air-void spacing factor is less than 0.25 mm. Previous studies [9,10] also reveal that, except for air-void structures, surface conditions also have great eects on the salt-frost resistance of concrete. Even if the air-void spacing factor of the concrete is smaller than Appl. Sci. 2020, 10, 632; doi:10.3390/app10020632 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 632 2 of 9 0.25 mm, salt-frost durability can be dramatically reduced if the surface performance of the concrete is attenuated in the process of plastering and usage. Furthermore, salt-frost durability of concrete under coupling eects in the load environment is dierent from that in a single environment [11]. This study mainly analyzed the air-void structure in concrete by using an air-void analyzer (AVA) and a tester for parameters of the air-void structure in hardened concrete, and discussed the influence laws of air content and parameters of the air-void structure on the salt-frost durability of concrete. In addition, in view of the salt-frost durability of concrete, control indexes (i.e., appropriate air content and air-void spacing factor) were put forward. 2. Materials and Test 2.1. Test Materials The P O 42.5 ordinary Portland cement (Table 1) and well-graded medium sand with fineness modulus of 2.67 were used. The syenite was used as coarse aggregate (Table 2) and the two graded aggregates with particle sizes of 5~20 mm and 20~40 mm were adopted. The ratio was 3:7. Moreover, naphthalene-based superplasticizer and MicroAir aqueous surfactant solution-type air entraining agent were utilized. Mixing water was tap water. Table 1. Chemical composition of cement/%. SiO CaO MgO Fe O Al O K O Na O SO Alkali Content 2 2 3 2 3 2 2 3 22.71 66.10 1.90 2.85 4.57 0.68 0.15 1.37 0.50 Table 2. Test results of limestone aggregate. Bulk Voidage/% Apparent 3 Saturated Surface Dry Grain Density/(kg/m ) Density/(kg/m ) Water Absorption/% Size/mm Loose Dense Loose Dense 5~40 2670 51.68 41.2 1290 1570 1.27 2.2. Mixing Ratio of the Concrete The ratio of cement, water, sand and pebble sand, pebble and water was 447:170:674:1,099,674:1099:170. Air entraining agent was added according to mass percentage of cement content and its amount of mixing was adjusted in accordance with air content of fresh concrete. Moreover, the slump of the concrete was controlled in the range of 30~50 mm. 2.3. Test Equipment and Methods An AVA produced by the Sanyo Company in Japan was used to test air content of fresh concrete, as shown in Figure 1. The RapidAir 457 image analyzer for the air-void structure in hardened concrete, from the Germann Instruments Company in the Kingdom of Denmark, was utilized to test the air-void spacing factor, as shown in Figure 2. Furthermore, salt-frost scaling resistance of the concrete was tested by using the Collider Detector at Fermilab (CDF) experimental machine, as shown in Figure 3. Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 9 Figure 1. Air content analyzer of fresh concrete. Appl. Sci. 2020, 10, 632 3 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 9 Figure 2. Air-void analyzer of hardened concrete. Figure Figure 1. 1. Air Air content analyzer of fresh content analyzer of fresh concr concrete. ete. Figure 1. Air content analyzer of fresh concrete. Salt-frost test was conducted in light of the CDF test method recommended by RILEM (International Union of Laboratories and Experts in Construction Materials, Systems and Structures) [12] and concrete specimens were molded in accordance with the designed ratio of mixing. After inserting a PTEF (polytetrafluoroethylene) partition into the sample, molding was performed through vibration. There were five molded specimens with the dimension of 150 × 150 × 150 mm in each group. After curing for 28 d in a standard curing room, the specimens were split into half and the surfaces (Figure 4) formed with the partition were used in the salt-frost test, with the dimension of 150 × 150 mm. After drying the specimen outside, butyl rubber with aluminum foil was used to seal the specimens for 28 d. After that, they were immediately immersed into salt solution of 3% concentration in a test chamber in the depth of 5 mm. Freezing an d thawing test was carried out immediately afterwards. Figure Figure 2. 