Highway Materials | Civil Engineering SSC JE (Technical) - Civil Engineering (CE) PDF Download

Chapter 4

HIGHWAY MATERIALS

Materials to be considered

• Soil characteristics
• Stone (aggregate) characteristics
• Bituminous materials

SOIL CHARACTERISTICS

Desirable properties of subgrade soil

• Stability - resistance to deformation under traffic loads.
• Incompressibility - limited settlement under load.
• Permanency of strength - retain strength over time and under environmental changes.
• Minimum change in volume and stability under adverse weather and groundwater conditions.
• Good drainage - rapid removal of excess water.
• Ease of compaction - ability to attain required density with available equipment.

Important terms

Field Moisture Equivalent

• Moisture content at which the demands for absorbed water are fully satisfied for a soil, as determined by specified laboratory or field methods.
• Also defined as the moisture retained by a soil specimen in a centrifuge against a force of 1000g for one hour in some standard procedures.

Group Index of Soil (GI)

• The Group Index (G.I.) was introduced by the Highway Research Board (HRB) to provide a simple numerical expression of the quality of a soil for use as a subgrade. G.I. is a function of the percentage passing the 0.074 mm (74 µm) sieve, Liquid Limit (L.L.) and Plasticity Index (P.I.).
• G.I. values vary between 0 and 20. Higher G.I. indicates poorer subgrade quality.
• The HRB expression for Group Index is:
  G.I. = 0.2a + 0.005ac + 0.01bd
  where the parameters a, b, c and d are derived from sieve and Atterberg limits as described below.
• Definitions of parameters:
  a = portion of material passing 0.074 mm sieve greater than 35% and not exceeding 75%, expressed as a whole number from 0 to 40 (= P - 35 when P between 35 and 75).
  b = portion of material passing 0.074 mm sieve greater than 15% and not exceeding 35%, expressed as a whole number from 0 to 20 (= P - 15 when P between 15 and 35).
  c = value of Liquid Limit in excess of 40 and up to 60, expressed as a whole number from 0 to 20 (= WL - 40 when WL between 40 and 60).
  d = value of Plasticity Index exceeding 10 and up to 30, expressed as a whole number from 0 to 20 (= PI - 10 when PI between 10 and 30).
• HRB classification divides soils into groups A-1 to A-7 with subgroups, and the Group Index is used to further indicate quality. Typical characteristics:
  • A-1, A-2 and A-3 are granular soils with less than 35% passing 74 µm sieve.
  • A-4, A-5, A-6 and A-7 are fine-grained soils with more than 35% passing 74 µm sieve.

Do you know?
Group index is shown in brackets: e.g., A-6(4) means the soil is classified A-6 with Group Index 4; A-6(4) is superior to A-6(16).

Factors on which soil strength depends

• Soil type (grain-size distribution and mineralogy)
• Moisture content
• Dry density (degree of compaction)
• Internal structure and fabric of the soil
• Type and mode of application of stress (static, dynamic, repeated)

Evaluation of soil strength

• Shear tests - laboratory tests on samples to measure shear strength parameters (cohesion and angle of internal friction).
• Bearing tests - field loading tests to determine bearing capacity and settlement characteristics.
• Penetration tests - small-scale bearing tests such as CBR used for pavement design.

Plate Bearing Test

• Used to determine supporting power of the subgrade and to estimate the modulus of subgrade reaction, k.
• Westergaard (often cited in pavement literature) used plate tests to define k as the pressure sustained per unit deformation of the subgrade at specified deformation using a specified plate size.
• Standard plate diameter used for k is 75 cm; a smaller plate of 30 cm diameter is sometimes used when testing is limited by available load.
• The pressure p corresponding to a settlement of 0.125 cm is read and k is calculated using the specified relation for the test arrangement.

• For consolidated and soaked specimens the pressure p_s required for the same deformation Δ is noted and the modulus of subgrade reaction for the soaked condition is denoted K_s.

