Dense Bituminous Macadam is a close-graded premix material with a void content of 5-10 per cent, working as a binder course used for road construction. DBM design should be according to the standard method considering heavy commercial vehicles.
The full-scale laying and compaction of Dense Bituminous Macadam shall be carried out with the help of approved material and mix design proposed for the permanent works using the Plants, Equipment, Machinery and approved methodology.
DBM density mainly depends on the Specific Gravity of aggregates and bitumen content(%), and the range varies from 2.30gm/cc to 2.33gm/cc. Optimum values need to be determined by the Marshal method.
Also, Read: Wet Mix Macadam Construction and Quality Control
Acceptance Criteria of Construction Materials for Dense Bituminous Macadam:
In order to satisfy Design criteria, the DBM mix shall consist of coarse aggregate, fine aggregate and filler in suitable proportions with the required amount of binder content, which is bitumen.
The following acceptance criteria shall be followed during the construction of dense bituminous macadam.
Bitumen for DBM work:
Paving Bitumen of penetration grade S-65 complying with Indian standard specifications for Paving bitumen as described in IS:73 and contract technical specification shall be used.
Specification of Coarse Aggregates for DBM:
- Coarse Aggregates shall be of crushed rock retained on the 2.36 mm sieve.
- Preferably, it should be hydrophobic and of low porosity, as porous aggregates absorb extra bitumen.
- Aggregates shall be as specified in tables 500-8 of MORTH for DBM and satisfy the physical requirements.
Specification of Fine Aggregates for DBM:
- Fine Aggregates shall be of crushed stone passing on a 2.36 mm sieve and retained on a 75.00-micron sieve.
- It should be clean, hard, durable, dry and free from dust and soft or friable matter, organic and other deleterious matter.
- Fine Aggregates shall have a sand equivalent value of not less not than 50 when tested as per IS:2720 part-37, and PI shall not exceed 4.0 when tested following IS:2720 part-5.
- Fine aggregate shall comply with the requirements as per clause 507.2.3 of MORTH.
Filler Specification for DBM:
Filler material shall conform to clause 507.2.4 of MORTH specification and consist of finely divided mineral matter such as cement, rock dust, and hydrated lime.
The filler gradation limits for DBM:
|% Passing by weight
The filler material shall be free from organic contaminants and have a plasticity Index not greater than four. The plasticity index(PI) requirement shall not implement if the filler is cement or lime.
Aggregate Gradation for Dense Bituminous Macadam:
The combined coarse, fine aggregates and filler shall satisfy the grading requirements as indicated in tables 500-9 of MORTH. As used in work, the grading of the aggregate mix shall be a smooth curve within and approximately parallel to the envelope in Table 500-9.
Job Mix Design Requirements for Dense Bituminous Macadam :
In addition to the grading and quality requirements of individual ingredients, the Job Mix shall meet the following requirements as per Table 500-10 of MORTH technical specification.
|Marshall stability with 75 blows
|Minimum 820 kg
|2 to 4 mm
|3% to 5%
|Voids in mineral aggregates (VMA)
|12% to 14%
|Voids in mineral aggregates filled with bitumen (VFB)
|65% to 75%
|Not less than 4%
|Stability to Flow ratio
|205 to 410
|Not less than 75%
|Filler Bitumen ratio
|0.6 to 1.2
The permissible variations of the actual mix from the job mix design shall be within the specified limits of Table 500-13 of MORTH specification.
Prime Coat / Tack Coat for Dense Bituminous Macadam:
It shall be a Bitumen Emulsion of grade- CSS 1h as per ASTM D – 2397 from an approved source (As per clauses 502.2.3 & 503.2.1 of Contract Technical Specification).
Construction Requirement for Dense Bituminous Macadam Laying:
Plants and Equipment for DBM Operation:
The following Plants, Equipment and Machinery shall be deployed for the execution of the DBM works.
- Self-propelled Bitumen pressure sprayer equipped for spraying emulsion of 7000 ltr capacity: 01 No.
- Mobile Air compressor: 01 No.
- Hot Mix Plant of capacity 150 TPH: 01 No.
- Automatic Sensor Paver: 01 No
- Smooth wheel Rollers(Deadweight capacity 8-10MT): 02 Nos
- Pneumatic tyre Rollers(Deadweight capacity 12-15MT): 02 Nos.
- Tipper/Dumpers of capacity. 15mt: Adequate nos as per the site requirement
- Water Tanker (capacity 10,000 Lit): 1 No.
- Survey-related accessories and equipment
- Other accessories as per the site requirements
Site Trials for DBM:
The full-scale laying and compaction trials shall be carried out with the approved mix design of DBM materials proposed for the permanent works using the Plants, Equipment, Machinery and methods proposed.
- The trials shall be done with the consent and under the Engineer’s supervision or his representative’s supervision.
- The trial shall be carried out to demonstrate the suitability of the Plants, Equipment, machinery and work procedures to provide and compact the material to the specified proportions, density and voids content and to confirm the other specified criteria of the finished Asphalt Pavement can be achieved.
- The trial length shall be for the layer’s full construction width and depth.
- The pavement layer trial shall be undertaken before the proposed work to commence full-scale work on that layer.
Data to be recorded during the site trial of DBM:
- The type and grade of bitumen used shall be recorded.
- The composition and grading of the construction material, including bitumen content, should be recorded.
- The temperature of bitumen and aggregate shall be recorded immediately prior to entering the mixer, and the temperature of the mix after discharge from the mixer prior to dispatch from the plant, on commencement of laying, on commencement of compaction and on completion of compaction.
- The type, mass, number of wheels, frequency of vibration and the number of passes of the compaction equipment should be noted as appropriate for each roller type.
- The percentage of air voids in the mix, voids in mineral aggregate (VMA), voids filled with bitumen (VFB), stability and flow should be recorded at the laboratory.
- The air voids achieved at the site, the compacted thickness of the layer, per cent compaction achieved, and any other important, relevant information as required by the Engineer should be recorded.
- The site trial shall be deemed to be successful if all specified sets of results over the range of combined effort meet the specified requirements for the material.
- As recorded in the site trial, the above data shall become the agreed basis on which the particular material shall be provided and processed to achieve the specified requirements.
Surveys and Setting out Works for DBM:
After completion of the Wet Mix Macadam layers as per the construction drawings, the limits of Dense Bituminous Macadam shall be marked as per established grid lines and Bench Marks etc. and as per approved construction drawings.
Preparation of Base for DBM:
- Where DBM is to be laid over the WMM surface, Prime coat shall be applied over the granular surface before laying DBM following provisions of clause 502 of MORTH.
- The tack coat shall be applied over the previously primed surface or even previously laid Bitumen surface before laying DBM in accordance with clause 503 of MORTH.
- The base on which DBM is to be laid shall be prepared, shaped and conditioned to the specified lines, grades and cross-sections following Clause 501 & 920 of MORTH or as directed by the Engineer.
- Loose materials and foreign matters shall be removed by thorough brushing with mechanical brooms and/or blown off by compressed air or other acceptable methods.
- Any surface defect shall be made good as instructed by the Engineer.
Edges Protection Work:
- Edges on the shoulder side or on the side where kerb has not been provided shall be well protected and aligned by fixing angles of size 100 mm x 100 mm properly anchored.
- It shall be ensured that the mix at edges is well compacted and proper line and levels are maintained.