0016 Study on the Rational Thickness of Surface Course on Semi-rigid Base Asphalt Pavement
Study on the Rational Thickness of Surface Course on Semi-rigid Base Asphalt
Pavement
Ji-shu Sun1Tian Xiao2 Yuan-ming Dou3 Chun-feng Yang3
1Doctoral student, Dept. of Civil Engineering, Hebei University of Technology, Guangrong Road #8, Tianjin 300130, People’s Republic of China; Tel:(86)022-********; E-mail: sunjishu76@https://www.wendangku.net/doc/f65556524.html,
2 Senior Engineer, Tianjin Municipal Engineering Design ＆Research Institute, 239 Yingkou Street, Tianjin 300051, People’s Republic of China; PH (86)022-********; E-mail:sjshebut@https://www.wendangku.net/doc/f65556524.html,
3Professor, Dept. of Civil Engineering, Hebei University of Technology, Guangrong Road #8, Tianjin 300130, People’s Republic of China; Tel: (86)022-********; E-mail: douyuanming@https://www.wendangku.net/doc/f65556524.html, ABSTRACT:Proceeding from the mechanical characteristics of semi-rigid base asphalt pavement, using pavement calculating program, flexural tensile stresses at the bottom of subbase in the condition of different thickness of surface course, base and subbase were calculated respectively in this paper. The influence of different material modulus on the flexural tensile stress at the bottom of every layer was studied. Then, the state of shear stress distribution in the asphalt surface layer under different surface course thickness was analyzed. The purpose was to investigate the effect of surface course thickness to the stress characteristics of semi-rigid base asphalt pavement. The results indicated that the influence of surface course thickness on the bottom flexural tensile stresses of pavement structural layers was not significant. The surface course thickness required by the mechanical property was not bigger. According to these results, for the economic benefit, a rational surface course thickness scope of semi-rigid base asphalt pavement that met the functional requirements was proposed in this paper.
INTRODUCTION
Due to the superior performances of higher bearing capacity, good water stability, driving comfort and lower construction cost; semi-rigid base asphalt pavement has been one of the most popular pavement structures in high-class highways for many years. But in recent years, with the constant increase of traffic volume and axle load, semi-rigid base asphalt pavement is easy to produce dry shrinkage cracks and low-temperature cracks during operation period, which affects its service
performances. Qisen Zhang, et al. (1992) studied the cracking mechanism of semi-rigid base asphalt pavement, and proposed some measures to reduce cracks. Feng Li and Lijun Sun (2006) analyzed the influence of base course modulus on the mechanical property of asphalt pavement. Based on the fatigue fracture theory and finite element method, Tao Yang (2005) studied the expansion rules of semi-rigid base asphalt pavement reflection cracks and the temperature fatigue cracks, and proposed the application of a stress absorption band to minimize cracking.
In order to deal with the higher axle loads, and prevent early cracking, the design thickness of pavement surface layer enlarges unceasingly. However, for the semi-rigid base asphalt pavement, where the main supporting course is a semi-rigid base; surface course mainly plays a function role. Thicker surface course may produce rut problem.
Proceeding from the mechanical characteristics of semi-rigid base asphalt pavement, the stresses at the bottom of the subbase layer of different surface course thickness were calculated respectively in this paper, and, the stresses change rules were analyzed. The purpose was to investigate the influence of surface course thickness on the mechanical properties of semi-rigid base asphalt pavement structure, and, put forward the rational thickness ranges of the asphalt surface course on semi-rigid base asphalt pavement that meet the functional requirements.
INFLUENCE OF SURFACE COURSE THICKNESS ON FLEXURAL TENSILE STRESS AT THE BOTTOM OF THE SUBBASE
In the system of semi-rigid base asphalt pavement structure, the main supporting course is the semi-rigid base, and, surface course plays a function role to ensure driving comfort, protect the base course and lengthen its service life, etc. Through calculation, we can know that the bottom of asphalt surface course is generally in the compressed stress condition or low tensile stress condition in the semi-rigid base asphalt pavement structure. The requirement of its flexural tensile strength is not high. In order to investigate the influence of surface course thickness on the mechanical property of semi-rigid base asphalt pavement structure, consult the typical semi-rigid base asphalt pavement structure figure (shown in Figure 1), the flexural tensile stresses at the bottom of subbase course were calculated by pavement calculating program in the condition of different surface course thickness, such as 4cm, 7cm, 10cm, 14cm and18cm. The calculating results are shown in Table 1.
FIG. 1. Sketch map of typical pavement structure.
Table 1．The Change of Subbase Flexural tensile stress along with Surface
Course Thickness
Table 1 indicates that surface course thickness increases each 1cm; the subbase flexural tensile stress will reduce less than 0.005MPa when other parameters are the same. Surface course thickness can not significantly influence the subbase flexural tensile stress of semi-rigid base asphalt pavement. Increasing surface course thickness is not able to improve the mechanical properties of semi-rigid base asphalt pavement structure. However, the increase of surface course thickness may increase the construction cost of road project. We should reasonably select the surface course thickness of semi-rigid base asphalt pavement structure according to the actual situation.
INFLUENCE OF MODULUS ON FLEXURAL TENSILE STRESS AT THE BOTTOM OF THE SUBBASE
Besides the surface course thickness, the base modulus and subgrade modulus are also the primary factors which affect the mechanical properties of semi-rigid base asphalt pavement structure. Reference to the pavement structure shown in Figure 1, taking base course thickness 18cm and subbase thickness 30cm as an example, flexural tensile stresses at the bottom of subbase course of different base modulus and subgrade modulus were calculated. The results are shown in Table 2.
