Construction of Water Bound Macdam Road

8/2/2019 Construction of Water Bound Macdam Road

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BY

CHAITHRA .N. KOWSHIK

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John Loudon McAdam

Term Mcadam in the present day means, the

pavement base course made of crushed or broken

aggregate mechanically interlocked by rolling and thevoids filled with screening and binding material with

the assistance of water.

Base, subbase.

The thickness of WBM ranges from 10cm to 7.5cm.

Design details.

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When used as a surfacing course,Deterioted rapidly

under adverse traffic conditions of traffic and

weather.

It is desirable to provide a bituminous surfacingcourse over WBM layer in order to prolong life.

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Types of course aggregates

Hard varieties of crushed aggregate or broken stone.

Soft aggregates like over burnt bricks metal or

naturally occurring soft aggregates like kankar maybe used.

Weaker varieties of aggregates gets crushed during

rolling. Hence no need of filler materials.

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Property Sub-base Base course Surfacing course

i) Los angelsabrasion valuemax value in %

60 50 40

ii) Aggregateimpact maxvalue in %

50 40 30

iii) Flakinessindex max valuein %

15 15

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Hard,durable and of acceptable shape

Free from flaky and elongated particles.

IRC specifies the following physical requirements of

course aggregates for WBM construction.

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Gradingno Size range, mm Sieve size, mm % passing thesieve, by weight1 90 to 40 100

806340

20

10065-8525-600-15

0-5

2 63 to 40 80635040

20

10090-10030-700-15

0-53 50 to 20 63

50402010

10095-10035-700-100-5

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Classification ofgrading Size ofscreenings inmmSieve size, mm % passing thesieve, by weight

A 12.5 12.510.0

4.750.15

10090-100

10-300-8

B 10.0 10.04.750.15

10085-10010-30

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Fine grained material

Kankar nodules or lime stone

Plasticity index value 4 to 9%

WBM layers used as base course or sub base course,with bituminous surfacing.

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Apprroximate loose quantities of materials in m3 for 10cm compacted thickness of WBM sub base using

course aggregate of grading no 1 per 10m2 area are

a) Course aggregate size 90 to 40 mm = 1.21 to 1.43

b) stone screening type A, 12.5 mm size= 0.40 to 0.44

Or

Crushable type screenings(moorum/gravel)=0.44 to 0.47

c) Binding material for sub-base course = 0.88 to 0.10

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Preparation of foundation for receiving the WBMcourse.

The foundation which is receiving either the sub grade or sub

base course.

Foundation layer is prepared to the required grade and camber

and the dust either loose materials are cleaned.

On existing road surfaces deprssions and pot holes are filled

and the corrugations are removed.

If the existing roads is bituminous surface means 50mm depthand 50 mm cut at 1.0 m interval and at 45 degrees to the

centre of line of the carriage way before laying the course

aggregate.

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Lateral confinement is to be provided before stating

WBM construction.

Construction shoulders in advance

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uniform spreading

10 cm compacted thickness

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By using three wheeled power roller 6 to 10 tones or

alternatively by an equivalent vibratory roller.

Rolling started from the edges, the roller being run

forward and backward until the edges are compacted.Then the run of the roller is shifted towards the centre

of the line roads.

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Used to fill up the voids in the compacted layer of the

course aggregates.

Sprinkling and groutingAfter screening the water is sprinkled over the

surface, swept into the voids using hand brooms.


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Binding material is applied at a applied at a uniform

and slow rate at two or more successive thin layers.

After each application of binding material surface is

sprinkled with water and wet slurry swept withbroom to fill the voids.

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After final compaction, the WBM course is allowed

to set over night.

On the next day spots were filled with screenings or

with the binding material, lightly sprinkled withwater and rolled if necessary

In case of WBM base course, the layer is allowed to

dry completely without permitting traffic.

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Surface evenness of longitudinal direction ischecked by 3.0m straight edge and the number of

undulations exceeding 12mm of grading no 1.

10 mm in the case of grade 2 and 3. Maximum number of undulations permitted in each

case is 30.

The cross profile is checked using camber template

and the max variation from special profile should notexceed 12mm.

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Construction of Water Bound Macdam Road