Pavement Evaluation, Strengthening of existing Pavements and it's benefits

Function of Pavement Evaluation?

The main function of pavement evaluation is to assess the condition of the pavement so that the maintenance and strengthening jobs can be planned in time. The pavement evaluation can also be referred to as diagnosis of distress and to a large extent, it is subject to the knowledge, experience, and judgment of the Individual for maintenance.

What are the reason for pavement evaluation?

Following are the four reasons for deciding that a road requires to be maintained:

(1) Poor riding quality: This condition indicates that the riding comfort has deteriorated below an acceptable standard. It can be measured by roughness measuring devices such as roughometer or unevenness integrators.

(2) Slipperiness: This condition indicates that the surface no longer provides adequate resistance to skidding. It can be measured by means of skid trailers.

(3) Structural deterioration: This condition indicates that the structure of pavement has deteriorated to such an extent that its load-carrying ability is seriously affected. The plate bearing test may be used for the structural evaluation of both flexible and rigid pavements. The structural capacity of the pavement is assessed by the load carried at a specified deflection of the plate or by the amount of deflection at a specified load on the plate.

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The Benkelman Beam device is used for the structural evaluation of flexible pavements. The beam was originally devised by A. C. Benkelman in U.S.A. and it is used for measuring the deflection of flexible pavement under the action of the moving loads. The higher rebound deflection values indicate good performance of the pavement and such pavements are more durable and can withstand comparatively heavy loads. The rebound deflection data is employed to indicate the present performance as well as the durability of the road structure.

***Fig2. BENKELMAN beam apparatus for use- cementconcrete.org***

(4) Other surface deterioration: This condition indicates pot holes, ravelling, spalling of concrete joints, etc. Its assessment, although subjective, can be guided by the prepared check-lists or guidelines.

It may be noted that two or more of the above reasons occur at the same time and can probably be rectified by a single treatment.

The pavement serviceability concept was introduced at the AASHO (American Association of the State Highway Officials) Road Test at Ottawa, Illinois, U.S.A.. It was decided that the road user should determine whether or not a pavement is satisfactory. The Present Serviceability Index (PSI) was obtained by co-relating user opinions with measurements of road roughness and the extent of cracking, patching, and rutting.

A panel of users drives over selected pavements and rates the pavements using an appropriate scale. The rating scale for this study varies from 0 to 5. A rating of zero denoted an impassable pavement whereas a rating of 5 indicated a perfectly smooth pavement. The raters are asked to mark on the scale the number which indicated their opinion of the road at the time when it is rated.

In addition, the raters are asked to give their opinions to the objective features like rutting and cracking of the pavement which influence their rating and are further asked to state whether the road is acceptable for traffic.

The PSI is then established from regression equations which relate user opinions with objective measurements. Many agencies in U.S.A. have developed their own equations based on a correlation between rating panels and roughometer readings and observations of cracking and patching on their highways.

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Strengthening of existing Pavements

The pavements should have adequate stability to withstand the designed traffic load under prevailing climatic and subgrade conditions. If the intensity of traffic has increased due to some reasons, the existing pavements are to be strengthened by suitable measures so as to adjust to the changed conditions or alternatively, new routes will have to be developed to divert the flow of traffic.

It should also be remembered that strengthening of highly deteriorated existing pavement will not serve any purpose. The best solution under such circumstances would be to remove the existing pavement structure and rebuild the same.

***Fig.2. pavement Strengthening process***

The strengthening of the existing pavement is carried out by providing an overlay which indicates the additional thickness of pavement of adequate thickness in one or more layers over the existing pavement. It is thus a corrective action that is applied when the pavement has reached a particular stage.

For instance, if the agency having jurisdiction has predetermined the minimum tolerable level of PSI as 2.5, the overlay is provided on the existing pavement as soon as that level is reached.

There are various methods available for the determination of overlay thickness. But the results obtained by these methods are likely to differ to a great extent because of the following facts:

(i) All the design methods are based on the inherent assumption that the proper specifications and construction procedures will be used.

(ii) Extreme care should be taken for the selection of coefficients for reflecting the load-carrying capacity of the existing pavement. The improper selection of coefficient may lead to large errors in the overlay thickness. It is also to be noted that the selection of coefficient is tied to pavement condition rather than load-carrying capacity.

