amudu: 2026

Saturday, February 7, 2026

Double Bituminous Surface Treatment (DBST)

Double Bituminous Surface Treatment (DBST)—also called Double Surface Dressing—is a widely used low-cost, flexible pavement surfacing technique, especially popular in countries like Sri Lanka for rural and provincial roads .

Here’s a clear, engineering-style explanation you can use for technical notes, estimates, or approvals.

1. What is DBST?

Double Bituminous Surface Treatment is a pavement surfacing method where:

Two layers of bituminous binder
Each followed by a layer of crushed aggregates
are applied one after the other on a prepared base.

👉 It is not a structural layer; it acts as a wearing course and waterproofing layer.

2. Typical Pavement Structure (Top to Bottom)

Second Aggregate Layer (Smaller size)
Second Bituminous Binder Spray
First Aggregate Layer (Larger size)
First Bituminous Binder Spray
Prepared Base Course (ABC / WMM / Existing Road)

3. Materials Used

(a) Bituminous Binder

Commonly used:

Cationic Bitumen Emulsion (CRS-1 / CRS-2)
or
Paving Grade Bitumen (60/70 or 80/100)

(b) Aggregates

First Layer: 12.5 mm or 10 mm crushed aggregates
Second Layer: 6.3 mm or 5 mm crushed aggregates
Clean, angular, well-graded, and dust-free

4. Construction Procedure (Step-by-Step)

Step 1: Surface Preparation

Repair potholes, edge failures
Clean dust and loose material
Ensure dry surface

Step 2: Prime Coat (if required)

Applied on granular base
Helps binder penetration and bonding

Step 3: First Binder Application

Spray bitumen/emulsion uniformly
Typical rate: 1.5 – 1.8 kg/m²

Step 4: First Aggregate Spreading

Spread larger aggregates
Immediately rolled with pneumatic / steel roller

Step 5: Second Binder Application

Applied after first layer sets
Typical rate: 0.9 – 1.2 kg/m²

Step 6: Second Aggregate Spreading

Spread smaller aggregates
Rolling to achieve proper embedment

Step 7: Curing & Opening to Traffic

Excess loose aggregates swept
Light traffic allowed after curing

5. Typical Application Rates (Guide Values)

Item	Approx. Quantity
First Binder	1.5 – 1.8 kg/m²
First Aggregate	0.015 – 0.018 m³/m²
Second Binder	0.9 – 1.2 kg/m²
Second Aggregate	0.010 – 0.012 m³/m²

(Actual rates depend on surface condition and specifications)

6. Advantages of DBST

✅ Low construction cost
✅ Quick execution
✅ Improves skid resistance
✅ Seals surface against water
✅ Suitable for low to medium traffic roads
✅ Easy maintenance

7. Limitations

⚠️ Not suitable for heavy traffic
⚠️ Requires good weather (dry conditions)
⚠️ Surface roughness compared to asphalt concrete
⚠️ Needs proper aggregate quality control