ROAD RESEARCH LABORATORY

'~ Ministry of Transport

RRL REPORT LR 327

A SYMPOSIUM ON THE METHOD OF COSTING

THE CONSTRUCTION AND MAINTENANCE OF ROAD PAVEMENTS

HELD AT THE ROAD RESEARCH LABORATORY DECEMBER 1969

Road Research Laboratory

Crowthorne, Berkshire

1970

CONTENTS

Abstract

i. Introduction

2. Introductory Papers

2.1 The pavement as a structure By M.E. Burt (Head of Design Division) 1

2.1.I The background of LR 256 2

2.1.2 Pavement structural performance 4

2.1.3 The revision of Road Note 29 4

2.1.4 The study of LR 286 5

2. i. 5 Conclusion 7

2.2 Techniques and costs of maintenance and traffic opera- tions by J.H. Nicholas (Head of Construction Division) (In the absence of Mr. Nicholas this paper was intro- duced by Dr. R.R.H. Kirkham) ii

2.2.1 Skidding resistance ii

2.2.2 Sealing of joints 14

2.2.3 Lane and edge markings 14

2.2.4 Costs of delays to traffic during maintenance 15

2.2.5 Conclusion 16

2.3 Method of analysis; needs for further re&earch and development by Dr. R.S. Millard (Deputy Director of Road Research)

2.3.1 Structural lives

2.3.2 Maintenance operations and costs

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"~ 3'.1

3.2

3.3

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5.

Appendix 1

Appendix 2

2.3.3 Traffic delays

2.3.4 Method of analysis

2.3.5 Conclusion

Discussion

Structural lives

Maintenance and traffic delays

Method of cost analysis

Summary of Discussion

Acknow ledgements

O CROWN COPYRIGHT 1970 Extracts from the text may be reproduced

provided the source is acknowledged

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Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on I st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

A SYMPOSIUM ON THE METHOD OF COSTING THE - CONSTRUCTION AND MAINTENANCE OF ROAD PAVEMENTS HELD AT THE ROAD RESEARCH LABORATORY DECEMBER 1969

ABSTRACT

T h i s r e p o r t r e v i e w s t h e p r o c e e d i n g s a t t h e o n e - d a y symposium on "The methods and c o s t s o f c o n s t r u c t i n g and m a i n t a i n i n g road p a v e m e n t s " h e l d a t t h e Road R e s e a r c h L a b o r a t o r y on 10 December 1969.

It includes the text of the three papers presented and a record of the discussion covering the main themes considered, i.e. structural lives, methods of maintenance, traffic delays and the method of cost analysis.

i. INTRODUCTION

The purpose of the symposium was to allow debate of Report LR 256 - The cost of constructing and maintaining flexible and concrete pavements over 50 years - and of the implications arising from the report. Attendance at the symposium was by invitation and the audience consisted of senior representatives from all sides of industry and highway authorities together with members of R.R.L, staff; the names of those attending are given in Appendixl, and of those contributing to the discussion are given in Appendix 2.

The symposium was opened by the Chairman, Mr. D.J. Lyons, (Director of Road Research) who outlined the problems to be discussed. The aim of this work is to develop methods of selecting construction materials and thicknesses on which the total cost, including the subsequent cost of maintenance, is minimised. To some extent engineers have arrived at this using judgement - the object now is to make the decision numerate. The work described in the report is the result of the deliberations of two Panels, working to a time limit of six months and using only the available data and not carrying out any further research.

The next step is to discuss the method used in the report of accounting for the expenditure on roads, pointing out the weaknesses in the method and areas where more information is required. It was hoped that the discussion would be confined to this aspect rather than to the policy which might be implemented at a later stage.

The proceedings continued with three papers prepared at R.R.L.

2.1

2. INTRODUCTORY PAPERS

The pavement as a structure by M.E. Burt (Head of Design Division)

My main task this morning is to deal with the road pavement as a

structure; Mr. Nicholas will talk about surfacemaintenance and traffic problems, and Dr. Millard will bring things together and look a little way into the future.

However before considering the structural aspects it may be help- ful to some of those present, who may not be as familiar with LR 256 as those of us who were concerned with it from~the beginning, to hear a brief recapitulation of its aims, method and conclusions.

2.1.1 The background of LR 256 The work was started just two years ago, and a time limit of six months was set and achieved. The bulk of the work was done by two Panels, one of the Research Committee on Concrete and the other of the Research Committee on Bituminous Materials. Their reports were co-ordinated and edited into LR 256 and considered by the Research Committee on Design and Construction.

The aim was to determine whether long,term maintenance costs should be considered in selecting the form of construction. Although the main aim concerned maintenance costs, initial construction costs were also evaluated, partly to help the estimation of maintenance costs, and partlylto assess their significance. Four types of road were selected with widely different traffic intensities:- a rural motorway, a peri- urban road, a rural secondary road and a housing estate road. Both flexible and concrete constructions were considered: for flexible construction the rural motorway and the peri-urban road had rolled asphalt surfacings and the rural secondary and housing estate roads dense bitumen macadam surfacings; the bases were of wet-mix, dense bitumen macadam and lean concrete. For the concrete~construction reinforced pavements were considered for the rural motorway, peri-urban and rural secondary roads with variations of hot or cold poured joint sealing compounds; for the housing estate road the alternatives were reinforced or unreinforced construction with hot poured joint sealants.

The standards of initial construction were necessarily based upon current practice, in particular Road Note No. 29,(Second Edition.) However it was realised thatthese standards were not necessarily the best for minimising the total cost of construction and maintenance over longperiods and that the investigation should provide information that could be used in a revision of Road Note 29 aiming at such designs for each form of pavement construction.

The costs of the initial constructionwere derived from the Ministry of Transport's records of billed unit costs for a number of accepted tenders. Themaintenance operations considered were strength- ening for structural reasons, restoration of skidding resistance, road marking, joint sealing, etc. Maintenance schedules for a period of 50 years were constructed, including the necessary periods of road closure. From these schedules the maintenancecosts and traffic delay costs were tabulated year by year. Costs of some maintenance operations were obtained from various highway authorities but, when applicable, the costs were taken as those for new construction increased by 15% Traffic delay costs were taken as £0.45 per p.c.u, per hour.

•

Maintenance and delay costs incurred in the future were reduced to present day values at the then current discount rate of 8% p.a. This process has a powerful effect of reducing the value of future costs, as is shown by the followingTable, which includes figures for the present rate of 10%.

Years hence Present value

8% 10%

i0 0.46 0.38

20 0.21 0.15

30 0.i0 0.06

40 0.05 0.02

50 0.02 0.01

The main indications of the work were:-

I. The average estimated construction costs tended to be higher for concrete than for flexible construction.

2. For concrete pavements the main contributions to the maintenance costs arose from re-texturing, surface markings and joints. For flexible pavements the main items were resurfacing for structural reasons, surface markings and surface dressing.

3. The total maintenance costsbefore discounting ranged up to about 100% of the initial construction costs, depending upon the type and construction of the road.

4. Traffic delay costs were comparable to the maintenance costs on the peri-urban road and were also significant on the rural secondary road.

5. After discountingat 8% per annum the total maintenance and delay costs ranged up to about 20% of the construction costs.

6. On the rural secondary roadand the housing estate road the dis- counted maintenance plus delay costs over 50 years indicated little difference between flexible and concrete construction.

7. On the rural motorway and peri,urban road flexible construction was cheaper::over the first 15 to 20 years, but concrete for the later years. The crossover was caused by the first re-surfacing required for the flexible pavements. For concrete ones such treatment was not required in the 50 year period.

8. The u n c e r t a i n n a t u r e o f some o f t h e d a t a was s t a t e d and t h e n e e d f o r f u r t h e r r e s e a r c h e m p h a s i s e d .

A

2.1.2 Pavement structural performance I turn now to the structural performance of the road pavement, i.e. its ability to withstand traffic and other loadings for a reasonable period of time. It seems clearly correct in principle to base the choice of pavement construction on the total cost of construction and maintenance over some period of years. This is easy to say, but more difficult to do because'data are needed on three aspects:-

1. The p e r i o d o f y e a r s to be c o n s i d e r e d , t h e numbers and t h e a x l e loads o f t h e commerc ia l v e h i c l e s , and t h e economic a s s u m p t i o n s . Al though p r e c i s e numbers can be chosen f o r t h e s e i tems i n i t i a l l y , changes may o c c u r o v e r t h e y e a r s , and I w i l l r e t u r n to t h i s p o i n t l a t e r .

2. The structural life of the pavement under given traffic, both for the initial construction, and after strengthening by an overlay.

3. The choice of the optimum balance between astrong, durable and possibly expensive initial construction which requires little main- tenance over the years, and a lighter and cheaper initial construction which requires more maintenance.

The p r e s e n t i s s u e o f Road Note 29, on which t h e t h i c k n e s s e s o f pavement used i n LR 256 were b a s e d , l a y s down recommendat ions f o r new construction but does not give any direct indication of the structural lives to be expected and says nothing about subsequent maintenance. The design is however based on the traffic expected 20 years after construction and it is understood that thePanel responsible for the recommendations had in mind a life of about 20 years for flexible construction and perhaps 30 years or more for concrete.

