FLEXIBLE PAVEMENT DESIGN, CONSTRUCTION AND QUALITY CONTROL
Tam-Jones Atuboyedia PhD, FCIHT, FNSE
Good roads do not come cheap in terms of money, time and materials. The design elements for a road can be numerous but it takes a lot of ‘thinking through’ to achieve the ultimate goal to providing comfortable, aesthetics and safe riding surfaces. The design parameters in the current usage of our roads seem inadequate or outdated; work methodology is near absent and quality control at source is hardly monitored. If the design life of these roads should attain their minimum 20-year span, we need to revise existing laws and reinforce others such as road taxes on overloading, indiscriminate abuse of the pavement. Good roads depend on proper traffic classification, route selection, soil management, aggregate blending and quality asphalt concrete. Each of these legs, when improperly handled, will give defective result and thereby nullify the design. It is important to note that the world has moved on from 20-year life span to Perpetual roads and the Superpave designs.
The topic of this lecture as highlighted encompasses what could make for a good road, when controls for these three elements are properly followed. The choice on route selection, geometric design, pavement design, construction method and quality control are very important if the basis of design must be achieved.
- Highway Classification System
Usually, the first place to start is to decide on what type of road that is required – Earth roads, Single Carriageways, Dual Carriageways, Controlled Access Highway, Motorway or Trunk Roads. Highways can be classified by different classification schemes:The classification of highways into different operational systems, functional classes, or geometric types is necessary for communication among engineers, administrators, and the general public.
- Different classification schemes have been applied for different purposes in different rural and urban regions. Classification of highways by design types based on the major geometric features (e.g. freeways and conventional streets and highways) is the most helpful one for highway location and design procedures. Classification by route numbering (e.g. NGR, State, and LGA) is the most helpful for traffic operations. Administrative classification (e.g. National Highway System or Non-National Highway System) is used to denote the levels of government responsible for and the method of financing, highway facilities. Functional classification, the grouping of highways by the character of service they provide is consistent with the “Policy on Geometric Design of Highways and Streets”.
- Classification by design types based on the major geometric features (e.g. expressways, conventional streets and highways), which is most useful for highway location and design procedures.
- Administrative classification (e.g. Federal Trunk A, Trunk B, Trunk C, State Primary and Local Government Feeder Roads) used to denote the level of government responsibility for, and method of financing facilities.
- Functional classification, which is the grouping of highways by the character of service they provide, particularly their relative emphasis on travel mobility versus access to property (e.g. principal arterial, minor arterial, collector, Local Street, etc.)
Functional classification has been used as an important planning tool for comprehensive transportation planning schemes and is now accepted as the predominant method of classifying highway facilities and systems.
The classification of Roads in Nigeria and the Nigerian road network from the colonial days to the present day, have been majorly classified into three, namely Trunks A, B & C.
- Hierarchy of Roads
ROAD CLASSIFICATION SUMMARY BY SELECTED COUNTRIES
|S/NO.||UNITED STATES||UNITED KINGDOM||EUROPEAN UNION||NIGERIA|
|1.||Freeways||PRN: Motorways (A-Class)||Motorways||Trunk-A*|
|2.||Arterials (Expressways)||SRN: Motorways(A-Class)||Expressways||Trunk-B|
|3.||Collectors||Secondary Roads:||Other Roads||Trunk-C|
|4.||Distributors||– B Class|
– Classified un-numbered roads
– Unclassified roads
* See further classification of the Trunk roads into ‘F’ and ‘A’ roads
|2.5.1 United States and Canada|
At the top of the hierarchy are limited access roads Freeways or Expressways, including most toll roads. These roads provide largely uninterrupted travel, often using partial or full access control, and are designed for high speeds.
Arterials are major through roads that are expected to carry large volumes of traffic. Arterials are often divided into major and minor arterials, and rural and urban arterials.
Collectors (not to be confused with collector lanes, which reduce weaving on freeways), collect traffic from local roads, and distribute it to arterials. Traffic using a collector is usually going to or coming from somewhere nearby.
4) Local roads
The network is based on a grid, with even numbered routes running east to west, and odd numbered routes running north to south. Lower numbered routes are in the southwest while higher numbered routes are in the northeast. Major routes such as coast to coast or border to border routes end in a 0 or a 5.
