It is common knowledge that China is rapidly developing its national infrastructure to support its economic growth. Well-known projects such as the Three Gorges Dam are symbolic of the interdependence of economic growth and civil infrastructure. Less well known, but of significant importance, is recent construction and expansion of the Chinese national highway network. Since 2001, China has spent more on transportation infrastructure than in their previous 50 years. Specifically, expressway mileage grew 65%, from 15,350 miles to 25,480 miles between 2001 and 2005, and it is expected that the expressway mileage will approach 53,000 miles by 2020.
With the rapid growth comes the need for a dependable transportation network with quality pavements to support the economy and a rapidly mobilizing population over the next 50 years. Pavements designed and built today must perform under heavy and high-volume traffic that is expected to only increase in the coming years.
Historically, Chinese provincial governments have relied upon both flexible and rigid pavements on their expressways. However, as traffic volume and weights have continued to increase, their conventional designs are no longer performing at the desired level. For example, pavement failures in five to eight and three to five years for rigid and flexible pavements, respectively, are not uncommon. Expecting their system to expand by 25,000 miles in the next 14 years, pavement engineers in China are looking for ways to meet the traffic demands and improve the life-cycle cost of their transportation system.
In November 2004, a team of U.S. pavement engineers and researchers visited Shandong Province to discuss the application of flexible perpetual pavements under the extreme traffic conditions in China. The U.S. team represented the Federal Highway Administration (FHWA), the Indiana Department of Transportation, the Virginia Transportation Research Council, the National Center for Asphalt Technology (NCAT) and Heritage Research Group. The Chinese delegation represented the Shandong Highway Bureau and the Shandong Transportation Research Institute. During the course of this visit, a perpetual pavement experiment was proposed for construction as part of the Bin-Bo expressway. This expressway, when completed, will connect Shanghai to the major port city of Tianjin. The main objective of the experiment was to construct a number of perpetual pavements, along with some control test sections, so that a better understanding of perpetual pavements could be developed. More specifically, the experiment will test the perpetual pavement concept under more extreme Chinese loads. From this, a set of recommendations for further perpetual pavement design could be established and applied to other projects in China. Additionally, it is expected that the findings will provide valuable data that will greatly assist the Shandong Highway Bureau in future pavement designs as they continue to expand the expressway system.
The project is located in east central China, about 200 miles southeast of Beijing. Generally speaking, Shandong Province is low-lying, with an elevation less than 330 ft above sea level. The soil in the area is mainly river-deposited silt that is used to build the 10- to 20-ft-high embankment on which the pavement structure is constructed.
Traffic data collected from an onsite weigh-in-motion (WIM) station clearly illustrate the extreme loading conditions in China. The WIM was installed as part of the study and will gather valuable data regarding traffic volume and load distributions. The average weights of the larger trucks (four-plus axles) exceed 70,000 lb. Of particular interest are the S1.2.2 vehicles, a standard five-axle semi-trailer comparable to Class 9 vehicles in the U.S., as they represent 33% of the heavy traffic volume and have an average gross vehicle weight of 122,000 lb. This is nearly double the gross vehicle legal limit in the U.S.
The average single-axle load is approximately 19,000 lb, which is nearly the legal limit for U.S. federal routes. Even more significant is the average tandem-axle load of 43,000 lb, which is 9,000 lb heavier than the U.S. legal limit. Translated into damage using the generalized fourth-power rule, an empirical relationship between axle load and pavement damage, the average tandem axle in China would cause 2.7 times more damage than the maximum legal tandem axle in the U.S. The extreme traffic is one of the unique aspects of this project, challenging traditional western pavement design and materials and allowing for performance evaluation under extreme conditions.
Licking the leaks
The Shandong Highway Bureau dedicated five pavement sections, each 1 km long, to the perpetual pavement experiment. The sections shown in Figure 3 were developed using load distributions and a limited amount of material property data provided by the Chinese team.
The two control sections comprised relatively thin HMA on top of cement-stabilized granular layers. This design is commonly used on expressways in China. After investigation and discussion, the team came to believe that the early pavement failures may be due to excessive stresses imposed on this weak “concrete” layer that cracks and rapidly causes reflection cracks to appear at the pavement surface, allowing moisture infiltration and subsequent moisture-related damage.
