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
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.
