Highway Materials and Construction

Subgrade soil

The subgrade is the natural or compacted soil that supports the pavement. Pavement thickness depends largely on subgrade strength, so its evaluation and compaction are critical.

Desirable properties: stability (strength), incompressibility, permanency of strength (little loss when wet), minimum volume change (swelling/shrinkage), good drainage, ease of compaction.

Soil classification for roads

• Indian Standard Soil Classification (IS 1498) — coarse-grained, fine-grained and highly organic soils.
• HRB/AASHTO classification — groups A-1 to A-7 (granular A-1 to A-3 are good subgrades; silt–clay A-4 to A-7 are poor).
• Group index (GI) — rating of fine-grained soils:

California bearing ratio (CBR) test

Measures the strength of subgrade soil and granular materials relative to a standard crushed stone.

A correction is applied to the load–penetration curve if it is initially concave upward (seating).

Plate load test and modulus of subgrade reaction

Used for rigid pavement design. A rigid plate (standard 75 cm diameter) is loaded in steps and settlements are recorded.

$p$ = pressure (kg/cm²) at a settlement of 1.25 mm; $k$ in kg/cm³.

• Correction for smaller plates: $k_1{a}_1=k_2{a}_2$ (approximately, for $a$ = plate radius).
• Correction for soaking: $k_s=k\frac{p_s}{p}$ from laboratory consolidation tests on soaked and unsoaked samples.

Resilient modulus

The resilient modulus $M_R$ is the elastic modulus under repeated loading — used in mechanistic–empirical design (IRC:37-2018). Where test data are not available, IRC:37-2018 relates it to CBR:

($M_R$ in MPa.)

Road aggregates

Aggregates form the bulk (about 90–95% by weight) of bituminous and granular layers and must resist traffic loads and weather.

Desirable properties and tests

Indicative maximum values commonly specified (lower values for surface courses, higher permitted for base/sub-base): crushing value about 30% (surface course) to 45% (base course); impact value about 30% for bituminous surface courses and up to about 40% for WBM bases; Los Angeles abrasion about 30% for bituminous concrete surface course to 50% for sub-bases. Current values are prescribed in the MoRTH Specifications for Road and Bridge Works for each layer.

Bituminous binders

Types

Viscosity grading

Paving bitumen in India is graded by absolute viscosity at 60 °C as per IS 73: VG-10, VG-20, VG-30 and VG-40. VG-30 and VG-40 are commonly used for highways in hot climates and heavy traffic; VG-10 for spraying and cold regions. (Earlier penetration grades such as 80/100 and 60/70 were used.)

Tests on bitumen

Bituminous mix design — Marshall method

The Marshall method determines the optimum binder content for hot-mix asphalt (dense graded mixes such as DBM and BC).

Volumetric properties

Selection of optimum binder content (traditional): plot stability, flow, unit weight, $V_a$, VMA and VFB against binder content; the optimum is the average of binder contents at maximum stability, maximum unit weight and a target air void content (about 4%), then checked against criteria. Current practice selects the binder content at 4% air voids and checks other criteria.

Typical MoRTH criteria for bituminous concrete (heavy traffic): minimum stability about 9 kN at 60 °C; flow about 2–4 mm; air voids 3–5%; VFB about 65–75%; minimum VMA depending on nominal aggregate size.

Superpave (Superior Performing Asphalt Pavements) — performance-graded binders and gyratory compaction; used increasingly.

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