White Toping

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EXECUTIVE SUMMARY

1. The bulk of India's highway pavements are deficient in thickness and the

specifications for various layers forming the pavement are sub-standard.Drainage of pavements is a serious problem and the -axle loads coming on themare very high (para 1.2).

2. Deficiencies in pavement thickness and scarcity of resources to strengthen thepavements to withstand the loads to be encountered during the design life, haveresulted in poor riding quality and consequent loss due to high vehicle operatingcosts (para 1.3).

3. Even the National Highway system has a backlog of strengthening of 19,250kms. The cost of strengthening this length is a huge figure of Rs. 14,450 crones(para 1.4).

4. As pavement strengthening is expected to be a major activity in the coming

years, the specifications for strengthening deserve close scrutiny (Para 1.5).

5. Strengthening of pavement can be accomplished by an "overlay" or an "inlay"(para 2.3).

6. Overlays can be of flexible type or of cement concrete, the latter being known bythe term "white-topping" (para 2.4). White-topping can be of the conventionaltype (thickness: 200-300 mm), thin (thickness: 125-200 mm) or ultra-thin

(thickness: 50-125 mm).

7. So far, the overlay choice was of flexible type because of abundant supply ofbituminous binder, resources constraint and manageable traffic volume (para

2.5).

8. The present practice of flexible overlays has many shortcomings, such as

provision of relatively weaker layers over existing pavement of insufficient

thickness (para 2.6).

9. Interest in cement concrete overlays is increasing because of various factors

such as uncertain future of bitumen availability, favourable cost of concreteoverlays, trend towards choice based on life-cycle-costs and improvements in

technology (para 2.7).

10. Concrete overlays enjoy many advantages such as long life, maintenance-free

performance, fuel saving, good riding quality, hard surface, no effect of spillageby oil, design precision, impenetrability to water, good reflectivity characteristics,availability of binder and favourable cost economics (para 2.8).

11. There are some drawbacks with concrete overlays such as difficultiesexperienced in locating utilities, poor rideability at joints and problem ofmaintaining traffic flow during their comparatively long construction period. Thereare, however, fairly simple ways of dealing with these problems (para 2.9).

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12. International experience on white-topping is encouraging. Countries like France,Belgium, U.S.A, U.K., etc. have successfully designed and constructed concreteoverlays. Their performance is good (chapter 3). Ultra-Thin-White-topping is anew technology that holds promise.

13. Various types of concrete overlays are possible: plain concrete, conventionallyreinforced concrete, continuously reinforced concrete, fibre-reinforced concreteand prestressed concrete (paras 4.1-4.6), besides Ultra-Thin-White-topping.Interest is presently centred on plain concrete and continuously reinforced

concrete. The choice between the two is essentially one of economics (para

4.7).

14. Ultra-Thin-White-topping (UTWT) is a new technology that has emerged. The

thickness is in the range of 50-125 mm (pars 5.1).

15_ In UTWT, the thin concrete overlay is bonded with the underlying bituminous

layers so that a composite action takes place. As a result, a major portion of theexternal load is transferred to the lower layers of the pavement and the stresses

in concrete are kept within acceptable limits (para 5.2).

16. The essential features of UTWT are: smaller dimensions of panels, high strengthconcrete, use of fibres to reinforce the concrete (para 5.3).

17. UTWT is laid on an existing bituminous layer after milling it so that a good bonddevelops between the concrete and the bituminous courses. Since high strengthconcrete is used, traffic can be allowed after one day of laying (para 5.4).

18. The performance of UTWT abroad has been good and design guidelines are

being developed (para 5.5).

19. Design of conventional concrete overlays is essentially a blend of engineering

analysis and experience of past performance (para 6.1).

20. The strength of an existing pavement may be evaluated directly by a platebearing test. Where it is not possible to carry out such a test to determine

strength, CBR values may be used to approximately arrive at the modulus of

subgrade reaction. The practice abroad is to prescribe a maximum limit to the

value of modulus of subgrade reaction. The limit is 500-600 psi/in. or 13.85-

16.61 kg./cm3 (para 6.2).

21. For Indian conditions, the wheel load selected should cater for the overloadingphenomenon (para 6.3).

22. Concrete of good strength is needed for overlays. A 28-day flexural strength of

4.4 MPa is appropriate for Indian conditions (para 6.4).

23. Jointed plain concrete overlay thickness varies from 190-270 mm for variousintensities of traffic (para 6.8), as against a thickness of 60-140 mm for flexibleoverlay (Para 7.7).

24. No doubt bituminous overlays are cheaper in initial cost than concrete overlays,almost half the cost of concrete overlays (Para 7.11). But it should beremembered that bituminous overlays are based on short design life, and henceinitial cost comparison alone is not equitable.

(iii)

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25. When life-cycle-costing is done, concrete overlays emerge as a very attractive

option, because of reduced maintenance and overlay costs, lower vehicleoperating costs and fuel savings (para 7.12). The NPV of construction andmaintenance costs of a flexible overlay (in a zone where stone aggregates are

cheap) is almost double that of a concrete overlay, thus bringing out very

decisively that flexible overlays are not at all economical. The first year fuel

savings amount to Rs. 23 lakhs per Km. (para 7.13).

26. Because concrete overlays are now cheaper than flexible overlays on a life-cycle-cost basis, the government should issue instructions for adoption ofconcrete overlays in all future projects (para 7.13).

27. The existing pavement must be corrected by a profile correction course before

superimposing the concrete overlay (para 8.1).

28. Drainage of the existing pavement must be improved (para 8.3).

29. Pavement widening should be done with strips having the same strength as the

existing pavement (para 8.4).

30. Traffic diversions must be constructed before taking up concrete overlays (para

8.5).

31. The temperature of the bituminous surface should be kept low by watering (para8.6).

32. Concreting and finishing must be done by mechanical pavers and appliances

such as joint-cutting machines (para 8.7). Use of weigh batchers arerecommended for the production of concrete mixes.

33. Quality control of workmanship is of prime importance (para 8.9).

34. Recent advances in cement technology render it possible to reduce thehardening period considerably (para 8.10).

35. Since "White-topping" is a new technology in India, it may be desirable to

undertake construction of a few trial lengths as a beginning.

36. R&D should be taken up in India on all aspects of concrete overlays so as to

develop design guidelines appropriate to Indian conditions.

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CHAPTERI

PAVEMENT STRENGTHENING PROBLEM

1.1. Effect of load on pavements

As a loaded wheel passes over a flexible pavement, a downward deflection occurs. Thegreater the magnitude of the load, the more is the deflection. The deflection also

depends upon the thickness of the pavement and its constituent layers, and on the

supporting strength of the subgrade soil. If the pavement is well-designed, the deflection isalmost entirely elastic and the pavement should return to its original level after the

wheel load has passed, the permanent deformation being negligible. But, when loads

are heavier than the design loads, a certain amount of permanent deformation is

caused, the cumulative effect of which results in rutting and shoving under the wheelpaths, and cracking, which are the principal indicators of pavement deterioration. Thepavement thus suffers serious deformations, affecting the riding quality. Normal

maintenance solutions like patching and resealing then act as temporary palliatives,unable to arrest the malady at its roots. The only solution is to provide an overlay of

suitable thickness and specifications, such that the structural strength of the pavement isincreased and its life is further prolonged.

The problem is more serious when pavements are not initially constructed to the desiredthickness and structural strength to meet the traffic requirements. The bulk of India's

highway pavements fall into this category. The embankments are very old, and were

formed years ago with local earth and practically no compaction. The pavements weregradually thickened as traffic grew. Till very recently, the layers were of stone soling,brick paving, or unbound aggregates (mostly Water Bound Macadam), surfaced with athin bituminous wearing course. The introduction of bituminous bases and dense

wearing courses has been comparatively recent, (since the late seventies) and that toorestricted to the heavily trafficked sections of arterial roads. Shortage of funds has_

forced the engineers to adopt the "boxed section" option for pavements, in which anyentrapped water has no outlet. A "bath-tub" situation is thus common in Indian

pavements as effective drainage is almost non-existent. As irrigation becomes widelypractised, the water table has been rising, creating capillary rise of water in the subgradethereby further affecting the pavement performance. The embankments are slightly

above or almost level with the adjoining fields. However, the worst blow to pavementscomes from overloaded vehicles. As against a legally permitted axle load of 10.2

tonnes, axle loads of 15-20 tonnes are quite common. A repetition of 150=300 millionstandard axles during the design life of a pavement is common on arterial highways.

The Vehicle. Damage Factor, which represents the damaging power of commercial

vehicles is very high (in the range of 3-10).

