Bitumen is a critical material in civil engineering, especially in road construction and maintenance. Its performance directly affects the durability and safety of pavements, making thorough testing essential. This chapter focuses on the identification and testing of bitumen to ensure it meets the required specifications for Kenyan infrastructure projects. Understanding the tests and the equipment involved enables engineers and technicians to verify quality and suitability for various applications.

2.1 Bitumen Tests Identification

Bitumen testing involves a range of procedures designed to determine its physical and chemical properties. These tests help ascertain the suitability of bitumen for specific environments and traffic conditions encountered on Kenyan roads. Proper identification of these tests is crucial for quality control in construction projects.

2.1.1 Penetration

The penetration test measures the hardness or softness of bitumen by determining the depth a standard needle penetrates under specific conditions. This test is vital because it indicates the consistency of bitumen, which affects pavement flexibility and resistance to deformation.

Significance of Penetration Test

• Assessing Bitumen Hardness: It provides a quantitative measure of bitumen’s hardness, guiding the selection of the right grade for different climatic conditions in Kenya.
• Quality Control: Ensures uniformity in bitumen supplied to projects such as county road upgrades.
• Predicting Performance: Helps estimate how bitumen will behave under traffic loads and temperature changes.
• Standard Compliance: Aligns with Kenya Bureau of Standards (KEBS) requirements for construction materials.
• Mix Design Input: Data from this test inform asphalt mix design, influencing pavement strength and durability.

Equipment for the Penetration Test

Steps for Carrying Out the Penetration Test

1. Heat the sample: Heat the bitumen gently until fluid, stirring to remove air bubbles, without overheating.
2. Pour into the container: Pour the bitumen into the sample cup to a depth at least 10 mm greater than the expected penetration.
3. Cool and condition: Allow the sample to cool in air, then place it in the water bath at 25°C for the specified time, usually 1 to 1.5 hours.
4. Set the needle: Place the sample under the penetrometer and lower the needle until it just touches the bitumen surface.
5. Release the needle: Release the needle for exactly 5 seconds under the 100 g load.
6. Read the dial: Record the penetration in tenths of a millimetre (dmm).
7. Repeat: Take at least three readings at points at least 10 mm apart and from the container edge, cleaning the needle each time, and report the mean.

Worked Example: Penetration Grade

Three penetration readings on a bitumen sample delivered to a contractor upgrading an access road for a district hospital were 63, 65 and 64 dmm. Find the penetration value and grade.

Penetration = Sum of readings ÷ Number of readings

Penetration = (63 + 65 + 64) ÷ 3

Penetration = 192 ÷ 3

Penetration = 64 dmm, so the bitumen is grade 60/70

2.1.2 Cleanliness

The cleanliness test, also called the solubility test, determines the percentage of bitumen that dissolves in a solvent such as trichloroethylene or toluene, and therefore how much insoluble impurity, such as mineral matter, carbon or salts, the bitumen contains. Pure paving bitumen is almost entirely soluble, and specifications usually require at least 99% solubility, because impurities reduce the binding power of the bitumen and its adhesion to aggregates. The test is done by dissolving a weighed sample, filtering the solution through a weighed filter and weighing the insoluble residue left behind. Example: the materials laboratory of a road contractor working for Kenya Urban Roads Authority rejected a delivery of bitumen after the cleanliness test showed 98.2% solubility, below the specified minimum.

Equipment for the Cleanliness Test

Steps for Carrying Out the Cleanliness Test

1. Prepare the crucible: Dry and weigh the crucible with its filter pad.
2. Weigh the sample: Weigh about 2 g of bitumen into the flask.
3. Dissolve: Add about 100 ml of solvent, stopper the flask and swirl until the bitumen dissolves.
4. Stand the solution: Let the solution stand for about 15 minutes so insoluble matter settles.
5. Filter: Pour the solution through the weighed crucible, washing the flask with clean solvent until the filtrate runs clear.
6. Dry the residue: Dry the crucible and residue in the oven, then cool in the desiccator.
7. Weigh and calculate: Weigh the crucible and residue and calculate the percentage soluble.

Worked Example: Solubility of Bitumen

A 2.000 g sample of bitumen left an insoluble residue of 0.012 g. Find the percentage insoluble and the solubility.

Insoluble % = (Mass of residue ÷ Mass of sample) × 100

Insoluble % = (0.012 ÷ 2.000) × 100

Insoluble % = 0.6%

Solubility = 100 − 0.6

Solubility = 99.4%, which meets a 99% minimum

2.1.3 Viscosity

Viscosity is the resistance of a liquid to flow, and for bitumen it describes how easily the binder can be pumped, sprayed and mixed with aggregates at working temperatures, and how well it resists flow in the pavement. The test is especially important for cutback bitumens and bitumen emulsions used in priming and surface dressing, whose viscosity at spraying temperature must be right for even coverage. For cutbacks and emulsions, viscosity is commonly measured with an efflux viscometer, such as the tar viscometer or Saybolt Furol viscometer, which records the time for a fixed volume to flow through a standard orifice, while penetration-grade bitumen may be tested with a rotational or capillary viscometer at 60°C and 135°C. Example: a contractor priming a new road in Kajiado County tested the MC-30 cutback prime to confirm it would flow and penetrate the base properly at the planned spraying temperature.

