Flexible Pavement Construction

In the 1970's the long practiced continental technique of flexible paving came to the UK. It has grown strongly since then and has now become the most widely used method of laying clay pavers.

The method of constructing a flexible pavement is straightforward. Pavers are placed upon a bed of uniform thick sharp sand within firm edge restraints, over a prepared, sub-base. Sand is brushed into the joints and the paved construction is compacted by several passes of a vibrating plate compactor.

The stability of paved areas is provided by frictional restraint of the sand grains in the joints. Applied loads on one paver are transferred to adjacent pavers; an effect known as interlock.

Their speed of construction and their ability to be trafficked immediately upon completion make them popular with specifiers and contractors alike. Flexible clay pavements can withstand heavy traffic loadings and are resistant to contaminants.

Flexible clay pavements provide excellent long term performance when care is taken in the structural design and construction of the underlying layers and the wearing surface. Reference should be made to BS7533-2: 2001 Guide for the Structural Design of Lightly Trafficked Pavements and BS7533-3: 2005 "Code of Practice for laying Flexible Pavements".

If excavation is required, such as laying of cables, pavers can be uplifted and reinstated once the work has been completed. Clay pavers out perform concrete block pavements because they have a permanent through colour which does not fade.

Clay pavers can also shrug off stains, in trafficked locations tests indicate that a 65mm thick clay paver will perform in an equivalent manner to an 80mm concrete block.

Sub-Base Design

The thickness of the sub-base is dependent upon the type and amount of traffic to be carried by the paved area, and the likelihood of the underlying subgrade to deform in use. To establish this factor a knowledge of the Californian Bearing Ratio (CBR) is necessary. The CBR can be estimated using Table 1 which has been extracted from BS7533-2 2001. The corresponding recommended sub-base thickness can then be estimated from Table 2, also taken from BS7533-2:2001.

If the sub-base is likely to be prone to frost then the sub-base should be adjusted to give a total pavement thickness of not less than 450mm. Wherever practicable the water table should be prevented from rising to within 600mm of the pavement surface. This can usually be achieved with the installation of subsoil drainage. The sub-base drain should be laid at low points below the level of the sub-base and perforated or open jointed pipes should be used. These should then be wrapped with a suitable filter fabric known as geotextile.

Where there is no roadbase, the surface of the compacted sub-base should be close textured to prevent migration of sand from the bedding course. It may be necessary to use "blinding" materials to achieve this. Roadbase Design

For lightly trafficked paved areas a roadbase is not normally required, provided that they are not expected to carry more than 0.5 million standard axles over their design life.

Where vehicle use is greater than this, or if the paved area is subject to channelised or constantly turning traffic then a roadbase should be constructed in accordance with the Department of Transport Specification for Highway Works 1994.

BS7533-2 :2005 requires that the roadbase should be levelled so that its surface level does not differ from the design level by ± 15mm.

The Bedding Course

There are two main methods of constructing the sand bedding course, precompaction and compaction after placing the pavers. It is now widely accepted that the precompaction method should be used.

For each site the bedding course sand should be obtained from a single source in order to maintain consistency. If the sand is stored it should be sheeted, to prevent saturation and to help maintain a uniform moisture content.

A method of laying the bedding course is as follows:-

1. The suitability of the road base or sub base should be checked. Consideration should be given to the use of waterproofing or overlay with a suitable geotextile. 
2. Sand is spread in a loose uncompacted layer. 
3. Screeding boards or rails are set up on site and the area is roughly screeded to a depth of 40-50mm. 
4. The sand bedding course is then compacted with two to three passes of a vibrator. The thickness of the uncompacted sand to achieve the finished surface profile after compaction needs to beestablished by trial and error at the start of the operation. As a guideline the uncompacted layer of sand will be approximately 15mm more than the desired finished compacted thickness. When compacted the bedding course sand should ideally be 30-35mm and no more than 50mm. 
5. After initial compaction a further thin layer of sand of approximately 10mm is placed and screeded. This provides a loose surface on which the clay pavers can be placed. It is recommended that at the end of the working day, the boundary of bedding course sand left should not exceed one metre beyond the laying course.