Lecture 6 Characterization of Asphaltic Materials. Part 1-General Concepts

Transcription

1 Lecture 6 Characterization of Asphaltic Materials Part 1-General Concepts

2 Introduction Asphalt concrete is basically a mixture of asphalt binder and aggregates, hot-mixed in an asphalt plant and then hot-laid to form the surface course of a flexible pavement. The mix proportion is approximately 95% aggregate by weight, 75% aggregate by volume and ideally 3%-5% air voids. The properties of asphalt concrete depend on: Quality of its components (i.e., asphalt binder and aggregates). Mix proportions (void content, binder content, aggregate geometry and etc.). Construction process. Asphalt concrete must provide a stable, safe, and durable road surface.

3 Asphalt Concrete: Stability Stability of the asphalt concrete depends on the strength and flexibility of the mixture which is influenced by mix properties and field compaction. The strength must be sufficient to carry the load without developing excessive plastic deformation. The structure must remain intact to provide good ride quality. The main contributor to strength is friction between particles as well as adhesive and cohesive bonds between aggregates and mastic. A dense-graded mixture, composed of particles with angular rough faces, with a relatively thin asphalt film between them is best for high strength mixes. Flexibility is also an important phenomena in pavement engineering as the pavement is expected to rebound (recovery of elastic strains) during unloading process without cracking or brittle fracture. This is imporant for fatigue performance under repeated traffic loads. Therefore it is imperative to consider both strength and flexibility to account for the stability of flexible pavements.

4 Asphalt Concrete: Safety Its important to design the asphalt mixes so that they provide proper frictional properties between the tire and the pavement surface. Safety is achieved by making the surface course skid resistant and able to allow quick drainage of water from the surface. Hydroplaning happens when a layer of water builds between wheels and the pavement surface which leads to loss of traction.

5 Asphalt Concrete: Durability Durability of the asphalt concrete is critical to ensure that it maintains the stability and skid resistance properties during service life. As asphalt ages, the HMA layer becomes less flexible and more prone to loss of service with time and repeated traffic loads. Pavements fail (i.e., loss of serviceability) due to: Changes in the aggregates (loss of surface macro-texture, aggregate breakage due to excessive loads or environmental conditions, and etc.). Permanent deformation or rutting (improper compaction, bad mix design (high air voids or tender mixes which leads to vertical lateral movement of materials). Cracking, either due to fatigue, or low temperatures (can be controlled with proper mix design). Bleeding of asphalt (excess binder content in the mix which compromises the surface frictional properties).

6 Refinery Operations During the distillation process, lighter molecules vaporize and asphalt binder remains. The consistency of the residual is highly variable and depends on the source and the distillation process.

7 Chemical Composition of Asphalt Binder - Large organic molecules of varying size and polarity Carbon 80-87% Nitrogen 0-1% Hydrogen 9-11% Sulfur 0.5-7% Oxygen 2-8% Heavy metals 0-0.5% Heavy metals play an important role as they contribute to polarity. - Molecular structure very complex Asphaltenes Resins Oils - Colloidal model Asphaltenes surrounded by resins Oils continuous medium - largest and most polar - intermediate, also polar - smallest, paraffin - like, non - polar Asphaltenes Resins Oils

8 Asphalt Concrete Mixtures Mixtures of aggregate and asphalt binder. About 95% aggregate by weight About 75% aggregate by volume Ideally, 3-5% air voids

9 Challenges for Characterization of Asphaltic Materials 20 year old pavement

10 Variation of layer Moduli with Time

11 Mechanical Behavior of Asphalt Mixes Thermoplastic Material Material properties change with temperature. Rheological Material Material properties change with time or frequency of loading.

12 Consistency Temperature Regimes where Distress Predominates Plexiglas Salt Water Taffy Molasses Low-temperature thermal Shrinkage cracking Intermediate-temperature traffic-associated fatigue High-temperature rutting Approximate Temperature, C

13 Binder Characterization

14 Binder Grades (Classification) The main purpose of binder grading systems is to classify binders based on their rheological and mechanical properties, assuming that these properties relate to field performance. The asphalt grading systems are: Penetration Grading (ASTM D 946) Based on a penetration of a needle in 0.1 mm in five seconds. Five Grades: Pen 40-50, Pen 60-70, Pen , Pen , and Pen Viscosity Grading (ASTM D 3381) Based on absolute viscosity at F (60 0 C). Five Grades: AC-2.5, AC-5, AC-10, AC-20, and AC-40. (the number following AC indicates the absolute viscosity in hundreds of poise. (e.g. AC-5: binder with absolute viscosity of 500 poise). Viscosity of Aged Residue Grading (ASTM D 3381) Based on absolute viscosity of RTFO aged binder at F (60 0 C). Five Grades: AR-1000-AR-2000, AR-4000, AR-8000, and AR Superpave Performance Grading (most commonly used). Based on temperature range and climatic conditions of a specific geologic location.

