Steel slag is regarded as one of the most widespread solid by-products of steel smelting with little commercial value. It can play a vital role in the construction industry especially in the field of transportation infrastructure construction. However, there are few evaluation systems established on the high-temperature deformation and low-temperature fracture behavior of steel slag rubber asphalt mixture (SSRAM). This study explores the performance of SSRAM by uniaxial penetration test, Semi-Circular Bending (SCB) test and evaluates test data through regression analysis. The uniaxial penetration test results shows that the failure deformation of SSRAM increases with the increase of steel slag content. According to the minimum allowable permanent deformation (RTS-min), the deformation of SSRAM should be controlled within 3 mm. Meanwhile, the cracking index of the SSRAM surface layer calculated at low temperature can meet the design requirements. The SCB test results show that the stress peak degradation rate (specimens with 10 mm notch are compared with 0 mm) of SSRAM with 40% steel slag content is 20.04%. That means proper steel slag content makes the stress peak degradation rate of SSRAM reaches the lowest value. The calculation results of fracture energy density (J1C) show that the steel slag additive reduced the fracture energy density of SSRAM. However, it is still proved that SSRAM with 40% steel slag has the best low-temperature fracture performance based on critical fracture toughness (K1C) and fracture stress peak. Furthermore, the crack propagation velocity parametric equation of SSRAM is proposed through fracture mechanics theory and the increase of velocity is exponential. Considering the high-temperature deformation resistance and low-temperature fracture property, the SSRAM surface layer with 40% steel slag content showed a batter application potential.
Steel slag is regarded as one of the most widespread solid by-products of steel smelting, and it has been regarded as a challenge to environmental management in countries worldwide owing to the low recovery rate (less than 50%). Thus, a series of environmental issues are caused, such as land occupation, and groundwater pollution, which in turn endangers human health [
Numerous studies were conducted on steel slag which used in asphalt mixture, Ziaee et al. selected Electric Arc Furnace Slag (EAFS) as coarse aggregates in Warm Mix Asphalt (WMA) mixtures, and their test results showed that EAFS can improve the mechanical properties of WMA mixtures [
Over the past few decades, many investigation methods for asphalt mixtures was developed to evaluate the permanent deformation of asphalt mixtures, such as indirect tensile strength, Laboratoire Central des Ponts et Chaussees (LCPC) wheel tracking [
The fracture phenomenon can be emerged in asphalt pavement, especially under a low-temperature environment. Marshall test or field sampling are widely used to conduct indirect tensile test (IDT), which was usually adopted to evaluate the anti-cracking performance of HMA at intermediate and low temperatures [
The main purpose of this paper was to conduct the uniaxial penetration test and SCB test to evaluate high-temperature deformation and low-temperature fracture behavior of SSRAM, which is also aimed at taking forward this research area for implementation in expressway pavement designs. This study provides technical support and theoretical basis for the application of steel slag to road surface layer.
Steel slag and basalt are used as coarse aggregate (The particle size is greater than 4.75 mm) in this paper. The immersion expansion rate of the steel slag meets the requirements of the specification. And the material composition design of steel slag and basalt is adapted from previous studies [
The AC-13C gradation was adopted to prepare SSRAM (
Content of steel slag (%) | Gross volume density (g/cm3) | VV |
VFA |
VMA |
Stability |
Flow value |
Asphalt-aggregation ratio (%) |
---|---|---|---|---|---|---|---|
0 | 2.564 | 3.9 | 74.3 | 15.2 | 9.4 | 30 | 4.95 |
20 | 2.582 | 3.9 | 74.8 | 15.5 | 9.6 | 28 | 5.15 |
40 | 2.617 | 3.8 | 75.6 | 15.6 | 9.5 | 27 | 5.34 |
60 | 2.633 | 3.9 | 75.3 | 15.8 | 9.5 | 29 | 5.48 |
80 | 2.641 | 3.8 | 76.2 | 16.0 | 9.2 | 31 | 5.57 |
100 | 2.646 | 3.8 | 76.4 | 16.1 | 9.1 | 33 | 5.65 |
The fluorescence micromorphology of rubber asphalt stirred by an impeller agitator for 1 h is presented in
Following the standard test method in specification JTG D50-2017, the test temperature and loading rate were 60°C and 1 mm/min, respectively. Following the standard test method T 0702-2011, the asphalt mixture specimens with different steel slag contents were prepared.
