Department For ILI~I InternationalDevelopment TITLE: by: Concrete roads in developing countries J DParry Transport Research Laboratory Crowthorne Berkshire RG45 6AU United Kingdom PA1 150185 PAI 150/85 PARRY, J D (1985). Concrete roads in developing countries. Highways and Transportation, 32 (8), 13-]17. Concrete Roads in Developing Countries' J.D. Parry. MiMechE., MIHT. INTRODUCTION Phillippines, the Caribbean. and the struction of some flexible pavements by OVER the past 30 years many thousands USA, together with reports and manuals the progressive application of overlays, of kilometres of bitumen road have been written in Australia and the UK. *.'- and the definition of acceptable or built in developing countries. 'This ever desirable levels of serviceability of the enlarging and ageing network COSTS,*.. . pavement at some time in the future. represents an increasing burden on the IT has been very. difficult to obtain cost. From the evidence that has been col- road authorities responsible for. main- information which is relevant to lected, there appears to be no outstan- taining it. In many of these countries the' developing .; countries in. the tropics. ding differences in the cost of the two time has come when there are no longer What has been obtained has tended to methods of road construction. Initial sufficient resource's in terms of equip- concentrate on construction costs. Little. costs tend to favour bituminous ment, money or organisation, to prevent quantitative .information appears to be. materials in most cases but, in the long major deterioration over, much of the available on lifetime, maintenance costs, term, concrete roads appear to be network, with a consequent loss of both' and nothing at all has been found on dif- cheaper. The trend is in favour of con- amenity and investment. . ~ferences in road user costs on the alter-. crete because of the increase in the price In the past, bituminous surfaced native types of construction. ofbitumen, which could possibly con- roads have been che'aper'jto build than Further difficulties in comparing costs tinue after the recent respite.. The comn- concrete ones, but the'raipid rise in oil' have been highlighted by Hodgkinson,, parison of costs will also vary according prices uring te last ecade geatly ,who states that the picture is comn- to. thetype of ~road to be built, the type reduced this cost 'differential:' Addi- pcaebythe selection of'desidhAlive'e,' of funds available to pay for it, and the tionally, 'the normal mainte neopera- discount rates (which can be made' to available, machinery and labour. tions on the two types of road'.are dif- show' that 'either -o~ncr~ete 'or; fle'xible 'Table 1 gives examples of the prices ferent in character, and e'xp'erie'nce f'ro'mi pavements cost less), the 'stage `con- of road surfacing materials in five areas the industrialised counitries 'suggests '' that concrete roads re'qu ire subs'tantially, TABLE 1. COMPARISON OF TYPICAL MATERIAL COSTS US DOLLARS less maintenance' than bitur miho'us -sur- ' "- ~'" "' 1983'"M face roads. In view of the difficu'lty tha't' many develping countries are.'experie'nc- l owa' . Caribbean ing in maintaining their ro~ad networks, USA Austr alia Philippines Islands UK the prospect of roads tha't'are com'-'' paratively mainteniance-free ' is~ver `at- ` Asphaltic concrete tractive'. ";' 100 mni/fim 2 1 12 ,- 11 1,15 1 3 This paper is based on- a"'s~tidy"'that' Ma~ss PC concrete.M 3 80-1,00 65 r.' '0 100-130 56 was carried out by the"~Over~s,'a'' nit'f OOC concrete laid the TRIRL' for the 'British 'Overseas 225 min/m 2 15 15'-'- 10- 17.5 15 Development Administr ation, land 's25 mm/inm 29-2 bae'ma'inly, on inform'ati6'n,"frofn ihe' -",' ' ' mechanical engineering and-- studied at.-Reading, College. of, Technology: He abec~ame h mem ber' 'of.'the" InstituiAon o6f 'Mec-hanical Engin'eeroi n~ 16 1972 -. Mosatof his. career,~apart from three years as' .engin'eer.' to' 'a 'refulg ee 'team in Norfth ~Africa has been s9p-ent at' t'i;e~TARL He was' responsible for~ the:, in~strum entai n o u r r i development and minirounidabout. experimeOn'ts', 'n~d carried out trials in the use~of r~kadio ~'c'o~MmmuJnticiation' for bs route. contr~ol I'n'1979 'he joined theiOverseas, Unit at TRRL and has 'reported on the ''design -'o~f modular ' timb'er"' bridge;lo technology 'road'. maintenance 'and the 'application 'of ~conc'rete, road 'technology in' of portland cement concrete'~ pavements for developing, coun- tries.~ ~The. comparsative costs .of conistuction and maintenance of bituminous and concrete roads~ are ~considered and design, constrc,-' tion and maintenance aspet are', compared at different 'levels of ttchnology. ' The -dhcfnclslon' Is.- drawn'that concrete roads'.m'ay~ be" viable In many countiries that uintil" now have uised' only ~biitmi~nous; surfaces. 