2. Air-v Air-void oid analyzer analyzer of of hardened hardened concr concrete. ete. Figure 2. Air-void analyzer of hardened concrete. Salt-frost test was conducted in light of the CDF test method recommended by RILEM Salt-frost test was conducted in light of the CDF test method recommended by RILEM (International Union of Laboratories and Experts in Construction Materials, Systems and Structures) (International Union of Laboratories and Experts in Construction Materials, Systems and Structures) [12] and concrete specimens were molded in accordance with the designed ratio of mixing. After [12] and concrete specimens were molded in accordance with the designed ratio of mixing. After inserting a PTEF (polytetrafluoroethylene) partition into the sample, molding was performed inserting a PTEF (polytetrafluoroethylene) partition into the sample, molding was performed through vibration. There were five molded specimens with the dimension of 150 × 150 × 150 mm in through vibration. There were five molded specimens with the dimension of 150 × 150 × 150 mm in each group. After curing for 28 d in a standard curing room, the specimens were split into half and each group. After curing for 28 d in a standard curing room, the specimens were split into half and the surfaces (Figure 4) formed with the partition were used in the salt-frost test, with the dimension the surfaces (Figure 4) formed with the partition were used in the salt-frost test, with the dimension of 150 × 150 mm. After drying the specimen outside, butyl rubber with aluminum foil was used to of 150 × 150 mm. After drying the specimen outside, butyl rubber with aluminum foil was used to seal the specimens for 28 d. After that, they were immediately immersed into salt solution of 3% seal the specimens for 28 d. After that, they were immediately immersed into salt solution of 3% concentration in a test chamber in the depth of 5 mm. Freezing and thawing test was carried out concentration in a test chamber in the depth of 5 mm. Freezing and thawing test was carried out Figure 3. Collider Detector at Fermilab (CDF) test machine. Figure 3. Collider Detector at Fermilab (CDF) test machine. immediately afterwards. immediately afterwards. Salt-frost test was conducted in light of the CDF test method recommended by RILEM (International Union of Laboratories and Experts in Construction Materials, Systems and Structures) [12] and concrete specimens were molded in accordance with the designed ratio of mixing. After inserting a PTEF (polytetrafluoroethylene) partition into the sample, molding was performed through vibration. There were five molded specimens with the dimension of 150 150 150 mm in each group. After curing for 28 d in a standard curing room, the specimens were split into half and the surfaces (Figure 4) formed with the partition were used in the salt-frost test, with the dimension of 150 150 mm. After drying the specimen outside, butyl rubber with aluminum foil was used to seal the specimens for 28 d. After that, they were immediately immersed into salt solution of 3% concentration in a test chamber in the depth of 5 mm. Freezing and thawing test was carried out immediately afterwards. Figure 3. Collider Detector at Fermilab (CDF) test machine. Figure 3. Collider Detector at Fermilab (CDF) test machine. Appl. Sci. 2020, 10, 632 4 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 9 Figure 4. Isolation face. Figure Figure 4. 4. Is Isolation olation face face. . 3. Result Discussion and Analysis 3. Result Discussion and Analysis 3. Result Discussion and Analysis 3.1. The Relationship between Air-Void Parameters of Fresh Concrete and Hardened Concrete 3.1. The Relationship between Air-Void Parameters of Fresh Concrete and Hardened Concrete 3.1. The Relationship between Air-Void Parameters of Fresh Concrete and Hardened Concrete Firstly, the relationship between air contents of fresh concrete and hardened concrete Firstly, the relationship between air contents of fresh concrete and hardened concrete determined Firstly, the relationship between air contents of fresh concrete and hardened concrete determined by the RapidAir 457 image analyzer was studied and the results are shown in Figure 5. by the RapidAir 457 image analyzer was studied and the results are shown in Figure 5. Air content of determined by the RapidAir 457 image analyzer was studied and the results are