Highway Materials & Testing

Subgrade: It is the natural foundation or fill which directly receives the loads from the pavement.

Functions:

• It should give adequate support to the pavement.
• It should posses sufficient stability under adverse climate and loading conditions.

Important tests for subgrade soil:

1) Modulus of subgrade reaction (K): Defined as the pressure sustained per unit deformation of subgrade at specified deformation or pressure level, in plate bearing test.

USE OF PLATE BEARING TEST:  Plate bearing test is used to evaluate.

• The supporting power of sub grade for use in pavement design by using relatively large diameter plates.
• Modulus of sub grade reaction of soils (K)
• Modulus of Elasticity of soils (Es).

Apparatus:

1. Plates of dia 75, 60, 45 and 30cm.
2. A loading device consisting of jack and proving ring arrangement.
3. A reaction frame against which the jack can give a thrust to the plate.
4. A datum frame resting far from the loaded area and dial gauges from this frame are used to measure the settlement of the loaded plate.

Plate bearing test setup:

• Seating load : 320 Kg for 75 cm dia plate.
• Seating : 0.07 Kg/cm²
• Standard plate size: 75 cm dia (commonly used size is 30 cm dia.)
• Unit of K: kg/cm³ or N/mm³

Method 1: 

• Standard settlement 1.25 mm.
• K = P/0.125 kg/cm²/cm

Method 2: (Revised U.S. Corps of Engineers method)

• Standard Pressure = 0.7 kg/ cm²
• K=0.7/ö Kg/cm²/cm

Boussinesq settlement equations for subgrade modules:

For flexible plate:

• Es = Modulus of elasticity of soil
• p = pressure on plate
• a = radius of plate
• 𝛿 = maximum vertical deflection at the centre of the flexible plate.

NOTE: Wheel loads through inflated rubber tyres may be considered as flexible plate loading or loading with uniformly distributed pressure.

For a rigid plate:

• The pressure on the surface is not uniformly distributed.
• p = pressure at a level of design deflection 𝛿 (If  𝛿 is theoretical maximum deflection, ‘p’ is the corresponding pressure)
• a = radius of plate.

NOTE:

1. A mild steel plate is considered relevant to the condition of rigid plate.
2. Boussinesq’s equation above are valid only for homogeneous single layer only.

Correction to ‘K’ value for small plate size :

As per IRC the standard plate size is 75 cm diameter.

• If 30 cm diameter plate is used for the purpose of experiment then K of soil = K based on 75 cm dia plate = 0.5 (K based on 30 cm dia plate)

CBR Test: A test for evaluating the stability of soil sub grade and other flexible pavements. Flexible pavement thickness is determined based on this test. It may be conducted in the laboratory as well as in the fields. It is an arbitrary strength test and  hence cannot be used to evaluate the soil properties like cohesion or internal friction or shearing resistance. Material passing 20 mm sieve is only used in the test presence of coarse grained particals  would result in poor reproductibility of results. The test is meant for soil but it is also carried out on sub base and granular base course materials.

Test Apparatus: Cylindrical mould (150 mm dia), Base plate, cylindrical plunger (50 mm dia), surcharge weight (147 dia) and compression testing machine.

Test Procedure: Specimen in the mould is subjected to four days soaking load is applied in the cylindrical plunger of 5 cm dia so that the penetration rate is 1.25 mm/minute. The loads for 0, 0.5, 1.0, 2,0, 2.5, 3.0, 4.0, 5.0, 7.0, 10.0, and 12.5 mm penetration are recorded. The load penetration curve is then plotted. If the curve is concave upwards initially due to surface irregularities, correction is applied by drawing a tangent to the curve at the point where the tangent meets the horizontal axis. The test load for 2.5 mm and 5.0 mm penetration are recorded

Normally CBR 2.5mm > CBR 5mm

(CBR reported is higher of the two).

If CBR5mm  > CBR 2.5mm , test is to be repeated for checking. If it again gives the same result, the higher value is repeated as CBR value.

Standard load values on crushed stones

Penetration(mm)	Standard Load (kg)	Standard Stress (kg/cm2)
2.5	1370	70
5.0	2055	105

North Dakota test: Another arbitrary strength test having a cone whose semi vertex Angle is 7⁰ 45 min

Aggregate: Main constituent in the pavement used as sub base as base courses. The strength of a flexible pavement depends upon the strength of aggregate

Hard aggregates: They are used for wearing course of superior pavement types. Used for base courses also. Ex : Granite

Toughness : Resistance against impact ‘aggregate impact test’ is conducted.

Specimen passing 12.5 mm sieve but retained on 10 mm sieve - 3 layers - 25 times tamping — hammer of 13.5 to 14 kg – free fall 38 cm- 15 blows.