Equipment for the Viscosity Test

Steps for Carrying Out the Efflux Viscosity Test

1. Clean and set up: Clean the cup and orifice and place the cup in the water bath at the specified temperature.
2. Close the orifice: Insert the ball valve to close the orifice.
3. Fill the cup: Pour the sample into the cup up to the levelling mark.
4. Condition the sample: Allow the sample to reach the test temperature, stirring gently.
5. Place the receiver: Put the graduated receiver under the orifice.
6. Start the flow: Lift the valve and allow the sample to flow into the receiver.
7. Time the efflux: Start the stopwatch when the liquid reaches the 20 ml mark and stop it at the 70 ml mark, so the time for 50 ml is recorded in seconds.
8. Report: Report the efflux time, orifice size and temperature as the viscosity.

Uses of Viscosity Results

• Selecting spraying temperature: Shows the temperature at which the binder can be sprayed evenly.
• Classifying cutbacks: Distinguishes grades such as MC-30, MC-70 and MC-3000 by their flow properties.
• Checking emulsions: Confirms that emulsions are fluid enough to spray but not so thin that they run off.
• Setting mixing and compaction temperatures: Guides temperatures for hot mix asphalt.
• Quality control: Detects deliveries that are too thick or too thin compared with the specification.

2.1.4 Ductility

The ductility test measures the ability of bitumen to stretch without breaking, indicating its flexibility. Flexibility is significant for accommodating pavement expansion and contraction due to temperature variations.

Relevance of Ductility Test

• Flexibility Assessment: Ensures bitumen can elongate to accommodate stresses without cracking.
• Durability Indicator: High ductility correlates with longer pavement life, reducing maintenance costs.
• Performance in Cold Regions: Critical for roads in highland areas around Eldoret where low temperatures prevail.
• Mixing and Laying Guidance: Guides the preparation of asphalt mixes to optimize workability.
• Detecting Aging: Identifies bitumen that has become brittle due to oxidation or weathering.

Steps for Carrying Out the Ductility Test

1. Prepare the mould: Assemble the brass briquette mould on a plate coated with a release agent such as glycerine and talc.
2. Pour the sample: Pour hot bitumen into the mould until it is slightly overfilled.
3. Cool and trim: Cool the briquette in air, then trim off the excess with a hot knife.
4. Condition: Place the briquette in the water bath at 25°C for about 85 to 95 minutes.
5. Mount the briquette: Remove the side pieces and fix the clips onto the ductility machine pins in the water bath.
6. Pull the sample: Start the machine to pull the clips apart at 50 mm per minute.
7. Record the elongation: Record the distance in centimetres at which the bitumen thread breaks.
8. Report: Report the mean of three results; paving grades commonly require a minimum of about 75 cm to 100 cm.

2.1.5 Flash and Fire Point

These tests determine the temperatures at which bitumen vapors ignite (flash point) and sustain combustion (fire point), essential for safe handling and storage.

Safety Considerations of Flash and Fire Point Tests

• Workplace Safety: Prevents accidents during bitumen heating and transportation on sites like Nairobi’s construction yards.
• Storage Guidelines: Dictates safe temperature limits for bitumen storage tanks in county depots.
• Transport Regulations: Ensures compliance with hazardous materials transport laws.
• Fire Hazard Assessment: Helps in emergency preparedness and risk mitigation.
• Supplier Verification: Confirms that bitumen meets safety specifications before delivery.

Flash and Fire Point Test Procedures

To fully understand the safety implications, it is important to know how the flash and fire point tests are conducted. These procedures are standardized to ensure consistency and reliability in Kenyan laboratories such as those operated by the Kenya National Highways Authority (KeNHA).

• Sample Preparation: Bitumen is heated in a test cup, typically using the Pensky-Martens closed cup or Cleveland open cup apparatus, to ensure it is in a fluid state without overheating.
• Controlled Heating: The sample is heated at a specified rate, usually 5–6°C per minute, while being stirred gently to maintain uniform temperature.
• Application of Ignition Source: At regular temperature intervals, a small test flame is introduced just above the surface of the bitumen vapors.
• Flash Point Determination: The temperature at which a visible flash appears on the surface is recorded as the flash point. This is critical for setting safe heating limits in facilities such as the Nairobi County asphalt plant.
• Fire Point Determination: Heating continues, and the temperature at which the vapors sustain combustion for at least five seconds is recorded as the fire point. This information is used by transporters and storage facilities, for example, at Kenya Pipeline Company depots, to develop emergency protocols.