'The, recomme~ndation is made'that the conc~rete alfternative should be investigaited 'and co.sted7 for'all road pro~acts In developing countries.' AUGUST/SEPTEMBER 1985THE JOURNAL OF THE INSTITUTION OF HIGHWAYS AND TRANSPORTATION 13 U On lew stu~d ¾ ad AUG1U.ST/SEPTEMBER 1985 of the world. These prices are of the materials used in these places and do not represent the casts of identical specifications. For example, in Iowa the concrete is of high strength with an ex- tremely low slump whereas, in the Caribbean, the concrete is as-mixed on site in small machines, or the local premix. Quantities are relatively small in the Caribbean and the material is often used on very narrow roads and on steep inclines, so the unit cost is high. Although both cement and bitumen are imported to the smaller .Ca ribbean islands;- there 'are many developing countries with their own cement in- dustries. Many of these countries find it difficult to raise foreign exchange to im- port bitumen for road construction and maintenance. Concrete roads may be built in these countries, using semi- manual methods as in the Philippines, which require almost no foreign ex-, change from the road building Ministry. Cement factories, however, use im- ported equipment and often burn im- ported fuel, so the foreign, exchange costs to the. country may still be signif i- cant.In the Philippines, there is no signifi- cant difference in the costs of construc- ting bituminous -and concrete roads. Forty per cent of the paved roads in the country are of concrete and represent about 13,"O kilometres. In ~practice, roads constructed by thegovernment tend to be of concrete, while. those., sponsored by outside agencies are more usually. of flexible'construction.' ~7'. -' ?",~ The costs of rebuilding the Baths Road in 'Virgin Gorda in the Caribbean were estimated for both a flexible (crushed stone base with double surface dressing) and rigid -pavement. This- showed that there was little difference in capital outlay', between concrete and bitumen.' However, it was concluded by,, the author that concrete was preferable because 'it is more durable and produces a far better long-term road base. It is maintenance-fre for a much longer - period than bitumen, given similar quali- ty control. Table 2-summarises-the cost..- estimates. .' TABLE 2 -CONSTRUCTION COST INj $ PER SQUARE METRE, *~VtRGINi GORDA.' 1980 1981 ,flexible pavement 19.6 25.7,,, rigid pavement' 18.0 2. In the UK, it is usual for bills' ofquari-' tities to be prepared using both rigid and flexible materials for most major con-_, struction contracts. Ten or more years, ago, these cost comparisons would, have shown the rigid, pavement, to be more costly to construct by up to 20, per, cent. Now either material ,may .be'~. cheaper depending on local conditions'. Of five motorway. contracts let .by. the11 Department of Transport in 1982, two,' were cheaper using bituminous con- struction and two were cheaper using concrete.' The remaining contract was similarly priced using either material. Further Investigations Required to Compare Overall Costs The most effective tool available to date for comparing the costs of alternative pavement designs and maintenance strategies in developing countries is the computer model RTIM2 (2). Unfor- -tunately~ -there~-~is not---enough data i~iab~~j't t ~j~lythis' mohdel'-tb-co'n- crete pavements. The Overseas Unit, TRRZL is currently monitoring maintenance and construc- tion projects in Zimbabwe and Thailand in order to obtain more precise data on the most economic maintenance in- tervention levels for gravel and bitumen surfaced roads, to further improve the relationships in the model. Trial sections of concrete pavement were constructed this year and 'existing pavements are already being 'monitored.. The purpose of this work is to obtain the following in- formation, so that' RTIM2 may also be used to predict the costs and benefits of proposed 'concrete pavements of dif- ferent designs. Constru"ction~ ,costs: '(in non-financial. terms) ') .Materials quantitiesf or each layer of ,the pvement.n Patcosts for each layer of 'the ~-pavement.Labour costs for each layer o~f the' pavement.Foreign exchange component of each.. Deterioration: Cumulative traffic anid cracking rela- tionships.Cumulative traffic'and joint sealing relationships. Road user costs: Tyre and spare, parts, consumption related to roughness; the effects of step- -ping and potholing may also have to be measured.,,. Road m'aintenance O'costs: Routine mai W6tinice costs are expectei to Ibe-'similar to 'those" for, flexible pavements."Recurrent mnainternance.'should 'be minimal after 'the 6'orrectibn 7of, early faults., bdt .the cost of repairing .,t em may be 's'ig'nif'ic'ant. .'Fa'ult's' will' include spalling~. sdue to. 'poor laying or..irap- propiiate. materials, and tlhe, prem"at'ure~ deteriortion of joint seals.. Periodic maintenance and upgrading costs are not difficult to meas~ure in terms of'plant. hours and materials, but the pavement ,life' betwee~n these ac- tivities is much mo're difficult td'predic't as it is affected, by climate, siub-base, quaity of the,.concrete materials and workmanship during sa aig n h ",number.of,heavy 'axle loads carried,.'