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dense bituminous macadam DBM

Dense Bituminous Macadam (DBM) – Grading, Testing and Method of Construction

Dense Bituminous Macadam(DBM) is a binder course used for roads and highway works with a greater number of heavy commercial vehicles and a close-graded premix material vehicle and a close-graded premix material having a voids content of 5-10 percent.

Function

  • 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.
  • Should be able to withstand the higher shear stress that occur below the bearing course.

Material Requirement for Dense Bituminous Macadam (DBM)

Bitumen (Binder):

  • Viscosity grade bitumen-VG10, VG20, VG30, VG40 etc.
  • Modified Bitumen- Polymer Modified Bitumen (PMB), Crumbled Rubber Method Bitumen (CRMB) etc.

Coarse Aggregate:

  • Coarse aggregate shall consist of crushed rock or crushed gravel.
  • Separate stock pile for DBM purpose to be maintained for each size of aggregate.

Fine Aggregate:

  • Fine aggregate shall consist of crushed or naturally occurring material or a combination of two.

Filler:

  • Filler shall be consisting of finally divided mineral matter such as rock dust, hydrated lime of cement.

Anti-Stripping Agent:

  • Anti-stripping available either in liquid form or powder form (hydrated lime, cement, rock filler). It will improve the moisture sensitivity of aggregates.
  • Anti-striping agent is the special item and the codes requirement to be satisfied before use.
  • 10 min test to be performed as per ASTM D3625 to evaluate the stripping of aggregate.

Grading Requirement for Dense Bituminous Macadam (DBM)

Grading I II
Nominal Aggregate Size 37.5 26.5
Layer Thickness 75-100 50-75
OBC Min 4.0% Min 4.5%
IS Sieve (mm) Cumulative % by weight of total aggregate passing Cumulative % by weight of total aggregate passing
45 100
37.5 95-100 100
26.5 63-93 90-100
19 71-95
13.2 55-75 56-80
4.75 38-54 38-54
2.36 28-42 28-42
0.3 7-21 7-21
0.075 2-8 2-8
  • In case aggregate have specific gravity more than 2.7, minimum bitumen content can be reduced proportionally.

Grading Requirement for DBM

Requirements of Fines in DBM (MORTH V):

Layer Thickness Fines Requirements
(75 mic passing)
Remarks
75-100 mm 2-8 % Grading 1
(MORTH V Table 500-10)
50-75 mm 2-8 % Grading 2
(MORTH V Table 500-10)

Plasticity Index≤ 4%                                      Sand Equivalent Value < 50%

Layer Thickness Requirement (300mic passing) Remarks
75-100 mm                 7-21 % Grading 1 (MORTH V Table 500-10
50-75 mm                 7-21 % Grading 2 (MORTH V Table 500-10

Plant & Machinery Required

  • Hot mix plant
  • Asphalt Paver
  • Tandem Roller
  • Pneumatic Tired Roller
  • Prime/Tack Spray Tanker
  • Survey Instrument

Tests and Frequency of testing for Bitumen

Test Specification Ref. Code Frequency
Absolute Viscosity 2400-3600 IS 1206 Part 2 1 test per lot
Kinematic Viscosity Min 350 IS 1206 Part 3 1 test per lot
Softening Point Min 47 IS 1205 1 test per lot
Penetration Test Min 45 IS 1203 1 test per lot
Flash Point Min 220 IS 1448 Part 69 1 test per lot
Ductility Min 40 IS 1208 1 test per lot
Viscosity Ratio at 60° Max 4 IS 1206 Part 2 1 test per lot

Acceptance Criteria & Frequency

Test Specification Ref. Code Frequency
Mix Grading Within JMF IS 2386 Part 1 1 Test/400 MT
Combined EI & FI Max 35% IS 2386 Part 1 1 Test/350m³
Los Angles Value Max 35% IS 2386 Part 4 1 Test/350m³
Aggregate Impact Value Max 27% IS 2386 Part 4 1 Test/350m³
Soundness Test Max 12% Na2SO4 Max 18% Mg2SO4 IS 2386 Part 5 1 Test for Each Source/Source change
Sp. Gravity & Water Absorption WA- Max 2% IS 2386 Part 3 1 Test for Each Source/Source change
Stripping Value Min 95% IS 6241 1 Test for Each Source/Source change
Sand Equivalent Test Min 50% IS 2720 Part 37 1 Test for Each Source/Source change
Plasticity Index Max 4% IS 2720 Part 5 1 Test for Each Source/Source change

Acceptance Criteria for DBM & Frequency

Test Specification Ref. Code Frequency
Marshal Stability Min 9.0 Is 2386 part 1 1 test/400 MT
Marshal Flow 2-4 Is 2386 part 1 1 test/400 MT
Marshal Quotient 2-5 Is 2386 part 4 1 test/400 MT
% Air Voids 3-5 Is 2386 part 4 1 test/400 MT
% Void filled with Bitumen 65-75 Is 2386 part 5 1 test/400 MT
Tensile Strength Ratio Min 80% IS 6241 One test per mix
% Voids in mineral Aggregate Table 500-13 AASTHO T283 1 test/400 MT
Fines to Bitumen Ratio 0.6-1.2 MORTH V & MS2 1 test/400 MT
Optimum Bitumen Content OBC±0.3% MORTH V 1 test/400 MT
Rate of spread of mix MORTH V After every 5th truck load
Density of Compaction 92% of GMM MORTH V 1/700 sqm

Construction of DBM Layer

Construction of Dense Bituminous Macadam (DBM) layer is to be carried out by following activities-

Preparation of WMM surface to receive Prime Coat:

Approved WMM surface shall be swept clean by power brooms or mechanical sweepers. All the loose particles to be removed.

