What Is DBM?

Dense Bituminous Macadam (DBM) is a type of road surface course that plays a crucial role in the construction of durable and resilient highways. Comprising a mix of graded aggregates, filler, and bitumen, DBM is designed to provide a strong and wear-resistant layer to withstand the stresses imposed by vehicular traffic. The aggregates used in DBM are carefully selected to achieve high density and excellent interlock, resulting in a compact and stable road surface. The bitumen content binds the aggregates together, enhancing cohesion and preventing the ingress of water, which could otherwise lead to structural damage. DBM serves as an intermediate layer between the coarse base course and the final asphalt layer, contributing to the overall strength and longevity of the road. Its ability to resist deformation and provide a smooth, skid-resistant surface makes Dense Bituminous Macadam a vital component in the construction of high-quality and long-lasting road infrastructure.



Preparation of base

The surface will be thoroughly swept clean free from dust and foreign matter using tractor compressor. Laying shall be suspended while free standing wateris present on the surface to be covered, or during rains, fog and dust storm. Laying of bituminous mixture shall not be carried out when the air temperature at the surface on which it is to be laid is below 10°C for mixes or when the wind speed at any temperature exceeds 40km/h at 2m height. Prime & Tack coat (SS-1/ RS-1) Emulsion with VG-10 Gr. Bitumen) Will be applied as per clause- 502 & 503 of MORT&H 5th revision over WMM and BT surface at the rate of 0.7-1.0,0.25-0.3 & 0.2-0.3Kg/Sqm for WMM and BT surface respectively will be applied through plate test sprayer just on or before laying of Bituminous mix

Preparation and Transportation of Mix

Asphalt mix (Dense Bituminous macadam DBM) will be prepared in the hot mix plant for getting proper and uniform Quality mix. The job mix formula (JMF) will be in accordance with clause- 507 of MORT&H 5th (Ref. Table 500-10 & 500-17 for Grading-1&2). Binder content shall be whiten The +/ 0.3 of JMF OBC Value. The proposed job mix formula to be used in HOT MIX PLANT during the production of BT Mixes with crushed Aggregate (Velgatoor Stone Crusher and Bitumen VG-30 & VG 40 Grade) by wt. of mix for Main Carriage way and service Road

Temperature of binder in case of VG-30 at the time of mixing will be in the range of 150-165°C, Aggregate temparature-150-170°C, Mix temperature-150- 165°C, Laying temperature-Min 140°C and Rolling temperature will be Min 90°C. And for VG-40 at the time of mixing will be in the range of 160°C-170 °C, Aggregate Temperate 160-175C, Bitumen Temperature 160-170C Rolling Temperature Min. 100°C and Laying temperature Min. 150°C. And the difference in temperature between the Binder and Aggregate at no time will exceeds 14°C. Mixing will be done in Hot Mix Plant as per job mix formula, to obtain a homogenous mixture in which all the particles of aggregates are uniformly coated. Regular record of temperature of bitumen, aggregates and mix will be maintained by using the temperature measuring devices attached with plant. Mix temp will be maintained less than 165°C. The mix will be transported form the hot mix plant to the site with suitable tipper vehicles if required specially in case of rainy weather covered with tarpaulin so that mix is not exposed to surrounding environment.

3 Spreading & Compaction:

The transported mix from the hot mix plant to the site will be spread by means of senor asphalt paver finisher to the specified grade, lines and cross-section. However, the mix will be laid manually in the restricted locations and in narrow widths.
The temperature of mix at the time of laying will be minimum 140°C for VG-30. The temperature of the mix at the time of transportation from plant, at the time of reaching at laying site and during laying at site- will be recorded. Bituminous material with a temperature greater than 170°C shall not be laid or deposited at site.
Longitudinal joints and edges will be constructed true to the delineating lines parallel to the centreline of the road.
Longitudinal joints will not be allowed to fall within the wheel path.
All transverse joints will be cut vertically to the full thickness of previously laid mix with asphalt cutter/pavement breaker and the surface painted with emulsion tack before placing fresh material. While starting the day work, previous day joint will be cut vertically and emulsion tack will be applied on the joint.
Longitudinal joints will be painted with emulsion tack (after removing the loose material) before laying adjacent layer. (Total width 8.5for MCW and Total Width 6.for SR) Mtrs will be done at a time
After the spreading of mix, rolling will be done immediately with speed not more than 5km per hour with due care to avoid unduly roughening of the pavement surface.
Initial or break down rolling will be done with 8-10 tonne smooth wheeled tandem roller.
The intermediate and final rolling shall be done with 8-10 tone static weight or vibratory roller up to the required compaction & for unevenness surface.
The finish rolling will be done with 6-8 ton weight smooth wheeled tandem roller. (One pass).

During the initial and finish rolling, no vibratory compaction will be resorted to the wheels of roller will be kept moist to prevent the mix from adhering to them, however no excessive water will be poured on the wheels.
Rolling of the longitudinal joints will be done immediately behind the paving operations. After this, the rolling operations will be commenced longitudinally from edges and preceded towards the center, except that on super elevated and unidirectional cambered portions, where it will be progressed from the lower to upper edge parallel to the center line of the pavement. The roller shall first compact material adjacent to joints and then work from the lower to the upper side of the layer, overlapping on successive passes by at least one-third of the width of the roller or in case of a pneumatic- tired roller, at least the nominal width of 300mm.
Rolling operations will be completed before the temperature of the mix falls below 90°C.
However if required extra passes will be given till the specified density is achieved.
Traffic will be allowed on DBM surface when the mix/surface cooled down to the surrounding temperature.
Density of compacted layer will be checked by tired depth gauge at regular interval the acceptance criteria for Density of DBM Grade I/II at site shall be more than 92% of the density based on theoretical maximum specific gravity (Gmm) as per ASTM D 2041.
Level of surface will be checked as per table 900-1 and surface regularity will be checked as per IRC SP-16-2004 Table-2.
Note: Loose thickness of layer (At site stretch) i.e 25-30% of compacted required thickness for DBM & BC layer respectively.