Highway Pavement: Layers, Functions, Types, Defects, Rigid and Flexible Pavements!
Pavement design, in general, consists of determining the thickness of the pavement or of the several layers of which it is composed in order to resist the wheel loads of the traffic and transmit them safely on to the foundation soil.
Thus, the strength of the pavement must be adequate to resist the contact pressure from the wheel loads, and the thickness must be sufficient to transmit this pressure on to a larger area of the foundation soil below to avoid excessive deformation or shear failure of the soil. This will ensure that the pavement structure is strong and stable during the entire design period to serve traffic needs.
The primary functions of a highway pavement are:
1. Provide a strong and smooth surface to resist traffic loads.
2. Distribute the loads safely on to a larger area of the foundation soil through the intermediate layers/courses
3. Carry traffic loads under repeated application during the anticipated design life without developing excessive or harmful deformations/strains.
In order to fulfill these functions, the requirements of a pavement are:
1. It should be strong enough structurally to withstand the stresses imposed by the traffic.
2. Its thickness should be adequate to transmit the applied loads and distribute them on to a larger area of the soil below so that the pressure transmitted is small.
3. It should provide a hard wearing surface so as to resist the abrasion caused by vehicle tyres.
4. It should be smooth enough to provide riding comfort, yet provide enough friction for tractive effort and to prevent skidding.
5. It should be impervious to water so as to prevent its deteriorating effect on the layers below.
6. It should have adequate durability to serve through its design period.
7. Its initial cost and maintenance cost during its design life should be a minimum.
Some of these requirements may appear to be conflicting; but a judicious compromise should be struck for good design of a pavement.
A pavement consists of layers or courses, in general, as shown in Fig. 7.1.
The courses/layers from bottom to top are:
i. Surface (or wearing) course
ii. Base course
iii. Sub-base course
iv. Subgrade soil.
The functions of each of these layers are given below in the reverse order:
i. Surface (or Wearing) Course:
This is the topmost layer; its function is to provide a smooth, strong, abrasion-resistant and reasonably impervious course. Since it is directly in contact with the vehicle tyres, it has to resist the imposed wheel loads and transmit them safely to the layer below. The material may be granular, bituminous or cement concrete depending upon the nature of the construction.
ii. Base Course:
This is immediately below the surface course and its function is to distribute the stresses transmitted through the surface course evenly onto the layers below. Invariably, it consists of granular or bituminous material, and acts as a structural part of the pavement.
iii. Sub-Base Course:
This comes just below the base course and provides additional help to the courses above it in distributing the loads. It also helps in preventing soil grains of the subgrade from intruding into the base course above, and counteracts frost action, if any. It may consist of stabilised soil or soil aggregate mixes, which facilitate drainage of free water from the pavement.
It is the compacted natural soil immediately below the pavement layers; this act as a foundation for the highway. The top surface of the subgrade is called the formation level.
Depending upon the alignment and the nature of the terrain, a roadway may be constructed over an embankment or a cutting, or at or nearly at the natural ground level. The formation of level, therefore, has to be properly decided to suit these conditions.
Evaluation of Subgrade Soil:
Subgrade soil, being natural soil compacted to the extent necessary, the criteria which govern its strength or bearing capacity are its ability to avoid shear failure and prevent harmful settlements (total as well as differential). At least top 50 cm of the subgrade soil is compacted to achieve the maximum dry density. The strength properties of the subgrade soil have to be determined for use in the design.
The common strength tests used for this purpose are:
1. Triaxial compression test
2. Plate bearing test
3. California bearing ratio (CBR) test
4. California resistance value test
The fourth involves the use of Hveem stabilometer and cohesion-meter; since this forms the basis of an empirical method of the design of what are known as ‘flexible pavements’.
The results from the triaxial compression test, the plate bearing test and the CBR test are used in some methods of pavement design.
Evaluation of Base and Sub-Base Courses:
These courses may contain primarily soil, or be made up of granular or stone aggregates. The above discussion relating to subgrade soil will be applicable for these courses also.
Evaluation of Surface Course:
If the surface course is of a bituminous mix, the Marshall Stability test is used. Evaluation of a cement concrete course may be done making use of a plate bearing test or Benkelman beam test.
Structurally speaking, pavements can be classified as:
A flexible pavement invariably consists of all the courses (component layers) as shown in Fig. 7.1. Thus, it is a multi-layered system with low flexural strength. The external loads are largely transmitted to the subgrade through the intervening layers-the base and the sub-base – by means of interlocking at the grain to grain contacts in the granular structure.
Lateral distribution of the compressive stresses on to a larger area with increasing depth is the basic mechanism of stress transfer. The thicknesses of the intervening courses are so designed as to keep the stresses transferred to the subgrade soil less than the allowable bearing pressure to ensure that deformations or settlements remain within permissible limits.