Correction for plate size
When the load available is insufficient to produce 0.125 cm deflection of a 75 cm plate, a smaller plate may be used and the k-value corrected. The relation between k and plate radius a is commonly given and used in correction procedures.

Relationship example: k₁ a₁ = k₂ a₂ where a is the plate radius; E denotes modulus of elasticity where relevant in theoretical corrections.

CBR Test (California Bearing Ratio)

• The CBR is a penetration test developed to evaluate stability of soil subgrades and other flexible pavement materials.
• The test uses a cylindrical plunger 50 mm diameter penetrating the material at 1.25 mm/minute.
• Standard reference loads obtained from tests on crushed stone are 1370 kg (70 kg/cm²) and 2055 kg (105 kg/cm²) at 2.5 mm and 5.0 mm penetration respectively for a 50 mm dia plunger.

• Normally the CBR at 2.5 mm penetration is higher than at 5.0 mm and the higher value is reported. If CBR at 5.0 mm exceeds that at 2.5 mm, the test is repeated to check the result.
• Presence of coarse graded particles in the sample gives poor CBR results; material passing 20 mm sieve is typically used in preparation of specimens for the test.

STONE (AGGREGATE) CHARACTERISTICS

Desirable properties of aggregates used in pavements

• Strength - to withstand stresses due to vehicular loads.
• Hardness - resistance to wear and abrasive action; attrition (mutual rubbing) should be minimal.
• Toughness - resistance to impact from moving wheel loads.
• Durability (soundness) - ability to withstand weathering, rain and groundwater action without disintegration.

Do you know?
Aggregates are exposed to physical and chemical action from water and atmosphere; hence road stones must be sufficiently sound to resist weathering.

Shape of aggregates

• Flaky and elongated particles have lower strength than cubical, angular or rounded particles.
• Rounded aggregates give better workability and lower specific surface in cement concrete; however, angular particles are preferred in granular base courses and bituminous layers for improved interlock and stability.
• WBM (Water Bound Macadam) and bituminous constructions often achieve better stability with angular particles than with rounded ones.

Adhesion with bitumen

• Aggregates with greater affinity to water (hydrophilic, e.g., silica-rich) may lose bitumen film in presence of water (stripping).
• Basic aggregates (e.g., limestones) are hydrophobic and generally have better adhesion to bitumen.
• IRC specifies stripping limits: stripping value of aggregate should not exceed 25% for use in bituminous surface when tested by immersing bitumen-coated aggregate in water at 40°C for 24 hours.

Tests for road aggregates

Common laboratory tests and purposes:

• Crushing test - resistance to crushing under gradually applied compressive load.
• Abrasion test - hardness and resistance to wear (Los Angeles, Devel, Dorry tests).
• Soundness test - resistance to weathering through chemical cycles.
• Impact test - toughness under repeated impact.
• Shape tests - flakiness index, elongation index and angularity number.
• Specific gravity and water absorption test.
• Bituminous adhesion (stripping) test.

1. Crushing Test

• Aggregates resistant to crushing are preferred; low Aggregate Crushing Value (ACV) is desirable.
• Procedure (summary): aggregates passing 12.5 mm IS sieve and retained on 10 mm sieve are filled in a cylindrical mould in three layers; a compressive load (40 tonnes) is applied at about 4 tonnes/minute; the crushed sample is sieved on 2.36 mm IS sieve and the fraction passing is weighed.
• Aggregate Crushing Value (ACV) = (W₂ / W₁) × 100, where W₂ = weight of crushed material passing 2.36 mm sieve and W₁ = original sample weight.
• Acceptable limits: For wearing (surface) course ACV should be < 30%;="" for="" base="" course="" acv="" should="" not="" exceed="" about="" 45%="" (typical="" limits;="" follow="" relevant="">

2. Abrasion Test

• Measures resistance to surface wear. Common tests: Los Angeles, Devel and Dorry abrasion tests.
• Los Angeles Abrasion Test - aggregates and steel balls are placed in a rotating drum; the machine is rotated at 30-33 rpm for 500-1000 revolutions and loss in weight is measured.
• Abrasion value is expressed as percentage loss in weight. For high-quality pavement aggregates the LA abrasion value should be less than 30%; for base course in WBM up to 50% may be tolerated.

