1. Introduction
Under the influence of repetitive traffic loading and changing climatic conditions, surface function distresses of asphalt pavement occur, such as cracking, roughness and insufficient skid resistance, which have significant influences on the comfort and safety of traffic [
1]. Therefore, preventive maintenance is prevalent due to the improvements of surface functions and lower initial costs. By conducting preventive maintenance in time, the surface conditions and the original pavement durability can be enhanced [
2]. At present, preventive maintenance techniques, including fog seal, slurry seal, micro-surfacing, chip seal, and thin overlay, are used widely [
3]. Of these surface treatments, thin overlays are beneficial to improving the defects of roughness and skid resistance. In addition, this surface treatment adopts hot asphalt binder and is more effective in shortening the open time compared with asphalt emulsion treatments, such as micro-surfacing [
4].
In this study, a kind of small particle porous ultra-thin overlay (PUTO) is proposed, which has a thickness of 1.5–2.5 cm and a large void ratio of 18–25% [
4], and combines the functions of the ultra-thin overlay and the porous drainage asphalt mixture. After the construction of PUTO, interconnected air voids are formed, which play an important role in draining away rainwater and thus improving driving conditions in wet weather. Furthermore, the high air voids in the small particle PUTO is beneficial to reducing traffic noise and improving driving comfort. Therefore, the structural characteristics of the small particle PUTO are prerequisites for achieving satisfactory performance, which are directly related to the construction quality [
5]. To ensure the satisfactory formation of the interconnected structure, the construction parameters including compaction equipment, compaction temperature, and compaction scheme, should be determined according to the design parameters.
As a paramount factor in asphalt pavement construction, the compaction process significantly influences the structural formation of asphalt mixtures and the durability of asphalt pavement. During ultra-thin overlay construction, the temperature of the asphalt mixture decreases quickly upon being placed on the original pavement due to the thickness of ultra-thin overlay, which put forwards stricter construction requirements than traditional asphalt mixtures. According to the existing studies, the improper compaction parameters will worsen the performance of pavement in several aspects [
6]. To be specific, lower compaction temperature and insufficient compaction energy result in greater air void content, which gives rise to the premature failure of the mixture due to the inferior strength [
7]. In contrast, excessive compaction contributes to the loss of drainage function due to the reduction of air voids and failure in the formation of an interconnected structure. Therefore, the construction process and the compaction parameters should be given great importance.
However, few studies have stressed the correlation between laboratory and in-field compaction parameters, as a result, the compaction strategies are generally determined based on construction experience [
8]. Currently, the Marshall, Hveem, Bailey, Coupe Aggregate Void Filling (CAVF) and Superpave design methods are the most common design methods for asphalt mixtures. Of these design methods, due to the easy access to test equipment, the Marshall method has been widely used throughout the world since the 1940s [
9]. Based on the Marshall design method, Xu [
10], Lee [
11], and Vackova et al. [
12] studied the influences of compaction energy and compaction temperature on specimen performance, including the volumetric properties, the Marshall stability, and the dynamic stability. Furthermore, several studies have been conducted to characterize the correlation of mechanical properties of laboratory specimens and site cores [
13,
14]. In addition, Micaelo [
15], Kassem [
16] and Masad et al. [
17] analyzed the relationship between different field compaction strategies and laboratory asphalt mixture properties based on digital image processing, field tests and X-ray computed tomography.
Considering the lack of recommended construction scheme in laboratory and on-site for PUTO in the existing specifications, it is necessary to conduct a comprehensive study on both laboratory fabrication and field compaction parameters to supplement the existing specifications. In this study, the laboratory fabrication and on-site compaction scheme were investigated and determined, respectively. Because of the dominant position of the Marshall method in Chinese asphalt pavement design standards and easy access to the experimental equipment, the Marshall method was adopted in this study to fabricate PUTO specimens. The influences of compaction parameters on the raveling resistance of a Marshall sample were analyzed by the Cantabro test. Then the correlation between laboratory and on-site compaction work was established based on the energy equivalent principle. On this basis, on-site compaction schemes were determined and a test section was paved.