In LR 256 therefore the lives of the initial Road Note 29 constructions were estimated from data from Alconbury Hill and other full scale experiments based on numbers of commercial vehicle axles, and the lives of the overlays estimated from the performance of similar experimental pavements with various thicknesses of surfacing. Although the former has a significant effect on maintenance costs, the latter fortunately does not because of the cost of the second overlay, if any, is so heavily discounted.

It is therefore pertinent to consider how accurate arethe estimates of the life of the initial construction, and whether the right balance was struck between construction costs and maintenance costs. Since LR 256 was completed two relevant activities have been going on. One is the Panel on the Revision of Road Note 29 under the Chairmanship of Mr. J.V. Leigh, County Surveyor of Hertfordshire. The other is the study reported in LR 286.

2 . 1 . 3 The r e v i s i o n . o f Road Note 29 The r e v i s i o n o f Road Note 29, which i s now a p p r o a c h i n g c o m p l e t i o n and w i l l be p u b l i s h e d some t ime d u r i n g 1970, i s l i k e l y t o c o n t a i n a number o f changes which a r e r e l e v a n t to t h e p r e s e n t d i s c u s s i o n .

For concrete pavements the data considered by the Road Note 29

Panel were the same as those used in LR 256, i.e. those published in LR 193 covering the performance over 10 years of the relevant sections at Alconbury Hill. The Panel's recommendations allow about 25 mm margin of reserve compared with the reinforced:concrete slabs at Alconbury Hill. Precise comparisons of the Panel's recommendations with the present issue of Road Note 29 are not possible because the method of specifying traffic has changed but in general, except for the most lightly trafficked roads, the recommended concrete thickness for a 40 year life may be up to 25 mm less than those in the present Road Note 29.

For flexible pavements the experimental data were brought up to date and are in process of publication. The recommended thicknesses take some account of the scatter of the experimental lives and therefore on average also include some reserve. In general some reductions in thickness of baseplus surfacing are likely, depending on the form of flexible construction, except for very heavily trafficked pavements where there is some increase.

The experimental data are not adequate to make any statistical estimate of the accuracy of the estimates of life for the experimental roads, let alone the bulk of new construction. The estimates of mean life for the experimental sections are however probably within + 20%, with lives of individual sections up tO + 50% about the mean. --The probable relationship between experimental roads and nominally identicil non-experimental ones is speculative; one cannot do better than the ~ sort of allowance made by the Road Note 29 Panel.

The d e s i g n s recommended by the Road Note 29 Pane l a r e b a s e d upon t h e t o t a l number o f commercial v e h i c l e s e x p e c t e d d u r i n g t h e d e s i g n l i f e o f the road and a l s o t a k e a c c o u n t o f t he t r a f f i c w e i g h t s p e c t r u m . The d e s i g n l i f e i s o p e n t o the c h o i c e o f t he d e s i g n e r , b u t v a l u e s o f p e r i o d s o f 2 0 y e a r s may be recommended f o r f l e x i b l e pavements and 40 y e a r s f o r c o n c r e t e o n e s . The new i s s u e w i l l t h e r e f o r e p r o b a b l y be in a form which w i l l a l l o w th e d e s i g n e r to c o n s i d e r w h e t h e r he s h o u l d pu t down i n i t i a l l y a s t r o n g pavement or whe the r he s h o u l d op t f o r a l i g h t e r i n i t i a l c o n s t r u c t i o n wi th the i n t e n t i o n o f s t r e n g t h e n i n g a f t e r some y e a r s , i . e . some form o f s t a g e c o n s t r u c t i o n .

2.1.4 The study of LR 286 In order to throw some light on the options open to designers, calculations were initiated at the RRL during the work which led up to LR 256 and these are reported in LR 286 "A study of the economics of stage construction for road pavements". In the time available today it is only possible for me to give a brief summary of the main indications from this study.

It is necessary to review briefly the relevant structural properties of concrete and of bituminous materials and the failure criteria used for road pavements. Considering concrete first, the failure criterion used in this country is based upon the length of cracking. The evidence from the Alconbury Hill experiment shows that for reinforced concrete slabsthe life in terms of the number of standard commercial vehicle axles to reach the critical crack length

increases ap~zoxima~ely with the sixth power of the slab depth, i.e. a 9 inch slab will have about twice the life of an 8 inch slab. The other relevant point is the difficulty of adding overlays of moderate thickness (say 50 to i00 mm) to concrete pavements whether the overlay be of concrete or bituminous material.

Pavements of bituminous materials (and I am for. the moment talking about fully flexible construction and excludingpavements with lean concrete or other cement bound bases) differ in two important respects. Firstly the life in terms of thenumber of standard commercial vehicle axles to give failure (which is defined as a total vertical deformation of 25 mm (i inch) from the initial surface level) appears to vary approximately only with the cube of the thickness of the bound layers. Secondly the application of overlays from say 25 mm thickness upwards is straight forward.

In LR 286 the data from the~Alconbury Hill experiment are used to calculate the mean lives of new constructions, and four models are used to represen~ the structural performance of a pavement strengthened by an overlay. It was necessary to consider these four-alternatives in view • of the uncertain knowledge of the behaviour of strengthened pavements. Today I am showing only a few results for one of the models which, for flexible pavements, involves adding thestrengthening layer (Stage 2) when the deformation of the original pavement (Stage i) reaches 19 mm (2 in), which is basically the same assumption that was made in LR 256. The overlay is intended to strengthen the original pavement before the latter is cracked sufficiently to impair its subsequent structural performance. The relevant results are summarised on Fig. 1 and'Table I. Part (a) of the table shows the initial constructionwhich would be needed to last for periods of 20, 40 and 80 years,.for a type of fully flexible construction and for traffic appropriate to a rural motorway. Fig. 1 shows the saving T as a proportion of the initial cost of single stage construction which would result from two-stage construction with the second stage added at t I years, for a discount rate of 8% per annum and a simple costing system which only takes account of the costs of the material and does not take account of additional costs such as raising~ the hard shoulders and traffic delay costs. It will be seen that there are "optimum" times at which to apply the second stage and that sub- stantial savings are possible compared with single.stage construction. The optimum time of application of the second stage, and the values of the first and second stage thicknesses are given in Table l(b). The simplified costing assumptions were necessary to limit the great volume of calculations involved. More detailed estimates were made for a limited number of conditions which included the one being considered. Table l(c) shows the results of including these extra costs; in general the optimum time of application of the second stage is delayed and the savings somewhat reduced. It is noteworthy that the detailed cost estimates for an overall life of 40 years indicate than an overlay of 56 mm should be. added at 20 years, which is not inconsistent With LR 256. The Table indicates savings compared, with single-stage construction for overall design lives as low as 20 years (the Stage 1 life being i0 years) but the second stage overlay is rather thin andthe structural model is suspect in this case. For an overall life of 80 years the

overlay would be 277 mm thick added at 25 years: the obvious indication from this (apart fromthe large extrapolation involved) is that for this period for flexible construction we should be considering three or Your stages rather than two.

LR 286 investigates the sensitivity of these savings to changes in traffic growthrate and discount rate during the~life of the road; the general conclusion is that the savings are substantially unaffected by such changes provided that they are taken into account when selecting the thickness to be applied at the second stage. In practice the timing of Stage 2 for flexible construction will depend on the performance of Stage 1 and may be earlier~or later than originally intended depending on the quality of construction and any unusual weather conditions, as well as the traffic actually experienced. Also, when selecting the thickness of Stage 2, the required overall life can be reassessed, and changes in traffic characteristics and economic conditions taken into account. This adaptibility of stage construction supports the indica- tion that for flexible pavements there is little point in designing for initial lives of more than about 20 years.

Two current developmentswill help decisions aboutwhen and how to overlay flexible pavements. The first is the development of deflection criteria and the~Lacroix Deflectograph which will enable the condition ~ of in-service roads to be surveyed and rational decisions taken about % the need for an overlay. Secondly the opportunity is being taken to overlay many of the experimental sections in the RRL's full-scale experiments which were nearing the end of their useful life, so that over the next few years we should be able to gain a useful knowledge of the behaviour of strengthened pavements.

I

Turning now to concrete pavements Fig. 2 gives information similar" to that of Fig. i. It is seen that the theoretical savings due to two2 stage construction for simple costing are substantially less than for • lexible construction. This is basically because the life of Stage 1 of a concrete road can be prolonged so much by a small additional thickness. In addition the Table 2 shows that the theoretical second stage thick- nesses are quite small, only 51 mm for an overall life of 40 years and 92 mm for an overall life of 80 years. Overlays of these thicknesses are unlikely to be satisfactory. For these reasons detailed costs have not been evaluated for concrete, but if done they would undoubtedly indicate smaller savings and longer Stage 1 lives than the simple costing.