1.2. Effect of poor pavements

With inadequate and ill-drained pavements in the country's network of roads, highwayengineers. are finding themselves at a great disadvaptage in keeping the pavements in atraffic-worthy state. Scarcity of resources for the road sector in the past has led to theadoption of stage construction strategies and short design periods. A design period ofmore than 10 years was almost a luxury and one had to do with a design period of 5

years in most situations. Roads in the country are, therefore, in a never-ending process ofevolution:. Strengthening' with thin flexible overlays is quite common. But such

strengthening is often overtaken by high traffic intensity and loading, causing quick

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rutting and deformations. Maintenance costs mount up as patching and crack-sealing

have to be attended to. The riding quality is very poor. A recent survey of the NationalHighway System has revealed that the average roughness of the network as measured

by a Bump Integrator is 4600 mm/km*, which is far too high (Ref. 2). A well-laidbituminous surface should have a roughness value of 1500-2000 mm/km*. Surface

roughness has a major impact on fuel consumption and vehicle operating costs (Ref. 3).Thus, the country loses thousands of crores of rupees every year because of the poor

state of its roads. At current prices, this may easily amount to Rs. 20,000 - 30,000

crores (Ref. 4).

1.3. Neglect of maintenance

Road transport has grown exponentially over the past 50 years. The total length of theroad network has increased from 0.4 million km in 1951 to 3.3 million km in 1999, (eight-fold increase), and the number of motor vehicles has increased from 0.3 million in 1951 to50 million in 2000 (170-fold increase) (Ref. 7). It is experienced that the maintenance ofroad assets already created does not receive adequate attention : even in the case ofNational Highways, allocations for maintenance in the last several years have not

exceeded 50-55 per cent of the requirements, and the situation in case of othercategories of roads is even worse (Ref. 5). This is a grave situation, which can only berectified by building road pavements which require minimum or zero maintenance.

1.4. Extent of pavement strengthening required in India

The cumulative effect of past neglect is that the country is now saddled with a road

network whose pavements are extremely deficient in thickness and are very poor in

riding quality. An estimate of the World Bank (Ref. 4) mentions that approximately 80-90per cent of the National and State Highways are not structurally adequate for the

permissible axle load of 10.2 tonnes. It also indicates that over 50 per cent of theNational and State Highways, and a higher percentage of other roads, are in bad

condition. The condition of city streets is much worse. A recent estimate in the TenthFive Year Plan (Ref. 6) puts the length of National Highways where strengthening of theweak pavements is involved as 19,250 km, at an estimated cost of Rs. 14,450 crores

(Rs. 145 billion). By the time this task is accomplished, which may take 10 to 15 years,additional lengths would again become due for further strengthening. The process

would thus go on perpetually, because flexible overlays last only for about 10 years orso. In the period 2011-2021, it is estimated that 24,000 km of National Highway

pavements, will need strengthening at an estimated cost of Rs. 18,000 crores (Rs. 180billion) (Ref. 7). In respect of State Highways, accurate data is lacking, but the Vision

2021 document (Ref. 7) puts *the requirement of strengthening of pavements as under:

2001-2011 2011-2021

Length 30,000 40,000

Cost (Rs.-Crores) 22,000 30,000

(Rs.-Billion) 220 300

* Note: The following are the equivalent International Roughness Index (IRI) Values (calculated from Ref.1)

Bump Integrator Roughness (mm/km) IRI

4600 5.9

2000 2.8

1500 2.2

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1.5. Need for care in selection of overlay specifications

It is obvious from the above, that pavement strengthening will be a major activity on

Indian roads for many years to come. Large outlays are involved. As such, great care isneeded in selecting the specifications for strengthening. A dominant factor in the choice ofspecifications for strengthening will be the need to cut down the recurring

maintenance costs, the never-ending requirement of additional overlays once every fewyears to keep the road in good condition, and the lack of drainage inherent in the presentpavements. One should also keep in mind the high traffic loadings and the rainfall

intensity which causes serious damage after every monsoon.

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CHAPTER 2

OVERLAY OPTIONS

2.1. Definition of Overlay

An overlay is a layer of substantial thickness provided on top of an existing pavement Itshould be distinguished from resurfacing or resealing, which are part of periodic

maintenance operations and which involve thin layers (say below 20 mm) intended toseal cracks, restore anti-skid property and smoothen rough riding surfaces. Overlays

are part of what are commonly termed 'rehabilitation' or 'strengthening' measures. In

India, resurfacing or resealing operations are financed from maintenance grants,whereas overlays are financed from capital budgets. Thus, funds for overlays comefrom plan outlays.

2.2. Purpose of Overlay

The principal purpose of an overlay is to restore or increase the load-carrying capacity, orlife, or both, of the existing pavement In achieving this objective, overlays also

restore the rideability of the existing pavements which have suffered rutting and

deformations. Besides, an overlay rectifies other defects such as loss of texture.

2.3. Overlay and Inlay

A pavement layer constructed on top of an existing pavement is termed as an overlay(Fig. 1). This results in an increase in the final finished level of a road. In some

situations, it may not be possible to raise the level of an existing road which might havedeteriorated and needs a strengthening layer. Many urban streets are examples of this.

The footpath level is fixed with reference to the adjoining property and cannot beincreased as it may cause drainage or access problems. There has to be a certain

minimum height of the roadside kerb, and this preludes the unending increase in streetlevels whenever strengthening layers are added. In such cases, the top layers of the

existing street which have undergone deterioration are excavated, and freshstrengthening layers are added to maintain approximately the same surface level. This

procedure is known by the term 'inlay' (Fig. 2)-

2A. Principal Overlay types

The two principal options for overlays in terms of specifications and binder are the

following:

(a) flexible overlay, considering of granular layers and bituminous layers.

(b)cement concrete overlay.

Cement concrete overlay on top of an existing bituminous surface is commonly knownby the term -white-topping-

Overlays are also provided on existing cement concrete pavements.. They can be of theflexible type or rigid type. Rigid overlays can be urubonded or bonded with the concretepavement below. This publication does not deal with overlays on top of existing

concrete pavements-

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White-topping can be further classified into (Ref. 8):

Conventional White-topping : Thickness range : 200-300 mm

Thin White-topping : Thickness range : 125-200 mm

Ultra Thin White-topping : Thickness range : 50-125 mm

2.5. Overlay practice in India so far

Bituminous overlays on existing black-topped surfaces have been the most common

strategy in the past, for pavement maintenance and rehabilitation. Many factors have

contributed to this. Notable among them are:-

1. Abundant supply of bitumen

Bitumen was available in plenty. at a comparatively low cost. Today, whileavailability is not yet a problem, its price has gone up considerably.

2. Resource constraint : Sta a construction strategy

In India, the roads sector has always had to exist under a serious resource

constraint. The road programmes were ambitious, but there was invariably ashortage of funds, so the meagre resources had to be spread thinly. It was

therefore unthinkable to adopt a high design life for the pavement since thisentailed a high investment. Under such circumstances, a 5-10 year design

period was all that the country could afford. Even in the latest (1997)guideline on strengthening of flexible pavements (Ref. 9), the design life formajor roads is taken as 10 years and that for less important roads, it is takenas 5 years only. For new flexible pavements, a design life of 15 years is

recommended for National Highways and State Highways, and a design lifeof 10-15 years is recommended for other categories of pavements (Ref. 10),with the disclaimer that very often it is not possible to provide the full

thickness of pavement right at the time of initial construction, and stage

construction techniques should be adopted in such cases. Stageconstruction was thus deliberately made the cornerstone of highwayplanning. Bituminous overlays, with their short life and amenability to

frequent renewals, fitted the situation very well.

3. Traffic volume and loads were manageable

In the past, the road traffic, both in terms of volume and axle loads was

manageable. Designs with thin bituminous overlays could cope with the

situation. And when signs of distress appeared, these could always be dealtwith by further overlays.

2.6. Shortcomings of flexible overlay procedures adopted in India

Inspite of. obvious advantages of amenability to stage construction, keeping to lowbudgets and little dislocation, flexible overlays suffer from many major disadvantages.

Many sections of heavily trafficked roads in India have pavements of thickness 300-500mm. Quite often, considerations of soil subgrade supporting strength and expected

traffic, dictated the provision of further 200-300 mm (or more) of strengthening layers onsuch pavements. Faced with such a situation, highway engineers have been making up theadditional thickness in one of the following manners:

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(a) adding substantial layers of water bound macadam (WBM) over the existingbituminous surface, and finally providing a bitumen-bound base and

bituminous surfacing (Fig. 3).

(b) making up the thickness deficiency with built-up spray-grout (BUSG) andfinally providing a bitumen-bound base and bituminous surfacing (Fig. 4).

(c) providing an ad-hoc thickness of bitumen-bound base and surfacing directlyon top of the existing surface, the thickness provided being much less thanthe requirement (Fig. 5).

Alternative (a) results in the provision of the full desired thickness, but it also results inthe placing of a weak granular base on top of an existing bituminous surface.

Alternative (b) accomplishes full thickness, but the BUSG layers can be even weaker

than water bound macadam layers in the absence of proper inter-lock. The quantity ofbitumen used in E3USG is too meagre to hold the aggregates together and as loadspass, the stone aggregates may start rocking in position. Settlements follow and largedeformation take place. In fact, the use of BUSG is now not followed on National

Highways, though the specification is still used for State roads.

