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ft HIGHWAYS DEPARTMENT
GUIDANCE NOTES
GLOSSARY OF TERMS COMMONLY USED IN FLEXIBLE PAVEMENT CONSTRUCTION
Research & Development Division RDIGN/010A Mrch 1895
1.0 , SCOPE
HIGQWAYS DEPARTMENT . GUIDANCE NQTES NQ. RD/GN/Oli}A
GLQSSARY OF TERMS COMMQNLY VSED IN FLEXIBLE PAVEMENT CQNSTRUQJON,_
This glossary includes terms most commonly used by engineers for flexible pavement construction.
2.0 AIM The aim is to clarify and standardise terms which practitioners may find confusing. This glossary will be extended from time to time to cover other aspects of highway. engineering.
3.0 DESCRIPTION QF TERMS
3.1 Payemeot Structure
3.1.1 Wearing Course Baw Courw. Roaclbew end Sub-base
A typical flexible pavement structure oonsists of a number of pavement layers which distribute wheel loads to the underlying subgrade. The names for these layers are widely accepted in civil engineering practice. A typical section is shown in Figure 1.
Wearing course, base course, roadbase and sub-base all distribute wheel loads. These courses are considered to be structural layers.
3.1.2 FrlctJon Course
Friction course layer is recommended as the uppermost layer on roads where the vehicle approaching speed is likely to exceed 70 km/h. It is a Water permeable layer which reduces the risk of aquaplaning. It also provides a Vl
In literature, friction course is . . some.times referred to as porous asphalt, porous wearing course or open textured wearing course. However in Hong Kong, the material should be referred to only as friction course material.
3.2 Bituminous Materia]s
3.2.1 In the General Specification for Civil Engineering Works, 1992 Edition (GS), the term "bituminous material" is used. In some other technical reference, terms like: asphalt, asphaltic concrete and dense bituminous macadam are used. These terms denote plant mixed materials of similar constituents ie. binder, aggregates and filler.
3.2.2 Except for friction course material, bituminous materials specified in the GS .are all continuously graded.
3.2.3 Roadbase material is often referred to as DBM which stands for dense bituminous macadam. However, DBM is a collective term for continuously graded dense bituminous materials. In general terms, the bituminous materials (except friction course material) specified in the GS. are all DBM type materials, .DBM is ~ot a . unique name for roadbase materials and the use of DBM for roadbase material description should be avoided. ;
3.2.4 Hot rolled asphalt (BRA) is a significantly different mix from the above mate~.: It is a gap graded mix consisting of sand, filler and binder. The binder content is high and the mecbanical.properties of the material are influenced primarily by those of the binder-sand-filler::mortar. This material should not be confused. with the bituminous material specified in the GS.
3 .2.5 Stone mastic asphalt (SMA) material is a gap graded material which is widely used in Europe. The material consists of high stone content, high binder and filler content with added fibres to stop bitumen run-off and give additional binder stabilitY. Overseas experience shows that SMA is particularly suitable for roads in regions of hot climate and heavy axle loads. It provides high skid resistance due to its coarse textured nmning surface which also reduce~ spray problems.
3.3 Payement Material
.3.3.1 . Very often the terms ~ for pavement materials are confused with terms for a pavement layer,. For example, wearing course and wearing course material have
.. different meanings. : The former refers to the layer whereas the latter refers to the material itself. 1n general, the bituminous materials specified in the GS are named according to their major application such as the 20 mm nominal size wearing course material is normally used for wearing course (refers to layer) construction. However if considered relevant the Engineer may elect to use the same material for another layer of construction.
3.3.2 The 10 mm nominal size wearing course material specified in the GS, apart from being used for wearing course construction in cycle tracks, is. very ,often used for surface regulation purpose particularly in cold milling and .resurfacing works.
RD/GN/OlOA Glossary of Terms Commonly Used in Flexible Pavemeot ConstNctioo J>a&e Z of9
Therefore the 10 mm nominal size wearing course material is also called 10 mm nominal size regulating ccur:se material.
3.3.3 There are currently two types of base course material of different nominal size (ie. 37.5 mm and 28 mm) for use in pavement construction. In choosing the type of base course material for use in base course construction, the recommendations made in Appendix 9-A to the Guidance Notes to the GS with regard to pavement layer thickness should be observed. The relevant recommendations are extracted and listed in Table 1:
Base Course (3_7 .. 5 mm) 60 80
Table 1 - Recommended Nominal Layer Thickness of Base Course Materia)
According to field experience, there are difficulties in controlling compaction of the 28 mm nominal size base course material. The air void contents achieved are largely .of low values thus increasing the chance of rutting when the base course material is subjected to heavy channelised axle loading. IIi comparison, the 37.5 mm nominal size base course material does not have the same problem. Therefore the 37.5 mm nominal size base course material is recommended to be used in base course construction if practicabli:.