2.5.2 United Kingdom
Motorways, similar to freeways, these high-speed roads are designated with an M prefix or (M) suffix. e.g. M1, A1 (M). The speed limit is generally 70 miles per hour (110 km/h) and there is a hard shoulder, an often slightly narrower lane next to lane 1, which is usually only to be used in cases of an emergency.
2) Primary A-road
Green on maps and signs. A main recommended route these can be single track as in North West Sutherland or more usually single carriageway or dual carriageway. The primary road network is fully connected, meaning you can reach any part from any other without leaving the network
3) B road
Regional in nature and used to connect areas of lesser importance. Usually shown as brown or yellow on maps and have the same white signs as non-Primary A-Class routes.
4) C road
C roads are used as local authority designations for routes within their area for administrative purposes. These routes are not shown on road maps, but have occasionally been known to appear on road signs.
1) Trunk-A Roads
These roads form the skeleton of the national road grid. They cut across regional boundaries in the country and even extend to the international borders of neighbouring West African countries. These categories of roads are under Federal Government’s ownership. They are designed, constructed, maintained and financed by the Federal government through the Federal Ministry of Works. The Federal Road Maintenance Agency (FERMA) is in charge of carrying out maintenance of this class of roads.
2) Trunk-B Roads
These roads are the second category of main roads in Nigeria. They link the major cities within States with the State capitals. These roads are designed, developed, financed and maintained by the State governments through their Ministries of Works, Transport or Infrastructure. The primary objectives of Trunk B roads are to enhance the socio-economic development of the various States in the country.
3) Trunk-C Roads
These roads are local feeder roads constructed and maintained by the Works Department of Local Government Authorities in Nigeria. This class of roads are primarily not concrete asphalted and are affected by seasonal weather changes. The roads link villages and communities in the remote parts of each local government region.
2.5.4 FEDERAL HIGHWAYS ACT
An Act to vest the powers of management, direction and control of Federal highways throughout Nigeria in the Minister of Works and Housing, in respect of planning (including research and designing of Federal highways), the construction and maintenance, the supervision of users of such highways and the regulation of traffic thereon.[1971 No.4.]
The Act has attempted to define the various roads as follows:
- Expressways – (using the prefix ‘E’) are what the Nigerian authorities consider the motorways in the country. From a European perspective, many of these roads lack the limited access nature which gives motorways their definition. When mapping expressways in Nigeria, pay attention to the details of the road (i.e. are junctions grade-separated? Are there gaps in the central reservation between the carriageways? ). This should provide enough information to map the roads as either highway=motorway or highway=trunk.
- Trunk roads – (using the prefix ‘A’) are what make up the bulk of the main network of roads in Nigeria. These roads should be represented as highway=trunk roads, or where replaced by parallel expressways, represented as highway=primary roads.
- Federal roads – (using the prefix ‘F’) F1xx roads run east-west whilst F2xx roads run north-south. These roads form the primary links between the main trunk routes across the country. Thus these roads should be mapped as highway=primary.
|E1||Expressway Lagos (Ojota Interchange) – Ibadan – Ojo (A1)|
|A1||Apapa Wharf – Iganmu – Western Avenue – Idi – Oro Roundabout across – Ikorodu Road – lkorodu – Shagamu – lbadan – Oyo – llorin – Jebba – Kontagora – Yelwa – Koko – Jega – Tambawal – Soko- – Illela Niger border|
|A1-1||Malu Road (Oval Interchange) (Al) – Kirikiri maximum security prison|
|A1-2||Junction of Broad Street with Marina – Eko Bridge – junction with Al at Western Avenue (Aorta Interchange)|
|A2||Kongolam (Niger border) – Daura – Kano – Zaria – Kaduna – Abuja – Lokoja – Auchi – Benin City – Warri|
|A3||Road over railway bridge on the Port Harcourt -township boundary – Aba – Umu Uyo – Owerrinta – Umuahia – Umu Duru – Okigwi – Awgu – Oji River – 14.4 kilometres Corner Otukpa – Oturkpo – Aliade – Makurdi – Lafia – Akwanga – Wamba – Jos – Bauchi – Kari Potiskum – Maiduguri – Gamboru|
