Includes introduction, Why DBM, design criteria, plants and equipment, weather and seasonal limitations, materials required and construction procedure.

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2. Introduction
Why DBM ?
Design Criteria
Materials
Construction
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3. Dense Bituminous Macadam(DBM) is a binder
course used for roads with more number of heavy
commercial vehicles and close-graded premix
material having a voids content of 5-10 percent.
It consists of one or more course of compacted
crushed aggregate premixed with a bituminous binder
and laid immediately after mixing and then
compacted.
This is used primarily for use as a bse course
deflections of the pavements are substantially lower
than WBM or gravel types.
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4. When this layer is exposed as a surface course, at least
a seal coat is necessary.
It is laid with usual thickness of 5-7.5cm, aggregate size
used are 37mm down and is open graded mix.
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5. This material has increased in popularity due to durable
surface which performs almost well as in all situations.
Suitable for all pavement layers and for all traffic
conditions.
DBM provides a good quality smooth surface and
improved skid resistance.
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6. Due to high cost of DBM it should be properly designed according
to standard design method so as to satisfy certain criteria needed
to assure adequate stability and durability.
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7. Bitumen of grade 30/40,60/70,80/100 penetration and tar RT-4,
cutbacks and emulsions may be used in cold mix construction
technique.
The binder content used varies from 3 to 4.5 percent by weight
of the mix.
The coarse aggregate should fulfill the following requirements:
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8. The grading of the aggregates for 75mm and 50mm thickness
for base and binder course construction are given in the table
below:
Grading for 75mm compacted thickness.
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9. Grading for 50mm compacted thickness.
The quantity of aggregates required for 10m2 of bitumen
bound macadam are 0.60 to 0.75 m3 and 0.9 to 1.0m3
respectively,for 50 and 75mm compacted thickness.
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10. Bitumen heating device
Hot mix plant
Sprayer
Mechanical mixer or improvised hand mixer
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11. Bituminous macadam shall not be laid or placed during
raining weather or when the subgrade or base course is
damp or wet unless emulsion is used or normally when
the atmospheric temperature in the shade is 16 C or
below.
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12.  Preparation of underlying layer
 The base on which dense bituminous macadam is to be
laid shall be prepared to the specified dimensions.
The surface shall be thoroughly swept and free from
dust and other materials.
If the base is irregular and wavy then proper corrective
course of adequate thickness is provided to rectify the
profile.
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13.  Application of tack coat or prime coat
4-7.5 kg/10 m2 for black top layer.
7.5-10 kg/10m2 for untreated WBM layer.
The tack coat shall be applied just ahead of the
spreading of premixed macadam.
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14.  Premix Preparation
The binder and aggregates to be used are separately
heated to specified temperature and then mixed
properly.
The mixing temperature for each grade and the bitumen
binder is also specified based on the laboratory result.
The mixture is then carried to the site for its placement
through a transporters or a wheel barrow.
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15.  Spreading of the mix
The mix shall be transported by tipper, truck and
spreading done by mechanical paver.
It should be spread in such a manner that after
compacting required thickness binder course is uniformly
laid.
Minimum time should be spent between the placement
of the mix and the rolling because compacting
temperature also influences the strength characteristics
of the resulting pavement structures.
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17.  Compaction
The mix after spreading shall be compacted by rolling by
a set of rollers at a speed not more than 5km/hr.
Typical roller position used in compaction is:
1. Initial or break down rolling  By 8 to 12 tonnes,three
wheel steel roller
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18. 2. Intermediate rolling  By 15 to 30 tonnes, smooth
wheel pneumatic roller
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19. 3. Finished Rolling  By 8 to 10 tonnes, tandem roller
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20. Wheel of rollers should be kept moist to prevent the mix
from adhering to them. Rolling shall commence from
longitudinally from edge and proceed towards center.
Rolling shall be continued till desired density is achieved.
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21.  Application of seal coat
Opening to traffic
Traffic can be allowed after completion of the final rolling
when the mix has cooled down to the surrounding
temperature.
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