• \b ot1b
UNITED NATIONS tr
ECONOMIC COMMISSION t
FOR AFRICA ,,-\---.L...---" ,
PAPER B
THE DESIGN AND CONSTRUCTION OF STRENGTHENING
OVERLAYS FOR ROADS
C.I. ELLIS
• ,
G. LIAUTAUD
)
GANISED BY THE ECONOMIC COMMISSON FOR AFRICA WITH
;".,,~l_? _
UNITED NATIONS ECONOMIC COMMISSION FOR AFRICA
Conference on Highway Engineering in Africa-Addis Ababa April 1974
organised by
the Economic Commission for Africa with the co-operation of the British and French Governments
PAPER B
THE DESIGN AND CONSTRUCTION OF STRENGTHENING OVERLAYS FOR ROADS
by
G. L1AUTAUD
Jng{mieurdet'tnstiun PoJytechnique d'HAITI Directeur du Lsboratoire des Travaux Publics du CAMEROUN
Centre Experimental de Recherches et d'Etudes du Batimentet des TravauxPublics (C.E.B.T.P.) 12, RueBrandon
75015 - PARIS - (France)
HI, equare MaxHymans 75015 - PARIS - (France)
C, I. ELLIS
BSc., M,l.C,E" MJn'St.H.E.
Overseas Unit,
Transport and Road Research Laboratory, Departmentof the Environment, Crowthorne,
Berks~, England.
This conference paper has been produced through the co-operation of the Overseas Unit of the Transport and Road Research laboratory, Department of the Environment, the Overseas Development Administration of the Foreign and Commonwealth Office, of Great Britain and Northern Ireland, and the Secretariat d'Etat aux Affairs Etrangeres
CONTENTS
Page Abstract
1.
2.
3.
4.
5.
6.
7.
8.
Introduction
Deflection techniques for pavement evaluation
The principle of the use of deflection measurements for pavement evaluation
The concept of deflection criterion curves
The method of measuring pavement deflection using a Benkelman Beam
The road strengthening design procedure 6.1 Design life
6.2 Deflection survey and classification of pavement conditions
6.3 Designing the overlay The strengthening operation Particular application
8.1 Strengthening of roads on expansive clays
1 2 2
3 4 5 6 6
6
8 910 10
16 19 References 19
3.1.1. CBR method 11
8.1.2. Deflection method 11
8.1.3. Remedial measures 12
8.2 Strengthening soil-cement roads 12
8.3 The effectiveness of different strengthening
materials in reducing deflections 13
8.3.1. Lateritic gravels 15
8.3.2. Lateritic gravels stabilised vith cement 15
8.3.3.
Quartz or siliceous gravels stabilised vith cement 15 8.3.4. Crushed rock - crusher run material 158.3.5. Bituminous materials 15
8.4 The change of deflection values with time and its effect on the estimated residual life of roads and on the optimum dates for their strengthening
Summary of practical applications
9.
10.
,. " ,~_.
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The design and cOl1l5truction of strengthening overlays for roads.
ABSTRACT
The strengthening of old roads which are inadequate to cexrr the rapidly increasing traffic is one of the 1I0st pressing problellll5 for the African highw8JT engineer.
This paper describes the use of deflection techniques for evaluating the condition of existing flexible roads and deteraining the IIOst economical method of strength- ening. The role of deflection criteria in the strength- ening design process is considered and a deflection survey and analysis procedure using hand deflection bea118 is outlined. Particular attention is given to the design of overlays for soil-cement roade and roads on expansive clays. Typical strengthening aaterials available in Africa exe discussed and their effectiveness in reducing deflections is asssssed. In the strengthening process, adequate attention anat be paid to associated remedial Ileasures such . . drainage and it is emphasized that, in the present state of knowledge, deflection measurements must not be regarded as supplying an unequivocal solution to the problem of oYerlay design. They must be backed up by engineering judgement firmly based on local experience.
1. INTRODUCTION
In almost every country in the world the volume of road traffic is growing year by year, and coupled with thi8 there i8 a steady trend for the commercial vehicle8 in use to become larger and heavier. The frequency and magnitude of the axle load8 being applied to road pavements are thu8 increa8ing and most highway authorities are faced with the task of strengthening their roads to meet the new demands made on them.
The problem i8 most acute in those countries in which for economic, social or geographical rea80n8, the growth in vehicle ownership started very 810wly and for
many years the main demand on the highway engineer was to provide as high a mileage of 'minimum' thickness bituminous-surfaced road and gravel road as resources would permit. TYpically the surfaced roads that were constructed to meet this initial demand consist of bases 15cm (6in) thick or less, sometimes provided with sub-bases on the weaker subgrades. The base material is usually natural or stabilised gravel, crushed stone, or hand-placed block-stone or brick and the common surfacings are penetration macadam or multiple surface dressings.
As long as the number of commercial vehicles using a road of this standard remains less than about 50 to 100 per day and the majority of the axle loads do not exceed about 5000kg (5 tons) the pavement can normally provide adequate service at an acceptable level of maintenance cost. Indeed roads with 15cm (6in) thick well- constructed dense bases and adequate sub-bases are capable of carrying up to 500 commercial vehicle, per day for many years with no more than a multiple surface- dressing surfacing •
However, many of these 'minimum' thickness roads are now carrying more traffic than they can sustain and, in the face of escalating maintenance costs, the highway engineers responsible for them have to strengthen or reconstruct the pavements.
The easiest and most convenient way to strengthen a flexihle road is to apply an overlay of premixed bituminous material to the surface, thus increasing the thick- ness of the pavement and improving the riding quality at the same time. However, premixed bituminous materials are relatively expensive and many of the highway authorities faced with the problem of widespread road strengthening cannot afford to adopt the policy. followed by some of the more affluent, of "lay 75= Oin) and see what happens". If the old pavement is very variable in strength it usually pays to vary the thickness of the overlay to match the residual strength of the underlying construction and in the weakest places it is often cheaper to reconstruct the old pavement rather than simply cover it with an extra-thick overlay.
The strength of a road pavement can be assessed by measuring the transient deflection of the road structure under load. This paper describes the use of deflection techniques for evaluating the condition of existing flexible pavements and determining the most economical method of strengthening them.
2. DEFLECTION TECIlNIQUES FOR PAVEMENT EVALUATION
Deflection techniques have been used for evaluating the structural condition of pavements for many years. Ever since A C Benkelman devised the simp12 deflection beam for measuring surface deflections on the WASHO test road in 1953 the use of the technique has become increasingly popular, first with research workers and
now with the highway engineers in charge of road design, maintenance and strengthening.
•
Seventeen years ago the UK Road Research Laboratory started to use the 3 4 deflection beam for monitoring the performance of full-scale road experiments' , and now a comprehensive picture of the relation between deflection and perforaance has been b~ilt up for the principal typeB of construction used for new _in roaa in Britain •
In Canada
6
and California7 the deflection technique is aneS8e~tial
P"Xioof the 11 pavement desi~ procedure and many highway authorities in the USA ' 'Europe Africa and Australia make use of deflection measurements in their highway maintenance procedures.Most authorities use the hand deflection beam for measuring the deflection of 13 the r'lll:d surface clf~sed by the passage of a 'standard' wheel load, but in California Texas and France mechanical measuring deviceB have been developed. The French machine has been adopted by the Department of the Environment (Ministry of Tr8JUlport) in Britain for determining overlay designs.
