Comparative analysis on the anti-wheel impact performance of steel fiber and reticular polypropylene synthetic fiber reinforced airport pavement concrete under elevated temperature aging environment

Construction and Building Materials 192 (2018) 818–835

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Comparative analysis on the anti-wheel impact performance of steel fiber and reticular polypropylene synthetic fiber reinforced airport pavement concrete under elevated temperature aging environment Yue Chen ⇑, Guoping Cen, Yunhua Cui Department of Airfield and Building Engineering, Air Force Engineering University, Xi’an 710038, China

h i g h l i g h t s
The reticular polypropylene synthetic fiber concrete and Steel fiber concrete have been prepared.
The experiments were carried out by the High-temperature aging anti-wheel impact test system.
Some specific damage indexes were selected to evaluate the Anti-wheel impact performance of the pavement concrete.
The mechanism of fiber modification on concrete matrix was studied by the XTH225 thermal imaging industrial CT instrument.
The steel fiber can control the aging and cracking of concrete matrix and the expansion growth of internal pore.

a r t i c l e

i n f o

Article history: Received 11 June 2018 Received in revised form 13 September 2018 Accepted 21 October 2018

Keywords: Reticular polypropylene synthetic fiber Steel fiber Anti-wheel impact Elevated temperature aging Pavement concrete

a b s t r a c t In this paper, the reticular polypropylene synthetic fiber concrete (RPSFC) (the representative of organic fiber concrete) and Steel fiber concrete (SFC) (the representative of inorganic metal fiber concrete) have been prepared. The experiments were carried out by the polyurethane high-temperature aging chamber, the xenon weather resistant test box and the modified wheel type impact test machine. The internal mechanical properties and external damage of concrete under the joint action of high temperature aging and dynamic wheel type impact were compared and analyzed, and the influence degree of fiber type and content is summarized. Some specific damage indexes were selected to evaluate the Anti-wheel impact performance of the pavement concrete, namely the loss of compressive strength, the loss of dynamic modulus, the growth size of internal pore and the surface impact damage. The results showed that adding fiber in the concrete can control the increase of indexes. The indexes of the specimen increase with the increasing test rounds, but decrease with the increasing fiber volume content. Through the comparative analysis of indexes, we found that the effect of steel fiber is more obvious under long-term action. Combined with the internal thermal imaging processing of fiber concrete, it is found that the steel fiber can obviously control the aging and cracking of concrete matrix and the expansion growth of internal pore. Generally speaking, the most effective way to improve the anti-wheel impact performance of concrete under elevated temperature aging environment is to add 1.5% metal fiber (SF). Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction Concrete due to its excellent mechanical performance and ease of construction, more and more applied to practical engineering. Nowadays, the pavement concrete becomes one of the research focuses. As an important carrier for the landing and skating of aircraft, the airport pavement is designed and constructed by concrete. The airport pavement is an open-faced sheet structure that is subjected to various environmental climates and aircraft repetitive loading [1]. In some arid areas, the precipitation is large, the ⇑ Corresponding author. https://doi.org/10.1016/j.conbuildmat.2018.10.175 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

temperature is high, and the sunshine is intense. The concrete on the airport pavement is exposed to high-intensity sunlight for a long time, resulting in high-temperature aging damage, leading to problems such as degradation of mechanical properties and durability, and seriously threatening the safety of the airport [2]. At low latitudes, airport pavements that have been used for a long time have been subjected to wheel impact after high-temperature aging and the damage has been deepened [3]. The increase in the degree of aging will lead to a decrease in the impact resistance of the concrete. After the impact, the internal structure of the concrete will also have a counter effect, aggravating the occurrence of high-temperature aging damage. Therefore, two kinds of

Y. Chen et al. / Construction and Building Materials 192 (2018) 818–835

damage phenomena will produce a comprehensive effect under the joint action, and its damage degree is much greater than that of simple addition [4]. Some typical pavement failure modes are shown in Figs. 1 and 2 as below. How to improve the impact and destruction of airport pavement in this particular environment is one of the hot issues at present, and there is no comprehensive study and specific measures. In recent years, fiber concrete has been widely used in the construction of various practical projects. It has become one of the most effective ways to improve engineering materials by adding various fibers in concrete to enhance its own performance to meet different engineering needs. As a result, the type of fiber added to the concrete, the specific content and the specific properties of the fiber concrete have become a pressing topic in the world. Studies have shown that in addition to the static mechanical properties such as compressive strength and flexural strength of concrete, the dynamic properties of the fiber are greatly improved [5,6]. Gong B, Kim S, et al. proposed that fibers can improve the energy absorption capacity of concrete when impacted by comparative tests and computer program simulations. In the study, several common engineering fibers were compared and analyzed. Combined with the characteristics of the fibers, different fiberimproving impact properties were proposed, including the bridging and supporting effects of the fibers [7,8]. Banthia N et al. performed testing using a home-made uniaxial tensile simple impact tester. The test results show the quantified relationship between the impact resistance of steel fiber reinforced concrete and quantitative relationship between pulse load and fiber content for three types of concrete with ordinary strength, medium strength, and high strength, respectively [9]. Mao L et al. studied the relationship between toughness and strain rate of fiber reinforced concrete and developed a mathematical model of different fiber reinforced concrete beams under impact loading [10]. These studies provide a certain theoretical basis for the application of fiber concrete, but with the constant adoption of new fibers in the project, new fiber materials and appearance patterns are also affecting the performance of concrete at all times. At the same time, the reinforcing effect of fiber on the weather-ability and durability of concrete has also been extensively studied. As a common engineering problem, the high-temperature aging of concrete has become one of the focuses of domestic and foreign scholars. For the various properties of fiber concrete under high-temperature aging conditions, Liu D, Miao C, et al. have systematically studied

Fig. 1. Surface aggregate exposure.

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Fig. 2. Impact abscission failure.

the results. The results show that the steel fibers can inhibit the decline of the dynamic elastic modulus of concrete, and can also change the destructive mode of specimens [11]. Nili M et al. proposed that the fiber has excellent thermal conductivity, and pointed out that the polypropylene fiber can increase the fracture toughness and fracture energy of the concrete when the volume content is 0.12%, thereby resisting the high-temperature aging damage [12]. For airport concrete pavement, Krishnan JM et al. studied the relationship between the penetration and interspace formation of concrete under continuous fatigue damage through a combination of laboratory tests and field tests. We learned that the incorporation of polycarbafil and other fibers can improve the frost resistance and water permeability of concrete and reduce the phenomenon of fracture and shedding [13]. According to AlKaissi Z A et al.’s research on the performance of conventional pavement concrete under high-temperature sunlight, steel fiber reinforced concrete has high heat resistance and reduces road surface swelling and cracking [14]. It can be seen that at present, the research on the high-temperature aging damage and impact resistance of fiber concrete has achieved certain results, but there are still blank areas for research. First of all, the current research on the impact resistance of fiber reinforced concrete is mostly focused on single factors, and there is a big difference between the coupling effects in the actual complex environment. Second, there is a certain difference between airport pavement concrete and pavement concrete. The starting points of the design of the two are different. The airport bears large aircraft loads and is highly affected by the weather, therefore, the strength and durability are higher than that of the road concrete. For the airport pavement concrete, the comparative analysis of the effects of different fibers is almost absent. Thirdly, the impact of aircraft wheels on the airport pavement is different from that of conventional impact methods such as. It is a dynamic wheel impact, with long time and strong friction. Therefore, the existing research results cannot be applied to the performance improvement of airport pavement concrete. In view of the above problems to be solved, the anti-wheel impact performance of organic fiber and inorganic metal fiber reinforced airport pavement concrete under elevated temperature aging environment was comparatively analyzed. Taking airport pavement concrete as the bearing carrier, steel fiber and reticular polypropylene synthetic fibers are selected, which represent inorganic metal fibers and organic synthetic fibers, respectively, blended in different amounts. Samples were all 6 types, 0%, 0.3%, 0.6%, 0.9%, 1.2%, 1.5%. In this study, combined with actual

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engineering practice, the self-designed high-temperature aging concrete wheel impact test system was used to study and analyze the impact resistance of fiber reinforced concrete at airport pavement under high-temperature aging conditions. At the same time, this paper puts forward the evaluation index of anti-wheel impact performance of concrete in high-temperature aging environment, and explores the effect of fiber content and type on the performance of concrete. Through the processing and analysis of the test results, this study summarized the equations for evaluating the indicators. The equations show the relationship between the numerical loss of the shock resistance index and the number of test operations after the impact of high-temperature aging on specimens with different fiber content. In addition, according to the comprehensive analysis of the actual damage conditions and the internal mechanical properties of the concrete damage index, this paper gives the best fiber variety and content of the airport pavement concrete. Finally, combined with the internal thermal imaging of the test piece, the improvement mechanism of the two fibers was compared and analyzed. The conclusion and analysis of this paper provide a theoretical and scientific basis for airport pavement construction and pavement material preparation in high temperature areas, which has great practical significance.

