TRL | Road Note 29 (3rd Edition) - A guide to the structural design of flexible and rigid pavements for new roads

Road Note No. 29 was published originally in 1960, and the first major revision was issued in 1965. As recently as 1968 the recommendations for unreinforced concrete pavements were reviewed by a Panel of the Road Research Laboratory's Research Committee on Concrete and, as an interim measure, prior to a further complete revision of the Road Note, these were issued as RRL Report LR 192.
The need to metricate has provided an opportunity to bring the Road Note up to date and to reconsider the basic layout originally adopted.
2 The present revision was drawn up within the Ministry of Transport in close consultation with a Panel set up by the Laboratory's Research Committee on Design and Construction. The Panel included county surveyors and consulting engineers.
3 This note deals solely with the construction of new roads and not with the resurfacing and maintenance of existing roads.
4 In the earlier editions, the designs catered for a limited selection of traffic categories defined in terms of the number of commercial vehicles per day to be carried by the road 20 years after construction. The use of a growth rate for commercial traffic of 4 per cent per annum was recommended.
To render the document more versatile, traffic is now defined in terms of the cumulative equivalent number of 8200kg (18 000 lb) axles (to be called standard axles) to be carried during the design life of the road. Simple means of estimating traffic on this basis from normal census data are provided. Using this approach the changes in minimum design thickness are continuous throughout the traffic range from residential roads to motorways.
From the initial traffic intensity and estimated growth rate, the engineer can prepare designs suitable for any 'life' up to 40 years.
The approach is also particularly applicable to the design of industrial access roads, or similar roads intended to carry specialized traffic. The accuracy of the designs will depend very largely on the accuracy of the traffic prediction over the chosen life.
5 The results of the latest research, and particularly those from full-scale experiments, have been incorporated. As a result, although for equivalent traffic the designs given are not markedly different from those in the previous edition, some economies result for certain traffic intensities.
6 The principal changes that have been made, apart from the metrication and the introduction of the new method of traffic assessment, are:
(a) The additional thickness of sub-base previously recommended under dense coated macadam and asphalt bases in flexible pavements is no longer required.
(b) Dense coated materials with tar binder and gravel aggregates for road-bases are included in the recommendations for flexible pavements regardless of the traffic to be carried. Gravel is also now included in the aggregates that may be used for roiled asphalt basecourses regardless of the traffic to be carried.
(c) The recommendations for unreinforced concrete pavements have been brought into line with RRL Report No. LR 192.
(d) Provision is made for varying the thickness of pave. Tents across the width of carriageways carrying uni-directional traffic, so that the engineer can, if he wishes, take into account the reduced commercial traffic flow on middle and fast lanes.
(e) A design procedure is included for private roads intended to carry specialized traffic.
7 For the benefit of engineers designing roads to the Ministry of Transport Specification for road and bridge works, 2 reference is made in the text to the relevant clause numbers of the 1969 edition of the Specification.
These clauses are subject to amendment from time to time by Technical Memoranda issued by the Department of the Environment.

Road Note 29 (3rd Edition) - A guide to the structural design of flexible and rigid pavements for new roads

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