The above discussion on flexible pavements excluded those constructions with lean concrete or other cementbound bases, which would be expected to behave in a manner intermediate between a concrete slab and a flexible pavementnot having a cement bound base.

2.1.5 Conclusion The overall conclusions from the study of LR 286, which agreewell with engineering practice, are that for concrete pavements a long initial life is economic and that two-stage construct- ion is unlikely to be worth-while for overall lives less than about 80 years. For flexible pavements two-stage construction offers greate r advantages, particularly due to their adaptibility to changing

~L ¸

conditions. It is doubtful whether it is worth designing for an initial life of more than about 20 years, even if much longer overall lives are intended. There are apparent advantages for overall design lives as low as 20 years (with Stage 1 about I0 years) but it is uncertain whether such advantages would be realised in practice.

For concrete, therefore, the choice in LR 256 of a pavement that requires no structural maintenance (apart from joints and local defects) for the 50 year period was correct. For the fully flexible pavement of the rural motorway the initial life was about right, but in view of the importance of the first overlay to the costs, further investigation of this pointis warranted. Calculations of the kind reported in LR 286 should also be made for other:types of road and other forms of flexible construction.

In conclusion therefore we shouldbe working towards minimising the total costs of construction and maintenance. Ourknowledge of structural performance will gradually increase from the results of existing full-scale experiments, from their extension to overlay experiments, from deflection surveys and from associated laboratory experiments. However our present knowledge of the widely different properties of concrete and bituminous materials indicates broadly the suitabilityof flexible materials for multi-stage Construction and the suitability of concrete for long periods without structural maintenance. This emphasises the importance of the choice of the overall design life and debate on this topic will be welcomed. Finally it is obvious that the structural aspects of the pavement cannot be considered in isolation but need to be integrated with the necessary surface maintenance, particularly for flexible construction.

Table 1

Flexible Construction

"a) Single stage c o n s t r u c t i o n

Overal des ign l i f e

20 40 80

years

Thickness of surfacing plus base for singlesta

mm

305 445 683

e construction

in

12.0 17 .5 26 .9

b) Two-stage construction - s imple costing

Overall design life- year s

20 40 80

Life of Stage 1 t I years

mm

224 272 328

8.2 12.5 19.0

Stage 1 thickness

in

8.8 10.7 12.9

Stage 2 t h i c k n e s s

mm in

38 l.S 147 5.8 340 13.4

Saving T

0.18 0.27 0.41

c) Two-stage c o n s t r u c t i o n - d e t a i l e d c o s t i n g

O v e r a l l des ign l i f e - years

20 40 80

Life of Stage 1 t I years

m m

246 335 376

i0 20 25

Stage 1 t h i cknes s

i n

9 . 7 13.2 14.8 •

Stage 2 thickness

mm

i i 56

277

• i n

0 .45 2 .2

10 .9

Saving T

0 .13 0 .17 0 .29

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Table 2

Concrete Construction

a) Single stage construction

Thickness of concrete slab Overall design life

years

20 40 80

mm

201 244 302

in

7.9 9.6 ii .9

b) Two-stage construction - simple costing

Overall design life - years

20 40 80

Life of Stage 1 t I years

mm

168" 181 203

7.5 ii.0 17.5

Stage 1 thickness

in

6.6

7.1 8.0

5tage 2 thicknes s

mm [ in

20 0.8 51 2.0 92 3.6

Saving T

0 . 1 0 0.15 0 .24

i0

2.2 Techniques and costs of maintenance and£raffic operations by J:H. NiCholas (.Head of Construction Division) ~In the absence Of Mr. N icho las t h i s pape r Was i n t r O d u c e d b ] Dr: R:R:H. Kirkham)

The total costs of the maintenance of road pavements are determined by:-

(a) t h e s t a n d a r d s adop ted , i . e . t h e minimum a c c e p t a b l e s t a t e o f t he road s u r f a c e ;

(b) t h e i n h e r e n t c o s t p e r u n i t a r e a o f t h e m a i n t e n a n c e p r o c e s s ; (c) t h e c o s t o f d e l a y to t r a f f i c w h i l e m a i n t e n a n c e works a r e i n (d) p r o g r e s s ;

t h e " l i f e " o f t he ma in tenance p r o c e s s , i . e . t h e p e r i o d b e f o r e t h e c o n d i t i o n o f t h e road f a l l s to t h e minimum a c c e p t a b l e s t a t e ; i n a d d i t i o n , t h e " l i f e " o f t he o r i g i n a l road s u r f a c e , i . e . t h e p e r i o d b e f o r e t h e c o n d i t i o n o f t h e road f a l l s t o t h e minimum a c c e p t a b l e s t a t e , a l s o a f f e c t s t h e t i m i n g o f s u b s e q u e n t m a i n t e n a n c e p r o c e s s e s .

(e)

Discussion of these factors is not really useful in the case of the hOusing, estateroad, as the standards are easily achieved and maintained, the traffic delay costs are negligible and the process costs and'!lives" are known with sufficient accuracy.

In the: case o f t h e r u r a l : s e c o n d a r y road t h e p i c t u r e becomes a l i t t l e more i m p r e c i s e : t r a f f i c d e l a y c o s t s n o w s t a r t to be an a p p r e c i a b l e p a r t o f t he o v e r a l l c o s t and some o f t h e s u r f a c e m a i n t e n - ance o p e r a t i o n s have u n c e r t a i n i n h e r e n t c o s t s and d u r a b i l i t i e s . On such roads s u f f i c i e n t i s p r o b a b l y known about s u r f a c e - d r e s s i n g c o s t s and d u r a b i l i t i e s , and t h e r e a re no problems i n o b t a i n 4 n g any r e a s o n a b l e l e v e l o f s k i d d i n g r e s i s t a n c e on a f l e x i b l e r o a d . But f o r a c o n c r e t e road o f t h i s t ype t h e d u r a b i l i t y o f a broomed t e x t u r e i s unknown and t h e r e i s no e x p e r i e n c e o f r e t e x t u r i n g - i n d e e d t h e f e a s i b i l i t y o f e x t e n s i v e g r o o v i n g o p e r a t i o n s on a l a r g e m i l e a g e o f r u r a l s e c o n d a r y roads has n o t y e t been c o n s i d e r e d . The s t a n d a r d s t o be a d o p t e d f o r r e s e a l i n g c o n c r e t e j o i n t s on such roads a r e a l s o u n c e r t a i n ; t h i s i s d i s c u s s e d f u r t h e r be low.

On the more hea~ily trafficked roads, which really form the main issue because, apart from housing-estate roads, they will constitute by far the greater mileage of new construction, there are several areas of much more serious imprecision. The remainder of this paper is therefore chiefly concerned with these outstanding areasof uncertainty, in which errors could significantly affect the Calculated costs of maintenance andthe deductions made from them, andwith the steps, includingresearch, that are needed to remove as much uncertainty as possible from these areas.

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2.2.1 Skidding resistance

The standards of skidding resistance adopted for the purpose of this exercise were those suggested by the Laboratory in consultation with Highway Engineers some years ago after a careful study of skidding- accident statistics: they were intended to provide a level of skidding resistance, related to the hazard s of a particular length of road, such that the risk that skidding would play a significantpart in wet-road accidents was reduced to an "acceptable" level. This level is necessar- ily partly subjective, although the cost of accidents, including those involving skidding, has of course been estimated and the economic argumentgiven due weight, on the assumption that the standards could be achieved using conventional surfacings , with no undue precautions. As mentioned below, such an assumption can no longer be made for intense modern traffic and the standards must therefore be looked at again for such conditions.

On motorways and similar high-speedroads built to improved standards of layout, where hazards such as junctions, roundabouts and sharp curves are eliminated or minimized, it would well be that that proportion of all accidents in the wet in which skidding is reported (the "skidding rate") will always be different from and possibly higher than that on ordinary roads. There could therefore be a case for a different level of "skidding rate" when considering what standard of skidding resistance is needed.

Logically the decision on the standards of skidding resistance should be made on a cost-benefit basis; instead of basing the standards on a chosen level of "skidding rate", i.e. accidents, a balance should be made between the cost of the accidents involving skidding and the cost of maintaining the road surface at the corresponding level of skidding resistance. This balance should be struck separately for different types of road. The introduction of a machine (SCRIM) capable of measur- ing quickly the skidding resistance of long lengths of road should enable much more accurate assessments of cost-benefit to be made.

