Pavement Overlay Design Methods

A well designed and constructed pavement should remain serviceable for the period for which the designer intended. During its service life, a pavement is subjected to applied surface stress from traffic and to internal stress caused by restraint to thermal and moisture movement. Even a well-designed pavement may be damaged by being overloaded or being subjected to abnormal internal stress during particularly severe weather. The pavement may remain serviceable throughout its design life or for only part of it but at some stage it may need to be strengthened, otherwise it will have to be taken out of service.

In many types of pavement, once deterioration commences, total un-serviceability is imminent and rapid degradation takes place over a short interval, particularly during severe weather. If remedial work is undertaken before deterioration becomes severe, the residual strength of the existing pavement can be taken advantage of so that lower cost strengthening can be undertaken which will extend the pavement’s life considerably. It is stressed that the time interval between the onset of degradation and complete failure is rapid and the difference in strengthening cost can rise substantially if strengthening is delayed.

Once the residual strength of a pavement has been assessed, the overlay design technique must be capable of selecting the thickness and properties of strengthening materials. The purpose of strengthening may be to extend the life of the pavement or to allow an existing pavement to carry heavier handling plant. The second reason for strengthening a pavement is of particular relevance to ports.

The term overlay refers to the addition of a new layer or layers without removal of any existing pavement (other than removing 10 mm or so to ensure a good key between old and new materials).

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

1. Deflection Beam Method (TRL)
2. Falling Weight Deflectometer Method or Pulse Load Method (Shell)
3. Component Analysis Method (The Asphalt Institute, USA)

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.

In this method, the chart specifies the thickness of overlay required to strengthen a pavement of given deflection (by Deflection Beam) in order to achieve a desired extension of life, expressed in standard axle load. The following information is required:

• Deflection (by Deflection Beam)
• Cumulative standard axle carried by the existing road
• Estimate of the future traffic expected
• Type and condition of pavement (remaining life etc.)
• Classification of sub grade structure to determine its CBR

Nevertheless, this method has limitations with regard to heavier loads in ports and it is applicable to only bituminous pavements. (Full details are given in TRL Reports (LR 833, LR 834 and LR 835).

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.

Component Analysis Method

The method was first proposed by the Asphalt Institute Maryland, USA. A major modification is that, whereas the Asphalt Institute method transforms each course in a pavement to its equivalent thickness of asphalt, in this section the transformation is to an equivalent thickness C Cement Bound Granular Material (CBGM), the standard material used in the new pavement design method.

The Modified Component Analysis Method is applicable to both rigid and flexible pavement. The transformation to an equivalent thickness of lean concrete is accomplished using Conversion Factors shown in the Table below.

Note: Concrete referred to as C means concrete with 28 days characteristic compressive cube strength of 20N/mm2. Where two numbers follow C, the first is characteristic compressive cylinder strength and the second is characteristic compressive cube strength.

The existing pavement is transformed into an equivalent thickness of CCBGM. The equivalent thickness of C is that which would be required to give the same load carrying capability as the existing pavement. The existing pavement constitutes a part of the strengthened pavement, so it is essential to determine accurately the thickness of each of the existing courses and the degree of degradation that each of those courses has undergone.

If records of original design of the pavement are not available it will be necessary to take either cores or trials holes to obtain this information. Even if records do exist, cores or trial holes should be taken to verify the actual situation. These tests should be carried out so that each one represents approximately 500 m2 of pavement. There should be a minimum of three tests and a maximum of 7 for larger pavements of uniform construction and condition. Where areas of a heavy duty pavement are used for dissimilar types of traffic, then each location should be considered as a separate area for analysis purposes. Similarly if the initial cores show that certain areas of pavement are stronger than others, it may be preferable to divide the overlay area into several zones and each zone should then have at least three cores taken.

In certain circumstances the properties of the materials may have changed since they were initially used, owing to cementing action or intrusion of materials from another pavement course, and it is essential to know whether this has occurred. Sampling should also be sued to determine the condition of each course so that the appropriate Condition Factors may be selected. It may be difficult to assess the condition of lower pavement courses, particularly with regard to cracking. In such situations, conservative assumption should be made.

Once each course has been identified, it is transformed to an equivalent thickness of C CBGM by multiplying its actual thickness by the appropriate Material Equivalence Factor from the above Table.

The transformed thickness is multiplied by two Condition Factors. Values for the first Condition Factor CF1 are given in following Table and are used for both rigid and flexible pavement.

Values for the second Condition Factor CF2 are shown in following table. CF2 takes into account the reduction in strength of each course as a result of rutting and settlement in the surface of flexible pavement. This is measured as a difference in levels under a 3-metre straight edge. If a pavement has deformed, cores should be taken to determine which courses of the pavement are affected. When there is no deformation or cracking, the Condition Factors are taken as 1.0 i.e. the material is as new.?

The transformation procedure is carried out for each course in the pavement and the sum of the transformed thicknesses is taken as the equivalent thickness of the pavement. The equivalent thickness is used in the design of the overlay.

The design procedure is similar for each type of material except that where pavement quality concrete is required as an over-slab, the thickness of equivalent C CBGM slab has to be multiplied by 0.62 before subtracting it from the thickness of the newly designed pavement.

In order to derive the thickness of the overlay it is first essential to design a new pavement structure for the design criteria required, using the new pavement design method. The design criteria are:

• Design life
• CBR of subgrade
• Single Equivalent Wheel Load (SEWL)
• Type of Overlay considered

The designed new pavement consists of a CCBGM base course with concrete block paving surfacing. The equivalent thickness of the transformed pavement is then subtracted from the thickness of the C CBGM base determined from the new pavement design chart. This gives the thickness of the overlay. If in situ concrete is to be used, the CBGM equivalent pavement thickness is multiplied by 0.62 to transform it into an equivalent thickness of C concrete in situ concrete. This is then subtracted from the thickness of C concrete required for the new pavement to give the thickness of over-slab required. Note that although the method produces an overlay thickness for CCBGM, other materials can be used as the overlay by using Materials Equivalent Factors from Table shown above.