Fig. 3. Reticular polypropylene synthetic fiber.

pavement to perform anti-slip groove processing, and ensure that aggregates were not exposed on surface.

3. Basic properties of test specimens 2. Raw materials and specimen preparation The basic properties of concrete include dynamic elastic modulus, static compression resistance and flexural strength. These indexes reflect the ability of concrete to resist deformation [17]. Therefore, this paper uses the WAW-1000G compression tester and DT-2 digital dynamic elasticity tester to test the main properties of concrete specimens after adding fiber. The main technical features are shown in Tables 4 and 5. Combined with the data in the table, the effect of organic synthetic fibers and inorganic metal fibers on the performance of concrete is shown in Fig. 5. It can be seen from Fig. 5 that the compressive strength, flexural strength and dynamic elastic modulus of inorganic metal fiber reinforced concrete increase with the increase of fiber content, and the rate of increase in early stage is greater, and the rate of late stage decreases. The compressive strength, flexural strength, and dynamic elastic modulus of organic synthetic fiber reinforced concrete increase with the increase of fiber content between 0 and 1.2%, and reach the peak at a content of 1.2%. After continuing to add the fiber, all three performance indicators have been reduced. This is mainly due to the phenomenon of inter-fiber twisting caused by the excessive density of organic fibers, which is not the case for metal fibers. At this time, the compactness of the concrete is reduced, cracks are generated inside, and the local strength is reduced, resulting in overall reduction of the overall performance. Longitudinal comparative analysis shows that when the fiber content is the same, the metal fiber increases the compressive strength and flexural strength of the concrete more obviously. With the increase of the content, the difference of the compressive strength and the flexural strength of the two kinds of specimens were getting larger and larger, and reached the peak of 4.1 MPa and 4.32 MPa respectively at the blending amount of 1.5%. The compressive strength increase rates of organic synthetic fibers and metal fibers for concrete were 16.77% and 24.95%, respectively, and the increase rates for flexural strength were 19.14% and

The test materials used in this paper were 42.5 R ordinary portland cement, with a density of 3.24 g/cm3, 3-days and 28-days flexural strength of 5.6 MPa and 6.9 MPa, and 3-days and 28-days compressive strength of 29.2 MPa and 55.8 MPa, respectively. Concrete aggregates are 5–10 mm, 10–20 mm, secondary limestone gravel, grading ratio is 40:60, gravel density is 1.62 g/cm3, aggregate density is 1.95 g/cm3, crushing index is 3.8%. Sand is medium sand with a fineness modulus of 2.88 and belongs to zone II gradation. Its density is 2.73 g/cm3 and its bulk density is 1.62 g/cm3. The water-reducing agent used is FDN high-efficiency waterreducing agent. The silicon powder density is 2.2 g/cm3, the average particle size is 0.15microns, and the specific surface area is 24 m2/g [15,16]. Test water is pure water. The benchmark mix ratio of pavement concrete in the test is shown in Table 1 below: This paper selects the reticular polypropylene synthetic fiber (RPSF) to represent the organic synthetic fiber into the pavement concrete, as shown in Fig. 3. The main technical performance of this fiber is shown in Table 2: Wave steel fibers (SF) were selected for the test to incorporate inorganic metal fibers into the pavement concrete, as shown in Fig. 4. The main technical indicators are shown in Table 3. In the test, the specimens have a size of 300 mm  300 mm  50 mm, with fiber content of 0%, 0.3%, 0.6%, 0.9%, 1.2%, and 1.5% were prepared. In order to fully ensure the dispersion and uniformity of the fiber distribution within the concrete matrix, this test uses a three-axis forced mixer to stir. The mixing and feeding sequence is divided into four steps: (1) The coarse and fine aggregates are uniformly mixed. (2) Add fly ash, silicon powder and FDN water reducer. (3) Stir the fiber. (4) Add cement and water and stir. Cover the surface of the specimen with a plastic film to prevent moisture loss. After standard curing for 28 days (T = 20 ± 2 °C, relative humidity > 95%), we remove the sample and smooth it out, then simulate the actual state of the airport

Table 1 Concrete reference mix ratio. Material consumption per m3 of concrete (kg)

Cement

Water

Sand

Medium stone

Small stone

Fly ash

Silicon powder

Water-reducing agent

350

156.4

610

819

547

215.7

35

6.03

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Y. Chen et al. / Construction and Building Materials 192 (2018) 818–835 Table 2 Main technical performance of reticular polypropylene synthetic fibers. Reticular polypropylene synthetic fibers

Length/ mm

Density/ kg/m3

Tensile strength/ MPa

Elongation at break

Elastic modulus/ GPa

Appearance

20

1220

511

12.3%

6–9

Reticulated monofilament

synthetic fiber concrete with the same content. Therefore, when the initial amount is small, the increase of the elastic modulus of the organic synthetic fiber is relatively obvious. With the increase of the amount, the effect of the metal fiber is more and more significant, and ultimately better than the organic synthetic fiber. In summary, the improvement effect of inorganic metal fibers is more remarkable. 4. High-temperature aging anti-wheel impact test system

Fig. 4. Steel fiber.

99.44%, respectively. For the dynamic elastic modulus of concrete, the dynamic elastic modulus of organic synthetic fiber concrete was higher than that of steel fiber concrete when the content was 0–1.03%, and the dynamic elastic modulus of organic synthetic fiber concrete was lower than that of steel fiber concrete when the content is 1.03–1.2%. Besides, the dynamic elastic modulus of organic synthetic fiber reinforced concrete reaches the peak at a content of 1.2% with a rise of 14.71%, which is 2.31% lower than that of the same content of metal fiber reinforced concrete. The increase rate of the dynamic elastic modulus of metal fiber reinforced concrete reaches a peak of 20.04% when the content is 1.5%, which is 7.26% higher than that of the reticular polypropylene

The whole test system consists of three test instruments, namely, polyurethane high temperature aging chamber, xenon weather resistance test box and wheel impact test machine. The three instrument combination acts on the specimen. Among them, the high-temperature aging environment of polyurethane hightemperature aging chamber simulation sample. Because the high temperature of the airport pavement is mainly caused by sunlight, the ultraviolet radiation in natural sunlight needs to be considered. Therefore, xenon arc lamps which can simulate the full sunlight spectrum are used to reproduce the destructive light waves in different environments. In this paper, an improved wheel impact tester designed by ourselves on the pavement rutting tester can simulate the actual state of wheel grounding, including load and speed. In terms of specimen size, 300 mm  300 mm  50 mm specimens were used in the experiment. 4.1. High temperature aging simulation test system The environmental conditions in the test were simulated in accordance with the standards in [18], and the

Table 3 The main technical performance of steel fiber. Steel fibers

Length/mm

Ratio of length to diameter

Density/kg/m3

Tensile strength /MPa

Elongation at break

Elastic modulus / GPa

Appearance

35

70

7800

1250

9.2%

220

Wavy

Table 4 Basic performance of reticular polypropylene synthetic fiber concrete. Index

Compressive strength (MPa) Flexural strength (MPa) Dynamic elastic modulus (Hz)

