Pavement Design Geotechnical & GeoEnvironmental Engineering CIVL4121 1 CIVL4121: Pavement Design: 1 1 GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING CIVL 4121 Structural Design of Pavements Martin Fahey School of Civil and Resource Engineering CIVL4121: Pavement Design: 2 2 1. AUSTROADS (2004). ‘Pavement Design: A Guide to the Structural Design of Road Pavements’. AUSTROADS Incorporated. (A version of this for on-line use has been placed in the ‘Notes’ page. It is a restricted licence, with up to 12 users at a time being permitted). See also: N:\CIVL4121 Geotechnical and Environmental Engineering\Pavement Design for the demonstration version of the CIRCLY program. References
GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING CIVL 4121
Structural Design of Pavements
Martin Fahey
School of Civil and Resource Engineering
CIVL4121: Pavement Design: 2
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1. AUSTROADS (2004). ‘Pavement Design: A Guide to the Structural Design of Road Pavements’. AUSTROADS Incorporated.(A version of this for on-line use has been placed in the ‘Notes’ page. It is a restricted licence, with up to 12 users at a time being permitted).
See also: N:\CIVL4121 Geotechnical and Environmental Engineering\Pavement Designfor the demonstration version of the CIRCLY program.
A pavement is usually a combination of several layers placed on top of an existing subgrade (usually soil) so that vehicle loads from traffic can be transmitted safely to the subgrade without failure or excessive damage (i.e, deformation, strain, cracking, rutting, etc.) that may affect the serviceability of the road during the design lifespan of the pavementIn general, there are two main types of pavement
Flexible Pavement: The top course is AsphaltRigid Pavement: The top course is Concrete
In WA, very little use is made of rigid pavements, so we will concentrate on Flexible PavementsFlexible pavements may consist of:
‘unbound’ granular layers only (with a thin asphalt or spray seal top layer providing water proofing and wearing resistance, but not contributing to the structural performance of the pavement)a combination of ‘unbound’ granular layers, and ‘bound’ granular layers, where the ‘bound’layer generally consists a thicker asphalt top course which contributes to the structural strength of the pavement, but may also include some cement-stabilised granular layers.
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Typical layers in a cross section of a conventional flexible
pavement
A combination of the above layers could be chosen for a specific pavement configuration
Flexible Pavement
Conventional Flexible Pavements are layered systems with better materials on top where the intensity of stress is high and inferior materials at the bottom where the intensity is low. Adherence to this design principle makes possible the use of local materials and usually results in a most economical design. This is particularly true in regions where high-quality materials are expensive, but local materials of inferior quality are readily available. (Huang, 2004).
Flexible Pavements with only Unbound Granular Materials
In WA, most ‘normal pavements’ do not have an asphalt layer as part of the structural system
there may still be a thin (<50 mm) coat of asphalt forming the sealing/wearing layerin many cases even this is omitted, and the sealing layer consists of a chipseal layer – a layer of bitumen into which is rolled stone chips (as in many country roads) – see example below
“Structure” comes from subgrade, sub-base and base coursesMajor highways and urban freeways will have a structural asphalt layer
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What is Asphalt?Asphalt (more correctly called ‘asphaltic concrete’) is similar to concrete – a mix of gravel or crushed stone and sand with added ‘binder’ –except that the ‘binder’ is bitumen (a petroleum product) rather than cementIt is delivered (and laid) hot (hence ‘hotmix’)Note that bitumen, from petroleum, is not the same as tar, which is obtained from coalOriginally ‘tarmacadam’ after its inventor John Louden McAdam, from Scotland
The Seal Coat is a thin asphalt surface treatment used to waterproof the surface or to provide skid resistance where the aggregates in the surface course could be polished by traffic and become slippery. The Surface Course is the top course of an asphalt pavement, sometimes called the wearing course. It is usually constructed of dense graded BMA (Bio-Mastic Asphalt). It must be tough to resist distortion under traffic and provide a smooth and skid-resistant riding surface. It must be waterproof to protect the entire pavement and subgrade from the weakening effect of water. If the above requirements cannot be met, the use of a seal coat is recommended.The Asphalt Course, sometimes called the (binder course), is the asphalt layer below the surface course. There are two reasons that a binder course is used in addition to the surface course. First, the Hot Mixed Asphalt (HMA) is too thick to be compacted in one layer, so it must be placed in two layers. Second, the binder course generally consists of larger aggregates and less asphalt and does not require as high a quality as the surface course, so replacing a part of the surface course by the binder course results in a more economical design. If the binder course is more than 75 mm, it is generally placed in two layers.
A Tack Coat is a very light application of asphalt, usually asphalt emulsion diluted with water, used to ensure a bond between the surface being paved and the overlying course. It is important that each layer in an asphalt pavement be bonded to the layer below. Tack coats are also used to bond the asphalt layer to a PCC base or an old asphalt pavement. The three essential requirements of a tack coat are that it must be very thin, it must uniformly cover the entire surface to be paved, and it must be allowed to break or cure before the HMA is laid.A Prime Coat is an application of low-viscosity cutback asphalt to an absorbent surface, such as an untreated granular base on which an asphalt layer will be placed. Its purpose is to bind the granular base to the asphalt layer. The difference between a tack coat and a prime coat is that a tack coat does not require the penetration of asphalt into the underlying layer, whereas a prime coat penetrates into the underlying layer, plugs the voids, and forms a watertight surface. Although the type and quantity of asphalt used are quite different, both are spray applications.
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Pavement Layers (contd.)
The Base Course and Sub-base Course: The base course is the layer of material immediately beneath the surface or binder course. It can be composed of crushed stone, crushed slag, or other untreated or stabilized materials. The sub-base course is the layer of material beneath the base course. The reason that two different granular materials are used is for economy. Instead of using the more expensive base course material for the entire layer, local and cheaper materials can be used as a sub-base course on top of the subgrade. If the base course is open graded, the sub-base course with more fines can serve as a filter between the subgrade and the base course. Sometimes the base course could be cemented.The Subgrade is the natural in situ material. The top 150 mm of subgrade should be scarified and compacted to the desirable density near the optimum moisture content. This compacted subgrade may be the in-situ soil or a layer of selected material.
The main element of a rigid pavement is a reinforced concrete slabA thin layer of asphalt is often placed on top of the concrete courseRigid pavements are rare in WA, but are used elsewhere in Australia (NSW)
There are numerous differences between flexible and rigid pavement in terms of:load distributionmodes of distress designconstruction overall performance and problems related to that.