The SCB test is used to characterize the fracture resistance of asphalt mixtures based on a fracture mechanics theory. For linear elastic materials, K is a function of the applied stress and geometric factor of the specimen, which increases as the applied stress increases and reaches a critical value (K1C) when the failure occurs. Besides, K1C also termed fracture toughness is an intrinsic property of the material, which helps explain the fracture resistance property of the material [
Following the specification JTG D50-2017, the uniaxial penetration strength of the asphalt mixture should meet the requirements of
where [Ra] is the allowable permanent deformation of asphalt mixture, Ne5 is cumulative equivalent single axle loads for design lane, Td is design temperature,
Content of steel slag (%) | The elastic stage | The yield stage | Uniaxial penetration strength (MPa) | ||
---|---|---|---|---|---|
Stress (kN) | Displace (mm) | Stress (kN) | Displace (mm) | ||
0 | 2.47 | 0.53 | 5.21 | 1.40 | 1.71 |
20 | 2.54 | 0.61 | 5.36 | 1.47 | 2.13 |
40 | 2.67 | 0.72 | 5.85 | 1.76 | 1.96 |
60 | 3.13 | 0.69 | 6.58 | 2.40 | 1.74 |
80 | 2.41 | 0.64 | 5.13 | 2.51 | 1.49 |
100 | 2.12 | 1.14 | 4.55 | 3.47 | 1.17 |
According to the specification, the penetration strength of the generally modified asphalt mixture is 0.7~1.2 MPa. The uniaxial penetration strength of SSRAM is higher than that. Based on the design parameters of the expressway in Hebei Province and the penetration strength of SSRAM, the minimum allowable permanent deformation (RTS-min) of the SSRAM surface layer can be obtained by
Content of steel slag (%) | RTS-min (mm) | Total rutting depth (mm) | Failure deformation |
---|---|---|---|
0 | 2.83 | 1.988 | 1.40 |
20 | 2.63 | 2.061 | 1.47 |
40 | 2.67 | 2.321 | 1.76 |
60 | 2.83 | 2.728 | 2.40 |
80 | 3.01 | 3.076 | 2.51 |
100 | 3.26 | 2.969 | 3.47 |
The data in
To make SSRAM meet the requirements of pavement structure design in some seasonal permafrost areas, the low temperature cracking index (CI) should be calculated by
where T is the mean of the lowest temperature for 10 consecutive years, St is the creep stiffness of asphalt BBR test at particular temperature range required by the specification when loading 180 s, ha is the thickness of asphalt mixture layer, b is the subgrade type parameter.
Referring to the results of creep stiffness in the BBR test of rubber asphalt [
Judging from the low-temperature stress-displacement curve, it is in line with elastoplastic fracture, which should be analyzed from the theories of elastoplastic fracture mechanics. The plane stress yield model proposed by Dugdale, which is similar to the Barenblatt model, assuming that the yield of the material is limited to a narrow area directly in front of the crack tip. In the fractured plastic zone, the cohesive force acts on the surface of the expanding crack to inhibit crack opening and crack growth. This distance can be regarded as the crack plastic zone (
where
The opening displacement of the crack tip can be solved from the stress-displacement curve and J integral (
When the asphalt mixture yields incompletely plastic in the fracture process, the stress-strain curve is shown in
where σmax is the maximum stress, Fmax is the maximum load, K1c is the fracture toughness, a is the initial notch depth and W is the specimen’s radius.
Combined with the SCB test results and
The stress peak of SSRAM at different depth notches is shown in
Content of steel slag (%) | 0 mm notch | 5 mm notch | 10 mm notch | |||||
---|---|---|---|---|---|---|---|---|
Force (N) | Stress (MPa) | Force (N) | Stress (MPa) | Decrease (%) | Force (N) | Stress (MPa) | Decrease (%) | |
0 | 4186.50 | 6.07 | 3157.16 | 4.58 | 24.59 | 2055.30 | 2.98 | 50.91 |
20 | 4447.39 | 6.52 | 3286.31 | 4.81 | 26.11 | 2697.61 | 3.95 | 39.34 |
40 | 4100.64 | 5.95 | 3582.71 | 5.19 | 12.63 | 3278.77 | 4.75 | 20.04 |
60 | 4048.08 | 5.87 | 3289.41 | 4.77 | 18.74 | 2830.54 | 4.10 | 30.08 |
80 | 3864.98 | 5.66 | 2830.54 | 4.15 | 26.76 | 2486.95 | 3.64 | 35.65 |
100 | 3813.59 | 5.53 | 2684.70 | 3.89 | 29.60 | 2028.20 | 2.94 | 46.82 |
Fracture toughness can characterize the strength of the stress field at the crack tip. The fracture toughness of SSRAM can be calculated by
Content of steel slag (%) | K1C(MPa · m1/2) | |||
---|---|---|---|---|
5 mm notch | COV (%) | 10 mm notch | COV (%) | |
0 | 0.3979 | 7.3 | 0.6107 | 4.5 |
20 | 0.4184 | 8.1 | 0.8098 | 5.8 |
40 | 0.4515 | 5.8 | 0.9742 | 4.6 |
60 | 0.4146 | 3.9 | 0.8410 | 2.2 |
80 | 0.3604 | 5.2 | 0.7466 | 5.4 |
100 | 0.3383 | 4.7 | 0.6026 | 3.7 |
In the SCB test, the covariance value of the test results of each group was less than 10%, thereby the test results were considered stable.