• Aggregate impact value is percentage fine passing 2.36 mm sieve to total aggregate weight.

Specifications:

• for wearing coarse <35%.
• for base coarse < 45%.

Soundness test: Resistance of aggregates to weathering action.

Solution of sodium sulphate or magnesium sulphate used.

Specifications: The average loss in weight after 10 cycles < 12% when tested with Sodium sulphate and < 18% when tested with magnesium sulphate.

Shape tests: better shape is angular

Flakiness index: Percentage by weight of aggregate particles whose least dimension thickness is less than 0.6 times their mean dimension. The test is applicable to sizes larger  than 6.3 mm.

• For road aggregate F.I shall be less than 15%.

Elongation Index: Percentage by weight of particle Greater dimension or length is greater than 1.8 times their mean dimension.

• For road metal not greater than 10 to 15%.

Strength: Resistance against gradual load.

• Assessed by aggregate crushing test. 
• Dry  aggregates passing 12.5 mm IS sieve and retained on 10 mm sieve is used. Three layers in mould — each layer tamped 25 times by 1.6 cm dia tamper — 45 to 60 cm fall — 40 t load applied at the rate of 4t /minutes.
• Aggregate crushing value = Crushed material pas sin g 2.36mm x 100/ Total weight of aggregate

IRC recommendations:

• Strong aggregates give low crushing value.
• For base course > 45 %.
• For surface course> 30 %

Hardness: Resistance to abrasion. Standard Abrasion tests are

1) Los Angle’s test

2) Deval abrasion test

3) Dorry abration test.

Los angel’s test : Resistance to abrasion and impact is evaluated by this method.

5 to 10 kg specimen — rotated at 30 to 33 rpm — 500 to 1000 revolutions depending on the grading of specimen

Wt of aggregate passing 1.7 mm sieve x 100/ Total weight

Specifications: 

• For cement concrete construction not equal to 16%.
• Surface or wearing course of bituminous mix not equal to 30%
• For base courses of bituminous bound Macadam not equal to 50%

Specific Gravity: Preferably between 2.6 to 2.9 for good road aggregate.

Water absorption test: not more than  0.6%

Angular number: For any aggregate it measures the voids in excess of rounded gravel For which voids ratio is 33%.

Angularity number = 67 - % solid volume
    = 67 - 100W/C.G

Where

W = mean weight of aggregates in the cylinder, g

C = Weight of water required to fill the cylinder, g

G = Specific gravity of aggregate.

• The higher the number, more angular is the aggregate.
• The range of the number for constructions is 0 to 11.
• The angularity number is expressed to the nearest whole number

Bitumen adhesion test: Adhering capacity of bitumen with aggregate ‘Static Immersion Test’ is very commonly used as it is quite easy and simple. As per IRC, the stripping value of aggregates should not exceed 5% for use in Bituminous surface dressing, penetration Macadam and Bituminous Macadam constructions, when aggregate is immersed in water bath at 40⁰ C for 24 hours.

BITUMINOUS MATERIALS

1. Bitumen: It is a hydro carbon of either natural or artificially obtained during fractional distillation of petroleum.
2. Asphalt: If the Bitumen contains some inert material of minerals, it is called ‘Asphalt’.
3. Tar: The viscous liquid obtained when natural organic materials such as wood and coal are destructively distilled in the absence of air.
4. Cut back: I f the viscosity of bitumen is reduced by volatile dilute, it is called cut back.
5. Emulsion: If bitumen is suspended in a finely divided condition in an aqueous medium and stabilized with an emulsifier, the material is known as ‘Emulsion’.

Tests on Bitumen:

Penetration tests:

• Determine Hardness or softness of bitumen. It can be measured by the depth in tenths of a millimeter (0.1mm) to which a standard loaded needle will penetrate vertically in 5 seconds.
• Sample is maintained at a temperature of 25°C.
• Total weight of needle assembly is 100 g.
• ‘Grading of Bitumen’ is done using this test value.
• 80/100 the range of 80 to 100 at standard test conditions. 30/40 and 80/100 are the more commonly used.
• In hot climate 30/40 grade is preferred due to its lower penetration value.
• As road tars are soft, penetration tests cannot be carried out on these.

Ductility tests: 

• It is expressed as the distance in ‘cm’ to which a standard briquette of bitumen can be stretched before the thread breaks. 
• The test is conducted at 27° C and at a rate of pull of 50 mm/mm.
•  The cross section at minimum width of specimen is 10 mm x 10 mm.
• The tests is believed to measure the adhesiveness and elasticity of bitumen.
• Desirable value; 50.