Application of Prime Coat:

  • Primer shall be Slow setting emulsion SS1 and it shall be sprayed uniformly at the rate of 0.7-1.0 kg/sqm.
  • Primer shall not be sprayed during heavy wind, rainy, foggy or when the ambient temperature is less than 10°c.
  • Primed surface shall be cured for at least 24 hrs.

Application of Tack Coat: 

  • Tack coat shall be Rapid setting RS1.
  • Surface to receive tack coat shall be clean and free from dust.
  • Tack coat shall not be sprayed during heavy wind, rainy, foggy or when the ambient temperature is less than 10°C.
  • Tack coat spraying temperature shall be 20°-70° C.
  • Rate of spray shall be 0.25-0.3 Kg/sqm.
  • Tack coat shall be cured until all the volatiles have evaporated.

Weather limitations for DBM laying:

  • Rain, fog or dust.
  • When the surface is wet.
  • Air temperature on the surface is less than 10°C.
  • Wind speed exceeds 40 kmph at 2m height.

Cleaning of Surface:

  • Surface shall be cleaned of all loose particles by air jet.

Mixing and Transporting:

  • Bituminous mix shall be prepared In hot mix plant of adequate capacity and appropriate temperature to be maintained for Aggregates, bitumen, mix temperature.
  • Normally, batch mixing time of plant begins with 30 sec mix cycle.
  • Bituminous material shall be transported in cleaned and Tarpaulin covered vehicle.

Mixing, Laying and Rolling Temperature for Bituminous Mixes 

Bitumen Viscosity Grade Bitumen Temp. (°C) Aggregate Temp. (°C) Mixed Material Temp. (°C) Laying Temp. (°C) Rolling Temp. (°C)
VG-40 160-170 160-175 160-170 150 min 100 min
VG-30 150-165 150-170 150-165 140 min 90 min
VG-20 145-165 145-170 145-165 135 min 85 min
VG-10 140-160 140-160 140-160 130 min 80 min

Precautions to be taken at Hot Mix Plant:

  • Calibrated Dial thermometer shall be installed at bitumen storage.
  • Insulation shall be provided for bitumen storage tank.
  • Frequency chart as per client requirement shall be at plant:

Aggregate ±1.5%    Bitumen  ±0.1%   Mineral Filler  ±0.5%

  • HMP shall not run more than rated speed.
  • Batch mixing time shall be 30 seconds.
  • Temperature of aggregates, bitumen and mix displaying at control panel shall be maintain as per (MORTH V Table 500-2).

Laying of DBM:

  • Metallic strings supported on pegs and arms to be used as level reference for paver. Interval between pegs shall be 10m.
  • Before commencing paving operations, paver parts to be completely checked. Like screed heater, auger, sensor etc.
  • Tire pressure of Pneumatic tired roller shall be maintained 0.56kg/cm².
  • Water Sprayer to be checked for adequate holes before spraying.
  • Temperature of the mix shall be checked before laying into the paver.
  • A thumb rule for determining the thickness of the loose mat is multiply the desired compacted thickness by 1.25 (Ref MS4 pg 265).

Compaction:

  • Once the material is spread by paver, compaction to start immediately and shall be completed within the specified rolling temperature.
  • Rolling of longitudinal joints shall be done immediately behind the paving operations.
  • After this, rolling shall start at the edges and progress towards the centre.
  • Tyre pressure of PTR shall be at least 0.56 Mpa.
  • Three types of rolling is used to achieve the required compaction

             1.Initial or breakdown rolling 2. Intermediate rolling 3. Finish rolling

  • Sample rolling pattern for DBM layer (1 plain pass + 2 Heavy vibration+ 1 Low vibration+ 3PTR+ 1Plain Pass)
  • Initial rolling shall be done with 8-10 T static wheel rollers.
  • Intermediate rolling shall be done with 8-10T static or 12-15T PTR.
  • Finish rolling shall be done with 6-8T smooth wheel tandem rollers.

 Joints:

  • There are two types of joints involved in DBM construction
  • Longitudinal       2) Transverse Joints
  • All joints shall be cut vertical to the full thickness of previously laid mix.
  • Vertical face to be coated with suitable viscosity grade bitumen.
  • While spreading the material along the joint, the material shall overlap 25-50mm on the previously laid mix beyond the vertical face of the joint.
  • Transverse joint begins at the end of the day’s paving operation.
  • At the end of day, the layer end has to be shape in the form of ramp. The end portion of layer shall be sawed on the next day before paving operation begin.

Surface Finish:

  • The surface of DBM layer shall not vary from ±6mm.
  • The longitudinal profile shall be checked with 3m long straight edge at the middle of each traffic lane along a line parallel to the centre line of the road.
  • Density of DBM layer shall be checked by taking cores after 24 hrs laying.
  • Min. load density equal to or more than 92% of the theoretical maximum sp. Gravity (Gmm).

Productivity

  • DBM Productivity             :            21000-25000 MT/Month/Team/Day
  • Paver Productivity            :            50 m³/hr
  • Roller Productivity            :            40 m³/hr