2.1.6 Float Test

The float test measures the consistency of bitumens that are too soft for the penetration test but too thick for an efflux viscosity test, such as some slow-curing cutbacks and road oils. A plug of bitumen is set in a brass collar screwed into the base of an aluminium float, chilled, and then floated on water at 50°C; as the plug warms and softens, water eventually forces its way through, and the time in seconds taken for this to happen is the float test value. A longer time indicates a stiffer material. Example: the laboratory of a county roads department in Nyandarua used the float test to check a slow-curing binder that gave penetration readings beyond the scale of its penetrometer.

Equipment for the Float Test

Steps for Carrying Out the Float Test

1. Prepare the collar: Place the collar, small end down, on a brass plate coated with release agent.
2. Fill the collar: Pour the heated bitumen into the collar until slightly overfilled, then level the top once cool.
3. Chill the plug: Place the collar and plate in the chilling bath at about 5°C for 15 to 30 minutes.
4. Fit the float: Remove the collar from the plate and screw it into the float.
5. Float the assembly: Immediately place the float on the water bath at 50°C and start the stopwatch.
6. Observe the plug: Watch until water breaks through the plug into the float.
7. Record the time: Stop the watch and record the time in seconds as the float test value.

2.1.7 Loss on Heating

The loss on heating test measures the percentage of mass lost when bitumen is heated for a fixed time at a high temperature, usually 163°C for 5 hours in the thin film oven test or rolling thin film oven test. It simulates the hardening that occurs when bitumen is heated in the asphalt plant and laid on the road, as light oils evaporate and the binder oxidises. A large loss shows that the bitumen contains volatile fractions and will harden quickly, so specifications usually limit the loss to about 0.5% to 1% and also require a minimum retained penetration after heating. Example: the materials engineer for an asphalt plant supplying an airport apron rehabilitation tested each bitumen batch for loss on heating to make sure the binder would not become brittle soon after paving.

Equipment for the Loss on Heating Test

Steps for Carrying Out the Loss on Heating Test

1. Weigh the pan: Weigh the clean, dry sample pan.
2. Add the sample: Pour about 50 g of heated bitumen into the pan to form a thin film.
3. Weigh the sample: Cool and weigh the pan and sample.
4. Heat in the oven: Place the pan on the rotating shelf of the oven at 163°C for 5 hours.
5. Cool: Remove the pan and cool it in the desiccator.
6. Reweigh: Weigh the pan and residue and calculate the loss.
7. Test the residue: Carry out the penetration test on the residue to find the retained penetration.

Worked Example: Loss on Heating

A bitumen sample weighed 50.000 g before heating and 49.850 g after 5 hours at 163°C. Find the loss on heating.

Loss % = ((Mass before − Mass after) ÷ Mass before) × 100

Loss % = ((50.000 − 49.850) ÷ 50.000) × 100

Loss % = (0.150 ÷ 50.000) × 100

Loss % = 0.3%, which is within a 1% limit

2.1.8 Specific Gravity

Specific gravity measures the density of bitumen relative to water, influencing mix design and volumetric calculations in pavement construction.

Functions of Specific Gravity Test

• Material Characterization: Differentiates bitumen types and detects adulteration.
• Volume Calculations: Assists in accurate determination of bitumen content in asphalt mixes.
• Quality Control: Verifies supplier consistency and compliance with standards.
• Design Optimization: Influences compaction and void content in pavement layers.
• Resource Management: Helps in estimating material quantities for budgeting and procurement.

Steps for Carrying Out the Specific Gravity Test (Pycnometer Method)

1. Weigh the empty pycnometer (A): Weigh the clean, dry pycnometer with its stopper.
2. Weigh with water (B): Fill with distilled water at 25°C, insert the stopper, wipe dry and weigh.
3. Add the bitumen: Empty and dry the pycnometer, then pour in hot bitumen to about half full without touching the sides above.
4. Cool and weigh (C): Cool to 25°C and weigh the pycnometer with bitumen.
5. Top up with water (D): Fill the remaining space with distilled water at 25°C, insert the stopper and weigh.
6. Calculate: Work out the specific gravity using the formula below.

SG = (C − A) ÷ [(B − A) − (D − C)]

Worked Example: Specific Gravity of Bitumen

A laboratory testing bitumen for a school access road recorded: A = 30.00 g, B = 80.00 g, C = 55.00 g and D = 80.40 g. Find the specific gravity.