3. THE PRINCIPLE OF THE USE OF DEFLECTION MEASUREMENTS FOR PAVEMENT EVALUATION
When a loaded wheel passes over a pavement a small transient vertical depression of the surface of the pavement occurs. The magnitude of the temporary 2epressions under a wheel load of say 4000kg (4 tons) ranges from about 0.25_ (10- in) for a stiff 'flexible' pavement in good condition, to ten times this figure for a weak pavement in danger of early failure.
The magnitude of the surface depreBsion or 'deflection' is a function of the wheel load, the area of contact between the tyre and the road, the speed of the wheel, and the stress-strain characteristics and thicknesses of the various pavement layers and the eubgrede , Thus if a standard wheel load, tyre Bize, and method of measure.ent are adopted, the magnitude of the surface deflection that occurs under the wheel enables comparisons to be made between the stiffness of different pavements and the change in stiffness over a period of time of a particular pavement.
Early in the history of the use of deflection measurements to monitor the
structural condition of highway pavements it was found that for a particular pavement the magnitude of1~hi7transient deflection correlated well with the subsequent 'life' of the pavement, t •
In order to explain this correlation it is necessary to consider the mechanism of failure of a flexible road.
Every vehicle that passes over a pavement induces transient strains in the pavement materials and the subgrade. The magnitude of these transient strains will vary greatly according to the magnitude of the wheel load and the effect of the tem- perature and moisture condition on the stress-strain properties of the pavement materials and subgrade at the time of the application of the losd.
When a transient strain due to a ~heel load exceeds a certain critical value in one or more of the pavement layers or the subgrade, it can be assumed that a small non-recoverable strain remains in that layer after the vehicle has passed.
Throughout the 'life' of the pavement these minute permanent strains accumulate and appear as deformation of the road surfacing.
The proportion of these transient strains that are large enough to produce permanent strains thus affects the length of time that the pavement can carry the applied traffic loading before 'failing'.
It is therefore reasonable to expect that the magnitude of the transient
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•
vertical strains of a pavement under a 'standard' wheel load should be anindicator of the magnitude of the whole range of strains that the pave.ent
experiences under traffic loading, and that this in turn is related to the n_ber of repetitions of the traffic loads that the pavement can sustain before accuaul- ating an unacceptable amount of permanent strain.
Thus it is not surprising that the magnitude of the deflection of a pave.ent under a 'standard' wheel load correlates well with the subsequent performance of that pavement under traffic.
It should be emphasised that the measurement of the surface deflection of a pavement under a 'standard' load doea not measure lIllY ab&olute properties of the pavement eystem. I t on~ has value when the cOllpOsition of the paveaent lqers ill defined and the properties of the wbgrade are known. It can be shown for instance that, when very flexible pavements are built on weak subgrades, the radius of
curvature of the deflected road surface is a better indicator of the strains imposed on1;he pavement by a wheel load than is the magnitUde of the vertical deflection • With most pave.ent BYstell8, however, deflection and curvature are
direct~ related and since curvature is difficult to measure, the measure.ent of deflection haa much to commend it.
4. THE CONCEPr OF DEFLECTION C1lITEIlIOlf CURVES
If the stresa-strain properties of pavement materiala and soila were better known, it would be possible to calculate the aurface deflection of a pavement ~stea
under a given load and to predict the amount of traffic loading that a paveaent could carry before 'failing'. Limited calculatione of this nature can be performed on the basis of present knowledge but there is still uncertainty about the behaviour of pavement materials when they are subjected to the wide range of loads imposed by traffic and about the effects of environmental factors and time-dependent changes in their properties.
The relation between the surface deflection of a pavement under a 'standard'
load and its future traffic-carrying capacity muat therefore be established eapirical~
by studying the deflection and performance characteristics of a specific type of pave- ment in a particular environment over a period of several years. This relationship can be convenient~ expressed in terms of a 'deflection criterion curve' derived by plotting the deflection of a pavement against the CUIIUlative traffic it has carried, the condition of the paveaent in the vicinity of the deflection measurement point also
being recorded.
Figure 1 illustrates a notional deflection criterion curve which is obtained by drawing a line between the plots of the deflection of failed or failing pavements and those in good condition. The magnitUde of the deflection that should not be exceeded if the pavement is required to have a long life is indicated by the ordinate
'a'. Deflections greater than 'a' indicate that a reduced pavement life can be expected. For instance if a deflection of 'y' is measured on a pavement after it has carried 'x1' amount of traffic (see Figure 1) it can be predicted that the pave- ment can carry approximate~a further 'X2' amount of traffic before failing.
Studies by the Bead Research Laboratory in Britain have established deflection criterion curves for the ~rincipal types of construction that are used for the new main roads in the country"'. Similar studies by the Onrseas Unit of the Transport and Bead Research Laboratory and by Centre Experimental du BIltiment et des Travaux Publics are in progress in many African countries in conjunction with national govern- ments to establish deflection criterion curves for other types of pavements in tropical enviroDllents.
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A tentative deflection criterion curve for bitumen sacadam surfacings on crushed stone bases in a tropical environment is shown in Fig 2(a)'8 This curve is based on the deflection studies that have been made in Malaysia for a period
of 7 years. The traffic is expressed in terms of the cumulative number of application~9
of an 8150kg (18ooo1b) equivalent axle load, the equivalency factors derived ~ Liddle having been used to convert the known axle load spectrl1lll of Malaysian traffic to
this standard axle load. Figure 2(b) shows similar relationships derived from experience in Madagascar.
5. THE METHOD OF I1EASUliING PAVEMENT DEFLECTION USING A BENKELMAN BEAM
There are several methods of measuring the deflection of a pavement under a wheel load but the simplest and IDOs~ popular is by means of the denection be. . originally devised by A C Benkelman. This consists of a slender pivoted beam (3.9m or 12ft long) supported in a low frame which rests on the road. The frame is fitted with a dial gauge for registering the movement of one end of the pivoted beam, the other end of which rests on the surface of the road.
When making a deflection measurement the tip of the beam is inserted between the twin tyres of the dual rear-wheel assembly of a loaded truck. The dial gauge is then zeroed and the truck moves slowly forward. As the wheels pass the tip of the beam, the road surface is depressed and the movement of the beam is registered by the dial gauge. The dimensions of the beam that has been adopted by the Transport and Road Research Laboratory are indicated in Fig 3 and a summary of the TRRL test procedure is given in Appendix 1. Plate 1 shows a deflection measurement being made. A
'standard' wheel load of 3175kg (70001b) is used by the Tranaport and Road Research Laboratory for its deflection studies both in Britain and overseas.
French practice makes use of maximua allowable axle loads in each particular country. Conversion factors shown in Table 1 are used to make comparisons with other measurements.
TABLE 1
Conversion factors for various axleloads
Axle load Multiplication factor for
deflection
13 tonnes 10
89 7 6.5
13T standard 1.00 1.20 1.35 1.50 1.70 1.80
6.5T standard 0.56 0.67 0.75 0.83
0.94
1.00
Since the stiffness of bitumen-bound pavement materials (and to some extent soils) is susceptible to changes in temperature it is necessary to allow for this when making measurements of deflection. The temperature of the road is recorded when the deflection measurement is made and a correction is made to the measured deflection to convert it to the equivalent deflec~ionat a 'standard' temperature.