(iii) The pavement evaluation procedure using deflection is also subject to error. It is likely that an acceptable deflection may vary with the thickness combinations for the various pavement structure layers. Hence, some combinations that present acceptable deflections may not be satisfactory from the stress point of view.

The I.R.C. has given guidelines for overlay thickness under various conditions of traffic and various types of pavement materials. They should be properly studied and examined before recommending a particular overlay thickness for the strengthening of the existing pavement.

Table 1. shows the usual combinations for overlay. In the majority of cases, it will consist of a bituminous overlay although concrete overlays are being used more frequently than in the past.

Table1. Combinations for Overlay

*No.*	*Existing pavement*	*Overlay*
1	Bituminous or flexible	Cement concrete or rigid
2	Bituminous or flexible	Bituminous or flexible
3.	Cement concrete or rigid	Cement concrete or rigid
4.	Cement concrete or rigid	Bituminous or flexible

IMPORTANT PRINCIPLES IN ROAD IMPROVEMENT

In making investigations to determine whether or not a certain road improvement is advisable and to select the most economical type of surfacing for the new road, the following three important principles should be kept in mind:

Economy of road improvement
Use of low cost roads
Wear of roads.

Each of the above principle will now be briefly described.

(1) Economy of road improvement: The improved roads are not luxuries to be enjoyed only if they can be afforded, but are essential to the welfare of the community. It is necessary for each civilized community to provide itself with routes for the easy and speedy movements of men and materials from place to place.

With the advent of the automobile and the steady increase in the number of motor vehicles in use, the highways have assumed ever greater importance as modes for transportation. A stage has now come when a community can no longer content itself with a badly maintained road or one that has an unimproved and rutted surface or one that runs up hill and downhill or one that twists and turns around every natural obstacle.

The operation of a motor vehicle on such roads would involve many wearing and time consuming delays and high operating costs in excessive petrol consumption and in wear and tear on the tyres and the various parts of the vehicle.

In order to facilitate travel and to reduce operating costs for every owner of a motor vehicle who uses the road, the hills should be cut, the valleys should be filled, substantial bridges should be built over the streams and a firm and durable surfacing should be provided. All such improvements would certainly involve considerable expenditures. But they are often more than offset by the savings in vehicular operating costs alone that result from the improvements.

. Use of low cost roads:

It is erroneous to assume that all roads should be hard-surfaced. As a matter of fact, it will be more logical to link the type of road with the intensity of traffic.

If but few vehicles are expected on a road, sufficient improvement may be made by reducing excessively steep grades and shaping the natural surface so as to provide for adequate drainage.

When the intensity of traffic increases and it becomes impracticable to maintain the unsurfaced road in continuous good condition, the road may be provided with a low type or intermediate type surfacing.

Still further when the traffic becomes heavier than an intermediate type surfacing can carry without excessive wear, it can be economically justified to provide a high type surfacing. Thus all the roads should be improved to the maximum degree as the traffic on them will justify.

If a relatively small number of vehicles are likely to travel over the road, the use of low cost roads would certainly result in less investment, easy maintenance and quite effective in reducing the cost of travel.

Wear of roads:

It should be remembered that there is no such thing as a permanent road. All roads, regardless of the type of surfacing, gradually wear out under the wheels of vehicles and the action of natural agencies.

In order to keep the road in continuously good condition and to prolong its useful life, immediate attention must be given to the necessary repairs.

However, no matter how well the repairs are made, an entirely new surface will eventually be required. Of course, the high type surfacings have a greater economic life than the lower types, i.e., they will last longer without resurfacing, provided each type receives the same careful and continuous maintenance and is subjected only to the intensity of traffic for which it is suitable.

Any road will be worn out quickly by traffic for which it was not designed and the rate of wear will be increased by the lack of maintenance.

What are benefits of Improved highways?

There are various benefits which are granted by the improved highways and for the purpose of convenience, they can be enumerated as follows:

(i) cheaper transport;

(ii) development of commerce, industry and agriculture;

(iii) development of intellectual and social life;

(iv) development of natural resources;

(v ) development of recreational facilities including touring;

(vi) good defence from military point of view;

(vii ) good transport facilities;

(viii) increase in fire protection;

(ix) increase in land values;

(x) increase in rural mail delivery;

(xi) increase in sanitary and medical protection; etc.

In considering the above mentioned benefits of the improved roads, it must not be overlooked that hundreds of thousands of wage earners are employed in building the roads and in manufacturing the vehicles that use them.