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X marks the spot

New research suggests that contrary to received wisdom the shape of pavers used for paving in ports and container terminals does affect structural performance. Chris Hodson and Colin Robinson explain

Why read this

Block pavers are established in port construction

Research indicates paver shape affects life and performance

Mechanical laying saves time and cost

Concrete block paving is well established for port, container and other heavy duty pavements. And tried and tested design guidance in Knapton and Meletiou's The structural design of heavy duty pavements for ports and other industries manual suggests that the type of paver - in terms of size and design - has little influence on overall structural performance.

However, recent research carried out in the UK by Professor Knapton at Newcastle University's department of civil engineering sheds new light on the influence of paver shape over the performance and design life of pavement surfaces.

The research project compared four different paver designs commonly used for mechanised laying.

This method of installation is expected to grow in popularity internationally, as it offers much faster laying rates (600m machine per day, compared with less than 50m 2manually).

This cost and time advantages are supported by recent independent evidence on the competitiveness of machine laid block paving compared with insitu concrete and asphalt pavements. In addition, European health and safety legislation now limits manual installation and this trend is expected to spread elsewhere.

The four pavers were tested using the NUROLF rolling load facility. They were laid in areas 2.5m square using the following construction: 80mm thick pavers, 30mm laying course material, 150mm lean concrete road base material and 250mm Type 1 granular sub-base material.

Each of the paver types was subjected to the same channelised full scale wheel loading up to 130,000 cumulative standard axles (csa).

NUROLF applies a vertical wheel load of 7,000kg/f through its offside wheel to the centre of the test area, as well as a horizontal force of 700kg/f.

The pavement base was designed to ensure that it remained undeformed, while allowing sufficient elastic deflection for the pavers to demonstrate their interlocking properties.

The deformations measured are all a result of the laying course material having either degraded or migrated as a result of the stresses imposed. Therefore, these deformations are a measure of the stress attenuation achieved by each of the test paver areas.

First, the trials clearly demonstrated the capabilities of larger pavers such as Types L and X to withstand substantial loading and trafficking without breakage.

At intervals, detailed surveys of the trial areas were carried out: characteristic maximum rut depths recorded at 130,000 csa are shown in Table 1, together with the approximate rate at which ruts developed, averaged from periodic measurements.

BS 7533 : 1992; the Guide for structural design of pavements constructed with clay or concrete block pavers, identifies that: 'It may be necessary to reset pavers during the life of a pavement if the rut depth exceeds 10mm'. Experience shows that for rectangular block paving (ie Type I) this generally occurs at about 8 million standard axles (msa).

The rate of rut development starts to reduce at 0.5 msa and continues to reduce with trafficking (in contrast to asphalt, which increases). Rut development rates can be interpolated against the experienced 8msa for rectangles to demonstrate that the Type X paver would not reach a 10mm rut depth until at least 12 msa.

This offers significant improvements in lifespan over the other three pavers and substantial reductions in whole life costs for heavily trafficked areas. Less trafficked areas can also benefit from the greatly reduced possibility of the need for premature, disruptive maintenance.

At the end of the trafficking trials, pull-out tests were executed with a rig which allowed the applied force and displacement of the paver to be measured.

Neighbouring pavers were held in place by application of the test equipment's reaction load. It was established that a deformation of 1.5mm indicated loss of interlock between pavers and pull-out forces at that point, together with paver footprint areas, are shown in Table 2.

Pull-out force values reveal that the Type X paver exhibits substantially enhanced resistance to vertical loads compared with the others.

Combining this attribute with a much larger footprint area (resulting in greater load dissipation) gives the Type X paver major capability advantages in dealing with concentrated point loads encountered at ports, such as container corner casings, trailer dolly wheels and crane stabilising jacks.

The research project report concludes: 'The Type X pavers achieve significantly greater levels of stress attenuation and surface stability than the other units and remain more stable under loading'.

Chris Hodson is managing director of XeneX International & Colin Robinson, is product development manager at RMC Concrete Products (UK).

The full report can be obtained from Hodsons@compuserve.com

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