Flexible pavements are modelled using layered theoryRigid pavements are modelled using plate theory
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Corner Break in Rigid Pavement Fatigue cracking in Flexible PavementReference: http://faculty.eng.fiu.edu/~cnunoo/classes/p_m&R/Lecture_01B-S05.pdf
Flexible pavements are dominant in WA, and hence will be our focus from now on
Compaction of the underlying subgrade, and of each subsequent layer of the pavement, is a major part of pavement constructionCompaction in general has been dealt with in an earlier part of this unitFor pavement construction, compaction is generally specified in terms of Modified CompactionCompaction control (measurement of the degree of compaction) is vital for quality control of the pavement construction processThe nuclear density metre is the most common method of compaction control in WA
requires stringent calibration procedureshealth-and-safety procedures are also rigid (because of the radioactive source in the instrument)
The aim is usually to achieve the highest possible density (this gives the highest strength and stiffness – key parameters for pavement performance)‘Drying back’ (see ‘Compaction’ notes) is often used
compaction to maximum possible density at about OMC (to give low permeability)allow drying to reduce water content (while density remains high)
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Increase asphalt in mix, Use heavy roller
Reduced lubrication difficult compaction
Low
Decrease asphalt in mixUnstable and plastic under roller
High
Quantity
Use light roller, decrease temperature
Particles move easily during compaction
Low
Use heavy roller, Increase temperature
Particle movement restrictedHigh
ViscosityAsphalt
Increase asphalt in mixDries mix: difficult to compactAbsorptive
Use heavy rollerHigh interparticle frictionRound surface
Factors Affecting Field Compaction (Asphalt) [cont.]
Roll before mix cools; increase mix temperature
Lose heat: less time to compactThin layer
Roll normallyHold heat: more time to compactThick layer
Course thickness
Increase mixing temperatureDifficult to compact: mix too stiffLow
Decrease mixing temperatureDifficult to compact: mix lacks cohesion
High
Mix Temperature
Increase filler in mixLow cohesion: mix may come apartToo little filler
Reduce filler in mix; use heave rollerStiffens mix: difficult to compactToo much filler
Reduce sand in mix; use light rollerToo workable: difficult to compactOversanded
Reduce coarse aggregate; use heavy roller
Harsh mix: difficult to compactExcess coarse aggregate
MixCorrectionEffectItem
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Challenges of Pavement DesignA pavement is a complex system, involving interaction between different layers A variety of materials are involved in a pavement, including unbound (granular), bound (cemented), asphalt, and concrete in addition to the in situsubgradeTraffic loads may vary significantly in terms of vehicles models, their loads and number and distribution of axlesProperties of pavement materials are rate-dependent and hence vary with the impulsive nature of traffic loadingA drastic assumption may need to be made (depending on the chosen model) in relation to material behaviour and stiffness anisotropy The presence of water in the pavement may affect the performance adverselyKeeping water out of the pavement (including the subgrade) is a major design requirement
This is the function of drainage systems in a pavement, but will not be covered here due to time limitations
Material properties may be affected by climate temperature (hot or cold) and deteriorate under repeated loads (the bitumen in asphalt is a viscous material, so properties change with temperature)
Excluding water from the pavement (all layers of the pavement, including the subgrade) is a primary requirement of good pavement designCareful detailing of drainage, hard shoulders, etc is paramount to ensuring the design life of a pavement system
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Main Elements involved in Pavement Design
The anticipated traffic loading over the design life of the pavement – this is called The Design TrafficThe properties of the in situsoil (the subgrade)The properties of the various materials used in constructing the layers of the pavement on top of the subgrade
the basic material propertiesthe properties as placed (compacted)
The Austroads “Cookbook” Approach to Pavement Design
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Traffic Considerations: Summary
For normal highways, the only traffic that has an impact on design is truck traffic – light vehicles (cars, car trailers, caravans, boats, etc) have no impact on the design
Reason: The magnitude of the damage caused by an axle load is proportional to the fourth power of the load: damage ∝ (L)4, where L is the axle load(For some types of damage, the powers are even higher – 5, 7, 12 – see later)
The wheels on a heavy vehicle are arranged on axles (which might have a single tyre at each end or dual tyres)Axles are arranged in groups (1, 2, 3 or 4 per group), as shown in the next slide; these are called Heavy Vehicle Axle Groups (HVAG)Loading over the design life of a road consists of a (very large) number of repetitions of each of these HVAGsPhilosophy in the Austroads Code for design of Flexible Pavements is to convert the number of repetitions of each HVAG into the number of “Standard Axle”repetitions that cause the equivalent amount of damage as each of these HVAGs
A “Standard Axle” is a “single axle, dual tyre” group (SADT) carrying a load of 80 kN, with a width between the dual tyres of 1800 mm. The Standard Axle is a single axle to which all other axles should be converted (for an equivalent effect). In calculating the design traffic for flexible pavements, the damage due to various axle groups under standard loads has to be converted into the number of Standard Axle Repetitions (SAR) that would cause the same damage ‘Equivalent Standard Axles’ (ESA) is thus defined as the number of repetitions of this Standard Axle that would cause the same damage as a given vehicle or axle group (thus it is a unit of loading, whereas a Standard Axle is a physical entity).
