Highway Materials

The subgrade is the top 500 mm of natural or compacted soil that supports the pavement. Its properties decide the pavement thickness, because the load finally reaches it.

Desirable properties

1. Adequate strength and stability - high CBR (design CBR measured at the field density and moisture, soaked 4 days; NRS 2070/IRC:37 use CBR to give the pavement thickness; minimum subgrade CBR is about 5%, and 8-10% or more is good).
2. Low volume change - low swelling and shrinkage, so low plasticity (low liquid limit and plasticity index).
3. Good drainage - permeable soil that does not retain water; high water table avoided.
4. Resistance to frost action - non-frost-susceptible soil in cold areas.
5. Ease of compaction - well-graded soil that reaches high dry density (maximum dry density and optimum moisture content) with ordinary rollers, giving good compaction (at least 97% of MDD, modified Proctor, by IRC/NRS).
6. Low compressibility - low settlement and consolidation, so pavements do not deform.
7. Durability - strength retained in wet condition; does not soften under rain or traffic.
8. Uniformity - uniform soil along the length so that the pavement has no differential settlement.
9. Free of organic matter and deleterious material.
10. Good modulus - high resilient modulus, which gives lesser deflection under repeated loads.

The Los Angeles abrasion test measures the resistance of road aggregate to wear (abrasion and impact) by a combined action of abrasion, attrition and impact in a rotating drum. It follows IS:2386 (Part IV) / AASHTO T96 / ASTM C131 and is used in NRS 2070.

Apparatus

• Los Angeles machine: hollow steel cylinder 700 mm dia and 500 mm long, closed at both ends, rotating about a horizontal axis, with one internal shelf (steel plate) 90 mm wide.
• Abrasive charge: cast iron or steel balls about 48 mm diameter, each 390-445 g.
• IS sieve 1.7 mm, balance (accuracy 1 g), oven, tray.

Procedure

1. Take a clean, oven-dried (105-110 degrees C) sample of 5 kg (gradings A to D) or 10 kg (gradings E to G), as per the grading of the aggregate in IS:2386.
2. Select the number of balls by the grading: A - 12, B - 11, C - 8, D - 6 balls, and 12 balls for gradings E, F and G.
3. Place the sample and charge in the drum, fix the cover, and rotate at 30-33 rpm for 500 revolutions (gradings A to D) or 1000 revolutions (E to G).
4. Remove the material, separate the balls, and sieve the aggregate over a 1.7 mm IS sieve.
5. Wash and dry the retained portion in the oven and weigh.

where $W_1$ = original weight and $W_2$ = weight retained on the 1.7 mm sieve after the test.

Significance

• It gives the hardness and toughness of aggregate and the ability to resist crushing, wear and abrasion in traffic.
• Low value means a strong aggregate. Desirable limits (IRC/NRS 2070, MoRTH): about not more than 30% for wearing course/bituminous concrete, not more than 35% for bituminous macadam, 40% for WBM base courses and granular sub-base (up to 50% for lower layers).
• Tests like this are quick, repeatable and widely used for quality control of aggregate sources.

The Marshall method (Bruce Marshall; developed further by the US Army Corps of Engineers) designs a bituminous mix by testing compacted cylindrical specimens for stability, flow, density and voids at different binder contents, and selects the optimum binder content (OBC). It is the method used in Nepal (NRS 2070, the Asphalt Institute MS-2 and IRC:111).

Specimen

Cylinders of 101.6 mm (4 in) diameter and 63.5 mm (2.5 in) height, about 1200 g of aggregate plus bitumen.

Procedure

1. Select materials - aggregates meeting the quality tests, grading within the specified envelope (for example, BC or DBM), and the bitumen (VG-30/60-70 penetration grade).
2. Prepare the trial mixes - at least 5 binder contents in 0.5% increments (for example, 4.5, 5.0, 5.5, 6.0, 6.5% by weight of mix), 3 specimens each.
3. Mixing and compaction - heat aggregates to about 150-160 degrees C and bitumen to 140-150 degrees C; mix; place in a mould at the mixing temperature (about 138 degrees C) and compact with the Marshall hammer (4.5 kg falling from 457 mm) with 75 blows on each face (heavy traffic; 50 for medium).
4. Density - cool, extrude, and find the bulk specific gravity $G_b$ (weights in air and water).
5. Stability and flow test - immerse the specimens in a 60 degrees C water bath for 30-40 minutes; load at 50 mm/min in the Marshall testing head. The maximum load in kN is the Marshall stability; the deformation at max load in mm is the flow value.
6. Voids analysis - calculate $V_v$ (air voids), VMA (voids in mineral aggregate) and VFB (voids filled with bitumen), and the corrections for volume/ height.
7. Plot graphs (binder content on X-axis): stability, flow, unit weight, $V_v$, VMA, VFB.

Optimum binder content

Take binder contents corresponding to:

• maximum stability,
• maximum bulk density,
• 4% air voids (median of 3-5%).

The mix is checked against the criteria (for 75 blows: stability not less than about 8 kN; flow 2-4 mm; air voids 3-5%; VFB 65-75%; VMA as per the nominal size). If any is not satisfied, the mix is redesigned (change the grading or the binder).

Road aggregates are tested in the laboratory for strength, hardness, toughness, durability, shape and bitumen affinity (IS:2386 and IRC/NRS 2070 specifications).

Why these are needed

• Strength tests (ACV, AIV, LAA) make sure the aggregate does not break under rollers and traffic.
• Hardness/abrasion tests prevent early wear of the surface.
• Soundness ensures durability against rain, frost and chemical weathering.
• Shape tests help in good packing and compaction.
• Affinity tests make sure the bituminous layer does not strip in the presence of water.

Bituminous binders are tested to check that they have the right consistency, safety, purity and durability for the climate and the traffic. The tests are given in IS:73 / IS:1201-1220 and adopted in NRS 2070 and IRC:SP:53.