|A3-1||The dual carriageway Enugu – Port Harcourt|
|A4||Calabar Township – Mbarakom – Ugep – Ikom – Ogoja – Katsina Ala – Jalingo – Yola Gombi – Bama – Maiduguri|
|F100||Lagos (Apapa Road Flyover south of Iganmu) – Badagry – Benin|
|F101||Ikorodu (A1) – Agbowa – Epe – Sunmage and terminating near Oso (A121)|
|F101-1||Sunmage (F101) – Iwopin|
|F102||Shagamu (Al) – Owode – Abeokuta – Meko – Illara – Benin (RNIE4)|
|F103||Effurun – Ughelli – Uwherum – Patani – Mbiama – Ahoada – Rumukoroshe – Nchia – Opobo – Eket – Ikot Ubo – Oron|
|F103-1||Okrika – Nchia (F103)|
|F104||Aba – Azumini – Etinam – Ndiya – Ikot Ubo (F103)|
|F105||Awbele (Benin border) – Oja Odan – Ilaro – Owode (Fl02)|
|F106||Owerri – Nguru – Etiti (A3)|
|F107||Umuahia – Bende – Ohafia (F234)|
|F108||Arochukwu – Ikot Okpora – Orira (A4)|
|F109||Amukpe (A2) – Eku – Abraka – Obiaruku – Umutu|
|F110||Port of Kolo – Ologbo (A2)|
|F.200||The road starting from a point near Marogbo, about nineteen Kilometres East of Badagry on F.100|
continuing thence to Ilaro‐Abeokuta Iseying‐Ago Are and terminating at Kishi on Trunk Road A.7.
|F.201||The road starting from Kaiama on Trunk Road A.7, continuing thence on Wawa‐New Bussa and terminating at Yelwa on Trunk Road A.1.|
|F.202||The road starting from Ago Are, continuing thence to Shaki‐Ilesha‐Okuta and terminating atKosubosu on Trunk Road A.7.|
|F.203||The road starting from Kamba on the frontier with the Republic of Benin, continuing thence to KalgoBirnin Kebbi and terminating at Argungu on F.132.|
|F.204||The road starting from Itokin on F.101, continuing thence to Ibefun‐Ijebu Ode and terminating at IdiAyunre on Trunk Road A.1.|
|F.205||The road starting from Ijebu Ode on A.121, continuing thence to Ijebu Igbo‐Ile Ife‐ShekonaOshogbo‐Offa and terminating at Ajasse on A.123.|
3.0 ROADWAY DESIGN
3.1 Geometric Design
Geometric roadway design can be broken into three main parts: alignment, profile, and cross-section. Combined, they provide a three-dimensional layout for a roadway.
- Route Location
184.108.40.206 The Environments of Highway Engineering
It is more usual than not to concentrate engineering on the computational aspects of a problem, with the result that people become conditioned to equating numerical solutions with problem-solving. Problem-solving, however, implies decision-making which in turn implies numerous compromises with prevailing environmental conditions and, particularly for public sector problems, with the politics of the budgetary process.
The engineer’s role in the decision-making process can only be enhanced by a full appreciation of how the environment facilitates or constrains alternative solutions to his/her problems. The engineer cannot stand by smugly on the side-lines with the ‘best’ scientific and technological solution, oblivious to the environment relating to its application. He/she must appreciate that the result of all of his/her computations is the establishment of technical boundary conditions, and that in the analysis he/she must recommend a ‘good’ or ’optimum’ course of action, i.e., one that is acceptableto the ultimate decision-makers.
220.127.116.11 Stating the problem
The highway engineering problem, like any other problem, exists in a complex environment that must be taken into account in decision-making. The engineer must analyse a particular problem and, on the basis of this analysis, make recommendations which are not only scientifically sound but are also adaptable to the physical environment, technologically achievable, economically viable, financially attainable, and in the final analysis, socially acceptable. What are these environments and how do they impinge on the engineering problem-solving process? To integrate all these parameters into severally acceptable highway takes time. In the UK, some motorway take more than 10 years for planning.
- a) The physical environment
This has traditionally been the environment of most concern to engineers, and includes the following factors: topography, geology, soils, materials, natural drainage, vegetation, land use, rainfall, and climate. Each of these factors may be further defined by various characteristics that assist the engineer in analysis and design. For example a soil through which a road may be located will be characterised by type, geotechnical properties, depth to bedrock, landform series, physical boundaries, etc; or rainfall might be characterised by intensity, duration and frequency per year.
