Indian Roads Congress | IRC:58-2015 | Bastukar.in Civil Engineering Reference
1. Modulus of Subgrade Reaction (k) vs CBR
k-value from plate load test (IRC:58-2015 Annex A) or estimated from CBR:
Soaked CBR (%)
k (MPa/m) — Subgrade only
k (MPa/m) — With 150mm DLC Sub-base
k (MPa/m) — With 150mm GSB + 150mm DLC
2
21
42
48
3
28
56
60
4
35
63
72
5
42
70
80
6
48
77
88
7
55
84
95
8
62
90
100
10
72
100
112
12
80
110
120
15
90
120
132
DLC: Dry Lean Concrete sub-base (M10 lean concrete, 150mm thick). DLC significantly increases effective k value and reduces slab bending stresses.
2. Key Design Formulae
Radius of Relative Stiffness (l)
l = [ E × h³ / (12 × (1 - μ²) × k) ]^0.25
Where:
l = radius of relative stiffness (mm)
E = Elastic modulus of concrete = 5000 √fck MPa
h = slab thickness (mm)
μ = Poisson's ratio of concrete = 0.15
k = modulus of subgrade reaction (MPa/m = N/mm³)
For M40 concrete: E = 5000 × √40 = 5000 × 6.32 = 31,623 MPa
For M35 concrete: E = 5000 × √35 = 29,580 MPa
For M30 concrete: E = 5000 × √30 = 27,386 MPa
Flexural strength (Modulus of Rupture): MR = 0.7 √fck MPa
Temperature Differential Stress (Westergaard)
S₁ (interior) = E × α × T / [2(1 - μ)] × C
Where: α = coefficient of thermal expansion = 10 × 10⁻⁶ /°C
T = temperature differential (°C) — varies 12–21°C across India
Based on CVPD (commercial vehicles per day), Modulus of Rupture (MR), and k-value.
CVPD
MR=3.5 MPa, k=42
MR=4.0 MPa, k=42
MR=3.5 MPa, k=80
MR=4.0 MPa, k=80
MR=4.5 MPa, k=80
150
200 mm
185 mm
190 mm
175 mm
165 mm
300
215 mm
200 mm
200 mm
190 mm
175 mm
500
225 mm
210 mm
210 mm
195 mm
185 mm
750
235 mm
220 mm
225 mm
205 mm
195 mm
1000
245 mm
230 mm
230 mm
215 mm
205 mm
1500
260 mm
240 mm
245 mm
225 mm
215 mm
2000
270 mm
250 mm
255 mm
235 mm
225 mm
3000
285 mm
265 mm
265 mm
250 mm
235 mm
Minimum slab thickness: 150 mm. Grade of concrete: Minimum M40 for expressways/NHs; M35 for state highways; M30 minimum for other roads. Use software (IITRIGID/IRC design charts) for accurate design.
4. Joint Design
Transverse Contraction Joints (Dummy Joints)
Slab Thickness h (mm)
Joint Spacing (m)
Groove Depth
Sealant Width
< 150
3.0 – 3.75
h/3
6 – 10 mm
150 – 200
3.75 – 4.5
h/4
6 – 10 mm
> 200
4.5 – 5.0
h/4
8 – 12 mm
Dowel Bar Design — Transverse Joints
Slab Thickness h (mm)
Dowel Bar Dia (mm)
Length (mm)
Spacing c/c (mm)
Material
150
20
400
250
MS Smooth Round
175
25
450
300
MS Smooth Round
200
25
450
300
MS Smooth Round
225
32
500
300
MS Smooth Round
250
32
500
300
MS Smooth Round
275
36
500
300
MS Smooth Round
300
38
500
300
MS Smooth Round
Dowel bars: one half painted/greased (free end) for load transfer. Half coated = sliding end; half bonded = fixed end. Dowel bars extend 200mm each side of joint.
Tie Bar Design — Longitudinal Joints
Lane Width (m)
Slab Thickness (mm)
Tie Bar Dia (mm)
Length (mm)
Spacing c/c (mm)
3.5
150–200
10 (deformed)
500
600
3.5
200–250
12 (deformed)
560
700
3.75
150–200
10 (deformed)
500
500
3.75
200–275
12 (deformed)
560
600
5. Sub-Base Specification
Type
Thickness
Grade
Compressive Strength
Purpose
Dry Lean Concrete (DLC)
150 mm (min.)
M10
≥10 MPa at 28 days
Uniform support, reduce k erosion
Granular Sub-Base (GSB)
150 mm
Type II
CBR ≥ 30%
Drainage, uniformity
Combination
GSB 150mm + DLC 150mm
—
—
Most effective sub-base for heavy traffic
6. Expansion Joint Details
Parameter
Specification
Spacing
120–180 m (where constrained; generally not required for slabs < 30m)
Gap width
25 mm
Dowel bars
Same as contraction joint; assembly with compressible filler