Pavement Design - Thickness Design

The thckness design of the pavement is the determination of the overall thickness of the road and the thickness of the individual layers. This is of course dependant on the type of material chosen for the road. This is explained in more detail below. The procedure described in this page is that in the Design Manual for Roads and Bridges, Volume 7.

The Map below is a clickable image.

Design Map - Thickness The map alongside can be used as a guide to the process of Thickness Design.

Clicking on the desired area of the map will take you to the desired section. When you have finished a particular section, if you wish to return to the map then use the back button on the browser or click on the map link.

It is highly recommended that you follow the logical progression of the map
Figure 1 - Foundation Design Process

As has been previously discussed, there are many methods of thickness design and nearly every country has adopted different methods. A review of several of these methods has been carried out by the Permanent International Association of Road Congress 1. The method discussed in these pages is that commonly used in the United Kingdom.

At this point, it is necessary to have ascertained the vehicle loading on the road surface. This is not an indication of the total traffic flow nor is it intended to design the road layout. It is solely relevant to the engineer and used to design the pavement thickness. This is covered in more detail and hopefully somewhat better explained on the traffic loading page. Thus you should now have two pieces of information, the CBR value and a vehicle loading in the left hand lane (right hand lane outside the UK) in millions of standard axles (msa).

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Design Implementation

As has been previously discussed, there are four standard designs in common use in the United Kingdom at present:

The design procedure for each type of pavement is essentially quite similar and it is standard to produce alternative designs for each of the types.

An explanation of the different materials that are used in road design is contained in the Materials section of this page. If you are unsure as to the meaning of any acronyms then holding the mouse over the highlighted text will produce an explanation in the dialogue box at the bottom of the page.

Flexible Pavement

A flexible pavement is constructed of several layers as shown below:

Figure 2
Flexible Pavement Layout

The sub-base has already been designed on the foundation page, a link to which is at the bottom of this page.

The total thickness of the combined bituminous layer is then determined from
Figure 2.1 - Design Thickness for Flexible Pavements.

This is on a separate page so as to reduce the page download time. It may take a few seconds for the page to download but please be patient. It is essential that you look at the chart before progressing any further.

The final design of the pavement is then dependent on the material chosen for the wearing course. It is standard to use a wearing course consisting of either

If PA is used then it's contribution to the bituminous design thickness is 20mm. A 60mm base course of either HRA , Dense Bitumen Macadam (DBM), or High Density Macadam (HDM) is required below a PA wearing course.

If HRA is used then a base course is optional. If used it may be of any permitted material and should be at least 50mm thick.

Base courses over HDM should also be HDM

Example

Design traffic75 msa
RoadbaseHDM
Design Thickness320mm

Design Options given that HDM is the chosen material and thus from the design chart the total thickness is 320mm

  1. 45mm HRA wearing course
    55mm HDM base course
    220mm HDM roadbase

    Choosing a HRA wearing course with a standard thickness of 45mm. Assuming the use of a HDM roadbase and an optional base course, the base course should be at least 50mm thick and must be of the same material as the roadbase. The remainder is made up of the HDM roadbase.

  2. 45mm HRA w/c
    275mm HDM r/b

    Choosing a HRA wearing course with a standard thickness of 45mm. Assuming the use of a HDM roadbase and no optional base course, the remainder is made up of the HDM roadbase.

  3. 50mm Porous asphalt w/c
    60mm HDM b/c
    240mm HDM roadbase

    Choosing a PA wearing course with a standard thickness of 50mm. Assuming the use of a HDM roadbase then a 60mm base course is required. The contribution of the PA wearing course is 20mm and the remainder is made up of the HDM roadbase.


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Rigid Pavement Design

A flexible pavement is constructed of several layers as shown below:

Figure 3
Rigid Pavement Layout

Again the sub-base has already been designed on the foundation page, a link to which is at the bottom of this page.

The total thickness of the combined bituminous layer is then determined from
Figure 2.2 - Design Thickness for Rigid Pavements.

Reinforcement Design

As can be seen from the design chart the thickness of the concrete slab is dependent on the area of reinforcement used. Normal design procedure is to produce a design for each of the four curves shown, that is the different areas of reinforcement. The area of reinforcement is generally 0.3% of the total cross sectional area. However if cracking is to be prevented then the area is increased to 0.6%. This is known as a continuously reinforced pavement.