Fracture energy means the energy absorbed by the specimen during the fracture process. Within a certain range, the higher the fracture energy of the asphalt mixture, the better the crack resistance of the mixture.
Content of steel slag (%) | Fracture energy (J) | COV(%) | J1C (J/mm2) | |
---|---|---|---|---|
5 mm notch | 10 mm notch | |||
0 | 8.94 | 7.33 | 4.36 | 0.0805 |
20 | 6.95 | 5.39 | 7.84 | 0.0780 |
40 | 7.89 | 6.51 | 8.89 | 0.0697 |
60 | 7.34 | 6.15 | 8.36 | 0.0598 |
80 | 5.87 | 4.64 | 6.13 | 0.0615 |
100 | 5.62 | 4.49 | 4.45 | 0.0565 |
During the SCB test, a laser displacement sensor was installed to record the displacement-time curve of the crack. Through the fitted curves, the crack propagation velocity more intuitively can be calculated. Velocity, acceleration, fracture stress peak, J1C values, and fracture toughness K1C can be more comprehensive to evaluate the low-temperature crack resistance of SSRAM. The crack’s Displacement-Time curves are shown in
The crack propagation velocity and acceleration continuously increased before the fracture stress reached the peak. It can be considered that the crack propagation velocity conforms to the exponential function image. Then, a parameter equation applicable to the crack propagation velocity of SSRAM was proposed (
It can be seen from
The low-temperature fracture properties of the SSRAM surface were evaluated from each of these perspectives, and the SSRAM surface layer with 40% steel slag content showed a batter application potential in highway construction engineering.
In this study, the uniaxial penetration test and SCB test were carried out. Following the test results and specification parameters, the uniaxial penetration strength, allowable permanent deformation, low temperature cracking index, fracture toughness, fracture energy and crack propagation velocity of SSRAM with different steel slag contents were calculated. The high-temperature deformation and low-temperature fracture behavior of SSRAM were analyzed from various aspects. The main conclusions are shown below: The dynamic stability of all specimens meet the specification requirements. The SSRAM with 40% steel slag content has the highest dynamic stability in rutting test while SSRAM with 20% steel slag content has the best anti-shearing strength in uniaxial penetration test. Considering the allowable deformation of asphalt mixture, the rutting depth and failure deformation of SSRAM need to be controlled within 3 mm to meet the requirements of the expressway. The low-temperature cracking index of the SSRAM surface layer is calculated as 1.37 and the SCB test results show that the stress peak degradation rate (specimens with 10 mm notch are compared with 0 mm) of SSRAM with 40% steel slag content is 20.04%. That means proper steel slag content makes the stress peak degradation rate of SSRAM reaches the lowest value. The fracture toughness of the specimen with 5 mm notch is less than specimen with 10 mm notch. The SCB test results proved that the fracture toughness of the specimen is not only affected by the notch depth, but the fracture stress of the crack has a significant effect on the fracture toughness of the SSRAM surface layer. With the increase of the crack depth, the fracture energy of the mixture gradually decreases. The aggrandization of steel slag content leads to the decrease of fracture energy density (J1C), it may be one of the main factors preventing SSRAM with high steel slag content from being used extensively in asphalt pavement. The crack propagation velocity and acceleration continuously increased before the fracture stress reached the peak. It can be considered that the crack propagation velocity conforms to the exponential function. Then, a parameter equation applicable to the crack propagation velocity of SSRAM was proposed in this study. Considering the high-temperature deformation resistance and low-temperature fracture property, the SSRAM surface layer with 40% steel slag content showed a batter application potential.