.. It is possible to make predictions now but more data is required to refine these relationships, particularly those relating to more lightly trafficked roads. DESIGN AND SPECIFICATION DESIGNS in current use vary greatly from country to country, both in terms of dimensions and materials. These two aspects of design are of course inter- related, and are in turn influenced by construction resources and methods, which also differ greatly from region to region. It is therefore not surprising that designs developed in different places vary considerably, and may be ap- propriate only in the region in which they evolved. To illustrate this, ex- amples from three very different areas are discussed. The Philippines All major projects are designed and drawn by consultants or engineers at the Ministry of Public Works and Highways, Bureau,~of Design. They use AASHTO 13 and British manuals 14 as references in addition to their own publications 15. Typically, a dual carriageway road, designed to carry- more than 500 vehicles per day with 15 per cent HGVs would be built -on a shaped subgrade with 180mm of sub-base, 150mm of crushed gravel, base, sometimes cement-treated,' and finally 230mm unreinforced concrete. The three layer design of s"ub-base, base and concrete slab is used except for reconstruction in which case the slab is cast onto the ex- isting bad~e, s'o long, as it is judged to be strong enough.. Contraction joints are sawn at 4.5m intervals and sealed usually with 60-70 pen. bitumen. In, recent years, these joints have been set at an angle of 800 to the centre-line so that both wheels on an axle do not impact the slab at the game tir~e. No expa'nsion joints are used, ex- cept on fixed structures. Dowels are not usually employed on transverse joints, but. may be used at the end of the day's construction run. Tie bars are set at .750mm. spacing. ,On large projects, where two lanes are cast together,. a 50mm plastic strip is. set by' the paver to iduce a crack'between the lanes." The Car'ibbean" Designs and methods of construction vary from island to island in the Carib- bean according to tradition, available materials and topography. Standards of' construction and maintenance are generally 'poor but the traffic is usually very light. Here comment is confined to four of the smaller islands. ANTIGUA. The general policy here has been to lay concrete pavements in low areas subject to flooding. There are several roads with 90 'to 100mm con- crete slabs that were built 20 and 30 years ago. These are cracked in places and due for upgrading, but the ride is acceptable at the low speeds dictated by tewidth and geometryy',,> 14 HIGHWAYS AND TRANSPORTATION AUGUST/SEPEMBER 198 K AUGUST/SEPTEMBER 1985 M Figure 1. General view of a concrete road and ditch in the West Indies. MONTSERRAT-.-The Montserrat ter- made of bitumen emuls'ion and a- rain is very hilly and some erosion results sand of 8mm maximum size. It is I ---from severe flooding of the roads. Con- without a tack coat in thicknesses fr crete is used where asphalt would. fail 12mm' 'Upwards and ' appears, to quickly on the steeper gradients. Crush- -- remfarkably' successful.7- Tortola I, ed beach rock, beach sand and river -- aboutL30 kilometres of concrete roac gravel are used in concrete Iaid to- a- nominal depth of 125mm with wood- CONSTRUCTION --- filled joints 10 to 13m apart. Concrete is - CRUSHING cores from remote ci mixed in .a two-bag mixer or smaller,, as--., , structkiri' site's,-often reveals that no readymix is available.~ The. heaviest..---cnce srgt-iwll-e loads on Montserrat roads'are~twin axle--- specificaition' This 'is the~'tes~ult 'of p1 gravel lorries weighing up to 30 tonnes., - ontroil of materials or p~boedures ant ST. KITTs.: Here, concrete is uised-for -- a waste of resources. -- - roads 6 'nly' where -asphalt -would fail. - At his stage strict quality contro This is 6n'tihe steep slopes and hairpin'. very-1 important, but' often' -difficult bends, 'arid those roads which act as; achieve.-- -Poor -cor~miunications, -~ic drains durin~g heavy stormhs.-- These~ are - hauls 'to' remote 'sites', lack 'of, resour mostly 200mm. thick:-- Nominal 20mm n- for, Maintenance -of-,equipmtent and stone and that passing the 6mnni-sieve is enough trained engineers fore site sur used, for-concretei of -1:3:6 itp'ro6rtions vision can all lead to a finished prod by volumfe, the remnainder being kep't for of low varciable .qulality: . surface dressing.~ Wood filled joints a~re-s - used at variable distances.- -- , - Mtras - --- TORTOLA. Concrete is'used fo'r -paiv-- The fin'hli re~pnsibilit~' for ~thequtalit ing steep gradients. It is lazid'nomfinally materia"l.s-a'nd 'the correcflbatchir~g' 125rimm thick and car~ries'axles-uip'-to 9 -with the site eng'ine'er: This i!!porti~u tonnes. 