Flexible roads - initial construction

On roads carrying light and medium traffic, bituminous surfacings which satisfy the present standards can be made with no unusual pre- cautions, provided a suitable choice is made of the natural (or slag) aggregate in the surface. As trafficdensity increases, however, it becomes more and more difficult to find suitable roadstones to resist the polishing action of tyres; indeed there is evidence to suggest that there is no natural roadstone which combines the necessary polish- resistance and abrasion-resistance against really intense urban and motorway traffic. The only artificial sources of adequate performance are currently too expensive and in inadequate supply to be considered for any but small specially important areas. Further, the maintenance of an adequate macro-texture for high-speed traffic becomes a problem under really intense traffic because the protruding stones are pushed into the plastic bituminous mixture particularly in hot weather. The current recommended material for surfacing such roads is rolled asphalt; evidence is accumulating that for roads of the rural motorway and peri- urban type considered in LR 256, the material cannot sustain the

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recommended skidding resistance on the most heavily trafficked lane for more than a few years - possibly only months. Research is in hand to improve this position, firstly by developing new artificial aggregates of adequate polish-resistance, secondly by modifying the asphalt to prevent the pushing in of surface stones. The periods (5-6 years) used in LR 256 for the initial lives of rolled-asphalt surfacings on the "slow" lane of very heavily trafficked roads are valid only if either the recommended standards are relaxed or it .is assumed that research will shortly providemore persistently non-skid surface treatments.

Surface dressing - costs and durability

On roads carrying all but the heaviesttraffic surface dressing is an accepted and well proven treatment, with an established range of costs. On very heavily trafficked and fast roads, however, experience with surface dressing is relatively limited: in particular, the problems associated with motorway surface dressing have yet to be evaluated fully and both costs and durability are far from definite. Recent contracts suggest that figures perhaps three-to six times con- ventional prices may be quoted. Whilst the high price undoubtedly reflects the special difficulties in working on very heavily trafficked roads, it seems likely that it could decrease as contractors gain more experience of working under these conditions. Surface-dressing durability is very dependent on the substrate: under very intense traffic the dressing is soon pushed into the richer roller-asphalt types of substrate and certainly in city streets experience indicates that only on a uniform hard substrate coulda surface dressing give a reasonable performance. There is reason to suspect therefore that the lives quoted in LR 256 for surface dressings on rolled-asphalt sub- strates may be optimistic for the motorway and peri-urban road. If research shortlyprovides a rolled-asphalt less prone to ',absorb" superimposed stones, then of course the subsequent surface dressings will also prove more durable.

Concrete roads - initial construction

The figures quoted in LR 256 indicate that the maintenance of a skidding resistance tothe recommended standard under intense traffic is apparently as difficult~on a concrete road as on a bituminous surface. Recent research suggests that the coarse aggregate plays little part but that the quality of the sand is decisive; further tests are in hand to confirm this. This does not necessarily mean an increase in cost; the type of sand giving an adequate result is quite likely to be readily available. The initial Coarse texture designed to ensure high-speed skidding resistance hasso far been provided by wire- brooming the plastic concrete; the more recent results suggest that this technique is not as durable as expected under intense motorway traffic and research is in hand to develop better techniques of texturing fresh concrete. If these are successful the figures given in LR 256 for initial texture life may well be pessimistic; otherwise they are somewhat optimistic.

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Concrete roads - retexturing

Retexturing concrete by cutting grooves is at present a slow and expensive process, although research is in hand to make it more efficient. There is a wide gap between the 5/- per yd 2 used in LR 256 and the tender price of some 30/- per yd 2 on a recent contract, and the figure used in LR 256 could well be unduly optimistic, even allowing for developments over the next few years. If this proves to be the case, the alternative of surface dressing, not considered in LR 256, may be more attractive: concrete has a uniform hard surface which should form a very suitable substrate for such a treatment, either single or double.

2.2.2 Sealing of jOints One of the perennial topics for discussion among highway engineers is the degree to which joints in concrete roads should be sealed against the ingress of water and grit. Despite much research, no convincing objective answer, particularly to the problem of water-sealing, has beenpossible, largely because other developments, such as the load-transfer device, have changed the Severity of the damage likely to arise if sealing is neglected. Hence unhappy experi- ences with older concrete roadsare not necessarily good indications of present dangers. There is a strong school of thought that, with modern concrete construction on important roads, where a good sub-base (often of lean concrete) is almost invariably provided, and where load-transfer devices are universally used, the risk of damage by ingress of small quantities of water is negligible, certainly if adequate sealing against grit is provided. It can be argued that sealing against grit, to prevent spalling of the joint arrisses, is the only real necessity; this can be done adequately by using preformed synthetic-rubber seals.

In current practice, as indicated by the figures given in LR 256, a compromise solution is reached; complete sealing against water is not attempted -it is doubtful whether this can be achieved for more than a short period even with careful workmanship - but the sealing compound is replaced at intervals chosen to ensure that large quantities of water donot pass down the joint. The compound is also intended to reject most of the grit that could cause spalling.

If a decision were reached to "seal" against grit only, using a preformed strip, such a "seal" being regarded as adequate also to prevent more than a very little water from passing down the joint, then the regime of maintenance given in LR 256 could be drastically changed, with substantial saving in direct maintenance costs and, particularly for the peri-urban road, in traffic-delay costs. It is clear that better information on the necessity or otherwise of using water-sealing compounds in concrete joints is of critical importance. Equally important is the question of the durability of preformed seals, and the ease and rapidity of their replacement if and when they become ineffect- ive.

2.2.3 Lane and edge markings Although there are some differences in techniques of marking of concreteandflexible road surfaces, they do not materially affect the cost or the pattern of operation of marking. On motorways, however, the cost of marking represents a very large fraction of the overall cost of maintenance; indeed, on the concrete

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motorway it is the biggest single item of carriageway maintenance. It is clear that improvements in the cost-effectiveness of marking mater- ials would be very well worthwhile. Clearly also the standard of carriageway marking should be looked at continuously to ensure that the balance between expenditure and return (in terms of safe and speedy traffic flow) is reasonable, and in accord with other similar economic balances.

2.2.4 COstS Of delays to:traffic:during maintenance Too little attention has been paid in the past to the effect of maintenance operations on traffic and the considerable loss to the community at large caused by unreasonable delays. On some motorways, and in the centre of large cities, operations have often been arranged to avoid the busiest times, and of course in certain areas events such as race- meetings dictate the "dead season" for road-works. Even so, the general impression given hasbeen that considerations such as minimum occupancy of road in space and time are wholly subordinate to those of the cost of the maintenance operationitself. LR 256 contains, as far as we know, the first serious attempt to quantify the cost of traffic delays. It is only an attempt, as the assumptions made cannot be rigorously justified and in practice each length of road will have its peculiarities that will affect delay - for example the availability, capacity and convenience of alternative routes, the precise daily and weekly pattern of traffic flow. Further experience of maintenance operations on motorways during the last year has shown that some of the assumptions made in LR 256 about occupancy of road space for particular maintenance operations were optimistic, particularly as some police forces rigorously insist that a lane being utilised by traffic must never be less than the full 12ft standard.

Much more in fo rma t ion i s needed on the r e a l cos t o f t r a f f i c de lays but even so enough i s known to make i t c l e a r t h a t a t l eas~ the o r d e r of magnitude i s given i n LR256. When t r a f f i c becomes r e a l l y i n t e n s e , the cost o f d e l a y s i s . s u c h t ha t makes every e f f o r t t o reduce i t we l l wor th- whi l e . On some roads i t can e x c e e d t h e t o t a l cos t o f a l l the main ten- ance ope r a t i ons on the car r iageway.

T r a f f i c de lay cos t s can be reduced by s e v e r a l means, among o t h e r s : -

(a) by c a r r y i n g out maintenance o p e r a t i o n s a t h igh speed, i f n e c e s s a r y by m u l t i p l i c a t i o n of machines and o p e r a t o r s , and by e n s u r i n g t h a t no hold-ups occur through inadequa te l o g i s t i c s ;

(b) by development of improved machinery which (with i t s a t t e n d a n t ope ra to r s ) occupies l e s s l a t e r a l space on the ca r r iageway;

(c) where p o s s i b l e by occupying r o a d s p a c e on ly dur ing pe r i ods of l e s s t r a f f i c flow;

(d) by improved methods of t r a f f i c management dur ing maintenance o p e r a t i o n s .

Research and development are u r g e n t l y r e q u i r e d on a l l t hese s u b j e c t s , i n c l u d i n g the o v e r a l l c o s t - b e n e f i t s from d i f f e r e n t t echn iques of main-

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taining and controlling traffic flows past road-works. For example on a busy commuter motorway, would there be any merit in narrowing the lanes past roadworks to increase capacity without unduly restricting the main- tenance operations, and diverting all heavy commercial vehicles to some alternative route,

2.2.5 ConcluSion An attempt has been made to pin-point the major areas of uncertainty in the figures used in LR 256. From what has been said, it will be seen that some of the uncertainties are sufficiently serious as to make precise conclusions about future surface maintenance costs impossible at the present time and much•more information is needed before deductions based on facts rather than judgement can be made. This cannot of course preclude the making of deductions on which to base policy but it must be recognised that "inspired guesswork" forms a significant part of such deductions at the present time.

In particular, the standards to which road surfaces are maintained in terms of skidding• resistance and sealingare critical. Any significant changes in these standards have a dramatic effect on maintenance costs. Secondly, the intrinsic costs of certain vital maintenance processes under conditions of intense traffic are not known, almost to an order of magnitude. Thirdly, the calculations of the cost of traffic delays is at present very imprecise and the possible effect of improved mainten- ance procedures and traffic management has yet to bedetermined.