Table 2．The Change of Subbase Flexural tensile stress along with Modulus
Table 2 indicates that modulus can significantly influence the subbase flexural tensile stress of semi-rigid base asphalt pavement. The subbase flexural tensile stress will reduce 0.011MPa if the subgrade modulus increases 10MPa. At the same time, the subbase flexural tensile stress will reduce 0.09MPa if the ratio of subgrade and subbase modulus increases 0.01. So, the subgrade modulus and the ratio of subgrade and subbase modulus have significantly influence on the flexural tensile stress at the bottom of subbase. It can improve the mechanical properties of semi-rigid base asphalt pavement structure and reduce pavement cracking by enhancing the subgrade modulus or the ratio of subgrade and subbase modulus.
INFLUENCE OF SURFACE COURSE THICKNESS ON ASPHALT PAVEMENT SHEAR STRESS
Under the repeating loads, shear stress may produce inside the asphalt pavement surface course, and may cause shear failure of the pavement. The shear stress will gradually reduce with the deep direction. When surface course is thinner, the shear stress among layers is relatively greater. Therefore, surface course thickness of the asphalt pavement must satisfy the requirement of anti-shear.
Shear Stress Analysis internal the Surface Course
Under the driving loads, surface course biggest shearing stress is located at the edge of wheel load contact face (Ministry of communications, 1995). The location would change with different pavement structural parameters. Based on the maximum shear stress coefficient , the shear internal the surface course could be calculated as
follow:
?τ?τταcos cos v m p == (1)
Where: m τ—Maximum shear stress (MPa); τ—Maximum shear stress coefficient; p v —Vertical uniform load (MPa); ?——Internal friction angle of the asphalt mixture.
Considering two kinds of asphalt mixtures anti-shearing structural strength coefficients of f =0.2 and f =0.5, internal shear stresses of the asphalt pavement were calculated in the condition of different surface course thickness. The results are shown in Table 3.
Table 3．Internal Shear Stress of Pavement in different Surface Course Thickness
Table 3 indicates that the influence of surface course thickness on the internal shear stress of semi-rigid base asphalt pavement is not significant. The surface course thickness will be enough to satisfy the pavement structure requirement generally. The shear failure will not occur inside the surface course of semi-rigid base asphalt pavement.
Interlayer Shear Stress Analysis
Under the driving loads, the horizontal direction will produce shearing stress on the interface between surface course and base course. According to elasticity theory and related standards, the interlayer shear stress τ could be calculated as follow:
H v p ττ= (2)
Where: τ—Interlayer shear stress; p v —Vertical uniform load (MPa); H τ—Interlayer shear stress coefficient.
Through the computation analysis, we can draw up the relation curve between interlayer shear stress coefficient and surface course thickness as shown in Figure 2.
FIG. 2. The relation curve between interlayer shear stress coefficient and
surface course thickness
From Figure 2, we can know that surface course thickness has significant influence on the interlayer shear stress. Therefore, surface course thickness must satisfy the requirement of interlayer shear stress.
THE RATIONAL SURFACE COURSE THICKNESS
According to the method of new Chinese Specifications for Design of Highway Asphalt Pavement (Ministry of communications, 2006), the traffic grade was divided into four levels, such as light traffic (cumulative equivalent axle load action times are less than 3×106), medium traffic (3×106～1.2×107), heavy traffic (1.2×107～2.5×107) and super-heavy traffic (more than 2.5×107). Comprehensively considering the flexural tensile stress and shear stress of semi-rigid base asphalt pavement structure, the rational surface course thickness ranges that satisfied the mechanical properties are proposed in Table 4.
Table 4．The Proposed Ranges of Surface Course Minimum Thickness
CONCLUSIONS
Through systematic mechanical calculation of semi-rigid base asphalt pavement, the flexural tensile stress and shear stress rules were studied in this paper. The main conclusions obtained are summarized below.
(1) Surface course thickness can not significantly influence the subbase flexural tensile stress of semi-rigid base asphalt pavement. Increasing surface course thickness is not able to improve the mechanical properties of semi-rigid base asphalt pavement structure.
(2) The influence of surface course thickness to the internal shear stresses of
semi-rigid base asphalt pavement is not significant. The surface course thickness will be enough to satisfy the pavement structural requirement. Generally, the shear failure will not happen in the surface course of semi-rigid base asphalt pavement.
(3) Through systematic mechanical calculation, the rational surface course thickness ranges that satisfied the mechanical properties were proposed in this paper. REFERENCES
Q isen Zhang et al. (1992). “Cracking Mechanism for Semi-rigid Based Asphalt Pavement Structure”. Chinese Journal of Civil Engineering, Vol. 25 (2):13-21. Feng Li and Lijun Sun (2006). “Analysis of the Influence of Base Course Modulus on the Mechanics Perfor mance of Asphalt Pavement”. Chinese Journal of Highway and Transportation Research and Development, Vol. 23 (10):41-43. Tao Yang (2005). “Analysis on the Cause of Reflection Crack of Asphalt Pavement Based on Semi-rigid Type Base and Anti-crack Method”.Wuhan University of Science and Technology master's degree paper, 2005
Ministry of communications. (1995). “Typical Structure Research of Semi-Rigid Base Asphalt Pavement for High Class Highway”. China Communication Press. Ministry of communications. (2006). “Specifications for Design of Highway Asphalt Pavement”. China Communication Press.