Consistency tests (main)

1. Penetration test - depth in 0.1 mm to which a standard needle (100 g, 5 s, 25 degrees C) penetrates; it classifies the grade (30/40, 60/70, 80/100).
2. Softening point test (ring and ball) - temperature at which bitumen softens; shows the temperature susceptibility.
3. Ductility test - distance in cm a briquette of bitumen can be stretched (25 degrees C, 50 mm/min) before breaking; shows cohesion and flexibility.
4. Viscosity test - flow time in seconds (Saybolt, orifice/tar viscometer) or absolute and kinematic viscosity; gives the grade (VG-10, 20, 30, 40) and the mixing and compaction temperatures.

Other tests

5. Flash and fire point - safe heating temperature.
6. Specific gravity (1.01-1.06) - for mix calculation.
7. Loss on heating (RTFOT) - ageing and volatile loss.
8. Solubility in trichloroethylene - purity (at least 99%).
9. Float test - consistency of tars, cutbacks and thick liquid binders.
10. Water content, Fraass breaking point, elastic recovery (modified binders), adhesion/stripping with aggregates.

Why needed

Penetration and viscosity indicate stiffness at service/mixing temperature, softening point shows high-temperature behaviour, ductility shows cracking resistance, flash point gives safety, and solubility and loss on heating show quality.

Definition

The aggregate crushing value (ACV) is the percentage by weight of fines (passing 2.36 mm IS sieve) produced when a sample of aggregate is crushed under a gradually applied load of 40 tonnes in a standard cylinder. It is a measure of the strength of the aggregate against crushing under a slowly applied (compressive) load (IS:2386 Part IV).

Apparatus

• Steel cylinder (mould) of 15.2 cm internal diameter with a base plate and a plunger (14.9 cm), a tamping rod of 16 mm diameter and 45-60 cm length with a rounded end.
• Compression testing machine of 40 t capacity, IS sieves 12.5, 10 and 2.36 mm, balance (to 1 g), measure of 11.5 cm diameter and 18 cm height.

Procedure

1. Take aggregate passing 12.5 mm and retained on 10 mm IS sieve, oven-dried; fill the cylindrical measure in three layers, each tamped with 25 strokes. Weigh the sample ($W_1$, about 3 kg for a measure).
2. Place the aggregate in the cylinder in three layers, each tamped 25 times, with levelled surface; insert the plunger.
3. Place the assembly in the compression testing machine and apply load at a uniform rate so that 40 tonnes is reached in 10 minutes (4 t/min); then release.
4. Remove the crushed material and sieve through 2.36 mm IS sieve; weigh the fraction passing ($W_2$) and retained.

The average of two tests is reported.

Significance and limits

• Low ACV = strong aggregate; high ACV = weak aggregate that crushes under load.
• Limits (IRC/NRS 2070): not more than 30% for wearing course (bituminous surface, cement concrete), not more than 45% for bituminous base and WBM base courses.
• Values: ACV below 10% is exceptionally strong, 10-20% strong, 20-30% satisfactory, above 35% weak.
• For aggregates with ACV above 30%, the 10% fines value is used.

Concept

Ductility of bitumen is the distance, in cm, to which a standard briquette of bitumen can be drawn out before the thread breaks, under specified conditions (25 degrees C, rate 50 mm/min). It shows the cohesion, elasticity and flexibility of the binder (IS:1208).

Apparatus

• Standard briquette mould (brass, 75 mm length, 10 mm x 10 mm minimum cross-section at the neck).
• Ductility machine with a water bath at 27 or 25 degrees C where one clip is fixed and the other moves at 50 +/- 2.5 mm/min.
• Thermometer, knife, glycerine and dextrin release agent.

  fixed end                      moving end
   [clip]=====\            /=====[clip]
   | |         \___thread___/         | |
   water bath 25 C, pulled at 50 mm/min

Procedure

1. Heat the bitumen to 75-100 degrees C above the softening point to be fluid, and pour it into the mould (coated with the release agent glycerine-dextrin) placed on a brass plate; slightly overfill.
2. Cool in air for 30-40 minutes, then trim the excess with a hot knife so that it is level with the mould.
3. Place in a water bath at 25 degrees C for 85-95 minutes, with the specimen and mould on the plate.
4. Remove from the plate and fix the rings to the clips of the machine; fill the bath with water at 25 degrees C so that the specimen is at least 25 mm under water.
5. Pull at 50 mm/min; the distance from start to the breaking point is the ductility value (cm). Report the average of three tests.

Significance and engineering application

• A high ductility means that the binder can stretch without cracking, so the pavement stays flexible under temperature changes and traffic.
• Bitumen with low ductility cracks, especially in cold weather and after ageing; ductility also shows the adhesive strength with the aggregates.
• Minimum values (IS:73 / NRS 2070): at least 50 cm for 60/70 and 80/100 penetration grade; 75 cm for VG-30 (before ageing).
• Used for specifying and quality control of bitumen for surface dressing, bituminous concrete and DBM, and to reject bitumen with low ductility.

Bituminous materials are used as the binder in road construction. The main types are as follows.

1. Natural bitumen

Found in nature, as in lakes and rock asphalt (Trinidad lake asphalt, Dead Sea, Gilsonite). It is mixed with the minerals as natural mixtures, used as a modifier or for waterproofing. Not common in Nepal.

2. Petroleum bitumen (straight-run bitumen)

The residue left after distilling crude petroleum. It is the most common binder in Nepal (imported from India). Grades: penetration grades 30/40, 60/70, 80/100 and viscosity grades VG-10, 20, 30, 40. Used for bituminous macadam, DBM, bituminous concrete and for seal coat and tack coat. Variants: blown (oxidised) bitumen for roofing and waterproofing, and modified bitumen (PMB/CRMB) for heavy traffic.