5One-bag 'mixers!1-are :'tised bto. yimotnfrcncee constructi make concrete~at 1:2:41 proportions wvith.-i-. as a"check -must~ be' made 'for; segrc crushed~xrock -and beach sand. The bays --- tion, and frequent- slump tests car are 3m wide by 7m long and are laid..- out. -Should -there be an investiga, -alternately with staggered wooden joints some time 6after construction becaus or they, have 50mm x 25fymm 'wooden early failure, it is ,very difficulut to be crack indute'rs ;s~t' in tes6'h of a d-iagn'osis- and -remedial measi concrete tends -i6o polish on< th'e Very " unle'ss all -the 'in-aterials; -including sepgradients anci haiirpin b~;fid~.it is water, :are sampled regujlarly'alnd thnoverlaid -with a prermixe 'material, analyses recorde~d. --- lugaid be - las ow -, Dor:1 is I isto)ngcesnot)er-uct -: of liesilar- -- ion, tga-riedtion-e of~sureires-thethe ___________ Antigua - Morntserrat-A,.- 'St.. Kitts Tortola Propo~rtions,. . 1:3:6 or- ',,1.2 112513:' 1 by volume , remiixconcrete Aggregates crushed rock crushed beach rock river sand beach sand - river gravel-., crushed rock crushed rock - - - ¼ ~~~beach sand ---. :. - Thickness'mm -601I50 - 125 -150-200 - -125- Slab length m,. 12 101J3 - t.57- Joints", wo wo woo d ood - -wood- Methods and Equipment The choice of method of laying the pavement should be largely influenced by the ability of the machine operators and labourers. Because the life of the pavement is affected significantly by the quality of the workmanship, it is impor- tant to match the method and equip- ment to the skill and diligence of the lay- ing team. The hand laying procedure observed in the Philippines was suc- cessful because the labourers were both willing and skilful. iMore mechanisation -might b e -Justified-where the -labour is -either mnore- costly or less reliable. In broad terms, the laying of concrete by a paving machine is at the same technological level as laying a -bituminous overlay or carrying out sur- face dressing. Whereas both these bituminous treatments may be done by labour intensive methods, the results rarely compare well with the mechanis- ed results. However, ' concrete pavements of good quality can be laid by hand using only very basic equip- ment.In both developing and industrialised -countries, there is a larger pool of labour familiar with concrete than with bitumen. This, is, not to suggest that workers may be taken from a building site and set to lay pavements without some instruction, but good results have been obtained with the help of a trained overseer. - Site Management - -Cleanliness and good timing are as im- portant when laying concrete, as they are 'when surfacing with premnixed asphalt or when su rface dressing. The critical aspects of site manage- ment of concrete paving are:-, (a) THE S~UB-BASE. The suipervisor should ensure t hat it is evenly~c'or- .pacted. and thorojughly soaked but with 'noexce~ss. water, -or covered with heavy grade polythene. This conditidn is very rarely fulfilled 'at remote sites.- - - (b) FORMW~ORK: The forms, whether timber or steel, must be in first' rate -condto ndms e firmly laid on compacted sub-base. The rejection of damaged forms should have-the full support ofj-the chief engineer. (c), INSPECTION~ OF THE RAW ~MATERIAL: When the fresh con- crete is spread between the forms, all the team should work quickly to conclude the 'laying. However, 'samfples miust be taken and a slump test carried ouit firstr, Should the 'concrete -b6'sub-s~tandard in.- any way tn~ut berejected. This is a most unpopular decision to take as the premix supplier;' the -contract manager, the laying -team and the Ministry admfinistratorsa are all in- conivenecd :and some ofte lose Money as a result:- it always retards the' prbdress of the work -- and can provoke initerferdrnce by the media anid-politiciahns, who may be AUGUST/SEPEMBER 1985THE JOURNAL OF THE INSTITUTION OF HIGHWAYS AND TRANSPORTATION 15 AUGUSTISEPTEMBER 1985 under pressure from the local in- habitants or the suppliers. It is im- portant that rejection of, sub- standard material should have the support of the whole of the ad- ministration. IdI TIMING. It is particularly important in hot climates to carry out the lay- ing without delay. This means that all items of equipment, fuel and per- sonnel must be ready before the material arrives. Once approved, the concrete must be spread, struck off, compacted, small areas made good and the surface textured as quickly as is consistent with good work.When there is a break in supply, the supervisor must judge whether to install a transverse joint or continue laying when the next batch of material arrives. if joints are to be sawn, at least every fourth joint should be cut as soon as is possible without tearing the concrete, and the intermediate ones by the end of the following day. MAINTENANCE AS a lower maintenance requirement