Importance of Asphalt to Highway Engineering

1. When mixed with mineral aggregates to form the upper layers of the pavement structures holds the aggregates in position and give them resistance to displacement by traffic. It provides tensile strength to the asphalt mixture and enables the pavement to resist bending stresses due to traffic load.
2. It also gives flexibility to the pavement and enables it to conform to minor irregularities of the base without failure.
3. It is preferable to a cement concrete road due to its lower initial cost and more uniform riding surface.
4. In more developed countries, there are more asphalt roads than cement concrete roads. In their road life studies, the average lifespan of all types of asphalt roads is twenty (20) years.

Characteristics of Asphalt

1. Hydrocarbons that are soluble in carbon disulfide. At normal temperatures, they are in solid form, and when heated, they will soften and flow. When mixed with aggregates in their flaccid state, they solidify and bind the aggregates together forming a pavement surface.
2. Highly waterproof and can seal the road surface against excess surface water.
3. Highly cementitious. With the proper amount of bitumen at the road surface, it can bond granular particles together that will resist traffic wear and displacement.
4. Bituminous road surfaces are relatively flexible and will adjust to base variations.
5. When properly selected, the bitumen can maintain its ductility and bond which in return can withstand deterioration due to withering.

Sources and Nature

Bituminous materials used in road construction are from the distillation of crude oil. The distillation process can be accomplished by mechanical or natural means.

• Kinds of Bitumen
  • Natural/rock asphalt – formed when crude petroleum oils work their way to the earth’s surface through cracks. Then the action of the wind and the sun drives off the lighter oils and gasses leaving behind black and plastic residue.
  • Manufactured or petroleum asphalt – obtained by the distillation process from crude petroleum
    • Asphalt Cement – 40-300 penetration grade
    • Oxidized or Blown asphalt – 0-30 penetration grade

Classification, Chemical, and Physical Properties

Bitumen used in road construction is called asphalt cement, a highly viscous and sticky material.

1. Classification and Grade
   • Asphalt Cement
     • Penetration grade
   • Liquid Asphalt
     • Cutback Asphalt – asphalt cement with solvent
       • Medium Curing
       • Rapid Curing
     • Emulsified Asphalt – asphalt cement + water +emulsifier
       • Anionic – negative charge
       • Cationic – positive charge
2. Chemical Properties
   • It is basically composed of various hydrocarbons with traces of sulfur, oxygen, nitrogen and other elements. It is however composed of two main components such as the following.
     • Asphaltene – This component gives color and hardness
     • Maltene – This component is composed of resin which provides stickiness or adhesiveness and oil which influences viscosity and softness.
   • Existing test for analyzing chemical composition requires sophisticated laboratory equipment and technical expertise not readily available in most laborites where such testing is done.
3. Physical Properties
   1. Consistency of asphaltic material ranges from very thin liquid to stiff semi-solid state. It is also graded by its consistency at given temperatures.
   2. Temperature Susceptibility – All bituminous materials are thermoplastic. This means that it becomes harder as its temperature decreases and softer it becomes when its temperature increases. It also varies among bituminous materials from different sources even if they are of the same grade. It is important to determine the temperature susceptibility of the bituminous material to be used in a paving mixture because it indicates the proper temperature when to mix the bitumen and aggregates and the proper temperature at which to compact the mixture on a roadbed.
   3. Adhesion and Cohesion
      • Adhesion – It is the ability of the bituminous material to stick to the aggregates in a paving mixture.
      • Cohesion – The bitumen’s ability to hold the aggregate particles firmly in place in the finished pavement.
   4. Durability – The resistance of the bitumen to change in properties due to weathering or aging which involves chemical and physical reaction called oxidation.
   5. Hardening – Bitumen tends to harden in the paving mixture during construction which occurs when heated to higher temperature.
4. Physical Tests
   1. Consistency Test
      • Viscosity Test – Measures the resistance to flow
      • Penetration Test – Measures the distance that a standard needle will penetrate a sample at a given temperature in a specified time.
   2. Temperature Susceptibility Test
      • Softening Point Test – It indicates the temperature at which the harder type of bitumen reaches an arbitrary degree of softening.
   3. Loss of Heating
      • Determines the rate of loss of volatile oil of bitumen when heated and the change in penetration caused by excessive heating.
   4. Ductility Test
      • Measures the ability of the bitumen to stretch without breaking.
   5. Flash Point
      • Indicates safe heating temperature of bituminous materials.
   6. Specific Gravity
      • This indicates the uniformity of bituminous product.
   7. Spot Test
      • Determines if the bituminous material was overheated during production.
   8. Solubility Test
      • Determines the bitumen content that is soluble in petroleum solvent.
   9. Distillation Test
      • Determines the bituminous content of liquid asphalt.
   10. Sieve Test
       • Determines the presence of larger globules of bituminous material in the emulsion.
   11. Cement Mixing Test
   12. Storage Stability Test
5. Handling, Storage and Sampling – The inspector should be aware of the potential sources of contamination that might exist where the bituminous material is being stored and handled. It is because an inspector may be required to collect samples for testing. Inspectors must be able to identify and avoid practices that could lead to contamination of samples. Additionally, inspectors must understand the changes in volume that bituminous materials undergo when heated or cooled. It is especially important to be knowledgeable when comparing bituminous materials at quantities measured at different temperatures. 
   
