Quick Summary: Dense Bituminous Macadam (DBM) is a dense-graded hot mix used as a binder/base course. This article summarises the material requirements, mix design criteria, MoRTH Tables (500-9 to 500-13), and the Top 8 Proven Steps for execution and quality control as per MoRTH Clause 507.

Quick Reference — DBM (MoRTH 507)

Applicable Clause: MoRTH Clause 507 (DBM / Base / Binder Course)
Nominal Sizes: 37.5 mm (Grading 1) / 26.5 mm (Grading 2)
Compacted Thickness: 50–100 mm (single layer) or as per design
Bitumen: VG-30 / VG-40 or PMB as per Contract
Min. Binder Content: Table 500-10 (Min 4.0% for Grading 1; Min 4.5% for Grading 2)
Field Density: ≥ 98% Marshall density (or ≥92% Gmm as per contract)

Scope

This DBM construction methodology provides details of mandatory requirements for construction in single or multiple layers of Dense Bituminous Macadam (DBM) on a previously prepared Wet Mix Macadam base. The thickness of a single layer shall be between 50 mm to 100 mm or as per approved design and drawings.

Codal Provisions

Contract Agreement
IRC: SP: 84-2014
MoRTH Specifications for Road & Bridge Works (5th Revision)
Relevant Drawings

Plant & Equipment Required

The following major plants and machinery shall be deployed for the construction of bituminous works to ensure uniform quality and efficient execution:

Stone Crusher: For producing well-graded aggregates as per MoRTH specifications.
Hot Mix Plant (Batch Type – Recommended): For precise proportioning and uniform mixing of aggregates and bitumen.
Batching Plant & Trial Setup: To ensure calibration and verification of mix proportions before regular production.
Sensor Paver with String Line Arrangement: For achieving required layer thickness, line, and level accuracy.
Steel Tandem and Pneumatic Rollers: For effective compaction and attainment of desired density.
Bitumen Sprayer: For uniform application of tack coat or prime coat.
Dumpers, Air Compressor, and Mechanical Broom: For material transport, cleaning of surfaces, and general site operations.

Additional instruments: Thermometers (0–250°C) for temperature monitoring, thickness gauges for layer measurement, a survey kit for level and alignment checks, and a core cutting machine equipped with 100 mm and 150 mm diameter bits for sampling and density verification.

Requirement of Materials

Material	Requirement / Specification
Bitumen	VG-30 or VG-40 (or PMB as per CA / IS 73 / IS 15462). Grade as per contract / climatic requirements.
Coarse Aggregates	Crushed rock / crushed gravel retained on 2.36 mm; clean, durable, cubical; satisfy LA abrasion/AIV, flakiness & elongation limits.
Fine Aggregates	Passing 2.36 mm, retained on 75 µ; sand equivalent ≥ 50; PI ≤ 4.
Filler	Rock dust / hydrated lime / cement approved by AE; grading per Table 500-9.
Quality Tests	IS:2386 (parts), IS:6241 (coating), AASHTO T283, IS:2720 series, Marshall stability (MS-2 / ASTM).

Note: Submit material source & test reports for AE approval prior to use.

Table 500-9: Grading Requirements for Mineral Filler

IS Sieve (mm)	Cumulative % Passing
0.6	100
0.3	95–100
0.075	85–100

In bituminous mixes, filler refers to fine material (passing 0.075 mm sieve) added to fill the voids in the mineral aggregate and improve the stability, density, and binding of the mix.

Common Fillers Used in Bituminous Mixes

Ordinary Portland Cement (OPC)
Lime (Hydrated or Quicklime)
Stone Dust / Crusher Dust
Fly Ash
Baghouse Dust
Rock Dust / Mineral Filler

Specification Reference: As per MoRTH Clause 507.2.4 & IS: 73.

Table 500-10: Composition of Dense Graded Bituminous Macadam

Grading	1 (Nominal 37.5 mm)	2 (Nominal 26.5 mm)
Layer thickness (mm)	75–100	50–75
IS sieve 45 mm	100	100
IS sieve 37.5 mm	95–100	100
IS sieve 26.5 mm	63–93	90–100
IS sieve 19 mm	-	71–95
IS sieve 13.2 mm	55–75	56–80
IS sieve 4.75 mm	38–54	38–54
IS sieve 2.36 mm	28–42	28–42
IS sieve 0.3 mm	7–21	7–21
IS sieve 0.075 mm	2–8	2–8
Bitumen content (% by mass)	Min 4.0*	Min 4.5*

The bitumen content is based on aggregates with a specific gravity of 2.7. Adjust for aggregate specific gravity and climatic conditions as required by MoRTH.