Fiber content (%) 0

0.3

0.6

0.9

1.2

1.5

50.1 5.38 52.59

52.5 5.75 53.84

54.3 6.18 56.68

57.6 6.37 58.99

59.2 6.56 60.33

58.5 6.41 59.31

0

0.3

0.6

0.9

1.2

1.5

50.1 5.38 52.59

53.6 7.89 52.77

58.4 8.91 54.84

60.2 9.25 57.60

62.1 10.66 61.54

62.6 10.73 63.13

Table 5 Basic performance of inorganic steel fiber concrete. Index

Compressive strength (MPa) Flexural strength (MPa) Dynamic elastic modulus (Hz)

Fiber content (%)

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Fig. 5. The effects of organic fiber and inorganic fiber on the basic properties of concrete.

high-temperature aging of airport pavement under realistic conditions was simulated using polyurethane high-temperature aging chambers, as shown in Figs. 6 and 7. The airport pavement surface in high temperature areas continues to absorb heat under conditions of high intensity sunlight, and the surface temperature can reach 60 °C. Therefore, the aging chamber temperature is set to 60 °C and the control accuracy is ±0.5 °C. The temperature history was collected from the pavement temperature at 2:00 noon at Wuwei airport, Gansu, China. The collection time was once a day, lasting for 4 months, totaling 120 times. As can be seen from the table, the surface temperature of the pavement can reach 69 °C at the highest and 58 °C at the lowest. In this paper, the mean value of data acquisition is simplified, and the aging temperature in the test can be set to 60 °C. The temperature history is shown in Table 6. On this basis, this study will improve the aging test of concrete in line with the actual conditions of airport pavement aging. The high-temperature aging of airport pavements is almost due to long sunshine hours, high daylight intensity and high temperatures [19]. Therefore, in this test, the helium aging test box was used to simulate the full-band sunlight to illuminate the pavement surface specimens, thereby fully simulating the actual environmental climatic conditions. Test equipment is shown in Fig. 8.

Fig. 7. Interior of polyurethane high-temperature aging chamber.

4.2. Anti-wheel impact test system

Fig. 6. Polyurethane high-temperature aging chamber.

For the test of concrete impact resistance, there is no unified test method in the world, such as drop hammer method, SHPB and the latest air gun method [20]. However, the impact of aircraft wheels on airport pavements is special, and it is a dynamic rolling wheel impact. Therefore, the conventional test method cannot simulate the stress and load of airport pavement concrete, and the conclusion cannot be applied to the construction of airport pavement. Combining with the actual situation, this paper improves the concrete road rim tester, and a pressure pump and a speed sensor is installed, conducting the concrete wheel impact test by reproducing the grounding motion and speed of the wheel in the room, as shown in Fig. 9. The test system consists of a test stand, a rigid test wheel, two guide rails (2 m long horizontal rails, 75 cm long vertical rails), an air pump loading system, and an electronic control system. Due to the large tire pressure of the aircraft, the steel wheel is used instead of a solid wheel made of rubber as the impact loading wheel in order to simulate the instantaneous rigid load of the wheel when it was grounded. The outer diameter of the drum is 170 mm, the wheel width is 50 mm and the weight is 3 kg. The

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Y. Chen et al. / Construction and Building Materials 192 (2018) 818–835 Table 6 Surface temperature history of the pavement. Collection times

Temperature/°C

Collection times

Temperature/°C

Collection times

Temperature/°C

Collection times

Temperature/°C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

65 61 62 61 62 62 60 60 60 60 59 59 58 58 59 60 62 63 65 61 60 60 59 60 61 58 60 59 64 63

31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

62 61 60 63 60 60 58 59 61 65 65 66 68 69 64 63 62 61 58 58 58 58 59 58 61 61 62 60 62 61

61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90

62 63 64 65 65 65 62 61 60 60 60 60 60 61 58 59 59 59 60 60 61 58 59 62 63 62 63 61 60 61

91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120

61 63 63 63 64 65 66 67 65 64 62 61 60 60 60 59 58 58 59 60 64 63 62 63 61 60 60 59 59 59

Fig. 8. Helium weather test box.

and measurement, the ground speed of aircraft wheels is 180– 200 km/h [21], and horizontal direction is 5–6°. It can be seen that the speed in the vertical direction is about 5.18 m/s. Speed sensors are installed on both guide rails to detect the speed of the steel wheel at the end of the guide rail. The pressure of the pump is ensured by adjusting the pressure of the air pump. Besides, the movement of the steel wheel in the horizontal direction basically does not have an impact, so the pressure of the pump can be appropriately reduced according to the test conditions. In addition, the accelerating loading wheel started to contact the surface of the test piece 50 mm from the edge of the inner edge of the specimen, completing the grounding impact and simulating the wheel rolling forward and sliding 200 mm, and then disengaged from the specimen, thus a wheel impact is completed. The impact frequency is 40 times/h. According to the field surveys in Urumqi and Lanzhou, we learned that in the actual climatic environment, the process of absorbing heat from the pavement surface of the airport to reach the maximum temperature generally lasts for 3 h. Therefore, the high-temperature aging selected 120 h for one cycle (40 days), and all the test pieces were placed in the polyurethane hightemperature aging chamber and the helium gas weather resistance test chamber for high-temperature aging and light treatment for 120 h each. After completion, the specimen was subjected to a wheel impact of 60 times. The above steps were recorded as a round of tests. The total number of tests was 10 rounds.

4.3. Impact indicators and data processing impact wheel is pressurized by a high-pressure gas pump, and the load pressure can reach 1.7 MPa, which is in line with various aircraft tire pressure requirements. The rigid wheel exhibits a uniform acceleration linear motion on the guide rails in both directions, and a speed in a tangential direction of the motion curve is formed when the specimen are touched, and the specimen are subjected to wheel-type rolling impact. According to research

In order to comprehensively study the superposition effect of high-temperature aging damage and wheel impact, this paper selects two types of indexes after the high-temperature aging impact of the specimen. The first type is the apparent index, the loss of the surface quality of the specimen is used as the impact damage, and intuitively reflects the extent of concrete damage.

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(a) loading wheel

(b) transmission device

(c) loading device

(d) two-way guide rail Fig. 9. Wheel Impact Tester.

The second type is the internal impact resistance performance index. Three indexes, namely, compressive strength loss, dynamic elastic modulus loss and concrete crack growth, are selected to evaluate the impact resistance of airport pavement concrete under high-temperature aging conditions against wheel impact performance, and then establish a fitting function relationship between indicators and test rounds. Finally, through the comprehensive analysis of appearance and internal measurement, the improvement effect of fiber on concrete is quantitatively and intuitively analyzed. (1) The amount of impact damage can represent macroscopically the degree of impact shedding of the concrete. When the specimen is subjected to high temperature aging, the surface will lose moisture and embrittle, and a certain degree of expansion occurs. At this time, the impact was applied, the destruction of the concrete was intensified, and the surface of the specimen was shed. This is an intuitive damage that can be seen with the naked eye, and it is also the greatest damage to the airport road surface. Therefore, in this study, the damage value of the specimen before and after the impact is used as the impact damage to measure the damage degree of the test piece, which was recorded as Dmn . The data processing is Dmn ¼ m0  mn , where m0 and mn are the mass of the initial specimen and the remaining mass of the specimen after the n rounds of testing, respectively, and are measured using a high-precision electronic balance. (2) The compressive strength loss represents the difference in compressive strength before and after the high temperature aging impact test of the specimen. This index represents the

loss of concrete compressive capacity due to the external effects. It can also be used to reflect the rate and magnitude of damage when the concrete is subjected to high temperature aging and wheel impact. In this test, the WAW-1000G compression tester was used to measure the compressive strength of the test pieces. The loss of compressive strength after n rounds of tests is denoted by Df ckn , and the calculation formula is Df ckn ¼ f ck0  f ckn . Among them, f ck0 and f ckn are the compressive strength of the initial specimen and the compressive strength of the specimen after n rounds of tests, respectively. (3) The amount of dynamic elastic modulus loss refers to the difference in the dynamic elastic modulus before and after the high-temperature aging impact test of the test piece, denoted by DEcn , where n denotes the number of test rounds. This indicator can reflect the concrete’s ability to resist dynamic impact damage. The calculation formula of DEcn is DEcn ¼ E0  En , where E0 and En are the dynamic elastic modulus of the initial specimens and the dynamic elastic modulus of the test piece after n rounds of test. This paper uses the DT-2 digital dynamic elasticity tester to test the specimen. (4) The concrete is affected by high-temperature aging, resulting in slight expansion and an increase in the internal pore size. At the same time, under the action of the impact, the internal pores of the concrete and the original microcracks gradually expand into large cracks or even form through-breaking. In this process, the total size of cracks continuously increases, and its growth value can well reflect the increase in the number of microscopic cracks in the concrete and the change in density, reflecting the growth

