WBM Roads

Definition: The pavement base course made ofcrushed or broken aggregates mechanicallyinterlocked by rolling and voids filled by screeningand binding material with the assistance of water

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.

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.

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

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

10080634020

8363504020

6350402010

10065-8525-600-150-5

10090-10030-700-150-5

10095-10035-700-100-5

IRC Recommendations for Grading of Screenings

Classification Grading

Size of Screenings mm

Sieve sizemm

% by weight passing the sieve

A

B

12.5

10.0

12.510.04.750.15

10.04.750.15

10090-10010-30

0-8

10085-10010-30

Construction Procedure of WBM Roads

• 1. Preparation of Sub Grade

• 2. Preparation of Base Course

• 3. Intermediate Layer

• 4. Wearing Surface

• 5. Shoulders

Sub Grade

Base Course

Intermediate Course

Wearing Course ShoulderShoulder

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.