Engineers and technicians with a broad range of experience in materials testing and the design of HMA mixtures and flexible pavement structures need not read this chapter in detail. Individuals who are relatively new to asphalt pavement technology will find the information on materials, asphalt pavements, asphalt concrete mixtures, and mix design methods helpful when reading the later chapters of this manual.
Materials Used in Making Asphalt Concrete Asphalt concrete is composed primarily of aggregate and asphalt binder. Small amounts of additives and admixtures are added to many HMA mixtures to enhance their performance or workability. These additives include fibers, crumb rubber, and anti-strip additives. Figure shows a typical HMA laboratory specimen and the materials used to produce it.
Asphalt binder is the thick, heavy residue remaining after kerosene, gasoline, diesel oil, and other fuels and lubricants are refined from crude oil.
Asphalt binder consists mostly of carbon and hydrogen, with small amounts of oxygen, sulfur, and several metals. The physical properties of asphalt binder vary tremendously with temperature. At high temperatures, asphalt binder is a fluid with a consistency similar to that of motor oil. At room temperature most asphalt binders will have the consistency of putty or soft rubber. Many asphalt binders contain small percentages of polymer to improve their physical properties; these materials are called polymer- modified binders.
Much of the current asphalt binder specification used in the United States was designed to control changes in consistency with temperature. This specification and the associated test methods are discussed in more detail in Chapter 3. Because HMA mixtures are mostly aggregate, aggregates used in HMA must be of good quality to ensure the resulting pavement will perform as expected. Aggregates used in HMA mixtures may be either crushed stone or crushed gravel.
In either case, the material must be thoroughly crushed, and the resulting particles should be cubical rather than flat or elongated. Aggregates should be free of dust, dirt, clay, and other deleterious materials. Because aggregate particles carry most of the load in HMA pavements, aggregates should be tough and abrasion resistant. Properties of aggregates and the tests that technicians use to evaluate them are discussed in detail in Chapter 4 of this manual.
All HMA mixtures contain small amounts of air voids. However, if the pavement is not compacted adequately during construction, compaction under traffic will fail to reduce the air void content to the design value and, as a result, the pavement will be permeable to air and water, potentially leading to moisture damage and excessive age hardening.
Because asphalt concrete is much more flexible than portland cement concrete, asphalt concrete pavements are sometimes called flexible pavements. The visible part of an asphalt concrete pavement, the part that directly supports truck and passenger vehicles, is called the surface course or wearing course.
It is typically between about 40 and 75 mm thick and consists of crushed aggregate and asphalt binder. Surface course mixtures tend to have a relatively high asphalt content, which helps these mixtures stand up better to traffic and the effects of sunlight, air, and water. Surface course mixtures also are usually made using maximum aggregate sizes less than 19 mm, which helps to provide for a quiet ride.
Also, using aggregate sizes larger than 19 mm can make it more difficult to obtain mixtures with sufficient asphalt binder contents to provide adequate durability for surface course mixtures, since the lower aggregate surface area of these aggregates results in a lower demand for asphalt binder. On the other hand, the lower binder content needed for these mixtures can make them more economical than mixtures made using smaller aggregates.
Below the surface course of a flexible pavement is the base course. The base course helps provide the overall thickness to the pavement needed to ensure that the pavement can withstand the projected traffic over the life of the project. Base courses may be anywhere from about to mm thick. In general, the higher the anticipated traffic level on a pavement, the thicker Background 5 Figure A compacted HMA laboratory specimen and the aggregate and asphalt used to prepare it.
Thicker pavements will deflect less than thinner ones under traffic loading, which reduces strains within the pavement and makes them more resistant to fatigue cracking. Traditionally, base course mixtures have been designed using larger aggregate sizes than surface course mixtures, with maximum aggregate sizes ranging from about 19 to This helps to produce a lean mixture with low asphalt binder content, which helps keep the cost of these mixtures low.
Also, using larger aggregate sizes allows base course mixtures to be placed in thicker lifts, which can reduce construction costs.
Using these types of mixtures in base course mixtures can help improve both fatigue resistance and resistance to moisture damage, since increased asphalt binder contents in HMA tends to improve fatigue resistance and will also reduce permeability to water. Sometimes an intermediate course is placed between the surface and base courses of a flexible pavement system.
