Thursday 3 June 2010

ROAD CONSTRUCTION

Construction can be either: Flexible Pavement or Rigid pavement

Flexible Pavements

Purpose of the Road:

· Transmitting reduced stress to such a value that the ground below can support.

· Stresses are static and dynamic.

· Stresses are transmitted approx. 45° through the road construction.

· Flexible construction were designed to last 20 years (accepting 2% growth per year).

· Proper construction therefore important.

· DoT design is based on 40yr, subject to periodic maintenance.

· Drainage of ground upon which the road is built is important.

· Flexible so called because the construction allows a small amount of vertical movement.

Road Design:

· Generally four layers:-

1. sub-base

2. roadbase

3. basecourse

4. wearing course


Load Distribution Requirements:

· Angle of distribution not as constant as 45°

· Stresses are greatest near the surface.

· Lateral deflective stresses present, therefore upper and lower roadbase required.

· Uneven surface will cause greater and variable stresses

· Therefore layered construction implemented

· lower layers thicker and cheaper

· upper layer thinner and more expensive

· each layer must be compacted properly

· Manual of Contract Documents for Highway Works, Series 700 provides tolerances

DESIGN STANDARDS

Standard axle

An axle carrying a load of 8200kg (18,000 lb). The legal axle limit in the UK is 11250kg for a non steering axle and 6250kg for a steering axle. Vehicle designers usually employ all of the legal weights. In design, we need to express axle loads in terms of standard axles. We do this by dividing the each vehicle axle load by the standard axle value and raising the result to the power 3.75. This means that a legal maximum non-steering axle applies 3.3 standard axles and a legal limit steering axle applies 0.4 standard axles. A full double decker bus would weigh about 15,000 kg, with 6000kg applied through the front axle and 9,000kg applied through the rear axle. Calculations show the number of standard axles for such a vehicle to be 2.3. An empty double decker bus would weigh 10,000 kg with 4000kg applied through the front axle and 6,000kg applied through the rear axle. This vehicle would apply 1.1 standard axles. It might be concluded that a service of double decker buses would each apply 2 standard axles as an average.

Cumulative traffic

The number of standard axles a pavement is designed to carry, measured in millions of standard axles (msa). Sometimes, the term cumulative number of standard axles (csa) is used, particularly when the total number of standard axles which will traffic a road throughout its life is being expressed. These figures are determined by multiplying the anticipated number of vehicles per hour by the number of trafficked hours in a day, then by the number of days for which the road is being designed. Finally, the figure is multiplied by the number of standard axeles per vehicle. It may be necessary to consider a mix of vehicles.

SUB-BASE

· assist in load spreading

· make up pavement to 450mm on frost susceptible soils

· provide protection to sub-grade

· provide surface on which to lay road base and kerbs/kerb foundation

· DoT type 1 - crushed rock or slag to grading curves in DoT Clause 802

· DoT type 2 - natural sands and gravel, DoT Clause 804

· Wet-mix macadam, DoT Clause 805

ROADBASE

· main load spreading layer

· materials very important

· binding characteristics of material important:

· natural interlock - dry bound macadams

· water bound - wet mix

· cement bound - wet-lean concrete

· bituminous bound - dense macadams and hot rolled asphalt

· Requirements are for a flexible base that will not crack - dense bituminous materials are used

· Dense Roadbase Macadams, DoT Clause 903

· Rolled Asphalt, DoT Clause 904


Sub-base and Roadbase Materials

Unbound materials

Clause 803 Granular sub-base, Type 1: This must be crushed rock, slag or other hard material graded as shown in the Table below.

Clause 804 Granular sub-base, Type 2: This is a much smaller sized material than type 1, therefore natural sand and gravels may be used as this material. The Table shows the grading requirements and variations between type 1 and type 2 stone.

Sieve size

Proportion passing sieve

Type 1

Type 2

75 mm

100 %

100 %

37.5 mm

85-100

85-100

10 mm

40-70

45-100

5 mm

25-45

25-85

600 μm

8-22

8-45

75 μm

0- 10

0- 10

μm = micrometer = one millionth of a metre

Clause 805 Wet-mix macadam: This is a ‘plant manufactured’ material using crushed rock or slag accurately graded and batched, and mixed with 2-6% water according to the nature of the aggregate.

The material is usually laid in compacted layers not exceeding 200 mm, the aggregate grading being within the stated limits. Care needs to be taken to keep the moisture content within the optimum limits. Drying out, or excess moisture will have a serious detrimental effect.

Other Unbound Material

Quarry Waste or Scalpings

Hardcore

Hoggin (or similar self-binding gravel)

Clinker

Shale

P FA: Cement-stabilized

Dry Bound Macadam

50 or 37.5 mm single size crushed rock or slag is spread in layers 75- 100 mm thick and rolled. A 25 mm thickness of 4.7 mm down similar material is then spread on the layer and vibrated into the coarse aggregate, the process being repeated until no more fines can be worked in.