The load distribution capacity of each of these layers depends upon the nature of the materials and the mix design aspects. The top layer or the surface (or wearing) course, which is in direct contact with the traffic loads has to be necessarily the strongest, while the layers below can be of relatively lower strength.
The surface course, therefore, consists of a mix with a binder material like bitumen and mineral aggregates. The base and sub-base courses consists of granular materials like crushed stone aggregate, gravel and aggregate-soil mixes.
The base and sub-base courses may consist of more than one layer of slightly different materials and specifications. Another important characteristic of a flexible pavement is that the deformations (especially if excessive) of the subgrade are transmitted and reflected to the surface and vice versa; that is why it needs a strong subgrade for successful performance.
A rigid pavement, in contrast to a flexible one, derives its capacity to resist loads by virtue of its flexural strength. Flexural strength allows the pavement to bridge over minor irregularities or weak spots in the subgrade or other courses such as the base or sub-base upon which it rests. Thus, the inherent strength of the pavement slab itself plays a major role in resisting the wheel loads; this, however, cannot under-rate the need for a strong subgrade.
It simply means that, provided a certain minimum support is derived from the subgrade, the performance of the rigid pavement is governed by the strength of the pavement slab rather than by that of the subgrade. Rigid pavements consist of cement concrete (OPC), which may be plain, reinforced or pre-stressed concrete.
The primary difference between a rigid pavement and a flexible one is in the structural behaviour; the critical condition of stress is the maximum flexural stress in the pavement slab not only due to the wheel load, but also due to warping caused by changes in temperature in the summer and winter seasons, and during the day and night. The warping of the slab is caused by the temperature gradient between the top and bottom, and the consequent flexure.
Further, temperature changes tend to cause stresses due to friction at the interface between the slab and the layer below, which opposes the movement of the slab. A rigid pavement can serve the dual purpose of a base and a wearing course. However, it is not normally laid directly over the subgrade when the latter consists of fine-grained soil. Providing a base or a sub-base below the pavement can enhance the life of the pavement significantly, and may prove economical in the long run.
A semi-rigid pavement is intermediate between the flexible and the rigid types. In this type, a base course of lean cement concrete, soil-cement (soil mixed with cement for a binder), or lime-pozzolona concrete (lime-fly ash-aggregate mix) is provided. A suitable surface course is provided as in a flexible pavement. The semi-rigid pavement derives some flexural strength, but much less than that of a cement concrete pavement; however, the phenomenon of lateral distribution of loads through the pavement depth, provides support.
Under certain circumstances of traffic and availability of materials, a semi-rigid type of pavement may prove to be economical.
A composite pavement comprises multiple, structurally different layers of heterogeneous nature. A typical example is a concrete pavement of two layers, sandwiching a brick layer. A base of roller compacted concrete and surface course of bitumen is another example.
Pavements of bricks, stone blocks, and precast cement concrete blocks laid over granular bases may also be considered to come under this category.
Low-cost roads, in our country, consist of roads constructed primarily with soil using stabilisation techniques. From the structural point of view, it is of the flexible type. Loosely, a road with at least one stone-aggregate course may be said to be a paved one.
If no bituminous or concrete wearing course is provided over the granular soil or aggregate course, it is said to be an unsurfaced road.
Defects in Highway Pavements:
Defects in Flexible Pavements:
Flexible pavements, include earth, gravel, stabilised earth, WBM and bituminous roads.
Some defects arise solely because of lack of quality in the surfacing’s:
4. Plastic deformation
7. Loss of skid-resistance
Defects attributable to the deficiencies in base course/sub-base course/subgrade are:
i. Localised depressions/pot holes
ii. Road bumps due to frost heave
iii. Consolidation deformation/settlement
iv. Wavy surface
vii. Deformation due to lack of bond between layers
viii. Edge damage
ix. Streaking in bituminous surfaces.
A brief explanation of some of these is given below, along with the causes and symptoms:
This is the progressive dislodging of aggregates due to insufficient binder or its failure. This is a result of the action of traffic on a weak surface.
This is the separation of bitumen coating in the presence of moisture, leading to loss of binding action and loss of aggregate under the action of traffic.
Several different types of cracking occur: hair-line cracks and short and fine cracks at close intervals on the surface apart from those that appear due to insufficient binder, excessive filler or inadequate compaction.
Alligator Cracks (Map Cracks):
Cracks which are interconnected and which form a series of blocks (Fig. 10.1).
Transverse cracks, usually interconnected to form a series of large blocks.
Cracks along straight lines in the longitudinal direction along the road. Frost action or volume changes in the subgrade may result in these.
Cracks near the edges and parallel to them. Lack of shoulder support and poor drainage may result in these.
This is observed in bituminous overlays over existing cement concrete pavements. The crack pattern from the underlying surface is invariably reflected on the bituminous surfacing. Although this does not amount to structural failure as such, surface cracks allow rain water to percolate and cause weakening of subgrade support or result in what is known as ‘mud-pumping’ (Fig. 10.2).