15 Asphalt Penetration Test (Consistency Test) The penetration test started out using a No. 2 sewing machine needle mounted on a shaft for a total mass of 100 g. This needle was allowed to sink into a container of asphalt binder at room temperature (25 o C) for 5 seconds. The consistency (stiffness) of a given asphalt binder was reported as the depth in tenths of a millimeter (dmm) that the needle penetrated the asphalt binder. Based on penetration test asphalts are categorized into five grades: Pen Pen Pen Pen Pen

16 Penetration Graded Binders- Disadvantage Temperature susceptibility (i.e., the rate of change in material properties with a change in temperature) can be estimated by determining the penetration at two (or more) temperatures. The most commonly used temperatures are 4 o C and 25 o C with a 100 g load for 5 seconds. This slide highlights one of the major problems with the penetration grading system. For example, three sources of asphalt binder can have the same penetration at 25 o C but decidedly different properties above and below this temperature. Medium Low This helps explain the differences in observed pavement performances even though the same penetration grade of asphalt binder is specified. High 25C (77F) Temperature

17 Viscosity, 60C (140F) Comparison of Grading Systems Penetration Grades AC 40 AC 20 AR AR AC 10 AC 5 AC 2.5 AR 4000 AR 2000 AR

18 Superpave Performance Grade (PG) Asphalt Binder Specification The PG Binder designation is based on expected extremes of hot and cold pavement temperatures. PG Performance Grade Min. pavement temperature Average 7-day max. pavement temperature

19 Performance Graded (PG) Binder Specification Maximum Temperature (ºC) Minimum Temperature ( 0 C) ºC) PG PG PG PG PG PG PG As an example, a PG is acceptable for use in a

20 Observed Air Temperatures Topeka, KS 36 very cold winter 40 average winter > standard deviation of 4 C

21 Calculated Pavement Temperatures Topeka, KS For the low temperature use the air temperature

22 PG Asphalt Binder Grades Topeka, KS PG (98% minimum reliability) PG (50 % minimum reliability) PG asphalt binder grades - six degree increments

23 Typical Asphalt Binder Tests Flash Point Temperature at which a material will ignite with an open flame. Important for safety during the mix preparation. Viscosity Rotational viscometer measures the viscosity at a standard temperature (e.g C) Complex Shear Modulus Dynamic Shear Rheometer (DSR) to determine viscoelastic material properties (dynamic shear modulus and phase angle). Flexural Creep Bending Beam Rheometer (BBR) as an indicator of creep stiffness properties of the binder. Asphalt Aging Rolling Thin Film Oven Test ( RTFOT) as an Indicator of the aging effect of short term high temperatures when producing the asphalt mix. Tensile Strength

24 Binder Tests-Viscosity ASTM D 4402 or AASHTO T 316 Rotational Viscometer Torque (T) required to maintain a rotational speed ( ) at constant temperature (e.g C).. Rotational viscosity, : 2 T (1/ Ri 1/ 4 R 2 0 ) R i = Spindle Radius R o = Chamber Radius

25 Asphalt Binder Viscosity and Temperature

26 Viscosity, Pa. s Mixing and Compaction Temperatures Compaction Range Mixing Range Temperature, C Information from viscosity test can be used to estimate appropriate mixing and compaction temperatures.

27 Binder Tests- Dynamic Shear Modulus AASTHO T 315 Dynamic Shear Rheometer (DSR) to measure dynamic properties ( G*, d): max q r / h max / q max r / 2T r h 3 T= Maximum applied torque h= Specimen height q= Deflection angle r = Plate radius Parallel plate rheometer applies shear stress

28 Dynamic Shear Rheometer (DSR) Oscillating Plate B A C Fixed Plate B Test operates at 10 rad/sec or 1.59 Hz 360 o = 2 p radians per circle 1 rad = 57.3 o Motor A A Time Parallel Plates with Sample One Cycle C Area for Liquid Bath

29 Dynamic Shear Modulus and Phase Angle In a perfectly elastic material, there is no lag between the applied stresses and measured strains, therefore the stresses and strains are in phase and the phase angle is zero (d=0 0 ). The stresses applied on a fully viscous material is out of phase with strains therefore (d=90 0 ). Asphalts are viscoelastic materials therefore (0 0 <d<90 0 ).