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of concrete impact damage. In this paper, ultrasonic nondestructive testing technology is applied, and the sound wave propagation speed in concrete is used as a standard to measure the number of voids in the matrix. The instrument used was a CTS-45 non-metallic ultrasonic testing analyzer produced in Guangdong, and the test method was Ultrasonic Testing Method [22]. The instrument is shown as Fig. 10. The schematic diagram of the Ultrasonic Testing Method is shown as Fig. 11.

Specimen preparation

High temperature aging simulation test system

This study believes that the non-interspace and cracks inside the staggered stage of the specimen without impact damage are dense and belong to the compact area, and the rest containing pores and cracks are non-dense areas. After the wheel impact, the original dense part will produce certain cracks. The original non-dense part of the crack width will change, and the size of the non-dense part will change accordingly. Therefore, by calculating the change of the wave speed in the test piece, it can be concluded that the change of the internal pore width after the impact is recorded as the increase in the size of the concrete crack, denoted by DLcn , and the unit is mm. It can be derived from the following Eq. (1):

DLcn

vs

þ

Li

v0

¼

Li

vn

Anti-wheel impact test

mn

f ck

Ecn

Lcn

XTH225 imaging industrial CT Internal imaging processing

ð1Þ

Fig. 12. Research flow.

After finishing, DLcn can be found by Eq. (2):

DLcn ¼

v s  Li ðv 0  v n Þ v0v n

ð2Þ

In the Eq. (2), v 0 denotes the internal wave speed of the test piece that has not been damaged by high temperature aging and impact. After testing, it can be seen that the ultrasonic wave propagation speed in the initial sample is 4431 m/s; v n represents the wave velocity inside the test piece after n rounds of test; v s means the sound wave velocity of the non-dense part, which between the cracks in the compact area is approximately equal to the sound velocity in the air (340 m/s). The research flow of this paper is shown as Fig. 12 as below. 5. Test results and analysis 5.1. The loss of compressive strength

Fig. 10. CTS-45 non-metallic ultrasonic testing analyzer.

Fig. 11. The schematic diagram of the Ultrasonic Testing Method.

The compressive strength is the most important strength index of concrete, and it is declining under high-temperature aging conditions. Its loss will directly lead to the weakening of the impact resistance of airport pavement concrete. Df ck indicates the ability of the specimen to maintain compressive strength under the wheel impact. After testing, the relationship between the Df ck of metal fiber reinforced concrete with different fiber content and the number of test rounds t is shown in Fig. 13. According to the Fig. 13, the Df ck of metal fiber reinforced concrete with different fiber content increases with the increase of t. When the fiber content is 0% and 0.3%, the concrete compressive strength loss rate is relatively large. As the experiment progresses, the late loss still maintains a large growth rate. The 0.6% steel fiber content is a key point. It can be seen from Fig. 13 that when the fiber content is greater than 0.6%, the growth rate is significantly reduced. In addition, from the figure, it can be seen that as the fiber content increases, the mitigation effect against compressive strength loss becomes more and more obvious, but the decreasing

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Y. Chen et al. / Construction and Building Materials 192 (2018) 818–835

Fig. 13. Relationship between steel fiber concrete Df ck and t.

range becomes smaller and smaller. After the end of the test, with the increase of fiber content, the reduction of Df ck was 18.7 MPa, 16.1 MPa, 12.9 MPa, 10.1 MPa, 8.8 MPa, and 7.4 MPa, respectively. The loss of specimens with a content of 1.5% was the smallest, with a loss percentage of 11.82%, 25.5% lower than that of ordinary concrete. Consequently, the relationship between Df ck and t can be described as an exponential function group, as shown in Eq. (3).

8
t  > Df ck ð0%Þ ¼ 26:73  exp 8:07 þ 26:9 > >
t  > > > Df ck ð0:3%Þ ¼ 21:37  exp 5:89 þ 20:65 > > > < Df ð0:6%Þ ¼ 16:37  exp
t  þ 15:74 ck
5:21  t > Df ck ð0:9%Þ ¼ 15:52  exp 8:69 þ 15:31 > >
t  > > > D f ð 1:2% Þ ¼ 16:34  exp þ 15:95 > ck 11:37 > > : Df ð1:5%Þ ¼ 12:1  exp
t  þ 11:53 ck 8:4

ð3Þ

By comparison, it can be seen that the steel fiber can effectively control the compressive strength reduction of concrete under the superposition effects of high temperature aging and wheel impact, and the effect is best when the content is 1.5%. Thus, the relationship between the Df ck of organic synthetic fiber reinforced concrete with different fiber content and the number of test rounds t is shown in Fig. 14.

Fig. 14. Relationship between reticular polypropylene synthetic fiber concrete Df ck and t.

As can be seen from Fig. 14, the Df ck of the reticular polypropylene synthetic fiber concrete with different content increases with the increase of t. After 10 rounds of tests, the damage of ordinary concrete was the largest, and the reticular polypropylene synthetic fiber concrete with 1.2% content had the lowest damage, with a loss of 9.1 MPa and a loss percentage of 15.37%. The damage rate of ordinary concrete is basically maintained at a large stable value, while the damage rate of fiber concrete is gradually reduced. In the first 6 rounds of tests, the compressive strength loss of fiber concrete with 0.3% fiber content was always greater than that of ordinary concrete. This is mainly due to the fact that the fiber content is relatively small and the inhibitory effect on the cracking of the matrix is not obvious. At the same time, the concrete attached to the fiber will be torn off during the impact cracking process, leading to greater cracks and more loss of compressive strength. However, in the later stage of the test, the main internal cracks in the matrix have been basically formed. The inside of the concrete has been divided into smaller bulk structures. Low-content organic fibers are sufficient to prevent continuous cracking, so the loss rate at the later stage is reduced, and the loss is lower than that of ordinary concrete. In addition, from the data analysis, we learned that when the fiber content is greater than 1.2%, the loss of concrete resistance to pressure has picked up. This is mainly because excessive fiber content will cause the fiber inside the matrix to become agglomerate and entangled, which will cause local softening and reduce the compressive strength and become a weak area. When this area is impacted, it first cracks, and affects the adhesion of the surrounding matrix, leading to a significant decrease in the overall compressive strength. The relationship between Df ck and t can be described as an exponential function, as shown in Eq. (4).