shown in Figure 5. Air content of hardened concrete was linearly correlated with that of fresh concrete [13] and the hardened concrete was linearly correlated with that of fresh concrete [13] and the correlation coecient Air content of hardened concrete was linearly correlated with that of fresh concrete [13] and the correlation coefficient reaches 0.88. In addition, the air content of hardened concrete has an average reaches 0.88. In addition, the air content of hardened concrete has an average loss of about 16% correlation coefficient reaches 0.88. In addition, the air content of hardened concrete has an average loss of about 16% compared with that of fresh concrete and the maximum loss rate is about 30%. compared with that of fresh concrete and the maximum loss rate is about 30%. loss of about 16% compared with that of fresh concrete and the maximum loss rate is about 30%. Equation y=a+bx 10 Equation y=a+bx Adj. R-Square 0.87933 Adj. R-Square 0.87933 a 0.2103 a 0.2103 b 0.84103 b 0.84103 02 4 6 8 10 12 02 4 6 8 10 12 Air content of fresh concrete / % Air content of fresh concrete / % Figure 5. The relationship between the air contents of fresh and hardened concrete. Figure 5. The relationship between the air contents of fresh and hardened concrete. Figure 5. The relationship between the air contents of fresh and hardened concrete. The determined air content of fresh concrete and the air-void spacing factor of hardened concrete, The determined air content of fresh concrete and the air-void spacing factor of hardened The determined air content of fresh concrete and the air-void spacing factor of hardened tested by the RapidAir 457 image analyzer, were analyzed and the results are presented in Figure 6. concrete, tested by the RapidAir 457 image analyzer, were analyzed and the results are presented in concrete, tested by the RapidAir 457 image analyzer, were analyzed and the results are presented in Figure 7 demonstrates the relationship between air content and the air-void spacing factor of hardened Figure 6. Figure 7 demonstrates the relationship between air content and the air-void spacing factor Figure 6. Figure 7 demonstrates the relationship between air content and the air-void spacing factor concrete, tested by the RapidAir 457 image analyzer. The results in the two figures show that the air-void of hardened concrete, tested by the RapidAir 457 image analyzer. The results in the two figures show of hardened concrete, tested by the RapidAir 457 image analyzer. The results in the two figures show spacing factor reduces in the form of a power function with the rise of air content. When air content that the air-void spacing factor reduces in the form of a power function with the rise of air content. that the air-void spacing factor reduces in the form of a power function with the rise of air content. reaches 12% from 2.6%, the air-void spacing factor decreases from 0.280 to 0.120 mm. The correlation When air content reaches 12% from 2.6%, the air-void spacing factor decreases from 0.280 to 0.120 When air content reaches 12% from 2.6%, the air-void spacing factor decreases from 0.280 to 0.120 coecient between air content and the air-void spacing factor of fresh concrete reaches 0.59, while that mm. The correlation coefficient between air content and the air-void spacing factor of fresh concrete mm. The correlation coefficient between air content and the air-void spacing factor of fresh concrete of hardened concrete is 0.67. As air content rises, the decreasing amplitude of the air-void spacing reaches 0.59, while that of hardened concrete is 0.67. As air content rises, the decreasing amplitude of reaches 0.59, while that of hardened concrete is 0.67. As air content rises, the decreasing amplitude of factor gradually slows down. the air-void spacing factor gradually slows down. the air-void spacing factor gradually slows down. Air content of hardened concrete / % Air content of hardened concrete / % Appl. Appl. Sci. Sci. 2020 2020, , 10 10, x FO , 632 R PEER REVIEW 5 of 5 of 9 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 9 0.30 0.30 0.25 0.25 0.20 0.20 0.15 0.15 0.10 Equation y=ax 0.10 Equation y=ax Adj. R-Square 0.59334 0.05 Adj. R-Square 0.59334 a 0.36715 0.05 a 0.36715 b -0.41677 b -0.41677 0.00 0.00 02 468 10 12 02 468 10 12 Air content of fresh concrete / % Air content of fresh concrete / % Figure 6. The relationship between air content and the air-void spacing factor of fresh concrete. Figure 6. The relationship between air content and the air-void spacing factor of fresh concrete. Figure 6. The relationship between air content and the air-void spacing factor of fresh concrete. 