Alternative (c) no doubt conforms to the desirable property of a flexible pavement, viz.,successive layers on top to have higher strengths. But, the thickness is deficient. The

life of the pavement is thus shortened and soon additional overlays become necessary.

An ideal solution is to provide the full additional thickness with bituminous layers. Thispractice is being followed on National Highways for important works, but on State roadsand urban streets full overlays with bituminous layers are hardly ever adopted. Thus our

pavements are never adequately designed, with the result that they are constantly beingover-stressed. Their performance quite naturally, then falls below expectations, and

road users have to put up with the inconvenience caused by frequent maintenance

interventions and thereby incur higher vehicle operating costs.

2.7. New interest in Cement Concrete overlays

In the recent past, many changes have taken place, creating an enthusiastic awareness ofthe role of concrete overlays on bituminous pavements. Some of these are:-

1. Uncertain future of bitumen supply

The oil crunch has exposed the danger of depending on a material derived

from the dwindling oil reserves of the world. Though India has doneremarkably well in pushing forward its indigenous oil exploration programme,the demand for oil products has surged upwards. India imports about 70 percent of its oil needs and this over-dependance on imports is likely to increasefurther during the coming years, causing serious 'drain on the foreign

exchange reserves. To make matters worse, the bulk of the Indian crude

lacks in bitumen content. Thus even if we attain self-sufficiency in oil (which ishighly improbable), to meet our rising bitumen demand, we may have toimport the same from abroad. Hence in the coming years, the supply position of

bitumen is likely to become difficult (Ref. 11).

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Existing bitumen supplies should therefore be restricted to maintenance

needs, and preferably not utilized to construct new roads. And whenever

there is scope for substitution of bitumen with any other type of binder, eitherfor repairs and rehabilitation or original construction purposes, the alternativeshould be taken advantage of.

2. Favourable cost economics of cement concrete

Shortage of oil supply worldwide has increased the price of bitumen to veryhigh levels. In India, the price of bitumen was around Rs. 4,000 per tonne inthe year 1993, but now the rate has increased to Rs. 12,000 - 16,000 per

tonne, depending upon whether it is ordinary bitumen or polymer modifiedbitumen. With such high prices, the cost of the bituminous course in a road isnearly equal to the cost of an equivalent cement layer, for the samethickness. The following comparison indicates the cost of specifications:-

Rs/cum

1. Dense Bituminous Macadam 4000-5000

2. Bituminous Concrete 5000-6000

3. Cement Concrete (M-40) 3000-4000

4. Cement Concrete (M-40) with Flyash replacement 2900-3600

In Tables 11 and 13, a sensitivity analysis of cost of different bituminous andcement concrete pavements is presented.

The cost of bituminous material and cement concrete can be viewed in

another light. The quantity of bitumen used in bituminous overlay material is 5per cent by weight of mix. The quantity of cement used is about 15 per cent

by weight of mix. Thus, the quantity of binder in a concrete overlay

material is three times that of bituminous binder. But tonne for tonne,

bitumen costs 5 times as much as cement. Moreover, for producing abituminous mix, both bitumen and aggregates have to be heated, but this isnot so for concrete. Fuel requirement for heating adds to the cost of abituminous course. Thus, tonne for tonne or cum for cum, bituminous overlaymaterial will be costlier than cement concrete.

3. Trend towards selection of resurfacing based on life-cycle costs ratherthan initial costs

It is being increasingly appreciated by highway engineers and funding

agencies, that the selection of a pavement type (new or overlay) should be

done after considering all potential design alternatives, each capable of

providing the required performance. If all other things are equal, thealternative that is the least expensive over a laid down time period should beselected; that is, the planner should try to find the design that will serve theneeds of the traffic volume and loads, at a given level of service, for the

lowest cost for the required number of years. This technique, known as life-cycle costing, has been recommended by the Bureau of Indian Standards

vide IS:13174 (Part 1)-1991. It consists of an economic assessment,considering all significant costs of ownership over the pavement's economic

life, expressed in discounted money values (Ref. 12). If this practice is

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followed, cement concrete scores very well over bituminous overlays

because of its practically maintenance-free performance over a long life span.

4. Improvement in construction equipment procedures and reinforcing

techniques

Cement concrete pavement construction techniques have undergone rapid

technological advancements in recent years, making it possible to achieve ahigh rate of progress along with well-controlled quality. Improved reinforcingtechniques, such as continuous reinforcement and fibre reinforcement, havebeen evolved, which give cost-effective pavements of excellent riding qualityand structural strength.

2.8. Advantages of cement concrete overlays over bituminous overlays

Cement concrete overlays enjoy many advantages over bituminous ones. These are

discussed below.

1. Practically maintenance-free long life

Cement concrete surfaces are practically maintenance-free. The onlyattention needed is at joints where resealing may have to be done every fiveyears or so. However, joint sealants that last for the life of the concrete

pavement (25-30 years) are now available in India. If continuously reinforcedconcrete pavements (CRCP) are provided, the joints are totally avoided andthere will be virtually no maintenance required. This is because, unlike in a

bitumen pavement, there are generally no potholes, ruts or cracking, wherecement concrete is concerned.

A well-designed and well-constructed cement concrete overlay has a life ofabout 40 years. Even after this long period, another overlay can be laid, (butthis must be done before cracks develop) and the pavement's life can be

further prolonged. It can thus be rightfully claimed that the life of a cementconcrete pavement can be almost endlessly renewed with minimum trouble tothe users. On the other hand, though bituminous overlays are designed for alife of 10-15 years, even well-made ones quickly age with traffic andenvironmental effects. A periodic renewal layer (say, about 25-40 mm thick)thus becomes unavoidable after 4-6 years of service; and after a few more

years, a fresh evaluation of the condition of the entire pavement becomes

necessary depending upon the traffic, and it could be likely that a substantialrehabilitation course of 100-200 mm may become necessary. In between,

work on routine repair like filling potholes and ruts goes on. Thus, therepair/rehabilitation works on a bituminous pavement are an almost endlessaffair, causing traffic disturbances and annoyance to road users.

2. Good riding quality

The initial riding quality in-built in a concrete pavement at the time of its

construction lasts for a long time. Pavement rideability deterioration is veryslow, in fact, almost absent (Fig. 6). This quality of maintaining a smoothsurface saves fuel, tyre wear and maintenance costs of vehicles. This is in

sharp contrast to a flexible pavement, where even the best of surfaces

deteriorates fairly quickly under traffic. Ruts are formed, settlements take

place and smoothness gradually disappears.

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3. Hard surface

Cement concrete is a material with a high resistance to abrasion and gives ahard surface, very little affected by wear and tear of traffic. Its abrasion

resistance also makes the pavement ideal for sharp curves, junctions and hillroads, where cornering, braking and accelerating forces tend to disintegratethe wearing surface. In addition, the hard surface becomes a greatadvantage in heavy-load trafficked sections near ports, container depots,

storage areas and similar locations.

4. No effect of spillage of oil

Concrete is unaffected by spillage of oil and lubricants from stationary andmoving vehicles. Bitumen, on the other hand, is easily dissolved by spilled

petroleum products, and bituminous surfaces degrade quickly when this

happens. Thus, cement concrete is ideal for bus depots, aircraft aprons,

truck parking bays, fuel stations and garages, and even busy road

intersections where vehicle waiting time tends to be long, since chances ofspillage at these locations are high.

5. Design precision

A cement concrete pavement is amenable to much better and more precisestructural analysis than a flexible pavement. This is because of the fact thatthe flexural strength of concrete, which is used as the main basis for design,can be readily determined through precise scientific tests. On the other hand,flexible pavement designs are mainly empirical, and characterization of

materials in the various layers that they have, is beset with difficulties.

6. Penetration of water

A cement concrete slab is practically impervious to moisture except at the

joints. If the joints are well sealed and adequately maintained, or if there areno joints as in CRCP, water can be totally excluded from the sub-grade. Onthe other hand, a bituminous pavement permits ingress of water though its

cracks and pores. Such water, by damaging the subgrade, can easily impairthe stability of the pavement. Water in bituminous courses can also cause

stripping and loss of adhesion between the stone particles and bitumen.

Thus cement concrete surfaces are beneficial for locations where there is f

looding or water-logging, as well as where there is a lack of drainage. Citystreets and roads through towns and villages, especially in high rainfall areas, arewell suited for concrete surfaces because of this factor.

7. Good reflectivity characteristics

Cement concrete has a light-coloured surface. Hence its reflectivity

characteristics are very superior when compared to those of dark coloured

bituminous surfaces. Thus, from the point of view of street lighting, cementconcrete is an economical surface (Ref. 13), since it requires a much lowerlevel of illumination.

