Asphalt Concrete (AC) Mix Design

Asphalt Concrete is a controlled mixture of:

• Mineral aggregates (coarse + fine)

• Mineral filler

• Bitumen (binder)

 

The goal of mix design is to get the right balance between:

• Strength & stability

• Durability

• Flexibility

• Workability

• Resistance to moisture and rutting

Main Objectives

A good AC mix must:

• Carry traffic loads without permanent deformation

• Resist cracking and moisture damage

• Provide adequate skid resistance

• Be economical and constructible

Standard Asphalt Concrete Mix Design Methods

1. Marshall Mix Design Method (Most common in Sri Lanka & South Asia)

This is the default method used in most government and provincial road works.

Key Parameters Checked

• Marshall Stability (kN)

• Flow value (mm)

• Air Voids (Va)

• Voids in Mineral Aggregate (VMA)

• Voids Filled with Bitumen (VFB)

• Optimum Bitumen Content (OBC)

Basic Steps

1. Select aggregate gradation

2. Prepare trial mixes with varying bitumen content

3. Compact specimens (75 blows per face for heavy traffic)

4. Test for stability and flow

5. Select an OBC satisfying all criteria

Typical OBC: 4.5% – 6.0% (by weight of total mix)

2. Superpave Mix Design (Advanced / Expressways)

Used mainly for:

• High-speed roads

• Expressways

• Heavy axle load corridors

Features

• Performance-based

• Uses gyratory compactor

• Considers climate and traffic loading

• Uses PG (Performance Grade) bitumen

Design Factors

• Traffic (ESALs)

• Temperature (max & min)

• Rutting and fatigue resistance

3. Hveem Mix Design (Rare in South Asia)

• Uses stabilometer

• More common in parts of the USA

• Less popular today

Types of Asphalt Concrete Mixes

1. Dense Graded Asphalt Concrete (DBM / AC)

Most commonly used mix.

Characteristics

• Well-graded aggregates

• Low air voids

• Good load distribution

Applications

• Binder course

• Wearing course

Examples

• AC 20 (Binder course)

• AC 14 (Wearing course)

2. Asphalt Concrete Wearing Course (AC-WC)

The top layer is exposed to traffic.

Key Requirements

• High skid resistance

• Good surface texture

• Durable against the weatherNominal size

• 10 mm

• 14 mm

3. Asphalt Concrete Binder Course (AC-BC)

An intermediate layer between the base and the wearing course.

Characteristics

• Larger aggregate size

• Higher thickness

• Structural strength provider

4. Stone Mastic Asphalt (SMA)

Premium, high-performance mix.

Characteristics

• High stone-to-stone contact

• High bitumen content

• Uses fibres (cellulose/mineral)

Advantages

• Excellent rut resistance

• Long service life

Applications

• Urban roads

• High traffic intersections

• Expressways

5. Open Graded Asphalt (OGFC)

Characteristics

• High air voids (15–25%)

• Permeable surface

Advantages

• Reduces water spray

• Improves skid resistance

• Reduces noise

Limitations

• Lower durability

• Requires good drainage

6. Dense Bituminous Macadam (DBM)

Often considered a base/binder layer.

Characteristics

• Coarser than AC

• High strength

• Used below wearing course

7. Warm Mix Asphalt (WMA)

Produced at lower temperatures.

Benefits

• Reduced fuel consumption

• Lower emissions

• Better compaction

8. Modified Asphalt Concrete

Uses:

• Polymer Modified Bitumen (PMB)

• Crumb rubber

• Anti-stripping agents

Benefits

• Better fatigue life

• Improved moisture resistance

• Higher rut resistance

Typical Layer Arrangement (Flexible Pavement)

Asphalt Concrete Wearing Course (AC 14)
Asphalt Concrete Binder Course (AC 20 / DBM)
Granular Base
Sub-base
Subgrade

Key Specifications Commonly Checked on Site

• Bitumen content tolerance (±0.3%)

• Temperature at laying (140–160°C for HMA)

• Compaction (≥ 95–98% of Marshall density)

• Surface evenness and thickness

1. Sri Lanka RDA / ICTAD Standard Gradations

(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

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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

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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