沥青路面结构设计的低成本农村道路

沥青路面结构设计的低成本structural Design of Asphalt Pavement for Low Cost

农村道路

Rural Roads

structural Design of Asphalt Pavement for Low Cost Rural Roads

In developing countries ，rural road construction 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 complex. 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 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 after 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 structure design for low-cost asphalt pavements for rural roads.

1 Traffic Composition of Rural Road

Rural 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 developed 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 carrying 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 trucks 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 overloaded 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 in

accordance with the following formula:

4.3512(/)s i s i

N C C P P N 错误！未找到引用源。和错误！未找到引用源。-the axle weight of an i-level axle in kN and the action frequency;

错误！未找到引用源。-the axle weight of standard axle in 100 kN and the action frequency; If the distance between axles is less than 3 m ， axle loads are calculated as

a double-axle or multi-axle loads ，and

()11 1.21C M =+-

If the flexural-tensile stress at the bottom of semi-rigid base is taken as the design parameter, the axle load is calculated in accordance with the following formula:

''212(/)s i s i

N C C P P N = If the distance between axles is less than 3m ，

()11 1.21C M =+-

2 Traffic Volume on Rural Roads

Minibuses are adopted as the standard vehicle for the design of rural roads. Table 1 shows its external dimensions.

Table 1 External dimensions of the passenger car m

Length Width Height Front overhang Distance between axles Rear overhang

6.0 1.8 2.0 0.8 3.8 1.4

The typical vehicle types on rural roads are listed in Table 2. And others such as non-power-driven vehicles ，animal-drawn vehicles ，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 specified 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，

()[]ηγγ11365-+=t s e N N

Ne refers to the cumulative equivalent axle load action frequency;

Ns refers to the equivalent 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;

η refers to lane coefficient, and 1.0 for a single lane and 0. 6一0. 7 for a dual lane. 3 Strength of Roadbed

The modulus of resilience 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 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 classification 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 cumulative 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 classification 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 strength 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 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 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 deflection，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，the 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 thickness of road surface. It is recommended that the thicknesses of the roadbase and the subbase should be equal to or larger

than 18 and 20 cm, respectively，in design of asphalt pavement structures of rural roads. Fig. 1 shows the effects of the changes in the thickness of each layer on road surface deflection.

(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 of 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 that 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 used 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 equivalent 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(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 capacity 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 cm

5 Calculation of the Thickness of Road Surface

5.1 Deflection

(1)Road surface deflection

Road surface deflection is a vertical distortion caused by vertical 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 deflection

The design deflection is the index representing the stiffness of the pavement 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 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 lowcost rural roads are as follows:

semi-rigid base:

2

629d s e L A N -=

flexible base: 0.21134d s e L A N -=

where 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 asphalt surface treatment road surface is1 .2.

(3)Allowable deflection

Allowable deflection is the maximum deflection

allowed at the end of the road's service life under lim-

iting conditions in poor season. Through thoretical

analysis and experimental study ，the calculation for-

mulas for the allowable deflection of road surface

which are applicable to the pavement structure design

of low-cost rural roads are as follows}2}:

0.2786d s e L A N -=

0.21418d s e L A N -=

When designing the asphalt pavement structure of low-cost rural roads, we should use formula (6) or (7 ) according to the types of roadbase to determine the thickness of asphalt pavement structure.

5.2 Tensile stress

Because 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 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 ，

/m r sp s K σσσ=≤

For the stabilized aggregate base with an inorganic binder-

0.110.35, 1.2s c c e

K A A N == For the stabilized fine-grained soil base with an inorganic binder:

0.110.35, 1.2s c c e

K A A N ==

5.3 Pavement thickness

To 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 frequency of equivalent axle load are shown in Figs. 3，4 and 5.