3. Impact Test

• Evaluates toughness and resistance to fracture under repeated impacts.
• Procedure (summary): a metal hammer weighing about 13.5-14.0 kg is allowed to fall freely from a height of 38 cm through vertical guides for 15 blows; the crushed material is sieved on 2.36 mm sieve and percentage passing is calculated.
• Total impact energy delivered = 15 × 38 × hammer weight (kg-cm). Example: 15 × 38 × 14 = 7,980 kg-cm.
• Acceptable values: Impact value normally should not exceed 30% for wearing surface; maximum permissible values may be 35% for bituminous macadam and 40% for WBM base course.

4. Soundness Test

• Assesses resistance to weathering by subjecting aggregate to cycles of immersion in saturated solutions of sodium sulphate or magnesium sulphate followed by drying.
• Procedure (summary): dry aggregate is immersed for 16-18 hours, then dried at 105-110°C to constant weight - this constitutes one cycle. After 10 cycles the average loss in weight is measured.
• Acceptable limits: average loss after 10 cycles should not exceed 12% with sodium sulphate and 18% with magnesium sulphate for aggregates used in pavement construction.

5. Shape Tests

(a) Flakiness Index

• Flakiness Index is the percentage by weight of aggregate particles whose least dimension (thickness) is less than three fifths (0.6) of their mean dimension.
• Applicable for sizes larger than 6.3 mm.
• Desired values: flakiness index preferably less than 15% and normally not exceeding 25% for road work.

(b) Elongation Index

• Elongation Index is the percentage by weight of particles whose largest dimension (length) is greater than 1.8 times their mean dimension (i.e., greater than 9/5 of mean dimension). Not applicable to sizes smaller than 6.3 mm.
• Elongation index values in excess of 15% are undesirable.

(c) Angularity Number

• Represents degree of angularity and packing of aggregate particles.
• Angularity number = 67 - % solid volume, where 67 represents the solid volume percentage for most rounded gravels in a well-compacted state (33% voids).
• Thus, angularity number measures voids in excess of 33%; higher angularity number means more angular aggregate.
• Typical range for angularity number for construction aggregates is 0 to 11.

In the angularity measurement: W = weight of aggregate in a cylinder; C = weight of water in the same cylinder; G_a = specific gravity of aggregate.

6. Water Absorption Test

• Specimens exhibiting water absorption greater than about 0.6% are generally considered unsatisfactory for many pavement uses (higher absorption indicates porosity and possible durability problems).

7. Bitumen Adhesion (Stripping) Test

• Aggregates that are electronegative (e.g., silica) tend to be hydrophilic and can lose bituminous coating in presence of water; basic aggregates (e.g., limestone) are hydrophobic and adhere better to bitumen.
• Stripping of bitumen from coated aggregates is a key durability problem, especially in bituminous mixtures permeable to water.
• IRC requirement: stripping value of aggregate should not exceed 25% for use in bituminous surfacing when tested by immersing bitumen-coated aggregate in water bath at 40°C for 24 hours.

BITUMINOUS MATERIALS

Bituminous materials for pavements are principally of two kinds: bitumen (petroleum asphalt or native asphalt) and tar. Other commercially important forms are cutback bitumen and bituminous emulsions.

Note: Native asphalts associated with a large proportion of mineral matter are called rock asphalts. When the viscosity of bitumen is reduced by adding a volatile diluent the product is called a cutback. When bitumen is dispersed in water with an emulsifier the product is a bituminous emulsion.