13.8221QADTQuad axle, dual tyres15.1181TRDTTriaxle, dual tyres16.3130TADTTandem axle, dual tyres22.590TASTTandem axle, single tyres20.080SADTSingle axle, dual tyres26.553SASTSingle axle, single tyres
Nominal load per tyre (kN)
SL: Standard Load (kN)DesignationAxle Configuration These Standard Loads are the
loads on each axle groups that cause the same damage as a standard axle (using the criterion that they cause the same deflection)Note the differences in equivalent nominal tyre load (due to the effect of overlapping loads)
For pavements consisting of unbound layers, topped with a thin sealing bitumen layerDesign traffic – calculation of Equivalent Standard Axles (ESA)Method based on the California Bearing Ratio (CBR) test to determine the elastic properties of the various components of the system
Direct measurement of CBRIndirect methods
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Design Traffic for Flexible PavementsWhen a load from a heavy vehicle is applied repetitively to the pavement it causes certain damage (cumulative strains in various layers of the pavement)For each repetition of this load, we can replace it with the standard axle load, but with a different number of repetitions, to induce the same damage. This number of repetitions is called SAR (Standard Axle Repetitions: non-dimensional)Important definition: When the damage is taken as the overall damage that can occur in a granular pavement topped by a thin layer of asphalt, then the SAR is called the Equivalent Standard Axle (ESA). This is the quantity we will be using in the simple empirical design approach (following section).For a particular HVAG, the design ESA (the DESA) is given as:
DESA = (ESA/HVAG)×NDT where NDT is the cumulative number of HVAGs (see later)Thus, calculation of DESA requires the average ESA per HVAG to be determined:
4
ji, all i
ijiji SL
LPP
HVAGESA ∑ ⎟⎟
⎠
⎞⎜⎜⎝
⎛××=
Pi = the proportion of all Axle Groups that are of type iPij = the proportion of all Axle Groups of type i that have loads of
magnitude jLij = load magnitude j on Axle Group iSLi = Standard Load for Axle Group of type i (see table previously)
Note 4th power: 10% overload results in 46% increase in damage: (1.1)^4 = 1.46
Design of Flexible Pavement with no Structural Asphalt layer
This is the most basic type of design – very straightforward empirical methodVery widely used in WA for normal roads (i.e. roads where there is no structural asphalt layer)The factors in the design are:
the design traffic, in terms of Equivalent Standard Axles (ESA)the properties of the subgrade, expressed using the CBR (California Bearing Ratio) valuethe properties of the sub-base (if used) and the base course, again in terms of CBR
Much of flexible pavement design is based on the CBR test
Important Note (for later): ESA is used only in the empirical method, where the overall damage is related to the 4th power. In the Mechanistic Design method, fatigue damage to the various components is related to different powers (5, 7, 12) for each material type. In this case, the equivalent to ESA is SAR (standard axle repetitions).
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California Bearing Ratio (CBR)
The California Bearing Ratio (CBR) test is a simple strength test that compares the bearing capacity of a material with that of a well-graded crushed stone (thus, a high quality crushed stone material should have a CBR ~ 100%).It is primarily intended for, but not limited to, evaluating the strength of cohesive materials having maximum particle sizes less than 19 mm .It was developed by the California Division of Highways around 1930 and was subsequently adopted by numerous states, counties, U.S. federal agencies and internationally (including UK, Canada, Australia, NZ, etc).The basic CBR test involves applying load to a small penetration piston at a rate of 1.3 mm per minute and recording the total load at penetrations ranging from 0.5 mm up to 7.5 mm
rock crushed quality-high for npenetratio mm 0.25 at Loadnpenetratio mm 0.25 at Load(%)CBR =
Though CBR is apparently a ratio of forces, in reality it’s a stiffness ratio, since it gives the ratio of forces required for a given penetration.
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Field & Laboratory CBR Tests
CBR tests are carried out on compacted samples in the laboratory, or in the fieldSamples for laboratory CBR tests are prepared by compacting the soil to density and water content equivalent to those of the various layers in the pavement – in situ conditions for the subgrade, and design densities for the basecoure and sub-base course (with allowance for possible water uptake ‘in service’ – see ‘soaking’on next slide).
In situ CBR test: rarely carried out (too slow, expensive)
Laboratory CBR test setup. Note steel surcharge on the sample surface
To allow for possible softening due to exposure to water, compacted samples may be ‘soaked’ for various periods prior to CBR test.Soaking is carried out under a surcharge, and swelling is measured.
Dynamic Cone Penetration Test (DCP) used to find in situ CBR. Similar to Perth Sand Penetrometer, except for conical tip
Calibration should be checked for new projects against actual CBR tests
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Clegg Hammer for CBRClegg Hammer, invented by Dr Baden Clegg at UWA in the 1970s, consists of a steel hammer (like a Modified Compaction hammer), equipped with an accelerometer.When dropped onto a compacted surface, from a predetermined height, it provides a direct readout of acceleration (deceleration), which is directly related to the ground stiffnessThis has been correlated with CBR : e.g. in WA, MRWA use the following correlation for cohesive soils:CBR = 0.06 x CIV2 + 0.52xCIV + 1where CIV = Clegg Impact Valueand CBR = California Bearing Ratio (%)
CBR and Clegg Impact Values (CIV) for some compacted soils
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Design Procedure
This is the ‘empirical design’ procedureCan be used only for flexible pavements, with unbound granular material and thin bituminous surfacing (spray chipseal, or thin asphalt seal)Can also use more sophisticated design (see ‘mechanistic design’ later)This simple procedure very often used, at least in ‘first pass’ design
Design of Flexible Pavement with no (structural) Asphalt Layer
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Design ExampleA granular pavement with a sprayed seal surface is to be constructed to carry 2 x 106 ESA. The subgrade (the in situ material) has a CBR = 3%. Materials available for the construction are shown in the table.Design chart shows the required total thickness over the subgrade with CBR = 3% is 560 mmChart shows at least 145 mm of basecourse material required, leaving 410 mm of sub-base to chooseIn fact, could use a full 560 mm of basecourse material (but this would be very expensive!)Instead, use as much as possible of the cheapest available sub-base material (the pit rubble). Since it has a CBR = 10%, we can use it to fill up to the CBR-10 line. The next cheapest material has CBR = 25%. If we use this next, we can only use it to fill up to the CBR-25 line. This is just short of the CBR-30 line, so we can fill in this gap either with the quarry rubble (CBR = 40%) – not practical to have thin layer (15 mm) - or simply use a slightly greater thickness of the basecourse material (i.e. about 160 mm ).Alternative (as shown) is to use the quarry rubble (CBR = 40%) to fill in between the pit rubble and the basecourse, allowing the minimum thickness of basecourse to be used
Design of Flexible Pavement with no (structural) Asphalt Layer
Total thickness: 560 mm
145 mm
Subgrade
Pit rubblePit rubble (CBR 10): 560-280 = 280 mm
Quarry rubble (CBR 40): 135 mm
280 mm
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Mechanistic Design MethodUsed with layered flexible pavement systems, including ‘bound’ layers, asphalt layers, and ‘unbound’ layersBased on a rigorous evaluation of elastic strains at critical locations under a single static load from a Standard AxleRequires values of elastic modulus for each layer
Can be directly measured, but generally obtained empirically
‘Damage’ to each layer evaluated in terms of the number of repetitions of the Standard Axle load (number of standard axle repetitions’, NSAR) to cause fatigue failure (cracking)Check if the actual (design) number of repetitions is less than NSAR - if so, OKLooks rigorous, but:
Modulus values often empirically derivedCalculating NSAR for each material type is very empirical:where K is related empirically to material stiffnessε is the strain calculated for a static load (above)b is a “damage exponent”, which varies from 5 to 12 !
