(Based on RDA Standard Specifications for Construction & Maintenance of Roads – commonly used by ICTAD/CIDA projects)
(a) Asphalt Concrete Wearing Course – AC 14
IS Sieve (mm)
% Passing
19.0
100
14.0
90 – 100
10.0
70 – 90
6.3
50 – 70
4.75
35 – 55
2.36
20 – 40
1.18
15 – 30
0.600
10 – 25
0.300
7 – 20
0.075
4 – 8
Binder content: 5.0 – 6.0 %
(b) Asphalt Concrete Binder Course – AC 20
IS Sieve (mm)
% Passing
26.5
100
19.0
90 – 100
14.0
65 – 85
10.0
50 – 70
6.3
35 – 55
4.75
25 – 45
2.36
15 – 30
0.075
3 – 7
Binder content: 4.5 – 5.5 %
(c) Dense Bituminous Macadam (DBM – 37.5 mm)
IS Sieve (mm)
% Passing
45.0
100
37.5
95 – 100
26.5
60 – 80
19.0
40 – 60
13.2
30 – 50
4.75
15 – 25
0.075
2 – 6
2. Typical BOQ Descriptions (RDA / Provincial Council Style)
(a) Asphalt Concrete Wearing Course (AC 14)
Providing and laying Asphalt Concrete Wearing Course (AC 14) of compacted thickness 40 mm, produced at approved hot mix plant using 60/70 penetration grade bitumen, laid and compacted to not less than 98% Marshall density, including heating, mixing, transporting, laying, rolling, joint formation, testing, all materials, labour, plant, and incidentals complete as per RDA specifications.
Unit: m² or m³
(b) Asphalt Concrete Binder Course (AC 20)
Providing and laying Asphalt Concrete Binder Course (AC 20) of compacted thickness 60 mm, including tack coat, temperature control, compaction to minimum 98% of Marshall density, complete as per RDA specifications.
(c) Tack Coat
Providing and spraying bitumen emulsion (CSS-1 / RS-1) at a rate of 0.25–0.30 kg/m², including cleaning of surface, complete.
3. Worked Marshall Mix Design Example (Simplified)
Given:
Mix type: AC 14
Traffic: Medium to heavy
Compaction: 75 blows per face
Trial Bitumen Contents
4.5%, 5.0%, 5.5%, 6.0%
Test Results (Typical)
Bitumen %
Stability (kN)
Flow (mm)
Air Voids (%)
4.5
11.5
2.1
5.5
5.0
13.2
2.4
4.3
5.5
14.0
2.8
3.8
6.0
12.8
3.3
3.0
Selection of Optimum Bitumen Content (OBC)
Max stability ≈ 5.5%
4% air voids ≈ 5.2%
Acceptable flow ≈ 5.3%
✅ OBC = 5.3%
Final Mix Properties @ OBC
Stability ≥ 9.0 kN ✔
Flow: 2–4 mm ✔
Air Voids: 3–5% ✔
VMA ≥ 14% ✔
4. Comparison Table – AC vs DBST vs SMA
Item
Asphalt Concrete (AC)
DBST
Stone Mastic Asphalt (SMA)
Structure
Dense graded
Surface dressing
Gap graded
Thickness
40–60 mm
10–20 mm
40–50 mm
Cost
Medium
Low
High
Durability
High
Medium
Very High
Rut Resistance
Good
Poor
Excellent
Traffic
Medium–Heavy
Low–Medium
Heavy
Life (years)
8–12
4–6
12–15
Maintenance
Moderate
Frequent
Low
5. Inspection Checklist for Site Engineers (Very Practical)
Before Laying
✔ Approved Job Mix Formula (JMF)
✔ Aggregate & bitumen test reports
✔ Clean, dry base
✔ Correct tack coat rate
✔ Weather suitable (no rain)
During Mixing & Transport
✔ Mixing temperature: 150–165°C
✔ Bitumen temperature: 140–160°C
✔ No segregation
✔ No excessive smoke/burning
During Laying
✔ Laying temperature ≥ 140°C
✔ Correct thickness using screed
✔ Proper longitudinal & transverse joints
✔ No cold joints
Compaction
✔ Breakdown rolling immediately
✔ Finish rolling before temp < 90°C
✔ Density ≥ 98% Marshall
✔ No roller marks/cracks
After Laying
✔ Surface evenness (3 m straight edge)
✔ Core density checks
✔ Thickness verification
✔ No bleeding / segregation
✔ Opening to traffic after cooling