3. Cutback bitumen

Bitumen dissolved in a volatile solvent (petroleum distillate) to reduce viscosity so that it can be used cold:

• Rapid curing (RC) - gasoline/naphtha solvent,
• Medium curing (MC) - kerosene,
• Slow curing (SC) - diesel/heavy oil. Used for prime coat, tack coat, surface dressing and patching, when heating is difficult, or for cold weather. Solvent evaporates.

4. Bitumen emulsion

Fine droplets of bitumen dispersed in water with an emulsifier. Cationic (positive) types are used for most aggregates. Types by setting speed: rapid (RS), medium (MS) and slow setting (SS). No heating needed; used for tack coat, surface dressing, slurry seal, cold mix and in wet conditions; it is environmentally friendly and safe.

5. Tar

Obtained by destructive distillation of coal, wood or similar organic matter (coal tar). Road tar grades RT-1 to RT-5 (RT-1 for surface painting, RT-2 for surface dressing, RT-3 for penetration macadam). It is more temperature susceptible and ages quickly and is carcinogenic, so now rarely used; has good adhesion with aggregate.

Meaning

The penetration value of bitumen is the depth, in units of 0.1 mm, to which a standard needle of total load 100 g penetrates vertically into a bitumen sample in 5 seconds at a temperature of 25 degrees C (IS:1203). It is the measure of hardness/consistency: a lower value means harder bitumen. The grade is named by penetration, such as 60/70 (value 60 to 70).

Apparatus

• Standard needle (1 mm diameter, taper) and penetrometer with a 50 g weight on the needle assembly (total 100 g).
• Container (55 mm diameter, 35 mm depth for penetration below 200), water bath at 25 degrees C (capacity 10 litres, depth 100 mm), a transfer dish, thermometer and stop clock.

Procedure

1. Heat the bitumen sample gently to a pouring consistency (at 75-100 degrees C above the softening point), stir to remove air bubbles, and pour into the container to a depth of at least 15 mm more than the expected penetration.
2. Cool in air at 15-30 degrees C for 1 to 1.5 hours; then place the container in the water bath (at 25 degrees C) for 1 to 1.5 hours.
3. Fill the transfer dish with water from the bath, put the container in it, and place on the penetrometer stand.
4. Adjust the needle so that its tip just touches the sample surface (use the reflection). Set the dial to zero.
5. Release the needle for 5 seconds, then read the penetration from the dial in 0.1 mm.
6. Make at least three readings, at points at least 10 mm apart and 10 mm away from the edge of the container; clean the needle every time. The mean of three, with a difference not more than the allowed tolerance, is the penetration value.

Significance

• It measures consistency and grading, and so decides the suitability of bitumen for the climate: harder grades (30/40, 60/70) for hot climate and heavy traffic; softer grades (80/100) for cold regions.
• It is used for quality control and for estimating the temperature susceptibility with the softening point (penetration index).
• NRS 2070 and IS:73 specify penetration grades of 60/70 and 80/100 for roads in Nepal (in hills 80/100 is used for lower temperature).

Road aggregates form the main part of a pavement (about 90-95% by weight), and so must have the following properties (IRC, NRS 2070 and IS:383).

1. Strength - resistance to crushing under wheel loads, particularly in the base and surface (aggregate crushing value not more than 30% for the wearing course).
2. Hardness - resistance to wear and abrasion by traffic (LA abrasion value below 30-40%).
3. Toughness - resistance to impact, in the surface and base (aggregate impact value below 30% for wearing courses; 45% for lower layers).
4. Durability / soundness - resistance to weathering, frost and chemical action (soundness loss not more than 12% with sodium sulphate).
5. Shape - angular, cubical, well-shaped particles that interlock; avoid flaky and elongated particles (combined flakiness and elongation index below 30-35%).
6. Surface texture and skid resistance - rough texture to give good skid resistance (polished stone value at least 55 to 60 for wearing course).
7. Affinity with bitumen (adhesion) - hydrophobic aggregate that resists stripping in the presence of water (limestone, basalt, granite are better than quartzite or siliceous gravel).
8. Low water absorption and porosity - not more than 2% (porous aggregates absorb bitumen and are weak).
9. Good grading - proper particle size distribution for dense packing.
10. Cleanliness - free from clay, silt, organic matter and weak materials.
11. Specific gravity - sufficient (2.5-3.0), uniform.
12. Economy and availability near the site.

Different pavement types

• Flexible pavement surface - hard, tough, skid resistant, good shape and good bitumen adhesion.
• Base and sub-base - strength, grading, and drainage.
• Rigid pavement - good strength, low alkali reactivity, low thermal expansion and clean.

Definition

Bituminous premixes (bituminous mixes) are mixtures of graded aggregates and bituminous binder that are prepared and mixed (at a plant or in place) before laying, then spread and compacted on the road to form a base or surface course. Examples: bituminous macadam, dense bituminous macadam, bituminous concrete.

Types

1. By aggregate gradation:
   • Dense-graded (continuous grading): bituminous concrete (BC), dense bituminous macadam (DBM), semi-dense bituminous concrete (SDBC).
   • Open-graded: bituminous macadam (BM), open-graded premix carpet (PC).
   • Gap-graded: stone matrix asphalt (SMA).
   • Sand mixes: sand asphalt, mastic asphalt.
2. By mixing temperature:
   • Hot mix (150-165 degrees C binder-aggregate),
   • Warm mix,
   • Cold mix (emulsion or cutback with aggregates at ambient temperature).
3. By the place of mixing:
   • Plant mix (hot mix plant, drum mixer) - best quality control,
   • Mix-in-place (road mix) - for low-volume roads.
4. By binder: bitumen, cutback, emulsion, tar and modified bitumen mixes.

Principle

The softening point of bitumen is the temperature at which a standard steel ball, sinking through a bitumen disc held in a brass ring, drops a distance of 25 mm when heated at a specified rate in a water or glycerine bath (ring and ball test, IS:1205). It shows the temperature at which bitumen changes from a solid to a flowing state, and so measures temperature susceptibility.