The physical environmental factors and characteristics of interest are shown in Fig. 3.1 along with an illustration of the attendant analyses. Highway engineers quickly come to know the effect of topography on alignment and gradient or of geology on location and depth of cuts, or of materials on pavement design or of vegetation on clearing and grubbing. They also come to appreciate the ‘remote’ relationships between the factors themselves, for example: a knowledge of the type of vegetation only might give some clue as to the type of soil: the parent geology of an area interacts with the rainfall and climate to produce certain types of soils (particularly the lateritic soils found in tropical areas): topography is not independent of geology, and drainage patterns reveal information about soil and geological strata.
Figure 3.1 Environmental Analysis of Physical Factors
- b) The Technological Environment
The technological environment in which a solution may he prescribed seriously affects the success of its implementation, whether success is measured in terms of quality, completion or time. Technological factors that affect highway engineering decisions include: technological literacy and awareness, skilled manpower, management manpower, manpower training capacity, tools and equipment, construction materials, construction technology, hauling capacity, analytical and computational capacity.
- c) The Economic Environment
Some of the factors that define the economic environment in so far as it affects highway engineering decisions are as follows: gross national product (GNP) or gross domestic product (GDP), recessionary trends, unemployment rates, inflation rates, per capita income, vehicles per capita, size of informal sector, cash versus food crop cultivation, agricultural versus industrial production, distribution of economic activity, cost of labour, equipment hire rates, cost of living, debt service ratio, competing demands for government funds, foreign exchange reserves; oil import bill; export-import ratio.
- b) The Financial Environment
In many ways finance is so closely allied to economics that it is debatable whether it should be identified as a separate environment for consideration. However, the reason for this separation is to emphasise the distinction between economic viability and financial feasibility, particularly from the point of view of engineering economics. The fact that an engineering economic analysis, which properly considers the economic and technological environment, clearly indicates that the benefit-cost ratio of a particular maintenance programme over the next five years exceeds any other alternative investment in the economy is one thing Whether the financing will be available to realise this benefit is another. The financial environmental factors include:
National credit rating; international credit rating; aid recipients; national debt; bank lending policies; bonding; insurance.
- c) The Social Environment
As mentioned earlier any highway engineering decision must ultimately be socially acceptable if it is to be successfully implemented. This is particularly the case with road programmes in areas characterised by subsistence farming, non-market economics, low population densities and non-motorised local transportation—the so-called rural areas. In this context, the following factors are suggested as those defining the social environment:seasonal unemployment; perennial underemployment; population density and distribution; technological development; skill level; educational level; per capita income; means of transport;location and distribution of social services; location and distribution of markets; food storage capacity; trip propensity; latent travel demand; educational opportunities.
Social acceptance is a vital factor in highway planning and engineering. In the rapid expansion of highways in developed countries following the Second World War, many engineering solutions were frustrated into abandonment because of socially-unacceptable consequences, such as neighbourhood disintegration, noise and air pollution, defilement of the natural environment, and so on. Environmental impact statements became the order of the day and any road development plan could be tied up in years of litigation.
Having known the type of road and route of traffic, you now apply the basic design controls. These controls and criteria include but not limited to the following:
- Design Vehicles: There are basically two classes of vehicles – passenger cars and trucks to be used. AASHTO has about ten design vehicles for this purpose.
- Traffic volume and composition
- Design Speed
- Capacity and level of service
- Access Control
3.2 PAVEMENT DESIGN
3.2.1 Flexible Pavement: The flexible pavement is that which may consist of a relatively thin wearing surface built over a base course and subbase course, and they rest upon the compacted subgrade.
Flexible Pavement (US Type)
3.2.2 Definition of Highway (UK)
A highway is “a way over which the public have a right to pass and repass as of right and not by sufferance or by licence”. It is an interesting point to note that a waterway can be a ‘highway’.
Some Terminology changes in the UK and EU Standards as from January 2002:
‘Surface course’ ` – replaces – ‘wearing course’
‘binder course’ – replaces – ‘base course’
‘base (roadbase)’ – replaces – ‘roadbase’
‘materials to BS 4887’ – replaces – ‘macadam’
|SECTION THROUGH PAVEMENT (UK TYPE)|
3.2.3 Axle Loads
In Nigeria, the axle load is about 80kN (8158kg (8.2tons)). This load is no more realistic as we have varying higher axle configurations and load. However from the Highway Act several axle loads have been suggested ranging from 11tonnes to 32 tonnes. However, the ECOWAS treaty has allowed various axle load limits for different type of vehicles ranging from 18tonnes to 84 tonnes. It important to note that our roads were not designed and constructed with this parameter in mind. For instance the cement tractor-trailers and fuel tankers have axle loads of between 45tonnes (441.3kN) and 60tonnes
(588kN). Definitely, these loads will rapidly damage the road. The sad aspect is what the Highway Act says:
Section 26 (2). Vehicles not to exceed 32 tonnes gross weight, etc.
