Presentation on theme: "LECTURE NO. 17 (Handout) PAVEMENTS AND BITUMINOUS MATERIALS Objectives: To introduce types of pavements To explain various bituminous materials, emphasizing."— Presentation transcript:
LECTURE NO. 17 (Handout) PAVEMENTS AND BITUMINOUS MATERIALS Objectives: To introduce types of pavements To explain various bituminous materials, emphasizing asphalts To explain properties of asphalts
PAVEMENT: Definition Pavement is defined as a road or highway with hard, smooth, and leveled surface made using a suitable material such as Portland cement concrete or asphalt concrete Asphalt concrete pavementPortland cement concrete pavement
PAVEMENT: Classification Pavements are classified as “flexible” or “rigid” depending on how they distribute surface loads Flexible pavements are those which are surfaced (i.e., paved) with bituminous materials such as asphalt concrete Asphalt concrete possess a lower stiffness (EI) than Portland cement concrete due to lower modulus of elasticity of asphalt concrete as compared to Portland cement concrete Due to lower stiffness of asphalt concrete the total structure of a flexible pavement "bends" or "deflects" under traffic loads. This is the logic behind calling such a pavement as "flexible pavement”. Flexible pavements distribute the wheel load over a cone-shaped area under the wheel, reducing the imposed unit stresses as depth increases, as shown below: Flexible pavements comprise about 93 percent of U.S. paved roads Load distribution under a flexible pavement Stress at lower depth is higher than the stress at higher depth
PAVEMENT: Classification----contd. Rigid pavements are those which are surfaced (i.e., paved) with Portland cement concrete Portland cement concrete possess a substantially higher stiffness (EI) than asphalt concrete due to higher modulus of elasticity of Portland cement concrete as compared to asphalt concrete Due to high stiffness of Portland cement concrete the total structure of a rigid pavement "bends" or "deflects" very little under traffic loads. This is the logic behind calling such a pavement as “rigid pavement”. Rigid pavements act as flexural members and distribute the wheel load fairly uniformly over the area under the pavement slab, as shown below: Rigid pavements comprise only 7 percent of U.S. paved roads Load distribution under a rigid pavement
PAVEMENT: Elements of flexible pavement A flexible pavement consists of four elements, as follows: –Subgrade: Prepared roadbed consisting of natural or imported soil –Subbase course: A layer between the subgrade and the base course, made from materials superior to that of subgrade. In case of a good quality of subgrade, the subbase course is omitted. –Base course: This is the layer directly below the asphalt concrete layer and generally consists of aggregate (either stabilized or unstabilized). –Surface course: This is the top layer and the layer that comes in contact with traffic. It may be composed of one or several different asphalt concrete sublayers. The above four elements of flexible pavement are shown below:
PAVEMENT: Elements of rigid pavement A flexible pavement consists of four elements, as follows: –Subgrade: Prepared roadbed consisting of natural or imported soil –Subbase course: This is the layer (or layers) under the base layer. A subbase is not always needed and therefore may often be omitted. –Base course: This is the layer directly below the Portland cement concrete layer and generally consists of aggregate or stabilized subgrade. –Surface course: This is the top layer and the layer that comes in contact with traffic. It consists of the Portland cement concrete slab. The above four elements of rigid pavement are shown below: Typical rigid pavement cross-section
BITUMINOUS MATERIALS: Introduction Bituminous material (or bitumen) is a solid, semisolid, or viscous cementitious material (i.e., binder) natural or manufactured, and composed of “hydrocarbons” Bitumen are usually fairly hard at normal temperatures. When heated, they soften and flow. Bitumens possess a number of properties that make them useful in the construction industry When mixed with aggregates in their hot fluid state, and then allowed to cool, they solidify and bind the aggregates together, forming a pavement surface. The most common bituminous materials are, as follows: –Asphalts (available as natural deposits or are produced from petroleum processing) –Tars (obtained through the destructive distillation of materials such as wood, coal, and shale, i.e., by heating wood or coal or shale in absence of air) –Pitches (obtained through further processing of tars)
BITUMINOUS MATERIALS: Classification Classification of bituminous materials is shown below:
ASPHALTS Asphalts are the most common and most widely used bituminous materials The term “asphalt” refers to a black cementitious material which varies widely in consistency from solid to semisolid at normal temperatures Asphalts are found as natural deposits or are produced from petroleum crude The classification of asphalts is already presented in the previous slide Natural Rock asphalts are rock deposits containing bituminous materials which have been used for road surfaces in localities where they occur Native asphalts are obtained from asphalt lakes in Trinidad and other Caribbean areas, these were used in some of the earliest pavements in North America Petroleum asphalts are products of the distillation of crude oil. These asphalts are used as the most common bituminous paving materials
PETROLEUM ASPHALTS Petroleum asphalts are produced from the residual matter (i.e., impurities) present in the crude oil Higher-grade crude oil (i.e., crude oil with less residual matter) may contain as little as 10% asphalt, whereas lower-grade crude oil (i.e., crude oil with more residual matter) may contain as much as 90% asphalt A flow chart showing production of different types of asphalts are shown in the adjoining figure.