0.30 0.30 0.25 0.25 0.20 0.20 0.15 0.15 0.10 b Equation y=ax 0.10 b Equation y=ax Adj. R-Square 0.6675 Adj. R-Square 0.6675 0.05 a 0.36292 0.05 a 0.36292 b -0.44638 b -0.44638 0.00 0.0002 468 10 12 02 468 10 12 Air content of fresh concrete / % Air content of fresh concrete / % Figure 7. The relationship between air-content and the air-void spacing factor of hardened concrete. Figure 7. The relationship between air-content and the air-void spacing factor of hardened concrete. Figure 7. The relationship between air-content and the air-void spacing factor of hardened concrete. 3.2. The Relationship between Air-Void Parameters and Salt-Frost Resistance of the Concrete 3.2. The Relationship between Air-Void Parameters and Salt-Frost Resistance of the Concrete 3.2. The Relationship between Air-Void Parameters and Salt-Frost Resistance of the Concrete 3.2.1. Air Content of Fresh Concrete 3.2.1. Air Content of Fresh Concrete 3.2.1. Air Content of Fresh Concrete Figure 8 demonstrates the changes of the scaled mass of fresh concrete, with dierent air contents, Figure 8 demonstrates the changes of the scaled mass of fresh concrete, with different air with the number of salt-freezing cycles [14]. It can be seen that an increase in the scaled mass of Figure 8 demonstrates the changes of the scaled mass of fresh concrete, with different air contents, with the number of salt-freezing cycles [14]. It can be seen that an increase in the scaled the benchmark concrete specimens, without mixing air entraining agent, accelerates after reaching contents, with the number of salt-freezing cycles [14]. It can be seen that an increase in the scaled mass of the benchmark concrete specimens, without mixing air entraining agent, accelerates after 14 cycles, and reaches about 1.3 kg/m after 28 cycles. It is followed by the increase trend of the scaled mass of the benchmark concrete specimens, without mixing air entraining agent, accelerates after reaching 14 cycles, and reaches about 1.3 kg/m after 28 cycles. It is followed by the increase trend of mass of the fresh concrete specimens with air content of 3.9%, which reaches about 0.7 kg/m after reaching 14 cycles, and reaches about 1.3 kg/m after 28 cycles. It is followed by the increase trend of the scaled mass of the fresh concrete specimens with air content of 3.9%, which reaches about 0.7 28 cycles. Moreover, the scaled mass of the fresh concrete specimens with air content of 8% shows a the scaled mass of the fresh concrete specimens with air content of 3.9%, which reaches about 0.7 kg/m after 28 cycles. Moreover, the scaled mass of the fresh concrete specimens with air content of small change trend and is only about 0.2 kg/m after 28 cycles. kg/m after 28 cycles. Moreover, the scaled mass of the fresh concrete specimens with air content of 8% shows a small change trend and is only about 0.2 kg/m after 28 cycles. 8% shows a small change trend and is only about 0.2 kg/m after 28 cycles. Air Air -voi -voi d s d s pacing pacing fact fact oror s /s / mm mm Air Air -voi -voi d s d s pacing pacing fact fact oror s /s / mm mm Appl. Sci. 2020, 10, 632 6 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 9 2.0 2.0 Benchmark concrete Benchmark concrete 1.8 1.8 Fresh concrete with air content of 3.9% Fresh concrete with air content of 3.9% 1.6 1.6 Fresh concrete with air content of 8% Fresh concrete with air content of 8% 1.4 1.4 1.2 1.2 1.0 1.0 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0.0 0.0 0 7 14 21 28 0 7 14 21 28 Salt-freezing cycles Salt-freezing cycles Figure 8. The change of scaled mass of the concrete with different air contents due to salt-frost. Figure 8. The change of scaled mass of the concrete with dierent air contents due to salt-frost. Figure 8. The change of scaled mass of the concrete with different air contents due to salt-frost. By further analyzing data of the salt-frost scaling of the concrete with different air contents after By further analyzing data of the salt-frost scaling of the