ASPHALT MIX DESIGNER

Transcription

1 ASPHALT MIX DESIGNER EFFECT OF MIX DESIGN ON PERFORMANCE AND CONSTRUCTION OF HMA Module 8 Module 8 1, July

2 QUALITY Meets or Exceeds the Expectations or Needs of the Customer Module 8 2, July

3 HMA Quality =? Module 8 3, July

4 Quality HMA Mixtures Constructability Conforms to specifications Satisfies functional requirements Module 8 4 the definition of quality must be customer-related or customer-driven. In terms of HMAC mixtures: 1. Is it constructable? 2. Does it conform to or meet specifications? 3. Does it satisfy functional or performance requirements?, July

5 Customer Driven Expectations of HMA Mixtures 1. Smooth surface 2. No hydroplaning 3. Minimization of traffic disruptions 4. Adequate friction at surface 5. Minimization of overall costs Module 8 5 The importance of the mixture depends on the type of facility or roadway used, i.e., freeway versus county road, roadway versus apron or parking areas. 1. Smooth surface: Rough surfaces result in higher vehicle operating costs and higher pavement maintenance costs or shorter service lives. 2. Minimization of traffic disruptions-durable mixtures: Inferior mixtures result in increased maintenance/rehabilitation activities (or traffic delays) and increased maintenance costs. 3. Adequate friction at surface: Reduced friction results in increased accidents producing increased or more frequent traffic delays 4. Minimize overall costs (taxes)., July

6 HMA Mixture Characteristics Resistance to Permanent Deformation Fatigue Resistance Durability Permeability Workability/Compactability Module 8 6 For HMAC mixtures to function well, they must be designed, produced and placed to give them certain required properties. These properties typically include: stability or strength, fatigue resistance, durability, fatigue resistance, permeability, and workability.! Stability - Ability to resist distortion! Fatigue Resistance - Ability to resist cracking under repeated loading! Durability - Ability to resist physical changes of the material in time! Impermeability - Ability to resist the passage of water and air through the mix! Workability - The ease with which a mixture can be placed and compacted, July

7 HMA Mixture Characteristics Resistance to Permanent Deformation Fatigue Resistance Durability Permeability Workability/Compactability Module 8 7, July

8 Rutting Longitudinal Surface Depression in the Wheel Path Module 8 8 Rutting can be divided into three categories, which relate to the cause of the ruts. 1. Plastic movement is a depression near the center of the applied wheel load with slight humps on either side of the depression. Plastic movement is generally caused by an insufficient amount of total voids in the HMA mat, thereby allowing the asphalt to act as a lubricant rather than a binder during hot weather. This type of rutting can also be caused by moisture trapped in the mix after construction and/or stripping. This type of rutting can occur immediately after compaction. 2. Densification (sometimes referred to as consolidation) is a depression near the center of the applied wheel load without an accompanying hump on either side of the depression. Densification is usually caused by excessive air voids or inadequate compaction after placement of the mat, allowing the mat to densify in the wheel paths when subjected to traffic in hot weather. This type of rutting is more of a long-term characteristic of the HMA mixture. 3. Mechanical deformation is a depression near the center of the wheel path that is caused by additional densification, consolidation, and/or lateral movements of the unbound materials below the surface. This type of rutting can be accompanied by a longitudinal cracking pattern adjacent to the wheel paths at the surface. This type of rutting is more localized and is more of a longer term characteristic of the pavement structure than the mix is., July

9 Shoving Horizontal Displacement Under Wheel Loads Module 8 9 Shoving is another type of distortion. Shoving is the horizontal displacement of an HMA mix under wheel loads. Shoving can occur shortly after compaction and usually occurs at intersections where increased horizontal forces from wheel loads are found. This type of distortion can also be caused by slippage between the HMA layer and supporting layer, either from the use of an excessive tack coat or from a dirty surface., July

10 Shoving Horizontal Displacement Under Wheel Loads Module 8 10 This slide shows another photo of shoving., July

11 Bleeding/Flushing Extrusion of AC at the Pavement Surface Module 8 11 Bleeding or flushing depending on the severity lead to reduced skid resistance of the HMA surface. Bleeding is the extrusion of asphalt cement at the pavement surface or the presence of a film of asphalt on the surface of an HMA pavement. This condition can occur during or shortly after compaction. It is caused by excessive amounts of asphalt cement in the mix, low air void contents, and/or too low viscosity asphalts for the climate where the mixture is placed. Overdensification of the HMA mat and moisture damage or stripping within the mixture can also cause bleeding or flushing of the mix, July