4. Plastic Deformation:
This tends to occur in wet clay subgrade soil under excessive traffic loading and is not recoverable after removal of loads. This gradually gets reflected on to the surface, causing permanent deformation and surface defects.
This involves loss of aggregate in some portions. This could also lead to stripping, ravelling, edge damage and pot-holing.
This is due to collection of binder on the surface because of the presence of excessive binder in premix, spray, or tack coat, loss of aggregate cover, or heavy axle loads. The surface appears fatty.
7. Loss of Skid-Resistance:
This is due to the polishing of aggregates under traffic or due to excessive binder.
i. Localised Depressions/Pot Holes:
These are shallow depressions caused due to the presence of inadequate compaction in certain pockets.
ii. Road Bumps:
These are localised upward movements of the pavements due to frost heave or due to high clay content in sub-base/subgrade and consequent swelling on absorption of moisture.
This results in large deformation of the pavement owing to poor subgrade containing clay, as also inadequate compaction.
iv. Wavy Surface:
This consists of longitudinal/transverse undulations in the pavement surface with crests and troughs. This could be due to uneven subsidence of the base or the subgrade.
These are longitudinal depressions in the wheel tracks. Heavy channelized traffic, heavy steel-tyred traffic, or poor quality pavement materials and inadequate compaction could lead to this condition.
These form regular undulations, especially in the longitudinal direction. Faulty surface course, oscillations from vehicle springs and unstable mixes could lead to this condition, shown schematically in Fig. 10.3.
vii. Lack of Bond between Layers:
This leads to the formation of crescent-shaped cracks pointing in the direction of wheel thrust, causing slippage. This is due to lack of bond or its failure between the surface course and the underlying pavement courses.
This comprises irregular breakage of edges of the pavement. Poor shoulder support, percolation of water into the edge areas through shoulders and lack of adequate strength at edges could be the reasons.
ix. Streaking in Bituminous Surfaces:
The presence of alternate lean and heavy lines of bituminous binder on the pavement is known as streaking. Non-uniform application of bitumen or application at a temperature lower than the appropriate one for the particular grade could result in this condition.
The following are the typical and basic kinds of defects which are known to occur in rigid pavements or cement concrete pavements:
1. Scaling of concrete
2. Spalling of joints
4. Shrinkage cracking
5. Cracking due to warping
6. Structural cracking
The causes for the occurrence of these defects are:
(a) Deficiencies in pavement materials.
(b) Structural inadequacy of the pavement.
(c) Poor workmanship.
Material deficiencies could be in respect of the requirements for cement, fine aggregate, coarse aggregate, mixing water, joint fillers and sealing compounds.
Structural inadequacy could be due to inadequate pavement thickness, poor subgrade support, and inadequate joints.
Poor workmanship could be in respect of mixing, laying, compaction, screeding/finishing, and curing of the cement concrete; this could also be in respect of the provision of joints including load transfer devices such as dowel bars and tie-bars, and filling and sealing of the joints with appropriate materials and compounds.
A brief description/explanation of the defects listed is given below:
1. Scaling of Concrete:
Scaling means flakes coming off the surface. This is attributed to improper mix design, the effect of chemical impurities on the mix, and excessive vibration of the mix causing cement mortar to come to the surface and getting abraded by the traffic. Scaling causes rough surface, affecting riding quality.
2. Spalling of Joints:
When a joint filler is placed in position ahead of concreting, faulty alignment of the filler can occur. This may leads to a projection of concrete on one side, which may get chipped off by the impact of traffic and result in cracking and subsidence at the joint.
When the pavement slab tends to settle downward under heavy traffic, the sub-base/subgrade material which forms a slurry in the presence of water tends to work up through the joints and cracks, if any, on to the surface. This phenomenon is called mud-pumping and is common in rigid pavements. This is usually observed in rainy season in the case of clayey subgrades.
The pavement damage because of mud-pumping may appear as shown in Fig. 10.4:
4. Shrinkage Cracking:
Soon after construction, shrinkage cracks develop during the curing period of cement concrete pavements. These cracks may occur in both longitudinal and transverse directions. However, this may not cause permanent damage, especially under initial traffic loads.
5. Cracking due to Warping:
Warping cracks may occur if the warping joints meant to control the stresses due to warping are not properly designed. Properly designed hinge joint, along with adequate reinforcements at the longitudinal and transverse joints, will provide structural adequacy and prevent warping cracks.
6. Structural Cracking:
Structural inadequacy, arising out of the inadequate thickness of pavement for the traffic volume and repetitive loading that is imposed, leads to the formation of structural cracks.
Such cracks generally occur at the edges and corners of pavement slabs; in case such cracks are not due to other causes like spalling or mud-pumping, they may be attributed to structural inadequacy. Cracks in the middle/interior regions of the pavement slabs are due to temperature stresses.