SG = (C − A) ÷ [(B − A) − (D − C)]

SG = (55.00 − 30.00) ÷ [(80.00 − 30.00) − (80.40 − 55.00)]

SG = 25.00 ÷ [50.00 − 25.40]

SG = 25.00 ÷ 24.60

SG = 1.016

2.1.9 Softening Point

The softening point test determines the temperature at which bitumen softens enough to allow a steel ball to pass through it. This test is essential to understand bitumen's temperature susceptibility, particularly in Kenya's varying climates.

Importance of Softening Point Test

• Temperature Sensitivity: Identifies the temperature threshold beyond which bitumen loses its rigidity.
• Suitability for Hot Climates: Ensures bitumen used in regions like Mombasa can withstand high ambient temperatures.
• Avoidance of Rutting: Helps prevent pavement deformation by selecting bitumen with an appropriate softening point.
• Material Specification: Allows engineers to specify bitumen grades for different road categories.
• Quality Assurance: Provides a consistent benchmark for suppliers and contractors.

Steps for Carrying Out the Ring and Ball Test

1. Prepare the rings: Place two brass rings on a plate coated with release agent.
2. Fill the rings: Pour heated bitumen into the rings, allow to cool for 30 minutes, then trim level.
3. Assemble the apparatus: Place the rings in the holder with ball guides, 25 mm above the base plate.
4. Fill the bath: Place the assembly in a beaker of freshly boiled distilled water at 5°C for 15 minutes.
5. Place the balls: Put a 3.5 g steel ball, also at 5°C, centrally on each sample.
6. Heat the bath: Heat at a uniform rate of 5°C per minute.
7. Record the temperatures: Record the temperature at which each softened sample, carrying its ball, touches the base plate.
8. Report: Report the mean of the two temperatures; for 60/70 grade bitumen this is typically about 48°C to 56°C.

2.1.10 Spread Rate

The spread rate, also called the rate of spray or application rate, is the quantity of bituminous binder applied per unit area of road surface, usually expressed in litres per square metre (L/m²), during priming, tack coating and surface dressing. Applying the correct rate is critical: too little binder fails to hold the chippings, causing stone loss, while too much causes bleeding and a slippery surface. The spread rate is checked on site with the tray test, in which shallow trays or pads of known area are placed in the path of the bitumen distributor, then weighed to find the binder deposited. Example: on a surface dressing contract for a rural road serving a tea estate in Kericho, the resident engineer carried out tray tests at the start of each day to confirm the distributor was spraying the designed 1.2 L/m².

Equipment for the Spread Rate Test

Steps for Carrying Out the Tray Test

1. Weigh the trays: Number and weigh each clean tray.
2. Place the trays: Set the trays across the road in the path of the distributor, clear of nozzle overlap edges.
3. Spray the binder: Drive the distributor over the trays at the normal operating speed and spray pressure.
4. Remove the trays: Lift the trays carefully after the distributor passes, avoiding spills.
5. Weigh the trays: Weigh each tray with binder and find the mass of binder collected.
6. Convert to volume: Divide the mass by the density of the binder at spraying temperature.
7. Calculate the rate: Divide the volume by the tray area to obtain L/m², and compare with the design rate.
8. Adjust if needed: Change the distributor speed or pump output if the rate is outside tolerance, and retest.

Spread rate (L/m²) = Volume of binder (L) ÷ Area (m²)

Worked Example: Spread Rate and Binder Cost

A tray measuring 0.5 m × 0.5 m collected 0.30 kg of binder. The density of the binder at spraying temperature is 0.97 kg/L. The design rate is 1.2 L/m². Find the spread rate, then estimate the binder cost for 1 km of road 7 m wide at the design rate if binder costs Ksh 120 per litre.

Area = 0.5 × 0.5 = 0.25 m²

Volume = Mass ÷ Density

Volume = 0.30 ÷ 0.97

Volume = 0.309 L

Spread rate = Volume ÷ Area

Spread rate = 0.309 ÷ 0.25

Spread rate = 1.24 L/m², slightly above the 1.2 L/m² design rate

Area of road = 1,000 × 7 = 7,000 m²

Binder required = 7,000 × 1.2 = 8,400 L

Cost = 8,400 × 120

Cost = Ksh 1,008,000

Chapter Summary

This chapter covered the identification of various bitumen tests essential for assessing the quality and suitability of bitumen in construction. Key tests such as penetration, cleanliness, viscosity, ductility, flash and fire point, float test, loss on heating, specific gravity, softening point, and spread rate were explained in detail. It also outlined the necessary tools and apparatus required to perform these tests accurately. Proper sampling techniques were emphasized to ensure representative bitumen samples for testing. The chapter detailed the procedures for conducting each test to obtain reliable results. It further discussed the analysis and reporting of test data to support informed decision-making in material selection. Finally, the importance of maintaining testing equipment and ensuring their calibration and cleanliness was highlighted to guarantee consistent and precise measurements over time.