Table 500-11: Requirements for Dense Graded DBM (Marshall / Modified Marshall)

Property	Modified Bitumen (Hot climate)	Modified Bitumen (Cold climate)	Test Method
Compaction Level	75 blows on each face of specimen
Minimum Stability (kN)	12.0	10.0	AASHTO T245
Marshall Flow (mm)	2.5–4	3.5–5	AASHTO T245
Marshall Quotient	2.5–5	—	MS-2 / ASTM
% Air Voids	3–5%	3–5%
% VFB	65–75%	65–75%
Coating of Aggregate	95% min — IS:6241
TSR	≥80% — AASHTO T283

Comparison of Standard and Modified Marshall Methods

Parameter	Standard Marshall Test	Modified Marshall Test (ASTM D5581 / MS-2)
Applicable Aggregate Size	Up to 26.5 mm	Up to 38 mm (1.5 inch)
Specimen Diameter	101.6 mm (4 inch)	152.4 mm (6 inch)
Specimen Height	63.5 ± 1.3 mm	95.2 ± 1.3 mm
Hammer Weight	4.5 kg	10.2 kg (mechanical only)
No. of Blows per Face	50 or 75 blows	1.5× standard (75 or 112 blows)
Minimum Stability Criteria	As per MoRTH Table 500-12	2.25 × standard value
Flow Value	2–4 mm	3–6 mm (1.5× range)

The Modified Marshall Method is adopted for bituminous mixes containing aggregates up to 38 mm nominal size (such as DBM Grading 1).

Table 500-13: Permissible JMF Variations

Parameter	Permissible Variation
Aggregate passing 19 mm or larger	±8%
Aggregate passing 13.2 / 9.5 mm	±7%
Aggregate passing 4.75 mm	±6%
Aggregate passing 2.36 / 1.18 / 0.6 mm	±5%
Aggregate passing 0.3 / 0.15 mm	±4%
Aggregate passing 0.075 mm	±2%
Binder content	±0.3%
Mixing temperature	±10°C

MoRTH allows tolerances in production to ensure performance — see Table 500-13 for values.

Table 500-2: Mixing, Laying and Rolling Temperatures for Bituminous Mixes (°C)

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

Note: Rolling must be completed before the mat cools below the minimum rolling temperature indicated above. The difference between binder and aggregate temperatures should not exceed 14°C.

Table 500-12 — Minimum Percent Voids in Mineral Aggregate (VMA)

Nominal Max Particle Size (mm)	Min VMA @ 3.0%	Min VMA @ 4.0%	Min VMA @ 5.0%
26.5	11.0	12.0	13.0
37.5	10.0	11.0	12.0

Interpolate VMA for intermediate air voids values. Example: if air voids = 3.5% between 3.0 and 4.0, interpolate linearly.

Construction Operation — Top 8 Proven Steps

Step 1 — Weather & Planning: Follow MoRTH Clause 501.5.1 for seasonal/weather limitations.
Step 2 — Preparation of Base: Ensure underlying layer is prepared, prime/tack coat applied.
Step 3 — JMF Approval & Plant Trials: Produce and submit Job Mix Formula (JMF) to AE and perform plant trials.
Step 4 — Mixing & Temperatures: Mix in batch-type plant; maintain design temperatures ±10°C.
Step 5 — Transportation & Laying: Transport mix in covered tippers; lay with sensor-controlled paver.
Step 6 — Compaction: Begin compaction immediately using steel tandem rollers.
Step 7 — Field Density & Quality Checks: Take cores/NDT; ensure minimum density ≥98% of Marshall.
Step 8 — Surface Finish & Acceptance: Verify surface evenness, check for segregation, flush/tender spots.

Joints

Transverse Joints

When the laying of the mixture is to be suspended long enough to permit the mixture to become chilled, a transverse joint shall be constructed. All transverse joints shall be cut vertically to the full thickness of the previously laid mix using an asphalt cutter or pavement breaker, and the surface shall be painted with hot bitumen before placing fresh material.

The joint shall be constructed square to the lane alignment, and all excess materials shall be removed and cut back into the previously constructed pavement to the point of full pavement depth. The exposed edge of the previously constructed pavement shall then be lightly coated with tack coat.

Longitudinal Joints

The paver used covers the required width, so typically there will be no longitudinal joints. However, if longitudinal joints are necessary, the edges shall be constructed true to the delineating line parallel to the center line of the road. All joints shall be cut vertically to the full thickness of the previously laid mix and the surface painted with hot bitumen before placing fresh material.

Longitudinal joints shall preferably be hot joints. Cold longitudinal joints shall be properly heated with a joint heater to attain a suitable temperature before laying the adjacent material.

Surface Finish and Quality of Work

Final surface finish must meet MoRTH Clause 902 tolerances. Verify compaction, evenness and absence of segregation before acceptance.

