WBM Roads

1. WBM Roads
Definition: The pavement base course made of
crushed or broken aggregates mechanically
interlocked by rolling and voids filled by screening
and binding material with the assistance of water

2. Salient Features of WBM Roads
• WBM means Water Bound Macadam.
• The word Macadam comes from the name of
Scottish Engineer John L. Macadam.
• Wearing surface is formed by cleaned,
crushed aggregates which are spread and
rolled by sprinkling water.

3. Salient Features of WBM Roads
• Low cost
• Thickness range: 8cm to 30 cm
• Camber : 2.5% to 3% OR 1:36 to 1:48
• Surface is better than earthen road but rough
as compared to bituminous or Concrete roads
• Can be used as Base Course for Bituminous or
Concrete roads.

4. Materials to be used in WBM roads
• Crushed trap rock – Most Suitable
• Crushed slag
• Burnt Brick bats
• Kankar Aggregate
• Laterite
• Screenings to be used as filler material

5. IRC Recommendations for Course
Aggregate Grading for WBM Roads
Grading No Size Range (mm) Sieve Size (mm) % Passing the sieve by
weight
1
2
3
90 to 40
63-40
50-20
100
80
63
40
20
83
63
50
40
20
63
50
40
20
100
65-85
25-60
0-15
0-5
100
90-100
30-70
0-15
0-5
100
95-100
35-70
0-10

6. IRC Recommendations for Grading of
Screenings
Classification
Grading
Size of Screenings
mm
Sieve size
mm
% by weight
passing the sieve
A
B
12.5
10.0
12.5
10.0
4.75
0.15
10.0
4.75
0.15
100
90-100
10-30
0-8
100
85-100
10-30

7. Construction Procedure of WBM
Roads
• 1. Preparation of Sub Grade
• 2. Preparation of Base Course
• 3. Intermediate Layer
• 4. Wearing Surface
• 5. Shoulders
Shoulder Wearing Course Shoulder
Intermediate Course
Base Course
Sub Grade

8. Precautions in Rolling WBM Roads
• Roller to be used- 8-10 Tonnes
• Rolling should be done from edges to the centre
• The successive rolling strips should be overlapped
• Length of each rolling should be <=200m.
• Water should be sprinkled and not poured by
buckets
• The rolling should be done for 80 passes or until
the aggregates are broken.