30 Binder Tests- Flexural Creep ASTM D 6648 or AASTHO T 313 Bending Beam Rheometer (BBR) Used to measure creep caused by a load applied in the middle of the beam.

31 Sample Results of BBR and DSR Tests on the Same Binder

32 Binder Tests- Tensile Strength AASTHO T 134 Direct Tension Test (DTT) Direct tension load applied to maintain a constant displacement rate at a standard low temperature. Result: tensile strain and stress at maximum load.

33 Binder Tests- Asphalt Aging ASTM D 2872 or AASTHO T 240 Rolling Thin-Film Oven (RTFO) test to simulate short term aging (aging during mixing and construction). Bottles with asphalt placed in a rotating rack at a high temperature (mixing temperature) to simulate short term aging of asphalt. ASTM D 6521 Pressure Aging Vessel (PAV) test to simulate long term aging: asphalt binder subjected to oxygen at high pressure and high temperature.

34 Types of Asphalt Mixes

35 Types of Asphalt Concrete Asphalt concrete mixtures can be classified into following two types based on whether hotmixed, hot laid or cold-mixed, cold-laid: Hot-mixed, Hot-laid Asphalt (HMA) Concrete Mixture. Cold-mixed, Cold-laid Asphalt Concrete Mixture. Asphalt concrete mixtures can be classified into following two types based on whether in-situmixed or plant-mixed: Road-mixed Or In Place-mixed Asphalt Concrete Mixture. Plant-mixed Asphalt Concrete Mixture. HMA concrete mixtures can be classified into following three types based on type of aggregate grading used: Dense-graded HMA Concrete Mixture. Stone Matrix Asphalt (Sma) Concrete Mixture. Open-graded Hma Concrete Mixture. Asphalt concrete mixtures can be classified into following three types based on the type of additives used: Rubber-modified Asphalt Concrete Mixture. Polymer-modified Asphalt Concrete Mixture. Sulfur-modified Asphalt Concrete Mixture.

36 Hot Mix Asphalt (HMA) Production Process HMA is produced and laid in the following steps: Both aggregate and asphalt are heated prior to mixing to drive off moisture from the particles and make the asphalt sufficiently fluid (maximum temperatures for heating asphalt and emulsified asphalt are F and 82.2 F, respectively) After heating, all the raw materials are mixed in the plant, and the hot mixture is transported to the paving site and spread on a loosely compacted layer to a uniform, even surface with the help of a paving machine. While the mixture is hot it is compacted by heavy, motor-driven rollers to produce a smooth, well-compacted paving course. Since the aggregates are thoroughly dried prior to mixing, stripping of asphalt (i.e., disintegration from the pavement) is expected to be minimal or nonexistent for hotmixed, hot-laid asphalt pavements

37 Warm Mix Asphalt (WMA) Warm asphalt mixes are separated from half-warm asphalt mixtures by the resulting mix temperature. If the resulting temperature of the mix at the plant is less than 100 C (212 F), the mix is considered a half-warm mix. If the temperature of the mix at the plant is above, 100 C (212 F), the mixture is considered a warm mix. There is still a wide range of production temperatures within warm mix asphalt, from mixes that are 20 C to 30 C below HMA to temperatures slightly above 100 C (212 F).

38 Cold Mix Asphalt Similar to hot-mixed asphalt concrete, cold-mixed asphalt concrete is also a mixture of asphalt, fine aggregate or both fine and coarse aggregates, and mineral filler (optional). Cold-mixed asphalt concrete is produced and laid at normal temperature, however, some heating of both the aggregates and asphalt may be required during winter season. Drying of aggregates is not necessary except when the particles have surface moisture. Commercial additives are needed in this type of asphalt concrete to improve bonding.

39 Road-Mixed and Plant-Mixed Asphalt Mixes A bituminous surface or base course produced by mixing aggregates and asphalt at the jobsite is called road-mixed or mixed-in place asphalt concrete. A mixture of aggregates and emulsified or cutback asphalt prepared at a central mixing plant and spread and compacted at the jobsite at near ambient temperature is called plant-mixed, cold-laid asphalt concrete.