TYPES OF PETROLEUM ASPHALTS Following types of petroleum asphalts are used in pavement construction: –Asphalt cement –Emulsified asphalt –Liquid or cutback asphalt
TYPES OF PETROLEUM ASPHALTS: Asphalt cement Asphalt cement (also called paving asphalt) are the primary asphalt products produced by the distillation of crude oil. At ambient temperatures asphalt cement is a black, sticky, semisolid and a highly viscous material It is strong and durable cement with excellent adhesive and waterproofing characteristics. It is also highly resistant to the action of most acids, alkalis and salts The largest use of asphalt cement is in the production of asphalt concrete, which is primarily used in the construction of flexible pavements throughout the world The asphalt cement can readily be liquefied by applying heat for mixing with mineral aggregates to produce asphalt concrete
TYPES OF PETROLEUM ASPHALTS: Asphalt cement---contd. Several standard grades of asphalt cement, based on consistency, are available commercially. Two methods, viscosity and penetration are used to classify asphalt cements into different grades, as follows: –The viscosity grades based on original asphalt cements (AC), as specified in ASTM D3381 are: AC– 5; AC– 10; AC– 20; AC– 30; and AC– 40 (The numerical values indicate viscosity at 140 ºF in hundreds of poise) –The penetration grades, as specified in ASTM D946 are: ; ; ; 60-70; and (higher the penetration, the softer the asphalt cement, therefore, is the hardest grade and is the softest grade
TYPES OF PETROLEUM ASPHALTS: Asphalt cement---contd.
TYPES OF PETROLEUM ASPHALTS: Emulsified asphalt Emulsified asphalt (also simply called emulsion) is a mixture of asphalt cement, water, and emulsifying agent Because the asphalt cement will not dissolve in water, asphalt cement and water exist in separate phases as shown in the following figure: To mix the asphalt cement with water, an emulsifying agent (usually a type of soap) is added
TYPES OF PETROLEUM ASPHALTS: Emulsified asphalt-----contd. Process of manufacture of emulsified asphalt consists passing the hot asphalt cement and water containing the emulsifying agent under pressure through a colloid mill, as shown in the following figure: The colloid mill breaks up the asphalt cement and disperses it, in the form of very fine droplets, in the water carrier The emulsified asphalt when used, the emulsion sets as the water evaporates The emulsion usually contains 55-75% asphalt cement and up to 3% emulsifying agent, with balance being water
TYPES OF PETROLEUM ASPHALTS: Emulsified asphalt-----contd. Two most commonly used types of emulsified asphalts are specified in ASTM D977 and ASTM D2397: –Anionic – electro-negatively charged asphalt droplets –Cationic – electro-positively charged asphalt droplets Anionic emulsions adhere better to aggregate particles with positive surface charges (e.g., limestone) Cationic emulsions adhere better to aggregate particles with negative surface charges (e.g., sandstone, quartz, siliceous gravel). Cationic emulsions also work better with wet aggregates and in colder weather The anionic emulsified asphalts include rapid setting (RS), medium setting (MS), and slow setting (SS), as specified in ASTM D977 The cationic emulsified asphalts include rapid setting (CRS), medium setting (CMS), and slow setting (CSS), as specified in ASTM D2397
TYPES OF PETROLEUM ASPHALTS: Emulsified asphalt-----contd. Selection and uses of emulsified asphalts are given in ASTM D3628. Generally, they are used as follows: Type of emulsified asphalts Uses Rapid-settingSurface treatments and penetration macadam's Medium-settingOpen-graded cold asphalt- aggregate mixtures Slow-settingTrack coat, fog seal, dense-graded cold asphalt-aggregate mixtures, and slurry seals
TYPES OF PETROLEUM ASPHALTS: Liquid or cutback asphalt Liquid asphalts or cutback asphalts are asphalt cements mixed with a solvent to reduce their viscosity to make them easier to use at ordinary temperatures They are commonly heated and then sprayed on aggregates Upon evaporation of the solvent, they cure or harden and cement the aggregate particles together Types and grades, as given below, are based on the type of solvent, which governs viscosity and the rates of evaporation and curing –Rapid-Curing (RC). Produced by adding a light diluent of high volatility (generally gasoline or naphtha) to asphalt cement. These are used primarily for tack coat and surface treatments. Specifications for RC type given in ASTM D2028. –Medium-Curing (MC). Produced by adding a medium diluent of intermediate volatility (generally kerosene) to asphalt cement. These are generally used for prime coat, stockpile patching mixtures, and road-mixing operations. Specifications for MC type given in ASTM D2027. –Slow-Curing (SC). Produced by adding oils of low volatility (generally diesel or other gas oils) to asphalt cement. They are also called road oils. They are generally used for prime coat, stockpile-patching mixtures, and as dust palliatives. Specifications for SC type given in ASTM D2026.
