Since 1983 the Department of Transport has used the TRRL deflection design method to aid the planning and design of structural maintenance. Deflection surveys provide essential data for this design method but on busy motorways and trunk roads the slow nature of the surveys can cause significant delays particularly during daytime. Deflection surveys are normally confined to daytime hours because the design method has been validated fully only for daytime conditions. To reduce the traffic congestion associated with surveys on busy roads the Department is considering carrying out these surveys at night. The Transport and Road Research Laboratory awarded a contract to David Croney Associates to examine the validity of using night-time deflection surveys in the deflection design method. The work carried out within this Contract comprised: (1) Theoretical analysis of the relationships between deflection and temperature for typical road pavements; (2) Twenty-four hour Deflectograph surveys at an experimental road site incorporating a range of roadbase and surfacing materials; (3) Re-analysis of deflection beam measurements made on full-scale pavement design experiments over the past 30 to 40 years; and (4) Development of tentative recommendations for future day and night deflection surveys. The theoretical effect of temperature changes on pavement deflection were modelled using a finite element computer program and some of the results are shown in graphical form. The results show that there is no unique theoretical relationship between pavement deflection and temperature measured at a depth of 40mm (the depth recommended in the present design method). Results from the twenty-four hour surveys suggest that diurnal changes of temperature do not affect deflection measurements on bituminous pavements to the same extent as seasonal changes. This has led to proposals for an alternative temperature correction procedure that could be applied to both day and night time surveys. The approach involves making corrections to the measured deflections using a seasonal pavement temperature at 40mm depth. This temperature can be derived either from historical records of average daily temperatures or from pavement temperatures little affected by diurnal changes for example those at a depth of 350mm. The report concludes that deflection surveys could be made outside daytime hours using the proposed method of temperature correction but that further validation of this approach is required.

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