8. Availability of binder

Cement is locally produced in our country, with Indian capital and local rawmaterials (limestone and coal). Its availability is assured over the years to

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come since the country has plentiful reserves of these materials. The cementindustry in India is continuously augmenting its capacity and upgrading itstechnology. The installed capacity in 2004 is 146 million tonnes, and theproduction was 118 million tonnes (Ref. 14). As such, the availability of

cement would pose no problem to meet the requirement of pavementconstruction, or rehabilitation. On the other hand, bitumen is derived frompetroleum crude, whose supply world-wide is comparatively limited and may bein doubt in a few years from now. A recent study (Ref. 11) indicated that thebitumen production capacity in our refineries was 2.8 million tonne perannum. Though this was adequate to meet the existing requirements,

increasing demand which is likely to go up to over 4.0 million tonnes by the

year 2007 would require refinery capacity to be suitably augmented.Otherwise, shortage of bitumen is likely to seriously affect the upkeep of ourroad system.

Economics of overlay types

Till a few years ago, cement concrete pavements were initially costlier thanflexible ones. With the steep increase in the price of both bitumen and oil,

and the need for heating both aggregates and bitumen before mixing in a hot-mix plant, bituminous layers have become costlier than cement concrete

layers for the same thickness. This is explained in detail in a subsequent

chapter. If "shadow-pricing" is done to delete the tax element and to accountfor foreign exchange outgo, the price differential between bituminous and

concrete surfaces becomes even more favourable to the latter. And when

whole-life-cycle costs are considered, cement concrete surfaces enjoy yet

greater advantage, because of lower maintenance cost and avoidance of

frequent strengthening layers during the design life. Finally, if road user costs

are included in the comparison, the cement concrete option becomes almostirresistible.

Utilisation of fly-ash, a waste material

Fly-ash can replace cement to an extent of 20-50 per cent in concrete for

almost all usages. It is well-known that fly-ash is a waste-material producedin thermal power plants where coal is used as the fuel. Fly-ash is pozzolanicand reacts with the lime set free when cement hydrates, thereby producingcementitious materials. Fly-ash improves the properties of concrete as

under:

greater long-term strength

improved workability

increased durability

In addition, the use of fly-ash results in a reduction in the cost of concrete.

1. Fuel Savings by trucks

When a heavy wheel load is imposed on a pavement, it deflects. The amount ofdeflection depends upon various factors such as:

The wheel load

The flexural strength of the pavement

The soil support strength

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A flexible pavement has low flexural strength, whereas a concrete pavementhas high flexural strength. Because of its low flexural strength, a flexible

pavement deflects quite a lot as the wheel of a vehicle passes over it. In

case of concrete pavement, this deflection is very little. As a result, in the

former case, the wheel has to overcome the larger deflection bowl created inthe flexible pavement as it moves along (see Fig. 7). This consumes asignificant part of the energy that would otherwise be available to propel thevehicle. The consumption of fuel is consequently more on pavements whichdeflect excessively than on those which deflect less. Rigid pavements are

thus more fuel efficient than flexible pavements, when the riding quality ofboth is the same. This effect is noticeably pronounced when heavy wheel

loads pass. Commercial vehicles, which have heavy wheel loads, can thus

derive the benefit of lower fuel consumption. American experiments have

proved this theory and fuel savings upto 20 percent are reported (Ref. 16).Limited experiments carried out in India have also shown fuel saving on

concrete roads upto 14 percent (Ref. 17). If the second carriageway of theentire NHDP programme covering 14,000 Km of National Highways had beenmade with white-topping as an overlay option, it would have resulted in anannual saving of 0.5 mil tonnes of diesel, worth Rs. 1000 crores assuming2500 trucks per day, a fuel efficiency of 4 Km/litre and 14 per cent savings!

14,000 Km x 2500 Trucks x 365 x 0.14

4 Km/litre x 1000 = 447,000 Tonnes,

costing around Rs. 1000 crores

2.9. Some drawbacks of Concrete Overlays and ways of overcoming them

There are some drawbacks in the use of cement concrete overlays as given below, butthese can be easily overcome.

Firstly, breaking of concrete slabs to locate or alter utilities is troublesome. The brokenslab cannot be easily repaired as in the case of flexible pavements. This drawback is

particularly applicable to urban roads. However, it can be overcome by shifting existingutilities into conduits that are laid below the road and laying some additional conduits tocater for expansion of utilities in future. Besides this, utilities can be catered for by

leaving gaps in the concrete pavement; such gaps being constructed as bituminous orwith precast concrete blocks. With these measures, the disadvantages can be turned to anadvantage since indiscriminate digging of roads is eliminated. The example of the city ofMumbai in this regard is a success story of how careful planning of utilities before white-

topping can be carried out.

Secondly, the joints in a normal concrete pavements result in lowering down of rider

comfort, especially when driving fast. This disadvantage can be overcome by sawing of

joints (thus keeping them narrow) and careful maintenance of the same with sealant.Continuously Reinforced Concrete Pavements (CRCP) which do not have joints,

completely eliminate this defect.

Thirdly, concrete surfaces can sometimes get polished and become smooth in texturewith passage of time under heavy traffic. Such pavements may then become

skidprone. This defect can be got over by carefully selecting polish-resistant aggregatesand in texturing the slab properly at the initial stage. There is also the remedy of grooving

or diamond grinding of the pavement again, if smoothness becomes unacceptable.

14


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Fourthly, many express the apprehension that concrete slabs may not be able towithstand the heavy overloading endemic to our country. The remedy for this lies in

designing them for a heavy axle load, say 15-18 tonnes, instead of the legally permitted

axle load of 10.2 tonnes. The actual axle load to be considered for design will, ofcourse, have to be selected for each road on an analysis of the likely actual axle load

spectrum.

And finally, there is the problem of maintaining traffic flow when concreting is being done

and the slab is left for curing. Ordinary cement concrete needs several days to hardenbefore it can be thrown open to traffic. The period of traffic diversion however, can bekept to a minimum if rapid hardening and high early strength cements are used. In fact,

"fast-track" construction can permit opening to traffic within 24-48 hours (Ref. 15). Ifsufficient demand builds up, the cement and concrete industry in India can also be

expected to bring similar technology to India at affordable prices.

There is a feeling that concrete pavements generate higher levels of noise than

bituminous surfaces. This may be true,' especially in urban locations. But researchabroad has come out with solutions to overcome this defect. "Whisper Concrete" withexposed aggregates and longitudinal grooving can be reduce the noise level to less thanthat generated on a bituminous pavement.

Concrete overlays, unlike bituminous overlays, do not require a minimum thickness ofunderlying pavement for structural stability. What is required is a firm and uniform

support, which can be provided by the existing bituminous layers, if any overlay is

proposed, and by the existing granular layers if an inlay is proposed after removing thebituminous layers. The lack of internal drainage of the pavement, which is one of the

endemic problems of most of Indian pavements built over the past in stages, does notseriously affect the performance of a concrete overlay as it does in the case of a flexible

overlay.


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CHAPTER 3

INTERNATIONAL EXPERIENCE

3.1. General

The increasing price of bitumen worldwide has led to the adoption of concrete overlayson top of bituminous surface in several developed countries abroad. Other factors whichhave favoured the use of concrete overlays are the technological advance inconstruction of concrete pavements and the increasing awareness of whole-life-cycle costing concepts. In this Chapter, the experiences in some countries are given.

3.2. France

In France, nearly two-thirds of the National Highway network showed inadequate

pavement thickness at the end of the 1960s. This was as a result of insufficient

maintenance and rapid development of heavy vehicular traffic over the period19501965. The country, therefore, took up a coordinated programme of overlays.

Nearly 18,000 kms out of 28,000 kms of National Highways received overlays during theperiod 1969 to 1982 (Ref. 18). Several lengths of these overlays were constructed incement concrete. The performance of these converted pavements is reported to beremarkable (Ref. 18) and "is an encouragement to the Highways Department to give a freshimpetus to utilisation of this technique on appropriate sites" (Ref. 19).

3.3. Belgium

Belgium introduced concrete overlays in 1960. Comparing the economic and technicalaspects of concrete overlays in 1984 Dermience (Ref. 20) says as under:-

"The cost of continuously reinforced concrete overlays remains higher than that ofa black overlay of the same thickness. On the other hand, the cost of an

overlay with dowelled concrete slabs is exactly the same. User costs are clearlylower for concrete that for black alternatives. Maintenance costs of concrete

overlays are practically non-existent".

Summarizing the Belgian experience Dermience (Ref. 20) says:-

"The main reason for the choice in Belgium of continuously reinforced concreteoverlays for major roads is the excellent performance of this type of pavement in thelong term, the virtual lack of maintenance costs and the reduced user cost"_

3.4. U.S.A.

Concrete overlays on flexible pavements have been in use in the USA for many yearsnow. The first such project was undertaken as early as 1918 (Ref. 21). During 1940s

and 1950s plain concrete resurfacings were used extensively at both military and civilairports, as aircraft loadings and traffic increased. The performance of many such

resurfacings was monitored by the Corps of Engineers and it has been reported that

they performed well (Ref. 21). In 1966, Westall (Ref. 22) presented design andconstruction details for concrete overlays on flexible pavements based on experience of 10years. He concluded:-

"Concrete overlays built on asphalt pavements have demonstrated the flexibility ofthis type of construction when a change in pavement type is planned and it is

practicable to re-use an existing asphalt pavement".