In Britain, the 'standard' temperature used is 20 C but studies made by the Overseas Unit suggest that 350C is a more suitable 'standard' temperature for use in the tropics. The relation between temperature and deflection for a particular type of pavement is obtained by studying the change in deflection of one part of the pave- ment as the temperature rises from early morning to mid-day on a hot day. A typical
5
\
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...__ ..
temperature deflection relationship for a
bitUD~n
macadam surfacing on a crushed stone or bitUllen-bound base in Malaysia is shown in Fig 4.6. TIlE ROAD STRENGTHENING DESIGN PROCEDUllE
When a decision has been made to strengthen a particular road on the basis of its current or probable future aaintenance cost and traffic flow, a procedure along the lines indicated in Fig 5 is required.
6.1 Design life
The first step is to decide how long the strengthened pavement is desired to last. This is largely an economic question, the answer to which depends on several factors, such as the predicted traffic growth, the cost and effectiveness of the strengthening, the cost of road maintenance etc. Detailed economic analyses are not often made for individual road reconstruction projects but general guidelines can be drawn up for the desirable design lives of pavements in a particular econo~.
Depending on the economic circumstances, the optimum design life for a reconstructed flexible road in a developing country is likely to be between five and fifteen years.
6.2 Deflection survey and classification of pavement condition
Having determined the pavement reconstruction policy, the next step is to carry out the evaluation of the structural and superficial condition of the old pavement.
The question arises as to how aany deflection measurements are required to
obtain a satisfactory indication of the condition and variability of the road atructure.
The number of deflection measurements it is feasible to -.ke with a Benkelaan Be. . to evaluate a specific length of road is obviously limited. Different aut~3ities
have different ideas on how many measurements are necessary. In California
measuremegts are made at 15.2m (50ft) intervals in one wheel-track of two-lane roada;
In Canada ten measurements are made per 305m (1000ft) length of road at random spacings in the outer wheel-track; in
9Brazil measure.ents at
50m
(164ft) intervals in both wheel-tracks are recommended •As well as measuring the deflection properties of an existing road, it is
desirable to record its surface condition and any deficiencies in drainage which may be contributing to pavement failure. A simple pavement condition rating system bas
been adopted for deflection studies overseas in which deformation and cracking are
measured on a quantitative basis by the deflection survey team. At each point on the road surface where a deflection measurement is made the deformation and cracking are rated in one of the five categories shown in Table 2.
For a deflection survey method to be practical, the manual labour content of the survey must be kept to a m~nJ.mum. This can be done by making IIll%imum use of visual evidence of the pavement surface condition.
It is suggested that the following deflection measurements are made in both wheel-tracks of the slow lane on dual carriageways and in both lanes of two-lane carriageways; this latter recommendation could be modified with further practical experience.
1. Tests on a basic pattern of ten equally-spaced tests per kilometre.
2. Additional tests on any area showing surface distress.
3.
If any measurement exceeds a predetermined value, which is related to established deflection criteria for the type 0f pavement concerned, the extent of the area involved should be delineated by additional tests.4. Additional tests should be made when the variability of the above measurements exceed a certain value.
•
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I
I
\ PAPER B
The design and construction of strengthening overlays for roads
TABLE 2
Classification of road surface condition
Degree of cracking
r
Transverse deformation
under a 2m (6.5ft) long atraightedge (viBible cracks)
Clo.Bsification Index Deformation Classification Index Crack length/unit area
D1 Less than 10mm (3/8in) C
1 NIL
.-
2D2 10mm (3/8in) to C
z
Not greater than 1m/m15mm (9/16in)
-
D3 15mm (9/16in) to C
3 Greater than 1m/m2 but
20mm (13/16in) not greater than 2m/m2
, D4
zOmm
(13/16in)to C4 Greater 2than 2m/m ,regarded as 'failed'
25mm (1in) but not greater than
in UK) 5m/mZ
D5 Greater than 25_( 11n) C
5
Greater than 5m/m2
(suggested failure (ravelling
&
potholingcl"i teria for main imminent, immediate
roads in Malaysia) maintenance required)
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The deflections used both to check variability and to design the overlay should be the largest readings from either wheel-track at each chainage.
A simple and adequate check on variability can be _de in the following way.
Ten consecutive measured deflection values are considered as a group and their mean value is calculated(for this purpose all the deflection values at the regular 100m spacings are used but only the maximum reading is taken froa &n1 one area tested on account of its surface distress or large deflection). If the largeat and .-allest nlues of the group differ from the mean by DlCre than one-third of the aean then four additional tests should be aade, each test being made mid-Way between the test points which registered the largest and smallest deflections and their t.mediate adjacent
test points. If any of these four measurements would reduce the distance between test points to less than 10m (say 30ft) then the extra measurement need not be carried out.
The process is then repeated on another group of ten consecutive deflection values formed by taking into the previous group the next deflection value along the road, whilst omitting the first deflection value from the previous group. The process is repeated as each new deflection measurement is made so that a n,n n 1ng check on variability is maintained.
This survey method was applied to the field results obtained on five sites in Malaysia and it was found that no significant area of pavellent weakness remained undetected. Using this method a team consisting of a supervisor, three technicians, two drivers and five labourers equipped with one truck and two deflection beaas should be able to cover up to
5
kilometres (3 miles) of two-lane road in a day.After all measurements have been made, it is convenient to plot for each lane the maximum deflection (corrected for temperature effect) and the largest pavement surface condition ratings for each cross-section against chainage along the road site.
Any area showing exceptional weakness which may require reconstruction or special treatment is delineated. The remainder of the road is then divided into sections by inspection in such a way as to minimise variation in deflections within each section. The minimum length of a section should be compatible with the method of resurfacing that it is intended to employ. It is unlikely that the thickness of a machine-laid surfacing would be varied over lengths less than about ZODa (660 feet).
If the deflection values are very high, implying that base reconstruction is required, then short lengths of pavement can be considered separately, since hand labour
(aasisted by machine) would normally be used for such an operation.
The final stage is to design the overlay for each section of the road and to
determine thos@: areas which merit further investigation. On those areas of
pavement where deflections are very high or failure has occurred, inspection holes should be dug and a careful note of drainage conditions made.
If it is assumed that the distribution of deflection is normal, then only 2.5 per cent of the pavement would be expected to have a deflection in excess of the sample mean plus twice the standard deviation of the sample. The survey method proposed will tend to separate out the very high deflections on areas warranting special treatment or reconstruction and so reduce the effect of skewness, thus the distribution of deflections in each selected section should not be far reDOved from a normal distribution.
Some authorities recommend an overlay design deflection of the mean plus
twice the standard deviation of a sample of readings. It is felt that in developing countries it is not appropriate to go to the expense of such a high degree of
certainty since this might lead to the expensive over-design of large areas of
pavement. Use of the proposed survey method in conjunction with a design deflection of the mean plus 1.0 or 1.5 standard deviations for the section is recommended.
Examples of the surve" and anal,-sis procedure described above are illustrated' in Fig
6,
in which the over1&,- design is based On the mean plus one standard deviation.The deflection surve" should be carried out at the time of "ear when the pavement is at its weakest; in the tropics this is usuall,- after the rainy season when the sub- grade is at its wettest.