Each of these areas of uncertainty will of course be studied and new figures for various standards and costs will be made available from time to time. Computer programmes are being prepared at the Laboratory to enable calculations of total maintenance costs to be made for any set of conditions and the new figures, as they become available, can quickly be inserted into these calculations.

2.3 Method of analysis; needs for further research and development by Dr. R.S. Millard (Deputy DirectQr of Road Research)

Road engineers have for a long time recognised that maintenance operations should be considered in selecting the initial design for road pavements but there has been no systematic study to determine how important they are.

We started the work reported in LR 256 with very little hope of obtaining a definitive answer immediately; the aims were to determine the order of the maintenance costs with different forms of construction, and if they were shown to be large and important, then to consider how they could be used

(a) to derive with each form of construction the most economical design, i.e. minimising the total costs of construction and maintenance, and

(b) to provide a means in any given situation of selecting the most economical form of construction, again aiming to minimise total costs.

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The LR 256 study showed that maintenance costs are important. With some forms of construction the maintenance costs totalled over 50 years exceed the initial cost of Constructing the pavement; and on busy roads the associated costs of traffic delays may be even higher. The study also showed that there can be marked differences between the total costs of maintenance and delay with different forms of construction. These differences are in line with the commonly held opinion of engineers, that maintenance costs on concrete roads can be considerably less than on flexible roads. Expressed another way, one of the advantages of flexible construction is the convenience with which the pavement can be successively strengthened to meet growing:traffic needs.

But the data obtained in this initial study fell short of what was necessary to refine the methods of design.

Further, on present information, the confidence limits on the discounted total costs of the different forms of construction are quite wide. On heavily trafficked roads that concern us most, they are wider than the apparent difference between the discounted total costs of flexible and concrete construction. So it is clearly desirable to move on to the next stage of refining the data, in order to provide the engineer with a practical method of taking maintenance and delay costs into account in his initial designs. The purpose of this Symposium today is to bring out the important aspects of the problem in order to guide us in the further research that is needed, and the intention in .~ these notes is to provide a basis for our discussion by indicating what we believe are the important issues. You may well have other issues : you wish to raise. If you have we want to hear them.

The subject divides into four main fields:-

(I) structural lives of pavements

(2) maintenance operations and costs

(3) associated traffic delays and costs

(4) method of analysis, including the period over which future costs should be considered and how these costs should be reduced to present-day values.

2.3.1 Structural lives The issues are whether we can predict with the required certainty the structural life of a pavement in a given situation, and whether we have reliable techniques to determine when they should be strengthened and by how much.

Here we are fortunate in having a series of full-scale road experiments over the past twenty years to provide good basic data on the structural lives of almost all the normal forms of road construction. Evidence on the lives of concrete roads as indicated in the Alconbury Hill experiment has recently been presented in LR 193 and a similar review is in progress on the lives of flexible roads.

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Obvious weaknesses in this approach are that extrapolation is required when considering forms of construction that may have lives of 40 years or more before streng£hening or reconstruction is required; and that it is not possible to examine in full-scale road experiments the behaviour of all the wide range of materials that may be offered partic- ularly for flexible construction. There is also the allegation that materials laid under the carefully controlled conditions of full-scale road experiments may not fairly represent those materials as used in normal practice. All these points merit discussion. There are two developments worth notingwhich, by supplementing the data obtained from full-scale experiments, will increase the certainty with which we can estimate the structural lives of pavements. One is the increasing knowledge of the physical properties of road making materials particu- larly under dynamic loading that is being obtained in the RRL and in the other laboratories all over the world; and the other is the advent of methods of assessing the remainingstructural lives of normal pave- ments in service by means of the deflection beam and now, more practic- ally, by the Lacroix Deflectograph. This last is going to prove of great value in meeting a further need, to build up knowledge on the effects of overlays in prolonging service life and to establish a method of determining the strength and thickness of overlays in given situations.

How accurately does one need to be able to predict the structural lives of pavements? With concrete roads, as my colleague Mr. Burt has told you, the analysis is showing clearly that lowest costs are achieved by designing for at least 30 years and probably longer; con- firmation for an intuitive judgement that is commonly made. Only very slight increases in initial cost are necessary to extend the design life well beyond 40 years. At this level and using the most optimistic assumptions possible on discount rates there is room for considerable error in the assumptions on structural life without having any practical effect on total costs at present day values.

With flexible roads designed to the current recommendation of Road Note 29, structural lives were found to be in the region of 20 years, and the major strengthening required at this time is an important factor in the total cost of flexible construction. Errors in estimating the structural life mean that the cost of strengtheningwill have to be met that much earlier or later and this could well have an important effect on total cost. Does the information from full-scale road experiments provide a reliable means of estimating the structural lives of the different forms of flexible construction? I believe that for the main forms in current use, it does and that, supplemented by information from deflectograph surveys and from laboratory work we shall be able with more and more accuracy to indicate representative structural lives for flexible construction.

In practice of course there will be quite a wide range in structural lives with the same form of construction in different situations. Site conditions and constructional standards will vary and traffic conditions may well vary from that predicted. Here the adaptability of flexible construction shows its value, since if for any reason the structural

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life turns out to be different from that assumed, there is the opport- unity to reassess when and what strengthening is required. Thus, should traffic flows increase morerapidly than is assumed in the initial design, the need to strengthen will come that much earlier in time and there will then be the opportunity to re-assess the form and thickness of strengthening required to meet the heavier-than-anticipated traffic loading.

2.3.2 Maintenance operations and costs Apart from strengthening to extend structural life, the operations having most effect on the total cost figures are those necessary to maintain a surface with a high resistance to skidding, and, with concrete roads the difference in costs associated with different joint sealing materials and varying standards of~joint maintenance. If for instance the price of grooving concrete roads is raised from the 5/- per sq yd used in LR 256 to 15/-, the total costs of maintaining the concrete pavement on the rural three- lane motorway are increased considerably and exceed the costs on the comparable flexible construction at all times over a 50 year life. But if the price of surface dressing flexible construction on motorways is raised from 1/3 per sq yd as in LR 256 to say 4/-, the total costs overtake those on a concrete road with grooving at 15/- per sq yd. The total cost calculations are clearly very sensitive to variations in

standards and prices for anti-skid treatments.

Will there be changes in the requirements for skid resistance of surfacings?

Will improved techniques be developed for providing surfacings with a high resistance to skidding that are more durable or cheaper than at present?

I believe that the answer to both these questions is yes. Present recommendations on sideway force coefficient ar~rightly based on estimates Of the likely savings in reduced accidents. But they take no account of the cost of providing and maintaining surfacings with a high resistance to Skidding. And it is becoming evidentthat onvery heavily trafficked roads, this cost may be very high indeed. The need is for information on the levels of SFC that are being obtained on roads of different type, in order to derive standards of resistance to skidding that are based on a comparison of costs With benefits. (These costs must incidentally take account of other factors, for instance the extra tyre wear deriving from the use of surfacings with a rough macro texture). Just as the Deflectograph can provide a basis for determining the need to strengthen roads, so can SCRIM the routine skid testing machine, provide the data from which soundly based standards for skid resistance can be derived for different classes of road. Both machines may then be usedby engineers to make routine measurements on their roads in order to derive maintenance programmes that meet the need in the most economical way.

On less havily trafficked roads present methods of providing surfacings of high resistance to skidding arereasonably adequate. But on high speed roads carrying more than about 5000 commercial vehicles

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per day, it is proving difficult to maintain even amiddle-range resistance to skidding. Neither the hot roller asphalt with precoated chippings nor the heavily raked concrete surface is proving to have a resistance to skidding that persists at a very high level under intense traffic. There are developments both current and in prospect. On bituminous surfacings there are the use of calcined bauxite aggregate and of other cheaper synthetic aggregates with a high resistance to polishing, and the greater use of surface dressing on motorways and other heavily trafficked roads, with possible changes in the nature and the cost of the substrate on which the surface dressings are laid. There are the possibilities of grooving concrete as it is laid and of developing concrete surfaces with exposed aggregate of high resistance to polishing. Certainly further developments are possible in the technique of grooving old concrete roads and there is also the possibil- ity of developing satisfactory methods of surface dressing concrete surfaces.

It will probably be two or three years before it is possible to derive new SFC criteria based on a reliable estimate of costs, - and probably about the same time for the present confused situation on techniques of anti-skid treatment on heavily trafficked roads toclear. In the meantime it should be possible to continue calculating mainten- ance costs for anti-skid treatments based on current information but always setting confidence limits exercising the best judgement possible on the developments that are likely in the immediate future.

It is no more than common sense to seek to combine the different forms of pavement maintenance into a minimum number of operations e.g. to combine strengthening with resurfacing to improve skid resistance and riding quality where this is possible.

This is of no immediate practical importance with pavements having design lives of twentyyears or more before strengthening is needed. But there is the possibility with flexible construction of operating with initial lives of ten years or even less. Then the maintenance schedules would be consciously planned to combinestrengthening and improving riding quality with a restoration of the anti-skid qualities of the surfacing.