(1) No vehicle shall be used on a Federal highway if the single axle and tandem axle weight exceeds
10 tonnes and 16 tonnes respectively or if the gross weight of the vehicle exceeds 32 tonnes except as specified in Parts I and II of the Schedule to these Regulations. [Schedule.]
(2) Notwithstanding the foregoing provisions of this regulation, any authorised officer may grant a permit for a particular occasion or occasions for the use of a motor vehicle or trailer which does not conform to the requirements of these Regulations and such permit shall be subject to such conditions as may be imposed thereon and shall be carried by the driver of the motor vehicle on all occasions and be produced by him on demand by a police officer or an authorised officer.
(3) An owner of a motor vehicle above ten tonnes but not exceeding 32 tonnes shall obtain an annual permit in such form as the Minister may determine to operate the said vehicle on any Federal highway.
Section 26 (3). Establishment of weigh‐bridges
(1) The Minister may install at such locations on any Federal highway as he may determine, weigh‐bridges for the purpose of weighing of vehicles in pursuance of these Regulations.
(2) An authorised officer may, if he so requires with the assistance of a police officer, at any time and for reasonable cause require a vehicle affected by these Regulations to be driven to a weigh‐bridge location to have its net, gross or axle weight ascertained and the person driving the vehicle or in charge thereof shall comply with such requirement.
(3) The provisions of these Regulations shall not apply to an omnibus, being a vehicle capable of carrying a load of not less than 750 kg designed and constructed for the sole purpose of carrying passengers and their luggage.
Section 26 (4). Penalty
(1) No person shall cause or permit to be used on any Federal highway or thereon have charge of a motor vehicle or a trailer which is not in all respects in accordance with the provisions of these Regulations.
(2) Any person who operates a motor vehicle in contravention of these Regulations shall pay at the weigh‐bridge location a penalty of N100 and in addition, the owner shall at his own risk and expense, remove the excess load from his vehicle.
(3) Any person who fails to pay the penalty in paragraph (2) of this Regulation or who fails to comply with or acts in contravention of any provision of these Regulations shall be guilty of an offence andliable on summary conviction to a fine of N200 or to imprisonment for a period not exceeding six months or to both such fine and imprisonment.
3.3 Various Design Methods
California Bearing Ratio (C.B.R.) Design Method; Nigeria (CBR) Design Method; British Design Procedure;The Asphalt Institute Design Method;National Crushed Stone Association Design Method;Hveem Method;AASHTO Design Guide;Mechanistic Design Methods;
3.3.1 Information Required for Pavement Design
18.104.22.168 Traffic Loads
US: Tyre Loads’ Axle and Tyre Configurations, Typical Axle Load Limits, Repetitions of Axle Loads, Traffic Distribution (By Direction and Lane) and Traffic Projections.
UK: The Number of commercial vehicles expected to use the road initially, the estimated annual rate of commercial traffic growth, the designed life period of the road, the level of the natural water table in relation to the proposed formation level, the type of road structure which will be used in building the pavement.
22.214.171.124 Serviceability Index
Serviceability is a concept derived during the AASHO Road Test. This concept is related to the primary function of a pavement structure: to provide the travelling public with a smooth, comfortable, and safe ride. The PSI is obtained from measurements of roughness and distress, e.g. cracking, patching and rut depth (for flexible) during the service life of the pavement. Since roughness is such an important consideration for the design of pavements, the change in roughness will control the life cycle of pavements. In this regard, the quality of construction will influence performance and the life cycle of th3e designated pavement. The initial pavement smoothness is an important design consideration. For example, the life cycle of a pavement initially constructed with a smoothness or PSI of 4.5 will have a significantly longer life cycle than one constructed to a PSI of 4.0. Thus quality control in the construction of a pavement can have a beneficial impact on performance (life cycle). A scale ranging from 0 to 5 is used to evaluate a pavement’s present serviceability index (PSI); pavement with a rating of zero is impassable and a rating of 5.0 would be perfectly smooth.