It is of considerable value in ansl,-sing surve" data to have information on the deflection histor,- of the pavement but initiall,- readings at tvo points in tillie, sa"
twelve months apart, will indicate the rate of change of deflection, which ~ indicate priority of treatment for some roads.
In interpreting deflection survey data it is necessary to have a broad knowledge of the pavement structure and the subgrade. Trial pits dug in the old pavement at one kilometre (say
i
mile) intervals should be sufficient for this purpose. At the same time attention should be given to the improvement of drainage conditions along the road. Drainage improvements should preferabl,- be completed at least one year before carT1ing out an over1&,- design survey so that the full benefits of the improvement~ be realised before pavement strengthening is undertaken.
6.3
Designing the overlayThe final step is to determine the thickness of the over1&,- required to reduce the mean deflection plus one standard deviation of each section to the 'design level'.
The design deflection level is determined from the appropriate deflection criterion curve and the required design life. Unfortunatel,- there 18 verr little information available on the relation between deflection and performance for bituminous surfacings in tropical environments and, for the present, highway engineers designing for tropical conditions muet rel,- l18.inl,- on data obtained in temperate cliaates. Figure 2 shovs tentative deflection criterion curves based on information gathered by the Transport and Bead Research Laboratorr and by Centre Experimental du Batiment et des Travaux Publics in a tropical environment and Fig 7 showe deflaction eriterion curves derived in Britain for a range of different types of pavementB"".
The thiekness of over1&,- that is required to reduce pavement deflections to the 'design level' is determined on the basis of experience. Caleulations based on elastic theorr can be made of the amount of reduction in deflection that should result when an over1&,- is added to a pavement, but experience has shown that in practice such calculations tend to underestimate the reduction in defleetion that actuall,- occurs.
Figure
8
shows the relation between reduction in deflection and overlay thic~ess derived from Transport and Bead Research ~boratorr investigations in Britain, and published data from California and Brazil •However, reeent results from West Malaysia18 suggest that, in solie environments at least, macadam type bituminous overlays have markedl,- less effect in reducing pave- ment deflections than denser mixes achieve in temperate climates (see Fig 9).
Using diagrams sillilar to Figs 8 and 9 but derived from local experience, the thickness of overlay required for each section of the road can be determined and a decision on reconstructing parts of the base can be made. This latter decision is largel,- a matter of comparing the eost of reconstructing the base with the cost of laying a thick overlay. If a length of road includes a number of localised veT1 weak areas, it is usually cheaper to reconstruet these areas rather than to lay a thick overlay over the whole area. At this stage of the design proeess, the condition of the road drainage should be reviewed and, where deficiencies have weakened the existing pavement, improvement should be specified.
< ".,f'~l. " " - - -
7. THE STRENGTHENING OPERATION
This paper is intended to draw attention to the key role that deflection techniques, and specifically the use of hand deflection beams, can play in road maintenance and strengthening measures. The actual construction operations involved are a separate and much wider issue that can only be touched on here.
The first construction operation that should be undertaken when strengthening a road ia to rectify faults in the drainage of the road. This work should preferably be completed twelve months before the main strengthening works are put in hand and a deflection survey should be csrried out after at least one wet season has put the improved drainage to the test. One of the most common drainage faults is the accumul- ation of surface water in the road base or sub-base. This fault is particularly preval- ent in roads with impermeable earth shoulders which have hand-placed block stone bases containing large voids in which water can accumulate. Characteristically, pavement failures occur at low points in the longitudinal profile. in cuts, and along the hillside edge of roads in side-long ground. Remedial measures should include the provision of drainage paths through the shoulders, the widening and deepening 9f the side ditches, and the improvement of the shape of the cross-section of the road •
Perhaps the second most common drainage fault is insufficient height of the road bed above ground water-table level. Where this situation exists over a relatively short length of road, it is best to raise the level of the pavement rather than lay excessive thicknesses of high cost pavement materials in an attempt to counter the weakness of a saturated subgrade. If a long length of road is involved, possible alternative routes should be BOught or, failing this, complete reconstruction of the pavement at a higher level should be considered.
Following the drainage works, the next operation should be the reconstruction of those areas of base which the deflection survey haa shown to be excessively weak.
Using a hand deflection beam, the exact extent of the weak areas can be marked out on the road surface. These areas are then excavated and reconstructed with suitable pavement materials the thickness of which can be detel1lined by reference to appropriate pavement design recoJlllendations. Again i t is preferable to complete this part of the strengthening operation some months before the premix overlay is due to be laid, so that any deficienciesin the compaction of the reconstructed areas can be remedied by the action of the traffic.
Finally the bituminous premix iteelf is laid. The points along the road at which the overlay thickness is to be changed are marked out in advance, preferably by the deflection survey team. Assuming that the premix overlay is to be laid by machine, the control of the thickness of the carpet lies in the handa of the machine operators who will adjust the thickness from section to section, making the transitions gradual so that the riding quality of the surface is not affected.
The mix design of the overlay material can be discussed only in general teras in this paper. The choice of the mix is often governed by the availability of materials and the type of mixing plant. In the great majority of situations the most satisfactory overlay will consist of a hot-mix made with penetration grade
bitumen and containing at least 20 per cent of aggregate in the 'stone' size (greater than 2. . or 1/12in). This general description embraces a wide range of hot-mix types, ranging from continuously-graded asphaltic concretes, through open-graded bitumen
macadam~, to gap-graded stone-filled sheet asphalts like the British hot rolled asphalt • All of these types of mix are used for overlays and each has its own advantages and disadvantages. If the existing bituminised road surface is generally intact and is impermeable, there is no need to place too much emphasis on impermeability in the overlay mix design. This means that low-voids-content mixes with their
attendant mix-control and compaction difficulties can be avoided in favour of high- voids bitumen macadams, which are generally cheaper and easier to manufacture.
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8. PARrICULAR APPLICATIONS
The first part of this paper has outlined the general principles which underlie the application of deflection methods to the problem of strengthening roads. The steps and procedures recoaended apply to the general case of traditional road structures.
This second part relates to specific aspects of the use of deflection measure- ments for strengthening, which warrant certain modifications or adaptations to the general rules stated in the first part. The two parts DUst be considered complementary.
These specific aspects are dealt with in order as follows:- i. the strengthening of roads on expansive clays,
ii. the strengthening of roads I118de of soils stabilised with hydraulic binders (notably cement) and which can be considered as semi-rigid structures,
iii. The ability of different types of material to reduce deflections when used as a strengthening layer,
iv. the changes in deflection values with time and the effects of these changes on the residual life of roads and on the opti.um dates for their strengthening.
8.1 Strengthening of roads on expansive clays
The problems of roads constructed on expansive clays is well known to highway engineers working in Africa.
The solution to this problem is less well known.
It DUst be remembered that such roads, particularly when mistakes have been made during design and construction, can show, very quickly, serious W-shaped deform- ations; ruts in he wheeltracks may reach depth of the order of 1Q-15cm.
These deforlll8tions are due essentially to the strains imposed on the supporting layers and to the variations within the subgrade; the variations are caused by the shrinkage and swelling to which the clays are SUbject as a result of saturation in the rainy Beason and desiccation in the dry season.
