Design & construction of bituminous & concrete roads

Types of pavements

Other types: composite pavements (bituminous over concrete or cement-treated base), semi-rigid pavements (cement-treated layers), interlocking concrete block pavements.

Layers of a flexible pavement

Also: prime coat (between granular base and bituminous layer) and tack coat (between bituminous layers).

Stresses in flexible pavements

• A wheel load is applied over a contact area (often assumed circular) with contact pressure roughly equal to tyre pressure.
• Boussinesq's theory (homogeneous elastic half-space): vertical stress below the centre of a uniformly loaded circular area of radius $a$:

• Burmister's layered theory — two or three elastic layers of different moduli; basis of modern analysis (a stiffer upper layer reduces stresses below).

Equivalent single wheel load (ESWL)

The single wheel load that produces the same effect (stress or deflection) at a given depth as a group of wheels (dual or tandem).

Boyd–Foster equal stress method (graphical): on a log–log plot of load against depth, ESWL = $P$ (one wheel of the dual) at depth $d/2$ and $2P$ at depth $2S$ ($d$ = clear gap between tyres, $S$ = centre-to-centre spacing), with a straight line between:

Design approaches

CBR method (empirical)

$t$ = total pavement thickness (cm) above the layer of given CBR; $P$ = wheel load (kg); $p$ = tyre pressure (kg/cm²). Thickness above each layer is found using that layer's CBR.

Design traffic

Standard axle and equivalency

The standard axle in IRC:37-2018 is a single axle with dual wheels carrying 80 kN. Damage by other axle loads is expressed by the fourth power law:

• Vehicle damage factor (VDF) — number of standard axle repetitions caused by one passage of a commercial vehicle; found from axle load surveys (or indicative values).
• Lane distribution factor (LDF) — proportion of commercial vehicles in the design lane: single-lane road 1.0; two-lane single carriageway 0.50; four-lane single carriageway 0.40; dual carriageway — two lanes in each direction 0.75, three lanes 0.60, four lanes 0.45.

Only commercial vehicles (laden weight above 3 tonnes) are considered. Design periods of about 20 years are commonly used for National and State Highways (longer for expressways and high-density corridors, shorter for lower categories). IRC:37-2018 is applicable for design traffic of 2 msa and above; low-volume roads are designed as per IRC:SP:72.

IRC:37-2018 mechanistic–empirical design

Design criteria

1. Bottom-up fatigue cracking of the bituminous layer — controlled by the horizontal tensile strain ${\epsilon }_t$ at the bottom of the bituminous layer.
2. Rutting (permanent deformation) — controlled by the vertical compressive strain ${\epsilon }_v$ on top of the subgrade.
3. Additional checks for cement-treated bases (fatigue of CTB) and top-down cracking considerations.

Material inputs

• Subgrade — effective CBR (considering embankment and subgrade layers) → $M_R$ from CBR relations.
• Granular layers — modulus depends on thickness and support: $M_{R,gran}=0.2h^{0.45}{M}_{R,support}$ ($h$ in mm).
• Bituminous layers — resilient modulus from IRC tables depending on binder grade (e.g. VG-30, VG-40) and pavement temperature (commonly 35 °C in design).
• Cement-treated layers — modulus and flexural strength as given in the code.
• Poisson's ratio — typically 0.35 for bituminous and granular layers and subgrade (0.25 for cemented layers).

Procedure

1. Estimate design traffic (msa) and subgrade effective CBR.
2. Select pavement composition (e.g. bituminous surface + DBM + WMM + GSB, or with CTB/CTSB layers) with trial thicknesses.
3. Analyse the pavement with the IITPAVE multilayer elastic program under the standard axle (dual wheels, 20 kN each, tyre pressure 0.56 MPa, 310 mm spacing).
4. Compute allowable ${\epsilon }_t$ and ${\epsilon }_v$ from the transfer functions for the design traffic.
5. Adjust thicknesses until computed strains do not exceed allowable strains; check minimum layer thicknesses and drainage.
6. IRC:37-2018 also provides design catalogues of layer thicknesses for different CBR and traffic combinations.

Components of a rigid pavement

Unlike flexible pavements, the thickness of a rigid pavement depends mainly on the flexural strength of concrete; subgrade support has a relatively smaller effect.

Design factors

• Wheel load and axle configuration — axle load spectrum, tyre pressure, contact area.
• Traffic — number of repetitions of different axle loads over the design period (commonly 30 years for rigid pavements).
• Modulus of subgrade reaction $k$ — from plate load tests (or from CBR correlations), increased for the effect of DLC/granular sub-base (effective k).
• Concrete properties — flexural strength (modulus of rupture), modulus of elasticity (about 30 000 MPa), Poisson's ratio (0.15), coefficient of thermal expansion (about $10\times 10^{-6}$ per °C).
• Temperature differential between top and bottom of the slab — causes warping.

Radius of relative stiffness

It measures the stiffness of the slab relative to the subgrade — the distance over which the slab spreads the load.

