Ed Lyon - HMA Compaction
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Presentation delivered at the CalAPA Spring Asphalt Pavement Conference April 9-10, 2014 in Ontario. Topic: The critical role compaction plays in the HMA construction process, plus methods and standards of measurement and problems to be avoided.
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Ed Lyon - HMA Compaction
- 1. Compaction of Hot Mix Asphalt Pavements (HMA) Ed Lyon, PE, GE April 9, 2014
- 2. Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014 • “Compaction is the process by which the volume of air in an HMA mixture is reduced by using external forces to reorient the constituent aggregate particles into a more closely spaced arrangement. This reduction of air volume in a mixture produces a corresponding increase in HMA unit weight, or density (Roberts et al., 1996). “ Compaction – Defined
- 3. Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014 Quality Characteristics Affected by Compaction – Stability / Strength – Fatigue Life – Durability – Moisture Sensitivity – Raveling Importance of Compaction
- 4. Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014 • “Compaction is the process by which the volume of air in an HMA mixture is reduced by using external forces to reorient the constituent aggregate particles into a more closely spaced arrangement. This reduction of air volume in a mixture produces a corresponding increase in HMA unit weight, or density (Roberts et al., 1996). “ Compaction – Defined
- 5. Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014 • “Compaction is the process by which the volume of air in an HMA mixture is reduced by using external forces to reorient the constituent aggregate particles into a more closely spaced arrangement. This reduction of air volume in a mixture produces a corresponding increase in HMA unit weight, or density (Roberts et al., 1996). “ Compaction – Defined The volume of air in an HMA pavement is important because it has a profound effect on long-term pavement performance.
- 6. Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014 – Fatigue Resistance “A reduction in air voids from 8% to 3% could more than double pavement fatigue life” (Scherocman, 1984a). – Durability “compacting a well-designed paving mixture to low air voids retards the rate of hardening of the asphalt binder, and results in longer pavement life, lower pavement maintenance, and better all-around pavement performance.” McLeod (1967) – Raveling. Kandhal and Koehler (1984) found that raveling becomes a significant problem above 8% air voids and becomes a severe problem above 15% air voids. Positive Effect of Compaction – Moisture Damage. “Air voids in insufficiently compacted HMA are high and tend to be interconnected with each other. Numerous and interconnected air voids allow for easy water entry which increases the likelihood of significant moisture damage.” (Kandhal and Koehler, 1984; Cooley et al., 2002)
- 7. Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014 Negative Effects of Excess Compaction Balance - For dense-graded HMA, for use on high traffic pavements in place air voids between 3 and 8 percent generally produce the best compromise of pavement strength, fatigue life, durability, raveling, rutting and moisture damage susceptibility. – Decreased Stability and Strength. Kennedy et al. (1984) concluded that tensile strength, static and resilient moduli, and stability are reduced at low air void content. – Rutting. The amount of rutting which occurs in an asphalt pavement is inversely proportional to the air void content (Scherocman, 1984a)
- 8. Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014 Aggregate Gradation Aggregate Gradation – Gradation has a direct affect on the effort required to properly compact a pavement Fuller’s maximum density line:Passing (%) = (d/D)n ; n = 0,45 (FHWA)
- 9. Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014 Aggregate Gradtaion – VMA VMA – Voids in Mineral Aggregate – The volume of the intergranular void space between the aggregate particles of a compacted paving mixture that includes the air voids and the effective binder content AASHTO R-35 HIGH VMA = High Voids, harsh or stiff mix • High stability • Difficult to compact LOW VMA = Low Voids, densely graded mix • Lower stability • Easier to compact
- 10. Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014 Relative Compaction How are common practices set up to ensure that the finished pavements have in place air voids between 3 and 8 percent ? Relative compaction is the ratio expressed as a percentage between the field in place density and a laboratory test standard. Relative to What Standard ?