Viscosity test: 

• Viscosity is the general term for consistency and it is a measure of resistance of flow.
• ‘Orifice type viscometer’ may be used.
• It is the number of seconds required for 50 ml of material to flow through an orifice of specified size at specified temperatures.
• Suitable to test cut backs and tars (10 mm orifice viscometer)

Float test: 

• Measures consistency. 
• The test is used for those bituminous materials for which neither orifice viscometer nor penetration test could be used to define consistency of material.
• The time taken in seconds by water to force its way into the float through the bitumen plug is noted as the float test value.
• The higher the test value, the stiffer is the material.

Specific gravity tests:

• Specific gravity is used in. classifying bitumen, bituminous mix design.
• Pure bitumen has the values 1.02 to 0.97. Cut backs will have lower value depending on proportion.
• Tars have 1.10 to 1.25.

Softening point test: (Ring and Ball test)

• It is the temperature at which the substance attains a particular degree of softening under specified conditions of test.
• Usually determined by Ring and ball test. 
• Hard grade — bitumen higher softening point. S
• oft grade bitumen—lower softening point. 
• It usually varies from 35 to 70°C.
• Higher the softening point the lower will be temperature susceptibility and is preferred in warm climates.

Flash and fire point test: 

• These are primarily safety tests.
• Pensky — Martens closed cup or open cup are used for conducting the tests.

1. Flash point: The lowest temperature at which the vapour of a substance momentarily takes fire in the form of a flash. The minimum specified value for bitumen used in pavement construction in pensky martens closed cup test is 175°C.
2. Fire point: The lowest point temperature at which the material gets ignited and burns under specified conditions of test

Solubility tests: 

• Pure bitumen is completely soluble in solvents like carbon disulphide and carbon tetra chloride
• If the black residue is over 0.5%, the bitumen considered is specified as 99% to be cracked.
• The minimum proportion of bitumen soluble in carbon disulphide is specified as 99%.

Spot test: 

• Test for detecting over heated or cracked bitumen.
• ‘Naptha’ solution is used.

Loss on heating tests:

• Heated to 163⁰ C for 5 hours.
• For bitumen used in pavement mixes, loss on heating> 1 %

Water content: 

• Maximum water content in bitumen > 0.2% by weight.

Cut back Bitumen: Used in surface dressing, soil bitumen stabilization particularly at low temperature.

Types of cut backs:

1. Rapid curing (RC): Are bitumen fluxed with a petroleum distillate such as naphtha or gasoline. Which will rapidly evaporate.
2. Medium curing (MC) : Fluxed with kerosene or light diesel oil.
3. Slow curing (RC): Blending with high boiling point gas oil.

The classification is based on rate of curing or hardening after the application.

• The grade of cutback or its fluidity is expressed by a figure which follows the initials : For ex: MC-2 means that is medium curing cut back of grade 2.
• The most fluid cut back is designated by numerical ‘0’. Such as RC-0, MC-0 and SC- 0. Suffix numerals 0, 1, 2, 3, 4 and 5 designate progressively more viscous cut backs as the number indicates a definite viscosity irrespective of type of cut back.

Bitumen Emulsion : Emulsions are used in bituminous road constructions, especially in maintenance and patch repair works.

• It can be used in wet weather even when it is raining. Also can be used for stabalisation  of sands in desert areas.

Tar: Obtained by Destructive distillation of wood or charcoal.

Five grades of tar viz  Rt- 1, RT-3, RT-2, RT-4, and RT-5 in the increasing order of viscosity.

1. RT- 1:  is used for surface painting under - exceptionally cold weather as this has very low
2. RT-2:  Recommended for standard surface painting under Normal Indian Climatic conditions.
3. RT-3:  Used for surface pointing, renewal coats and per premixing chips for top course and light carpets.
4. RT-4:  Used for premixing Tar Macadam in base course.
5. RT-5 :  Used for grouting purposes.

MARSHALL STABILLITY TEST:

• Applicable to hot mix-paving mixture design using penetration grade bitumen and containing aggregates with maximum size of 2.5 cm.
• Not suitable for open graded mixes and is doubtful for course graded mixes.
• It is a type of unconfined compressive strength test, in which a cylindrical specimen 10 cm dia and 6.3 cm height is compressed radially at a constant rate of strain of 5 cm per minute.
• Standard temperature 60°C the corresponding load carried by specimen is called Marshal stability value and the deformation at failure in units of 0.25 mm is recorded as Marshal flow value.
• Prior to stability flow test density voids analysis is carried out on the test specimens.
• The optimum binder content for the aggregate mixture and anticipated traffic conditions is a compromise value which meets specified requirements for stability flow value and voids content.