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Plain concrete has been used extensively for resurfacing in California since 1960, it hasbeen reported in 1981 that: "Plain concrete resurfacings without dowels or reinforcementhave given excellent service in California" (Ref. 23).

Federal funding has been available for many years in the USA for 4 R projects:restoration, rehabilitation, resurfacing and reconstructing (Ref. 24). White-topping is one ofthe types of resurfacings. Some of the projects whose white-topped surfaces are stillgiving good service despite being more than 20 years old are described below:

In Kansas, the existing 20 year old bituminous pavement was in distress due to thermalcracking at regular intervals. The cracks were 50-80 mm wide in the 250 mm thick

bituminous layer and could not be satisfactorily repaired. The State decided therefore toresurface 13 km of four-lane divided roadway with 200 mm of plain, undowelled concrete to

bridge over the cracks in the pavement. 100 mm of bituminous layer was initially milledoff and the material salvaged was recycled and used to resurface the shoulders. The 200mm concrete overlay was slip-formed on the remaining 150 mm bituminous

layer. Skewed joints were sawn at 5 m spacing (Ref. 25).

On an interstate highway near Dallas, Texas, the flexible pavement consisted of 280 mmsub-base, 200 mm base and 150-200 mm of bituminous surfacing from two or three

resurfacings. The State had unsuccessfully tried cold milling to restore the riding quality.They decided to go in for a concrete overlay. A levelling course of 25 mm in bituminousmaterial was laid to correct the profile. Thereafter, a 280 mm plain cement concrete slabwas laid. The transverse contraction joints were at 5 m spacing. Dowels and deformedtie-bars were fastened to the old pavement ahead of the slip-form paver, which placed12 m of concrete in one pass, two 4 m mainline lanes, a 3 m outer shoulder and 1 m

inner shoulder (Ref. 25).

In 1983, Oregon State tried its first inlay, a variation of white-topping. Iowa and Idahohad built concrete inlays in bituminous pavements in 1979 and 1981. In Oregon, theymilled off all the existing outer (or truck) lane 4 m wide and 330 mm deep. This trenchwas then filled with a 330 mm CRCP slab (Ref. 25).

Some of the Midwestern states-Iowa, Minnesota and Nebraska, have now built manymiles of white-topping on light-traffic rural roads (Ref. 25). These are two lane ruralroads, 6-7 m wide, carrying farm-to-market traffic. The concrete overlays range from130 to 180 mm thickness. Overlays are plain concrete with joint spacings of 5 m or less.Usually, the new concrete is slipformed directly on the existing bituminous surface. Incases of serious existing distortion, the surface is milled to establish a more uniform

cross-section for the concrete overlay.

In Iowa, in the nineteen seventies, several country engineers analysed the economics ofresurfacing of their bituminous-surfaced secondary road system in an attempt to

decrease maintenance costs and lengthen the required maintenance cycle. Their

analysis, which was widely publicized has resulted in the construction of cement

concrete overlays over old bituminous country roads in a number of counties (Ref. 27,-28). Thickness of these ranged from 100 mm to 200 mm. Two fundamental

construction procedures were adopted; one involved the complete removal of the old

bituminous material prior to concrete overlay and the other retained the existing

bituminous surface as a base under the new concrete slab. The former technique was

adopted where the existing pavement surface exhibited extensive deterioration and

distortion. The latter technique was adopted where the existing surface was in a

tolerably acceptable condition. The experience of Iowa is that "Portland cement


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concrete overlays can be successfully constructed over existing asphaltic concrete

roads, with a minimum of surface preparation, and can contribute a long-termeconomical solution to the ever-increasing cost of maintenance" (Ref. 28).'

In his article "Concrete Overlays Challenge Asphalt", Renier said (Ref. 29) that thetechnology of concrete overlays is undergoing rapid changes. No longer will lanes to beoverlaid with concrete, have to be closed for days or weeks. He added that the concreteindustry believed that quick-setting concrete overlays would soon be a viable option oneven the busiest of highways, where closing lanes for long periods caused unacceptabletraffic congestion. Since then, two decades have passed and even quicker setting mixeswhich can become hard enough to take traffic within hours, have been developed.

Another example of a full-depth inlay is reported from Iowa (Ref. 29). 25 mm of anexisting 330 mm bituminous pavement was cold-milled and removed. In its place a 250mm plain concrete slab was slipformed. The removed bituminous material was recycledto raise the existing bituminous shoulder to match with the surface of the new concrete

pavement.

Since 1976, Utah has used plain concrete to resurface certain sections of Interstate

routes which had been badly distorted. Based on extensive research and economic

analysis, it was established that plain concrete resurfacing of distressed flexiblepavements could result in savings through less frequent sealing and resurfacing (Ref.23). The minimum thickness for concrete resurfacing over bituminous pavementadopted by Utah has been 250 mm.

Fibre reinforced concrete resurfacings on flexible pavements have also been carried out inUSA (Ref. 21). These relate mainly to airport aprons. Good performance has been

reported.

As far back as 1982, the NCHRP Synthesis (Ref. 21) summed up the USA's experiencewith white-topping as under:

"All five types of concrete (as described in Chapter 4 of this publication) have been usedsuccessfully to resurface existing flexible pavements. The existing flexible pavement isused as a high-quality foundation and the design and construction of the resurfacing areessentially the same as for the same type of concrete pavement. Unless the existing

surface is badly distorted, no surface preparation is necessary; however, isolated, failedor badly distressed areas should be repaired to maintain uniformity. Should the surfacebe badly distorted, it can be levelled with a levelling course or by surface grinding (cold-milling etc)".

In a later report (1994) (Ref. 15), covering the performance of a number of white-toppingprojects, it is concluded that:

White-topping is an increasingly popular use of PCC resurfacing as arehabilitation or structural strengthening alternative on AC (Asphaltic Concrete theUS term for bituminous surfacing) pavement".

A 200 mm plain concrete white-topping overlay laid in 1966 in California was reviewed in1989, when the road had carried more than 10 million ESALs (Equivalent Standard AxleLoads) and was considered to be in excellent condition. Another overlay 150 mm in

Iowa had carried truck traffic to a grain elevator for some 20 years and was rated in faircondition with some mid-panel cracking of 12 m long slabs.


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Many inlay projects have been implemented since then and it has been concluded that toenhance the performance of inlay projects, positive load transfer and positive

drainage features are essential.

The American Concrete Pavement Association, in a recent report has summarised thestate of practice of Whitetopping and one can infer that it is by now an accepted option ofoverlay in USA (Ref. 30).

A detailed study of white-topping projects in the USA revealed that Life-cycle economicanalysis was a major consideration, besides lane closure traffic delays. The economicanalysis included:

se rvice life

initial cost

maintenance costs

future rehabilitation requirements

cost of maintenance of traffic

The AASHTO Pavement Design Guide (Ref. 26) recognises that a jointed concrete

pavement (plain or reinforced) or continuously reinforced concrete pavement as an

overlay can be placed on an existing bituminous pavement to improve both structuralcapacity and functional conditions. It mentions that a PCC overlay is a feasiblerehabilitation alte rnative for bituminous pavements for practically all conditions, and it ismost cost effective when the existing pavement is badly deteriorated. It also says thatPCC overlays have been successfully constructed as thin as 125 mm and as thick as

300 mm. 175-250 mm has been the typical thickness for most highway overlays.

3.5. U.K.

The first application of concrete overlays on a flexible pavement in UK in 1981 wasreported by Gregory (Ref. 31). In U.K., much of the major road network has already

been built. Thus, the strengthening of the existing pavements, particularly those of

flexible construction, is likely to play a much larger part than new construction, in theroads programme. Though the strengthening practice hitherto was flexible overlays, thepossibility of overlaying in concrete has been under consideration in that country. As anexperimental project, a CRCP overlay was constructed on certain sections of Truck

Road A2, aggregating to 5.5 km in length. A thickness of 200 mm was used. The steelpercentage adopted was 0.65 per cent. A conventional paving train was used for

construction in one-lane widths. The following comparison of relative costs is presented:a) 200 mm CRCP overlay - 100

b) 300 mm bituminous overlay - 101 (same traffic capacity as (a) above)

c) Reconstruction - 159

It is seen that CRCP overlay is slightly cheaper than a bituminous overlay for an

equivalent design. A more competitive bid rate for CRCP could have been possible if alonger length was involved. Gregory concluded (Ref. 31) that:-

"...CRCP overlays appear to be very competitive in cost of construction when

compared with other forms of strengthening".


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The Report of Great Britain to the XVII World Road Congress at Sydney, 1983 contained the

following (Ref. 32):"Since the last Congress, experience has been gained using CRCP overlays

(essentially add-on layers) to old concrete and fatigued flexible road pavements asa strengthening operation. More work of this nature is inevitable as road

pavements reach the end of their initial design life. Consideration is also beinggiven to the use of other methods for pavement strengthening and partialreconstruction. These include a variety of concrete pavement slab types using aCRCB (continuously reinforced concrete base with a flexible wearing course)inlay set into the old pavement structure."