The purely empirical approach is very useful for routine design, where there are no bound layers (particularly no structural asphalt layer)A more fundamental approach is to consider the properties of the layers within the context of elastic theory, taking account of the strains induced by the wheel loading, and the fatigue characteristics of the various layers. This approach is called MECHANISTIC DESIGNThough the approach is founded in fundamental solid mechanics, the process specified in the Austroads Guide for determining the input parameters is generally quite empirical
thus the Mechanistic Design approach is a mixture of empiricism and rigourMechanistic design in the Austroads Code involves both testing procedures and empirical relationships for determining input parameters The design procedure uses the CIRCLY program (Wardle, 1977), from MinCad, to do the actual calculationsCIRCLY has evolved to take account of the design requirements within the Austroads Code
Tensile and compressive strains in flexible pavement
The basis of flexible pavement design is that the actual traffic volume over the design life of the pavement should not lead to any of the strains below exceeding given values.
Modes of Distress in Pavement Layers: Design Criteria
Strains of interest in the design approach are:Tensile strain at bottom of the asphalt layerTensile strain at bottom of any bound (cemented) layerCompressive strain at the top of the unbound (granular) base (?)*Compressive strain at the top of subgrade
* Not considered in Austroads code, or in CIRCLY
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Critical Strain Locations in the Mechanistic Design Method
Failure of a flexible layered pavement system is considered to be due to strains at critical locations reaching a design limit. These locations are as shown:
-165 0 165 1635 1965 XCo-ordinates (mm) assumed in CIRCLY
Elastic Parameters for Mechanistic DesignMechanistic Design method is based on elastic layer theory the pavement materials are characterised by the elastic parameters Young’s Modulus E and Poisson’s Ratio ν.Values required for: Unbound granular material; Modified granular material (still treated as unbound); Bound (cemented material); AsphaltFor unbound granular materials, elastic parameters are recognised to be anisotropic. In Mechanistic Design procedure, vertical stiffness is taken to be twice the horizontal stiffness for such materials – i.e. Ev = 2 EhModulus of unbound granular materials depends on confining stress
In CIRCLY, this is allowed for by dividing granular layers into a number (up to 5) sublayers, with each assigned different stiffness values
Pavement materials are subjected to repeated loads, which tend to ‘soften’ the response (i.e. reduce the stiffness) – the relevant ‘elastic’ modulus is called the ‘resilient modulus’, obtained from cyclic loading test:
the portion of the vertical strain that is recovered when the stress is removed
The resilient modulus for the top granular layer is determined either:by direct measurement, using a special cyclic triaxial test, where the sample is prepared in a representative state, and subjected to repeated loading at the appropriate stress level Correlation with CBR values (e.g. Ev (MPa) = 10 CBR(%)by using ‘presumptive’ values, as shown in the following tables (from Austroads, 2004)
E values for lower sublayers are assigned according to the CIRCLY procedures (i.e. taking account of stress level, and stiffness of overlying and underlying layers) – see later
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Back-analysis of Pavement Stiffnesses: Falling Weight Deflectometer
The magnitude and shape of pavement deflection is a function of traffic (type and volume), pavement structural section, temperature affecting the pavement structure and moisture affecting the pavement structure. Thus, many characteristics of a HMA pavement can be determined by measuring its deflection in response to load. Surface deflection is measured as a pavement surface's vertical deflected distance as a result of an applied (either static or dynamic) load. The more advanced measurement devices record this vertical deflection in multiple locations, which provides a more complete characterization of pavement deflection. The area of pavement deflection under and near the load application is collectively known as the "deflection basin".The most common type of measurement equipment is the falling weight deflectometer (FWD). The FWD can either be mounted in a vehicle or on a trailer and is equipped with a weight and several velocity transducer sensors. To perform a test, the vehicle is stopped and the loading plate (weight) is positioned over the desired location. The sensors are then lowered to the pavement surface, the weight is dropped, and the surrounding pavement vertical deflection is recorded.
Back-analysis of Pavement Stiffnesses: Benkelman BeamThe Benkelman Beam, developed at the Western Association of State Highway Organizations (WASHO) Road Test in 1952, is a simple device that operates on the lever arm principle. The Benkelman Beam is used with a loaded truck - typically 80 kN on a single axle with dual tires inflated to 480 to 550 kPa. Measurement is made by placing the tip of the beam between the dual tires and measuring the pavement surface rebound as the truck is moved away. The Benkelman Beam is low cost but is also slow, labor intensive and does not provide a deflection basin.