- 11. Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014 Common Practice – Specifying Relative Compaction LAB TEST MAXIMUM DENSITY (LTMD) • Specified Air Voids in the lab between 3% and 5% • Compacted Briquette ÷ Theoretical Max SpG • Specified Relative Compaction 95% to 96% of LTMD • Field Density ÷ Compacted Briquette In Place Air is equal to the sum of the air voids in the lab and air voids from the field relative compaction tests Example: 3.5% Air Voids in the lab, 96.4% Compaction In Place Air = 3.5% + 100% – 96.4% = 7.1 %
- 12. Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014 LTMD - Marshall Method • Mechanically Compacts Briquettes in steel molds using the Impact from the Marshall Hammer. • Mix Design based on optimum asphalt content that yields air voids in the compacted briquettes of 4% when comparing briquette density to Rice Theoretical Density The density of the compacted briquette is used as the specified Lab Test Maximum Density (LTMD ) for comparison to determine the relative compaction
- 13. Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014 LTMD California Kneading Compactor Hveem Method • Mechanically Compacts Briquettes in steel molds using the California Kneading Compactor. • Mix Design based on optimum asphalt content that yields air voids in the compacted briquettes between of 4% when comparing briquette density to Rice Theoretical Density The density of the compacted briquette is used as the specified Lab Test Maximum Density (LTMD ) for comparison to determine the relative compaction
- 14. Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014 Common Practice – Specifying Relative Compaction Theoretical Maximum Specific Gravity: – Specify a minimum in place relative compaction equal to 92% of the Theoretical Maximum Specific Gravity, commonly referred to as the Rice Test – Field Tests are performed by Nuclear Gauge or Cores – % Relative Compaction is the ratio expressed as a percentage between the in place density of the compacted pavement and the Rice maximum specific gravity. Agencies May specify a maximum in place relative compaction on the order of 96 to 97% to preserve stability in areas of heavy traffic
- 15. Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014 Theoretical Maximum Specific Gravity - Rice • Measures the specific gravity of a loose mixture of the HMA after all of the air in the mix has been removed under vacuum. • Used in all mix design methods to select the design asphalt content that yields air voids in compacted briquettes between 3 and 5%
- 16. Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014 Gyratory Compactor Super Pave Method • Mechanically Compacts Briquettes in larger steel molds using the Gyratory Compactor. • Mix Design based on optimum asphalt content that yields air voids in the compacted briquettes of 4% when comparing briquette density to Rice Theoretical Density. • Specifies a Minimum VMA – stiffer mixes Theoretical Maximum Specific Gravity (Rice) test used for comparison to field density to determine the relative compaction
- 17. Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014 YOUR QUESTIONS Thank you for your attention.
- 18. Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014 Hamburg Wheel Tracking Device Super Pave Method
Hinweis der Redaktion
- Good afternoon, my name is Ed Lyon and I am a registered civil engineer and registered geotechnical engineer in California. I am thrilled to be here today to discuss compaction of hot mix asphalt pavements. This is a very broad subject so this afternoon I will be focusing my presentation on three areas. First I want to spend some time discussing the importance of compaction and its impact on the long term performance of a pavementSecond I will be discussing aggregate gradation and its impact on compaction and pavement performance And Finally I am going to review the common practice in use to specify relative compaction and how they are used to achieve long term pavement performance.
- First lets start off with a good working definition of compaction. Read definition:Compaction is the process of rearranging the aggregate and binder in the loose mix into a dense interlocking matrix while driving air out of void spaces the mix increasing the density or unit weight of the mix and creating a stable finished surface
- It has been said that compaction is the single most important factor that determines long term pavement performance. That is due to the fact that the degree of compaction accomplished affects many of the key pavement properties that drive performance.Compaction will affect the Satbility / Strength of the pavemnetIt has a direct relationship to the pavements ability to resist repetitive fatigue loadsIt will directly affect the pavements durability or resistance to weathering and it ability to resist moisture damage and proper compaction will prevent raveling of the pavement.To understand how compaction can positively or negatively impact these quality characteristics lets go back to our definition
- Compaction is the process by which the volume of air in the HMA Mixture is reducedSo essentially the extent to which the HMA is compacted in the field will determine the volume of air left in the completed pavementsPoorly compacted pavements will have a high volume of air remaining in the completed mixDensely compacted pavements will have a low volume of air in the completed mix
- And the volume of air in an HMA pavement is really the characteristic that drives the long term performance of the pavementSo when we talk about compaction and how it affects pavement performance we are really talking about the volume of air, or air voids, in the completed pavements and how that will affect the long term performance of the pavement