It is obvious that on such a weak subgrade strengthening techniques which
involve overlaying with thin layers of rigid or semi-rigid material are inappropriate and in practi
3e
doomed to failure. In effect the horizontal strains that occur (in excess of 10- or 1000 microstrains) are such that the tensile stresses and strains created within the strengthening layer would be far in excess of any acceptable values and would initiate the appearance of new cracks almost immediately.Consequently the most promising solution appears to be the use of a atrengthening layer of particularly flexible material; the techniques of cement-stabilised I118terial or dense-graded bituminous mixtures such as asphaltic concrete must be eliminated right from the start.
In regions of expansive clay soils, resouces of gravel materials are generally rare. As a result there is a tendency to use crusher run material from the nearest quarry as a new strengthened base, preferably with a covering of impervious surface dressing.
The calculation of the thickness of strengthening material can be carried out in two ways:
by the traditional CBR method by the deflection method.
••• .,... _1 _
8.1.1. CBR method. The method cowsistB of measuring the bearing capacity of the BUbgrade in the IIIOBt unfavourable conditions of compaction and moisture content. The compaction will obviously be that of the ground in-situ since the phenomenon of swelling causes the subgrade to undergo a certain "deco.paction" and hence the equilibrium densities are often much less than those .easured at the time of construction.
The moisture content of the soil at the time of testing must correspond to the IIIaXiIIrwI moisture content of the soil during the vet season. If _ples are taken during the dry season, then CBR test specimens IIUst be maintained at the required moisture content for at least four days or until there is no significant variation in moisture content throughout the _ple. Once the density and the reference moisture content have been chosen the CBR values are measured according to the standard proced- ures. Probably the values obtained vill be between 0 and 5 per cent but vith a
maximum of 10 per cent for slightly sandy clay soils.
Several design methods baaed on the CBR test are available in the literature and they can be divided into two categories:
the first takes account of the CBR down to very' lov values, and the thickness of the pavements is regulated by consideration of these CBR values (Peltier, TrlUl8port and Bead Research Laboratory').
the second does not differentiate between CBR values of less than 4 and provides a cOlllJl\On thickness of road pavement for all such values: these methods only provide a method of regulating the thickneBB of road pavement for CBR values greater than 4 per cent (~ons Associates, South Africa and to a certain extent
CEBTP method).
The particular experience of the construction of the Waza-Maltam Baad (North Cameroun) has confirmed that COnsideration IlUst be given to CBR values less than 4 or 5 per cent and that the corresponding regulation of pavement thickness is completely
justified. Therefore we propose that the calculation of road thicknesses in these circumstances should be based On Fig 10, which has been derived from the work of Peltier of LOPC and of the Transport and Baad Research Laboratory.
If the existing material is sound and the actual thickness of the existing pavement is known, the thickness of the strengthening layer can be calculated by
subtracting the actual thickness from the total thickness indicated by the chart.
8.1.2. Deflection method. The deflection method consists of measuring the deflections on the old road, dividing the road into homogeneous zones on the basis of the deflection values and then calculating the necessary thickness of strengthening
for each zone in order to reduce the measured deflections to a value considered to be acceptable. The procedures to be followed in measuring the deflections have been explained earlier in the report (Section
5).
Naturally the choice of the maximum acceptable value of deflection depends On the expected den~~ty of traffic o~2the road. However, the value will undoubtedly lie between 50 x 192 lIIIIl and 100 x
22
IlIDI for deflections under a 13 tonne axle load or between 28 x 10 mm and 55 X 10 mm under a 6.5 tonne axle load.The graphs in Fig 11 shows the thickness of strengthening required to reduce 2 deflections measured under a 13 tonne axle ~o a maximum acceptable value of 75 X 10- . .
(corresponding to a value of about 42 x 10- mm for deflections under a 6.5 tonne axle).
This curve has been derived from the deflection measurements made before and after strengthening with crushed rock (0-4omm) on the Waza-MaltaJII Bead. The calculations have been made using a
'k'
value of 78 for the crushed stone and substituting this value in the general formula relating to the reduction of deflections, which is usually considered to be of the form:----_.
-~-=
k logstrengthening thickness (cm) (deflection before stre henin) (deflection after strengthening
where k i8 considered as the "strengthening power" or "coefficient of reduction in deflection" of the strengthening layer.
Hence, by taking values of k approx:i.Jllately equal to 80 for crushed stone (0-4oma) other charts may be derived for maxi~ acceptable values of deflection different ' from those used for the Waza-Kaltllll Road. However, we would point out that the use of the value given above has given an excellent correlation between the thickness designed by the
eRR
method on the one hand and by the deflection method on the other.8.1.3. Remedial measures. Road strengthening by increasing pavement thickness is generally insufficient by itself; this is particularly true for roads on expansive clays, Associated remedial measures are also necessary. The most important are as follows:
treatment of the surface to make it as impervious as possible. We consider that a multiple surface dressing is better suited to this purpose than premixed bituminous materials md it also has the advantage of greater flexibility.
the use of wide shoulers (at least 2m) made from free-draining materials;
the slope of the shoulders should be at least 4 per cent.
waterproofing of the shoulder surface in order to avoid shrinkage cracking in the underlying embankment. Where widening of the shoulders requires an increase in width of the embankment, it is useful for the upper 10-15cm of the increased embankment width to be stabilised with lime.
remoTal of trees situated too close to the road; the aint.ua distance between the shoulder and the tree line should be 5 metres.
construction of deep ditches with a well-designed transverse profile and longitudinal slope.
and finally strict control of traffic and axle loads in order to keep them within the design conditions.
Of course it must not be forgotten that particular attention must be paid to subsequent maintenance.
8.2 Strengthenins soil-cement roads
Soils stabilised with cement are often criticised, particularly in hot cliaates, on account of their susceptibility to cracking. Transverse and longitudinsl cracks at lDOre or less regular intenals and forming a rectangular pattern are due essentially to hydro-theraal shrinkage. Sometimes longitUdinal cracks start as a result of
excessive tensile forces at the bottom of the semi-rigid stabilised layer; these forces aggravate the cracking and sooner or later lead to crazing and disintegration of the surface.
A satisfactory economic solution to the problems of cracking in cement-stabilised materials has not yet been found but full-scale experiments of controlled overstressing are being carried out in Malawi. At best the problem can be reduced by strict
sdherence to appropriate techniques at the time of construction, ie by limiting the maximum moisture content to prevent excessive shrinkage; double compaction in order to control the fine cracks; early curing; deferred laying of the surfacing.
There is a divergence of opinion about the seriousness of cracking: some engineers feel that if there is no significant deformation of the road then there is no need to worry about the cracking; others feel, on the contrary, that cracks
are weak points where water can enter and cause deterioration of the road pavement.
In either case, the cracking certainly produces an unpleasant psychological effect, especially on a client or user who has not been warned of the problems in advance.
So how do we solve the problem of strengthening these cracked roads? In our opinion two conditions must be distinguished; the first where the cracks appear to be stable and do not affect the cross-section of the road; the second where the cracks are developing and occur as a result of strains set up within the pavement layers and which cause surface deformations.
In the first case it is not a matter of strengthening but rather of camouflaging the cracks. This can be done by placing a thin carpet (3 to 4cm) of a fine-textured bituminous material designed to be very flexible and self-healing: a basic formula aay use 6 to 7 per cent of a 80/100 or 60/70 pen bitumen. It is essential to use neutral
fillers and at all costs to avoid the use of active fillers (such as cement) which make the bitumen-coated material rigid. Surface dressing is another alternative which can seal the finer cracks very effectively provided that the base is stable.