The review of costs in LR 256 revealed a considerable difference in the cost of maintaining concrete roads, depending on whether hot- applied or cold applied sealing compounds are used. The latter though more expensive in first cost are considerably more durable. In the maintenance schedules on which the costs were based, it was assumed that joints are resealed with hot applied compounds every 4 years, or alternatively, with cold applied compounds every ten years. These assumptions were made to represent common practice. But common practice varies enormously; some engineers attempting to reseal as soon as it is evident that the seal against water has broken down, whilst others neglect resealing for many years, often apparently without prejudicing the performance of the road. If sealing were necessary only against the penetration of grit and not against water, the costs of maintaining concrete roads and the associated traffic delays would be significantly reduced.

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The other routine maintenance operations- traffic lane marking, surface patching, etc either affect all forms of surfacing equally or cost so little that their effects on comparative maintenance costs can be neglectedj But there is one aspect of maintenance that has so far received too little attention. This is the organisation of the work so that traffic delays are minimised. Broad decisions are made, for instance to confine maintenance operations on motorways to less busy periods. But are there possibilities in controlling present methods of maintenance and developing new methods so that width is occupied for less time?

2.3.3 Traffic delays As far as we are aware, the calculations in LR 256 are the first of their kind inwhich an attempt was made to include the costs of traffic delays associatedwith road maintenance. In the absence ofmeasured data, assumptions had to be made and the figures quoted cannot be regarded as very accurate. But their order is sufficient to indicate delay costs can be important onheavily trafficked roads and that the total costs can be considerably affected by differ- ences in maintenance procedures acting through their effect on traffic delays. The calculations are clearly sensitive to differences in delay costs arising in this way and this suggests the need for more information on the scale and cost of traffic delays associated with different main- tenance processes, both to indicate where the maintenance processes might be speeded up, and possibly also to show how traffic management during maintenance operations can be made more effective.

2.3.4 Method of analysis The comparison of the overall costs involves reducing them to present day values and the main issues seem to concern

(a) the period over which the estimates should~be made and (b) the assumptions to be made in discounting, future costs

to present values.

A further consideration is that whilst construction and maintenance costs fall on the national or local exchequer, the major part of traffic delay costs falls directly on the road user.

In LR 256 the costs were presented over 25 and 50 years, and the 50 year period was used in making the generalised conclusions. What is the right period to employ?

The first consideration is on how long we expectnew roads built now to continue to functionas routes for motor vehicles of a similar nature to those now in use. New modes of transport are currently being discussed and promoted, and one can confidently expect that the next half century will bring some Of these newmodes into use in cities and possibly on long distance routes between towns. It seems likely too that some major roads will before long have lanes designated for specific vehicles andpossibly carrying special equipment. But it is still reasonable to presume that in 50 years time our road network, somewhat expanded will still be carrying self, propelled vehicles not markedly different from now intheir speed and loading characteristics. Many routes will be carrying much more traffic than now and though most

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of the traffic growth is likely to be in passenger-carrying vehicles a steady increase is to be expected both in numbers and weights of commercial vehicles. Thus in planning most new roads it is reasonable to believe that they are being built for continued, indeed increasing , use for periods of at least fifty years, probably more. There will be exceptions where a more limited life can be foreseen e.g. forest roads or roads serving factories with limited life, but for most roads the initial investment is made in the belief that a new facility is being created that will<continue in use for well beyond the half century.

The next consideration involves our responsibility for expenditure many years ahead. How much should we be concerned with the bills our grandchildren will have to meet? Tempting though it may be to treat this as a moral issue, reasonable resolution is possible only by approaching it in economic terms. One of the functions of the discotmt rate is to make possible decisions between short-term and long-term investment. As Mr. Burr has illustrated in his paper, if one discou/Its to present day values at 8%, expenditure expected beyond forty years from now is so small as to be scarcely relevant to present day decisions. This means incidentally that predictions on the pattern of road mainten- ance for that distance ahead can be subject to considerable error without prejudicing the accuracy of costs at present value.

With discount rates at a high level many of those concerned with planning new durable facilities such as roads and bridges, are concerned at the possible effect on present standards. They puzzle about the effects of inflation, and changes in productivity and in the relative costs of labour and materials. More consideration is clearly needed on how the discount rate should be applied in testing the viability of projects that are going to continue to give service for many years. The lines of possible development are suggested by modifications proposed by the Economic Planning Directorate of the Ministryof Transport to employ the currentrate (now 10% per year) and assume

(1) t h a t t h e l a b o u r e l e m e n t o f m a i n t e n a n c e c o s t s and t h e c o s t s o f d e l a y s due t o t r a f f i c i n c r e a s e a t 3% p e r y e a r .

(2) that the cost of equipment and materials increases at 1% per year and that these increases are partly offset by

(3) an assumed increase of 1% per year in the productivity of labour.

These assumptions have the effect of increasing long term costs in relation to early costs, but with analyses of road costs the effect is not large and is important only in a finely balanced comparison between two forms of construction.

Thus the conclusion we offer for discussion is that calculations on road building costs should be made over a period of 50 years, noting that with current discount rates the significance of expenditure 20 years ahead is reduced to less than i/Sth of present day costs, and 50 years ahead is very small indeed.

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2.3.5 Conclusion To help to retain the discussion on useful lines suggestions are made•below as to what are the more important issues and on the order in which it would be logical to consider them. The aim•in the symposium is not to discuss the merits of different forms of con- struction on present knowledge; • it is to bring out the issues that must be considered in arriving at a better understandingof the interaction of construction and maintenance costs and so to establish the most economical uses for all materials.

i. Structural lives

Evidence on initial lives and on lives of overlays with different forms of construction.

Techniques of determining strength of existing roads and the need for overlaying.

2. Maintenance operations

Anti-skid treatments, standards and techniques.

Maintenance of c o n c r e t e joints

Road marking and other operations

Improvements in operational techniques to reduce traffic delays.

3. Traffic delays•

Methods of a s s e s s m e n t

Traffic management to. reduce delays

4. Method of analysis

Period over which maintenance costs should be considered

Discounting to present values

Treatment of exchequer costs and direct user costs

3. DISCUSSION

The method o f c o s t i n g t h e c o n s t r u c t i o n and t h e m a i n t e n a n c e o f r o a d s as e s t i m a t e d i n L a b o r a t o r y Repo r t LR 256, was u s e d as a b a s i s f o r t h e d i s c u s s i o n which was b r o a d l y d i v i d e d i n t o t h r e e s e c t i o n s as b e l o w : -

(a) Structural lives (b) Maintenance and traffic delays (c) Method of cost analysis

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3.1 Structural lives

Possible changes in methods of transport likely to occur in the next fifty years were mentioned and although it was considered that wheeled vehicles would still be used, the loading characteristics would probably change. It was stressed that the estimates of life derived in LR 256 were based upon present-day traffic spectra and uniform growth rates up to a maximum which was below that now being used for bridge design (6000 commercial vehicles per day in each direction).

The value of full-scale experiments in providing data on lives of different forms of construction was mentioned with particular reference to the Alconbury Hill experiment. Some doubts were expressed on the extrapolation of results of these experiments; the length of time before results were obtained was stressed and the work at the Laboratory on accelerated testing and correlation with road performance was mentioned. It was pointed out that there could be marked differences in the per- formance of experimental and non-experimental materials and that it would be worthwhile examining the cost-benefit of differing degrees of supervision and control after making an allowance for savings in pavement thickness.

It was generally agreed that for concrete roads it was desirable to design for long life (forty years and greater) but for flexible roads it was advantageous to construct in ~tages. The economics of stage con- struction had been theoretically studied in Report LR 286; this report was, in the main, favourably received as confirmingwhat many engineers were already practising but it was pointed out that the mathematical treatment used in the report h~d not considered the condition of the sub-base and the subgrade. The major advantages of stage construction were stated as the saving in initial costs, the possibility of ration- alisation of the strengthening layers with the maintenance schedule and the fact that it was possible to take into account changes in traffic volume and spectra during the life of the road. It was also stated that with stage construction the performance of the road structure at the end of the first or subsequent stages could be measured by the deflection beam and by ultrasonic wave propagation methods.

The criteria for failure of the flexible and concrete roads adopted in LR 256 were not questioned but it was suggested that "croco- dile" cracking in flexible roads or an uneconomic frequency of patching should also be considered as factors defining the end of the useful life of the pavement.

3.2 Maintenance and traffic delays

It was agreed that the costs of maintenance and of traffic delays should be included in any consideration of the economics of pavements but much emphasis was placed upon the paucity and unreliability of the presently available information. Obtaining more information on both maintenance and delay costs was considered of paramount importance as also was the development of computer programmes to optimise the main- tenance operations.