12 Bleeding/Flushing Extrusion of AC at the Pavement Surface Module 8 12 Fat spots are probably construction related - possible causes are fuel drips, tack picked up on tires,, July

13 CAUSES Instability Excessive Amounts of Asphalt VMA too low Rounded Aggregates Low Air Voids Low Viscosity Asphalt for Climate Moisture Damage/Stripping Module 8 13, July

14 Bleeding and Flushing Isolated Areas Termed Fat Spots where Asphalt Cement has come to the Surface during the Compaction Process Excessive fluids in mix (asphalt and/or moisture) Wide fluctuation in mix temperature Module 8 14, July

15 Mixture Resistance to Rutting Asphalt Binder Stiff and elastic at high temperatures Aggregate High interparticle friction Gradation acts like one large elastic stone Module 8 15, July

16 HMA Mixture Characteristics Resistance to Permanent Deformation Fatigue Resistance Durability Permeability Workability/Compactability Module 8 16 Fatigue or load-related cracks are caused by placing stiff mixtures over weak pavement structures (or mixtures with inadequate structural support for the repeated traffic loads and tire pressures). HMA mixtures with inadequate tensile and/or shear strength are susceptible to fatigue failure., July

17 Alligator Cracking Any resemblance? Module 8 17 This slide shows a photo of alligator cracking., July

18 Block Cracking Module 8 18 This slide shows a photo of block cracking., July

19 Distress in Wheelpath Progressive Damage longitudinal cracking alligator cracking potholes Affected by asphalt binder aggregates pavement structure Fatigue Cracking Module 8 19, July

20 tensile stresses HMA must be strong & resilient Module 8 20, July

21 HMA Fatigue Behavior Longer Fatigue Life flexible materials low stress/strain level Shorter Fatigue Life stiff materials high stress/strain level Exception thick pavements non deflecting support layers Heavy Trucks Module 8 21, July

22 Cures for Fatigue Cracking Design for actual number of heavy loads Keep subgrade dry (i.e., low deflections) Use thicker pavements Use non moisture susceptible materials Use paving materials that are resilient Module 8 22, July

23 HMA Mixture Characteristics Resistance to Permanent Deformation Fatigue Resistance Durability Permeability Workability/Compactability Module 8 23, July

24 Raveling Loss of Adhesion between Asphalt and Aggregate Module 8 24, July

25 Raveling Loss of Adhesion between Asphalt and Aggregate High Air Void Content Inadequate Density Asphalt Film Thickness Low Asphalt Content Asphalt Cleanliness Aggregate Stripping Potential Aggregate Absorption Module 8 25 Raveling is a type of mix disintegration, and is more of a long-term performance characteristic of the mix. Raveling is defined as the removal of the HMA surface caused by the dislodging of the aggregate particles. There are three primary factors that cause raveling: 1. Inadequate density 2. Low asphalt content or high air void content 3. Too many fines and/or dirty aggregate The following in combination with the above factors, can result in accelerated and severe raveling: application of high shear stresses or horizontal loads, and the presence of water. Raveling is related to the adhesion between the asphalt and aggregate. Those factors that affect the adhesion include: the density or air voids of the mixture, asphalt content (film thickness), aggregate cleanliness, stripping potential or moisture susceptibility, and/or absorption., July

26 HMA Mixture Characteristics Resistance to Permanent Deformation Fatigue Resistance Durability Permeability Workability/Compactability Module 8 26, July

27 What causes high permeability? How do you reduce it??? Module 8 27, July

28 Laboratory Permeability Testing Module 8 28, July

29 HMA Mixture Characteristics Resistance to Permanent Deformation Fatigue Resistance Durability Permeability Workability/Compactability Module 8 29, July

30 Workability Module 8 30 Workability, July

31 Poor Workability Possible Cause Coarse graded mixes Modified Binders (Elastomers & asphalt rubber) Possible solutions Increase temperature Minimize handwork Module 8 31, July