The three experimental sections were designed as full-depth hot-mix asphalt (HMA) sections with some common perpetual pavement features. The team believes that this design, which omits the cement-stabilized granular layer, will avoid the overstressing and consequent early cracking. The stone-matrix asphalt (SMA) and dense-graded Superpave layers commonly used in China were chosen for their rut resistance. An open-graded drainage layer was used within each section to help remove moisture and mitigate stripping problems. Moisture damage (stripping of asphalt binder from the aggregate) has been observed in several failure investigations of pavements in the Shandong Province that had been in service for five to eight years.
Reducing moisture infiltration or providing an outlet for internal pavement drainage has been shown effective in reducing moisture-related distress. Therefore, the drainage layer was included in the perpetual pavement sections. It should be noted that Control Section 1 also included an open-graded drainage layer. Each of the three experimental sections included a so-called rich bottom layer that was placed at 0.6% above the optimum asphalt content. Based upon previous studies, it is believed that the added asphalt content will improve the density and overall fatigue resistance of the pavement where the tensile strains are the highest. The “125 me (modified)” section has a polymer-modified binder that is expected to further improve the fatigue resistance.
The thicknesses of the experimental sections were designed using the perpetual pavement design software PerRoad 2.4. This mechanistic-empirical pavement design software enables engineers to incorporate critical pavement response thresholds in the design of flexible pavements. The first experimental section was designed with a critical tensile strain threshold of 70 me to control fatigue cracking at the bottom of the asphalt layer. At the top of the lime-stabilized layer, 200 me was used to control rutting. Given the materials and traffic conditions, this resulted in 20 in. of asphalt pavement. The next two experimental sections used less conservative tensile strain thresholds (125 me), which resulted in 15 in. of asphalt pavement. The 125 me(modified) section utilized a higher grade of asphalt in the rich bottom to evaluate its effectiveness in controlling fatigue cracking.
A major aspect of the experiment was instrumentation that was embedded during construction. As shown in Figure 3, strain gauges and pressure plates were placed at depths to measure pavement responses critical to perpetual pavement performance (i.e., horizontal strain at the bottom of the asphalt layer and vertical stresses at the top of granular layers or soil). The temperature sensors, also shown in Figure 3, provide a critical link between environmental conditions and pavement response. The embedded instrumentation will provide comparisons between theory and field as well as among the five different designs without having to wait for observed damage.
The test sections were constructed during the summer of 2005, and engineers from NCAT and the Heritage Research Group were onsite to provide technical assistance. Rolling patterns were established on trial mixes and two pavers were used in tandem to pave widths up to 40 ft. During construction, HMA samples were obtained from the delivery trucks and shipped to the U.S. for further testing.
The project opened to traffic in December 2005. Since then, data collection efforts have focused upon characterizing live traffic distributions and pavement response analysis under a known test vehicle in various environmental conditions. The use of control vehicles allows for a more direct comparison among sections as well as providing reliable traffic to evaluate the design and performance of the test sections among one another and over time. Some of the important research goals supported by control-vehicle testing are:
- Compare response among test sections;
- Validate structural design methodology;
- Predict structural performance of the test sections;
- Enhance theoretical models;
- Characterize seasonal and temperature effects on pavement response; and
- Evaluate load-response interaction.
The control-vehicle testing will be followed by testing under live traffic conditions in addition to quarterly testing with the control vehicle. Laboratory testing is currently being performed in both the Shandong Province and in the U.S. to determine fatigue and stiffness properties. This testing was expected to be completed during the summer of 2006.
While the core objective of this project is to better understand traffic-loading conditions and their effect on pavements in China, it is expected that the results also will directly benefit perpetual pavement research and practice in the U.S.
Testing under the heavy traffic loads will give U.S. researchers and practitioners a better understanding of flexible pavement performance in extreme traffic conditions. Findings from this research may help guide future decisions toward legal limits and overload permitting here in the U.S.