is often an important reason for deciding to build a road in concrete, it is worth examining the types of maintenance ap- propriate to concrete pavements, and the likely effects of neglected maintenance. Routine maintenance The only work required in this category is the cleaning of ditches and culverts. The result of neglecting this work will depend largely on the sub-base material. Concrete pavements exist in the Philip- pines and the Caribbean that have been subjected to flooding every year for several decades. Roads like these, built on stable sub-bases, will not suffer if the side drains are neglected, but the sur- rounding country may be damaged by erosion due to lack of control of the run- off. However, when the sub-base con- tains a significant amount of fine material, it is important to keep out the water and it is then necessary to main- tain good side drains well below the level of the sub-base. Periodic maintenance The frequency of periodic works depends mostly on the intensity of the traffic, but also partly on the severity of the rainfall. Resealing of joints and crack sealing These are important on the weaker sub- bases to prevent the ingress of water. Many major concrete roads in the Philip- pines contain cracks several hundred metres long progressing through many slabs, but the pavements are not con- sidered to have failed. These cracks are sealed with hot bitumen or a rubber compound to keep out the rain and the pavements remain stable. New cracks should be sealed as soon as they open sufficiently to allow the sealant to enter. Depending on climate and traffic, reseal- .-~ -, 1 M Figure 2. Road widening and rehabilitation in Manilia. ing may be required every five to 15 years. Failure to carry out this type of maintenance leads to deterioration in the form of further cracking and rocking slabs, with possible pumping out of the fines from the sub-base. Hole repairs Hole patching in concrete is similar to hole patching of flexible roads. The hole is excavated, sound material is placed in the sub-base and compacted, and the surface is finished with concrete at least as thick and strong as the existing slab. Alternatively a bitumen mixture may be used. If it is suspected that an adjacent slab is rocking, a flexible patch of bituminous premix will last longer, but should be expected to fail again and so require frequent inspection, until the rocking is cured permanently. Stabilising rocking slabs When a concrete slab rocks under the action of traffic, it acts as a pump, ejec- ting fine material with the water that has entered by the joints which must have failed. With the loss of material from the sub-base the rocking becomes worse. The problem is to stop the movement and then seal the joints. Having ensured that poor drainage will not lead to further failure, the slab may be treated in, two ways. The loss of sub-base may be made good by injec- ting cement slurry or bitumen through holes drilled near the joints. This re- quires several items of equipment, which may not be readily available, and well-trained operators. The simpler solu- tion is to break out the old slab, make good the sub-base with granular material and cast a new slab, which may be keyed to the adjacent ones with dowels and tie-bars. Shoulder maintenance This is less critical for concrete roads than for flexible roads because the pave- ment edge is less susceptible to damage. However, a pronounced step down from the pavement presents a hazard to traffic and a possible channel for water, so shoulders should be main- tained to the same standard as for flexi- ble roads. Eroded shoulders may be treated with a bituminous fillet sloping from the level of the pavement at 200 or 3Q0 down to the level of the shoulder. Retexturing This may be required when the surface becomes polished. Grooving, milling and scabbling are all relatively expensive remedies and require an investment in machinery. Short, steep sections, such as on the hairpin bends on Tortola could be. treated by sawing shallow grooves across the road. They are, in fact, overlaid with a thin premix as described earlier.By the time a major road becomes polished, it may well be expedient to carry out other repairs or strengthening. In the Philippines, severely eroded or polished concrete is often overlaid with asphalt. Cracks and joints from the underlying concrete pavement reflect through the overlay after a period of time depending on the thickness, the traffic intensity and the flexibility of the overlay. Concrete overlays had been tried, but it was difficult to align the joints closely with those in the old pave- ment. Rehabilitation When concrete slabs become broken to the extent that an overlay is not the solu- tion, or surrounding levels preclude an increase in pavement thickness, the pavement must be removed and relaid. A good example of this is a recent pro- ject in the Philippines. The Manila South concrete road sustained virtually all the traffic load south of the city from around 1950 to 1970, when a bypass was built From 1970 to date it has carried only part of the traffic, but is now busy enough to warrant upgrading from one to two lanes in each direction. To do this a new lane was laid either side of the old road, which was then broken up and relaid. Reclaimed material such as this may be used as coarse aggregate or sub-base material if crushing facilities exist, or 16 HIGHWAYS AND TRANSPORTATIONAGUTSPMBR18 -- . .1 --- 1. 11~ ' --- d
