Highway Materials TR 331 PDF

TR 331 – HIGHWAY MATERIALS PART II – BITUMINOUS MATERIALS, AC MIX DESIGN, H/W DRAINAGE  BITUMINOUS MATERIALS INTRODUCTION Also known as Asphaltic Materials. Are materials that contain bitumen (US: asphalt), resemble bitumen, or constitute a source of bitumen. They include bitume...

TR 331 – HIGHWAY MATERIALS PART II – BITUMINOUS MATERIALS, AC MIX DESIGN, H/W DRAINAGE  BITUMINOUS MATERIALS INTRODUCTION Also known as Asphaltic Materials. Are materials that contain bitumen (US: asphalt), resemble bitumen, or constitute a source of bitumen. They include bitumen (asphalt) and tar binders. Bitumen (Asphalt) A solid or semi-solid (viscous) material, black or dark-brown in colour, having adhesive properties (cementitious), and consisting essentially of hydrocarbons, derived from petroleum or occurring in natural asphalt deposits, and soluble in carbon disulphide, CS2. Bitumen dissolves in petroleum oils Tar A viscous liquid, black in colour, with adhesive properties, obtained by destructive distillation of coal or wood. In H/E we mostly refer to tar derived from bituminous coal. NB: Tar can also be obtained from petroleum by chemical treatment (cracking), not physical processes such as fractional distillation used for production of bitumen (asphalt) Bitumen (Asphalt) - Two main categories: Natural asphalt & Petroleum asphalt Natural Asphalt Occurs naturally in natural deposits (as native asphalt e.g. in Trinidad lake, etc; or rock asphalt in sandstone or limestone) Petroleum Asphalt Also known as refinery asphalts. It is produced by industrial (fractional) distillation of crude petroleum (crude oil) It is the heaviest fraction and the one with the highest boiling point, boiling at 525 °C (977 °F). Trinidad lake asphalt  3.3 PHYSICAL PROPERTIES Those which directly affect the performance of asphalt in a mixture while being mixed, laid and in service. Challenge: To develop physical tests that satisfactorily characterize key asphalt binder parameters and how they change throughout the lifetime of a mix. Asphalt is a rheological material → Its stress-strain characteristics are time-dependent Asphalt is a thermoplastic material → Its stiffness (or consistency) varies with temperature → Physical properties of asphalt are time and temperature dependent 3.3.1 RHEOLOGY Study of deformation and flow of matter Important in determining physical properties of asphalt HMA deformation closely related to rheology of asphalt →Rheology determines performance of HMA pavement Example: - High HMA deformations and flow → rutting and bleeding - High asphalt stiffness → fatigue cracking and thermal cracking Thus: Comparison of diff binders must be done at some common reference temperature Characterizing asphalt binder properties should involve examining rheological properties over the range of temp that may be encountered in its lifetime. Rheological Properties (Consistency parameters) Measure of hardness or degree of fluidity. 1. Penetration Test (AASHTO T49, ASTM D5) Penetration depth of a standard needle under specified cond. of weight (100g), time (5 sec) and temperature (25°C) - Pen units expressed in 0.1 mm (e.g., 8 mm → 8/0.1 = 80) - Assumption: same penetration deform similarly NB: Based on field performance, no relation with test parameter Penetration test equipment Asphalt Penetration cont… - It is an empirical parameter used in grading asphalt - The harder an asphalt cement, the lower will be its penetration & vice versa - Five stand. penetration grades exist: 40-50, 60-70, 85-100, 120-150, 200-300 - The values represent the min & the max pen for each pen grade - Two common grades are 60-70 (for hot regions) and 85-100 (for cold regions) - Asphalt of lower pen grades are used at bus-stops or parking places where traffic stresses are very high. 2. Softening Point Also known as the “ring and ball” softening point. Temp. at which asphalt begin to show fluidity At this temp. asphalt can no longer support the weight of a 3.5g steel ball. The ball, enveloped in binder, falls through h = 25 mm (touches the base plate) At the softening temp., many asphalt types have a penetration of 800 (= 80 mm).  