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Presumptive Values of Elastic Values for Unbound Granular Materials
Similar tables in the Code for other types of materials
separated from asphalt layer by uncemented material, to avoid ‘reflection cracking’ (cracks in the cemented layer ‘reflecting’ through to the asphalt layer
In Mechanistic Design, parameters required for cemented materials are:elastic modulus of the cemented materiala fatigue cracking relationship, to relate the applied tensile strain to the number of load repetitions to cause crackingthe ‘post cracking’ modulus is also required – cracking of the cemented layer does not mean the pavement has failed (hence the comment about preventing cracks from being ‘reflected’ to the asphalt layer)
asphalt
granular material
cemented material
subgrade
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Properties of Bound (Cemented) MaterialsDetermination of Modulus
One of the following methods is used:1. Direct measurement using a flexural beam test (Why Flexural?)2. Modulus correlations with UCS*
Eflex = k × UCS (in MPa)where UCS = Unconfined Compressive Strength in MPa (after 28 days)k = 1000 - 12503. Assigning presumptive values
The approach used in Austroads for determining the fatigue life (the number of load repetitions to failure) for each material (layer) is as follows:Determine the strain (ε) in the critical locations (as illustrated previously) under the (static) application of the standard axleThe fatigue life of the various materials is then represented by an equation of the form:
NSAR = allowable number of repetitions of the load (i.e. allowable SAR)ε = strain from application of the standard axle (horizontal tensile strain in
bound layers; vertical compressive strain in unbound layersK = the “fatigue constant” - material property (usually related to modulus) b = the ‘damage exponent’ of the material
7Rutting and shape loss (subgrade)12Fatigue of cemented materials5Fatigue of asphalt
Damage exponent, bDamage Type
b
SARKN ⎥⎦⎤
⎢⎣⎡ε
=Note: Austroads 2004 calls this N; I have added the subscript SAR to avoid confusion
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Fatigue in Cemented Materials
The Austroads relationship for cemented materials is:12
804.0 191113000
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
με
+= ERFNSAR
NSAR = allowable number of SAR (repetitions of the standard axle loadμε = tensile strain at base of cemented layer produced by the static
load (in microstrain)E = cemented material modulus (MPa)RF = reliability factor for cemented materials fatigue
•After fatigue cracking: material is treated as granular material with the following properties:
Ev = 500 MPa; ν = 0.35; Ev = 2 Eh•The remaining fatigue life of the pavement is then calculated, with fatigue in the asphalt and in the subgrade being the governing criteria
According to Austroads, 2004:“The Project Reliability is the probability that the pavement, when constructed to the chosen design, will outlast its Design Traffic before major rehabilitation is required”
Hence it is a measure of how important the project is
Road class Project reliability %
Freeway 95-97.5Highway: lane AADT > 2000 90-97.5Highway: lane AADT ≤ 2000 85-95Main Road: lane AADT > 500 85-95
Other Roads: lane AADT ≤ 500 80-90
AADT: Average Annual Daily Traffic
TABLE 50
Fatigue in Cemented Materials
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One of the following methods is used:1. Lab testing (indirect tensile test) (e.g.,
Marshal test)2. From Bitumen properties and mix
volumetric properties using Shell nomographs (will be used here)
3. Using published data
Properties of Asphalt: Determination of modulus
Asphalt modulus varies with pavement temperature (because of the viscous nature of bitumen). The Shell Nomographrequires the operating temperature at the site. Austroads (Appendix 6.1) gives tables of Weighted Mean
Average Pavement Temperatures (WMAPT) for a wide range of locations in Australia and NZ. The table for WA is shown here.
Modulus values obtained from laboratory testing must also be corrected for temperature, and for traffic speed (impulse loading duration), and for air voids.
Asphalt Modulus: Shell NomographsTwo main steps are
involved:1. Determine modulus of
the bitumen binder2. Determine modulus of
the asphalt mixture
Note:
1. All parameters are explained on the chart
2. See example on the chart
Step 1
1
2
3
WMAPT (ºC)
Assume = 1/V, where V is heavy vehicle design
speed (in km/hr)
=E of bitumen binder
Perth: 58 – 29 = 29º:SB ~ 20 MPa
Kununarra: 58 – 42 = 16 º:SB ~ 2 MPa
The Penetration Test measures the distance a weighted (100g) needle sinks into bitumen in five seconds. T800pen is the temperature at which the needle will penetrate 80 mm.
Deriving Input Values A, PI and T800penfor Shell Nomograph
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Step 2
Step 2 (Shell Nomograph)
Determine modulus of the asphalt mixture
require the relative volumetric proportions of aggregate (Vg), bitumen binder (Vb), and air (AV)typical air voids 5-10%in the example, Vb = 11% (bitumen) and Vg = 82% (aggregate)using the value of 20 MPa for Ebitumen (i.e. SB) from Step 1, the result is a mix modulus of 3000 MPathis value is already corrected for vehicle speed and temperature (WMAPT)this modulus is referred to as Smix in the asphalt fatigue equation.
3000
E o
f bitu
men
bin
der
(fro
m S
tep
1)
Note: If we used the SB value of 2 MPa (Kununurra), modulus would be about 10
times less – probably means this bitumen is not suitable for the North of Australia
NSAR = allowable number of SAR (repetitions of the standard axle load)με = tensile strain at base of asphalt produced by a single application of the load (in
micro-strain)VB = percentage by volume of bitumen in the asphalt mix (%)RF = Reliability factor for asphalt fatigue (different from cemented materials) –
Table 6.13Smix = Asphalt modulus (MPa) – e.g. from Shell Nomograph
Permanent Deformation (‘Rutting’) of Subgrade79300⎥⎦⎤
⎢⎣⎡με
=SARNexponent = 7 (c.f. 12 for cemented material;
5 for asphalt)
NSAR = allowable number of SAR (repetitions of the standard axle load)με = compressive strain produced by a single repetition of the load (in
micro-strain)(Note: No ‘reliability factor’ RF for subgrade fatigue)
This gives the allowable number of repetitions of the standard axle load to produce permanent rutting in the subgrade (which reflects upwards as permanent rutting of the pavement)
Note: In Austroads 2004, the subgrade is the only non-cemented granular material to be considered to have a finite fatigue life. Thus, while other granular layers in the pavement are part of the CIRCLY analysis, the strains in such layers are not considered, and not even reported by CIRCLY. For any cemented granular layer, once the fatigue life has been calculated, the post-fatigue calculation assumes this is a granular layer, which is treated like any other granular layer (but is not sublayered)
Fatigue Relationships - GeneralForm of fatigue relationship for asphalt (left) and cemented granular material (centre) shows that both stiffness (Smix for asphalt, and E for cemented material) and strain (με) are below the line. If these stiffnesses were not raised to a power, we would have stiffness × strain = stress. So, in a crude sense, the allowable number of repetitions of SAR for each case is inversely proportional to the applied stress – as would be expected.
( )( )
5
36.008.1856.06981
⎥⎦
⎤⎢⎣
⎡
με+×
=mix
BSAR S
VRFN
:SARallowable Asphalt,12
804.0 191113000
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
με
+= ERFNSAR
:SARallowableCemented,
79300⎥⎦
⎤⎢⎣
⎡με
=SARN
:SAR allowable Subgrade,
These relationships give the allowable number of SAR (standard axle repetitions) for each material. This is not the same as ESA. Presumptive relationships between ESA and SAR are presented later (Table 7.8, Slide 67).