Apparatus

• Brass ring (inside diameter 15.9 mm, height 6.4 mm), steel balls (9.5 mm diameter, 3.5 g),
• Ball guides, a support frame holding the ring in a horizontal position, with a bottom plate 25 mm under the rings,
• Beaker (800 ml) containing distilled water (for softening point below 80 degrees C; glycerine for higher), thermometer (0 to 100 degrees C), stirrer and a heater.

   thermometer
      |   ball
   ===|===(o)===  ring + bitumen
      |    |
      |    | 25 mm
   ___|____v_____  bottom plate
   water bath, heated 5 C/min

Procedure

1. Heat the bitumen to fluid state, pour into the heated ring placed on a metal plate (smeared with glycerine), cool for 30 minutes, trim the surface flat with a hot knife.
2. Fix the rings in the holder, place a steel ball on each, and place in the beaker of distilled water at 5 degrees C (for 15 minutes).
3. Heat the bath so that the temperature rises at a uniform rate of 5 +/- 0.5 degrees C per minute, with stirring.
4. Note the temperature at which the bitumen touches the bottom plate with the ball. The average of the two balls (difference not above 1 degree C) is the softening point.

Significance

• Higher softening point means a bitumen with less temperature susceptibility - suitable for hot climates, to avoid bleeding and flushing in summer.
• It is used for grading (typical 45 to 55 degrees C for 60/70; 40 to 50 for 80/100; VG-30 at least 47 degrees C) and for finding the penetration index.
• Also used to decide the heating temperature for mixing and compaction (the mixing temperature is about 100 degrees C higher than the softening point).

The aggregate impact value (AIV) measures the resistance of an aggregate to sudden shock or impact (toughness), and is the percentage of fines (passing 2.36 mm) produced when the sample is subjected to 15 blows of a standard hammer (IS:2386 Part IV).

Apparatus

• Impact testing machine: a metal base, a cup (steel, 102 mm inside diameter, 50 mm deep), a hammer of 13.5-14 kg (usually 14 kg) with a free fall of 380 mm.
• Cylindrical measure (75 mm diameter, 50 mm depth), tamping rod (10 mm dia, 230 mm long), IS sieves 12.5, 10 and 2.36 mm, balance.

        | hammer 14 kg
        |  falls 380 mm
     ___v___
    |  cup  |  <- aggregate in 1 layer 
    |_______|
      base

Procedure

1. Take dry aggregate passing 12.5 mm and retained on 10 mm sieve (about 300-400 g, to fill the measure in three layers).
2. Fill the cylindrical measure in three layers, tamping each layer with 25 strokes; weigh the sample ($W_1$).
3. Transfer to the cup of the machine (fixed on the base) and tamp 25 times.
4. Raise the hammer to 380 mm and let it fall freely; give 15 blows, at an interval of not less than 1 second.
5. Remove the crushed material and sieve through the 2.36 mm sieve. Weigh the passing portion ($W_2$) and the retained portion ($W_3$); $W_2 + W_3$ should not differ from $W_1$ by more than 1 g.

Report the mean of two tests.

Interpretation

• Below 10%: exceptionally strong; 10-20%: strong; 20-30%: satisfactory for roads surfacing; above 35%: weak.
• Limits (IRC/NRS 2070): not more than 30% for wearing course and not more than 35-45% for bituminous base and WBM.

A bituminous mix (aggregate plus bitumen) should have the following properties.

1. Stability - resistance to deformation (rutting, shoving, bleeding) under traffic load, resulting from the internal friction of aggregates, interlock and the cohesion of binder. Depends on the aggregate shape, grading and binder content.
2. Durability - the mix must resist weathering, ageing of bitumen, water, and the abrasive action of traffic for a long time. It needs a sufficient binder content, with adequate film thickness, low voids and a good compaction.
3. Flexibility - ability to adjust to settlement and deflection of the base and subgrade without cracking; requires a higher binder content and a proper binder grade.
4. Fatigue resistance - resistance to cracking under repeated load applications.
5. Skid resistance - rough texture and angular aggregate give a good grip on wet and dry surfaces.
6. Impermeability - protects the lower layers from water; adequate air voids (3-5%) but not interconnected.
7. Workability - ease of mixing, spreading and compaction, depends on grading and temperature.
8. Resistance to moisture damage (stripping) - good adhesion between aggregate and bitumen.
9. Resistance to low-temperature cracking and high-temperature rutting.
10. Economy - using local materials with minimum cost.

The properties conflict (for example, more binder gives durability and flexibility but reduces stability), so the mix design (Marshall) finds an optimum balance.

The Marshall stability test finds the load-carrying capacity and deformation of a compacted cylindrical bituminous specimen. It is the basic test of the Marshall mix design method (ASTM D6927, Asphalt Institute MS-2, adopted in NRS 2070).

Apparatus

• Cylindrical mould 101.6 mm dia and 75 mm high, with base plate and collar.
• Marshall compaction hammer (4.536 kg, free fall 457 mm) and compaction pedestal.
• Breaking head (two semicircular segments of 101.6 mm inside diameter), Marshall testing machine with a proving ring/load cell and a flow dial, loading rate 50 mm/min.
• Water bath at 60 degrees C, ovens, mixing bowl, thermometers.

Procedure

1. Heat aggregates (blended to the specified gradation, about 1200 g per specimen) and bitumen (grade 60/70 or VG-30) to the mixing temperature (about 150-160 degrees C for aggregates, 150 degrees C for bitumen); mix thoroughly.
2. Place in a preheated mould (and a paper disc at the bottom) and compact with 75 blows on each face for heavy traffic (50 for medium; 35 for light) at about 135-140 degrees C.
3. Cool, extrude, and measure height; find weight in air and in water and calculate the bulk density and voids.
4. Immerse the specimens in the 60 degrees C water bath for 30-40 minutes (to represent the hottest condition).
5. Remove, place in the breaking head, and put it in the Marshall machine; apply load at 50 mm/min until the maximum load is reached, then the load drops.
6. Record the maximum load (Marshall stability, kN) with correction for the specimen thickness, and the flow value (vertical deformation at the max load in 0.25 mm units or mm). The test must be done within 30 seconds after removing from the bath.

