DEFLECTOGRAPH SURVEY, A SUMMARY

INTRODUCTION

THE DEFLECTOGRAPH PROCESS EXPLAINED

Surprisingly enough, in its basic form, a DEFLECTOGRAPH survey of a "flexible" road pavement will give you a graph of deflection values of a road pavement, when subject to a standard load. Quite logical really!

The deflection is measured in one hundredths of a millimetre.

The deflection is measured whilst the road is subject to a standard load, this is provided by loading dual wheels on the rear axle of a suitable vehicle to 3175kg., a sufficient space is left between the wheels for the measuring arm of the deflectograph to pass, hence recording the deflection when the load is imposed, and obviously the weakest parts of the road pavement will deflect the most.


The deflection is measured in the inside and outside wheel tracks.
Equipment and procedures for its use in the measurement of road pavement deflection in the United Kingdom are described in the,
Transport and Road Research Laboratory Reports L.R.834 and L.R.835.

If we consider the survey in its basic form the information you obtain will show the areas of highest deflection which would suggest these areas are the weakest lengths of road surveyed, and therefore the lengths of road to receive treatment first.

That is the basic principle, BUT other factors have to be considered to adjust found deflection values to a set of standard values that will be able to bear direct comparison  with each other.


It must be made clear that standard deflection values quoted in deflectograph reports are those obtained by the DEFLECTION BEAM, this is the original "stand alone" piece of equipment developed by the T.R.R.L..

The deflectograph is a machine based on a normal lorry chassis with all the relevant loading of wheels, pivoting deflection arms, appropriate measuring devices, and the means of accurately recording the deflection information produced.

The deflectograph can survey approx. 4 kilometres of highway per hour, far in excess of the rates that can be achieved with a deflection beam which has to be manoeuvred and recorded by hand for each reading.

The "beam" is not a beam but a trailing arm as shown in the picture to the left. The two "beams", one on the inside wheel track and one on the outside wheel track are contained in a "cradle" to provide additional stability and ease of operation. The "cradle" is winched to a position underneath the lorry and then released so that it is stationary and the twin wheels on each end of the rear axle pass either side of the measuring head/skid as the vehicle moves forward. The deflection caused in the road pavement by the passing wheels is measured by a unit in the "beam" pivot and recorded in the lorry cabin. The measuring head is stationary at the time of measurement.

Here you can see the main "cradle" skid situated centrally between the two set of wheels, this provides stability to the whole apparatus. The skid passing between the inside wheeltrack twin wheels is shown by a yellow circle.

The measuring arm on the outside wheel track in the forward position before the winch is released.

The rear of the unit showing signing on the vehicle and on the side of the road, also out of picture is a workman with a stop go board which is sufficient traffic control in many rural areas. On major roads and dual carriageways, a safety "buffer" vehicle is required. See the relevant code of practice for particular safety requirements.

The results from a Deflectograph Survey DO have to be converted back to deflection beam results.

This may seem confusing but in fact the results are only marginally different, except at the higher levels of deflection.

It is necessary to adjust these values further still to allow for temperature variation in the bituminous layers of the road pavement. 
Adjustment will also be made in relation to the type and depth of construction in the pavement. i.e. coated materials, lean concrete roadbase, granular, etc..

Road construction details will be established by a road coring programme, the cores being taken at regular intervals along the lengths to be surveyed.

All the necessary information to make these adjustments and to process deflectograph information is contained in,
TRRL REPORT L.R.833, "PREDICTION OF PERFORMANCE AND THE DESIGN OF OVERLAYS"

When we have obtained the corrected standard deflection values we can bring further factors to bear, such as :-

1) Estimation of past traffic, (million standard axles, m.s.a.'s), from this we can predict expired life.

2) Estimation of future traffic (m.s.a.'s), from this we can design the thickness of overlay to extend the life of the pavement under that traffic for a given period.
The information to be able to do this is in T.R.R.L. L.R.833, it is in the form of tables and graphs, which have been developed over 20 years of use of the deflection beam on full scale road experiments conducted by the T.R.R.L..
This information has now been included in computer programmes, so that by inputting the corrected standard deflection values and road traffic figures, the print out will display predictions of remaining life and the design of overlay required to extend the life of the road to carry any given traffic.

The D.Tp. issue the following publications relating to calibration, use, and maintenance of deflectograph equipment, and the processing and interpretation of results obtained from a deflectograph survey.

HD 29/94:Structural Assessment Methods
HD 30/94:Structural Assessment Procedures
to be found in the,
D.Tp. DESIGN MANUAL FOR ROADS AND BRIDGES, VOL.7 : Pavement Design and Maintenance

The above documents have received revision since 1994, but still contain much of the 1994 content, for the most up to date edition of these documents visit the website, www.standardsforhighways.co.uk.

It is always wise to thoroughly investigate a site that a deflectograph survey may indicate is weak before you commit yourself to large expenditure, deflectograph is just one tool in many in determining the condition of the road pavement, it does not always give a true reflection of the strength of the road pavement for a number of reasons.


PERSONAL NOTE
Before you start believing sweeping and alarmist statements printed in the press that "X" percentage of the road network has no residual life left, putting the suggestion in uninformed minds that the roads are falling apart, may I suggest you read the Road Notes and Design Guides I have mentioned.
As I understand it, and I could be wrong, so check it out in the documents I mention, that when a Deflectograph Survey indicates no residual life it is time to investigate further the results of the survey with on site investigation before you start overlaying or reconstructing road pavements that are still in a visual sound condition.
A Deflectograph Survey is a tool amongst many used by Highway Maintenance engineers and technicians, used wisely it is an excellent guide to the state of the highway network, but do not let it become your sole parameter, or you may be repairing road pavements prematurely.

I would imagine the same advice holds good for "SCANNER".

I would also add that having been in charge of the Deflectograph Survey programme for a number of years during my career I found the database of road pavement construction details obtained from the coring programme and in later years (with modern technology) an actual database of core photographs very useful indeed, especially if you able to identify the actual materials in the construction.

I try to refer to as few commercial sites as possible in compiling my site, but when a site offers particularly useful information about a subject I make an exception.

For further information on the use of deflectograph, press ------------> HERE

For further information on various road pavement surveys,
and road pavement management systems, press --------------------------> HERE

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