The objection of this paper is to determine field compaction parameters of porous ultra-thin overlays proposed in [
4], which can be helpful for its construction plan designing. This paper is organized into the following sections.
Section 2 shows properties of materials used in this paper and related materials preparation works. The determination of laboratory and on-site compaction schemes is presented in
Section 3 and
Section 4.
Section 5 validate the performance of the paved test section, and
Section 6 concludes the study.
4. Correlations of Laboratory and On-site Compaction Schemes
Based on the above studies, the optimal compaction schemes, including number of blows and compaction temperature, of the four kinds of mixtures are determined. To further confirm the on-site compaction schemes, the principle of energy equivalence was used to establish the correlation between laboratory and on-site compaction energy, as shown in Equation (1). In particular, the compaction energy of Marshall samples was calculated and the compaction energy of the construction equipment was determined. Then, the compaction schemes were calculated according to the necessary compaction energy, which enables the mixtures to be compacted with a void ratio of 20%.
where
E0 is the energy produced by the compaction hammer falling once, J;
m is the quality of the hammer;
g is the acceleration of gravity, and 9.8 m/s
2;
h is the height at which the hammer is dropped, m.
4.1. Calculation of Compaction Energy in Laboratory
A standard Marshall compactor was used to compact the samples in the laboratory. During compaction, a hammer is freely dropped in the vertical direction and continuously compacts the sample to provide the energy required for the Marshall sample to reach a predetermined degree of compaction. The energy calculation method produced by a single compaction is shown in Equation (1). According to the calculations, the energy generated by a single compaction of the hammer is 20.32 J. To achieve a target void ratio of 20%, the number of blows for PAC-1-I, PAC-2-II, PAC-2-I, and PAC-2-II samples were 53, 59, 50 and 50 respectively, and the corresponding compaction works were 1077 J, 1199 J, 1016 J and 1016 J respectively. Obviously, to achieve the same level of compaction, an asphalt mixture with coarse gradation requires more compaction than one with fine gradation. On one hand, the asphalt-aggregate ratio used in the fine gradation is large, and the asphalt film wrapped on the aggregate surface plays a certain lubricating role in the compaction process, which makes the rearrangement between particles easier. On the other hand, due to the larger proportion of coarse aggregates in the coarse gradation, free movement, and mutual intrusion between aggregates with the same volume are more difficult, increasing internal frictional resistance.
4.2. Calculation of Compaction Energy on Site
The compaction of asphalt pavement is generally divided into three stages: preliminary compaction, re-compaction, and final compaction. Since the purpose of preliminary compaction is to make the loose mixture relatively stable, small-tonnage compaction machines are mainly used. Re-compaction is a key process in compaction and plays a major role in the compaction of the asphalt mixture. Static compaction or vibratory compaction is generally selected, as needed. The final compaction is to eliminate previous rolling traces and further stabilize the aggregate. Generally, a rubber or steel roller is used to roll the surface pavement.
4.2.1. Compaction Energy Generated by Paver
Before being compacted by a steel roller or rubber roller, the loose asphalt mixture is pre-compacted by the screed at the rear of the paver. It also plays a significant role in the process of mixture compaction. A study [
22] indicates that the energy provided by the paver screed to the asphalt mixture is equivalent to the energy produced by a Marshall sample being compacted 15 blows in the laboratory. Therefore, the pre-compaction energy provided by the paver for the mixture is calculated and shown in Equation (2).
where
Ep is the energy of paver pre-compacting, J; and
n is the number of blows.