Significance

• Stability (typically not less than 8 kN for heavy traffic; or as per NRS 2070/IRC:111 for BC) shows the resistance to rutting and shoving.
• Flow (2 to 4 mm) indicates the plasticity and flexibility; high flow means a plastic mix that tends to rut; low flow means a brittle mix that cracks.
• Marshall quotient = stability/flow (kN/mm), an indicator of the stiffness of the mix.
• Together with density and voids data, the results are used to find the optimum binder content, to check the mix against standards, and for quality control of the plant mix.

Hardness of aggregate is its resistance to wear (abrasion and attrition) caused by the rubbing action of vehicle wheels and by the movement of the particles themselves. It is measured by the Los Angeles abrasion test (IS:2386 Part IV; ASTM C131), and also by the Deval abrasion test.

Los Angeles abrasion test - apparatus

• Los Angeles machine: hollow steel cylinder 700 mm diameter and 500 mm long, closed at both ends, with a shelf fixed inside, rotating about the horizontal axis.
• Abrasive charge of steel balls (about 48 mm diameter, 390-445 g each; 6 to 12 balls).
• IS sieve of 1.7 mm and a balance.

        axis (30-33 rpm)
     .-----------------.
    /   o o  <- balls    \
   |  aggregate  | shelf  |
    \               ____ /
     '-----------------'
         cylinder 700 x 500 mm

Procedure

1. Take oven-dried aggregate of 5 kg or 10 kg, according to the grading (A to G).
2. Place it with the specified number of balls in the cylinder; close the cover.
3. Rotate at 30 to 33 rpm for 500 revolutions (gradings A-D) or 1000 (E-G).
4. Remove the material, separate the steel balls, and sieve on a 1.7 mm IS sieve; wash and oven-dry the retained portion and weigh it.

where $W_1$ = original weight and $W_2$ = weight retained on 1.7 mm sieve.

Significance

• It indicates the ability of the aggregate to withstand the wear from traffic, and the crushing and impact in rolling.
• Good aggregate has a low value: below 30% for the wearing course, up to 35-40% for the base, 50% for the lower layers (IRC/NRS 2070). A value below 20% is considered hard.
• It is a simple, reliable test used in aggregate source selection.

The soundness test (IS:2386 Part V) measures the resistance of aggregate to disintegration by weathering action, such as freezing and thawing, wetting and drying, and salt crystallisation. It uses repeated immersion in a saturated solution of sodium sulphate or magnesium sulphate.

Apparatus

Wire-mesh baskets, containers for the solution, IS sieves (as per the aggregate size), oven (105-110 degrees C), balance and a hydrometer. Solution: saturated sodium sulphate (or magnesium sulphate) with specific gravity 1.151 to 1.174 (1.295-1.308 for MgSO4).

Procedure

1. Wash and oven-dry the aggregate, separate into the fractions (for example 10-12.5 mm, 12.5-20 mm, 20-25 mm...) and weigh a specified weight (usually 500 g of each coarse fraction, or 100 g for fine).
2. Immerse the sample in the solution for 16 to 18 hours, at 27 +/- 1 degrees C, so that the solution covers it by at least 15 mm.
3. Remove, drain for 15 minutes, and dry in the oven at 105-110 degrees C until constant weight (a drying cycle of about 4 to 8 hours); cool to room temperature.
4. This forms one cycle; repeat the cycle five times.
5. After the last cycle, wash the sample free of the salt (with barium chloride test), dry, and sieve over the sieve on which it was originally retained.
6. Weigh the retained material and find the loss.

where $W_1$ = initial weight and $W_2$ = final retained weight. The total weighted average loss is reported.

Significance

• It shows the durability of aggregate under climatic changes; useful in places with freezing weather (the hills and high altitude of Nepal).
• Limits: loss not more than 12% with sodium sulphate and 18% with magnesium sulphate after five cycles (IS/IRC/NRS 2070).

Viscosity is the property of a fluid that resists flow. For bituminous binders it determines the temperature at which the material is suitable for spraying, mixing and compaction, and so it is a measure of the flow property or consistency.

Methods

1. Orifice (efflux) viscometers - Saybolt Furol and Standard Tar Viscometer (STV), which measure the time taken for a fixed volume of binder to pass through a standard orifice (used for cutbacks, tars and emulsions).
2. Capillary viscometers - kinematic viscosity at 135 degrees C and absolute viscosity at 60 degrees C (vacuum capillary viscometer, used for VG grading in IS:73).
3. Rotational viscometer (Brookfield) - for modified binders.

Standard Tar Viscometer (road tar viscometer) test (IS:1206)

Apparatus: A brass cup (about 100 mm dia) with an orifice of 10 mm (4 mm for the harder grades) at the bottom, closed by a ball valve; the cup sits in a water jacket with a heater, thermometer, a stirrer, a 50 ml receiving flask and a stop clock.

     thermometer
        |   __ __ 
     |  v  |    | cup (binder)
     |_____|    |
     | water jacket (heated) |
        \ _ /   <- orifice 10 mm
          |
        [flask 50 ml]

Procedure:

1. Select the orifice size and the test temperature (e.g. 25, 40 or 60 degrees C for cutbacks/tars; 135 degrees C for thick bitumen).
2. Heat the water bath to the temperature and keep the binder in the cup at that temperature (stir to uniform temperature); close the orifice with the valve.
3. Fill the cup to the mark with the binder.
4. Place the receiving flask under the orifice, lift the valve and start the stopwatch.
5. Stop the stopwatch when the binder reaches the 50 ml mark in the flask.
6. The time in seconds is the viscosity value (Saybolt Furol seconds or STV seconds). Repeat the test and take the average.

