Pavement Evaluation Devices

A number of devices are used for evaluation of existing pavement. The most commonly used ones are discussed below.

SCRIM (Sideways Force Coefficient Routine Investigation Machine)

SCRIM is the current standard skid resistance test device for trunk roads and motorways in the UK, as well as several other countries. It operates by dragging a wheel at an angle to its direction of travel. Though the wheel is free to rotate, the angular difference leads to a sideway force, which is measured, and which gives a coefficient of friction. The test is carried out on a wet road surface since the presence of water leads to minimum skid resistance.

TRACS (Traffic Condition Surveys)

The National Highways UK employs the TRACS survey for checking serviceability or surface condition of the trunk road network. TRACS surveys utilise traffic-speed image collection and laser profile measurement to automatically assess cracking, rutting, texture and ride quality.

SCANNER (Surface Condition Assessment for the National Network of Roads)

Local authorities in UK commissioned the first TRACS Type Surveys (TTS) on local roads in 2004/05, covering their principal (A) roads. The results from TTS surveys were reported as in 2005. Following the first round of research completed during 2004, a revised specification was introduced under the name of SCANNER.

The introduction of SCANNER technology on local authority roads was overseen by the SCANNER Implementation Group, on behalf of the Roads Board. The new specification expanded the application of traffic speed carriageway condition surveys to include all road types and surveys covering B and C class roads in UK commenced in 2005/06.

With the introduction of the new specification, SCANNER surveys became mandatory for the assessment of the classified road network.

TSD (Traffic Speed Deflectometer)

The TSD is a fully functional research tool capable of measuring the structural condition of asphalt road pavements at traffic speed. It has proven to be capable of assessing the structural strength of asphalt road pavements at speeds up to 80km/h. This has been achieved by developing an empirical relation between the deflection velocity (recorded by the TSD) and the vertical deflection (recorded by the Deflectograph).

FWD (Falling Weight Deflectometer)

The FWD is a non-destructive technique used to assess the structural capacity and performance of pavements by measuring their deflection under a load. The FWD is mainly used on highways, airports, ports and hardstanding areas, but can also be applied to subgrades and sub-bases during construction. The FWD is trailer mounted and is operated via a laptop from within a towing vehicle. Each individual test takes approximately one minute to conduct and distances between test points vary depending on survey requirements.

The basic principle of an FWD test is to determine the deflection (i.e. the vertical movement) of the pavement under a dynamic load. The load is created by dropping weights onto a series of buffers that transfer the force to a loading plate that sits on the pavement surface. The resulting downward deflection of the pavement is recorded by geophones positioned at known distances away from the loading plate which can measure deflections to an accuracy of ± 1 micron (1 thousandths of a millimetre). The measurement recorded at each geophone can then be used to measure an accurate deflection bowl of the whole pavement rather than just a central deflection. This then allows for an accurate picture of the entire pavement construction, from the bound layers to the condition of the underlying subgrade, to be calculated.

In addition to performing back-analysis of FWD data, other processes can be carried out such as joint condition assessments on concrete roads, void assessment, surface modulus comparison, ACN/PCN calculation of airfields and foundation performance assessment.

GPR (Ground Penetration Radar)

GPR is a geophysical method that uses radar pulses to image the subsurface. This non-destructive method uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) of the radio spectrum and detects the reflected signals from subsurface structures. GPR can have applications in a variety of media, including rock, soil, ice, fresh water, pavements and structures. In the right conditions, practitioners can use GPR to detect subsurface objects, thickness of pavement layers, density variations, sub surface moisture, steel reinforcement and voids.

Deflectograph?

The Deflectograph is used to assess the structural condition of flexible pavements. It works on the principle that as a loaded wheel passes over the pavement, the pavement deflects and the size of the deflection is related to the strength of the pavement layers and subgrade.

The structural condition of other flexible pavements is assessed in terms of residual life using a long-established Deflection Design Method based on deflection and traffic loading.

The Deflectograph is an automated deflection measuring system. It is a fully self-contained lorry-mounted system, whereby measurements of deflection are taken at approximately 4m intervals in both wheel-tracks while the machine is in motion. It is regarded by the Overseeing Organisation as the standard deflection measuring device for use on flexible pavements.

The Deflectograph measures the amount a flexible or flexible-composite road pavement bends under the weight of a standard axle. This information can be integrated with details of the pavement construction and present and future traffic flows to determine the residual life of the pavement and the recommended overlay.

Because the deflection is measured in the nearside and offside wheel tracks, it is easy to determine if the whole width of the road requires strengthening or just repair of the (generally) nearside wheel track.

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