What Is Flexible Pavement?

According to the different working characteristics of roads under load, it is divided into three types: flexible pavement, rigid pavement and semi-rigid pavement.

Flexible pavement refers to less rigidity and lower bending and tensile strength, which mainly depend on compression,

Flexible pavement has low bending strength. It mainly depends on compressive strength and shear strength to resist driving load. It can produce a certain amount of bending and deformation under the action of repeated driving loads. For example, a layered pavement structure composed of plastic materials such as asphalt and aggregates. Its bearing capacity depends on the load-diffusion characteristics of the entire layered system, and is greatly affected by the strength and stability of the soil foundation. In the design, the factors such as subgrade and pavement must be considered comprehensively. ^[1]

The design parameters of the road shoulder are basically the same as those of the main line of the road surface. The design traffic volume of the road shoulder is obviously less than the main line. This is the main difference between the two. Huang (2007) believes that the traffic volume on the shoulder is mainly composed of three parts, namely, the puppet traffic volume, the number of parking times and the normal traffic volume. Normal traffic refers to the traffic generated when the shoulder is used as a temporary lane during peak hours. When the shoulder is not used as a temporary lane, the designed traffic of the shoulder only includes incidental traffic and the number of stops.

When there is no obstacle on the paved shoulder, the truck will enter the shoulder about 30 cm or more due to the occasional deviation from the lane in the lane adjacent to the shoulder, thus forming occasional traffic on the shoulder. California (1972) uses 1% of the traffic volume of the lane adjacent to the shoulder as the incidental traffic volume, which is about 104 times of 18 kip standard axle load. Barksdale (1979) believes that this proportion is low, and occasional traffic accounts for at least 2% to 2.5%. ^[3]

China is one of the earliest countries in the world to use the elastic layered system theory for pavement design, but it is different from any other mechanics-based design method because China uses road surface deflection as a design index and uses layers Multi-index design system with bottom bending tensile stress as the checking index.

Pavement design methods based on road surface deflection appeared in the 1940s and 1950s, such as the US Kansas and US Navy design methods, but they are a single index design method, which only uses road surface deflection as the only design index. To determine the total thickness of the pavement, its obvious disadvantage is that there is no direct relationship between the deflection index and the main damage of the pavement such as cracking and permanent deformation. For this reason, the design method of our country supplements the bending stress index of the bottom layer in addition to the deflection design index, which is used to control the bottom cracking of the asphalt layer and the inorganic combination base layer (called a semi-rigid base layer in our country), thereby forming The characteristics of China's design method, that is, the design method with the main purpose of controlling the overall stiffness of the pavement and cracking under the load as the index, and the road surface deflection and the layer bottom bending stress as the index.

The rebound deflection of the road surface under the load reflects the overall stiffness of the subgrade pavement structure. This results in the weakening of the material strength and stiffness and the increase in road surface deflection, and the fatigue cracks that eventually appear on the road surface are the microcracks that accumulate from quantitative changes to qualitative changes. Therefore, there is a certain inherent relationship between road surface deflection in theory and pavement fatigue cracking, but the model (design equation) between the road surface rebound deflection and the number of traffic load interactions still depends on the support of reliable data .

In China, the bottom bending tensile stress of the asphalt surface layer and the inorganic combination (semi-rigid) base layer is used as the check indicator. In fact, it has played a role in controlling the thickness of the two layers. Strain indicators have the same effect. However, China has not adopted another strain index in other mechanical design methods-the top surface compressive strain. In the traditional mechanical design system, the compressive strain on the top of the soil foundation is not only used to control the total thickness of the pavement, but also an additional assumption is made that the elastic strain and plasticity in each deformation process should become a certain ratio, so According to this assumption, the rutting (permanent deformation) of the road surface can be predicted by the compressive strain of the top surface of the soil. This method is used in the famous AI and Shell design methods. Our design system does not reference this index and its plastic deformation. Proportional assumptions, no design indicators, models or prediction methods specifically for rut damage. ^[3]

The design of flexible pavement can be divided into two parts: structural design and thickness calculation. Only when the pavement structure design work obtains correct results, it is meaningful to calculate the thickness on this basis, so the success or failure of the pavement structure design is directly related to the success of the pavement design work, and it is in a key position in the pavement design work. In engineering practice, there are many concrete examples that fully illustrate this point.

Pavement structure design includes the following two.

(1) Design of each structural layer: It is mainly based on local specific conditions and use requirements to select the type of structural layer and composition materials.

(2) Combined design of structural layers: According to the principles of local materials and phased construction, different types of structural layers are combined into a structure that can withstand the repeated actions of driving loads and natural factors, and can give full play to the maximum effectiveness of each structural layer. Economic and reasonable subgrade pavement structural system.

In the design of flexible pavement structures, the following technical and economic principles should be followed.

(1) Take measures according to local conditions, pay attention to characteristics, take materials locally, and apply them according to their conditions.

The design of the pavement structure must be guided by the general design principles, taking full account of the specific conditions and some special requirements of each place. At the same time, we must pay attention to the successful experience of local pavement design and apply it to local pavement structures that have been effective. In road engineering, material costs account for a large proportion, generally around 60% to 70%. Therefore, materials should be collected locally to the maximum extent possible, and all natural materials, industrial wastes and processing materials that can be obtained locally should be used as much as possible. Give full play to the advantages of local materials and achieve the application according to the material.

(2) Construction in phases and gradually improving.

The volume of traffic is the most important factor in determining the grade and type of pavement, and the volume of traffic gradually increases with time. In order to use the state funds reasonably, pavement design should generally be carried out according to the recent use requirements, and firstly meet the recent needs. In the future, with the increase of traffic volume, the increase of vehicle models and the increase of investment, gradually increase the pavement level and increase the pavement thickness. However, care must be taken during construction to make the pre-projects lay the foundation for later projects and be fully utilized in later projects.

(3) Overall consideration and comprehensive design.

In the design of the pavement structure, the soil base, cushion, subbase, base and surface should be regarded as an organic whole. According to the requirements of stable soil foundation, solid base and durable pavement, give full play to the role of each structural layer, rationally select pavement materials, determine the appropriate structural layer thickness, so that the design of the pavement can meet the requirements of strength and stability as a whole. Be economical, reasonable and durable.

(4) Convenient for construction and good for maintenance.

The selection of each structural layer should also consider the equipment and construction conditions. Where possible, mechanized construction should be used as much as possible. In addition, maintenance work after opening to traffic should also be considered. The maintenance workload of high-grade and sub-high-grade roads is much less than that of medium- and low-grade roads. The higher-grade roads require less maintenance work as much as possible, so as not to affect the driving of large traffic, so when designing the pavement structure, Consideration should be given to ease of construction and maintenance, to ensure the need for long-term traffic.

(5) Consider the influence of climatic factors and water temperature conditions.

The pavement structure design should ensure that the natural structure and the wheel load repeatedly affect the overall structure of the pavement with sufficient water stability, dry stability, freezing stability and high temperature stability. Therefore, the natural climate and water temperature conditions may affect the road surface. Give full attention to it. On sections with poor hydrogeology, efforts should be made to improve the wet and dry conditions of the sections. For the base course of the asphalt pavement, technical measures shall be taken to ensure the water stability of the structural layer in the wet and medium-wet sections. In frozen areas, measures must also be taken to ensure the freezing stability of pavement structural layers. ^[4]