CIVL4121: Pavement Design: 66
66
Austroads Approach: ESA & SAR
Calculate N (i.e. allowable NSAR) for each material, as in previous slide. Calculate the design traffic in terms of ESA (i.e. DESA), and converts this into design SAR (DSAR), which is different for each material type (see Slide 67): Asphalt: DSARa = 1.1 × DESACemented: DSARc = 12 × DESA (it is only coincidence that this is same as damage exponent)
Subgrade: DSARs = 1.6 × DESAThus, for a given DESA – say 107 – the DSAR for the three materials would be different:Asphalt: DSARa = 1.1 × 107
Cemented: DSARc = 12 × 107 = 1.2 × 108
Subgrade: DSARs = 1.6 × 107
The ‘damage’ is expressed as the design SAR (DSAR) compared to the allowable SAR (NSAR). CIRCLY uses the term ‘cumulative damage factor’ (CDF) to express this. If CDF for a layer is less than 1, this layer will not fail for the design traffic.For each material, Cumulative Damage Factor (CDF):
Step 1: Calculate the number of heavy vehicles for the most highly trafficked lane on the road
The equation to derive the Design Traffic (NDT) in cumulative heavy vehicle axle groups (HVAG)traversing the design lane during the specified period (NDT = Number of HVAG)
NDT = 365 × (AADT × DF) × %HV/100 × NHVAG × LDF × CGFwhereAADT = Annual Average Daily Traffic in vehicles (all vehicles, light and heavy) per day in the first year;%HV = average percentage of all traffic comprising Heavy VehiclesNHVAG = average number of Axle Groups per Heavy Vehicle; DF = Direction Factor is the proportion of the two-way AADT travelling in the direction of the design
lane;LDF = Lane distribution factor (the proportion of traffic in that direction travelling in the design lane) CGF = Cumulative Growth Factor
NDT is the design number of HVAG for the design life of the road.Empirical method of Flexible Pavement design requires extra step – derive DESA from NDT
Calculation of Design Traffic for Flexible Pavement Design
(On many highways, heavy truck traffic mostly uses the left-hand land, so this lane dictates the pavement design)
Calculation of Design Traffic for Flexible Pavement DesignDetermination of AADT: This will be determined from historical data in the design area, or from vehicle surveysDetermination of %HV: This will be determined from historical data in the design area, or from vehicle surveysDetermination of NHVAG: This will be determined from historical data in the design area, or from the following Presumptive values
Determination of DF: DF = 0.5 for a two-way road and 1.0 for a one way roadDetermination of LDF:
CIVL4121: Pavement Design: 70
70
Determination of CGF (Cumulative Growth Factor):
P = design period = 20-40 years for flexible pavement (unless otherwise reasoned)R = Annual growth (%)
(1 0.01 ) 1 for R >00.01
= P for R = 0
PRCGFR
+ −=
Calculation of Design Traffic for Flexible Pavement Design
In the empirical method of flexible pavement design, we used the ESA (equivalent standard axle) concept – an ESA being the number of load repetitions of a Standard Axle to cause the same damage as the actual HVAG. This conversion is based on the assumption that damage in the overall sense to the pavement is governed by the 4th powerlaw:
4
ji, all i
ijiji SL
LPP
HVAGESA ∑ ⎟⎟
⎠
⎞⎜⎜⎝
⎛××=
Pi = the proportion of all Axle Groups that are of type iPij = the proportion of all Axle Groups of type i that have loads of
magnitude jLij = load magnitude j on Axle Group iSLi = Standard Load for Axle Group of type i (see table previously)
This information comes from the Traffic Load Distribution (TLD), which is obtained either from weight measurements, or from presumptive values (see presumptive values for TLDs for urban and rural roads on next slide).In Mechanistic Design, we consider the damage to the individual components, and we have seen that the ‘damage exponent’ is different for each damage mode (tension in asphalt layer, tension in cemented layers, compression in the subgrade)The equivalent quantity for mechanistic design is SAR (Standard Axle Repetitions) and DSAR, which differs for each component: DSARa (asphalt); DSARs (subgrade rutting); DSARc (cemented layers). These SAR are related to ESA using different factors for each damage type, as outlined previously (see also next slide)
For asphalt: SARa = ESA × (SARa/ESA) = 4.5 × 107 × 1.1 = 4.95 × 107
For subgrade: SARs = ESA × (SARs/ESA) = 4.5 × 107 × 1.6 = 7.2 × 107
For cemented layers: SARc = ESA × (SARc/ESA) = 4.5 × 107 × 12 = 5.4 × 108
Because of the different fatigue damage exponents (5, 7, 12) for each material type, for a given total design traffic (given number of HVAG), the equivalent number of repetitions of the standard axle (SAR) is different for each material type. Thus, for 5 × 107 HVAG, have to design cemented layer to take 5.4 × 108 standard axle repetitions, but have to design asphalt to take only 4.95 × 107 standard axle repetitions.
Design ApproachCalculate the design traffic in terms of the number of design equivalent standard axles, DESACalculate the design SAR (i.e. DSAR) for each material from DESA, using appropriate factorChoose a pavement structure (number of layers and layer thicknesses)Assign stiffness values to each layer (or sublayer, as shown later)Using layered elastic theory (i.e. the CIRCLY program), calculate the strains at the critical points (tensile strains in bound layers, and vertical compressive strain in the subgrade) under a single static application of the Standard AxleUse these strains to calculate the allowable number of SAR for each material, using the generic formula:
Compare allowable SAR (NSAR) to the design SAR (DSAR)If all design DSAR ≤ allowable NSAR, pavement is OK. If not, change one or more layers until this condition is met. In CIRCLY, this is expressed as the Cumulative Damage Factor (CDF) for each layer:
Optimisation involves choosing layer thickness such that the allowable ESA for each layer is as close as possible to the design ESA.
b
SARKN ⎥⎦⎤
⎢⎣⎡ε
== SAR of number Allowable
SARNDSARCDF =
CIVL4121: Pavement Design: 76
76
Design Approach (Contd.): Pre- and Post-CrackingBecause of the severe “damage exponent” in the fatigue relationship for cemented layers (12), a cemented layer often fails at much lower SAR than the asphalt and subgradelayers. However, this is not the end of the pavement, since the cemented layer, post-cracking, is considered to continue to act as an unbound granular layer, with Ev = 500 MPa, Eh = 0.5Ev and ν = 0.35. (This is not sublayered).The approach is:
use CIRCLY to find the strains for the pavement containing the original cemented layer, and, from this, find the NSAR for all layers, including the cemented layer (this is the ‘1st layout’)repeat the CIRCLY calculation, but now treating the cemented layer as cracked (i.e. unbound), with the above elastic parameters, and again find NSAR for each material in this pavement (this is the ‘2nd layout’)convert all NSAR to NESA using the appropriate multiplier for both of these casescombine the 1st layout and 2nd layout results, to calculate the total fatigue life of the asphalt and subgradelayers, as follows (equation 8.5 in Austroads 2004):
ndSstS
CCSndA
stA
CCA N
NNNNN
NNNN 2
12
11 and 1 ×⎟⎟
⎠
⎞⎜⎜⎝
⎛−+=×⎟⎟
⎠
⎞⎜⎜⎝
⎛−+=
NA and NS are the total allowable ESA for asphalt and subgrade, respectively. NC is the allowable ESA for cemented layer (1st layout), N1stA and N2ndA are the numbers of ESA for the asphalt using the 1st and 2nd
layouts, respectively. Similarly, N1stS and N2ndS are the numbers of ESA for the subgrade using the 1st and 2nd layouts, respectively.