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The major maintenance opemation for both flexible and concrete roads was considered to be the improving, of the resistance to skidding. This is dependent upon the.recommended criteria for different classes of roads which have been developed from a study of wet road skidding accidents. The difficulties in quantifying resistance to skidding in terms of accidents with all the economic and social implications were stressed and it was also stated that driver behaviour might be an addition- al factor which should be taken into account in the analysis of accidents. The introduction of SCRIM for monitoring the resistance to skidding of long lengths of many-roads will enable a clearer indication to be obtained of the present condition of roads and make it possible to derive improved criteria for the skid resistance of roads of different types. It was commented that there was greater danger in having a wide range of levels of resistance to skidding in a road than in having a uniform level even if it was somewhat below the recommended standard. On flexible roads the practice of surface dressingby lanes, as included in LR 256, may cause drainage problems.

It was suggestedthat there should be means of feeding-in the improvements in the technology of maintenance at an early stage so that maintenance could be planned in detail to make the best use of all resources; this would also haveto consider the practicability of carrying out the maintenance operations particularly on very heavily trafficked roads. It was thought that the development of cheaper synthetic aggregates or of a surfacing with skid resistance "in depth" or, for concrete roads, a method of wet grooving would all have a marked effect on the costs of maintenance. Another point made was that the data used in LR 256 for defining the maintenance schedule for motorways had been derived from information Obtained from trunk roads and this procedure was open to some doubt.

One opinion expressed was that any economic comparison of forms of construction should be based on maintenance costs because, at present, all forms of construction are considered to be in direct competition. If this were done it might be difficulttotake into account changes in the original construction; for example, the grooving of the plastic concrete or changes in the design of rolled-asphalt mix- tures. The system of maintaining roads should take into account some of the wider implications, for example , the problems .of adding layers both under and over bridges, the standards of workmanship attained, and the requirements of the road user. The importance of the noise problem was also stressed and it was proposed that improvements in vehicles, tyres and road surfacings should be co-ordinated to obtain the maximum advantage. There was a general expression of willingness to co'operate in the experimental and development work.

Calculations of delays and their costs are extremely sensitive to the "queueing" time which in turn is dependent upon the road network available near the obstruction. The cost of delays is also important at traffic intersections where there is a high probability of accidents occurring andthe use of expensive materials now to reduce the accident probability and the associated delays may well be justified. The marked effect of delays on driver behaviour was mentioned and it was thought desirable to take into account accidents occurring after passing an

25

obstruction which might be directly attributed to the frustration of queueing. A plea was made to rationalise over the whole country methods of dealing with traffic for the different maintenance operations. For surface-dressing work the importance of planningthe work was stressed, taking into account the types of aggregate and binder to be used and the climatic conditions. Adequate provision for controlling the traffic was very important since slow-moving traffic was the best medium for achieving adequate compaction.

It was felt that the problem of delays to traffic was greatly accentuated by the work of statutory undertakezs and this work should be co-ordinated with the highway authorities.

3.3 Method of cost analysis

In LR 256 all the costs incurred in future years are corrected by t h e a p p l i c a t i o n o f a d i s c o u n t f a c t o r t o y i e l d an e q u i v a l e n t p r e s e n t v a l u e . Thus £1 in t e n y e a r s t ime i s v iewed as wor th c o n s i d e r a b l y l e s s t h a n £1 now. This has n o t h i n g t o do w i t h i n f l a t i o n as t h i s e f f e c t i s a l l o w e d f o r by work ing t o a c o n s t a n t p r i c e l e v e l which r e f l e c t s t h e two basic economic principles, firstly, that £i today can be invested to yield more than £i in ten years time in real resource terms on a compound-interest basis and, secondly, that both individuals and society have a time preference, for example, for a road to be built now rather than at some time in the future. Both these considerations lead to a higher valuation being put on the present than on the future. If the discount rate is high, as at the present time (i0 per cent), any- thing that happens beyond twenty years in the future is of little practical significance providing there is not a strong countervailing effect from, for example, traffic growth. Although this philosophy is alien to traditional engineering practice the logical conclusion is that one builds for a relatively short life.

Given the discounting principle then maintenancecosts are relevant to the decision on the type of construction to be adopted in the way that has been demonstrated in LR 256. The only substantial query to raise is that of exchequer versus user costs. The correct approach should be to think in terms of the community and the resources at the community's disposal. In principle taxes merely represent transfers within the community and are thus not costs to it; therefore, they should be ignored and a social cost basis used. User costs can be taken to be social costs if externaleffects such as amenity loss are not large. Accepting social costs as the basis the calculations are still not easy to perform with sufficient accuracy. This is one of these common analytical situations in which the conclusions are very sensitive to present technological uncertainty, for example, the cost of main- taining adequate skid resistance, and it is obvious that research should continue on such topics. This is also true for the morepurely economic elements (inflation and relative cost changes) in the calculation although, except for the discount rate, the sensitivity to these is not so high.

Economic logic and the present economic circumstances make for a downgrading of long-term needs. The analysis in LR 256 covered a period 26

of 50 years but there is a distinct possibility of obsolescence because of technological changes or of limited lengths being obsolete or less important because of changes in the road network. On this basis the calculations in LR 256 assume complete certainty regarding the future or probabilities of I. Any level of uncertainty can, in principle, be expressed by probability factors of less than i which can be used with the costs inmore distant years in addition to the discount factors. This explicit recognition of uncertainty regarding the future could be used in further work. Preliminary calculations using this concept on the results given in LR 256 indicate that the work is valid in this wider perspective.

Although it was felt that an acceptable method of costing mainten- ance and delay costs over a period of time was required, conflicting views were expressed on the application of present discounting techniques. Factors such as the residual value of a road at the end of the agreed period had not been taken into account in the study carried out in LR 256.

An argument was put forward that the cost of delays and congestion should be estimated for the whole road network of the country and the benefits estimated for increasingthe size of the road system and hence reducing congestion generally. The considerable cost benefits possible with stage construction under this scheme were mentioned.

Thepoint was made that finance for the public sector was always restricted and that, therefore, the best use had to be made of the available funds. If the stage construction concept was adopted there would have to be some revision of the method of allocating money for major roadworks to take into account the considerable expenditure required at the subsequent strengthening operations. Also involved in this problemwas the decision as to whether the money would come from the national exchequer or from the local authority rate funds.

4. SUMMARY OF DISCUSSION

i. It is essential to consider maintenance and delay costs in any economic study of pavement design and construction; means of costing social benefits should also be sought.

2. More reliable data on the lives of road structures and surface treatments and the costs of construction and maintenance are required so that the method used in Report LR 256 may be refined.

3. It is essential to take into account improvements in technology, for example, stage construction probably has design and economic advantages for flexible pavements but not for concrete roads. A computer programme is desirable to enable continuing loptimisation of costs to be undertaken. \

4. Areas requiring further research are:-

(i) The study of road structures to give the greatest economic benefits for an acceptable structural 'life'.

27

([2)

(3)

(4)

(s)

(6)

The development of road surfacings which maintain a high resistance to skidding for long periods under heavy traffic.

The study of new materials (e.g., artificial aggregates), new construction methods and mix design to obtain improved performance of surfacings, particularly on heavily trafficked roads. Environmental factors (e.g., noise) should also be taken into account.

The techniques and organisation of pavement maintenance operations and the costs of the work and of the delays to traffic.

The control of traffic to minimise delays, and the use of slow moving vehicles in the consolidation of surface dressings.

A method of costing accidents which takes into account social factors.

5. ACKNOWLEDGMENTS

The arrangements for the symposium were co-ordinated by Mr. J.W. Tyler. A record of the proceedings was made by Mrs. J.L.B. Campe and this report was prepared by Messrs. D.M. Colwill and J.M. Gregory.

28

APPENDIX i.

LIST OF DELEGATES

Mr. B. Arthur Assistant County Surveyor Bucks C.C.

Mr. W.T.F. Austin Freeman Fox & Partners

Mr. M.E. Butt R.R.L.

Colonel Bush I n s t i t u t e of Works & Highways Supts.

Mr. J. Apse Borough Surveyor London Borough of Kingston upon Thames

Mr. F.G. Ballard Plant Advisory Services John Laing Construction Co.

Mr. H.G. Barnes Sydney Green & Son Ltd.

Mr. G.R. Baxter Tarmac Roadstone Holdings Ltd.

Mr. J.M. Beckett Tarmac Roadstone Holdings Ltd.

Mr. J.F.S. Best Ministry of Transport St. Christopher House

Mr. H. Bowdler County Surveyor Kent C.C.

Mrs. J.L.B. Ca~e R.R.L.

Mr. C.R. Chadwick County Surveyor Wiltshire C.C.

Mr. A.T. Clarke Technical Director British Reinforced Concrete E ng.Co.

Mr. J.G. Clee Refined Bitumen Assoc.

Mr. R.H. Clements Meteorological Office

Mr. L.G. Clugston Clugston Holdings

Mr. T.M. Coburn R.R.L.

Mr. K.D. Coe Limmer & Trinidad Co. Ltd.

Mr. R.J. Bridle Midland Road Construction Unit Ministry of Transport

Mr. D.C. Broome Asphalt 8 Coated Macadam Assoc.

