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DBM (Dense Bituminous Macadam) specificatins,layer thickness, physical requirements, aggregate size, grades and its measurements


DBM acts as a bituminous  base course and thickness of a single compacted  layer shall be 50 mm to 100 mm. 

In this article we shall discuss about
Technical specifications of DBM (Dense Bituminous Macadam), layer thickness, physical requirements, aggregate size, grades and its measurements

A-Scope: The work shall consist of construction in a single layer of DBM on a previously prepared base or sub-base. The thickness of a single layer shall be 50 mm to 100 mm.

B - Material: The choice of binder material shall be stipulated in the contract or by the engineer, where viscosity grades of bitumen are specified, they are referred to by a designation in accordance with IS: 73. Where modified bitumen is specified, it shall conform to the requirements of IRC:SP:53 and IS:15462.

Selection criteria for viscosity grade bitumen based on highest and lowest daily mean temperatures at a particular site are ]given in table no -1.

1-Coarse Aggregates: - It shall consist of crushed rock, crushed gravel or other hard material retained on the 2.36mm sieve. They shall be clean, hard, and durable, of cubical shape, free from dust and soft or friable matter, organic or other deleterious matter.
Where crushed gravel is proposed for use as aggregate not less than 90% by weight of the crushed material retained on the 4.75mm sieve shall have at least two fracture faces, except that in case of bituminous concrete the requirement in this regard shall be 95%.

2- Fine Aggregates- it shall consist of crushed or naturally occurring material, or a combination of the two, passing 2.36 mm sieve and retained on the 75micron sieve. They shall be clean, hard, durable, dry and free from dust and soft or friable matter, organic or other deleterious matter. Natural sand shall not be allowed in binder course. However, natural sand up to 50% of the fine aggregates may be allowed in base courses. The fine aggregate shall have a sand equivalent value of not less than 50 when tested in accordance with the requirement of IS:2720 (Part 37). The plasticity index of the fraction passing the 0.425mm sieve shall not exceed 4, when tested in accordance with IS:2720 (Part 5).

3-Filler: Filler shall consist of finely divide mineral matter such as rock dust, hydrated lime or cement approved by engineer. The filler shall be grade within the limits indicated in table no.-2.
The filler shall be free from organic impurities and have a plasticity index not greater than 4, The PI requirements shall not apply if filler is cement or lime. Where the aggregates fail to meet the requirements of the water sensitivity test in table no.-3, then 2% by weight of aggregates, of hydrated lime shall be used and percentage of the fine aggregate reduced accordingly.

** if the min. retained tensile test strength falls below 80. Use of antistripping agent is recommended to meet the requirements.

4-Aggregates grading and Binder content (Grades of DBM)

When tested in according with IS:2386 Part 1(wet sieving method. The combined grading of the coarse and fine aggregates and filler for the particular mixture shall fall within the limits given in table no. -4  for grading 1or 2 as specified in the contract. To avoid gap grading, the combined aggregates gradation shall not vary from the lower limit on one sieve to higher limit on the adjacent sieve.

  Grades of DBM
Nominal size of aggregate - 37.5 mm
Layer thickness= 75-100 mm
Bitumen content % by mass of total mix should not be less than 4%.
Nominal size of aggregate - 26.5 mm
Layer thickness= 50-75 mm
Bitumen content % by mass of total mix should not be less than 4.5%.           

*    The nominal max. particle size is the largest specified sieve size upon which any of the aggregate is retained.

**    Corresponds to specific gravity of 
aggregate being 2.7. in case aggregate have sp. gravity more than 2.7, the min. bitumen content can be reduced proportionally. Further the region where highest daily air temperature is 30°C or lower and lowest daily air temperature is -10°C or lower, the bitumen content may be increased by 0.5%.
Bitumen content indicate in table no - 4 is the minimum quantity. The quantity shall be determined in accordance with clause(
C - Mix Design.)

C - Mix Design
The bitumen content required shall be determined following the Marshall mix design procedure contained in Asphalt Institute Manual MS-2.

The Fines to Bitumen (F/B) ratio by weight of total mix shall range from 0.6 to 1.2.

(1) Requirements for the Mix
 Apart from conformity with the grading and quality requirements for individual ingredients, the mixture shall meet the requirements set out in  table no. -5 (Table 500-11 as per MORTH 5th revision).

