Guidance

Section 5: measuring road structural condition

Published 17 December 2024

Applies to England

Deflectograph

It is considered useful to complement information on the visual condition of major roads with information about their structural condition. Many highway authorities obtain information about the structural condition of parts of their major road networks by carrying out Deflectograph surveys.

The Deflectograph (pictured in Figure 1) is a machine for assessing the structural condition of flexible pavements. The survey vehicle employs a beam, which is laid onto the surface of the pavement. The survey vehicle drives over the beam and the amount the beam tip deflects under the load of the vehicle is measured. The survey vehicle then draws the beam along the road to the next measurement point.  As a result of this survey method, the Deflectograph survey is carried out at slow speed (2.5km/h).

Figure 1: Example of Deflectograph vehicle

Until 2000, routine Deflectograph surveys were carried out on the trunk road network at a rate of approximately 25% of Lane 1 in each year. However, because of the increasing traffic delays caused by traffic management, and with the introduction of routine TRACS surveys, routine Deflectograph surveys were discontinued, and the device is now used for scheme level assessment only, covering around 10% of the National Highways (NH, formerly Highways England) strategic Lane 1 road network each year.

On local roads, there has also been a reduction in the use of Deflectograph as a routine tool, following the introduction of SCANNER surveys. Again, the technique is still used for scheme level assessment. However, some local authorities still undertake routine assessments of certain lengths of their road networks with this device.

As a result of the discontinuing of the Deflectograph as a routine assessments tool on trunk roads, it is not possible to collate representative national statistics for this device for trunk roads. The significant decline in its use on English local roads, and the lack of nationally collated data, also makes it impossible to collate representative national statistics for this device for local roads. However, summary information has been provided in this technical note because of the widespread use of the method as a scheme level tool.

Because of its use for scheme level assessment, National Highways continues to implement an annual correlation trial for Deflectograph survey vehicles, which is currently carried out by TRL. All Deflectograph vehicles undertaking surveys on trunk road network are required to pass this. Other Deflectograph survey operators are also able to attend the trials, although it is not compulsory. However, as local authorities commissioning these surveys expect that the Deflectograph vehicles used will have passed the trials, it has been found that the entire UK Deflectograph fleet attend the trials. During the trials, the Deflectographs are required to undertake comparative surveys on a number of sites having different levels of structural strength. The trials therefore aim to ensure consistency across the Deflectograph fleet in the UK.

Deflectograph data

Deflectograph data are recorded as point values describing the deflection of the measurement beam in each wheel path, reported at intervals of approximately 4m.

Because the deflection measurement is affected by temperature, the survey procedures require that the pavement temperature is also reported at regular intervals, and that surveys are not carried out outside a certain temperature range (this range is dependent on the type of pavement being surveyed). The measured deflection data is then corrected to a standard temperature of 20°C.

Other data required to enable the further processing of Deflectograph data includes the pavement construction (type and age of materials and their thicknesses). This information is often missing or of poor quality for many parts of the network. Therefore, Deflectograph surveys are frequently accompanied by coring and Ground Penetrating Radar measurements to assist in determining the pavement construction.

The analysis also requires information on the past traffic (in terms of standard axles) since the original construction of the road section under assessment, or since the last structural maintenance.

Assessing structural condition using Deflectograph data

Methods have been developed to allow the structural condition of the road to be derived from the measured deflections, given the past traffic loading and the construction of the road pavement.

It has been shown that there are 2 main phases of ‘structural behaviour’ during the life of a flexible road. In the first phase, there is a very slow increase in deflection until the onset of ‘investigatory’ conditions when the road pavement begins to deteriorate more rapidly and unpredictably. This increase in deflection is related both to the traffic loading since the date on which the road was built or last strengthened and the actual level of deflection.

In the second phase of ‘structural behaviour’ it is increasingly difficult to predict how the pavement will deteriorate. This second phase ends when the pavement finally reaches a failed condition when reconstruction is likely to be needed. The “residual life” of the pavement is defined as the life of a pavement (in traffic or years) until the onset of ‘investigatory’ conditions, for example, until the end of the first phase. This is the point at which more intensive monitoring of structural condition should start as the residual life falls below zero.

Relationships have been developed between deflection, cumulative traffic and the onset of ‘investigatory’ conditions for both flexible and flexible composite pavements. These relationships take into account both the type and thickness of the road structure. Essentially for a given pavement type the higher the initial level of deflection the shorter the residual life in terms of cumulative traffic (until the onset of ‘investigatory’ conditions). By knowing the projected traffic levels on the length under assessment it is possible to convert the predicted residual life, in terms of cumulative traffic, to years. The above analyses are provided in computer programmes such as HAPMS and PANDEF.

More recent research has shown that the deflection deterioration relationships described above do not apply to thick, well constructed pavements with asphalt bases on strong foundations. These pavements do not suffer from structural deterioration, but rather from environmental factors that cause cracking to occur at the surface of the road. Therefore the terminology “determinate” and “indeterminate” (or “long”) life has been introduced to describe pavements that typically would be expected to follow the behaviour assumed in the deflection models (determinate life) and those where the measurement of deflection cannot be used directly (using traditional methods) to determinate the life of the pavement (indeterminate life or “long life”).

Further information on deflection data

The above paragraphs provide summary information on the Deflectograph and the data provided. Further information can be obtained in the following references:

• Design Manual for Roads and Bridges (DMRB), CS 229, “Data for pavement assessment”, (National Highways)

• HAPMS Document 104 (2001). Processing of Deflection Data. Version 01.05. (National Highways)

• DMRB Chapter 5. (2008). Design Manual for Roads and Bridges (DMRB) –, Volume 7, Section 3, Part 3 HD 30/08 Chapter 2 “Pavement Deterioration Mechanisms” and Chapter 6 “Interpretation”. (National Highways)

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