8 t Df ck ð0%Þ ¼ 26:73  expð8:07 Þ þ 26:9 > > > t > > D f ð 0:3% Þ ¼ 21:37  expð Þ þ 20:65 ck > 5:89 > > < Df ð0:6%Þ ¼ 16:37  expð t Þ þ 15:74 ck 5:21 t > > Df ck ð0:9%Þ ¼ 15:52  expð8:69Þ þ 15:31 > > > t > D f ð 1:2% Þ ¼ 16:34  expð Þ þ 15:95 > ck 11:37 > : t Df ck ð1:5%Þ ¼ 12:1  expð8:4Þ þ 11:53

ð4Þ

Through the comprehensive analysis of the two types of fiber reinforced concrete, it can be seen that in the early stage of testing, the difference between Df ck for ordinary concrete and Df ck for fiber reinforced concrete is not significant. This is mainly due to the short time of high-temperature aging, the moisture inside the concrete has not been completely evaporated, no large expansion pressure has been formed, and no stable thermal stress has been formed inside the specimen, and the interspace inside the concrete is small. In addition, the degree of aging is low, and the material of the concrete matrix itself is not embrittled or aged. Under the combined effects of the above-mentioned various reasons, the incorporation of fibers has less influence on the compressive strength of the specimen at the early stage. At the later stage of the test, the improvement of compressive strength loss of both fibers after impact was significantly better than that of ordinary concrete. The main reason is that high temperature aging causes local expansion and cracking of the concrete matrix, and fibers can prevent or delay the occurrence of cracking. Thus, Longitudinal comparison of the results of two kinds of fiber concrete test shows that the Df ck of steel fiber concrete is always lower than that of organic synthetic fiber concrete same fiber content, the maximum difference is 2.7 MPa, the minimum is 0.3 MPa. This shows that under the same conditions, the effect of steel fiber is more obvious. The thermal stress caused by high temperature increases the interspace inside the matrix and distributes unevenly. Some interspace generates expansion stress due to moisture evaporation, and the matrix material is aged, and the powdered fractures and bonding failure

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occur in the transition zone [23]. Since organic fibers have a certain degree of flexibility, they stretch and rebound more easily than steel fibers, so that the organic fibers undergo elastic deformation upon impact. The matrix to which the fibers are attached cracks within the deformation range and is always larger than steel fiber concrete. At the same time, the organic fibers themselves have poor heat resistance and are susceptible to aging hardening and breaking. In addition, the steel fibers are wavy and have a relatively rough surface, and have better adhesion to concrete than organic fibers. Therefore, the impact resistance and crack resistance of steel fiber are more obvious, and the corresponding concrete Df ck is smaller. In summary, it can be seen that the incorporation of fiber can not only improve the compressive strength of the airport pavement concrete itself, but also can significantly reduce the compressive strength loss of the concrete under high temperature aging and impact, and reduce the loss rate. Metal fiber and organic synthetic fiber had the best improvement effect when the content was 1.5% and 1.2% respectively, and SF was better than RPSF. 5.2. The loss of dynamic modulus

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inside. The longitudinal comparison shows that for SFC, DEcn decreases with the increase in the amount of metal fibers added. The DEcn of the test specimen with 1.5% addition content reached a minimum value of 21.6 Hz, which was only 66.67% of the loss of ordinary concrete. Comparing the dynamic elastic modulus values before and after test specimens, it can be seen that with the increase of the content, the proportion of a to the initial elastic modulus is 44.48%, 42.95%, 41.61%, 36.51%, 26.78%, and 23.79%, respectively, among them, the numerical range difference is 20.69%. This shows that with the increase of the content of steel fiber, the improvement effects of the high temperature aging and wheel impact of concrete on the dynamic elastic modulus loss are more and more significant. Thus, the relationship between DEcn and t can be described as an exponential group, as shown in Eq. (5).

8 t DEcn ð0%Þ ¼ 43:81  expð7:52 Þ þ 44:19 > > > t > > D Ec ð 0:3% Þ ¼ 41:62  expð Þ þ 41:81 n > 8:57 > > < DEc ð0:6%Þ ¼ 45:44  expð t Þ þ 45:46 n 9:72 t > D Ecn ð0:9%Þ ¼ 43:98  expð10:29 Þ þ 44:31 > > > > t > DEcn ð1:2%Þ ¼ 47:83  expð24:51Þ þ 49:07 > > : t DEcn ð1:5%Þ ¼ 46:49  expð26:02 Þ  46:46

ð5Þ

The environment with high temperature aging has a great influence on the elastic modulus of concrete. In addition, the rolling impact of the wheel is a dynamic impact, therefore, the loss of the dynamic elastic modulus is used to quantify the ability of the test piece to resist deformation. After testing, the relationship between the DEcn of metal fiber reinforced concrete with different fiber content and the number of test rounds t is shown in Fig. 15. According to the Fig. 15, the DEcn of metal fiber reinforced concrete with different fiber content increases with the increase of t. When the fiber content was 0–0.9%, the increase rate of DEcn decreased gradually with the increase of the number of test rounds, and the rate decrease was most obvious after the sixth round of test. When the fiber content was between 0.9% and 1.5%, the increase rate of DEcn remained essentially constant as the number of test rounds increased. The process of the sixth to seventh round of tests is the key stage, during which the loss of dynamic elastic modulus of the specimen suddenly increases, and then gradually stabilizes. This is mainly due to the fact that the sixth round of tests is the critical point of cracking inside the specimen with low fiber content. At this point, the specimen produced larger cracks and the elastic modulus was greatly reduced. For a sample with a fiber content greater than 0.9%, the upward trend of DEcn is relatively stable because there is no large cracking

By comparison, it can be seen that the incorporation of metal fibers can effectively reduce the loss of dynamic elastic modulus under the combined action of high-temperature aging and wheel impact of concrete, and the effect is best when the content is 1.5%. After testing, the relationship between the DEcn of organic synthetic fiber reinforced concrete with different fiber content and the number of test rounds t is shown in Fig. 16. Similar to metal fiber reinforced concrete, as can be seen from Fig. 16, the DEcn of organic synthetic fiber reinforced concrete with different fiber content increases with the increase of t. From the figure, it can be seen that when the fiber content is more than 0.6% and the fiber is continuously added, the inhibitory effect of the growth of DEcn on the test piece becomes weaker and weaker, but the final DEcn still continues to decrease. When the fiber content is 0.9%, 1.2% and 1.5%, the curves in the figure are almost coincident, and the fitting curve with 1.2% addition is slightly lower. After ten rounds of tests, the DEcn of the test specimen with a fiber content of 1.2% reached a minimum of 23.9 Hz, which is 73.31% of the normal concrete. Continuing to increase the fiber content will result in dense fiber distribution in the concrete. Due to the organic fibers are soft and foldable [24], a large amount of air is entrained in the concrete vibrating process. As a result, the porosity inside

Fig. 15. Relationship between steel fiber concrete DEcn and t.

Fig. 16. Relationship between organic synthetic fiber concrete DEcn and t.

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the matrix is large, and the hydration reaction is not complete, and the free water content is high. Under the effect of continuous high temperature, the water evaporates to form a large stress, thermal stress will also increase, resulting in pore expansion, thinning of the surrounding matrix and micro-cracks. Under multiple dynamic impacts, the internal pores of the matrix ruptured, micro-cracks penetrated, and the loss of elastic modulus rebounded. Therefore, the DEcn of the sample with a fiber content of 1.5% is greater than that of the specimen with a content of 1.2%. From the comparative analysis of the slope of the curve at each node in the figure, it can be seen that the growth rate of the specimen with 1.2% content is the slowest. Thus, the relationship between DEcn and t can be described as an exponential, as shown in Eq. (6).

8 t DEcn ð0%Þ ¼ 43:81  expð7:52 Þ þ 44:19 > > > t > > D Ec ð 0:3% Þ ¼ 40:66  expð Þ þ 40:8 n > 7:56 > > < DEc ð0:6%Þ ¼ 46:42  expð t Þ þ 46:38 n 12:9 t > D Ecn ð0:9%Þ ¼ 41:29  expð17:79 Þ  41:24 > > > > t > D Ec ð 1:2% Þ ¼ 89:57  expð Þ - 89:82 n > 41:49 > : t DEcn ð1:5%Þ ¼ 178:04  expð76:04 Þ  178:01