(See Austroads 2004, Appendix 8, section A8.3.2 for example of application of this procedure)
Actual Area (Ac) = Tyre load/ Tyre Pressure = P / q
Modelling Wheel Loads for Determination of Stresses and Strains
The Standard Axle modelled in CIRCLY as 4 circular uniformly loaded patches, with 750 kPa vertical pressure, and radius 92.1 mm, which equates to 80 kN equally distributed to the four circular patches.
CIVL4121: Pavement Design: 78
78
Assumptions (Boussinesque solution)1. Linear elastic, isotropic, homogeneous half space2. Circular loaded area
Vertical (normal) stressNotations
Stresses and Strains in a Flexible Pavement
Not directly applicable to anisotropic layered materials, but diagram gives a representative picture of vertical stress (normalised by the applied surface pressure q) reducing below the centreline of the load (r/a = 0), and also the variation at different radii away from the centre of the loaded area.
Design of New Flexible Pavements: Critical Strains
1. Tensile strain at bottom of asphalt2. Tensile strain at bottom of cemented material3. Compressive strain at top of subgrade
Unit stress (750 kPa)
Granular material
Cemented material
Asphalt
Critical locations
1800 mm330 mm 330 mm
33
2
1
Standard Axle
CIVL4121: Pavement Design: 80
80
Sublayering of Granular Material for Damage CalculationThe modulus of granular materials depends not only on the intrinsic characteristics of these materials, but also on the stress level at which they operate and the stiffness of the underlying layers. Because of the varying stress levels within an individual granular (i.e. uncemented) layer, these layers must be divided into 5 sublayers.
For granular materials placed directly on to a stiff cemented sub-base, no sublayering is required. This allows full advantage to be derived from good-quality granular basecoursematerials
In case of sublayering the following procedures should be followed:Divide the total thickness of the selected granular layer into 5 equal-thickness sublayersEv of the top sublayer of the granular layer is the lower of E = 10 times the design CBR of the selected granular material and E determined using the following Equation
The ratio of moduli of adjacent (i.e. underlying) sublayers is determined using the following equation
15
v top granular sublayer
subgrade
ER
E⎡ ⎤
= ⎢ ⎥⎣ ⎦
(total granular thickness/120)v v subgradeE top granular sublayer E x 2 =
The modulus of each sublayer may then be calculated from the modulus of the adjacent underlying sublayer, beginning with the in situ subgrade, the modulus of which is known.For all granular materials, the other stiffness parameters required for each sublayer may be calculated from the following relationships:EH = 0.5 EV
f = EV / (1 + νV).
Note: Sublayering for granular layers is not required if the granular layer rests directly on a cemented subbase layer.
This is one of the big advantages of placing a cemented layer lower in the profile, rather than directly below the asphalt layer. See example later.
CIVL4121: Pavement Design: 82
82
Example:A road is to be constructed in which there will be a granular layer of 475 mm thickness on top of the in situ subgrade. The subgrade has a CBR value of 5. Determine the elastic parameters to be used in design, assuming a high-standard crushed rock for the granular material
Solution:1. Subgrade:
CBR = 5, then Ev = 10 × CBR = 50 MPaEH = 0.5 EV = 25 MPa, νv = νH = 0.45f = Ev/(1+νv) = 34.5 MPa
2. Sub-base: Top granular sublayer (1st of 5 sublayers):Minimum of:a) EV top of base = Ev subgrade × 2 (total granular thickness/125)= (50) (2)(475/125) = 696 MPa
b) EV top of base = 500 MPa (see Table 6.3, Slide 50 in these notes using High Standard Crushed Rock)
Then, Ev minimum = 500 MPaLet us now subdivide the granular material into five layers and calculate the elastic parameters for each layer.
Sublayering of Granular Material for Damage Calculation
Ratio of modulus of an underlying layer: R = (500/50)(1/5) = 1.585
Depth above subgade (mm)
Thickness mm
Ev (MPa)
Remarks Ev = Ev (below) * R
Fifth (Final) sublayer of granular material 475 95 500
Fourth sublayer of granular material 380 95 316 = 199 * 1.585
Third layer of granular material 285 95 199 = 126* 1.585
Second sublayer of granular material 190 95 126 = 79 * 1.585
First sublayer of granular material 95 95 79 = 50 * 1.585
Subgrade 0 95 50
Sublayering of Granular Material for Damage Calculation15
v top granular sublayer
subgrade
ER
E⎡ ⎤
= ⎢ ⎥⎣ ⎦
Note: One of the big advantages of placing a cemented layer lower in the profile (below a granular layer), is that, in such cases, sublayering of the granular layer is not required.
Thus, in the example above, sublayering forces the reduction of the modulus of a good quality base material in the lower 4 sublayers. However, if this layer rested directly on a cemented layer, sublayering is not required, so that the full thickness of the granular material can be assigned the same value as the top sublayerin a sublayered system – 500 MPa in this case.