Highway Construction Quality Control Tests | DBM & Bituminous Pavement

Highway Construction Quality Control Tests

A. Binder Tests

S. No.	Characteristic	Purpose	Test Method (IS Code/Standard)	Typical Requirement (VG30)	Test Frequency
i	Absolute Viscosity @ 60°C	Measures resistance to flow at high pavement temperatures	IS 1206 (Part 2)	2400 – 3600 Poises	Per lot / consignment
ii	Kinematic Viscosity @ 135°C	Ensures proper coating during mixing and compaction	IS 1206 (Part 3)	350 cSt (min.)	Per lot / consignment
iii	Penetration @ 25°C	Measures hardness under standard load	IS 1203	45 mm (min.)	Per lot / consignment
iv	Flash Point (COC)	Safe heating limit to prevent fire	IS 1209	220°C (min.)	Per lot / consignment
v	Solubility in Trichloroethylene	Checks purity of bitumen	IS 1216	99.0% (min.)	Per lot / consignment
vi	RTFO Residue Tests	Measures short-term aging resistance	IS 9382	Viscosity Ratio ≤ 4; Ductility ≥ 40 cm	Per lot / consignment
vii	Elastic Recovery	Fatigue resistance	IS 15462	50–75% (min.)	Per lot / consignment
viii	Separation Test	Storage stability of modifier	IS 15462, App. A	Softening Point difference ≤ 2.5°C	Per lot / consignment
ix	Softening Point	Temperature susceptibility & rutting resistance	IS 1205	50–75°C (depending on grade)	Per lot / consignment

B. Aggregate Tests

S. No.	Test	Purpose	Minimum Frequency
ii	Aggregate Impact Value / Los Angeles Abrasion	Aggregate strength	1 test per 350 m³ per source or on quality change
iii	Flakiness & Elongation Indices	Shape of aggregate	1 test per 350 m³ per source or on quality change
iv	Soundness (Na₂SO₄ / MgSO₄)	Weathering resistance	1 test per source or on quality change
v	Water Absorption	Checks porosity	1 test per source or on quality change
vi	Sand Equivalent	Fines content / cleanliness	1 test per source or on quality change
vii	Plasticity Index	Soil plasticity	1 test per source or on quality change
viii	Polished Stone Value	Skid resistance	1 test per source or on quality change
ix	% Fractured Face	Aggregate angularity	1 test per 350 m³ when crushed gravel is used

C. Bituminous Mix Tests

S. No.	Test	Purpose	Minimum Frequency
x	Mix Grading	Gradation of mix	1 set per 400 t of mix, minimum 2/day/plant
xi	Stability & Voids Analysis	Strength, flow, density, void content	3 tests per 400 t of mix, minimum 2/day/plant
xii	Moisture Susceptibility (MSHTO T283)	Resistance to moisture damage	1 test per mix type or on aggregate/source change
xiv	Binder Content	Correct proportion in mix	1 set per 400 t of mix, minimum 2/day/plant

D. Process Control & Field Tests

S. No.	Test	Purpose	Frequency
xiii	Temperature (Binder, Aggregate, Mix)	Ensure proper mixing and compaction	At regular intervals
xv	Rate of Spread	Proper layer thickness in field	After every 5th truck load
xvi	Density of Compacted Layer	Ensures compaction meets specification	1 test per 700 m²

💡 Frequently Asked Questions – Dense Bituminous Macadam (DBM)

1. What is Dense Bituminous Macadam (DBM)?

DBM is a dense-graded bituminous mix used as a binder course in flexible pavements, placed below the Bituminous Concrete (BC) layer. It provides structural strength and distributes traffic loads to the lower layers.

2. What are the key materials used in DBM construction?

The main materials include crushed aggregates, filler, and VG-30 or VG-40 bitumen. Sometimes, modified bitumen (PMB/CRMB) is used for higher traffic conditions.

3. What is the typical thickness of DBM layers?

The DBM layer is generally laid in thicknesses of 50 mm to 100 mm, depending on the pavement design and traffic category.

4. What are the key quality control tests for DBM?

Major QC tests include: aggregate impact/abrasion value, flakiness index, binder content, mix grading, stability and flow, density, and temperature control during mixing and compaction (as per MoRTH 507).

5. What are the compaction requirements for DBM?

The target field density should not be less than 98% of the Marshall density. Proper rolling with tandem, pneumatic, and vibratory rollers is essential for achieving uniform compaction.

6. What is the minimum temperature requirement during DBM laying?

The mix temperature at the time of laying should be around 140–160°C, and compaction should begin before it drops below 120°C. Binder temperature should be checked at regular intervals.

7. What MoRTH clause governs DBM construction?

Dense Bituminous Macadam is governed by Clause 507 of MoRTH Specifications (5th Revision), covering materials, mix design, laying, compaction, and quality control requirements.