This document provides information on the construction of water bound macadam (WBM) roads. It discusses the specifications of materials used, including coarse aggregates, screenings, and binding material. It also outlines the construction procedure, including preparation of the foundation, spreading aggregates, rolling, application of screenings and binding material, and allowing the WBM layer to set and dry before permitting traffic. The document serves as a reference for the construction of WBM pavement layers, which can be used as sub-bases, bases, or surfacing courses in road construction.

1. Transportation Engineering-I_BCE5012
Highway Construction
Er. Kamal Kumar, Assistant Professor, Faculty of Civil Engineering
Email: kmalkumar.ce@srmu.ac.in
Prof. (Dr.) Abhishek Saxena, Dean, Faculty of Civil Engineering
Email: dean.ce@srmu.ac.in
Shri Ramswaroop Memorial University, Barabanki, 225003

2. Transportation Engineering-I_BCE5012
Topic Covered: Construction of Water Bound Macadam Roads
Presented By:
Er. Kamal Kumar
Assistant Professor
FoCE, SRMU

3. Transportation Engineering-I_BCE5012
CONSTRUCTION OF WATER BOUND MACADAM ROADS
General
The water bound macadam (WBM) is the construction known after the name of John
Macadam. The term macadam in the present day means, the pavement base course made of
crushed or broken aggregate mechanically interlocked by rolling and the voids filled with
screening and binding material with the assistance of water. The WBM may be used as a sub-
base, base course or surfacing course. The thickness of each compacted layer of WBM ranges
from 10.0 cm to 75 cm depending on the size and gradation of the aggregates used. The
number of layers and total thickness of WBM construction depends on the design details of
the pavement.
When used as a surfacing course, WBM gets deteriorated rapidly under adverse conditions of
traffic and weather, therefore it is desirable to provide a bituminous surfacing course over the
WBM layer in order to prolong its life.

4. Transportation Engineering-I_BCE5012
Specifications of Materials for WBM Pavement
Type of coarse Aggregate
The course aggregates used in WBM generally consists of hard varieties of crushed aggregates
or broken stones. However, soft aggregates like over burnt bricks metal or naturally occurring
soft aggregates such as kankar and laterite may be used. Crushed slag obtained from blast
furnace may also be used. The weaker varieties of aggregates which get crushed during rolling
need not strictly conform to the requirements of gradation and there is no need of using
screenings for filling the voids in the coarse aggregates.
Properties of Coarse Aggregates
The crushed stone aggregate should be generally hard, durable and of acceptable shape, free
from flaky and elongated particles. The IRC specifies the following physical requirements of
coarse aggregates for WBM construction, in terms of the test value for the three pavement
layers.

5. Transportation Engineering-I_BCE5012
Size and Grading Requirements of Coarse Aggregates
The coarse aggregates for each layer of construction should, as far as possible confirm to any
one of the three gradings. Grading No. 1 consists of coarse aggregates of size range 90 to 40
mm and is more suitable for sub-base course. Thickness of compacted layer is usually 100
mm. Grading No. 2 consists of aggregates of size range 63 to 40 mm and grading No. 3 of
range 50 to 20 mm and compacted thickness of each layer is normally 75 mm.
Screenings
The screening are used to fill up the voids in the compacted layer of coarse aggregates. The
screenings consist of aggregates of smaller size, generally of the same material as the coarse
aggregates. The grading requirements of screenings for WBM construction are given below:
Classification of
grading
Size of screenings,
mm
Sieve size, mm Percent passing the
sieve, by weight
A 12.5 12.5 100
10.0 90-100
4.75 10-30
0.15 0-8
B 10.0 10.0 100
4.75 85-100
0.15 10-30

6. Transportation Engineering-I_BCE5012
The IRC has suggested that from economic considerations, predominantly non-plastic
materials such as kankar nodules, moorum or gravel (other than river borne rounded
aggregate) may be utilised as screening material, provided the liquid limit of the material is
less than 20%, plasticity index is less than 6.0% and the portion of fines passing 0.075 mm
sieve is less than 10%. However, if crushable type of materials are used as coarse aggregate,
the use of screenings may be dispensed with.
Binding Material
Binding material consisting of fine grained material is used in WBM construction to prevent
raveling of the stones. Kankar nodules or lime stone dust may also be utilised, if locally
available. The binding material with plasticity index value 4 to 9% is used in WBM surface
course construction; the plasticity index of binding course material should be less than 6.0% in
the case of WBM layers used as base course or sub-base course, with bituminous surfacing. If
the screenings used consist of crushable material like moorum or soft gravel, there is no need
to apply binding material, unless the plasticity index value is low.