Significance

• Gives the proper grade of binder (for example, the VG-10 to VG-40 classification by viscosity) and identifies whether it is suitable for the climate.
• Gives the mixing and compaction temperatures (for example, the temperature at which viscosity is 170 +/- 20 cSt for mixing and 280 +/- 30 cSt for compaction).
• Shows the suitability of the cutback or emulsion for spraying and the control of quality.

Highway materials are all the natural and manufactured materials used in the construction, strengthening and maintenance of roads and pavements. They include soil (subgrade, embankment), aggregates (stone, gravel, sand), binders (bitumen, cutback, emulsion, tar, cement, lime), bituminous mixes, concrete, geosynthetics and steel; the engineering of highway materials deals with their properties, tests and use in each pavement layer.

The California Bearing Ratio (CBR) test (IS:2720 Part 16) measures the strength of subgrade soil by the resistance to penetration of a plunger and is used with IRC:37 and NRS 2070 to find the pavement thickness.

Preparation of dynamically compacted specimen

1. Take about 5-6 kg of soil, pass through the 20 mm sieve (material above 20 mm is replaced by an equal weight of 4.75-20 mm fraction).
2. Find the optimum moisture content (OMC) and maximum dry density (MDD) by the standard/heavy compaction (Proctor) test; the specimen is prepared at OMC and at the required field density (generally 97% of MDD of heavy compaction).
3. Mix the soil with the calculated water.
4. Mould: 150 mm dia and 175 mm high, with collar and a base plate; the spacer disc (148 mm dia, 47.7 mm high) is placed at the bottom with filter paper above it.
5. Compact the soil dynamically in the mould in five layers (heavy compaction: each layer with 56 blows of a 4.89 kg rammer falling 450 mm; for standard compaction: 3 layers of 56 blows with a 2.6 kg rammer from 310 mm).
6. Remove the collar, trim the top surface level, take out the spacer disc, and invert the mould on the base plate with filter paper.
7. For soaked CBR: place surcharge weights (2.5 kg annular weights, total 5 kg, representing the pavement weight) on the top, and immerse in water for four days while measuring the swell with a dial gauge.

Method of conducting the test

1. After soaking, drain for 15 minutes and place the mould with the surcharge weights under the plunger of the loading frame.
2. Seat the plunger (50 mm diameter) under a small load of 4 kg; set the load and penetration dials to zero.
3. Apply the load so that the plunger penetrates at 1.25 mm per minute.
4. Record the load at penetrations of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10 and 12.5 mm.
5. Plot the load-penetration curve; if the curve is concave upward at the start, correct by shifting the origin.
6. Read the loads at 2.5 mm and 5.0 mm penetration.

Standard loads for crushed stone: 13.34 kN (1370 kg) at 2.5 mm and 20.0 kN (2055 kg) at 5.0 mm. The CBR at 2.5 mm is normally adopted; if the CBR at 5 mm is greater, repeat the test, and if the same, adopt the higher value.

Take the average of three specimens; the design CBR is the soaked value at the field density.

A flexible pavement consists of layers, each made of materials suited to the stress at that level. Materials are specified in NRS 2070 / DoR Standard Specifications and IRC.

Rigid pavement layers

• Subgrade, granular sub-base, dry lean concrete (DLC) base and pavement quality concrete (PQC) slab of M30-M40 concrete, with dowel and tie bars and joint sealants.

Binders and other materials

• Bitumen (VG-30, 60/70), cutback, emulsion for tack coat and prime coat; cement, lime, fly ash for stabilisation; geotextiles and geogrids; steel in reinforcement.

The materials get stronger and costlier upwards, so cheaper local materials are used in lower layers.

Descriptive tests of aggregate are simple tests (visual or physical) that describe the character of the aggregate rather than its strength.

1. Particle shape tests - flakiness index, elongation index and angularity number; show the proportion of flat, long and angular particles.
2. Specific gravity and water absorption - show density and porosity of the aggregate.
3. Bulk density and voids - used for mix proportioning.
4. Surface texture (smooth, rough) and colour - indicate the skid resistance and the affinity with bitumen.
5. Petrographic examination - rock type and mineral content, including alkali-silica reactive minerals.
6. Gradation (sieve analysis) - particle size distribution.
7. Stripping/adhesion test and cleanliness (clay, silt and organic content) tests.

These tests are descriptive in the sense that they give the physical description to the aggregate, and are used with the strength tests (crushing, impact, abrasion) for acceptance.

Hardness and toughness

• Hardness is the resistance of aggregate to wear (abrasion and attrition) by the rubbing action of the wheels of vehicles. Test: Los Angeles abrasion test (limit about 30% for a wearing course) or Deval attrition test.
• Toughness is the resistance of aggregate to failure by impact (sudden shock) from moving loads. Test: Aggregate impact value test (AIV not more than 30% for a wearing course; 45% for base).

Aggregate blend by graphical method

Given: Aggregates P and Q with a specified grading envelope. Let $x$ be the fraction of P and $(1-x)$ the fraction of Q. At every sieve:

In the graphical method (Rothfuch's method) the % passing of P is marked on one vertical axis, that of Q on another, and a straight line joins the two points on each sieve; the vertical lines for the specification limits are drawn on it, and the common range of the proportion is read. The same range is obtained by solving for $x$ at each sieve:

Common range: P between 37.5% and 62.5% (controlled by the 2.36 mm sieve), so Q is 62.5% to 37.5%.

Best proportion: the mid-point, P : Q = 50 : 50. Check:

Answer: Aggregate P : Aggregate Q = 50 : 50 (any proportion of P between about 38% and 62% satisfies the specification, and 50% gives the gradation near the middle of the envelope).

Distinguishing the three values

All three are low for good aggregates.

Why flakiness and elongation indices are needed

• Flaky particles (least dimension less than 0.6 times the mean size) and elongated particles (length more than 1.8 times the mean size) have a poor shape.
• They break easily under rollers and traffic, which changes the grading, and are weak.
• They do not interlock or pack well, leaving more voids, and give lower stability and compaction.
• They tend to orient in the same direction, giving weak planes.
• They need more binder for coating in bituminous mixes. So their quantity is controlled (combined flakiness and elongation index not more than about 30-35%) by determining the indices.

