Highway Materials & Tests — Soil, Aggregates & Bitumen — Transportation Engineering Notes

Last reviewed 16 Sept 2026 · 11 min read

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:

FormulaGroup index

G I = 0.2 a + 0.005 a c + 0.01 b d

$a$ = percentage passing 75 µm sieve − 35 (0 to 40)
$b$ = percentage passing 75 µm sieve − 15 (0 to 40)
$c$ = liquid limit − 40 (0 to 20)
$d$ = plasticity index − 10 (0 to 20)

GI ranges from 0 (good subgrade) to 20 (very poor); higher GI → thicker pavement.

California bearing ratio (CBR) test

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

FormulaCBR test

Soil compacted in a mould (150 mm diameter) at OMC; soaked for 4 days (soaked CBR) under surcharge weights.
A 50 mm diameter plunger penetrates at 1.25 mm/min.
Loads recorded at 2.5 mm and 5.0 mm penetration.

C B R = \frac{Test load at given penetration}{Standard load at that penetration} \times 100

Standard loads: 1370 kg at 2.5 mm and 2055 kg at 5.0 mm (of the standard crushed stone).

Normally the CBR at 2.5 mm is reported; if the CBR at 5.0 mm is higher, the test is repeated, and if again higher, the 5.0 mm value is adopted.

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.

k = \frac{p}{Δ} = \frac{p}{0.125 cm}

$p$ = 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} = 10 \times C B R (for C B R \leq 5%), M_{R} = 17.6 \times C B R^{0.64} (for C B R > 5%)

( $M_{R}$ 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

Property	Test	Indicates
Strength	Crushing value test — 40 t load on 10–12.5 mm aggregates in a cylinder; % passing 2.36 mm	Resistance to crushing under gradually applied load (lower value → stronger)
Hardness	Los Angeles abrasion test — aggregates with steel balls rotated in a drum (500 revolutions at 30–33 rpm); % passing 1.70 mm; also Deval and Dorry tests	Resistance to wear and abrasion (lower is better). The LA test also includes impact action
Toughness	Aggregate impact value — 13.5–14 kg hammer falling 380 mm, 15 blows; % passing 2.36 mm	Resistance to impact (sudden loads)
Durability	Soundness — 5 cycles in sodium or magnesium sulphate solution; % loss	Resistance to weathering (limits about 12% with sodium sulphate, 18% with magnesium sulphate)
Shape	Flakiness index (thickness < 0.6 × mean size), elongation index (length > 1.8 × mean size), angularity number (= 67 − % solid volume; 0 for rounded to about 11 for angular)	Flaky and elongated particles break and reduce interlock; angular particles give better interlock
Specific gravity and water absorption	Pycnometer/wire basket methods	Quality and porosity (water absorption commonly limited to about 2%)
Adhesion to bitumen	Stripping value — bitumen-coated aggregates immersed in water at 40 °C for 24 h	Resistance to stripping of bitumen film by water
Polishing resistance	Polished stone value (accelerated polishing + British pendulum)	Skid resistance of surface aggregates

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

Binder	Description
Bitumen (asphalt cement)	Black or dark brown hydrocarbon material obtained from the fractional distillation of crude petroleum; soluble in carbon disulphide
Tar	Obtained by destructive distillation of coal or wood; more temperature-susceptible and brittle; soluble in toluene; rarely used now
Cutback bitumen	Bitumen dissolved in a volatile solvent to make it fluid at lower temperatures: rapid curing (RC) — petrol/naphtha; medium curing (MC) — kerosene; slow curing (SC) — high-boiling oils. Used for prime coats and cold mixes
Bitumen emulsion	Fine droplets of bitumen dispersed in water with an emulsifier; rapid setting (RS) for tack coats and surface dressing, medium setting (MS) for premixes, slow setting (SS) for fine mixes and slurry seals. Can be used on damp surfaces; energy-efficient; cationic emulsions adhere well to most aggregates
Modified bitumen	Polymer modified bitumen (PMB), crumb rubber modified bitumen (CRMB) — improved resistance to rutting, cracking and ageing
Natural asphalt	Lake asphalt (Trinidad), rock asphalt

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

Test	Procedure	Significance
Penetration test	Depth (in units of 0.1 mm) that a standard needle loaded to 100 g penetrates in 5 s at 25 °C	Consistency/hardness — higher penetration → softer bitumen
Ductility test	Briquette with 1 cm² cross-section pulled at 50 mm/min at 27 °C; elongation (cm) at breaking	Cohesion and ability to deform without cracking
Softening point (ring and ball) test	Bitumen in a brass ring with a steel ball, heated in water/glycerine at 5 °C/min; temperature at which it softens enough for the ball to touch a base plate 25 mm below	Temperature susceptibility — higher softening point → less susceptible
Viscosity test	Absolute viscosity at 60 °C (vacuum capillary) and kinematic viscosity at 135 °C	Grading, flow at pavement and mixing temperatures
Flash and fire point test	Pensky–Martens apparatus; temperature at which vapours flash momentarily (flash) and burn continuously (fire)	Safety during heating
Specific gravity test	Pycnometer (paving bitumen about 0.97–1.02)	Quality and mix calculations
Solubility test	Solubility in trichloroethylene (commonly at least 99%)	Purity
Float test	For soft bitumen/tar too soft for penetration test	Consistency
Thin film oven / rolling thin film oven test	Heating a thin film to simulate short-term ageing, followed by tests on residue	Hardening during mixing and laying
Water content, loss on heating, spot test		Presence of water, volatiles, cracked 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).

FormulaMarshall test

Cylindrical specimens 101.6 mm diameter × 63.5 mm high, prepared at different binder contents.
Compacted with a 4.54 kg hammer falling 457 mm — 75 blows on each face for heavy traffic (50 blows for lighter traffic).
Specimens immersed in water at 60 °C and loaded at 50.8 mm/min on the curved faces.
Marshall stability — maximum load (kN) at failure; flow value — deformation (mm) at maximum load.

Volumetric properties

FormulaVolumetrics

Bulk specific gravity of compacted mix $G_{mb}$ ; theoretical maximum specific gravity $G_{mm}$
Air voids: $V_{a} = \frac{G _{mm} - G _{mb}}{G _{mm}} \times 100$
Voids in mineral aggregate: $V M A = 100 - \frac{G _{mb} P _{s}}{G _{s b}}$ ( $P_{s}$ = % aggregate by weight of mix; $G_{s b}$ = bulk specific gravity of aggregate)
Voids filled with bitumen: $V F B = \frac{V M A - V _{a}}{V M A} \times 100$
Theoretical specific gravity of a mix: $G_{t} = \frac{100}{\sum \frac{W _{i}}{G _{i}}}$

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.

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Highway Materials & Tests — Soil, Aggregates & Bitumen