32 Compactability Aggregate Properties Aggregate Gradation Asphalt Cement Module 8 32 The compactability of the of an HMA mixture is dependent on the aggregate properties, the aggregate gradation and the asphalt cement., July

33 Compactability Problems Shoving Tender Zone Checking Module 8 33 The three major compactability problems are shoving (the movement of the mix under the rollers), the tender zone (a subset of shoving associated with Superpave mixes) and checking (small hair line cracking in the surface of the mix)., July

34 Shoving Displacement of Mix in the Direction of Compaction Unstable or tender mixes Hump in the gradation curve Excess fluids in mix High mat temperatures Module 8 34 Shoving is the displacement of the mixture in the direction of compaction. Shoving is usually visible by a large bow wave in front of the breakdown roller. Shoving can occur together with mix checking Shoving is characterized by an unstable or tender mix (i.e., an excess of fluids in the mix, a hump in the fine aggregate grading curve, or excessive mat temperatures during rolling). A mix with a high stability can still be a mix that will distort longitudinally under the compaction equipment and later under traffic., July

35 Cures for Shoving Place mix in thinner lifts Increase internal stability of mix Reduce fluids content Increase percentage of crushed material Reduce natural sand content Module 8 35 The cure for a mix that shoves under the compaction equipment is to increase the internal stability of the mixture. One method is to reduce the fluids content. Another solution is to increase the internal friction among the aggregate particles (change the aggregate gradation or increase the amount of crushed particles). If used, these mixtures (tender mixes) should be placed at lower laydown temperatures consistent with the ability to obtain sufficient density under the rollers., July

36 Checking Short transverse cracks spaced at 1 to 3 inches apart that develop after compaction equipment passes over mat Excessive deflections of pavement under compaction equipment Deficiency in mix properties High mix temperatures Module 8 36 Checking can be defined as short transverse cracks, usually 1 to 4 inches in length and 1 to 3 inches apart. These surface cracks are not visible behind the paver. The cracks usually occur after the compaction equipment passes over the mix and are restricted to the surface (usually ¼ to d inch in depth). The causes for checking can be separated into three factors: (a) excessive deflection of the pavement structure under the compaction equipment, (b) a deficiency in the asphalt concrete mix, and/or (c) high mix temperatures. Excessive deflections from the weight of the rollers causes the pavement layers to bend excessively, placing the new mix in tension. The check marks are then formed with the surface of the new mixture being pulled apart as the pavement deflects under the compaction equipment. [, July

37 Mix Properties Related to Checking Excess fluids in mix (asphalt and/or water) Excess middle size sand (hump in the gradation curve) Characteristic of mix that is susceptible to checking mix shoves under roller Module 8 37 A more prevalent cause of checking is a deficiency in the asphalt concrete mixture, typically a tender mix. The two most common causes are: 1. An excess of fluids in the mix (too much asphalt cement, too much moisture, or both). 2. A non-uniform sand gradation (too much middle-size sand - No. 16 and No. 30 sieve-size material, and too little fine-sized sand - No. 50 and No. 100 sieve-size material), in other words a hump in the fine aggregate gradation curve. The mix tends to be shoved by the roller instead of being tucked under by the compactor and is characterized by a bow wave that occurs in front of the rollers on a steel wheel roller. The amount of checking that occurs is increased when the mix temperature is too high for the particular asphalt cement grade being used in the mix. As the mix temperature increases, the viscosity of the asphalt cement decreases, causing an increase in the tenderness of the asphalt concrete mixture. Checking can appear on the mat s surface either during breakdown rolling or during the intermediate rolling., July

38 Cures for Checking A wrong cure is to delay breakdown rolling A proper cure is to change the mix characteristics Module 8 38 If checking occurs during breakdown rolling, a wrong cure for checking is to delay the breakdown roller. Delaying compaction allows the mix to cool and the proper density may not be attained. The proper solution is to change the mix characteristics, not the rolling procedure. On a temporary basis, until the mix design can be altered, the mat can be compacted using a pneumatic tire roller for breakdown instead of a static steel wheel or vibratory roller. If checking does not occur initially during breakdown rolling, but is observed in the intermediate rolling zone, it is appropriate to delay the intermediate compaction until the mix drops below its critical temperature zone. This assumes that adequate densities can still be obtained., July

39 ANY QUESTIONS? Module 8 39, July