ð6Þ

Therefore, by comparison, we found that the increase of organic synthetic fiber content in the range of 0.3–1.2% has a great inhibitory effect on the increase of DEcn , and the effect is most significant when the content is 1.2%. Through comparative analysis of the two kinds of fiber reinforced concrete, it can be seen that, in different ranges, the increase of both fiber types can inhibit the dynamic elastic modulus loss of the concrete. With the optimal content of two fibers, the steel fiber concrete is 9.62% lower than the organic synthetic fiber concrete and 33.33% lower than the ordinary concrete. The order of the inhibitory effects on the elastic modulus of concrete under high temperature aging impact is: 1.5% SFC > 1.2% SFC > 1.2% RPSFC > normal concrete. Through analysis, the main reasons are the following two points: (1) For high temperature aging resistance, SF has excellent thermal conductivity, which can quickly diffuse the absorbed heat and avoid the accumulation of heat, thus preventing the aging phenomenon from intensifying. The RPSF endothermic performance is better than SF, resulting in difficult heat dissipation, increased internal molecular movement, easy softening, and ultimately brittle fracture. (2) For the wheel impact resistance, the steel fiber is not easily deformed under a continuous high temperature, and can maintain its original shape well when subjected to the impact, and maintain the control of the expansion size and the crack inside the base body. The RPSF will soften and deform under high temperature, the length of stretching will increase, and the network structure of fiber will be damaged, which will have additional effect and lager affected area. Under the effect of the wheel rolling impact, the surrounding concrete is detached from the fiber, the fixing effect is reduced, the crack density of the matrix increases, and the elastic modulus decreases. 5.3. The growth size of internal pore High temperatures can cause concrete to age, mechanical properties decrease, and internal pores increase or even crack. Wheel impact can increase the degree of cracking and increase the size of the crack. After testing, the relationship between the DLcn of metal fiber reinforced concrete with different fiber content and the number of test rounds t is shown in Fig. 17. As can be seen from Fig. 17, the DLcn of metal fiber reinforced concrete with different fiber content increases with the increase of t. The seventh round of test is a change node, and a sudden and substantial increase in DLcn in the case of a decrease in the rate of increase in the previous period, which indicates that the degree

Fig. 17. Relationship between steel fiber concrete DLcn andt.

of aging of the test specimens has accumulated to a threshold, and a continuous cracking has been formed with the continuous impact. From the longitudinal comparison of the curves in the figure, DLcn decreases as the fiber content increases. In addition, after ten rounds of tests, the fiber content of the specimen corresponding to the minimum DLcn was 1.5%, and the minimum value was 2.28 mm, which was 69.51% of the ordinary concrete. When the fiber content of the test piece is less than 0.9%, the decrease rate of DLcn gradually decreases. As the fiber content is more than 0.9%, the decrease rate of DLcn gradually increases. This shows that there is a tendency for high-content steel fiber concrete to continue cracking. The critical value for cracking is high and the process of reaching the critical value is slow. The difference between the test specimens with different dosages first increases and then decreases. This is mainly because the fibers can effectively prevent impact damage in the early stage, but with the accumulation of high-temperature aging damage, a fixed volume of expansion space is formed inside the test piece, and part of the fibers have been detached from the matrix under impact [25]. Moreover, it can be seen that in the seventh round, size fracture growth is changed. The surface and section images of specimen after the seventh test round are shown as below in Fig. 18. The process of impact damage can be understood as the process of crack development inside the specimen. There are primitive microfracture inside the concrete, and gradually expand and increase under the continuous impact. There is a threshold for the number of shocks. Before this time node, the crack is in a process of accumulation of size and density. After reaching this time node, the cracks of concrete subjected to a certain number of shocks form a penetration, which results in the centralized cracking of the specimen in this time period. This phenomenon will cause a change in the rate of internal damage change, resulting in a change in the standard in the seventh round of the test. The relationship between DLcn and t can be described as an exponential group, as shown in Eq. (7).

8 t DLcn ð0%Þ ¼ 7:12  expð16:99 Þ þ 7:21 > > > t > > DLcn ð0:3%Þ ¼ 7:32  expð18:38 Þ þ 7:38 > > > < t DLcn ð0:6%Þ ¼ 6:99  expð19:1 Þ þ 7:12 t > > DLcn ð0:9%Þ ¼ 15:25  expð59:06Þ  15:21 > > > t > > DLcn ð1:2%Þ ¼ 1:07  expð8:33Þ  1:02 > : t DLcn ð1:5%Þ ¼ 0:47  expð5:53 Þ  0:49

ð7Þ

By comparison, it can be seen that the addition of metal fibers in the concrete can reduce the crack growth of the specimen, and the corresponding content with the most obvious effect is 1.5%. After

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(b) section images

(a) surface images

Fig. 18. The surface and section images of specimen after the seventh test round.

testing, the relationship between the DLcn of organic synthetic fiber reinforced concrete with different fiber content and the number of test rounds t is shown in Fig. 15. As can be seen from Fig. 19, when adding reticular polypropylene synthetic fibers to the specimen, the DLcn of specimens increases with the increase of t, and its overall trend is similar to steel fiber concrete. After the end of the test, the DLcn of specimen with a content of 1.2% was the lowest, reaching 2.67 mm, 19.60% lower than the ordinary concrete. According to the extent of controlling the crack growth effect, the fiber content of the specimen was ranked as follows: 1.2% > 1.5% > 0.8% > 0.3% > 0%. Thus, the relationship between DLcn and t can be described as an exponential group, as shown in Eq. (8).

8 t DLcn ð0%Þ ¼ 7:12  expð16:99 Þ þ 7:21 > > > t > > D Lc ð 0:3% Þ ¼ 3:63  expð Þ þ 3:64 n > 12:5 > > < DLc ð0:6%Þ ¼ 1:74  expð t Þ  1:73 n 6:94 t > D Lc n ð0:9%Þ ¼ 1:54  expð9:9Þ  1:38 > > > > t > DLcn ð1:2%Þ ¼ 0:23  expð3:93 Þ  0:14 > > : t DLcn ð1:5%Þ ¼ 0:49  expð5:41 Þ  0:31

ð8Þ

From the above equation, it can be seen that the addition of metal fibers in the concrete can reduce the crack growth of the specimen, and the corresponding content with the most obvious effect is 1.2%. Comparing and analyzing the test results and fitting equations of the two fiber concretes, we can draw the following conclusion: The incorporation of both fibers can reduce the

increase in cracks when the concrete is subjected to wheel impact under high-temperature aging conditions, and the number of cracking critical actions of concrete as a whole has increased. In addition, comparing the two optimal contents of concrete, the improvement effect of metal fiber on DLcn of concrete is weaker than that of organic synthetic fiber in the early stage of the test. However, with the progress of the test, the crack growth of steel fiber reinforced concrete is getting smaller and smaller, until finally less than organic synthetic fibers, so after the end of the experiment, DLcn of RPSFC is greater than that of SFC. The main reason is that the strength and hardness of metal fibers are relatively high, and the aging rate at high temperatures is lower than that of organic synthetic fibers [26]. Organic synthetic fibers can absorb the kinetic energy generated by the impact and convert them into elastic potential energy in the early stage. When the thermal stress in the early stage of the concrete is small, the aging of the matrix is not obvious, and the matrix sticks to the RPSF, which can weaken the impact. At the end of the experiment, the mechanical properties of the matrix decreased, RPSFC quickly aged, the cohesive force decreased, the misalignment fracture occurred synchronously with the matrix, and the ability to control cracks was reduced, gradually lower than SFC. In summary, the 1.5% content of metal fibers can better control the internal crack growth of specimens under high temperature aging impact. 5.4. Effect of fiber type on the surface impact damage of concrete The external conditions of high temperature aging will cause the brittleness of concrete on the pavement surface to a certain extent, and the detached fibers pose a great safety hazard to the safe take-off and landing of aircraft. Moreover, the wheel impact of aircraft wheels on the road surface exacerbated the occurrence of concrete shedding damage. In order to fully study the apparent impact resistance index of concrete Dmn , the test pieces are grouped according to the amount, and the relationship between the impact shedding masses Dmn and the number of test rounds t of the specimen with different fiber content of steel fiber concrete can be obtained, as shown in Fig. 20. According to the Fig. 20, we can draw the following conclusions about the overall trend, single content and impact method:

Fig. 19. Relationship between reticular polypropylene synthetic fiber concrete DLcn andt.

(1) From the overall trend, the Dmn of the two types of fiber reinforced concrete with different amounts increase with increasing, and the rate of early rise is slow. When the test was carried out to the sixth round, almost all of the test specimens reached a critical value of surface shedding mass. As the test continues, the shedding mass increases abruptly and the rate increases significantly. As can be seen from

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Fig. 20. Relationship between steel fiber concrete Dmn andt.