In another report (Ref. 33) it was indicated that close control of the concrete and theconditions under which it was laid were necessary to obtain good performance. It was

also inferred that preliminary shaping of the existing pavement surface by milling wasdesirable.

Another example of concrete overlay in UK was on a 5 km length of motorway (M18)

(Ref. 34). A condition survey of the existing flexible pavement indicated that major

maintenance was required in the form of overlays to give the pavement the required

additional 20 years of life; besides which, in certain sections, the critical stage had

already been passed and hence total reconstruction was necessary. Alternative bidswere invited for the work, and it was found that CRCP was the least expensive, at 67 percent of the cost of flexible reconstruction, and 83 per cent of the cost of constructing a300 mm thick bituminous overlay. It was therefore decided to overlay the existing

pavement with 225 mm CRCP, having 0.65 percent of high-yield deformed steel

reinforcement.

3.6. Conclusion

As many countries are now facing serious problems of road maintenance andrehabilitation, engineers are constantly on the search for cost-effective solutions. Theuse of cement concrete overlays on bituminous pavements is one such promising

answer. It is technologically feasible and, based on whole-life-cycle-cost, is economical aswell. The performance of white-topped roads already constructed is encouraging. Inaddition, concrete overlays fit in well with the need to reduce bitumen consumption asthe cost of this material is constantly on the rise.


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CHAPTER 4

TYPES OF OVERLAYS

4.1. Common types of concrete overlays on bituminous surfaces

A review of successful white-topping projects indicates that the following are thecommon types of concrete overlays on bituminous surfaces:

1. Jointed Unreinforced Concrete Pavement (JUCP)

2. Jointed Reinforced Concrete Pavement (JRCP)

3. Continuously Reinforced Concrete Pavement (CRCP)

4. Fibre Reinforced Concrete

5. Prestressed Concrete

These are described in subsequent paras of this Chapter. Another type of overlay,Ultra-thin White-topping, is dealt with in Chapter 5.

4.2. Jointed Unreinforced Concrete Pavement (JUCP)

Plain unreinforced concrete overlays on bituminous surfaces are jointed slabs. Dowelbars and tie bars are provided as in a new pavement. The joints are generally sawn.

4.3. Jointed Reinforced Concrete Pavement (JRCP)

Reinforcement, when provided in concrete slabs, is intended to hold the fractured faces atcracks tightly closed together, so as to prevent deterioration of the cracks and to

maintain aggregate interlock thereat for load transfer (Ref. 35). It does not increase the

flexural strength of the unbroken slab when used in quantities that are consideredeconomical. Where slabs are provided adequately with joints to control cracking, suchreinforcement is not required. The nominal longitudinal reinforcement, if at all provided, isabout 0.2 to 0.3 per cent of the cross-sectional area. For example if the slab is 30 cm

thick, the area of longitudinal steel per metre width of slab

Provide 12 mm dia bars at 12 cm spacing giving 9.42

100 OOx0.3= = 9 cm2.

cm2per metre width.

Conventionally reinforced concrete slabs have no particular advantage over the plain

concrete slabs, except to hold the fractured faces at the cracks and increase the spacingof transverse joints (from 4-5 m to 200-250 mm). Hence they are not commonly

adopted.

4.4. Continuously Reinforced Concrete Pavement (CROP)

CRCP as an overlay, contains continuous longitudinal steel reinforcement with no

intermediate transverse joints. Transverse reinforcement may or may not be used (Ref.21). The thickness of CRCP overlay is substantially lower than that of a plain concreteoverlay, but the economy achieved by this reduction is often offset by the cost of extrasteel. The economics of both types of pavement need to be worked out in detail, to

establish which alternative is cheaper. The cost of aggregates, cement and steel are thecrucial factors in costing. However, it must also be borne in mind that CRCP eliminatesthe need to maintain the joints and the discomfort of too many joints. CRCP is thus able to

provide a very good riding quality, so desirable for superior facilities like Expressways, andhence is the preferred choice in many countries.


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4.5. Fibre Reinforced Concrete

Fibre reinforced concrete is a technique whereby short fibres of steel, polypropylene orother material are randomly dispersed into the concrete during mixing, to provide

reinforcement in all directions. Thus cracking is very closely controlled. The cost of

fibres is, however, substantial and hence the economics of fibre-reinforced concrete has tobe carefully worked out before its use is recommended.

When fibre reinforced concrete is used, it is possible to reduce the overlay thickness.

The following formula may be used (Ref. 36),

fPCChSFRC hPcc

fSFRC

where h SFRC required slab thickness of SFRC, cm

hPcc = required slab thickness of PCC, cm

fpcc = design flexural strength of PCC, MPa

fSFRC = design flexural strength of SFRC, MPa

The following example illustrates the use of the formula:

The 28-day flexural strength of plain concrete is 5MPa, whereas with the addition of 1per cent of steel fibres it increases to 8MPa. The thickness of overlay with plain

concrete is 250 mm. What is the thickness of the overlay with steel fibres?

hpcc =

fpcc

fSFRC =

hSFRC =

=

=

=

250

5

8

2508

250 0.625

250 x 0.79

198, say 200 mm

4.6. Prestressed Concrete

Prestressing dramatically increases the strength of concrete and its load-carrying

capacity and hence is generally used in bridges and tall buildings. The technique can

also be used for concrete pavements and white toppings. The inherent high strength ofprestressed concrete resurfacings makes it possible to use this technique in situations

where the overlay has to restore or increase the load-carrying capacity of existingpavements. However, as a resurfacing material, prestressed concrete can still be

considered to be in the experimental or developmental stage.


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4.7. Summing up

Out of the five types of concrete available for white-topping, interest is presently

centered on two, viz. Jointed Unreinforced Concrete Pavement (JUCP) and CRCP.

Both are widely used for the purpose. The choice between the two is essentially one oftheir comparative economics for a specific case.


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CHAPTER 5

ULTRA THIN WHITE-TOPPING

5.1. Definition

Concrete overlays on structurally deficient and distressed flexible pavements have beenin common use for over 60 years now. These overlays are designed as new concrete

pavements resting on the bituminous layers, by analyzing the stresses on the basis of

Westergaard theory, and taking advantage of the higher value of subgrade support

(modulus of subgrade reaction) provided by the existing pavement. These overlays aregenerally thicker than 125 mm, and may even be 200-250 mm thick. They are called

"conventional" white-topping, to distinguish them from a new form of white-topping ofthickness 50-125 mm which has emerged since the 1990s. Because of their lesser

thickness, these new type of concrete pavements go by the name of Ultra-Thin-White-Topping (UTWT).

The first project of UTWT reported in literature was built in Louisville, Kentucky, USA, in1991 (Ref. 37) as an experimental project. The successful performance of this projecthas given adequate confidence to highway engineers to adopt the technique on other

projects.

In fact, three,types of white-topping are now recogniized:-

(i) Conventional white-topping of thickness greater than 200 mm

(ii) Thin white-topping of thickness 125-200 mm

(iii) Ultra-thin white-topping of thickness less than 125 mm

Conventional white-topping has already been dealt with earlier. This Chapter deals with

UTWT, of a thickness of 50-125 mm.

5.2. Difference between Conventional White-topping and UTWT

In the conventional white-topping, the bituminous surface is considered as a sub-baseproviding a uniform and firm support to the overlay concrete slab. The concrete slab isdesigned in the same way as a new concrete pavement resting on a sub-base, using theclassical Westergaard's theory. The sub-base only provide vertical reaction and does

not take care of the bending stresses. Since the overlay slab acts independently, its

thickness is substantial.

In UTWT, the asphalt surface is milled and the concrete overlay is bonded to it.

Because of the resultant bonding action, a true composite pavement, results, reducing

the load-induced stresses in the concrete overlay. As a result, the overlay thickness isreduced. By bonding, much of the load is transferred down into the bituminous part ofthe pavement structure, thus keeping the stresses in the concrete well within acceptablelimits. As the bonded concrete slab thickness is reduced, less load is carried by it and theasphalt layer carries more load. As the thickness of the concrete slab is increased, itcarries more of the load, transferring less of it to the asphalt layer. The performance ofthe composite pavement depends on the thickness and strength of both the overlay andthe asphalt layer and hence, it is necessary that there must be a sufficient thickness ofasphalt layer for the technique to be economical and. successful. Usually a minimum

thickness of 75 mm of asphalt layer (after milling) is needed.


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When the concrete overlay bonds with the asphalt layer, the neutral axis of stress

distribution shifts from the middle of the concrete section towards the bottom of the

concrete. This shifting lowers the tensile stresses at the bottom of the concrete to a

value that the thin concrete can withstand (Ref. 37, 38).

The mechanism of load transfer in UTWT is different from the load transfer in a

conventional concrete pavement. A reference to Fig. 8 shows the difference. Traditionalconcrete pavements are designed to counter the load through bending action and henceare made thick enough to resist stresses induced by bending. In UTWT, short joint

spacings result in a small panel size. As a result, the slabs deflect instead of bending.