concrete with dierent air contents after By further analyzing data of the salt-frost scaling of the concrete with different air contents after 28 cycles, as presented in Figure 9, it is found that the scaled mass of the concrete due to salt-frost 28 cycles, as presented in Figure 9, it is found that the scaled mass of the concrete due to salt-frost 28 cycles, as presented in Figure 9, it is found that the scaled mass of the concrete due to salt-frost reduces [15] in the form of a power function with the increase of air content and the correlation reduces [15] in the form of a power function with the increase of air content and the correlation reduces [15] in the form of a power function with the increase of air content and the correlation coefficient is up to 0.78. When the air content of fresh concrete is higher than 6%, the trend of the coecient is up to 0.78. When the air content of fresh concrete is higher than 6%, the trend of the scaled coefficient is up to 0.78. When the air content of fresh concrete is higher than 6%, the trend of the scaled mass of the concrete decreasing with the rise of air content gradually slows down. mass of the concrete decreasing with the rise of air content gradually slows down. scaled mass of the concrete decreasing with the rise of air content gradually slows down. 1.8 1.8 1.6 1.6 Equation y=axb Equation y=ax 1.4 1.4 Adj. R-Square 0.77825 Adj. R-Square 0.77825 1.2 a 2.212 1.2 a 2.212 b -1.1912 b -1.1912 1.0 1.0 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0.0 0.0 02 46 8 10 12 02 46 8 10 12 Air content of fresh concrete / % Air content of fresh concrete / % Figure 9. The relationship between the air content and the scaled mass of fresh concrete due to salt-frost. Figure 9. The relationship between the air content and the scaled mass of fresh concrete due to salt- Figure 9. The relationship between the air content and the scaled mass of fresh concrete due to salt- frost. frost. Figure 10 demonstrates the compressive strength of the concrete linearly declined with the growth of air content in the fresh concrete, with the correlation coecient of 0.76. Moreover, the compressive Figure 10 demonstrates the compressive strength of the concrete linearly declined with the Figure 10 demonstrates the compressive strength of the concrete linearly declined with the strength of the concrete reduced by about 4% every time the air content rose by 1%, as shown growth of air content in the fresh concrete, with the correlation coefficient of 0.76. Moreover, the growth of air content in the fresh concrete, with the correlation coefficient of 0.76. Moreover, the in Figure 10. compressive strength of the concrete reduced by about 4% every time the air content rose by 1%, as compressive strength of the concrete reduced by about 4% every time the air content rose by 1%, as shown in Figure 10. shown in Figure 10. 22 22 Scaled mass / kg/m Salt-frost denudation / kg/m Scaled mass / kg/m Salt-frost denudation / kg/m Appl. Sci. 2020, 10, 632 7 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 9 Equation y=a+bx 30 Equation y=a+bx Adj. R-Square 0.76415 Adj. R-Square 0.76415 a 54.36735 a 54.36735 b -1.86277 b -1.86277 02 4 6 8 10 12 02 4 6 8 10 12 Air content in fresh concrete / % Air content in fresh concrete / % Figure 10. Effect of air content on the compressive strength of hardened concrete. Figure 10. Eect of air content on the compressive strength of hardened concrete. Figure 10. Effect of air content on the compressive strength of hardened concrete. 3.2.2. Air-Void Spacing Factor 3.2.2. Air-Void Spacing Factor 3.2.2. Air-Void Spacing Factor Based on further analysis of the data of the air-void spacing factor of hardened concrete and the Based on further analysis of the data of the air-void spacing factor of hardened concrete and the Based on further analysis of the data of the air-void spacing factor of hardened concrete and the scaled mass of the concrete due to salt-frost, as presented in Figure 11, it is found that the air-void scaled mass of the concrete due to salt-frost, as presented in Figure 11, it is found that the air-void scaled mass of the concrete due to salt-frost, as presented in Figure 11, it is found that the air-void factor shows a good relationship with the salt-frost scaling of the concrete [16], and their correlation factor