Designing and Construction of Pavement Overlays and Inlays | Civil Engineering Optional for UPSC PDF Download

• Using the empirical relationships established from the AASHTO Road Test, the required overlay thickness can be determined considering traffic loads, environmental factors, and the existing pavement condition.

• One common tool is the AASHTO Guide for Design of Pavement Structures which uses the concept of structural number (SN) to determine thickness.

Three techniques are normally used by the UK pavement engineers to assess the strength of existing pavements and overlay design are as follows:

1. Deflection Beam Method (TRL)
2. Falling Weight Deflectometer Method or Pulse Load Method (Shell)
3. Component Overlay Design Method

1. Deflection Beam Method (TRL)
The first method is used widely in the UK and allows engineers to design overlays for most types of bituminous highway pavements. A deflection beam is used to measure the elastic deflection of the pavement under a standard heavy wheel load moving at creep speed. The measured deflection may then be used to predict the future structural performance for the pavement and to select strengthening course thickness.

2. Falling Weight Deflectometer Method or Pulse Load Method
Falling Weight Deflectometer (FWD) or Pulse Load Method is based upon measuring the elastic deflection in the pavement beneath a 150kg mass which is dropped through 400mm on to the surface of the pavement. An arrangement of springs converts the impact load into an equivalent load of 600kg acting, for a short time, on the pavement. The deflection is recorded electronically, using equipment in a field vehicle.
The method suffers from the same limitation as the TRL method in that is applies only to bituminous pavements subject to highway traffic.

3. Component Overlay Design Method
This method is commonly used, and the steps involved in this design method are explained in the following.

• In the component overlay design method, the condition of each course in an existing pavement should be expressed as an equivalent thickness of a standard material. This allows the current condition of dilapidated materials to be assessed numerically. The standard material chosen could be DBM 100 pen.
• The thickness and condition of each course of the existing pavement should be determined.
• Once each type of material within the pavement is identified, Material Condition Factor (MCF) should be assigned to that material. The true thickness of each course in the pavement should then be multiplied by the appropriate MCF to obtain the equivalent thickness of DBM.
• In order to account for any degradation which may have taken place in each course of pavement since it was first constructed the equivalent thickness of each course should be multiplied by two condition factors (CF1 and CF2) which accounts for the less than perfect condition of the material in the pavement. CF1 should be applied to take account of cracking and spalling of the pavement materials and CF2 should be applied to account for rutting and settlement. (Note: If the deterioration of the surface or road base is a result of failure in the sub-base, the subgrade improvement or the subgrade, it may not be possible to overlay the existing pavement.
• Once the existing pavement has been transformed into an equivalent thickness of DBM, a pavement should be designed in accordance with the general method of pavement design and that theoretical pavement should be transformed into an equivalent thickness of DBM using the material conversion factors. By comparing the equivalent thickness of DBM in the existing pavement with the equivalent, thickness of DBM required for the new pavement, a determination can be made as to whether the provision of a clay or concrete block paver overlay will provide a satisfactory solution. It may be that an additional course of material should be provided beneath the clay or concrete block pavers in order to bring the overlain pavement to a satisfactory structural condition.
• If the levels or required falls for the finished surface after paving are different from those of the existing surface, planning out or regulation with appropriate bituminous material will be required. The effect of such removal or addition should be considered during the evaluation for overlaying. It is important that the surface on which laying course material is laid should be such as to ensure a uniform thickness of laying course material everywhere.

Both the AASHTO and UK procedures are rigorous and have been developed based on extensive research and field observations. It’s essential to follow the specific guidelines and standards for each region and consult the latest editions of their design manuals since the field of pavement engineering is continually evolving.