                       Table-5 Requirements for Dense Graded Bituminous Macadam

(2) Binder Content

The binder content shall be optimized to achieve the requirements of the mix set out in Table-5. The binder content shall be selected to obtain 4 percent air voids in the mix design. The Marshall method for determining the optimum binder content shall be adopted as described in the Asphalt Institute Manual MS-2.

Where maximum size of the aggregate is more than 26.5 mm, the modified Marshall method using 150 mm diametre specimen described in MS-2 and ASTM D 5581 shall be used. This method requires modified equipment and procedures. When the modified Marshall test is used, the specified minimum stability values in table-6 (Table 500-12 as per MORTH 5th revision)  shall be multiplied by 2.25, and the minimum flow shall be 3 mm.

            Table - 6 Minimum Percent Voids In Mineral Aggregate (VMA)

(3) Job Mix Formula

The Contractor shall submit to the Engineer for approval at least 21 days before the start the work, the job mix formula proposed for use in the works, together with the following details:
i) Source and location of all materials;
ii) Proportions of all materials expressed as follows:
a) Binder type, and percentage by weight of total mix;
b) Coarse aggregate/Fine aggregate/Mineral filler as percentage by weight of total aggregate including mineral filler;
iii) A single definite percentage passing each sieve for the mixed aggregate;
iv) The individual gradings of the individual aggregate fraction, and the proportion of each in the combined grading;
v) The results of mix design such as maximum specific gravity of loose mix (Gmm), compacted specimen densities, Marshall stability,flow, air voids, VMA, VFB and related graphs and test results of AASHTO T 283 Moisture susceptibility test; vi) Where the mixer is a batch mixer, the individual weights of each type of aggregate, and binder per batch;
vii) Test results of physical characteristics of aggregates to be used;
viii) Mixing temperature and compacting temperature. While establishing the job mix formula, the Contractor shall ensure that it is based on a correct and truly represent6tive sample of the materials that will actually be used in the work and that the mix and its different ingredients satisfy the physical and strength requirements of these Approal of the job mix formula shall be based on independent testing by the Engineer for whichsamples of all ingredients of the mix shall be furnished by the Contractor as required by the Engineer.

The approved job mix formula shall remain effective unless and until a revised Job Mix Formula is approved. Should a change in the source of materials be proposed, a new job mix formua shall be forwarded by the Contractor to the Engineer for approval before the placing of thematerial.

(4) Plant Trials — Permissible Variation in Job Mix Formula 

Once the laboratory job mix formula is approved, the Contractor shall carry out plant trials lo establish that the plant can produce a uniform mix conforming to the approved job nix formula. The permissible variations of the individual percentages of the various ingrecients in the actual mix from the job mix formula to be used shall be within the limits as specifed in table - 7( Table 500-13 as per MORTH 5th revision)  and shall remain within the gradation band. These variations are intended to apply to individual specimens taken for quality control tests in accordance with Secticn 900.

               Table -7 Permissible Variations in the Actual Mix from the Job Mix Formula
Description Base/binder Course

(5) Laying Trials
Once the plant trials have been successfully completed and approved, the Contractor shall carry out laying trials, to demonstrate that the proposed mix can be successfully laid and compacted as per relevant standard. The laying trial shall be carried out on a suitable area which is not to form part of the works. The area of the laying trials shall be a minimum of 100 sq.m of construction similar to that of the project road, and it shall be in all respects, particularly compaction, the same as the project construction, on which the bituminous material is to be laid. 

The Contractor shall previously inform the Engineer of the proposed method for laying and compacting the material. The plant trials shall then establish if the proposed laying plant, compaction plant, and methodology is capable of producing satisfactory results. The density of the finished paving layer shall be determined by taking cores, no sooner than 24 hours after laying, or by other approved method. The compacted layers of Dense Graded Bituminous Macadam (DBM) shall have a minimum field density equal to or more than 92% of the density based on theoretical maximum specific gravity (Grnm) obtained on the day of compaction in accordance with ASTM D 2041.

Once the laying trials have been approved, the same plant and methodology shall be applied to the laying of the material on the project, and no variation of either shall be acceptable, unless approved in writing by the Engineer, who may at his discretion require further laying trials.

5.Measurements of pavements-  DBM  material shall be measured as finished work either in cubic metres, tonnes or by the square metre at a specified thickness as  indicated in the contract drawing, or documents, or as otherwise directed by the enginee.

Thanks for reading
Mukesh kumar (Gyanofengineering)


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