Maintaining the Nation's Roads

Wednesday 04 February 2009

John Booth, Principal Engineer at Soil Mechanics looks at road pavement testing

The investigation of road pavement structure is becoming increasingly important as the UK road network we all take for granted continues to age. A recent 'Road Maintenance' study by the Asphalt Industry Alliance for instance, indicates that the majority of Local Authorities report an increase in the need for structural maintenance work over the last ten years with average regional increases ranging between 30% and 59%.

Over the years roads have been built using a wide variety of materials, particularly those that have evolved through successive maintenance treatments, and any investigation needs to take into account, and consider the implications of, this variety of construction.

Gaining a good understanding of asset condition and performance will mean that the potential for losses and deterioration is better quantified, and will ultimately lower total life costs.

In practice, the way in which road construction is investigated can vary widely. The key to selecting the most effective techniques and methods of dealing with a variety of road construction materials is engineering knowledge and experience.

The starting point, as with any site investigation is to be aware at the outset what information will be needed to produce a successful strengthening design.

New roads are designed using either the design charts provided by the Highways Agency in the Design Manual for Roads and Bridges, or by some form of structural analysis, normally linear elastic modelling.

When examining an existing construction the same options can be considered, either to compare the construction thicknesses to current design practice, or apply a modelling method. The latter is likely to produce the most economic remediation package but it does require more information, particularly about the strength and condition of the pavement layers. It is worth noting that current design charts are also based on linear elastic theory, but implicit within them are assumptions about the likely strengths of the various layers.

Possible approaches to data acquisition which have been successfully used fall into three categories. These are in-situ testing of the whole construction, invasive investigations, and laboratory testing.

The first category includes the deflectograph and the falling weight deflectometer. The first of these has been in use for many years both as a maintenance indicator and as a design aid, although its use as a national maintenance indicator has now been discontinued in favour of other devices that operate at traffic speed. The second is usually used on a scheme specific basis and can provide a detailed analysis of the road's carrying capacity. Both devices require accurate input of the existing construction details to produce meaningful results.

The second category; invasive techniques, include coring, usually using concrete core drill bits, and trial pits or trenches. High quality coring is essential for providing information about the bound construction, including material type and thickness. They also frequently provide the optimum means of obtaining samples of the bound layers for laboratory testing. Trial pitting is more useful for examining the unbound layers such as the sub base, capping and sub grade. Some in situ test techniques can also provide valuable data. The use of the TRL dynamic cone penetrometer through core holes can provide information about the sub grade as well as the unbound layers, and is especially useful in the absence of trial pits. The light weight deflectometer can be used to provide strength characteristics of unbound layers, whilst Californian Bearing Ratio tests (CBR) and plate load tests can also be used for in situ evaluation of the mechanical strength of pavement sub grades and base courses.

A primary decision will naturally be whether to test in the laboratory, in situ or a combination, however the relative merits of the former need to be aimed mainly at establishing accurate construction thickness details, precise descriptions, in road engineering terms, of the materials used, and the strengths of the various bound materials used in the construction. Thickness and material descriptions can also be achieved through examination and logging by experienced engineers and technicians.

Determining the compressive strength of concrete is straight forward, and tensile strength is also important. For bituminous bound materials there are mainly two strength related properties. Firstly there is what might simply be called load spreading ability, usually known as stiffness; secondly there is rut resistance which measures the ability of the material to resist displacement under traffic loadings.

Another issue that must be considered if the road construction is to be excavated relates to the presence of tar in older roads. Any road that contains bituminous materials placed before about 1984 may contain tar in one or more layers. Tar increases the health risks to workers and can only be disposed of at sites licensed to take hazardous waste.

This article really only scratches the surface of road pavement testing however does cover some of the main points for optimising road pavement investigations. Other matters that may come into consideration include road surface properties, such as skid resistance, and recycling materials that arise from road maintenance work, an important issue with the increasing costs of new imported materials and the higher costs involved in disposing of excavated material as waste.

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