In the second case it is important naturally to avoid using a strengthening material of cement-bound materials; a flexible material must be used.
If the volume of traffic is high and axle loads heavy it may be desirable to use a new bitumen-stabilised gravel base covered with a new surface (preferably surface dressing). The thickness of the bitumen-stabilised gravel can be between 10-20cm according to the intensity of traffic (not forgetting the thickness which is necessary for reshaping).
If the traffic is light (less than a few hundred vehicles per day) a strengthening layer of crushed gravel (O-3Qmm or o-4amm) should be adequate, covered with a thin carpet of bituminous material or double surface dressing. The thickness of this strengthening layer can be limited to 15-20cm (excluding the reshaping).
The crushed material will not necessarily make any marked improvement in the deflection of the old road since, on a road which is already semi-rigid its power to reduce deflection will undOUbtedly be very weak. This layer of crushed material must essentially be considered as a "cushioning" layer between the old cracked surface and the new; equally it would play the role of a reshaping layer intended to improve the smoothness of the road after laying of the new surface. If the level of traffic is low, a well-graded and well-compacted gravel material will satisfactorily resist the traffic stresses.
In the same way as for the roads on expansive clays the increase in thickness is not sufficient in itself.
The shoulders also must be resurfaced correctly in order to match up with the thickness of the strengthening layer. Attention to the shoulders is particularly important where the strengthening layers are of crushed gravel.
8.3 The effectiveness of different strengthening materials in reducin5 deflections
One of the essential properties of a strengthening layer is its ability to distribute the traffic stresses so that the stresses transmitted to the top of the old pavement are compatible with the residual strength of that pavement.
This ability to distribute the stresses is usually termed 'distributing power' or 'strengthening capacity' and is calculated from deflection measurements taken before and after construction of the strengthening layer.
where e = d =
0
k =
The method is based on the rule known as 'the logerithllic reduction of deflections' formalised in France by Lasalle and Langumier and then confirmed by Leger of LCPC and ChaAterau of SETIlA.
This rule shows that the deflection of the surface of the road is reduced in accordance with the following formula:
e = k log do
d1
thickness of strengthening layer (cm) mean deflection after strengthening
a coefficient dependent on the strengthening . .terial.
The formula may be used for designing the thickness of strengthening layers.
Knowing or having determined the 'k' value of the strengthening uterial and having fixed a maximum acceptable value of deflection 'dj ' , the mean deflection before strengthening 'd ' may be inserted into the formula and the necessary thickness of strengthening ca~culated. This will be the thickness required to reduce the deflection do to the maximum acceptable value d1•
It seellB appropriate for us at this conference to set aside a paragraph of this paper to these k values which indicate the ability of different types of strengthening material to reduce deflections.
We will consider some of the traditional strengthening materials available in Africa:-
ie - lateritic gravels (some plasticity) - lateritic gravels stabilised with cement
- quartz or siliceous gravels (low plasticity) stabilised with cement - crusher run material
and - bituminous materials (0-10m111 or 0-2011l11).
It may be seen from the following comments that the value of the coefficient 'k' for a given material ia not a unique valu.. and that it generally depende on nUlleroua factors, the most important of which are:
- the present level of deflection on the road to be strengthened - the thickness of the strengthening layer
- the age of the strengthening layer
and - the climatic environment (in so far as this influences the properties of the strengthening D1Bterials, particularly bituminous mixtures).
The following paragraphs give k values obtained on different road works in Africa.
"'~,",l " ' _
8.3.1. Lateritic gravels. For lateritic gravels (plastic) used_~sa strength- ening layer on a road ~owingmean deflections greater thlQl 100 x 10 . . under a 13 tonne axle (55 x 10- . . under a 6.5 tonne axle), a coefficient k in the region of 65 was obtained. This value was calculated ~ediatelyafter construction but one would i.IIagine that it will improve with time if one takes account of the harden- ing which takes place in most laterites.
The miniIIUII deflection that it is possible to obtain O~2a thick baBe of lateritic gravel is like~ to be of the order of ~50 x 10 . . under a 13 tonne axle load (22 to 28 x 10- _ under a 6.5 tonne axle).
8.3.2. Lateritic avels stabilised with cement. When a lateritic gravel baa been stabilised witb cement say 3- per cent , its co~essivestrength increaBe.
and the value of its coefficient k improves. Experillental data bas given 'k' _Illes in the region of
40
to50
after7
to 28 days treatment. The value does not improve further with time since in tropical climates the compressive strength tends to reach its JIlSld.suJI value after about 28 dars.The minimum deflection
2on a thick ~se of cement-stabilised lateritic grav!2 is of the 02der of 25 x 10- to 35 x 10- . . under a 13 tonne axle load (14 x 10 to 20 x 10- . . under a 6.5 tonne axle).
8.3.3. z or siliceous ave Is stabilised with cement. In this case we are considering gravels less plastic than those mentioned above.
These materials stabilized with cement (3-4 per cent) can give analagaus to the French •grave-ciaent" or British 'lean c2ncrete'.
compressive strengths may reach 40-60 bars (600-900 Ib/in ).
a performance The 28-dar Experience with this t1Pe of material shows that the value of k is of the order of 45 at
48
hours improving to30
after7
dars curing.-2 2
The minimum deflection on this
2aaterial wou2d be about 15 x 10 to 20 x 10- . . under a 13 tonne axle load (8 x 10- to 11 x 10-.. under a 6.5 tonne axle).
8.3.4. Crushed rock - crusher run material. Crusher run material cannot be
expected to significantly reduce deflectio~when placed on a road whose mean defl!~tion
before strengthening is less than 100 X 10 . . under a 13 tonne axle load (55 x 10 . . under a 6.5 tonne axle). For this reason one sometimes doubts the efficiency of these materials when they are placed as a base overlying a relatively long laterite.
However, on weak subgrade such as swelling clays or heavy clays, the crusher run material
(o-3Omm
or o-4omm) can lower the level of deflections. The experience of the strengthening work on the Waza-Maltam Iioad (Cameroun) on expansive clays anggests a k value of about 80.Th!2minimum defle:~ion on a base of crushed material is likely to be abou~2
50 x 10 mm
2t o
60
x 10 lSI for measurements with a 13 tonne axle load (28 x 10 . . to34
x 10- lIS under a 6.5 tonne axle).8.3.5. Bituminous materials. The values given for the materials quoted above apply to a range of strengthening thicknesses generally between 10 Cm and 20 cm. For economic reasons bituminous materials used in Africa rarely reach such thicknesses.
Figure 8 shows the order of magnitude of the reduction in deflection brought about by a dense bituminous overlay in s temperate climate and is for different levels of deflection before strengthening. The k value was between 35 and
40.
However, for bitumen macadam t1Pe overlays in Malaysia (aee Fig9),
k "alues could vary between 65 and 110.I t is worth emphasizing the following points:-
- on a medium strength pavement the use of too thin a layer (2-3cm) of bituminous surfacing made from soft or semi-soft bitumen (180/200 pen, 80/100 pen) has very little chance of being able to reduce the deflection in any appreciable vsy, especially in warm climates.
- in order to be effective a strengthening layer must be made of crushed aggregste, with hard bitumen of at least 60/70 pen and laid at least 4cm thick.