Bituminous Roadbases

Characteristics/Advantages:

Load-Spreading Properties

Speed of Construction

Imperviousness and Frost Resistance

Dense Roadbase Macadam to BS 4987 (SHW Clauses 902/903)

The need for strong but truly flexible bases that will not t crack has led to the use of these dense bituminous materials.

The main requirements in the composition of the dense macadam for use in roadbases are that the materials have a fines content (aggregate passing 3.35 mm sieve) of 38% and are made with high-viscosity binders, i.e. 50,54 or 58°C evt tar, or 50 pen, 100 pen or 200 pen bitumen.

‘evt’ = equi-viscous temperature which is the temperature in degrees Celsius at which a tar has a viscosity of 50 seconds as determined by the British Standard Test.

‘Pen’ is the abbreviation for ‘penetration grade’ and is a measure of the hardness of a bitumen binder.


Rolled Asphalt to BS 594 (SHW Clause 904)

Rolled asphalt is the oldest established bituminous material used for roadbase construction and has load-spreading properties superior to those of other flexible road bases.

Concrete and Cement-Bound Materials (SHW Clause 1001)

Cement–bound granular materials and lean concrete are all categories of the group known as cement bound material (CBM). The HA has renamed these as cement-bound materials 1,2 and 3 (CBM 1 CBM2 and CBM3). A new and stronger category, CBM4, has been introduced for roads of higher traffic category.

Cement-bound materials are mixtures of raw material and cement which have a moisture content compatible with compaction by rolling, and capable of meeting the requirements for surface level, regularity and finish.

CBM 1, weakest, min crushing strength of 4.5N/mm2 at 7 days

CBM 2, coarser and stronger, 7N/mm2 at 7 days

CBM 3 & 4 can be 40mm or 20mm aggregate size, 10N/mm2 and 15N/mm2 at 7 days.

SURFACING

This includes the wearing course and the basecourse.

The top layers of the road have the greatest work to do. The upper layers of the road have to carry the greatest intensity of traffic loadings.

Basecourse

· distribute the traffic loads over the roadbase (weaker material than basecourse)

· provide a good shaped material on which to lay the thin wearing course

· 45mm to 105mm thick

· type of material selected according to traffic density

· roads designed to carry 2.5 million standard axles basecourse will be rolled asphalt or dense coated macadams

· HA Clauses 905 and 906

Basecourse Materials

The type of material used for a base course is selected according to the intensity of traffic loading expected.

The nominal size of the stone (20, 28 or 40 mm) depends on the thickness of the layer. The thicker the basecourse, the larger the stone size.

On new roads being designed to carryover 2.5 million standard axles, a basecourse of rolled asphalt or dense-coated macadam should be used.

Similar to those for roadbase construction and provide the same benefits, particularly the excellent load-spreading soon as the material is cool.

Wearing Courses

· provide a skid resistant surface

· weather protection

· withstand abrasion and stresses from traffic

· good running surface

· wide variety of bituminous materials available

· 20mm - 40mm thickness

· DoT Clauses 909, 910, 911, 912, 913 and 914


Wearing Course Materials

A wide variety of bituminous materials is used for wearing courses, laid in thicknesses ranging normally from 20-40 mm.

Important points in new construction are the additional strength which the wearing course may add to the pavement and the extent to which it forms an impervious layer over the construction.

Hot Rolled Asphalt (BS 594) - This is the strongest and most durable wearing course. It is very dense, made with a high fines and asphaltic cement content with crushed rock, slag or gravel added, to provide additional stability and reduce the cost. Hot rolled asphalt is the only wearing course recommended for use on the more heavily trafficked roads. It is normally laid 40 mm thick with 20 mm coated chippings rolled into the surface to provide better skid resistance.

10 and 14 mm size Close Graded (formerly called ‘Dense’ ) ( BS 4987 ) – These materials are used in new construction and for resurfacing using crushed rock, slag, gravel or limestone, or C34 to C42 evt ‘ tar binder. Limestone and gravel tend to polish, resulting in loss of skid resistance and are, therefore, not always suitable.

Pervious ( BS 4987 ) – These surfacings have been developed in an effort to reduce spray from vehicle wheels and also to reduce the risk of aquaplaning in wet Weather. They are made in 10 mm and 20 mm sizes with a relatively hard binder and 2% hydrated lime to reduce ‘stripping’ under the action of water.

Preparation for Laying

· assume good drainage

· assume sub-base of adequate strength

· some form of lateral support

· longitudinal accuracy of surface to be overlaid by coated macadams - 3m straight edge parallel to centre line.

· transverse similar

· tack coat to secure adhesion between existing and new materials

Transportation of Materials

· DoT Clause 901, clean vehicles covered while in transit

· supplied continuously to paver without delay

· spread, levelled and tamped by approved self-propelled pavers: two types

· Caterpillar

· Wheeled

· Tamping bar operation very important - 80% total compaction

Surface Alignment and Regularity

· laid accurately and to constant thickness

· DoT Clause 702 provides tolerances for horizontal alignment ± 25mm from one edge

· surface levels: at any point, DoT Table 7/1

· regularity: rolling straight edge along line parallel to edge of pavement

· surface roughness - texture depth:

· sand patch method

· mini texture meter

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