126.96.36.199 Reliability (Confidence Level)
The concept of reliability as applied to pavement design can be defined as the probability that the pavement will perform as intended under the design traffic loading, and other crucial design inputs (material properties, environmental factors, etc.).
188.8.131.52 Material Characterization
- California Bearing Ratio (C.B.R.) Design Method
When the CBR of subgrade and other layers are known, the thickness of overlying material to provide satisfactory pavement can be determined. This method of designing is empirical.
- Nigeria (CBR) Design Method
The CBR method was first developed by the US Corps of Engineers and modified by the British Transportation and Road Research Laboratory (TRRL) and have been adopted by Nigeria and are contained in the Federal Highway Manual.
- Alternative way to find CBR
The Dynamic Cone Penetrometer (DCP) and other forms of cone penetration test can be used as an alternative means of measuring an approximate CBR. The relationship developed by Enoch George and Tam-Jones Atuboyedia (2007) could be used as shown below:
Log CBR = 3.06 – 1.45 Log S (Where, S = penetration and measuredin mm/blow)
From the Flexible pavement design curves, the various thicknesses of the layers can be determined.
184.108.40.206 Drainage Characteristics
In roadwork, the main drainage requirements fall into two categories a) Sub-soil Drainage b) Drainage of the carriageway.
- Sub-soil Drainage
This is usually associated with the construction of new roads. Great care must be taken to keep the water table below the road formation level. This in low-lying wet areas, may require the provision of a system of land drains, collector drains and a long outfallpipe or open ditch before the ground can be drained sufficiently for the structural work of the road building to be started.
- Drainage of the Carriageway
The surface of the water must be kept clear of standing water in order not to increase the dangers to road users. To accomplish this roads should be cambered when straight, laid to crossfalls on bends, and adequately provided with gullies or grips to dispose of rain water which falls on the carriageway.
4.0 THE CONSTRUCTION PHASE
This is a very important aspect of translating the design into physical reality. This aspect can invalidate all the hard work in the design phase, depending on a number of factors: material selection and characterization, right choice and use of equipment, control of “hold points”, especially at the batching plant and also the use of asphalt products.
4.1 Construction Materials
Soils may be divided into three main classes:
- Coarse-grained soils, also known as non-cohesive;
- Fine-grained soil, also known as cohesive
- Organic soils.
Compaction: It is important that fill material used to form embankments is compacted to the maximum dry density and to an accurate surface profile. The factors that determine this are:
- material characteristics of the fill
- moisture content of the material
- type of compaction equipment being used
- mass(weight) of the equipment in relation to its width of roller or base plate
- thickness of the layer being compacted
- number of passes required (The number of passes needed to achieve the desired compaction depends on the lift thickness, contact pressure, and soil moisture content.)
The aggregate in hot-mix asphalt provides a structural skeleton that will distribute traffic and environmental stresses. The individual fractions of the total aggregate gradation are designated as follows:
- Coarse aggregate (material retained on the 4,75 mm sieve) fine aggregate (material passing the 4,75 mm sieve and retained on the 0,075 mm sieve) and filler (material which substantially passes the 0,075 mm sieve).
- The coarse aggregate in an asphalt mix normally consists of crushed stone or blast-furnace slag. It should be uniform in quality, and free from deleterious matter and weathered and disintegrated particles.
- The particles of coarse aggregate should ideally be cubic. The texture and shape of the aggregate particles will influence the stability of the mixture. Angular rather than rounded particles are preferred.
- The fine aggregate may be a material such as sand, gravel, crushed stone or slag, or a blend of these. It should be clean and free from clay or any other deleterious matter.
- The shape of the particles of the fine aggregate significantly affects the workability of the asphalt mixture as well as its resistance to deformation.
- Filler is mineral matter composed of particles smaller than 0,075 mm. Mineral filler may consist of flue dust, stone dust, fly-ash, hydrated lime, Portland cement or blast-furnace cement, milled blast-furnace slag, or any combination of these.
In an asphalt mix the filler acts as an extender to the binder and performs two important functions. Firstly, it acts as a voids-filling material to help give a dense, durable mix. Secondly, it stiffens the mix and improves resistance to plastic deformation.