(1)When the accumulative frequency of equivalent axle load is within 500000 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 and 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 equivalent 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 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 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 analysis 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:

22.4121

E h H h E =- 6 Concluding Remarks

Compared with concrete pavement, asphalt pavements have a lower construction cost, which is suitable for the roads in relatively underdeveloped rural areas in China. The research in this paper proposed a method for structural design of low cost asphalt pavements. The method is to provide an guideline for the design of asphalt pavement structure in rural areas.

References

[1]Yuan G L , Zhang F , Chen S W , et al. Research on technical indexes of rural highway construction in Jiangsu province [ J ].Highway, 2005(6):135一139(in Chinese).

[ 2 ] Research Institute of Highway , the Ministry of Communications. Final Report on Low Cost Inter-township and Rural Road Construction Techniques 〔R].Beijing; Resdarch Institute of Highway, 2003(in Chinese).

[ 3 ] Liu Q Q. How to reduce the construction cost of the rural highway [ J ] .Journal of Highway and Transportation Research and Development, 2005(2):41一44(in Chinese).

[ 4 ] JTG B014-97. Specification for design of highway asphalt pavement[ S ](in Chinese ).

[ 5 ] JTG BO1-2003. Technical Standard of Highway Engineering [ S ](in Chinese).

[6] Deng X J. Engineering for sub-grade and pavement[ M].2nd ed. Beijing; People's Communications Press, Beijing, 2004(in Chinese ).

沥青路面结构设计的低成本农村道路

在发展中国家,农村道路建设主要是因缺乏资金而受阻。工程师最关心的农村问题是如何修 建的公路不仅成本低,而且满足交通需求。特别是在广大农村地区,有各种各样的交通方式,交通成分非常复杂。与其他国家相比,在中国农村地区交通组成有其自身的特征. 因此,没有足够经验对农村公路建设提供参考依据。近年来,中国的中央政府增加了对农村公路建设的力量。与此同时, 在中国，研究人员已经完成了关于中国农村道路的大量研究和一些结论。在作者的观点,选择不同的路面结构材料是关键措施，从而降低农村道路的工程造价。相比,沥青混凝土路面相对费用少、是第一个在中国农村道路的选择。然后,根据农村道路建设研究成果,作者也做了一些对低成本沥青路面结构设计的初步研究。

1 农村道路交通组成

农村道路包括县公路、城市道路和村的道路。

农村道路上，交通通常是喜忧参半。一个平常县的道路，交通量平均为的300至1500辆/ 天；一个经济发达的县,已经达到了1000到2000 辆/ 天。通信量县、镇之间是100到300辆/ 天，交通量之间通常低于城镇是100至300辆/ 天。在一个混合交通流、卡车占有40%到70%的交通量,主要为轻型卡车携带小于2.5吨(包括农业车辆如拖拉机等)、行走、中型卡车2.5 - 5吨。大部分的轻或中等卡车超载。重型卡车的比例小于9%。某些道路上的比例对县、超载的卡车是5%至32%,而在一些县公路连接到国家或省级公路的比例躯干,超载的比例通常数量的20%提高到32%。

交通量对农村道路不会很重。然而,考虑到中国的实际情况,以及车辆超载,要满足 100 kN,和 BZZ-100标准轴载作用次数。

人行道挠度和底部的拉应力作为沥青路面设计参数。轴载荷计算方式,依照下面的公式: 4.3512(/)s i s i

N C C P P N = 错误！未找到引用源。和错误！未找到引用源。为车轴重量和频率

为总车轴重量和频数

如果两轴之间的距离小于3米,轴载荷进行了计算：

()11 1.21C M =+-

如果弯拉型压力的半刚性基层底部作为设计参数计算、轴重依下列公式：

''212(/)s i s i

N C C P P N = 如果两轴之间的距离小于3米：

()11 1.21C M =+-

2 对农村公路交通量

农村道路以面包车为车辆的设计标准。表1显示了其外部尺寸。

典型的车辆类型农村道路都列在表2。如电动车,摩托车 和自行车可以考虑为边对农村道路交通量的计算的路边干扰。

根据交通组成和卷,农村道路被分成5个等级。每一等级通信量见表3。交通量在表3规定经以小巴为标准的车辆类型,并将不同类型车辆的车辆折算系数记录在所给的表2，表3中,

