1、外文文献structural Design of Asphalt Pavement for Low Cost Rural RoadsYuan Goulin(袁国林)12 Chen Rongshen(陈荣生)1. College of Transportation, Southeast University, Nanjing 210b9b, China2. College of Civil Engineering, Nanjing University of Technology, Nanjing , ChinaIn developing countries，rural road constru
2、ction is mostly cumbered by shortage of funds. Engineers concerns most in rural areas is how to build roads which not only cost less but also meet the traffic demands. Especially in vast rural areas of China, there are a great variety of transportation patterns, and the traffic composition is very c
3、omplex. Compared with other countries，the traffic composition in China rural areas have its own features. Therefore，there is no experience about the rural roads construction for reference. In recent years，the central government of China has increased the strength for rural road construction. At the
4、same time，a lot of researches about rural road construction have been done by researchers in China, and some conclusions about china rural roads have been made. In the authors opinion，the selection of the pavement structure material is the key measure to reduce the construction cost of rural roads a
5、fter the route has been determined. Compared with concrete pavement，asphalt pavement relatively costs less and is the first choice for rural roads in China. And then， according to the research achievements about rural roads construction，the authors have done some preliminary researches on the struct
6、ure design for low-cost asphalt pavements for rural roads.1 Traffic Composition of Rural RoadRural roads include county roads，town roads and village roads.The traffic on rural roads is usually mixed. On a county road, traffic volume is between 300 to 1500 veh/d in average，and in a county with a deve
7、loped economy， it reaches 1000 to 2 000 veh/d. The traffic volume between county and town is 100 to 300 veh/d，and the traffic volume between towns is usually less than 100 to 300 veh/d. In a mixed traffic flow，trucks account for 40% to 70% of the traffic volume, which are mainly light trucks carryin
8、g less than 2. 5 tons(including agricultural vehicles such as electro-tricycles，walking tractors etc.)and medium-size trucks of 2. 5 to 5 tons. Most of these light or medium trucks are overloaded. The proportion of heavy truck is less than 9%.On some roads to counties，the proportion of overloaded tr
9、ucks is 5% to 32 %，while on some county roads connecting to national or provincial trunk highways，the proportion of overloaded vehicles usually amounts to 20% to 32% .The traffic volume on rural roads is not heavy. However，considering the practical situation in China, as well as the exitence of over
10、loaded vehicles，100kN，or BZZ-100 was adopted as standard axle load in the research.The pavement deflection or the flexural-tensile stress at the bottom of asphalt surface is taken as the design parameter. The axle load was calculated inaccordance with the following formula:isN35.421)/(siPC和 -the axl
11、e weight of an i-level axle in kN and the action frequency; -the axle weight of standard axle in 100 kN and the action frequency;和 If the distance between axles is less than 3 m， axle loads are calculated asa double-axle or multi-axle loads，and1=1+1.2(1)If the flexural-tensile stress at the bottom o
12、f semi-rigid base is taken as the design parameter, the axle load is calculated in accordance with the following formula:=12()8If the distance between axles is less than 3m，1=1+2(1)2 Traffic Volume on Rural RoadsMinibuses are adopted as the standard vehicle for the design of rural roads. Table 1 sho
13、ws its external dimensions.3Table 1 External dimensions of the passenger car mLength Width Height Front overhang Distance between axles Rear overhang6.0 1.8 2.0 0.8 3.8 1.4The typical vehicle types on rural roads are listed in Table 2. And others such as non-power-driven vehicles，animal-drawn vehicl
14、es，and bicycles can be taken into account in the calculation of traffic volume on rural roads，in view of their roadside interference.In accordance with the traffic composition and volumes，rural roads are divided into five grades. The traffic volume of each grade is shown in Table 3.Traffic volume sp
15、ecified in Table 3 was obtained by taking the minibus as the standard vehicle type,and converting different types vehicles according to the vehicle conversion coefficients given in Table 2.In Table 3，1365tseNNe refers to the cumulative equivalent axle load action frequency;Ns refers to the equivalen
16、t axle load action frequency in the designed traffic lane in the beginning operation period of rural roads;y refers to the average annual growth rate of traffic volume; refers to lane coefficient, and 1.0 for a single lane and 0. 6 一 0. 7 for a dual lane.3 Strength of RoadbedThe modulus of resilienc