Fig. 20, the shedding mass on the concrete surface is a staged damage. In the 1–6 rounds of tests, the Dmn of both ordinary concrete and fiber concrete can be fitted with the logarithmic function of t, as shown in Fig. 20. In the 7–10 rounds of tests, the slope of the fitting function of Dmn for t suddenly increased, it was linearly increased with t, and the specific fitted function was shown in Fig. 20. This shows that after 6–7 rounds of tests, the micro-cracks inside the

concrete have gradually joined and become penetrating damage, and developed to the concrete surface. From Fig. 20(a), it can be seen that ordinary concrete also has this phenomenon, which shows the characteristic of the concrete matrix itself. In addition, the incorporation of fiber can reduce the occurrence of shedding phenomenon and fix the bonding matrix, but due to the non-uniformity of the fiber distribution, part of the matrix will still fall off. At this

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time, the through cracks bypass the fiber and cause surface damage [27]. Under the same conditions of the concrete matrix body itself, different fiber content leads to different complex tortuous levels of the through cracks. According to DEcn and DLcn of the internal indicators of the specimen, after 6–7 rounds of tests, the loss of both indicators also increased significantly, which also confirmed the occurrence of the matrix internal damage. (2) From the perspective of single content, when the fiber content is 0.3%, 0.6%, and 0.9%, in the first stage of the test, the Dmn of the reticular polypropylene synthetic fiber concrete is always smaller than that of the steel fiber concrete. In the second phase of the test, the Dmn of organic fiber reinforced concrete with above three different contents has increased significantly, and finally it is larger than steel fiber concrete. When the fiber content increased to 1.2%, the Dmn of the metal fiber reinforced concrete was greater than the reticular polypropylene synthetic fiber concrete throughout the test, and the difference between the two increased as the content increased. Meanwhile, in the second phase of the experiment, the difference in the rate of damage between the two types of concrete increases as the amount increases. According to the content from 0.6% to 1.5%, the slope of the fitting line is ranked as: 0.2, 0.3, 1.1, 1.3. It shows that the improvement effect of metal fiber is more and more obvious. The reason for the above phenomenon is that the specimen with smaller fiber content does not fall off during the first phase of the test, but there is a matrix that is about to fall off on the surface of the specimen. At this time, part of the concrete fibers has actually fractured. The concrete is fixed by the folding and winding deformation of the organic fiber inside the granules, and the two ends of the partial fibers are respectively located in the two sides of the matrix. However, the deformability of steel fibers is relatively poor. As the thermal stress and fatigue effect of high temperature aging in the current period make the concrete and some steel fiber lose the bond effect [28], the fiber cannot be connected by deformation, and it is directly shedding after the wheel type impact. When the content increases to more than 0.9%, the high-temperature aging resistance of the specimen increases, and the metal fiber reinforced concrete has a greater degree of reinforcement and cross-distribution in the interior, which directly solidifies the concrete on the surface of the specimen. When impacted, the net-like

(a) Steel fiber concrete

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organic synthetic fiber layers weaken the energy generated by the impact while the steel fibers can disperse the energy three-dimensionally. Therefore, in the second stage of the test where the high-temperature aging and the impact action strengthens, the metal fiber inhibits the shedding of concrete surface to be more effective. (3) From the point of view of the impact mode, there is a process of parallel slip by the inertia of the loading wheel after the impact, which is one of the differences between the wheel impact and the ordinary impact. This is also one of the differences between the wheel impact and the normal impact. When the fiber content is less than 0.9%, the specimens are not significantly aged at the early stage and the surface is relatively smooth. The incorporation of organic fibers will soften the surface of the specimen to a certain extent, while the surface hardness of the metal fiber reinforced concrete is high and the frictional force is large. Therefore, the masses of the reticular polypropylene synthetic fiber concrete specimen dropped less than that of the steel fiber concrete specimen. Through data processing, the specimens after ten rounds of tests are classified by type. The change between the Dmn of different types of fiber concrete and fiber content is shown in Fig. 21. As can be seen from Fig. 21, after the test is completed, for metal fiber reinforced concrete, Dmn decreases with the increase of the fiber content, and reaches the lowest value when the content is 1.5%, which is only 64.21% of the ordinary concrete. When the dosage increased from 0.9% to 1.2%, Dmn had the largest decrease, reaching 4.55 g. For reticular polypropylene synthetic fiber concrete, Dmn decreased first and then rebounded with the increase of fiber content. The lowest value was 20.63 g, and the content was 1.2%. Comparative analysis shows that the masses of steel fiber concrete surface shedding is smaller than that of reticular polypropylene synthetic fiber concrete. The ratio of the former to the latter is ranked according to the content of 0.3–1.5%: 95.7%, 96.5%, 95.5%, 79.9% and 70.1%. From this, it can be seen that when the blending amount is small, the anti-shedding performance of the two kinds of fiber reinforced concrete is similar, while with the increase of the fiber content, SFC inhibits the concrete exfoliation on the surface of the specimen more significantly. Based on the above analysis, two kinds of fiber concretes with the best content of fiber were tested, and then the optimal fiber types and content were compared. The relationship between the

(b) RPSF concrete

Fig. 21. Relationship between the Dmn of different types of fiber concrete and fiber content.

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Fig. 22. Comparison of the relationship between Dmn and t for concrete with two optimal fiber contents.

masses of surface shedding Dmn and the number of test rounds t is shown in Fig. 22. It can be clearly seen from Fig. 22 that the Dmn of the steel fiber concrete with a content of 1.5% is always less than that of the reticular polypropylene synthetic fiber concrete with a content of 1.2%. Moreover, as the number of test rounds increases, the difference between the two kinds of fiber reinforced concrete increases. This is a good indication that the inhibitory effect of SF is significantly greater than that of RPSF, and the optimal content is 1.5%. 5.5. Analysis of external damage status of specimen and its optimal improvement method Through the analysis of the internal indicators of the concrete and the results of the apparent indicators, the linear relationship between each indicator and the number of test rounds for different

fiber content was obtained. In addition, studies have summarized the types and content of fiber that have the best improvement effect on each single index under high temperature aging and wheel impact. According to the comparison, it can be seen that, for metal fiber reinforced concrete, the optimal content of SFC for reducing internal indicators Df ck , DEcn , DLcn and apparent indicators Dmn is 1.5%, and the optimal content of RPSFC is 1.2%. Combined with the quantitative analysis, this paper observes and analyzes the external damage status of the specimen and proposes the optimal improvement method. The wheel impact tests under high temperature aging were carried out with fiber contents of 1.5%, 1.2%, 0.9% blended in SFC and 1.2% blended in RPSFC. The damaged state of the specimen is shown in Fig. 19. From Fig. 23, it can be seen intuitively that the external damage degree of the steel fiber concrete specimen with a content of 1.5% is small, slight wear occurs, and the surface becomes rough, but large-grain concrete does not fall off. In contrast, the SFC and RPSFC pavement specimens with a content of 1.2% were all exposed to the impact of concrete debris, but the degree of SFC was relatively light. When the SF content drops to 0.9%, it can be seen from the Fig. 19(c) that the RPSFC specimen with 1.2% content is significantly less damaged than the SFC specimen. Through the comparison of these four pictures, we can find that the surface area of road interview is distinctly different. The surface of the SFC specimens concrete has only a slight powdery drop. The surface of RPSFC is damaged in a large area, the macro-texture of the surface is almost disappeared, and there is a large amount of granular shedding. Therefore, according to the degree of reduction of external damage, the order of fiber reinforced concrete in the experiment is: SFC with 1.5% of blends > SFC with 1.2% blends > RPSFC with 1.2% blends > SFC with 0.9% blends > ordinary concrete. Combining the quantitative analysis results of the two types of indicators and the apparent damage status of the test specimens, this paper can conclude that metal fiber SF with a content of 1.5% in the airport pavement concrete can greatly improve its high temperature aging environment and anti-wheel impact performance

(a) Concrete with 1.5% steel fiber

(b) Concrete with 1.2% steel fiber

(c) Concrete with 1.2% RPSF

(d) Concrete with 0.9% steel fiber

Fig. 23. Surface damage of concrete specimen.