40 Dense Graded HMA A dense graded HMA mix is produced using well-graded aggregates, and intended for general use. When properly designed and constructed, a dense-graded HMA concrete is relatively impermeable. Dense-graded HMA concrete mixes are generally referred to by their nominal maximum aggregate size. Fine-graded mixes have more fine and sand sized particles than coarsegraded mixes.

41 Stone Matrix Asphalt (SMA) Stone matrix asphalt (SMA) is a gap-graded HMA that is designed to maximize deformation (rutting) resistance and durability by using a structural basis of stoneon-stone contact. Because the aggregates are all in contact, rut resistance relies on particles interlock rather than binder adhesive and cohesive bonds. Since aggregates do not deform as much as asphalt binder under load, this stoneto-stone contact is believed to reduce rutting. SMA is generally more expensive than a typical dense-graded HMA (about percent) because it requires more durable aggregates, higher asphalt content and, typically, a modified asphalt binder. In the right situations it should be cost-effective because of its increased rut resistance and improved durability.

42 Open-Graded HMA An open-graded HMA mixture is designed to be water permeable. (dense-graded mixes usually are not permeable). Open-graded mixes use only crushed stone and a small percentage of manufactured sands. There are three types of open-graded mixes typically used in the U.S.: Open-Graded Friction Course (OGFC): Typically 15 percent air voids, no minimum air voids specified, lower aggregate standards than Porous European mixes (PEM). Porous European mixes (PEM): Typically percent air voids, specified minimum air voids, higher aggregate standards than OGFC and requires the use of asphalt binder modifiers. Asphalt Treated Permeable Bases (ATPB): Less stringent specifications than OGFC or PEM since it is used only under dense-graded HMA, SMA or PCC for drainage.

43 Modified Asphalt Mixes Asphalt rubber also called crumb rubber, which is a recycled product from old tires, is added ranging from 1% to 5% (by weight of asphalt) as an additive in the production of HMA for improving the flexibility and therefore fatigue performance of pavement systems. Rubber addition increases the viscosity and the softening point of the asphalt. Polymers (such as ethyl vinyl acetate, silicone, and epoxies) are added to asphalt as additive to produce polymer-modified asphalt concrete Polymer addition increases dispersion, ductility, and adhesiveness of asphalt. It s often used to reduce the temperature sensitivity of the stiffness properties of the mixes. Sulfur is added to asphalt concrete to provide higher stiffness at elevated temperatures.

44 Asphalt Mixture Weight-Volume Relationships

45 Asphalt Concrete Mixtures Mixtures of aggregate and asphalt binder. About 95% aggregate by weight About 75% aggregate by volume Ideally, 3-5% air voids

46 Effective Asphalt Content During the asphalt mix preparation, some of the asphalt is absorbed in the pores of the aggregate particles. The portion of asphalt absorbed by aggregate particles is called absorbed asphalt. The net amount of asphalt available to coat and bind aggregates together is called effective asphalt. It is important to account for the absorbed asphalt when using porous aggregates during mix design.

47 Asphalt Concrete Mixture-Phase Diagram The mass/volume relationships of a compacted asphalt mixture are illustrated in the following figure: Mass/volume relationships for an asphalt concrete mixture: Density ( ) Asphalt content (P B ) Effective asphalt content (P BE ) Asphalt absorption (P BA ) Air voids (AV) M Total mass (= M G + M B ) M G Mass of aggregate M B Mass of asphalt (binder) (= M BE + M BA ) M BE Mass of effective asphalt, the asphalt binder between particles M BA Mass of absorbed asphalt, absorbed into the pores of the aggregate particles V Total volume of the compacted mix V G Volume of aggregate, the bulk volume including the aggregate pores V BE Volume of effective asphalt V BA Volume of absorbed asphalt V B Volume of asphalt (= V BE + V BA ) V A Volume of air between the coated aggregate particles in the mix V GE Effective volume of aggregate (= V G V BA ) V MM Volume of voidless mix (maximum mix volume) = M/V P B = M B / M P BE = M BE / M P BA = M BA / M G AV = V A / V Voids in mineral aggregate (VMA) VMA = (V BE + V A )/V Voids filled with asphalt (VFA) VFA = V BE / (V BE + V A )

48 Asphalt Concrete Mixture-Example