17、e of roadbed varies greatly. For convenience，the strength of roadbed can be divided into four classes according to its moisture content and modulus of resilience，as shown in Table 4.The parameters in Table 5 are determined by combining design principles with practical experience. By applying elastic
18、 multilayer theory to the pavement structure specified in Table 5，the influence of Ne on the pavement thickness of rural roads was analyzed，and the result show that for given h，h2，E0，the roadbase thickness for neighboring traffic classes changes in a range of 4-5 cm. This result indicates that the c
19、lassification of traffic volume on rural roads shown in Table 3 is reasonable and feasible in terms of the design and construction of asphalt pavement structures.By using the elastic multilayer theory，the asphalt pavement structure of ordinary rural road in Table 5 is analyzed. When Ne，the cumulativ
20、e equivalent axle load action frequency，the thickness of road surface(h =3 cm)，and the thickness of subbase(h2 = 20 cm ) remain the same , the influence of neighboring roadbed strength classifications on the thickness of roadbase is 3 cm 一 5 cm. This conclusion indicates that the strength classifica
21、tion of roadbed is reasonable and applicable to the design and construction of asphalt pavement structure.4 Determination of Thicknesses of Asphalt Pavement Structure Sensitivity analysis of the design parameters of roadbed and pavement structures is to find out the relationship between structural s
22、trength of asphalt pavement structure and the design parameters of each layer, and determine the most sensitive layer in the pavement structure. The asphalt pavement structure of rural roads is generally composed of a road surface, a roadbase，and a subbase，as shown in Table 6. The pavement structure
23、 was analyzed according to elastic multiplayer theory under the double circular uniform load，with an assumption that there is continuous contact between the adjacent layers of the asphalt pavement structure. The basic parameters used in the calculation and analysis of asphalt pavement structure are
24、listed in Table 7. By analyzing the effects of the change of all the parameters of pavement structure on the distortion of the road surface，roadbase，and roadbed , the following conclusions have been drawn.(1)Increasing the thickness of the road surface effectively decreases the road surface deflecti
25、on，but raises the cost. The comparatively economical and effective method is to increase the thickness of the subbase, which is superior to increasing the thickness of roadbase，while increasing the thickness of the road surface is the last choice.(2)As the thickness of pavement structure increases，t
26、he change of road surface deflection will trend to be gentle. When the thickness of road surface reaches a certain value，the variance in the road surface deflection will not be obvious，and then it is ineffective to enhance the bearing capacity of asphalt pavement structure by increasing the thicknes
27、s of road surface. It is recommended that the thicknesses of the roadbase and the subbase should be equal to or largerthan 18 and 20 cm, respectively，in design of asphalt pavement structures 5of rural roads. Fig. 1 shows the effects of the changes in the thickness of each layer on road surface defle
28、ction.(3)Road surface deflection is very sensitive to the change of modulus of the roadbed. The increase in the modulus of roadbase or subbase is also effective to decrease the deflection of the road surface. On the other hand，the deflection of the road surface decreases gradually when the modulus o
29、f the surface increases，being the least effective factor. When the modulus of the road surface increases to a certain value， decrease in road surface deflection is not apparent. Fig. 2 shows the effect of the modulus of each layer on road surface deflection. From the above discussion，we conclude tha
30、t the most sensitive layer for road surface deflection is subbase，and the next is roadbase. To decrease the road surface deflection of low-cost rural roads，the strength and stability of the roadbed should be enhanced, and the materials with a certain thickness and relatively high density should be u
31、sed to pave the subbase.The traffic volume or the accumulative equivalent axle load action times(frequency)within the designed life of road is used to determine the type and thickness of the asphalt pavement road surface, and the results are listed in Table 8，where veh/d means the number of the equi
32、valent the passenger cars per day. For a low traffic volume rural road with Ne 500 000，graded broken stones(or gravel)can be used as a flexible base. The flexible base has good strength and effectively prevents reflection cracks of the asphalt pavement road surface, provided the graded broken stones
33、(or gravel ) meets the requirements for high density(degree of compaction ,100%. To ensure the sufficient strength and stability of the flexible base，its thickness is not less than 15 cm，the thickness of the aggregate subbase is not less than 20 cm，A semi-rigid base usually has a good bearing capaci
