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CHAPTER 3
WATER BOUND MACADAM (WBM) AS BASE COURSESalient Features
3.1 Methodology
Prepare the surface to receive the WBM course to the designed
lines, grade and
cross fall.
Make the surface free of dust and extraneous material. The
irregularities, if any
should be made good by providing profile corrective course.
Spread the coarse aggregate uniformly on the prepared surface to
proper
grade/camber/ super-elevation (by using templates placed across
at 6.0 m
intervals) in such quantities that would give the required
compacted thickness.
Check the surface with templates and all high or low spots
remedied.
Dry Roll the surface till aggregates are partially compacted
with sufficient void
space left for application of screenings. However, where
screenings are not to be
applied as in the case of soft aggregates, compaction shall be
continued until the
aggregates are thoroughly keyed.
Sprinkle water on WBM surface and again roll with plain
rollers.
Proceed the rolling from inner edge to outer edge at the
super-elevated portions
and from the edges towards the centre in portions for cross fall
on both sides. Theedge should be first compacted with roller
running forward and backward.
Check the profile transversely and longitudinally with
templates/ straight edge.
Correct the irregularities, if any, by loosening the surface,
adding or removing
the needed amount of aggregates and re-rolling until the entire
surface conforms
to the specified camber/ cross fall and grade.
Sprinkle screenings to completely fill the interstices
maintaining a slow and
uniform rate, in three or more applications. The screenings
should not be damp
at the time of application.
Do not apply screenings so fast and thick as to form cakes or
ridges on the surface.
Continue dry rolling and brooming till no more screenings can be
forced into the
voids of coarse aggregates.
Sprinkle water on the surface taking care that the underlying
layer is not
damaged.
Sprinkling, sweeping and rolling should continue till aggregates
are thoroughly
keyed, well bonded and firmly set in its full depth and a grout
has been formed of
screenings.
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In case the screenings are not of crushable type such as moorum
or gravel, addbinding material in specified quantity after
application of screenings.
Sprinkle the binding material in two or more layers at a slow
and uniform rate.
Continue the process of water sprinkling, sweeping and rolling
till the resultingslurry forms a wave ahead of roller.
Allow the compacted WBM course to completely dry and set before
the nextpavement course is laid.
Construct the earthen shoulders simultaneously with the WBM
construction.
3.2.1. Materials(i) Coarse Aggregate
(a) Physical requirements
Aggregate Impact Value of soft material should be tested under
wet conditions as perIS:5640.
(b) Grading :
3.2 Quality Control Requirements
TABLE 301.1: Physical Requirements of Coarse Aggregates for WBM
(Base Course)
Table 301.2 : Grading Requirements of Coarse Aggregate for
WBM
Test Base
Aggregate Impact value(IS : 2386 Part 4 or IS : 5640)
Less than 40
Flakiness index(IS : 2386 Part 1)
Less than 25
Soundness test(IS : 2386 Part 5)(To be carried out if water
absorption is > 2%)-Loss with Sodium Sulphate
-Loss with Magnesium Sulphate
Less than 12%Less than 18%
GradingNo.
Size Range
IS SieveDesignation
Per cent by weightpassing
2 63 mm to 45 mm
3 53 mm to 22.4 mm
90 mm
63 mm
53 mm
45 mm
22.4 mm
63 mm
53 mm
45 mm
22.4 mm11.2 mm
100
90-100
25-75
0-15
0-5
100
95-100
65-90
0-100-5
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