MIX DESIGN: The overall objective in the design of hot-mix asphalt is to arrive at an economical blend of aggregate and binder. The resultant mix will contain:
- sufficient binder of the correct type and a suitable aggregate grading to ensure a durable layer
- sufficient resistance to plastic deformation and to cracking to enable it to carry the expected traffic without significant distortion or cracking
- sufficient air voids in the compacted mix to allow for possible additional compaction under traffic without flushing, bleeding or loss of resistance to plastic deformation,
4.2 Construction Equipment
For soil compaction, we need:
- Rollers of various types – smooth wheel with or without vibration, grid, tamping, sheepfoot, pneumatic
- Vibrating plates – fibro tampers, power rammers, dropping weight compactors
- Construction Methods
For large projects, STAGE CONSTRUCTION could be encouraged. The appropriate use of right equipment for the right job will enhance the design parameters.
5.0 QUALITY CONTROL
During the compaction: How many times should the compactor pass? What type of compactor do you use for what type of soil? (Pneumatic-tyred roller, tandem roller, dead weight steel roller, sheepfoot roller, etc)
What is the quality of granite chippings? Different types of test on aggregate like impact test, abrasion test, crushing value test, soundness test, etc. There seems to be an abuse of the 0 – 50mm stone base materials. This needs to be checked. For instance, there could be more fines than the required 50mm. This could be a major source of failure of the road with dynamic load.
The entire design of the asphalt concrete can be changed and made irrelevant by the Operator (owner) of the batching plant if the various sizes of aggregates, asphalt cement quality, quantity and thickness to be placed are changed. This is where most dubious contractors make their so called ‘profit’. This is why there must be a ‘hold point’ between the client and the contractor, prior to loading the batching plant.
5.3.1 Asphalt Paving: Temperature & Timing
220.127.116.11 Mix Temperature
When it arrives at the job site, hot-mix asphalt is normally between 275 and 300 degrees Fahrenheit (1350C and 1490C). For the initial rolling, the temperature of the mix needs to be between 220 and 290 degrees Fahrenheit (930C and 1430C). If the mix temperature falls below approximately 185 degrees Fahrenheit (850C) before the final compaction, it will be too stiff to compact properly. How long the contractor has to complete compaction depends on the thickness of the course; thinner layers will cool faster than thicker ones. However, the ambient temperature, wind speed and ground temperature also affect the rate at which the asphalt will cool.
18.104.22.168 Ambient Temperature
The ambient or air temperature is the measurement with which most people are familiar. Most applications require that the ambient temperature is at least 50 degrees Fahrenheit and rising while the asphalt is being installed and compacted. If the wind velocity is high, the asphalt will cool faster, so on extremely windy days, the ambient temperature may need to be above 50 degrees.
22.214.171.124 Ground Temperature
The ground temperature is the temperature of the base on which the asphalt will be installed. This can be significantly lower than the ambient temperature, especially in the early morning following a cold night. Most contractors use an infrared thermometer to measure the ground temperature, which needs to be at least 50 degrees Fahrenheit and rising throughout the process.
126.96.36.199 The Importance of Temperature
The reason that the mix, ground and ambient temperatures are critical is that satisfactory results will not be achieved if the asphalt is not sufficiently compacted. Pavement that cools too rapidly will not have the density required, so the pavement will ravel, leaving the surface rough and prone to retain water. The pavement will not last as long as it should or provide the smooth, attractive surface for which asphalt is famous.
6.0 AREAS OF CONCERN
6.1 The Engineer seem to be a problem to himself. We undercut ourselves in our professional integrity by way of our quotations, construction techniques and lack of knowledge.
6.2 There has to be a minimum level of prices for elements of work to be done.
6.3 The younger Engineers are too much in a haste to “make it”. Engineering Consultancy is not easy. You can go to jail if found liable in either a professional misconduct or a wrong opinion expressed on a design.
6.4 As an engineering community, we need to advertise the various arms of engineering on TV and print media. The reason is that without engineering there is no civilisation.
6.5 If road projects are properly gazetted and the money made available by Houses of Assembly in their budget approval processes, the Executive arm shall be forced to continue projects that were started by their predecessors.
Good road means appropriate design, supervision on the construction site and quality controls at the batching plants. It means appropriate review of soil, traffic and asphalt parameters. A failure in each of these will not achieve the designers dream, hence the design life of the pavement.
- AASHTO (1994), A Policy on Geometric Design of Highways and Streets
- AASHTO (1993), AASHTO Guide for Design of Pavement Structures
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