7. Transportation Engineering-I_BCE5012
Construction Procedure
Preparation of Foundation for Receiving the WBM Course
The foundation for receiving the new layer of WBM may be either the subgrade or sub-base or
base course. This foundation layer is prepared to the required grade and cambar and the dust
and either loose materials are cleaned. On existing road surfaces, the depressions and pot-
holes are filled and the corrugations are removed by scarifying and reshaping the surface to
the required grade and camber is necessary. If the existing surface is a bituminous surfacing,
furrows of depth 50 mm and width 50 mm are cut at 1.0 m intervals and at 45 degrees to the
centre line of the carriageway before laying the coarse aggregate.
Provision of Lateral Confinement
Lateral confinement is to be provided before starting WBM construction. This may be done by
constructing the shoulders to advance, to a thickness equal to that of the compacted WBM
layer and by trimming the inner sides vertically.

8. Transportation Engineering-I_BCE5012
Spreading of Coarse Aggregates
The coarse aggregates are spread uniformly to proper profile to even thickness upon the
prepared foundation and checked by templates. The WBM course is normally constructed to
compacted thickness of 7.5 cm except in the case of WBM sub-base course using coarse
aggregate grading no. 1 which is of 10.0 cm compacted thickness.
Rolling
After spreading the coarse aggregates properly, compaction is done by a three wheeled power
roller of capacity 6 to 10 tonnes or alternatively by an equivalent vibratory roller, the weight
of the roller depends on the type of coarse aggregates.
Rolling is started from the edges, the roller being run forward and backward until the edges
are compacted. The run of the roller is then gradually shifted towards the centre line of the
road, uniformly overlapping each preceding rear wheel track by one half width. This process
is repeated by rolling from either edge towards the centre line until adequate compaction is
achieved.
On superelevated portions of the road, rolling is commenced from the inner or lower edge and
progressed gradually towards the outer or upper edge of the pavement.

9. Transportation Engineering-I_BCE5012
Application of Screenings
After the coarse aggregates are rolled adequately, the dry screenings are applied gradually
over the surface to fill the interstices in three or more applications. Dry rolling is continued as
the screenings is being spread and brooming carried out.
Sprinkling and Grouting
After the application of screenings, the surface is sprinkled with water, swept and rolled. Wet
screenings are swept into the voids using hand brooms. Additional screenings are applied and
rolled till the coarse aggregates are well bonded and firmly set.
Application of Binding Material
After the application of screening and rolling, binding material is applied at a uniform and
slow rate at two or more successive thin layers. After each application of binding material, the
surface is copiously sprinkled with water and wet slurry swept with brooms to fill the voids.
This is followed by rolling with a 6 to 10 tonnes roller and water is applied to the wheels to
wash down the binding material that sticks to the roller. When crushable type screenings like
moorum and gravel are used, there is no need to apply binding materials, except in the
surfacing course.

10. Transportation Engineering-I_BCE5012
Setting and Drying
After final compaction, the WBM course is allowed to set over-night. On the next day the
‘hungry’ spots are located and are filled with screenings or binding material, lightly sprinkled
with water if necessary and rolled. No traffic is allowed till the WBM layer sets and dries out.
In the case of WBM base course, the layer is allowed to dry completely without permitting
traffic to ply and then the bituminous surfacing is laid. Limited construction traffic may be
permitted to ply over the WBM layer taking proper care not to damage the layer.

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References
• S. K. Khanna & C.E.G. Justo – “Highway Engineering”, Nem Chand and Bros.
• L. R. Kadiyali – “Transportation Engineering”, Khanna publishers.

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