()[]ηγγ11365-+=t s e N N

Ne 指的是累积的等效轴重行动的频率

Ns 指的等效轴重行动的频率在设计交通车道上开始运行期间;

r 指的年增长率交通量

η指系数,一个车道为为1.0,双车道为0.6一0.7。

3路基强度

路基回弹模量的变化极大。为方便起见,路基的强度可以分为四类根据其含水率和模量的韧性,显示在表4。

表5的参数确定设计原则相结合的实践经验。采用多层弹性理论对路面结构表5中指定的影响,在暗夜精灵,农村道路铺装厚度进行了分析,结果表明,在给定的h,h2,E0、路面基层厚度变化为周边交通类一个范围内的4 - 5厘米。测试结果表明,该分类农村道路运输量对显示在表3是合理的、可行的设计和施工条件对沥青路面结构。

采用多层弹性理论,沥青路面结构的普通乡村公路表5中进行了分析。当北东向、累积的等效轴重行动频率,路面的厚度(h = 3厘米的厚度)、基层(h2 = 20厘米)保持不变,邻近的路基强度的影响分类基层的厚度为3 cm 一5厘米。这一结论表明,路基强度分类是合理的、适用于设计与施工的沥青路面结构。

4厚度测定沥青路面结构

设计参数的灵敏度分析路基和路面结构之间的关系,找出沥青路面结构强度的结构及设计参数,并确定各层的最敏感的路面结构层。沥青路面结构,农村道路通常是由一个基层路面,和统一,显示在表6。分析了路面结构的多人游戏根据弹性理论的双圆弧均布载荷下,假设接触有连续相邻层的沥青路面结构。使用的基本参数的计算与分析沥青路面结构都列在表7. 通过分析影响所有的参数变化对路面结构的扭曲,基层路面和路基,下面,结果显示。

(1)增加厚度路面有效减少路面的偏斜,可是饲养的成本。比较经济和有效的方法是提高基层的厚度,优于基层厚度的增加,而提高路面的厚度是最后的选择。

(2)随着厚度的增加,路面结构的变化将路面变形趋势是温柔。当路面的厚度达到一定值时,在路面挠度变化并不明显,然后它是无效的承载力提高沥青路面结构通过增加厚度的路面。建议在路面基层厚度和基层应等于或大于18至20厘米,分别对沥青路面结构设计,农村道路。图1显示变化的影响各层厚度对路面偏斜。

(3)路面位移的变化非常敏感对于路基系数E0。增加基层模量或基层也是有效降低路面的变形情况。另一方面,挠度路面模量逐渐降低的时候,作为表面增加最小有效因素。当路面模量的增大到一定数值时,降低路面变形不明显。图二显示效果的每一层的模量对路面偏斜。从上面的讨论,我们可以得出结论:最敏感层底基层路面位移,第二是基层。以降低路面变形,低成本的农村道路路基的强度和稳定性的,应加强对一定厚度的材料和相对较高的密度应使用统一铺平道路。

交通体积或累计的等效轴重行动的时间(频率)内的生活道路是设计用来确定型和厚度的沥青路面面层,结果列于表8,辆/天意味着每天的等效客车数。低交通量公路即Ne ≦500000，分级破碎的结石(或砾石)可以用来作为一个灵活的基地。柔性基础具有良好的强度和有效防止反射裂缝沥青路面的路面,提供了分级石头(或砾石)符合规定的要求对高密度(压实度(100%)(21。确保足够的强度和稳定性的灵活的基地,其厚度不得小于15厘米,厚度的骨料基层不小于20厘米, 一个半刚性基层通常具有良好的承载能力为农村道路与Ne ≦500000,或者这些低交通流量,但相对来说,设备的最小厚度碱或基层是16-18厘米