However the performance of this type of material is the subject of another paper to thiB conference and is dealt with in more detail there.
Table 3 summarises the values given above.
TABLE 3
Characteristics of various strengthening materials
Coefficient of reduction Minimum deflection obtainable in deflection on pavements totally cOllStructed Strengthening material
k of strengthen~gmaterial
(order of magnitude) x 10 _
(order of magnitude)
under 13 tonne under 6.5 tonne
axle axle
Crushed rock (cruBher run) 80 55 30
Lateritic gravel (plastic) 65 45 25
Lateritic gravel stabilised
45 30 17
with cement
Dense bitumen macadam or preferably asphaltic
40 25 14
concrete (in temperate climate)
Siliceous or quartzitic
gravels (low plasticity) 30 15 9
stabilised with cement
8.4 The change of deflection values with time and its effect on the estimated residual life of roads and on the optimum dates for their strengthening
It is generally agreed that while the road is behaving in an elastic manner the deflection value does not change and that it is only :Croll the onset of fatigue and plastic behaviour that the deflection starts to increase progressively until it reaches high levels at the time of failure.
Solie engineers consider that this constant deflection during the early life of the road (elastic phase) constitutes an important characteristic which justifies the interest in these measurements.
16
However, i t is equaJ.l;r agreed that under the effect of traffic and cl~te
the road fails progressivel;r by the attrition of the base, by coapression of the subgrade, by the ageing of the bituainous binder or by a breaking down of the stabilised sateria1s.
If there is fatigue or loss of strength, intuitivel;r one feels that the streues on the road must have some effect and that probabl;r the deflection vlllue IlU8t change gradually. Recently, in Senegal (1972), a general study on road strengthelling with bituminous ...terials was undertaken. The results were taken from the whole of tha territory on roads constructed in a suilar .anner on siJIilar subgrades and supporting a relstivel;r constant 8IIlOunt of traffic over a period of nearly 25 years.
The results of the deflection measurements are given in Table
4,
grouped according to the traffic carried and the years of service. The means of the groupll of values are plotted in Fig 12 which indicates the relation between change in deflection and traffic.TABLE
4
Results of deflection measurements in Senegal
I Cumulative number of Age of
Mean age Mean de!2ections D
m+ 1.3"
section (years) x 10-2•
commercial vehicles (vears) x 10 ...
2t05x103 2 2 60 60
5 x 103 to 104
- - - -
104
to 2 x 104
- - - -
4 57
7 92
2 to 5 x 104 5
7.5 59
8 42 70
10 65
11 102
8 97
9 76
5
94
5 x 104
to 105
5 120
10 7.7 90 89
10 74
8 105
7 80
8 73
7 83
8 148
105 to 2 x 105 7 123
6 9 83 103
10 77
14 86
Table continued •••••
---_.
Table 4 Continued ••••
Cumulative DUllber of Age of
Kean age Kean dd!,~ions D+ 1.3"
section .. -2
commercial vehicles
(years) (years) x 10 x 10 _
12 120
9 145
2 to 5 x 105
13 110
11 13.5 100 106
15 74
21 111
14 82
5 x 105 to 106 14
14.5 110
15 152 131
106
to 2 x 106 20 190
21 20.3 175 180
20 175
It is interesting to note that on he whole of the road network tested there was an increase in deflection with increase in traffic. Therefore we may conclude that on such roads the deflection is not constant but gradually increases.
For this particular case, Fig 12 app£srs to indicate a Budden increase after about 15 years and cumulative traffic of 5 x_~~ commercial vehicles: the corresponding deflection is of the order of 105 x 10 mm under a 13 tonne axle load.
The same curve is drawn again in Fig 13 where both acceptable deflection and change in deflection are plotted against cumulative traffic; it can be seen that the sudden growth of the deflection corresponds to movement into the plastic phase and this change takes place after about 15 years of service for the road pavement used in Senegal up to now.
During !~e elastic phase, the deflections under a 13 tonne axle load grow by about 4 x 10 mm per yea~2onaverage whilst in the plastic phase they grow much IDore quickly at about 16 x 10 mm per year, ie four tilDes as fast.
If this is accepted then we IDust take account of the change in deflection with time when determining the acceptable deflection used for calCUlating the residual life of the road. In other words, the true acceptable deflection is lower than that given by Fig 13 and one must subtract frOID i t the value corresponding to the change of deflection with time (see Fig 14). A similar procedure must be followed when one determines the residual life of a road, as shown in Fig 15. Entering the chert at
(1) if we follow the change in deflection with time towards (A) the true residual life is less than the value normally calculated. This means that the optimum date for strengthening is brought forward ahead of the date which would normally be fixed.
This result, when adapted to any particular set of conditions, should enable
the engineer to plan his road strengthening works in a more rational manner and reduce the risks of failure.
18
9. S100lAllY OF PRACTICAL APPLICATIONS
Deflection .easurements provide the most convenient .ethod of designing road strengthening measures. In the present state of knowledge they anat not be regarded ae supplying an unequivocal solution to the problem of oTerlay deeip and must be backed up by engineering jUdgeaent firmly baeed on local experience.
The effort put into the design of etrengthening meaeures may well be waeted unless adequate attention i6 given to the construction of the associated rea.dial measures. Particular attention IlUst be paid to deficiencies in drainage of the road paTement.
10. liEFERENCES
1. O'REILLY, M.P., and R.S. MILLARD. Road l18king materials and pave.ent design in tropical and sub-tropical countries. Ministry of Transport. Road Reeearch Laboratory, RRL Report No. LR 279, Crowthorne, 1969 (Road Research Laboratory).
2. HIGHWAY RESEARCH BOARD. The WASHO Road Test. Part 1. Design, Conetruction and Testing Procedures. Highway Research Board Special Report No. 18.
Washington, 1953.
3. LEE, A.R., and D. CRONEY. British full-scale paveaent desip experiaente.
Proc , Int. Conf. Struc. Deeign of Asphalt Paveaents at Univ. Michigan, Ann Arbor, USA 1962. Ann Arbor 1963 (Univereity of Michigan).
4.
SALT, G.F. Recent full-ecale flexible paveaent design experiaents in Britain.Proc , 2nd Int.Conf.Struc. Deeign ot: Asphalt Pavements at Univ. Michigan, Ann Arbor, USA 1967, Ann Arbor 1968 (Univereity of Michigan).
5. MILLARD, R.S., and N.W. LISTEll. The aeses.ent of _intenance needs for road pavements. Proc , Inst. Civil Engineers 1971,
48
(Feb) 223-244 Inst. CiT. Enge LONDON 1971.6. CANADIAN GOOD ROADS ASSOCIATION. A guide to the structural design of flexible and rigid paveaents in Canada. Canadian Good Roads Association Paveaent Design and Eftluation COllllittee. Ottawa 1965 (Canadian Good Roads Association).
7. BEATON, J .L.E. ZUBE, and R. FORSYTIl. Field application of the resilience design procedure for flexible pavements. Proc. 2nd Int. Conf. Structural Design of Asphalt Pavements at Univ. Michigan, Ann Arbor 1968 (University of Michigan).
8. HIGHWAY RESEARCH BOARD. Design of overlays and pavement rehabilitation.
Highway Research Record
300
Washington, 1969.9. HIGHWAY RESEARCH BOARD. Evaluation of pavements by deflection studies for maintenance purposes. Highway Research Record 129, Washington, 1966.