Joint Design

There are three types of joint, expansion contraction and warping. Typical designs are shown in Figure 4 below. Contraction and expansion joints are called Transverse joints and the warping joint is a longitudinal joint.
Contraction joints allow the slab to shorten as it's temperature drops.
Expansion joints allow the slab to expand as it's temperature increases above that at which it was cast.
Warping joints tie the slabs together and can be thought of as hinges in the slab.

Design Thickness for Flexible Paving
Figure 4
Pavement Joint Types

At the contraction joint, there is a crack inducer so that if cracking does occur then it is in the area of reinforcement.

Joint Spacing- The spacing of the joints depends on the type of pavement you wish to design.

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Flexible Composite Pavement Design

Flexible Composite pavements also have a similar design procedure to the other forms of pavement. Design charts however have not been included but can be found in the Design Manual for Roads and Bridges, Volume 7, Section2, Part3, Chapter3.

This design consists of a standard flexible pavement with a Cement bound roadbase. The thicknesses are dependent on the materials chosen but can be read directly from the design manual.

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Rigid Composite Pavement Design

Rigid Composite pavements have a similar design procedure to the other forms of pavement. The layout of the pavement is shown below. Design charts however have not been included but can be found in the Design Manual for Roads and Bridges, Volume 7, Section2, Part3, Chapter3.

Figure 4
Rigid Composite
Pavement Layout

This design consists of a standard rigid pavement with a bituminous overlay. The thicknesses are dependent on the materials chosen but can be read directly from the design manual.

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Materials

As has already become obvious, there are many different alternatives when it comes to choosing the materials to use in the pavement design. The material chosen depends on many factors, including availability, cost, and feasibility of design.

It is standard practice to design the road using each of the available options and then to compare them using the above criteria.

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Roadbase Materials

Flexible

Flexible Pavements are constructed of the following materials, details of which can be found in British Standards2,3.

Rigid

All rigid and rigid composite should be constructed using Pavement Quality Concrete, manufactured, cured and laid in accordance with the Specification for Highway Works (MCHW1)4 series 1000. The use of joints is discussed in the pavement design section above.

Surfacing Materials

This is also known as the wearing course and is described as such in the design charts.

There are three possibilities for the wearing course, Hot Rolled Asphalt (HRA), Porous Asphalt (PA) or concrete. The use of concrete has been described above should be used as described.

Design Criteria

The designs discussed above are based on the Design Manual for Roads and bridges, which in turn is based on the work of the Transport Research Laboratory. This design manual is only applicable to roads to be built in the United Kingdom and only for trunk roads, Motorways and other multi-laned roads.

When comparing the relative benefits of the types of road, it is necessary to compare both over a similar time period. This is normally the design life, which is typically 20 years for flexible and flexible composite pavements and 40 years for rigid and rigid composite pavements. As such, it is necessary to include all maintenance costs and residual values of the road.

Analytical Design

The design charts presented above are based mainly on empirical results and full scale experiments. Some work however has included an analytical approach to the design. This is based on the stresses and strains induced in the pavement by an applied wheel loading. It is however very complicated and rarely used and as such is not covered in these pages.

Problems

Using the same example as in the traffic analysis page, that is a design loading of 5.13 millions of standard axles (msa), calculate the following:

  1. Given the following soil test data suggest appropriate capping and sub-base layers for the road:
    Moisture Content (%)Bulk Density (Mg/m3)CBR (%)
    5.01.57523.0
    7.51.84420.0
    10.02.1895.2
    12.52.1322.1
    15.02.0821.0

  2. If the road were to be constructed using a flexible pavement design, suggest appropriate thicknesses for the wearing course, base course and roadbase layers.
  3. If the road were to be constructed using a rigid pavement design, suggest appropriate thicknesses for the concrete slab. Comment on the amount of reinforcement and the spacing of the expansion and contraction joints.

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References

1) Permanent International Association of Road Congress, Report to XVth congress, Mexico City, 1975, from the Technical Committee on Flexible Pavements

2) BS4987; Parts 1 and 2 Coated Macadam for Roads and Other Paved Areas, BSI, 1988

3) BS594; Part 1 Hot Rolled Asphalt for Roads and other Paved Areas, BSI, 1992

3) Department of Transport, Manual of Contract Documents for Highway Works, Vol 1, Specification for Highway Works, 1993

e-mail: D.G.Toll@Durham.ac.uk.

Last Updated: 25 February 1997