Dense Bituminous Macadam Road

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

1. Dense bituminous macadam road - Sarat Mishra 1

2. Introduction Why DBM ? Design Criteria Materials Construction Presentation Outline : 2

3. Introduction 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. 3

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. 4

5. Why DBM ? 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. 5

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. Design Criteria (IRC:94-1996) 6

7. Materials 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: 7

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. 8

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. 9

10. Plants and Equipment Bitumen heating device Hot mix plant Sprayer Mechanical mixer or improvised hand mixer 10

11. Weather and Seasonal Limitations 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. 11

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. Construction Procedure 12

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. 13

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. 14

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. 15

16. Process of spreading of mix 16

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: Initial or break down rolling  By 8 to 12 tonnes,three wheel steel roller 17

18. 2. Intermediate rolling  By 15 to 30 tonnes , smooth wheel pneumatic roller 18

19. 3 . Finished Rolling  By 8 to 1 tonnes , tandem roller 19

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. 20

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. 21

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