shows a good relationship with the salt-frost scaling of the concrete [16], and their correlation factor shows a good relationship with the salt-frost scaling of the concrete [16], and their correlation reaches 0.93. With the constant decrease of the air-void spacing factor, the scaled mass of the concrete reaches 0.93. With the constant decrease of the air-void spacing factor, the scaled mass of the concrete reaches 0.93. With the constant decrease of the air-void spacing factor, the scaled mass of the concrete specimens due to salt-frost rapidly reduces. When the air-void spacing factor decreases from 0.4 to specimens due to salt-frost rapidly reduces. When the air-void spacing factor decreases from 0.4 to specimens due to salt-frost rapidly reduces. When the air-void spacing factor decreases from 0.4 to 0.2 mm, the scaled mass reduces from 1.6 to about 0.3 kg/m , by about 80%. When the air-void spacing 0.2 mm, the scaled mass reduces from 1.6 to about 0.3 kg/m , by about 80%. When the air-void spacing 0.2 mm, the scaled mass reduces from 1.6 to about 0.3 kg/m , by about 80%. When the air-void spacing factor is smaller than 0.18 mm, its influence on salt-frost scaling resistance reduces. factor is smaller than 0.18 mm, its influence on salt-frost scaling resistance reduces. factor is smaller than 0.18 mm, its influence on salt-frost scaling resistance reduces. 2.0 2.0 Model y=a+bx+cx 1.8 1.8 Model y=a+bx+cx Adj. R-Square 0.92986 1.6 Adj. R-Square 0.92986 a 0.47225 1.6 a 0.47225 1.4 b -4.69319 1.4 b -4.69319 c 19.34709 1.2 c 19.34709 1.2 1.0 1.0 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0.0 0.0 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 Air-void spacing factors /mm Air-void spacing factors /mm Figure Figure 11. 11. The relationship The relationship b between etween the the air air--void void spac spacing ing factor and factor and the the scale scaled d m mass ass of of hardened hardened Figure 11. The relationship between the air-void spacing factor and the scaled mass of hardened concr concrete ete due due to to salt-fro salt-frost. st. concrete due to salt-frost. Figure 12 presents the relationship between the compressive strength and the air-void spacing Figure 12 presents the relationship between the compressive strength and the air-void spacing Figure 12 presents the relationship between the compressive strength and the air-void spacing factor. The lower the air-void spacing factor was, the denser the air voids and the lower the compressive factor. The lower the air-void spacing factor was, the denser the air voids and the lower the factor. The lower the air-void spacing factor was, the denser the air voids and the lower the strength of concrete, as presented in Figure 9. The correlation coecient between the air-void spacing compressive strength of concrete, as presented in Figure 9. The correlation coefficient between the compressive strength of concrete, as presented in Figure 9. The correlation coefficient between the factor and the compressive strength of the concrete was 0.57. air-void spacing factor and the compressive strength of the concrete was 0.57. air-void spacing factor and the compressive strength of the concrete was 0.57. Scaled mass / kg/m Scaled mass / kg/m Compressive strength / MPa Compressive strength / MPa Appl. Sci. 2020, 10, 632 8 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 9 Equation y=a+bx Adj. R-Square 0.56628 a 30.99461 b 62.21401 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 Air-void spacing factor / mm Figure 12. Figure 12. Effe Eect ct of of the air- the air-void void spa spacing cing facto factor r on th on the e compressive compressive strength of hardened concrete. strength of hardened concrete. 