Equivalent radius of resisting section

For a load of contact radius $a$ on a slab of thickness $h$ (thin-plate theory correction):

Load stresses (Westergaard)

Westergaard considered three critical positions of the wheel load:

• Interior — load well away from edges and corners.
• Edge — load at the edge, away from corners.
• Corner — load at the corner of the slab.

• Interior and edge stresses are tensile at the bottom of the slab; corner stress is tensile at the top.
• Edge load stress is generally the most critical of the load stresses (without load transfer).

Temperature stresses

Warping stresses

A temperature differential between the top and bottom of the slab makes it curl (warp); self-weight and subgrade restraint resist this, inducing stresses.

• Day (summer, mid-day): top hotter → slab tends to curl down at edges → tension at the bottom (adds to wheel load tension at the edge).
• Night (winter): top cooler → edges curl up → tension at the top (adds to corner load stress).

Frictional stresses

Uniform temperature change makes the slab expand or contract; friction with the sub-base restrains it.

($S_f$ in kg/cm²; $W$ = unit weight of concrete, kg/m³; $L$ = slab length, m; $f$ = coefficient of friction, about 1.5.) This governs the spacing of contraction joints.

Critical combinations

Earthwork, embankment and subgrade

1. Clearing and grubbing, removal of topsoil.
2. Embankment construction in layers (commonly not more than about 200–250 mm compacted thickness) with suitable soil at optimum moisture content (OMC), compacted by rollers.
3. Subgrade — the top portion (commonly 500 mm) compacted to a higher density.

Compaction requirements (MoRTH practice, modified Proctor): embankment about 95% of maximum dry density (MDD); subgrade and shoulders about 97% of MDD.

Field density checked by the sand replacement method or core cutter method (soft soils), or nuclear density gauges.

Compaction equipment: smooth-wheeled rollers (granular and bituminous layers), sheep-foot rollers (cohesive soils), pneumatic-tyred rollers (mixed soils, bituminous layers), vibratory rollers (granular soils), plate compactors and rammers (confined spaces).

Soil stabilisation

Improving soil properties (strength, stability, durability) for use as subgrade, sub-base or base.

Granular layers

Granular sub-base (GSB)

Well-graded or close-graded natural gravel, crushed stone or mixtures, laid at OMC and compacted; acts as a structural and drainage layer extending across the full formation width.

Water bound macadam (WBM)

Clean crushed coarse aggregates mechanically interlocked by rolling, with voids filled with screenings and binding material (e.g. fine soil) with the help of water.

Construction steps:

1. Preparation of the base (shaping to camber, compaction), provision of lateral confinement (shoulders).
2. Spreading of coarse aggregates in uniform layers (compacted thickness typically about 75 mm per layer).
3. Dry rolling from the edges towards the centre (overlapping) until aggregates are firmly interlocked.
4. Application of screenings to fill voids, with dry rolling and brooming.
5. Sprinkling of water and grouting with continued rolling (wet rolling).
6. Application of binding material with water to form a slurry filling remaining voids; rolling.
7. Setting and drying — the layer is allowed to dry overnight before the next layer or surface treatment.

Wet mix macadam (WMM)

Graded crushed aggregates and granular material pre-mixed with water at OMC in a pugmill/mixing plant, laid with a paver and compacted with vibratory rollers. Faster, more uniform and denser than WBM; widely used as base course on highways.

Bituminous construction

Interface treatments

Bituminous surfacing and layers

Hot mix construction

1. Hot mix plant — batch plant (weighs and mixes each batch) or drum mix plant (continuous mixing in a drum); aggregates heated and dried, binder heated, mixed to specified temperatures.
2. Transport in covered tippers to limit heat loss.
3. Preparation of the base — cleaning, correcting profile, tack coat.
4. Laying with a sensor paver to line, level and camber, at the specified temperature.
5. Compaction while hot:
   • Breakdown rolling — smooth steel-wheeled/vibratory rollers.
   • Intermediate rolling — pneumatic-tyred rollers (kneading action, sealing).
   • Finish rolling — tandem smooth-wheeled rollers to remove marks. Rolling proceeds from the lower edge to the higher edge, with longitudinal joints compacted first.
6. Quality control — temperature, binder content (extraction), gradation, density (cores), thickness, surface regularity (3 m straight edge), Marshall properties.

Warm mix asphalt technologies lower mixing temperatures, saving energy and reducing emissions.

Cement concrete pavement construction

1. Preparation of subgrade, drainage layer and DLC sub-base; laying of separation membrane.
2. Placing dowel bar assemblies (or automatic dowel inserters) and tie bars.
3. Batching and mixing (central batching plant); transport in tippers/transit mixers.
4. Paving with a slip-form paver (or fixed-form paving with vibrating screeds for smaller works); internal vibration.
5. Finishing — floating, straight-edging; surface texturing (brush or tining) for skid resistance.
6. Curing — application of curing compound immediately, followed by wet curing (e.g. wet hessian, ponding) for about 14 days.
7. Joint cutting with saws at the correct time (before uncontrolled cracking), then joint sealing.
8. Opening to traffic after the concrete gains required strength (typically about 28 days for full strength).

Failures of flexible pavements