Permeable asphalt pavement refers to the fully permeable pavement in which rainwater permeates into the ground through subgrade or discharges through buried drainage facilities [1
]. Porous asphalt pavement refers to the semipermeable pavement in which rainwater penetrates into the surface course and is horizontally drained by buried pipelines. However, referring to the literature in recent years, the difference between them is not obvious [2
]. Permeable asphalt mixtures (PAM), as sustainable pavement materials, are credited with various environmental benefits, such as urban floods alleviation by promoting the natural infiltration of rainwater, underground water quality improvement by treating pollutants, and heat-island effect mitigation by facilitating heat transfer processes [3
]. However, their poor low-temperature cracking resistance has become a barrier against the wide application of PAM [5
Steel slag is the main byproduct of the steel manufacturing industry [7
]. According to the data from the World Steel Association, the crude steel production output of China in 2019 has reached 996.3 million tons [8
], but the utilization rate of steel slag in China is only 29.5% [9
]. A large number of steel slags have been disposed of as waste, leading to a series of serious social and environmental problems, such as the occupation of limited land, heavy metal pollution of groundwater, and dust pollution of the air [10
]. Due to the excessive exploitation of natural aggregate, limited natural aggregates cannot satisfy the increasing demand for road construction [12
]. Therefore, it is urgent to find a high-quality and low-cost alternative for natural aggregate [14
]. The mechanical performance of PAM is poor and can be enhanced by high-quality aggregates [15
]. Fortunately, steel slags can be served as an attractive substitute for natural aggregates due to its superior characteristics including low cost, high wear resistance, toughness, multi-angularity, and good adhesion with bitumen, compared with other aggregate substitutes [16
Owing to the superior characteristics of steel slag, its application in PAM has received extensive attention in recent years. Skaf et al. [18
] carried out a series of tests to examine the effect of steel slag on the pavement performance of PAM, including mechanical performance (abrasion loss and indirect tensile strength), durability (cold abrasion loss, aging, and long-term behavior), water stability, skid and rutting resistance, and permeability. The results showed that the addition of steel slag significantly improves the permeability, skid resistance, durability, and rutting resistance of PAM. Rodríguez-Fernández et al. [19
] and Chen et al. [20
] found that steel slag can improve the economy and environmental sustainability of PAM by life-cycle cost analysis (LCCA). Existing research studies about the low-temperature behavior of PAM focused on natural aggregate, but few focused on steel slag [21
]. Therefore, the effect of steel slag on the low-temperature behavior of PAM needs to be studied. The low-temperature fracture behavior of PAM with steel slag was complicated, owing to its non-uniformly graded skeleton and interlocking structure. Therefore, a simple, feasible, and real-time fracture monitoring technique is needed.
Acoustic emission (AE), as a nondestructive testing (NDT) and real-time monitoring technique, has great potential for the identification of failure stage, the classification of cracks, and the location of cracking sources [23
]. AE is a phenomenon that generates transient elastic waves by the rapid releasing of internal energy from the localized source or sources of materials, which is related to the local irreversible changes, including plastic deformation, crack initiation and propagation, etc. [26
]. AE technique has been used extensively for damage assessment and health monitoring in diverse fields, including metal, rock, concrete, and bridge, etc. [28
]. However, relatively limited research works have focused on the damage assessment of asphalt materials using the AE technique. Behnia et al. [30
] proposed an AE-based method to determine the embrittlement temperature (TEMB
) of asphalt binders. The results showed that the amount of pavement cracks is proportional to the difference between TEMB
and performance grade (PG) low temperature of asphalt binders. Wei et al. [31
] utilized the AE parameters, including amplitude, ringing count, AE energy, and average frequency, to divide the failure stage of asphalt materials under low temperature. The spatial evolution process of internal micro-cracks was tracked by the AE localization technique. Qiu et al. [32
] utilized AE energy and b
-value to monitor and evaluate the fatigue damage behavior of asphalt mixtures. Then, the fractal dimension of the AE waveform after wavelet transform was used to identify the damage critical state. The results revealed that the energy distribution of AE waveform can be used to characterize the irreversible damage mechanism associated with Kaiser and the Felicity effects. Sun et al. [34
] studied the relationship between AE parameters and oxidation aging performance of asphalt mixture. The existing research mainly focused on the application of the AE technique in dense asphalt mixture (DGM), but it was rarely used in PAM. The characterization of the cracking mechanism of PAM is a complicated and challenging subject [35
]. Nowadays, some studies demonstrated the feasibility of the AE technique in PAM with heterogeneity and stone-on-stone interlocked structure, and reported that AE possesses great potential in real-time crack monitoring and fracture characterization of PAM, especially in complicated environments. Jiao et al. [37
] investigated the damage evolution and fracture modes of PAM under low-temperature splitting with the aid of the AE technique. Cai et al. [39
] found that the incorporation of lignin fiber can significantly delay the growth of shear cracks along the interface under uniaxial compression by employing the AE technique, and stated that AE can effectively compensate for the deficiency of mechanical experiments and theoretical models. Chai et al. [40
] carried out low-temperature splitting and AE tests on the specimens with different freeze-thaw cycles. The results showed that AE parameters can effectively identify the damage stage of PAM under freeze-thaw cycles. Therefore, the previous research provided experience and guidance for the application of the AE technique in PAM pavement.
However, the low-temperature fracture behavior of PAM with steel slag based on the AE technique has not been discussed in existing research, and especially the b-value analysis has not been involved. Therefore, the principal purpose of this paper was to investigate the improvement mechanism of steel slag on the low-temperature fracture behavior of PAM. Steel slag replacement level was designed from 0% to 100% (25% increment). Low-temperature splitting test with slow loading (1 mm/min) was used to obtain steady and slow crack growth, which can be detected employing the AE technique. Firstly, the relationships between mechanical characteristics and AE activities for each group were described by the sentry function. Then, the shear and tensile modes were characterized by the relationship between the rise angle (RA) and average frequency (AF). Finally, the relative variations of micro-cracks and macro-cracks were identified by the b-values. The results obtained in this study provided a basis for the superior low-temperature cracking resistance of permeable asphalt mixtures with 100% steel slag (SS-100).
In this study, the low-temperature fracture behavior of PAM with steel slag was monitored in real time by employing the AE technique. The sentry function, RA, AF, and b-values were used as indexes to evaluate the fracture behavior. Based on the test results, the following conclusions could be drawn:
1. The cumulative AE energy increased with the increase of steel slag replacement level. A slight bottom-up trend of sentinel function was observed in the 0.6–0.9 displacement level for SS-100, which is associated with an enhanced strain energy storing capability generated by the interlocking structure of steel slag coarse aggregates.
2. The fracture stages of the control group and SS-100 could be divided based on the slope changes of the cumulative RA and cumulative AF curves. The incorporation of 100% steel slag reduced the shear events and restrained the growth of shear cracking of the specimen in the macro-crack stage.
3. The considerable drops of three kinds of b-values in the final phase were registered in the control group, but there were significant repeated fluctuations in SS-100, which are associated with the brittle failure of the control group and the ductile failure of the steel slag group, respectively. The sharp drops and significant fluctuations of b-values could serve as a serious damage alert of the control group and SS-100, respectively.
4. The fracture behavior of PAM was improved significantly after adding 100% steel slag. AE technique possessed great potential for real-time characterizing of fracture behavior of PAM containing steel slag and was expected to be further applied in pavement engineering.