What are the different types of pavement used for construction of roads? Explain in details.
Types of pavement
Road pavements are generally classified into two types:
I. Flexible Pavement
II. Rigid Pavement
A third of pavement has become popular during recent times, known as semi-rigid pavements.
I. Flexible Pavements: The pavements which have very low flexural strength and are flexible in their structural behavior under the wheel loads are called flexible pavements. Because of the low flexural strength, the pavement deforms if the subgrade deforms. Inflexible pavement, the vertical load of traffic is transmitted to the lower layer by grain to grain transfer. A well-compacted granular structure forms a good flexible pavement layer.
The load transferring ability to the flexible pavement depends on the type of material and the thickness of the pavement. Its thickness is so designed that the stress on the subgrade soil are kept within its bearing capacity and the subgrade is prevented from excessive deformations. Inflexible pavements, the subgrades play an important role as it carries the vehicle load transmitted through the pavement.
All bituminous pavements, water-bound macadam (W.B.M), gravel pavement, etc. are examples of flexible pavements.
II. Rigid Pavements: Rigid Pavement are those which possess considerable flexural rigidity.
A rigid pavement derives its capacity to withstand loads from flexural strength. The rigid pavement has slab action and it is capable of transmitting the loads to a wider area. The rigid pavement does not get deformed to the shape of the lower surface as it can bridge the minor variations of the lower layer.
Cement concrete pavement is the best example of rigid pavement.
III. Semi-Rigid Pavement: it represents an intermediate state between the flexible and the rigid paverpent. The flexural strength of such type of pavement is much lower than the concrete slab, but it drives support by the lateral distribution of load through the pavement depth and in flexible pavement.
Lean cement concrete, soil cement, and lime-pozzolanic concrete construction are examples of semi-rigid pavements.
What are the different components of pavements? Discuss their functions also.
A road pavement consists of the following components:
(1) Sub Grade: It is a layer of natural soil or filled soil prepared to receive the pavements materials over it.
The functions of subgrade are to transfer the entire load coming to it to the earth mass and to provide good support to the pavement structure. It is always essential that the stress transmitted to the top of the subgrade should not exceed at any time beyond the allowable limit. To avoid any deformation of the subgrade at least the top 50cm layer of the soil is well compacted under the controlled conditions of optimum moisture content and maximum dry density.
TYPES OF PAVEMENT ||SSCJE||RRBJE||
The strength properties of the soil subgrade are necessary to evaluate. The following tests on soil are carried out to find out its strength:
e) CBR test
f) Triavial Compression test
g) Plate load Test
h) California resistance value test.
(2) Sub Base Course: It is a layer of pavement.materials.provided between Subgrade and Base course
It is provided as an additional layer when the subgrade is of poor quality. It consists of broken stones, slag, broken burnt bricks otc, at the sub-base course it is desirable to use small-size aggregates for proper interlocking.
The Sub-base course has the following function:
a. It improves the bearing capacity of subgrade
b. It checks the capillary rise of subsoil water
C It prevents sub-grade material from working up into the base course. It eliminates frost heave in frost-affected areas.
(3) Base Course: It is a layer of pavements materials between surface .course and sub base course. Generally, large size particles like boulders, bricks, etc. are used as base courses. This course is considered the most important component of pavement structure because it has to bear the impact of traffic transferred through the wearing course.
It serves the following functions:
a) It acts as a foundation for the road pavements and transfers the traffic load to the sub-base
b) It withstands the high shearing stress imposed upon it due to impact to traffic on the wearing course.
(4) Wearing Course: It is the topmost layer of the pavements structure. Inflexible pavement normally a bituminous surfacing is used as a wearing course and in rigid pavements, the cement concrete acts like a base course as well as a wearing course.
It has the following functions:a) It provides a smooth and dense riding surface
b) It resists the pressure exerted by tires
c) It takes up wear and tear due to the traffic
d) It provides a water-tight layer against infiltration of surface water.
What is Water Bound Macadam (WBM)? Explain detailed procedure for its constructions.
It should consist of clean, crushed, or broken aggregates mechanically interlocked by rolling and bounded together with screening binding material and water.
The material used for W.B.M
The most common and durable material for use as aggregates in WBM is broken stone aggregates, crushed slag, over burnt bricks aggregates, and naturally occurring aggregates such as Kankar or laetrile are also used.
Methods for construction: The sub-grade or sub-base on which WBM is to be constructed should be prepared to the specified grade and camber. The prepared base should be free of dust and ruts. When WBM is to be laid directly over the sub-grade, a 25 mm thick course of screening should be separated over the prepared sub-grade before spreading of coarse aggregates. This layer acts as an inverted choke.
The coarse aggregates are spread uniformly and evenly over the prepared base. The quantities of aggregates spread should be such that the thickness of the compacted layer does not exceed 75mm. the spread aggregates surface should be checked from time to time with templates and any variation greater than 12mm are corrected.
After spreading of courses aggregates dry rolling is started by using 6 to 10 tones three-wheeled power roller. The rolling should start from the edges and gradually progress towards the centerline of the road. The movements of the roller should be such that each successive pass uniformly lapping proceeding track by at least one half-width.
After dry rolling screening are spread uniformly to fill the voids. Dry rolling and brimming should be done during the spreading of screenings.
After the screenings have been applied, the road surface is sprinkled with water. Swept and rolled. The sweeping is done with me help of hand brooms to fill the void and the operation should be continued till the coarse aggregates are thoroughly keyed.
after sprinkling and grouting, the binding filer material is applied in a thin layer. after the application of each layer, the surface is sprinkled with water and rolled during rolling water is applied to the wheels of the roller to wash done the binding materials sticking with wheels.
the road surface is allowed to dry overnight after final completion and a layer of sand or soil 6mm thick is spread on the surface sprinkled with water and rolled to fill and depression the wbm base then allowed to dry completely.
Also, Check This.