   
   1. Safe Handling of Bituminous Materials – Bituminous materials are usually heated to a temperature exceeding 149 degrees Celsius to facilitate its use. Momentary contact can severely burn exposed flesh or skin. In case of skin burns, apply cold water or ice packs to reduce the heat. Use personal protective equipment to avoid exposure to fumes coming off from hot bituminous materials such as hydrogen sulfide. Hydrogen and sulfur are naturally present in these materials.
   2. Storage
      • Do not store in open areas exposed to rain.
      • Storage areas must be clean to avoid contamination.
      • Check storage drums for leakage and damage.
6. Sampling
   1. Samples must be representative of the entire shipment.
   2. Contamination or other alteration of sample before testing will produce misleading test results. Misleading results obtained leads to wrong decisions such as rejection of entire shipments when it does in fact meets the specifications.
   3. Best practices to observe when doing sampling
      • Use new and clean containers to avoid contamination.
      • Seal filled containers with clean, dry and tight-fitting lids.
      • Label containers clearly and properly.

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Sampling

1. Concrete coring shall be undertaken to obtain the required number of specimens.  It should be done after the concrete has become hard enough to permit sample removal without disturbing the bond between the mortar and coarse aggregate. In general, the concrete shall be 14 days old before specimens are taken.
2. Samples from concrete coring that show abnormal defects or samples that have been damaged in the process of removal shall not be used.
3. Samples for determining compressive strength that contains embedded reinforcement should be avoided or trimmed to eliminate the reinforcement, provided that a length-ti-diameter ratio (L/D) of at least 1.0 can be obtained. 

Core Specimens

The diameter of core specimens for thickness determination should be at least 3 times the nominal maximum size of the coarse aggregate used in the concrete and must be at least twice the nominal maximum size of the coarse aggregate in the core sample. The length of the specimens, when capped, shall be as nearly as practicable twice its diameter.

Concrete Coring for Thickness Determination

A completed pavement is accepted on a lot basis. A lot shall be considered as 1000 linear meters of single-lane pavement. Or 500 linear meters of pavement if the two-lane pavement is poured concurrently. While other areas such as intersections, entrances, cross-overs, ramps, etc. will be grouped to form a lot. Each lot will be divided into five (5) equal segments and one concrete core will be obtained from each segment. When the measurement of any core is less than the specified thickness by more than 25mm, additional cores will be taken at no less than 5-meter intervals parallel to the center line in each direction, from the affected location.

Concrete coring will only be stopped when a core that is not deficient in thickness by more than 25mm is found in each direction.

Evaluation of Concrete Samples for thickness determination

1. Concrete core samples should be measured at least nine (9) measurements of length, one at the center, and one each at eight additional locations spaced at equal intervals along the circumference of the circle.
2. The average length of the core is then obtained from the nine measurements. Each measurement will be subject to the following;
   • Individual measurements in excess of the specified thickness by more than 5mm will be considered as specified thickness plus 5mm.
   • Individual measurements less than the specified thickness by more than 25mm shall not be included in the average.
3. When the average thickness for the lot is deficient, the contract unit price is adjusted as follows:

Example:

Concrete coring was made on a section of a newly constructed pavement. A core specimen taken from the concrete pavement was measured for thickness determination. The required pavement thickness is 230mm.

Analysis:

The deficiency in thickness taken from the computed average of the corrected thickness is only 2mm. Therefore, the section where the concrete cores represent will be paid in full.

Concrete Coring Strength Determination

Concrete coring on pavements will only be done when the strength test results of concrete beam samples are not acceptable. At least three representative cores shall be taken from each area of concrete in-place considered deficient. Obtained concrete cores will be subjected to compressive strength tests wherein the average strength test results will be computed. Concrete in an area represented by cores can only be considered adequate when the average strength of the cores is equal to 85% of, and if no single core is less than 75% of, the specified strength.

Example:

In a section, the required concrete strength at 28 days is 24.1 Mpa (3500 psi). Concrete coring was made and three cores were taken from the section and were tested for strength verification. The results are the following;

Analysis:

• The required strength is 24.1 Mpa (3500 psi)
• The strength test result for Core ID no. 2 is less than 75 percent of the required strength.
• Concrete in the section represented by the cores is considered inadequate.

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Laboratory Tests

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Definition:

Standard, mm By Alternate US Standard
Gravel  ——————- 2 mm to 75 mm No. 10 to 3 in.
Sand ——————- 0.05 mm to 2 mm No. 270 to No. 10
Silt ——————- 0.005 mm to 0.05mm
Clay ——————- 0.001 mm to 0.005 mm
Colloids—————— less than 0.001 mm

Engineering Properties

Granular Soils:

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Eligibility Requirements

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