TYPES OF PETROLEUM ASPHALTS: Liquid or cutback asphalt------contd. Cutback asphalts are commercially available in different grades, as shown in the following Fig.: The thinnest and most fluid grade is designated by the suffix number 30, which is available in MC type only
TYPES OF PETROLEUM ASPHALTS: Liquid or cutback asphalt------contd. Emulsified asphalts are increasingly being used in lieu of cutback asphalts for the following reasons: –Environmental regulations: Emulsions are relatively pollution free –Loss of high-energy products: When cutback asphalts cure, the diluents which are high energy, high price products are wasted into atmosphere –Safety: Emulsions are safe to use –Lower application temperature: Emulsions can be applied at relatively low temperatures saving the fuel costs. Emulsions can also be applied effectively to a damp pavement, whereas dry conditions are required for cutback asphalts
PROPERTIES OF ASPHALTS Following are the properties of asphalt that are of great importance in pavement design and construction: –Consistency –Specific gravity –Ductility –Purity –Flash point –Rate of curing –Aging or hardening –Resistance to reaction with water –Durability
PROPERTIES OF ASPHALTS Consistency Consistency describes the degree of fluidity of asphalt cement at any particular temperature Consistency of asphalt cement is measured in terms of viscosity and penetration values Softening point is also measured to determine the temperature at which an asphalt changes its phase from solid to liquid
PROPERTIES OF ASPHALTS Consistency: Viscosity Viscosity is a measure of the resistance to flow and is the fundamental consistency measurement in absolute units There are many methods to measure the “viscosity” –Absolute viscosity (in Pa-s or poise, 1 poise = 0.1 Pa-s) measured at 140 ºF, as per the ASTM D2171 specifications –Kinematic viscosity (in cm 2 /s or stoke, 1 stoke = 100 centistokes = 1 cm 2 /s) measured as per the ASTM D2170 specifications –kinematic viscosity = absolute viscosity/density Viscosity decreases (i.e. materials become more fluid) as temperature increases A very viscous fluid is very thick
PROPERTIES OF ASPHALTS Consistency: Viscosity----contd. Following figure illustrates typical temperature-viscosity relationships for asphalts Grades of asphalt materials and temperatures at which they are used depend to a great extent on their viscosity Plant temperatures for mixing asphalt-paving materials are usually specified in terms of viscosity Temperature limits corresponding to viscosities of 1.5 to 3.0 cm 2 /s (150 to 300 centistokes) are sometimes used For example, the plant temperatures for heating the asphalts A, B, and C (as shown in the figure) will respectively be in the following ranges corresponding to viscosities of 150 to 300 centistokes: 153 ºC to 167 ºC; 148 to 163 ºC; and 137 ºC to 149 ºC.