5.3. Features of UTWT

5.3.1. The following are some of the features of UTWT

1. Smaller dimensions of panels

2. High strength concrete

3. Use of fibres

5.3.2. Smaller dimensions of panels

In order to minimize the stresses due to curling and warping, the panel dimensions arekept very small. As against 3-4 m x 3-5 m in a conventional concrete slab, the panels inUTWT are 0.6 m-1.2 m square. The thumb rule is to keep the maximum joint spacing inUTWT between 12 to 15 times the slab thickness (Ref. 38)_ Thus, if the slab thickness is100 mm, a pane! size of about 1.2 x 1.2 m is recommended.

5.3.3. Concrete strength

Since UTWT is laid on existing roads, traffic interruptions and delays involved in curingand strength gain of conventional concrete cannot be tolerated. Highway authorities donot tolerate a traffic delay of more than 24 hours. Due to this, the strength of the

concrete within 24 hours should be sufficiently high, ie., in the region of 25-30 MPa. Thiscan be achieved through the use of water reducers, fibres and a low water-cement ratio. Inone demonstration project in Louisville, Kentucky, (Ref. 39), in 1991, a UTWTpavement was constructed to be of a thickness that varied between 50 and 90 mm. Themix was produced to give a strength of 27.6 MPa in 18 hours, using high range waterreducers, polypropylene fibres and a water-cement ratio of 0.33.

5.3.4. Use of fibres

Since the thickness of UTWT is small, and the overlay is expected to be thrown open totraffic in one or two days, one of the means employed to increase the strength is throughthe use of fibres. Fibres can be of steel or polypropylene. These fibres improve the

UTWT in the following ways.

yield high strength concrete which can be opened to traffic in one or

two days

control cracking

provide better resistance to impact, shock and thermal stresses

improve fatigue resistance


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reduce the amount of intrusion by aggressive environment

increase the flexural strength by 20 to 150 per cent

improve resistance to abrasion

if steel fibres are used, the aspect ratio (ie. length/diameter) should be in the range of

50-100 (Ref. 36). The diameter of the fibres can vary from 0.5 to 1.0 mm. The quantity tobe used is 40-200 Kg/cum (0.5-2.5 per cent by volume).

Polymeric fibres are polypropylene or polyester and include-

rayon

nylon

polyethylene

cellulose acetate

PVA fibres

Fibrillated or triangular fibres provide a better bond than plain fibres. The diameter of thefibres is about 0.04 mm. The fibres must have a reasonably good tensile strength and

high elastic modulus. The quantity of fibres used is in the range of 1-2 Kg per cum ofconcrete.

5.4. Construction features

Three operations are involved in UTWT construction-

* preparation of the existing surface

placing, finishing and curing of the concrete

cutting/sawing of joints at the prescribed spacings

the performance of UTWT depends upon the formation of a good bond between the

overlay and the existing bituminous surface. To develop this bond, the existing surface ismilled, which creates a rough surface that "grabs" the concrete. Milling must be

followed by cleaning so that all loose particles that can hinder bonding are eliminated.

Paving for an UTWT can be done in the same manner as a conventional concrete

pavement. Conventional hand-held vibrating screeds, slip-form pavers or fixed-formpavers can be used. Normal finishing and texturing procedures are applied. Proper

curing is important to avoid shrinkage cracking and debonding between the bituminoussurface and concrete overlay. Curing compound is applied at twice the normal rate,

because the overlay, being thin, has a high surface area to volume ratio and there are

chances, that water may be lost rapidly due to evaporation. During the application of thecuring compound, care should be taken that the curing compound is not sprayed on theadjacent uncovered prepared bitumen surface, since that would prevent bonding.


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Joint sawing is carried out with lightweight portable saws as early as possible, (with in 6hrs) to control cracking. The depth of the saw cut should be approximately one-fourth to

one-third of the total depth of the overlay. The width of the joints is about 3mm.Typically, UTWT joints are not sealed. Test studies have shown that the UTWT overlayperforms well without sealants because the compactness of the slabs minimize joint

movement.

If the concrete develops a strength of 25-30 MPa within 24 hours, traffic can be allowedover the slabs when the concrete is one day old.

5.5. Looking to the future

UTWT has not been tried out in India so far (except for a very short, four-lane, 30 metrelength in Pune), though the Central Road Research Institute is planning to lay an

experimental stretch in Delhi. The study will be watched with great interest.

However, there have been many successful UTWT projects implemented abroad. TheLouisville experiment (see para 5.1 above) has successfully withstood traffic consisting of400-600 trucks per day. About 2,00,000 ESALs passed over the pavement in one year(Ref. 39). UTWT overlays have been constructed in Brazil and Canada (Ref. 38). TheCanadian project has a thickness of 100 mm with a concrete that developed a

compressive strength of 20 MPa in 24 hours and 35 MPa at 28 days. The traffic on thisroute is reported to consist of heavy trucks. The road is giving good service-

In Alabama, USA, two UTWT overlays have been constructed on heavily travelled

bituminous pavements, and the performance is reported to be good (Ref. 41).

The success of UTWT has prompted the introduction of this technique in the

forth coming revision of AASHTO Design Guide (Ref. 40).

Considering the serious problems faced by highway and city engineers in maintainingbituminous surfaced roads, UTWT holds great promise since it is economical, fast-trackand maintenance free.

Further research is needed to evolve a simple design methodology for this type ofoverlay.


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CHAPTER 6

DESIGN CONSIDERATIONS FOR CONVENTIONAL OVERLAYS

6.1. General requirements

The design of conventional concrete overlays is essentially a ' blend of engineering

analysis and experience of past performance. For concrete overlays on concretesurfaces, empirical equations have been developed and used extensively for airfield andhighway pavements. But for concrete overlays on existing bituminous pavements, the

procedure adopted is to treat the existing pavement as a high quality foundation and

calculate the slab thickness in the usual manner as for a new pavement. The followingrequirements must be kept in view:-

1. Thickness must be adequate for the anticipated loads and the number of theirrepetitions, over the design period.

2. Joints (longitudinal and transverse) must be able to transfer thesuperimposed loads without impairment of rideability. The joints should

minimize the migration of moisture and fine solids through the overlay as wellas between it and the underlying pavement.

3. Concrete must be of adequate strength to withstand the stresses induced by

loads and temperature. The fatigue characteristics of concrete must be

accounted for.

4. The mix design selected must be able to achieve the minimum desirable

strength of concrete in the field, taking into account statistical variability of thematerial.

6.2. Evaluation of the existing pavement and suitability criteria

The first step in the design of an overlay is the evaluation of the existing pavement

condition and structural capacity. A visual condition survey which covers distress

features like cracks, ruts, potholes, ravelling and settlements is generally able to provide asubjective assessment of the pavement condition. The thickness and composition of the pavement can be determined through trial cores. But a real knowledge of the

structural adequacy or otherwise of the pavement can come only through somestandardized tests. For designing flexible overlays, the common tests are CBR, DCP

(Dynamic Cone Penetrometer) of the subgrade at the field density and Benkelman Beam

deflection. More modem methods such as the Falling Weight Deflectometer andDynaflect or Deflectograph are still not in common use in India. The above testmethods, however, fail in giving the design thickness of concrete overlays. The basic

requirement for designing a concrete slab is the modulus of subgrade reaction, k. In thecase of a new pavement, the k value of the soil subgrade is determined. In the case of anexisting pavement which has to be resurfaced, the k value at the surface of the

existing pavement is determined. The usual method followed is a plate-bearing test. As kvalue is influenced by test plate diameter, the standard procedure is to use a 75 cm dia.plate. However, since it is more convenient to use a smaller dia. plate, e.g. 30 cm, thelatter is often adopted. The following approximate conversion then holds good:-


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k75 =0.5k 30

But this conversion is generally valid for homogenous foundations and may not holdgood for layered construction. Hence, in the case of an existing pavement, as one is

dealing with a series of layers, caution is needed.

Where it is not possible to carry out the plate-bearing test, the k value may be

approximately determined from the known CBR values. The following table (Table 1)can be used (Ref. 35).

Table I

Approximate Relationship between "k" Value and CBR Value

CBR %a 2 3 4 5 7 10 20 50 100

k (Kg/cm3) 2.08 2.77 3.46 4.16 4.84 5.54 6.92 13.85 22.16

A correlation between k value and the Benkelman Beam deflection has been presentedby Sherman and Hannon (Ref. 42). This is likely to be of great use to Indian engineersbecause Benkelman Beam deflections are easily determined. This correlation is given inFig. 9.

The practice abroad is to prescribe a maximum limit to the k-value. The U.S. Corps ofEngineers lay down that in no case a k value greater than 13.85 Kg/cm3 (500 psi/in) beused in (Ref. 21). Sherman and Hannon (Ref. 42) prescribe a limit of 16.61 Kg/cm3 (600psi/in). These practices appear to be arbitrary and the rationale behind fixing maximumlimits on k value is not known.