- A densely compacted pavement will have low in place air voids the benefits of low in place air voids have been demonstrated through numerous studies and they include: Improved Fatigue resistance: The tensile strength of the pavement is derived from the binder and in order to resist fatigue loads the pavement needs good tensile strength and the horizontal stresses imposed on the bottom of the pavement layer are pulling stresses that result in fatigue cracking. To more the mix is compacted the fewer air voids are present allowing the binder to carry those forces and remain elastic.Durability or the pavements resistance to weathering is improved by low air voids as it has been shown that low air voids slow the rate of hardening of the binder and extends the time during which the pavement remains flexible extending the life and performance of the pavement Poorly compacted mixes have a high volume of air that can lead to raveling. Studies have shown that raveling begins to become an issue at 8% air voids. Well compacted mixes will have less than 8% air and will not be subject to raveling Finally sensitivity to moisture damage can be improved by proper compaction
- However you can have to much of a good thing. There are negative affects to driving the air voids out of the mix or over compacting a pavement.Stability is the pavements ability to resist lateral deformation under vertical loads. It is primarily derived from the shear strength of the aggregates. As you begin to compact an HMA pavement from a loose state you increase the stability and strength of the mix as you force the aggregate into a tight matrix.As you continue to compact a pavement to low air void content, lower than 3%, the binder is called to carry larger portions shear strength and the pavement will experience reduced stability and reduced resistance to rutting.Therefore, to achieve long term pavement performance it is important to find balance between the positive benefits of low air voids and the air voids required to ensure the stability and strength of the mix
- Understanding the importance of air voids in the mix we can also demonstrate how aggregate gradation will impact the performance of the pavements and the effort required to compact the pavement. The FHWA Power 45 Curves define the ideal gradation that will yield the greatest density when compacted. These curves represent the aggregate gradation that will most easily combine as each of the various particle sizes in the mix interlock and the smaller fractions ideally fill the voids between the larger particles resulting in the tightest spacing possible between coarse to fine for a given maximum aggregate size.Mixes designed with gradations that fall along these curves will be very easy to compact in the field and will have low oil demands as the tight arrangement of the aggregates removes leaves a low volume of air remaining to be filled by binderAs with compaction though we again can have too much of good thing. Mixes that are designed along these lines may have too few voids to provide the necessary volume of air in the pavement to ensure stability.So some agencies will specify a minimum VMA to force the gradation away from the ideal curves
- The VMA or Voids In Mineral Aggregate is the volume of voids in the aggregate including the volume occupied by air voids and the volume filled by binder that was not absorbed by the aggregate.So in order to ensure that the mix contains sufficient voids to provide a stable mix agencies specify a minimum VMA. The higher the VMA the further away from the ideal power 45 curves the gradation will fall meaning that the aggregates will not interlock as well and will therefore be more difficult to compact.High VMA Mixes will be harsh or stiff mixes that tend to have higher stability but they will be more difficult to compact in the field Low VMA mixes will be densely graded they may have lower stability but they will be much easier to compact in the field The design aggregate gradation selected based on a VMA specification is driven by the same goals for long term pavement performance and the need for balance between stability and durability. Compaction of the pavement is the final step that determines how well we achieve that balance.
- So how are the specifications that we work with every day set up to verify that this balance have been achieved?Our specifications call out Relative Compaction. We defined compaction as the process of compacting the pavements and reducing the air content of the mix, but what is relative compaction.Percent relative compaction is the ratio between the field in place density So when we talk about relative compaction it is important to ask the question Relative to What ??
- There really are two general ways that our specifications have been developed The first is a two step process where we specify air voids in the lab and then specify relative compaction in the field based on a Lab Test Maximum Density In the mix design stage we compact briquettes at varying oil contents and compare the their density to whats called the maximum theoretical specific gravity to select the oil content that results in 4% air in the lab During construction we see specifications that require tests to determine the air voids and limit them to 2 to 5% We then we use the density of the compacted briquettes as the lab test max density the LTMD and compare that to the field densityto determine relative compaction
- There are three common mix design procedures in use today that include the marshall method, the hveem method and the super pave method of mix design.
- The maximum specific gravity is the combined specific gravity of the aggregate which are typically 2.6 to 2.8 and the binder which are typically 1.01 so the values are inversly related to binder content. The higher the percent binder the lower the maximum theoretical specific gravity.This test never compacts the mix – in fact its important that the test be run on a sample that is loose so that the air in the mix can be removed easily. The test measures the maximum specific gravity by removing all of the air in the mix under vacuum. It can be a sensitive test and it is driving the evaluation of the overall long term performance of the pavement so it is important that the test be run carefully.It is sensitive to the time that the test is run. It needs to be run long enough to completely remove the air from the test vessel, any air remaining in the container will be included in the calculated volume of the materials tested.All of our mix design procedures use the Maximum theoretical specific gravity test, commonly referred to as the Rice test, to determine the maximum density that defines the percent air voids in the mix.