General requirements of bitumen

• Adequate viscosity at time of mixing to coat aggregates and to resist flow under ambient conditions.
• Low sensitivity to temperature changes (stability of properties with temperature).
• Good adhesion to aggregates and resistance to stripping by water.

Common tests on bitumen

Typical laboratory tests include:

• Penetration test
• Ductility test
• Viscosity test (Furol / orifice viscometer)
• Float test
• Specific gravity test
• Softening point test (Ring and Ball)
• Flash and fire point tests (Pensky-Martens closed cup)
• Solubility test
• Spot test (for overheating/cracking)
• Loss on heating test
• Water content test

1. Penetration Test

• Measures hardness or consistency of bitumen by the depth (in tenths of millimetre) a standard loaded needle penetrates vertically in 5 seconds at 25°C.
• The standard needle assembly applies a 100 g load (or as specified) on the specimen maintained at 25°C.
• Penetration grades are reported as numbers (e.g., 80/100 meaning penetration between 80 and 100 tenths of a millimetre at 25°C).
• Typical penetration values of bitumen used in pavements in India range from about 20 to 225; commonly used grades are 30/40 and 80/100.

2. Ductility Test

• Ductility is the distance (in cm) a standard briquette of bitumen (cross-section 10 mm × 10 mm) can be stretched before the thread breaks.
• Test conditions: specimen temperature 37°C and rate of pull 50 mm/min.

• ISI recommendations: minimum ductility of 75 cm for certain grades (45 and above). For some regional uses grades A-65 to 200 may have minimum ductility of 15 cm; grade S-35 may require 50 cm, depending on specification and availability.

3. Viscosity Test

• Viscosity measures resistance to flow. Orifice-type viscometers are commonly used.
• Viscosity is often measured as time for 50 ml of material to flow through a specified orifice under standard conditions (Furol viscosity is a standard test for liquid bituminous materials).

4. Float Test

• Used for materials where penetration and standard viscometer tests are not suitable.
• Test specimen is cooled to about 0.5°C; a bitumen plug is placed in a float assembly and floated in water bath at 50°C. Time for water to force its way through the plug (seconds) is recorded as the float test value. Higher float time indicates stiffer material.

5. Specific Gravity Test

• Specific gravity of pure petroleum bitumen typically ranges 0.97 to 1.02. Tars have higher specific gravity, approximately 1.10 to 1.25.

6. Softening Point Test (Ring and Ball)

• Softening point is the temperature at which the material attains a particular degree of softening; tested by the Ring and Ball apparatus.
• Higher softening point generally indicates lower temperature susceptibility and is preferred in warm climates.
• Procedure (summary): a steel ball is placed on the bitumen sample in a brass ring; the assembly is heated at a specified rate (commonly 5°C/min) in a liquid bath and the temperature at which the ball passes through the sample is recorded as the softening point.
• Softening points for paving bitumens commonly vary between about 35°C and 70°C depending on grade.

7. Flash and Fire Point Tests

• Flash point - the lowest temperature at which vapour produced momentarily ignites (flash) under specified test conditions.
• Fire point - the lowest temperature at which vapour continues to burn for a specified time after ignition.
• Pensky-Martens closed cup apparatus is commonly used. Minimum specified flash point for bitumen in the Pensky-Martens closed cup test is typically around 175°C.

8. Solubility Test

• Pure bitumen is soluble in solvents such as carbon disulphide and carbon tetrachloride; insoluble matter (e.g., carbon, mineral residues) should be less than about 1%.
• Material is considered cracked if insoluble fraction or other indicators exceed specified limits (e.g., solubility <>

9. Spot Test

• Detects overheating or thermal cracking of bitumen. A small quantity of bitumen is dissolved in a solvent (e.g., naphtha), and a drop on filter paper is examined.
• Uniform colour indicates uncracked bitumen; a dark brown centre with lighter annular ring indicates overheating or cracking. Spot test can be more sensitive than solubility alone.