Since many of the flexible pavements in India have Water Bound Macadam as the basecourse and relatively thin bituminous surfacings, it would be appropriate to assume

k=300 lbstin2/in or 8 kg/cm3 in such cases. For roads with bituminous layers of thickness of100 mm and above, a k value of 11.1 can be considered. This corresponds to a

Benkelman Beam deflection of about 1.2 mm, which is common in India.


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Table 4

Stress Ratio and Allowable Repetitions in Cement Concrete

Stress Ratio

0.45

0.46

0.47

0.48

0.49

0.50

0.51

0.52

0.53

0.54

0.55

0.56

0.57

0.58

0.59

0.60

0.610.62

0.63

0.64

0.65

Allowable AllowableRepetitionsStress Ratio Repetitions

6.279x107 0.66 5.83x103

1.4335x107 0.67 4.41x103

5.2x106 0.68 3.34x103

2.4x106 0.69 2531

1.287x106 0.70 1970

7.62x105 0.71 1451

5.85x105 0.72 1099

3.26x105 0.73 832

2.29x105 0.74 630

1.66x 105 0.75 477

1.24x105 0.76 361

9.41x104 0.77 274

7.12x104 0.78 207

5.4x104 0.79 157

4.08x104 0.80 119

3.09x104 0.81 90

2.34x104 0.82 681.77x104 0.83 52

1.34x104 0.84 39

1.02x104 0.85 30

7.7x103

Since fatigue failure will always be at the end of design life of 20-30 years, it is felt thatthe 90 day flexural strength should be selected for checking design against fatigue

failure instead of the 28 days strength. The 90 day flexural strength can be taken as 1.2times the 28 day flexural strength.


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6.6. Stress due to temperature

The temperature stress at the critical edge region of the concrete slab may be obtainedas per Westergaard's analysis using Bradbury's coefficient from the following equation:

EATaC

Ste 2

Ste = temperature stress in the edge region, kg/cm2

E = modulus of elasticity of concrete, kgicm2

AT = maximum temperature differential during day between topand bottom of the slab, C

a = coefficient of thermal expansion of cement concrete per C

C = Bradbury's coefficient, which can be ascertained directlyfrom Bradbury's chart against values of U f and B/ e

(Fig. 10)

L = slab length, or spacing between consecutive contractionjoints, cm

B = width of slab, cm

f = radius of relative stiffness, cm

(1 _4

122

)k

w e Poisson's ratio

h = thickness of the concrete slab, cm

k modulus of subgrade reaction, kg/cm3

The values of Bradbury's coefficient C are presented in the form of achart in Fig.10

6.7. Corner stress

The load stress in the corner region may be obtained as per Westergaard's analysis,modified by Kelly, from the following equation:

1.213P

Sc 1-h2

where Sc = load stress in the comer region, other notations remaining

the same as in the case of edge load stress formula, kg/cm2

P = Wheel Load, kg

a = radius of equivalent circular contact area, cm

The temperature stress in the corner region is negligible, as the corners are relatively

free to warp and therefore, may be ignored.


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6.8. Sample design

A sample design is given in Appendix ITable 5 gives the thickness arrived at for the common traffic loads. It also gives the

thickness as per Portland Cement Association (PCA) method (Ref. 58).

Fig. 11 gives a best fit line for the values in Table 3, which can be used as a rough

guide.

Table 5

Wheel Loads Considered and Thickness Required

Traffic ThicknessClassification

1. Low

2. Medium

3. High

Wheel Load Thickness(Kg) (mm)

5000 kg 190

7500 kg 240

9000 kg 270

as per PCA

Method (mm)200

240

260

6.9. Comparison of thickness adopted in other countries

The French practice in designing concrete overlays on flexible pavements which has

been in vogue is by applying the analytical models (Ref. 19) for determination of

stresses. Traffic is determined in terms of repetitions of standard axles over the designlife. Fatigue performance of concrete is accounted for. The thickness of overlay isrelated to the deflections. The range of thickness is as under:

Table 6

Overlay Thickness

Traffic (during design life) Thickness of Overlay (cm)

9 million standard axles (msa) 23 - 28

3.5msa 21-27

2.0 msa 20 - 25

0.64 msa 19 - 24

The actual thickness of concrete overlays on flexible pavement, as constructed in

France, varies from 17 to 20 cm. Long length of these pavements, which were in servicefor more than 10 years were reported to be giving excellent performance. They had

carried more than 50 percent of the total planned for load repetitions, at the time of

examination.

The thickness adopted for CRCP overlays on flexible pavements in U.K. projects varyfrom 20 to 22.5 cm (Ref. 32, 33).

In Utah (USA), a thickness of 25 cm of plain concrete overlay on flexible pavements hasbeen the rule for the past few decades (Ref. 21). The thickness adopted for main roads inmany other American states is in the range of 20-30 cm.


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CHAPTER 7

ECONOMICS OF CONVENTIONAL WHITE-TOPPING

7.1. Purpose of Economic Analysis

Those familiar with the advantages and superiority of cement concrete pavements areconvinced that this pavement type should always be considered as an alternative to theconventional bituminous construction. But an impression exists in the minds of manyengineers that cement concrete roads are costly and the nation cannot afford such

expensive specifications. After all, they argue, there is a serious constraint of resourcesin the roads sector and whatever little is available should be used to construct the

maximum lengths. However, the point to be noted is that when scarce resources haveto be optimally utilized, all the available alternatives should be considered, and the onethat brings in the maximum benefits should be selected for implementation. Highwayengineering economic analysis is one useful tool that is available for a scientific

assessment of the consequences of adoption of the different options available itpavement construction.

7.2. Initial cost as decision criterion

Highway engineering designs were so far based almost exclusively on the initial costcriterion. The rationale was that, with the existing constraint of funds, the designs

selected had to be very economical even to start with. Hence low-cost specifications

and the resulting small design life period were invariably adopted. Stage constructiorstrategies i.e. building up the required pavement thickness, layer by layer, over ar

extended period of time, thus had an important role. While these policies fitted in wel

with the needs of the past, it is increasingly being felt that initial cost as the sole decisiorcriterion can no longer be accepted. The BIS has therefore issued instructions, (IS-13174:1991), keeping the future consequences of adoption of low initial cost solutions itview, that whole-life-cycle cost comparisons must invariably be made, when deciding between alternative techniques or materials. However, initial cost considerations cannot becompletely done away with in a resource-scarce economy such as still exists in India.

7.3. Whole-life-cycle-cost concept

A rational approach to selecting pavement types should start with the identification ofvarious potential design alternatives, each capable of providing the required

performance. Thereafter, the consequences of each design option over the required

period of time should be studied. The consequences should include the cost of routinemaintenance of the pavement over its life, the cost of major resurfacings that may be

needed at various stages in future and the cost borne by the users of the facility. The

future costs can be discounted to present values by selecting an appropriate discount

rate and these can then be added to the initial cost, to give the total life-cycle-cost.Once this is done, the alternative that has the least life-cycle cost should be the choicefor construction. In order to present the analyst with various scenarios based onchanges in major policy variables or cost inputs, a sensitivity analysis is generally carriedout.

The steps involved in whole-life-cycle-costing are:

Statement of the problem, which in the present case is pavement design for

conditions existing at the site (subgrade soil, materials, traffic, environment etc.).


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2. Generation of alternative designs, covering various specifications and materials.

3. Evaluation of the designs including estimation of initial costs, maintenance costs,rehabilitation costs and user costs, and converting them to today's monetary

value through a discounted cash flow analysis. A sensitivity analysis is carried

out to determine if any change in the assumptions has a significant effect on theanalysis.

4. The final step is the selection of the lowest-life-cycle-cost alternative.

7.4. Equitable comparison

For an unbiased selection of design, the comparison of life-cycle-cost of the various

alternatives must be equitable. In the case of pavement overlays, the comparison mustextend for a period equal to the life expectancy of the cement concrete alternative, since ithappens to be much higher than the flexible pavement life. In that period, allinterventions in the flexible pavement by way of resurfacings and major rehabilitation,must be accounted for, as should be any costs incurred on the concrete pavement (e.g.

replenishing joint filler). The overlay design itself must be on a common basis. Forexample, a concrete overlay is intended to strengthen the existing pavement forwithstanding the present and future traffic, making good the deficiency fully. For an

equitable comparison, the flexible pavement too must be designed to make up the

thickness deficiency fully. There is no room for incorporating ad-hoc overlay provisions ina scientific analysis. This must be borne in mind because decisions on flexible

overlay thickness tend to be rather tentative in this country. This is because of the

assurance given by proponents of bituminous surfaces that as and when deficiencies

crop up in future, they can always be made good by repairs or rehabilitation,

7.5. Traffic

The design of overlays is governed by the present-day traffic, the axle load spectrum

and the future rate of growth. Each case has to be examined in detail. For the purpose ofthe analysis here, three classes of traffic are considered:

Design Axle Load (single-axle, dual wheel assembly)

Heavy : 18 tonnes

Norma

White Toping

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