1 --.j, . 1 1 AUGUSTISEPTEMBER 1985 may be broken and compacted in situ if adjacent levels are not critical. DISCUSSION AND RECOM- MENDATIONS CONCRETE roads are used successfully in several developing countries, although the designs and construction methods differ considerably. Costs of concrete and bituminous construction are difficult to compare and also vary from region to region. in general, con- crete roads range from being similarly priced to being 30 per cent mor~e expen- sive. Maintenance costs for 'the two types of construction are even more dif- ficult to compare than the construction costs, but a concensus suggests that the maintenance requirement for con- crete roads is lower in terms of cost, materials and manpower by up to 70 per cent for the first 20 years. There are two approaches to the costs of maintenance. In the developed world it is assumed that the required tmaintenance operations will be carried * out more or less as needed, despite any economic difficulties, such as those ex- perienced in the UK in recent years. When projecting the long term cost of a road to be built in a developing coun- try, the same assumptions are often made. Should these maintenance or upgrading operations not be carried out on time, the road may fail or require ex- pensive rehabilitation much earlier than projected. Many developing countries do not have the ability in terms of funds, equipment or personnel to maintain their existing road network, and so cannot reasonably be expected to maintain more new roads in the way that planners would wish. In the medium and long term under these circumstances, roads designed to require minimum maintenance may last several times longer than traditionally designed roads, and thus would cost far less over the design life. Concrete roads that are not heavily-trafficked require very little skill- ed maintenance for 10 or 20 years. A further advantage of concrete roads is that they can be built by labour- intensive methods using skills and technology learned in the building trade. This is advantageous for both the mobility and the availability of skilled labour, and also to a limited extent for equipment. The introduction of con- crete technology in the road building sector can also do much to develop local skills and offers scope for the fostering of local contracting industries. From the evidence available, concrete should be considered as an alternative form of construction, particularly in those -developing countries with in- digenous cement manufacturing capability. Some countries are net im- porters of cement and some produce only sufficient supplies for their building trade, but world-wide there is surplus capacity, particularly in south-east Asia.' Even where the initial cost of construc- tion is higher than for a comparable bituminous surfaced road, the reduced maintenance requirement over the design life and may make this type of construction more economic in the long term. This should be considered par- ticularly in those countries experiencing difficulties maintaining their road net- work to an economic standard. Bilateral and multilateral aid donors should encourage the building of con- crete roads, where they can be shown to be economic. They should ensure that feasibility studies include the concrete option, taking into account the maintenance performance of the receiv- ing agency. REFERENCES(ll Hodgklnson. R.J. Concrete roads for Queensland. Cement and Concrete Association of Australia. 1980. 121 Parsley. L.L and Robinson. R. The TRRL Road investment Model for developing countries (RTIM2). Department of the Environment, tdepart- ment of Transport, TRRAL Report LR 1067. Crowthome. 1982 (TARLI 131 AASHTO. Interim guide for design of pave- ment structures. Washington, 1972 (American Association of State Highway and Transportation Offilcials).(41 Road Raaaarch laboratory.. A guide to the structural design of pavements for new roada. Department of the Environment, Road Note No. 29. London 1970. IHM Stationery Office). (51 Buraau of Public Highways. Standard specifications for highways and bridges. Manila, 1972. (Department of Public Works and Comn- munications, Republic of the Philippines). Crown Copyright 1985. The work described in this report forms part of the programme carried out for the Overseas 'Development Administration, but any views expressed are not necessarily those of the Ad- ministration. I