PROPERTIES OF ASPHALTS Consistency: Penetration Penetration test (ASTM D5) is used to measure the consistency of asphalt empirically (i.e., indirectly) Penetration is the consistency of asphalt expressed as the distance in tenths of a millimeter that a standard needle penetrates a sample of asphalt vertically under standard conditions of loading (100 g), time (5 seconds), and temperature (77 °F), as shown in figure below: Asphalt cements have an upper limit on penetration value of 300 (i.e., 30 mm)
PROPERTIES OF ASPHALTS Consistency: Softening point Softening point is measured by ring and ball (R & B) method in accordance with ASTM D36, as shown in the following figure: Softening point can simply be defined as the temperature at which asphalt cement cannot support the weight of a steel ball and starts flowing.
PROPERTIES OF ASPHALTS Specific gravity Specific gravity and density of asphalt can be determined using the procedure described in ASTM D70. Since the volume of asphalt changes with temperature (with the change from semisolid to liquid state), the specific gravity of asphalt is expressed at a given temperature (usually at 77 °F or 60 °F) Petroleum asphalts have specific gravity values close to unity (0.95 – 1.05) Specific gravity of asphalt decreases with increasing temperature. For example, asphalt cement has a specific gravity of at a temperature of 10 °F, 1.0 at 60 °F, and at 300 °F. A knowledge of specific gravity is essential to determine the percentage of voids in a compacted material. Also, its measurement is required to convert the volume measurement of asphalt to the units of mass
PROPERTIES OF ASPHALTS Ductility Ductility test is performed on the asphalt cement samples in accordance with ASTM D113. Ductility test measures the distance in centimeters that a standard briquette of asphalt cement will stretch 5 cm/min at 77 °F) before breaking, as shown in the following figure: Ductility is sometimes used as an indirect gage of adhesion and cohesion of asphalt Adhesion is the ability to stick to aggregate particles in the asphalt concrete Cohesion is the ability to hold the aggregate particles firmly in place
PROPERTIES OF ASPHALTS Purity Refined asphalt cements consist of almost pure bitumen, which by definition is entirely soluble in carbon disulfide. Only very little amounts of impurities are generally present in refined asphalt cements To determine the purity of asphalt cement, a “solubility” test is conducted in accordance with ASTM D2042
PROPERTIES OF ASPHALTS Flash point If asphalt cement is heated to a high enough temperature, it gives off enough vapor to flash (ignite) in presence of spark or open flame Flash point is the temperature below that of the fire point, which is the temperature where a material combusts. Flash point test is conducted in accordance with ASTM D92 to indicate the temperature to which asphalt cement may be safely heated without the danger of instantaneous flash in the presence of an open flame
PROPERTIES OF ASPHALTS Rate of curing The process of evaporation of solvents from cutback asphalts, and the attendant thickening of the material, is called curing. Curing can also be described as the change in consistency of an asphalt due to the progressive loss of diluents by evaporation The rate of curing, or the time required for a cutback asphalt to harden (from its original liquid consistency) and develop a consistency that is satisfactory for the function as a binder in pavements is an important property of cutback asphalts The rate of curing is influenced by the following factors: –Volatility or evaporation rate of the solvent –The amount of solvent –Penetration (or viscosity) of the asphalt base –Temperature and humidity of the environment –Wind velocity –Surface area of the pavement
PROPERTIES OF ASPHALTS Aging or hardening Aging or hardening is the process under which asphalt becomes harder and brittle due to increase in the viscosity of asphalt caused by: –the evaporation and oxidation of the lighter, oily constituents during mixing at high temperatures, called “volatilization” (short-term aging or hardening) –the oxidation of the oils to resins and resins to asphaltenes when used over a period of years (long-term or in-service aging or hardening) Durability of pavement is adversely affected by aging
PROPERTIES OF ASPHALTS Resistance to action of water Asphalt materials designed for pavements should be able to withstand the effects of water The durability of the pavement is greatly affected by the ability of asphalt to adhere to aggregate particles in the presence of water Loss of bond in the presence of water may lead to pavement deterioration
PROPERTIES OF ASPHALTS Durability Durability can be defined as the property that permits a pavement material to withstand the detrimental effects of moisture, air, and temperature. The performance and durability of an asphaltic pavement are affected by a number of factors, including mix design, properties of aggregates, workmanship, and the properties of the asphalt. Asphalt pavements fail or crack due to three main causes: –Permanent deformation or rutting occurring at high temperatures as the asphalts softens and the mix loses elasticity –Fatigue cracking due to high volume of load applications and aging of the asphalt –Low-temperature cracking as asphalt becomes brittle and the pavement shrinks in cold weather