The shape of aggregate particles is judged by two indices (IS:2386 Part I).

• Flakiness Index (FI) - percentage by weight of particles whose least dimension (thickness) is less than 0.6 times their mean dimension (flaky).
• Elongation Index (EI) - percentage by weight of particles whose greatest dimension (length) is greater than 1.8 times their mean dimension (elongated).

Flakiness index test

Apparatus: thickness gauge (a metal plate with slots of different width for each size fraction), IS sieves (63, 50, 40, 31.5, 25, 20, 16, 12.5, 10 and 6.3 mm), balance.

1. Take a sample of at least 200 pieces of each size fraction to be tested, and sieve it into the standard fractions (a fraction is taken only if it has more than 5% of the sample).
2. Weigh each fraction ($W$).
3. Pass each particle of the fraction through the slot of the corresponding width in the thickness gauge (slot width = 0.6 x mean size of the fraction, for example 10.8 mm for the 25-20 mm fraction).
4. Weigh the particles passing the slot ($w$).

Elongation index test

Apparatus: length gauge (a metal gauge with pins at spacings of 1.8 times the mean size of each fraction).

1. Use the same sieved fractions (at least 200 pieces each).
2. Try each particle in the length gauge; those that cannot pass through the gauge opening (length greater than the gauge) are elongated. Weigh them ($w$).

Limits

Combined FI + EI should not exceed about 30% for wearing courses and bituminous layers, and 35% for base layers (NRS 2070/IRC/MoRTH). Flaky and elongated particles lower the stability, break during compaction and need more binder.

Aim: find the proportions $a$, $b$, $c$ (in %) of quarries A, B and C, with $a + b + c = 100$, so that the combined % passing at every sieve falls within the specification. The mix passing at a sieve is

Assumption: the "Specification" column gives the allowable range of % passing; the target is near the middle of each range.

Step 1: Decide the main fraction by control sieves

• Quarry A is a fine material (100% passing even 0.30 mm; 40% passes 0.075 mm). Quarry B is a coarse sand-gravel. Quarry C is the coarsest.
• 4.76 mm sieve: only C is coarse (55% passing); $P = 100 - 45\,c/100$. The range 60-75 gives $c = 55.6$ to $88.9\%$, so C is the main component (about 65%).
• 0.075 mm sieve: $P = 40a + 3(b + c)$ over 100 $= 3 + 0.37a$. For 5 to 10, $a = 5.4$ to $18.9\%$, so A is a small proportion (about 10%).
• The remainder is B: $b = 100 - a - c$.

Step 2: Trial mix A : B : C = 10 : 25 : 65

All sieves are inside the specification. (A Python check of all combinations in 1% steps gives the feasible range as: A 6-15%, B 0-38%, C 56-85%; the least-squares fit to the mid-points is about 8 : 25 : 67, which is also acceptable. The trial mix 10 : 25 : 65 is rounded and convenient for site batching.)

Answer: Quarry A : Quarry B : Quarry C = 10% : 25% : 65%, giving a blend passing 100, 96.8, 70.8, 45.8, 31.2, 17.9, 6.7% on the sieves 25.4, 12.7, 4.76, 1.18, 0.30, 0.15, 0.075 mm.

Liquid bitumen is bitumen that is made fluid at ordinary temperature, so that it can be sprayed or mixed with aggregate without heavy heating. It is produced in two ways: by cutting back the bitumen with a solvent (cutback bitumen) or by emulsifying it in water (bitumen emulsion).

Cutback bitumen

Bitumen dissolved in a volatile petroleum solvent. After laying, the solvent evaporates (cures) and leaves the bitumen binder.

Grade numbers (0 to 3, for example MC-0, MC-3) show the viscosity: the higher the number, the thicker.

Bitumen emulsion

Tiny globules of bitumen (about 60% by weight) suspended in water with an emulsifier. Types: rapid setting (RS), medium setting (MS) and slow setting (SS); cationic (suited to most aggregates and wet conditions) and anionic.

Importance

• Used for prime coat, tack coat, surface dressing, patch repairs and cold mixes.
• Allows work in cold weather and where heating facilities are not available, such as remote hill roads.
• Emulsions are safe, save fuel and can be applied on damp surfaces.
• Disadvantages of cutbacks: solvent cost, fire risk and loss of solvent to the air.

Types of consistency tests on bituminous binders

1. Penetration test - hardness at 25 degrees C (0.1 mm).
2. Softening point test (ring and ball) - temperature at which bitumen softens.
3. Viscosity test (Saybolt/tar viscometer or capillary) - flow time at the specified temperature.
4. Ductility test - elongation before breaking at 25 degrees C.
5. Float test - for semi-solid tars and cutbacks (time for a plug of binder to be forced out by water).

Why different consistency tests are needed

• Bitumen is a visco-elastic material, and its consistency changes with temperature, so one test at one temperature can't describe its behaviour in all conditions.
• Penetration shows the hardness in service (25 degrees C); softening point gives behaviour at the high service temperature (resistance to flow and bleeding); viscosity gives the behaviour at spraying, mixing and compaction temperatures (60 to 150 degrees C); ductility shows the cohesion and cracking resistance at low temperature.
• Two bitumens may have the same penetration but different temperature susceptibility, so combined tests are used (for example the penetration index from penetration and softening point).
• Different types of binders (bitumen, cutback, tar, emulsion) have a different consistency range, so different tests suit them.

Ductility test

The ductility of bitumen is the length in cm to which a standard briquette is stretched before the thread breaks.