VISCOSITY (KINEMATIC AND ABSOLUTE) Viscosity is a measure of resistance to flow at a given temp. It is a fundamental property of fluid (asphalt), whereas penetration and softening point are empirical tests. Asphalt viscosity measured at two temp.:60°C &135°C Viscosity at 60°C is called absolute viscosity [poise] Corresp. approx. to viscosity of asphalt in HMA pvmnt during hot summer (most critical state in service) Visc. at 135°C is called kinematic viscosity [centistokes] At this temp. asphalt is sufficiently fluid to flow under gravitational forces alone. Corresp. approx. to viscosity of asphalt at mixing and laydown conditions. 3. Absolute Viscosity (ASTM D2171) Also known as dynamic viscosity Measured at 60°C, using a vacuum viscometer. μ = τ/γ Where: μ = viscosity; τ = shear stress γ = rate of shear NB: poise = 0.1 Pa.s Pa.s equivalent to Nꞏs/m2, or kg/(mꞏs)). Absolute Viscosity 4. Kinematic Viscosity Kinematic viscosity = absolute viscosity/density [centristroke = 1 mm2/s] Measured at 135°C to simulate the mixing process The cross-arm (capillary tube) viscometer is used A constant head is maintained, flow under gravity Measures time to flow between two timing marks -Rotational (brookfield) Viscometer (RV) test -Dynamic Shear Rheometer (DSR) test Will be discussed in the Superpave section. 5. Ductility Test Test involves stretching a standard-sized asphalt briquette (dumbbell) to its breaking point. The sample under water at 25°C is stretched at 50mm/min until it breaks. The distance at rapture, in cm, is reported as ductility.  3.3.2 DURABILITY (AGING) TESTS Short-term aging → During the mixing process Long-term aging → After pvmt construction due to environmental exposure and loading No direct measure for binder aging (age-hardening) Aging effects accounted for by subjecting asphalt binder to simulated aging, then conducting other standard physical tests to evaluate the changes Durability tests include: 1. Thin-film oven test (TFO) – 2. Rolling thin-film oven test (RTFO)→Superpave 3. Pressure aging vessel test (PAV)→ Superpave 1) Thin-film Oven (TFO) Test Determines the effect of heat and air on a thin film of a bituminous material. Indicates changes in asphalt properties during conventional mixing; the residue approximates binder condition in newly constructed pavement A thin film of asphalt heated in oven at 163°C for 5 hrs Changes in other properties – penetration, viscosity & ductility expressed as a %ge of the original values. - Retained penetration = Penetration of aged sample * 100% Penetration of original sample - Aging index = Viscosity of aged sample * 100% Viscosity of original sample  3.3.3 Safety Tests Measure temperature at which asphalt materials will burst or flash into flames Working temperatures must be controlled (kept below Flash Point by ~ 50°F) for safety purposes. Recommended safe temp for Pen-grade ~ 245-335°C Flash Point is the lowest temp at which the vapour from (heated) asphalt is ignited by an open flame Fire Point is the lowest temp at which asphalt continues to burn without further heat supply. Safety tests include: - Cleveland Open Cup (Flash and Fire Point test) - Tag Open Cup test and - Pensky Martin Flash Point test Safety Tests cont…  3.3.4 TEMPERATURE SUSCEPTIBILITY Refers to the rate at which consistency of asphalt changes with changes in temperature. Consistency is measured by penetration and viscosity Two common parameters for temp susceptibility are the Penetration Index (PI) and Penetration-Viscosity number (PVN) Penetration Index (PI) If logarithm of penetration, log(Pen), is plotted against temperature (T), a straight line is obtained; thus: log(Pen) = A*T + k where A (= slope) shows the temp susceptibility of the asphalt, k is a constant. TEMPERATURE SUSCEPTIBILITY Temperature susceptibility for different asphalts