34、ty For the rural roads with Ne)500 000，or those with low traffic volumes but relatively,the minimum thickness of semirigid base or subbase is 16-18 cm5 Calculation of the Thickness of Road Surface5.1 Deflection(1)Road surface deflectionRoad surface deflection is a vertical distortion caused by verti
35、cal load on the road surface. It not only reflects the whole strength and stiffness of asphalt pavement structure and roadbed，but also has a close internal relation with the service condition of the pavement.(2)Design deflectionThe design deflection is the index representing the stiffness of the pav
36、ement structure. It is also the deflection of the pavement which is established according to the accumulative equivalent axle load estimated to pass over a lane in the expected design life, road types, road classification，and the types of road surface and roadbase. The design deflection is not only
37、the main basis for the design thickness of the pavement structure，but also the necessary index for the examination and acceptance of the project. Through theoretical analysis and experimental study，formulas for the design deflection value which are applicable to the pavement structure design of lowc
38、ost rural roads are as follows:semi-rigid base:=6290.2flexible base:=11340.2where A, is the type coefficient of the road surface. The type coefficient of asphalt concrete road surface is 1.0;that of hot-mix asphalt macadam and that of emulsified asphalt macadam road surface are all 1. 1; and that of
39、 asphalt surface treatment road surface is1 .2.(3)Allowable deflectionAllowable deflection is the maximum deflectionallowed at the end of the roads service life under lim-iting conditions in poor season. Through thoreticalanalysis and experimental study，the calculation for-mulas for the allowable de
40、flection of road surfacewhich are applicable to the pavement structure designof low-cost rural roads are as follows2:=7860.2=14180.2When designing the asphalt pavement structure of low-cost rural roads, 7we should use formula (6) or (7 ) according to the types of roadbase to determine the thickness
41、of asphalt pavement structure.5.2 Tensile stressBecause the asphalt pavement structure of lowcost rural roads is not substantial enough and the heavy vehicles are allowed to pass over them, the maximum tensile stress should be checked by computing the stresses of the semi-rigid base and subbase. The
42、 tensile stress at the bottom of semi-rigid base or subbase，would be less than or equivalent to the allowable tensile stress of the materials of the semirigid base or subbase , namely，=For the stabilized aggregate base with an inorganic binder-=0.350.11 ,=1.2For the stabilized fine-grained soil base
43、 with an inorganic binder:=0.350.11 ,=1.25.3 Pavement thicknessTo make it simple and convenient for engineers to determine the desired thickness of rural road pavement, the curves of the thickness of the roadbase of low-cost rural roads according to typical pavement structures and accumulative frequ
44、ency of equivalent axle load are shown in Figs. 3，4 and 5.(1)When the accumulative frequency of equivalent axle load is within times per lane，asphalttreated or asphalt penetrated surfaces with thickness of 1. 5 cm 一 cm is recommended for road surface. For various accumulative equivalent axle loads a
45、nd the moduli(Eo)of roadbed，the equivalent thickness of roadbase is shown in Fig. 3.(2)When the accumulative frequency of equivalent axle load is within 500 001)一 1 000 000 times per lane，asphalt macadam or asphalt concrete with thickness of 3 cm -5 cm is recommended. For various accumulative equiva
46、lent axle loads and moduli(Eo)of roadbed，the equivalent thickness of roadbase is shown in Fig. 4.(3)When the accumulative frequency of equivalent axle load is within 1000 000-2 000 000 times per lane，asphalt concrete road surface of 5 cm-7 cm thick is recommended. For various accumulative equivalent
47、 axle loads and moduli(Eo)of roadbed , the equivalent thickness of roadbase is shown in Fig.S.In Figs.3-5，Ld is the designed deflection, Lo is the representative deflection of roadbed，E, is the modulus of resilience of the roadbase，in MPa , Eo is the modulus of resilience of the roadbed，in MPa ,and
48、H, in cm，is the equivalent thickness of the base (roadbase and subbase)，which can be obtained through calculation and in-site investigation for a trilevel-pavement roads(including road surface，base and roadbed).If a designed road has four layers，i.e. a subbase is added，according to the regression an
49、alysis of the extrapolated results of a number of multi-layer flexible systems and the available research findings，the thickness of the roadbase , h,，in cm, can be calculated from the following equation:1=22.4216 Concluding RemarksCompared with concrete pavement, asphalt pavements have a lower construction cost, which is suitable for the roads in relatively underdeveloped rural areas in China. T