10. LASSALLE, J., and G.
Proc. 2nd Int. Conf.
Ann Arbor USA 1967.
LANGUKIER. A method of strengthening flexible pavements.
Structural Design of Asphalt Pavements at Univ. Michigan,
Ann
Arbor 1968 (University of Michigan).11. DEHLEN, G.L. An investigation of flexure cracking on a major highway.
Proc. Int. Conf. Structural Design Asphalt Pavements at UniT. Michigan Ann Arbor USA 1962. Ann Arbor 1963 (University of Michigan).
12. MORGAN, J .R. and A.J. SCALA. Deflections in flexible pavements.
Australian Road Research Board, Vol. 2, No. 5 September, 1965.
Research Board, Melbourne, 1965.
Journal of the Australian Road
13. ZUBE, E., and R. BRIIJGES. The use of pavement deflections in asphalt pavement overlay design. Proc. Int. Conf. Structural Design Asphalt
Pavementls at Univ. Michigan. Ann Arbor 1962 Ann Arbor 1963 (Universit)' of Michigan).
14. SCRIVENER, F.H., G. SWIFr, and '0'.'0'. MOORE. A ne" research tool for measuring pavement deflection. High"a)' Research Board No 129, High"S)' Research Board, Washington, 1966.
15. PRANDI, E. The Lacroix-LCPC deflectograph. Proc. 2nd Int. Conf. Structural Design of Asphalt Pavements at Universit)' Michigan, Ann Arbor USA 1967.
Ann Arbor 1968 (Universit)' of Michigan).
16. HIGHWAY RESEARCH BOARD. The AASHO Road Test. Report 7. SWDlll')' Report.
High"a)' Research Board Special Report No 61G Washington 1962.
17. HVEEM, F.N. Pavement deflections and fatigue failures. High,,~Research Board Bulletin No 114. High"a)' Research Board, Washington, 1955.
18. BULMAN, J.N.,
c.r.
ELLIS,and H.R. SMITH. Pavement design in Mala)'sia,implications of deflection and axle load surve)'s. Conf. on Road Engineering in Asia and Australasia, Kuala Lumpur 1973.
19. LIDDLE, W.J. Application of AASHO Road Test results to the design of flexible pavement structures. Proc. Int. Conf. on Structural Design of Asphalt Pave.ent.
at Universit)' of Michigan, Ann Arbor 1963 (University of Michigan).
20. ELLIS, C.I. Axle-load distributions on roads overseas. Surve)' on roads in West Mala)'sia, 1967. Ministry of Transport, Road Research Laboratory.
RRL Report No LB 187, Crc;>wthorne, 1968 (Road Research Laboratory).
21. BRITISH STANDARDS INSTITUTION. Specification for rolled asphalt, (hot process).
British Standard 594: 1961 British Standards Institution London. 1962.
11• ACKNOWLEDGEMENTS
The authors wish to acknowledge that much of the first part of the paper is based directly on the work of Bulman
&
Smith, (see additional bibliography).12. ADDITIONAL BIBLIOGRAPHY
SMITH,
H.R. A deflection survey technique for pavement evaluation in developing countries. Department of the Environment, TRRL ReportLR
525, Crowthorne 1973 (Transport and Road Research Laboratory).BULMAN, J.N. Strengthening of flexible roads in the tropics: the use of deflection measurements. Department of the Environment,
TRRL
ReportLR
444, Crowthorne 1972(Transport and Road Research Laboratory).
SECRETARIAT D'ETAT AUX AFFAIRES £TRANGERES. Contribution to the study of the
strengthening of paved roads in tropical Africa and Madagascar (Contribution a l'etude du renforcement des chaussees revetues en Afrique tropicale et
a
Madagascar).CEBTP Avril 1971.
CEBTP Sept 71 Manual of road design for developing tropical countries (Mamiel de dimensionnement de chaussees pour les pays tropicaux en voie de developpement).
LIAUTAUD,G. Utilisation and effectiveness of deflection measurements on roads in
french speaking Africa and Madagascar (Utilisation et interet deB measures de d{flexion sur chaussees en Afrique francophone et
a
Madagascar). Revue generale des RoutesNo 484 Fe~ 1973.
DURRIEU, J, G. LIAUTAUD and ADAM. Non-destructive testing by deflectometer measurements on paved roads in Senegal, (Auscultation, par la m~thode deflectom{trique de chaussees revetues au Senegal. Revue generale des Routes No 485 Mars 1973.
LABORATCIRE DES TRAVAUX PUBLICS DU CAMEROUN. Waza-Maltam Road: The problem of its strengthening and deflection survey (Route Waza-Maltam; Probl~me de son renforcement et campagne de deflexion). Internal report Feb-March 1973.
LASSALLE, J, and G. LANGUMIER. Considerations of the strengthening design for flexible roads (consideration sur Ie culcul, du renforcement des chaussees souples). ·Revue generale des Routes No 392 1964.
LASSALLE, J, and G. LANGUMIER. Some examples of the application of the Colas
experimental method (Quelques examples d'application de la methode experimentale Colas).
Revue generale des Routes No 413 1966.
CHANTERAU, M, and P. LEXiER. The catalogue of typical road structures of the Department of Roads (Le catalogue de structures types de chauBsees de la Direction des Routes).
Bulletin de liaison du LCPC. No 61 Sept 1972.
LASSALLE.
J.
Strengthening with bituminous materials.(Renforcements en erobes bitumineux. Consid€rations et
Revue generale des Routes No 404 Nov 1968.
Consideration of new applications applications nonvelles).
LABORATOIRE DES TRAVAUX PUBLICS DU CAMEROUN.
Summary Report (ContrOle des travaux; Voieme Internal report June 1973.
Control of works; Roads department;
de Douala; Rapport de synth~se).
13. APPENDIX 1
Standard TllllL method of l18king daflection beam measuremante
1) Load a
5
ton lorry (or similar) fitted with twin rear wheels to give a load of 6350kg (14 OOOlb) on the rear axle (ie 3175kg or 7000lb On each pair of twin rear wheels).2) Inflate the rear tyPres to 585 kN/m2
(85lb/sq in).
3) Mark a point on the road at which the deflection is to be measured, and reverse the lorry until the rear wheels at 1.25m (4ft) behind the marked point.
4) Insert the deflection beam between the twin rear wheels until its point rests on the lllarked point of the road. If required, insert a second beam in a siailar way between the other pair of twin rear wheels.
5) I t is helpful in positioning the lorry and aligning the beSlls parallel to the lorry axis i f a pointer is fixed to the lorry 1.25_ (4ft) in front of each pair of twin rear wheels.
6) Check the beam pivot arms for free movement, adjust the footscrews if necessary, and zero the dial gauges whilst tapping the beams gently with a SJl8ll spanner.
7) Record the dial gauge reading. (Either zero or BOme BJl811 positive or negative reading).
8) Drive the lorry slowly forward and whilst gently tapping the beu note the maximum dial gauge readings and the final readings when the lorry wheels have IIOved 3m (10ft) or more clear of the tips of the beams.
9) For each beam calculate the deflection of the road surface by adding the difference between the first and maximum dial gauge readings to the difference between the lllaximum and final gauge readings.