10. Loss on Heating Test

• Sample is heated at 163°C for 5 hours and loss of weight is expressed as percent by weight.
• Acceptable loss: generally should not exceed 1% for pavement bitumens; for very soft grades (penetration 150-200) up to 2% may be allowed.
• Heated residue should not show reduction in penetration greater than about 40% compared to original.

11. Water Content Test

• Maximum water content in bitumen should not exceed about 0.2% by weight.

Cutback Bitumen

• Cutback bitumen is bitumen whose viscosity has been reduced by adding a volatile diluent (petroleum fraction) to allow work at lower temperatures.
• Types by rate of curing:
  • Rapid Coating (RC) - volatile diluent evaporates quickly (e.g., naphtha, petrol).
  • Medium Curing (MC) - intermediate volatility (e.g., kerosene, light diesel).
  • Slow Curing (SC) - low-volatility diluents (e.g., heavy gas oils) giving slow curing.
• Designation example: RC-1, RC-2 etc., with higher numerical value indicating progressively thicker/viscous products (e.g., RC-2 thicker than RC-1). RC-0 or SC-0 may contain ~45% solvent and 55% bitumen; RC-5 and MC-5 may contain ~15% solvent and 85% bitumen (approximate compositions vary by specification).
• RC diluents (e.g., naphtha/gasoline) produce penetration values typically in the 80-120 range for the diluted product. Choose cutback type according to workability, ambient temperature and curing time required.
• Tests on cutback bitumen include viscosity tests at specified temperatures and orifice sizes, distillation to determine fractions and residue up to 360°C, penetration and ductility on residue, and flash point on cutback itself.

Bituminous Emulsion

• A bitumen emulsion is a liquid product in which bitumen is dispersed as fine globules in an aqueous medium and stabilised by emulsifiers.
• Emulsions are two-phase systems of immiscible liquids; bitumen content typically ranges from 40% to 60%, the remainder being water and emulsifying agent.
• Average diameter of bitumen globules in a good emulsion is about 2 µm (micrometres), not mm.
• Emulsions are widely used for maintenance, surface dressing, tack coats, prime coats and where operations at lower temperatures or wet conditions are required (can be used in damp conditions).

TAR

Production stages

• Carbonisation of coal (or wood) to produce crude tar (destructive distillation).
• Refining or fractional distillation of crude tar to separate useful fractions.
• Blending of distillation residue with distillate fractions to produce road tars with desired properties.

Tests for road tars

• Specific gravity test
• Viscosity test on standard tar viscometer
• Equiviscous temperature (EVT)
• Softening point and softening point of residue
• Float test
• Water content
• Distillation fractions (up to specified temperature ranges: e.g., up to 200°C, 200-270°C, 270-330°C)
• Phenol content (percent by volume)
• Naphthalene content (percent by weight)
• Matter insoluble in toluene (percent by weight)

Typical designations of road tars by increasing viscosity: RT-1 (lowest viscosity) to RT-5 (highest viscosity). Usage examples:

• RT-1 - low viscosity, used for surface painting in very cold conditions.
• RT-2 - recommended for standard surface painting under normal climatic conditions.
• RT-3 - for surface painting, renewal coats and premixed chip applications for light wearing courses.
• RT-4 - generally used for premixing tar macadam in base course.
• RT-5 - high viscosity, used for grouting and heavy-duty premix operations where very viscous tar is required.

Comparison of tar and bitumen

• Bitumen is a petroleum product; tar is produced by destructive distillation of coal or wood.
• Bitumen is soluble in carbon disulphide and carbon tetrachloride; tar is commonly soluble in toluene (and may show different solubility behaviour).
• Bitumen is generally more resistant to water than tar.
• Tar is more temperature susceptible - larger variation of viscosity with temperature.
• Tar often contains higher free carbon content, as indicated by solubility and residue tests.