Indian Roads Congress | IRC:37-2018 | Bastukar.in Civil Engineering Reference
1. Design Traffic Calculation
N = 365 × A × [ (1+r)^n - 1 ] / r × F × D × L
Where:
N = Cumulative standard axles (CSA) over design life (in msa)
A = Initial daily commercial vehicles in both directions (CVPD) on the road
r = Annual traffic growth rate (decimal, e.g., 0.075 for 7.5%)
n = Design life in years (15 years for most new roads)
F = Vehicle Damage Factor (VDF) for the vehicle mix
D = Lane distribution factor
L = Lateral distribution factor (generally 0.75 for design lane)
Vehicle Damage Factor (VDF) — IRC:37-2018 Table 1
Vehicle Type
VDF (Indicative)
Rear Axle Config
2-Axle truck (rear single axle)
2.5 – 4.5
Single rear axle
3-Axle truck
3.5 – 6.0
Tandem rear axle
Multi-axle truck (4+ axles)
4.5 – 9.0
Tridem / multi axle
Semi-articulated vehicle (tractor-trailer)
4.0 – 7.0
—
Bus (2-axle)
1.5 – 3.0
Single rear axle
Light commercial vehicle
0.05 – 0.5
—
Standard Axle Load (reference)
1.00
80 kN single axle
Standard Axle: 80 kN (8.16 tonnes) on a single axle with dual wheels. VDF values should be derived from actual axle load surveys; above values are indicative only.
Lane Distribution Factor (D) — IRC:37-2018
No. of Lanes (each direction)
% of total two-way traffic on design lane
Single lane road (total 1 lane)
100%
2-lane single carriageway
75%
4-lane divided (2 lanes each direction)
40%
6-lane divided (3 lanes each direction)
33%
2. Pavement Design Catalog — IRC:37-2018 (Thickness in mm)
Based on subgrade CBR and design traffic (MSA). Layers: GSB = Granular Sub-Base, WMM = Wet Mix Macadam, DBM = Dense Bituminous Macadam, BC = Bituminous Concrete.
CBR (%)
MSA
GSB (mm)
WMM (mm)
DBM (mm)
BC (mm)
Total (mm)
2%
2
200
200
40
25
465
5
200
250
60
25
535
10
200
250
100
40
590
20
200
250
140
40
630
50
200
300
170
50
720
100
200
300
200
50
750
4%
2
150
200
40
25
415
5
150
225
55
25
455
10
150
250
80
40
520
20
150
250
120
40
560
50
150
275
150
50
625
100
150
300
185
50
685
6%
2
150
175
40
25
390
5
150
200
50
25
425
10
150
225
65
40
480
20
150
250
100
40
540
50
150
250
130
50
580
100
150
275
165
50
640
8%
2
150
150
25
25
350
5
150
200
40
25
415
10
150
200
55
40
445
20
150
225
90
40
505
50
150
250
115
50
565
100
150
250
145
50
595
10%
5
150
175
35
25
385
10
150
200
50
40
440
30
150
200
80
40
470
100
150
225
130
50
555
Note: These values are indicative from IRC:37-2018 Appendix-I design catalog. Actual design should use mechanistic-empirical method with IITPAVE software as per IRC:37-2018. DBM Gr-I/II; BC Grade-I (25mm on lower traffic roads). Granular Sub-Base as per MORD/MORT&H specifications.
3. Material Specifications Summary
Layer
Material Spec
Compaction
Min CBR / Strength
IS/IRC Reference
Subgrade
Good quality soil, CBR≥2%
98% MDD (IS 2720 Part 8)
CBR≥2%
IRC:SP-72
GSB (Granular Sub-Base)
Natural gravel/crushed stone, PI<6
98% MDD
CBR≥30%
MORTH Clause 401
WMM (Wet Mix Macadam)
Crushed aggregate, 0–45mm
98% MDD
CBR≥80% (10-day soaked)
MORTH Clause 406
DBM (Dense Bituminous Macadam)
60/70 bitumen + graded aggregate
Min 92% Marshall density
Marshall stability ≥9 kN
MORTH Clause 507
BC (Bituminous Concrete)
60/70 bitumen + aggregate, 19mm max
Min 92% Marshall density
Marshall stability ≥9 kN
MORTH Clause 509
4. Traffic Growth Rate & Design Life
Annual Growth Rate (r)
Formula Factor [(1+r)^15-1]/r
Effective CSA multiplier (15 yrs)
5%
21.58
365 × 21.58 = 7877
6%
23.28
365 × 23.28 = 8497
7%
25.13
365 × 25.13 = 9172
8%
27.15
365 × 27.15 = 9910
7.5% (typical)
26.12
365 × 26.12 = 9534
Design Life: 15 years for new construction (IRC:37-2018). 10 years for overlays/strengthening works.
5. Material Quantity Formula (per lane-km)
Volume of each layer (m³) = Length (1000 m) × Width (3.5 m for single lane) × Thickness (m)
Example for WMM 250mm thick, single lane:
Volume = 1000 × 3.5 × 0.25 = 875 m³
Allow 5–10% compaction factor: Supply = 875 × 1.07 = 936 m³ loose
Bituminous layers (tonnes/lane-km):
Weight = 1000 × 3.75 × thickness(m) × density(kg/m³) / 1000
(DBM density ~2350 kg/m³; BC density ~2400 kg/m³)