1. Heat the sample and pour it in a brass briquette mould (coated with glycerine-dextrin), and cool and trim it level.
2. Keep the mould in a water bath at 25 degrees C for about 90 minutes.
3. Fix the specimen in the ductility machine in the water bath (25 degrees C), remove the side pieces of the mould, and pull the ends apart at a uniform rate of 50 mm per minute.
4. Record the distance at which the thread breaks, in cm. Take the mean of three tests.

Significance

• A high ductility shows good flexibility and cohesion and so the bitumen can follow movements without cracking, and a good binder for the aggregate.
• Specification: at least 50 cm for 60/70 and 80/100 grades (IS:73/NRS 2070); low value means a brittle binder that cracks in cold weather and with age.

A bituminous binder for roads should have the following properties.

1. Adequate consistency - suitable viscosity and penetration for the climate and traffic: sufficiently fluid at the mixing and spraying temperatures (to coat aggregate) and stiff enough at service temperature to avoid bleeding and rutting.
2. Low temperature susceptibility - its properties should change little with temperature (high penetration index), so it neither softens in summer nor cracks in winter.
3. Good adhesion and cohesion - strong bond with aggregate (resists stripping when wet) and good internal cohesion (adequate ductility).
4. Ductility and flexibility - ability to deform without cracking under traffic and settlement.
5. Durability / resistance to ageing - low loss on heating, resists oxidation, hardening and weathering.
6. Water resistance - impermeable and not damaged by water; low water content.
7. Safety - high flash and fire point to allow safe heating; no foaming when heated.
8. Purity - solubility in trichloroethylene of at least 99%, free of water, wax and impurities.
9. Workability - easy handling, mixing and spreading.
10. Economy and availability.

Bituminous mixes are classified in the following ways.

1. By aggregate gradation

• Dense-graded mixes - continuous grading from coarse to fine, with low voids; strong and impermeable. Examples: bituminous concrete (BC), dense bituminous macadam (DBM), semi-dense bituminous concrete (SDBC).
• Open-graded mixes - mostly coarse aggregate with few fines, high voids (15-25%), good drainage but lower strength. Examples: bituminous macadam (BM), open-graded premix carpet, porous asphalt.
• Gap-graded mixes - some intermediate sizes missing, with a high coarse and filler content and a lot of binder; e.g., stone matrix asphalt (SMA), highly rut-resistant.
• Sand and mastic mixes - sand asphalt, mastic asphalt (fine aggregate in rich binder, used for bridge decks).

2. By mixing temperature and method

• Hot mix asphalt (prepared at 150-165 degrees C in a hot mix plant).
• Warm mix (100-130 degrees C using additives).
• Cold mix (with emulsion or cutback at ambient temperature).
• Plant mix and mix-in-place (road mix).

3. By use in the pavement

• Surface course - BC, SDBC, premix carpet, mastic, SMA.
• Binder/base course - DBM, BM.

4. By binder

Bitumen, cutback, emulsion, tar and polymer/rubber-modified bitumen (PMB, CRMB) mixes.

In the Marshall mix design, specimens are prepared at binder contents that differ by 0.5% (for example 4.5 to 6.5%) and tested for stability, flow, density and voids. Five graphs are plotted with the binder content on the X-axis.

Shape of the graphs

 Stability       Unit weight      Air voids (Vv)
   ^   .-.         ^    .-.         ^ \
   |  /   \        |   /   \        |  \
   | /     \       |  /     \       |   \___
   +-----------> % +--------------> +--------> %
   peak at B1      peak at B2        4% at B3

 Flow            VMA              VFB
   ^       /        ^ \     /       ^        /
   |     /          |  \___/        |      /
   |___/            |               |___ /

• Stability rises with the binder content to a maximum and then falls.
• Unit weight (bulk density) increases to a maximum (usually a little at higher binder content than stability) and then decreases.
• Air voids ($V_v$) decrease steadily with more binder.
• Flow increases steadily with the binder.
• VMA decreases to a minimum and then increases; VFB increases steadily.

Finding the OBC

1. From the graphs read the binder content for:
   • maximum stability, $B_1$;
   • maximum bulk density, $B_2$;
   • 4% air voids (the middle of the 3-5% range), $B_3$.
2. The optimum binder content is the average:

3. Read the stability, flow, voids, VMA and VFB at the OBC from the graphs and check against the specification (for example, flow 2-4 mm, $V_v$ 3-5%, VFB 65-75%). If the values are not satisfied, the mix is redesigned.

Example: $B_1 = 5.2\%$, $B_2 = 5.5\%$, $B_3 = 5.6\%$ gives OBC $= 5.43\%$ (illustrative values).

Bulk specific gravity ($G_{mb}$)

The bulk specific gravity of a compacted bituminous mix is the ratio of the weight in air of a given volume of the compacted mix (including the air voids) to the weight of an equal volume of water at the same temperature. It is found by weighing the specimen in air ($W_a$), in water ($W_w$) and SSD ($W_{ssd}$):

It is used to find the unit weight and the voids (air voids, VMA, VFB) of the compacted mix.

Maximum (theoretical) specific gravity ($G_{mm}$)

The maximum specific gravity is the specific gravity of the loose mix with no air voids: the ratio of the weight of the voidless mix (aggregate + binder) to the weight of an equal volume of water. It is measured on the loose mix in the vacuum-pycnometer (Rice) test, or calculated:

where $W_s$ and $W_b$ are the percentages by weight of aggregate and binder, $G_{se}$ the effective specific gravity of the aggregate, and $G_b$ the specific gravity of the binder.

Use

$G_{mb}$ is always less than $G_{mm}$ because of the air voids.

Given: weight in air $W_a = 1231$ g, in water $W_w = 715$ g, SSD weight $W_{ssd} = 1232$ g, and $G_{mm} = 2.56$.

Step 1: Bulk specific gravity of the compacted mix

Step 2: Percentage air voids

Answer: Air voids $V_v \approx 7.0\%$ ($G_{mb} = 2.381$). This is more than the usual 3-5% limit for a dense mix (NRS 2070/Asphalt Institute), so the specimen is too porous.