Mr. L.W. Budden The Amey Group Ltd.

Mr. D.M. Colwill R.R.L.

Mr. D.F. Cornelius R.R.L.

Mr. W.H.B. Cotton County Surveyor Durham C.C.

Mr. J.N. Bulman R.R.L.

Mr. A.E. Burks Chairman, Road and Pavings Committee Concrete Society

Mr. A.F. Coombs Chief Road Engineer Welsh Office

Mr. Cowlishaw Reinforcement Manufacturers Assoc.

29

Mr. B.E. Cox Highways Dept. Hertfordshire C.C.

Mr. J.A. Gaffney County Engineer West Riding Yorks C.C.

Mr. W.J. Cozens Ministry of Public Building & Works

Mr. I. Cram County Surveyor's Dept. Warwickshire C.C.

Dr. D. Croney R.R.L°

Mr. L.F. Crossley County Surveyor East Riding Yorks C.C.

Mr. R.F.P. David Robert McGregor & Sons Ltd.

Mr. F.R. Dinnis City Engineer Edinburgh Corporation

Mr. H. Dobson Refined Bitumen Assoc.

Mr. R.A. Downs County Surveyor Gloucestershire C.C.

Mr. R.C. Dorling County Surveyor Huntingdonshire C.C.

Mr. W. Downie D.R.E. S.W. Division Ministry of Transport

Mr. Gear County Surveyor's Dept. Essex C.C.

Mr. G.B. Gibson Chairman Roller Owners Assn.

Mr. J.M. Gregory R°R.L.

Mr. J.L. Hammond Ministry of Transport

Mr. L.W. Hatherly Dept. of Highways & Transportation G.L.C.

Mr. J. Hayes City Engineer & Surveyor Manchester Corporation

Mr. E.W. Hinchley Technical Manager Midland-Yorkshire Tar Distillers Ltd.

Dr. A.J.M. Hitchcock R.R.L.

Mr. A.D. Holland Deputy Chief Engineer Ministry of Transport St. Christopher House

Mr. L.C. Hopkins Amalgamated Roadstone Corp. Ltd.

Mr. D.E. Dyas Ministry of Transport

Mr. D.S. Elbourne John Laing Construction Ltd.

Mr. P.A. Fitzpatrick

Mr. G.W. Froggatt Engineer & Surveyor Eton Rural District Council

30

Mr. A.W. Jacomb Deputy County Surveyor Hampshire C.C.

Mr. S.S. Jardine Wimpey Asphalt Ltd.

Mr. H. Jennings Ministry of Public Building & Works

Dr. R.H.H. Kirkham R.R.L.

Mr. J.R. Lake Ministry of Transport

Mr. W. Mervyn Law Asphalt & Coated Macadam Assoc.

Mr. H. Mildenhall Cement & Concrete Assoc.

Mr. F.D. Lebish Robert McGregor & Sons Ltd.

Dr. A.R. Lee British Slag Federation

Mr. N.W. Lee Deputy County Surveyor West Sussex C.C.

Dr. G. Lees Dept. of Transportation & Planning Birmingham University

Mr. J.V. Leigh County Surveyor Hertfordshire C.C.

Dr. R.S. Millard Deputy Director R.R.L.

Mr. M. Milne Director S.E. Road Construction Unit Ministry of Transport

Mr. D.W. Moutrie B law Knox Ltd.

Mr. A.R. Middleton National Coal Board

Dr. M.H. Milloy R.R.L.

Mr. W.A. Lewis R.R.L.

Mr. R.H. Morray-Jones Kinston Minerals Ltd.

Mr. N.W. Lister R.R.L.

Mr. N.G. Mulroy British Quarrying Co. Ltd.

Mr; D.J. Lyons, Director, R.R.L.

Mr. D.B. McIntyre County Surveyor Cheshire C.C.

Dr. D. McNeil Director of Research Coal Tar Research Assoc.

Mr. J.T. Manuel H.M. Inspector of Constabulary (Traffic) Home Office

Mr. G. Margeson R.R.L.

Mr. F.R. Martin Ministry of Public Building & Works

Mr. D. Matthews R.R.L.

Mr. D.P. Maynard Cement & Concrete Assoc.

Mr. W.E. Murphy Cement & Concrete Assoc.

Mr. J.H. Nicholas R.R.L.

Mr. D.C. O'Leary Dept. of Civil Engineering Salford University

Mr. A. Orme Ministry of Transport

Mr. J.L. Paisley ChiefHighway Engineer Ministry of Transport

Mr. G.C.F. Palmer Limmer & Trinidad Co. Ltd.

Dr. K.R. Peattie Assistant Manager, Bitumen Divn. Esso Petroleum Co. Ltd.

Dr. P.S. Pell Dept. of Civil Engineering Nottingham University

31

Mr. C.H. Peters R.R.L.

Mr. C.D. Pike Sand & Gravel Assoc.

Mr. A. Please R.R.L.

Mr. K.D. Raithby R.R.L.

Miss B.E. Sabey R.R.L.

Mr. G.F. Salt R.R.L.

Mr. G.A.C. Searle Ministry of Transport

Dr. D.R. Sharpe Cement & Concrete Association

Mr. R'P. Sleep Ministry of Transport

Mr. S. Sloth Statens Vejlaboratorium Denmark

Mr. H.E. Snow Constable Hart & Co. Ltd.,

Mr. W.H. Spencer Director Eastern Road Construction Unit Ministry of Transport

Mr. Springett R. Travers Morgan & Partners

Mr. S.A. Stewart British Road Tar Association

Professor H.B. Sutherland Dept.of CivilEngineering Glasgow University

Mr. D.J. Swift Divisional Road Engineer Dept. (Eastern Region) Ministry of Transport

32

Mr. P.W. Tebbutt Worksworth Quarries Ltd.

Mr. P.D. Thomson R.R.L.

Dr. E.D. Tingle R.R.L,

Mr. J.W. Tyler R.R,L.

Mr. B.J. Walker Cement ~ Concrete Association

Mr. D. Walker Dept. of Civil Engineering Salford University

Mr. M.J. Walker Cement & Concrete Association

Mr. J. Weaver Cement & Concrete Association

Dr. A.C. ~iffin R.R.L.

Mr. O.T. Williams Sir Owen Williams & Partners

Mr. T. Wilson Director North Western Road Construction Unit Ministry of Transport

Mr. W.B. Wilson Sir Owen Williams & Partners

Col. P.J.F. Wingate R.R.L.

Mr. E.M. Winterbottom Johnston Bros. (Contractors) Ltd.

Mr. P. Witt Cement & Concrete Association

Mr. L.G. Wooton May Gurney & Co. Ltd.

APPENDIX 2

The following contributed to the verbal discussion:-

Mr. J.M. Beckett

Mr. M.E. Butt

Mr. K.D. Coe

Mr. F.R. Dinnis

Mr. D.S. Elbourne

Mr. L.W. Hatherley**

Dr. A.J.M. Hitchcock

Mr. L.C. Hopkins

Dr. R.H.H. Kirkham

Mr. W.M. Law

Dr. A.R. Lee

Dr. G. Lees

Mr. J.V. Leigh*

Mr. D.J. Lyons

Mr. D.B. McIntyre

Mr. J.T. Manuel

Dr. R.S. Millard

Mr. J.L. Paisley

Dr. K.R. Peattie

Dr. P.S. Pell

Miss B.E. Sabey

Mr. G.F. Salt

Mr. G.A.C. Searle***

Dr. D.R. Sharp

Mr. D.J. Swift

Mr. E.M. Winterbottom

Initiated the discussion on structural lives (Section 3.1)

l !

I !

maintenance and traffic delays (Section 3.2)

method of cost analysis (Section 3.3)

(Contribution read in his absence)

A written contribution was received from the following:-

Mr. A.E. Burks

(447) Dd 635271 3,500 4/70 H.P. Ltd. G1915 PRINTED IN ENGLAND

33

ABSTRACT

A symposium on the method ofcostingthe construction and maintenance of road pavements held at the Road Research Laboratory December 1969: Ministry of Transport, RRL Report LR 327: Crowtborne, 1970 (Road Research Laboratory). Thisreport reviews the proceed- ings at the one-day symposium on 'The methods and costs of constructing and maintaining road pavements' held at the Road Research Laboratory on 10 December 1969.

It includes the text of the three papers presented and a record of the discussion covering the main themes considered, i.e. structural lives, methods of maintenance, traffic delays and the method of cost analysis.

ABSTRACT

A symposium on the method ofcostingthe construction and maintenance of road pavements held at the Road Research Laboratory December 1969: Ministry of Transport, RRL Report LR 327: Crowthorne, 1970 (Road Research Laboratory). Thisreport reviews the proceed- ings at the one-day symposium on 'The methods and costs of constructing and maintaining road pavements' held at the Road Research Laboratory on 10 December 1969.

It includes the text of the three papers presented and a record of the discussion covering the main themes considered, i.e. structural lives, methods of maintenance, traffic delays and the method of cost analysis.