3.3 . 4 In preparing drawings, it would be more appropriate to specify the thickness and the material for each pavement layer. A typical road section with suggested description is shown in Fig. 2.
3.3.5 A general section of bituminous pavement (or new works construction is shown in Fig. 3 whilst the choice of material for the surfacing layer and the thickness of each layer are shown in Table 3.
3.4 Differentiation between Friction Coune Material. Wearing Course Materia). Textured Wearing Course Material. Stone Mmjc Asphalt Material and ED9'!I 'JYpe Anti-Skid Dress!ng
3.4.1 Table 3 shows that friction course material, wearing course material, textured weanng course material or stone mastic asphalt material could be chosen to be laid as the uppermost layer of the pavement. The different function of each application needs to be distinguished.
3.4.Z Friction course material and wearing course material are distinct materials. The former is non-structural and porous . whereas the latter .is structural and impermeable after proper compaction.
RD/GN!OlOA GloSSliJI of :renns Commooty Usc4 in l'texibte Pa.emeo~ CoDSinlCtiou Paa3 of9
3.4.3 Textured wearil?.g wu,rse material has been consid~ as. an al~Uve for special surface treatment when required in aceoJ;d~e with TPDM. (TPDM ref: Volume 2, Chapter 3 Cl 3.3.3.3 and Cl 3.3.3.4) to increase skid resistance. The material bas been used pn Uia1 for some years in Hong Kqng, but further evaluation is considered necessary as the performance observ~ ao.d the test results obtained so far 'are .WC.onslstent.
' .: ~ .. . . ' 3...4.4 !.Stone mastic asphalt (SMA) m.aterjal i~ . n,ow consid_ered as al~tive for special
surface treatment when required in accordance with TPDM (TPDM ref: Volume 2, Chapter 3 Cl 3.3.3.3 and Cl 3.3.3.4) to increase skid resistance. A few trials on ~~ ~pal bave been carried out recently in Hong. Kong ~ the results are
PfOJAiS.Ill8; ~::More trials are recoromeed on use o( this materlal to facilitate the develojunenC of a practicable specific~tion for ~ of . the rill\terial under local conditions.
.3.4.5 Textured wearing course material and SMA material are both struc1Ui:al and impermeable after proper compaction. These two materials are categorised as speCial bituminous materials and should only 'be laid by the specialist contractors included in the "List of ApP.fO~ed .~liers of Materials and Specialist Contractors for Public Works" uo.der ' ihe' caiegoiy of "Supply of Bituminous Pavement Materials and
C9nstmction,of,S~ia,l Bituminou~ Surfacing".
3.4.6 .. A CQmpar.ison between friction course .matet:ial, wearing course iliaiei:ial, textured . wearing C9~ material and $MA matepal by functions ~d properties is ~ised in Table 2>
Friction ' Course
. Wearing ~s.e
Non-structural
S~l
Structural , ' .
. ; .... . .
n .. :;:,l:one Mastic StrucluJ:ll). ,Asphalt , (SMA)
Permeable
lmpelJlleable
Impermeable
. . ....
GSrequ~: ' n~ less than 1. S rom ~ut 0.3. ~to 0.4 rom, but no Spicification requirement Draft speciftcation requireinen.t: .. : .. .
.average > :0.1mm . . & '
3.4.7 Very often, epoxy type anti-skid dressing is applied at locations recommended by Road Safety and Standards Division, TranSport Department. This maierial is not structural but serves to provide a very good microtexture and macrotexture surface, and could also be considered as an alternative for special surface treatment when required in accordance with TPDM (TPDM ref: Volume 2, Chapter 3 Cl3.3.3.3 and Cl 3.3.3 .4) to increase skid resistance. However, according to past observl!tions, the mate~al is not recommended to be applied on bituminous s~ing in view. of the high rate of failure at the interface caused by shearing of the surfaci_ng maier:ial.
3.5 Binder.
3.5.1 Bitumen
The type of binder specified in the GS is petroleum bitumen which is o\ltained l>Y refinery processes from bitumen rich crudes. Other types of bitUmen, not -covered iii the QS. ~' ;tlso (o].IJld as a natural dep~.it }>r .a co~ppnent of naturally occurring asphalt, in which it is. as.~ociated with min~ial matt~r.,. Bitumen found as a natural deposit is normally hilrd, brittle and expens~ve. It. is known as Native or Natural Bitumen.