4.2.2. Compaction Energy Generated by Steel Roller
The steel roller generally has the dual functions of static and vibration compaction. Static compaction relies on the static pressure generated by its own weight to cause the relative movement of the aggregate. Vibratory compaction causes the material to resonate with the steel wheel through vibration impact force, and the exciting force propagates further in the depth direction, so that a thick mixture can also obtain a better compaction effect. However, a large vertical exciting force may cause coarse aggregates on the surface of the pavement to be crushed, and “white spots” on the pavement surface appear. To avoid this phenomenon, and taking into account the thin structural features of the overlay, vibration compaction is not used in the compaction process. The energy absorbed by asphalt mixture for one pass (back and forth) can be obtained by changing the height of the asphalt pavement, as shown in Equation (3):
where
Es is the energy provided by the steel roller for 1 pass, J;
Fs is the weight of the steel roller, N; and
hs is the pavement thickness variation with the steel roller rolling once, m.
According to the measured density change of the mixture during construction process, the height change for one pass of the asphalt mixture can be calculated. After the mixture was pre-compacted by the paver, it was then compacted for 4 blows by a steel roller, and the height change (Δ
h) of the pavement was 0.2 cm, and the average height change (
hs) per compaction was 0.05 cm [
23].
4.2.3. Compaction Energy Generated by Rubber Roller
Due to the elasticity of the rubber pneumatic tire, a rubber roller applies horizontal and vertical forces to the pavement, thereby causing a smashing effect on the asphalt mixture. However, since the horizontal force does not directly affect the enhancement of the pavement density, only vertical force is considered in the calculation. Similar to the static pressure of a steel roller, the rubber roller also compacts the mixture by gravity. The work done on the asphalt mixture for one pass is
Er, and can be obtained by Equation (4):
where
Er is the energy provided by the rubber roller for 1 pass, J;
Fr is the weight of rubber roller, N;
hr is the pavement thickness variation of steel roller rolling once, m.
After the rolling of the rubber roller, it can also be calculated that the average height change of pavement compacted by the rubber roller is about 0.04 cm (
hr = 0.04 cm), based on the measured density change of mixture at the construction site [
22].
4.3. Determination of Compaction Schemes on Site
Based on the principle of energy equivalence, it is known that an asphalt mixture requires energy to achieve a predetermined degree of compaction, and the energy is absorbed by the asphalt mixture from the paver, steel roller and rubber roller, respectively. Therefore, the number of rolling passes of the steel or rubber roller is determined. A DD130 steel roller and XP301 rubber roller were used in the paving process of this test section, and their masses are 13.442 t and 30 t respectively. According to the above equation, the energy provided by rolling one pass with the steel roller and rubber roller is 65.9 J and 117.6 J, respectively. Taking the PAC-1-I asphalt mixture as an example, the required compaction work is 1077 J when the target void ratio is 20%, of which 304.8 J is provided by the paver, and the remaining 772.2 J is provided by the compaction machine. Therefore, the number of rolling passes of the steel and rubber roller can be calculated by Equation (5):
where
E is the energy required for the asphalt mixture to reach a predetermined density, J;
ns is the number of rolling passes of the steel roller; and
nr is the number of rolling passes of the rubber roller.
The on-site specific rolling number was calculated according to Equation (5), and the two best calculated compaction methods for each asphalt mixture were selected. As shown in
Table 7, the calculated number of passes for the PAC-1-I asphalt mixture is 3 and 5 or 5 and 4, respectively (
ns = 3,
nr = 5 or
ns = 5,
nr = 4). The calculated number of passes of the PAC-1-II asphalt mixture is 3 and 6 or 5 and 5, respectively (
ns = 3,
nr = 6 or
ns = 5,
nr = 5). The number of rolling passes for the PAC-2-I and PAC-2-II asphalt mixtures are 2 and 5 or 4 and 4, respectively (
ns = 2,
nr = 5 or
ns = 4,
nr = 4). The final on-site specific number of passes for the steel roller and rubber roller can be rationally determined according to engineering experience and the compaction machinery inventory.