4. Conclusions 4. Conclusions (1) The air content of hardened concrete was linearly correlated with that of fresh concrete and (1) The air content of hardened concrete was linearly correlated with that of fresh concrete and showed an average loss of about 16% in comparison with that of the fresh concrete. showed an average loss of about 16% in comparison with that of the fresh concrete. (2) The air-void spacing factor was closely related to air content. The air-void spacing factor showed (2) The air-void spacing factor was closely related to air content. The air-void spacing factor showed a decreasing trend in the form of a power function with the increase of air content, and was more a decreasing trend in the form of a power function with the increase of air content, and was more closely correlated with the air content of hardened concrete compared with that of fresh concrete. closely correlated with the air content of hardened concrete compared with that of fresh concrete. (3) The compressive strength of concrete linearly changed with increasing air content, with the (3) The compressive strength of concrete linearly changed with increasing air content, with the correlation coecient of 0.76. Moreover, there was a certain linear relationship between the correlation coefficient of 0.76. Moreover, there was a certain linear relationship between the compressive strength of concrete and the air-void spacing factor, showing the correlation coecient compressive strength of concrete and the air-void spacing factor, showing the correlation of 0.57. coefficient of 0.57. (4) In comparison with air content, the salt-frost resistance of the concrete was more closely correlated (4) In comparison with air content, the salt-frost resistance of the concrete was more closely with the air-void spacing factor. When the air-void spacing factor was smaller than 0.18 mm, correlated with the air-void spacing factor. When the air-void spacing factor was smaller than the eect of air-void parameters on the salt-frost resistance of the concrete showed a decreasing 0.18 mm, the effect of air-void parameters on the salt-frost resistance of the concrete showed a trend. If the air content of fresh concrete is adopted to control the quality of concrete, 6% is decreasing trend. If the air content of fresh concrete is adopted to control the quality of concrete, recommended as the standard. 6% is recommended as the standard. Author Contributions: Data curation, H.Z.; Formal analysis, H.Z.; Funding acquisition, P.G.; Project Author Contributions: Data curation, H.Z.; Formal analysis, H.Z.; Funding acquisition, P.G.; Project administration, Z.Z. and Y.P.; Resources, W.Z. All authors have read and agreed to the published version of the administration, Z.Z. and Y.P.; Resources, W.Z. All authors have read and agreed to the published version m ofathe nuscript. manuscript. Funding: This paper is supported by the project supported by the Jiangsu Natural Science Foundation for Youth Funding: This paper is supported by the project supported by the Jiangsu Natural Science Foundation for Youth Fund (Grant No. BK20180113), the Surface Project of Jiangsu Natural Science Foundation (Grant No. BK20181112), Fund (Grant No. BK20180113), the Surface Project of Jiangsu Natural Science Foundation (Grant No. the High-level Talent Project Funding Scheme of Jiangsu (Grant No. XCL-CXTD-007), the Post-Doctoral Fund BK20181112), the High-level Talent Project Funding Scheme of Jiangsu (Grant No. XCL-CXTD-007), the Post- of China (Grant No. 2018M630559, Grant No. 2014M551588), and the Project of Trac Construction in Shanxi Doctoral Fund of China (Grant No. 2018M630559, Grant No. 2014M551588), and the Project of Traffic Province (Grant No. 16-2-08). Construction in Shanxi Province (Grant No. 16-2-08). Conflicts of Interest: The authors declare that they have no conflicts of interest to this work and do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Conflicts of Interest: The authors declare that they have no conflicts of interest to this work and do not have any commercial or associative interest that represents a conflict of interest in connection with the work References submitted. 1. Mehta, P.K.; Monteiro, P.J. Concrete Microstructure, Properties and Materials; Tan, W., Wang, D., Ding, J., Eds.; References Power Press: Beijing, China, 2008. 2. Power, T.C.; Helmuth, R.A. Theory of volume change in hardened Portland cement pastes during freezing. 1. Mehta, P.K.; Monteiro, P.J. Concrete Microstructure, Properties and Materials; Tan, W., Wang, D., Ding, J., Eds.; Proc. Highw. Res. Board. 1949, 32, 285–297. Power Press: Beijing, China, 2008. 3. Du, L.; Folliard, K.J. 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This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Applied Sciences – Multidisciplinary Digital Publishing Institute
Published: Jan 16, 2020
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