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6. Analysis of fiber action mechanism The test results show that after the fiber is incorporated into the concrete matrix, the anti-wheel impact performance of the concrete is greatly improved. In order to study the mechanism of fiber modification on concrete matrix, the XTH225 thermal threedimensional imaging industrial CT instrument was used for internal imaging processing of concrete specimens. The distribution and modification mechanism of different fibers were analyzed from the internal state of the concrete. The improvement effect of different test pieces was compared. The instrument is shown in Fig. 24. The three specimens were plain concrete, organic synthetic fiber concrete with 1.2% content, and steel fiber concrete with 1.5% content. The imaging results are shown in Fig. 25. Since the specific heat capacity of the fiber is smaller than that of concrete, after the sample absorbs heat, the fiber warms up faster and its temperature is higher. It can be seen that the three specimens in Fig. 25 are imaged as follows: the light gray region is a non-dense region, the dark gray region is a dense region, and the black region is a pore region inside the matrix. Comparing the Fig. 25 (a), (b) and (c), it can be clearly seen that after the specimen is incorporated into the fiber, the volume occupied by the dense area is larger than that of the ordinary concrete, and the interior of the steel fiber reinforced concrete is more uniform and there is no obvious porosity. From Fig. 25 (a), it can be seen that the degree of compaction in ordinary concrete is very uneven, and the porosity is mostly present at the junction. Under the action

Fig. 24. XTH225 industrial CT instrument.

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of the sun and high temperature of the cycle, the moisture inside the matrix is evaporated in large amounts and is left inside the closed pores, creating pore pressure. Under the pressure of water vapor, an uneven pressure gradient is formed inside the matrix. When the impact energy is transmitted to the intersection of the pressure gradient, the strength and toughness of the matrix itself cannot withstand greater fracture energy, and internal damage occurs, thus the pores continue to develop and eventually form cracking. The incorporation of fibers can better connect the dense and non-dense regions and improve the physical properties at the junction. At the same time, the accumulation of moisture is reduced, the pore density and size are reduced, and the thermal stress and pore pressure formed at high temperatures are reduced. In contrast, the improvement effect of steel fiber is even more pronounced. For organic synthetic fibers, they are light in weight and have a low density, and cannot be sufficiently dispersed in the process of agitation of the concrete to form an uneven distribution pattern in the matrix. When the local content is too much, the fiber is wound around the aggregate and the surface of the aggregate is wrapped, making it unable to fully bond with the cement slurry, resulting in the formation of pores around the aggregate during the subsequent vibrating process. In addition, due to the different temperature expansion coefficients of the material and the cement slurry in the concrete, under the effect of high-temperature aging, the temperature deformation causes cracks on the interface of the aggregate material, and the mechanical properties become fragile. When impacted by the wheel, the cracking of the aggregate around the aggregates in the organic fiber pavement concrete was intensified, and the cracks developed respectively, eventually forming a network and causing damage. When the local organic fiber content is too low, the local condition of ordinary concrete in Fig. 25 (a) will be generated. The non-dense region will surround the dense region and form a divergent pressure outward from the pore. When the impact force acts internally, the matrix in the middle of the zone of the two action breaks first. Since the connection of organic fibers to the concrete matrix can be defined as a kind of soft bridging method, the softness of the organic fibers determines that the effect of improving the internal compactness of the matrix is not obvious, and the critical strength of the internal fracture of the matrix cannot be improved. However, after the steel fiber is added to the concrete, the hard bridging of the steel fiber limits the volume change of the concrete at high temperatures, reduces the development of micro-defects in the concrete, and moderates the degradation of the high temperature performance of the concrete to some extent. From Fig. 21 (c), it can be seen that the internal density of steel fiber reinforced concrete is relatively uniform and there are few pores. There are three main points to the role of steel fiber in improving high-temperature aging performance. First of all, the steel fiber itself can increase the cohesion of the

Fig. 25. Thermal three-dimensional imaging of the specimen.

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matrix, limit the expansion of the concrete in the hightemperature aging environment, reduce the cracking and extension of the concrete interior, and to a certain extent, mitigate the deterioration of high-performance concrete at high temperatures. Secondly, due to the better thermal conductivity of steel fiber itself, it can make the concrete more uniform in internal temperature at high temperature, thereby reducing the internal stress generated by the temperature gradient, reducing internal damage, and suppress the volume change of concrete due to rapid temperature change. Ultimately, it reduces the generation and development of micro-defects inside the material. Third, since the steel fiber itself has a high strength, it can act as a micro-agitating rod during the stirring process, so that the local matrix can be stirred evenly to ensure sufficient hydration reaction. In addition, steel fibers can form a uniform chaotic support system inside the concrete. In this support system, the original pores communicate with each other, which makes it easier for water vapor inside the material to diffuse from the interior, and the corresponding pore pressure decreases, showing less cracking of the concrete. For the impact resistance, steel fiber can play a more excellent energy absorption effect, its deformation critical value is high, and it is not easy to produce elastic deformation. When in the region of the density, steel fibers can act as aggregates under the action of impact loads to bear partial the impact energy and play a weakening role in momentum. Therefore, metal fibers (SF) can better improve the wheel impact resistance of airport pavement concrete under high-temperature aging conditions.

stronger inhibitory effect on Dmn than SFC in the first stage test. In the second phase of the test, the inhibitory effect of SFC is more and more obvious, better than RPSFC. When the fiber is greater than or equal to 1.2%, the improvement of SF is always better than RPSF. From the fiber type point of view, the SF pair of metal fibers improves better. (5) In the paper, actual observation and comparative analysis of the external damage status of the specimens are carried out. Results show that under high temperature aging and wheel impact, the damage of inorganic steel fiber concrete (SFC) with volume content of 1.5%is significantly less than that of reticular polypropylene synthetic fiber concrete (RPSFC) with volume content of 1.2%. Consequently, it is recommended that the inorganic steel fiber concrete (SFC) with volume content of 1.5% be adopted in the application of airport construction. (6) By sampling the specimen for internal imaging, it was found that the fibers can significantly reduce the density and amount of micro-cracks in the large pores inside the concrete, making the whole more compact. Metal fiber SF can better relieve thermal stress and internal pore pressure due to high temperatures. Conflict of interest None.

7. Conclusion

References

In this study, the reticular polypropylene synthetic fiber concrete (RPSFC) (the representative of reticular polypropylene synthetic fiber concrete) and Steel fiber concrete (SFC) (the representative of inorganic matal fiber concrete) have been prepared. The experimental study was carried out for the anti-wheel impact performance of fiber reinforced airport pavement concrete under elevated temperature aging environment. The loss of antiwheel impact performance was divided into 2 kinds of indexes, namely apparent loss index and internal mechanical property loss index. The characteristics of internal mechanical property loss index were investigated. Moreover, the effect of fiber type on the apparent impact damage of pavement surface was analyzed. Furthermore, the optimal fiber variety and content which is the most beneficial to the pavement concrete were put forward. The main conclusions are as follows:

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(1) Incorporating fibers into concrete can effectively reduce the apparent loss of wheel impact and the loss of internal mechanical properties of airport concrete under hightemperature aging conditions. Within a reasonable range, Df ck , DEcn , DLcn and Dmn are all decreased with increasing fiber content. (2) For reticular polypropylene synthetic fiber concrete (RPSFC), the adding of organic fiber with volume content of 1.2% can significantly improve the anti-wheel impact performance of airport pavement concrete under elevated temperature aging environment; (3) For inorganic steel fiber concrete (SFC), the adding of metal fiber with the volume content of 1.5% can significantly improve the anti-wheel impact performance of airport pavement concrete under elevated temperature aging environment; (4) For Dmn , this index increases linearly with the number of test rounds and is divided into stage of 1–6 rounds and 7– 10 rounds. When the fiber is less than 1.2%, RPSFC has a

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