CIVL4121: Pavement Design: 84
84
79
126
199
315
500
500
50
100
150
200
250
300
350
400
450
5000 100 200 300 400 500 600
Vertical modulus (MPa)
Dep
th o
f top
of s
ubla
yer
(mm
) fro
m S
ubgr
ade
3
4
5
2
1
Subgrade
Sublayer
Sublayer
Sublayer
Sublayer
Sublayer
Sublayering of Granular Material for Damage Calculation
10. Determine design number of SAR for each relevant distress mode (i.e. DSARa, DSARc, DSARs for the asphalt, cemented layer, and subgrade, respectively)
9. Use the fatigue strain criterion for an asphalt layer to calculate the allowable NSAR for the asphalt
8. Use the fatigue strain criterion for a cemented layer to calculate the allowable NSAR for this layer
7. Use the subgrade fatigue criterion to calculate the allowable NSAR for the subgrade
6. Determine the elastic parameters for asphalt
5. Determine the elastic parameters for cemented materials, pre- and post-cracking
(The Granular layer should be divided into 5 sublayers and a special interpolation is made for the modulus of each of these sublayers between the subgrade and the top sublayer)
4. Determine the elastic parameters and thickness of the other granular sublayers (if relevant)
3. Determine the elastic parameters (as above) of the top sublayer of the granular layer (if relevant)
EV ; EH = 0.5 × EV; νV = νH; f = EV / (1 + νV)
2. Determine the following elastic parameters for the in situ subgrade and selected subgrade materials:
1. Select a trial pavement and a desired project reliability
13. Input the above values into CIRCLY and determine the maximum vertical compressive strain at the top of the subgrade and top of selected subgrade materials and the maximum horizontal tensile strainat the bottom of each cemented and/or asphalt layer.
If the post-fatigue cracking phase of cemented materials life is being considered, the calculation is repeated, using the post-cracked material parameters for the cemented layer (Ev = 500 MPa = 2Eh; ν= 0.35)
12. Determine critical locations in the pavement for the calculation of strains as follows:ً * bottom of each asphalt or cemented layer
* top of in situ subgrade and top of selected subgrade materials.* on vertical axes through the centre of an inner wheel load and through a point midway between the
two wheel loads at a centre-to-centre spacing of 330 mm.
11. Approximate the Standard Axle wheel loading as four uniformly loaded circular areas at centre-to-centre spacings of 330 mm, 1470 mm and 330 mm; a vertical load of 20 kN is applied to each circular area at a uniform vertical stress distribution of 750 kPa.
* Radius of each loaded area R = 2523p –0.5 (about 92.1 mm for highway traffic), where R = radius (mm) and p = vertical stress (kPa).
18. Compare alternative acceptable designs for economic assessment.
17. If the pavement is unacceptable or additional pavement configurations are required for comparison, select a new trial pavement, return to Step 1 and repeat Steps 1 to 16.
16. If, for all distress modes, the allowable number of SAR exceeds the design number of SAR (DSARa, DSARs, DSARc) the pavement is acceptable. If not, it is unacceptable.
(In CIRCLY, this is expressed as Cumulative Damage Factor, CDF, for each layer: if CDF ≤1, that layer is OK.
15. For each distress mode, compare allowable number of SAR (NSAR) with the design number of SAR (DSAR)
14. Determine using the criteria selected in Steps 7, 8 and 9 the allowable number of SAR (Standard Axle Repetitions) for each of the relevant distress modes.
If the post-cracking phase of cemented materials life is being considered, calculate the total allowable loading of the pre-cracking and post-cracking phases of life.
CIVL4121: Pavement Design: 88
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Ensure that you have downloaded this solved example from the websiteNote: We use CIRCLY in this example. The example is similar to that in Appendix 8.3, Section A8.3.2,
except that I have used a different thickness of cemented layer, so that the total pavement thickness (t) above the subgrade falls within the range of thicknesses that can be analysed by the demonstration version of CIRCLY: i.e.:
t = 280 mm
421 < t < 499 mm
t = 580 mm
The example involves a flexible pavement (499 mm thick), with:
Asphalt: 175 mm
Cemented granular material: 124 mm
Granular base: 200 mm
Subgrade
EXAMPLE: Design of New Flexible Pavements
We will show that, for this pavement, the post-cracked phase is the most important. Also, we can show that putting the cemented layer below the granular base gives a much greater life, for the reasons explained earlier.
Main Roads in WAWhat ratios of MRWA pavement construction are of the following types (I am assuming that practically 100% is flexible pavement):
(a) full pavement where the asphalt is part of the structural system ~ 0.05%
(b) full pavement where there is minimum asphalt cover (40 mm or less ?) ~5% almost all in the metro area, some in major urban centres such as Albany and other sites are steep grades and intersections subject to heavy vehicles turning movements
(c) full pavement where there is only a spray seal / chipseal, but no asphalt ~92%
(d) unsealed pavement ~3% of the 37,445 lane km
Is it the case that that (a) is only used for major urban freeways/highways, or is it also used for major rural highways?
Rural asphalt is typically only short lengths of 30mm or 40mm thickness at intersections - type (b) above. Type (a) would be very rare in regional areas. The attraction is not so much the structural properties or long life, it is the rapid construction time that reduces the traffic control costs and other fixed overheads typically incurred with granular construction. Processes such as mixing, surface finishing and trimming, drying back layers before placing the next layer or applying a prime seal are all take time and drying back layers is only effective in warm dry conditions. Rain events can prolong the time required.
Do you have a notional cost per m2 of these pavement types (I know this depends on many factors, including pavement thickness, so just some general figures would be fine - or even figures that were in terms of thickness)?
The average cost of metropolitan pavements is $1.6M / lane km and regional costs would be in the order of $300, 000 / lane km.
How commonly do you use a cemented layer somewhere in the pavement (that is, apart from an asphalt layer)? It it's used, is it always separated from the asphalt layer by an unbound granular layer, or is it directly below the asphalt layer (or seal)?
We do routinely stabilise pavement materials with cement and lime to improve the properties of marginal material (usually after failure but sometimes at initial construction) or basecourse that will be used in poor drainage conditions. Those pavements are still considered as unbound layers with UCS values < 1MPa and modulus < about 1000MPa. Rural pavement repairs typically comprise in situ stabilisation of 200mm of pavement with 2% LH cement.
We have very recently constructed a short length of bound pavement under 175mm of asphalt at Leach /Orrong intersection as a construction expedience to allow traffic to be shifted and structural works to proceed. It is very much the exception. 175mm is the minimum thickness of asphalt over a bound layer recommended by Austroads.
(Thanks to Ross Keeley, MRWA, for supplying this information in response to my questions)
CIVL4121: Pavement Design: 90
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Road Statistics for WA (National, MRWA, and Local Authority)
Note:
• Most roads (in terms of total length) are local authority roads
• Most main roads are sealed
• Most local roads (in terms of km) are unsealed
• The average age of main road pavements is 29 years, with the seals having average age of 11 years
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