3.5.2 AsPhalt
Asphalt is a naturally occurri.oi inixrure iJi. :which bitumen is associated with a substantial proportion of inert mlnef!!.l ,.matter. Lake Asphalt is a .highly viscous natural asphalt found in well defmed surfacti deposits, the most important of which is located in Trinidad and is known as Trinidad Lake Aspha!! (TLA). Other types of asphalt, like Rock Asphalt, are not relevant to this glossacy.
The at>Pve:-mel}t,ioned terms ar.e usually confused with the American tenn ;, Asphalt" whjch,denotes bituminous material types,
3.5.l Polymer Modified Bihnnen
For some applications, polymer modified bituJ11en is used as binder in the bituminous material. Polymers can be in the form of different chemical composition for different applications. Performance of bituminous material using polymer modified bitumen as binder can be improved in the following ways:
by improVing the low temperature rheological (flow) properties of the binder thus improving its resistance to low temperature cracking (fatigue);
by thickening the binder film coating the aggregates (as a higher binder content can be used) thus enhancing its durability against binder hardening due to oxidation;
by alleviating the susceptibility of the mix to stripping and defonnation thus prolonging its serviceable life.
RD/GNIOlOA Glossary of Terms Commoo!y Used i4 Fluil>le Pavement ~oiU!II(Ction PageS of 9
3.6 Polymer Modified Bituminous Material
3.6.1 Polymer Modified Friction Course Material
Friction course material mixed with polymer modified bitumen is known as polymer modified friction course material. In comparison with normal friction course material, a high binder content can be used with modified friction course material thus enhancing its durability against binder hardening due to oxidation. In Hong Kong, polymer modified friction course material is increasingly used as noise reducing highway surfacing material. Its application is covered by Guidance Note . No. RD/GN/011/A issued by the Research and Development Division of Highways Department.
3.6.2 Cushion Course Material
This material is a fme and dense.graded mix blended with a polymer modified binder. It is recommended to be used.at the interface between rigid pavement and bi~ous overlay to act as a cushion layer which absorbs the stress induced .in the overlay by the movement of the underlying rigid pavement particularly at joints. In Hong Kong, the cushion course material is of 3. 35 mm nominal size and the nominal thickness of application is 20 mm. The material is commonly used when noise reducing highway surfacing material is applied on existing rigid pavement or structures. For details of application, please refer to Guidance Note No~ RD/GN/0111 A issued by the Research and Development Division of Highways Department.
3.6.3 Polymer Modified Wearin& Course Material
Wearing course material mixed with a polymer modified binder is known as polymer modified wearing course material. As wearing course material is a dense material, the main reason for using polymer modified binder in the material is to modify the mix behaviour and achieve a better resistance to fatigue, stripping and deformation. In Hong Kong, polymer modified wearing course material is not commonly used in highways works. To date, it has only been used in the project of "Reconstruction and Improvements of Lion Rock Tunnel Road from Toll Plaza to Shatin Road".
RD/GN/OIOA Glossary of Terms Commonly Used in Flexible Pavement Construction Page 6 of9
ww""l"wo'fi'~W>"""'"IIr 'IGN/OliA
1992 Edition).
For old bridges which have not .!>cCD deSigned 10 finish with bituminous $1\ffaciog, please refer to tbe details shown in HyD Drg,. No. GN/011/0IA for laying of bituinioous overlay on existing concrete decks.
For layer thicknesses b, c, d and e, please refer to Guidance Note No. RD/GN/017 Guidance Notes 10 Pavement Design.
For new works wbere the base course is newly constructed, tlie iextured wearing COW$C material or the stone mastic asphalt material i:ao hi laid directly on top of tbe base course layer without existence of the 20 mm nominal size wearing coune layer. For cold milling and resurfacing works, tbe cold milled surface should first be regulated with 2025 mm thick regulating course material before application of tbe textured wearing course material or stone mastic aspbalt material.
Glouary of Terms CCHDJDODiy Used iD Flexible Pavtment Construcdoo I'll 7 of9
RDICN/Ol'A
Wearing Course
Base Course
Roadbase
Sub-base
Sub grade
Typical Pavement Structure (without friction course)
Friction Course Wearing Course
Base Course
Roadbase
Sub-base
Sub grade
Typical Pavement Structure (with friction course)
Figure 1
Glosary ol Tmnl Commocly Uted ID. Flea.ible Pavement CoasrructioD . ... ... , 9
ISO mn
~PROVED JOIN1 SUI. ANT
ClASGRIO II 500 WI01 OR EOU IVALHH
TRAFFIC OIRECTIOH
150m
lO mm THict POLYMER MODI FR ICTION COURSC MATERIAL
Fl(O
HION -20 mn THICl