5计算路面的厚度

5.1挠曲量

(1)路面挠度

路面是一种垂直失真变形在垂直荷载作用下的路面。它不仅反映了整体强度和刚度的前提和路基、路面结构也有着紧密的内在联系和服务条件的人行道上。

(2)设计挠度

设计偏向指数代表路面结构的刚度。它也是的变形情况依法成立的路面累计的等效轴重估计越过一条小巷设计寿命的预期,道路类型、道路分类,和类型的路面和基层。不仅是设计挠度的主要依据设计厚度的路面结构,而且必要的检查验收指标的工程。通过理论分析和试验研究,设计弯沉值公式,适用于路面结构设计的隔间农村道路如下

半刚性基层：

2

629d s e L A N -=

柔性基层 : 0.21134d s e L A N -=

As 是路面类型系数,沥青混凝土路面系数为1.0,沥青混合料的级配碎石和乳化沥青碎石路面都是1.1;而路面沥青表面处理1.2。

(3)许用挠度

许用挠度的最大挠度在最不利季节条件下的允许在这条路的使用寿命。通过分析，实验研究和公式计算,对于允许偏转的路面,适用于路面结构设计的低成本的农村道路如下:

0.2786d s e L A N -=

0.21418d s e L A N -=

沥青路面结构设计时,低成本的农村道路,我们应该用公式(6)项或者第(7)根据不同类型人员的基层厚度的确定沥青路面结构。

5.2拉应力

由于沥青路面结构的隔间农村道路不是实体足够的和重型车辆经过允许的最大拉伸应力,应当检查通过计算应力的半刚性基层和底基层。在底部的拉应力的半刚性基层或底基层,将小于或等于允许拉应力的材料或设备,即统一基础

/m r sp s K σσσ=≤

对于稳定骨料与无机粘结剂：

0.110.35, 1.2s c c e K A A N =

=

为稳定细土基与无机粘结剂: 0.110.35, 1.2s c c e

K A A N ==

5.3铺装厚度

为了简单,便于工程师确定，理想的乡村公路路面厚度曲线和路面基层的厚度以及低成本农村道路根据典型路面结构和累计频率的等效轴重都显示在表3、4和5。

（1)当累计频率的等效轴重在500000次/年,处理过的沥青或沥青渗透表面厚度为1.5 cm

一4cm 被推荐用于路面。为各种各样的累计当量轴荷载及模(E0)的等效厚度路基、基层被显示在图3。

(2)当累计频率的等效轴重在500000一1000000次／车道、沥青碎石沥青混凝土厚度或3厘米,5厘米是被推荐的。为各种各样的累计当量轴载荷及模(E0)的路基、基层厚度是相当于如图4。

(3)当累计频率的等效轴重在1000000-2000000次／车道,沥青混凝土路面5厘米厚的cm-7是被推荐的。为各种各样的累计当量轴载荷及模(行政)的等效厚度路基、基层如图4所示

在图3-5,Ld 为设计数,L0为代表的挠度变形、E0是路基模量E1为模回升的路基,单位为MPa 、H,相当于在厘米,厚度的基层和底基层基地,可以通过计算得出了人行道、深入调查道路(包括道路表面、基地和路基)。如果一个设计的路四层,即是说,按照统一的推算,结果进行回归分析的多个多层灵活的系统和已有的研究结果,路面基层的厚度、H 、,厘米,可以计算出下列方程:

22.4121

E h H h E =- 6作为结束语

相比,沥青路面混凝土路面施工成本较低,适合于在相对欠发达农村地区的道路。通过本文的研究方法,提出了一种低成本的结构设计、沥青路面。这种方法是将提供一个指导设计的沥青路面结构在农村地区

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