This is sometimes called a binder course. Typically 50 to mm in thickness, it consists of a mixture with intermediate aggregate size and asphalt binder content. The surface, base, and intermediate courses together are referred to as bound material or bound layers, because they are held together with asphalt binder. In a typical asphalt concrete pavement, the bound layers are supported by a granular subbase that in turn lays over the subgrade. Granular subbase is crushed stone or gravel, usually to mm in thickness.
The nominal maximum aggregate size varies, but it should always be well compacted prior to placement of the base course. The subgrade is the soil on which the pavement is constructed.
If the soil is stable and strong, it may only need compaction prior to placing the granular subbase and remaining pavement layers. Before pavement construction, such a subgrade should be stabilized by blending in lime, portland cement, or other additives, or treating it with asphalt emulsion, and then thoroughly compacting the soil.
Sometimes the granular subbase is omitted from a pavement and a relatively thick base course is placed directly on the subgrade soil. Such a pavement structure is called a full-depth asphalt pavement. The advantage of this type of construction is that the overall pavement can be thinner because of the increased strength and stiffness of the supporting pavement.
However, it should be remembered that such a base course mixture will be significantly more expensive than granular subbase, since it contains asphalt binder. Figure is a cross section of a typical flexible pavement system. Typical asphalt concrete pavement structure. In many cases, the intermediate course is omitted; full-depth asphalt pavements do not include a granular subbase.
How Asphalt Concrete Pavements Fail Rutting Rutting often referred to as permanent deformation is a common form of distress in flexible pavements. When truck tires move across an asphalt concrete pavement, the pavement deflects a very small amount.
After the truck tire passes over a given spot in the pavement, the pavement tends to spring back to its original position. Often, however, the pavement surface will not completely recover.
Instead, there will be a very small amount of permanent deformation in the wheel path. Severely rutted pavements can have ruts 20 mm or more in depth. Rutting is a serious problem because the ruts contribute to a rough riding surface and can fill with water during rain or snow events, which can then cause vehicles traveling on the road to hydroplane and lose control.
Rut depths of about 10 mm or more are usually considered excessive and a significant safety hazard. Figure is diagram of rutting in an HMA pavement. Other related forms of permanent deformation include shoving and wash boarding. Shoving occurs at intersections when vehicles stop, exerting a lateral force on the surface of the hot mix causing it to deform excessively across the pavement, rather than within the wheel ruts. Wash boarding is a similar phenomenon but, in this case, the deformation takes the form of a series of large ripples across the pavement surface.
Excessive permanent deformation in one or more of the bound layers is the result of an asphalt concrete mixture that lacks strength and stiffness at high temperatures. Several problems with a mix design, such as selecting an asphalt binder that is too soft for the given climate and traffic level, can make it prone to rutting and other forms of permanent deformation.
Relationships between mixture composition and pavement performance are discussed in detail in Chapter 7. Fatigue Cracking Like rutting, fatigue cracking results from the large number of loads applied over time to a pavement subject by traffic. However, fatigue cracking tends to occur when the pavement is at moderate temperatures, rather than at the high temperatures that cause rutting.
Fractionate the resulting pan using a sieve shaker, making sure to limit the quantity Care must be taken to assure that the large of material on a given sieve so that all particles amounts of pan material shown are uniformly have an opportunity to reach the sieve openings blended and added to the batch sample. Make sure that each pan is the sieves: as homogeneously mixed as possible.
If the chosen sieves were For this example, the following trays were Example: used: When fractionating with fewer will need to be combined in proportion to the cold than the five tray slots typically present, fill the feed bin percentages and then sieved. Fractionate each pan, of all aggregate materials placing each resulting size fraction into a Fractionate each aggregate source with a sieve separate pan.
Using the in Table 3. Even though the particles are small, Fractionate each aggregate individually and keep there may be some level of segregation. If each fraction in a separate, labeled pan or bucket. Then batch 3 percent more of the Calculate the number of grams required of each moist fines than would be required of dry fines to size fraction to make a 5,gram batch.
The moisture will be First, calculate the number of grams needed for driven away when the aggregates are placed in the each aggregate source according to the percentage oven to bring them up to mixing temperature. Add Taring the scales after adding each new of 5, In the oven, In order to compensate for this conglomerate and may alter the binder properties potential error, a prepared trial batch should in the recycled materials.
Caution must be material added during batching. A thorough understanding of the material the temperature to a high enough level to assure being utilized by the designer is necessary when thorough mixing with the virgin aggregate. Mix designers are encouraged to Hydrated lime is often used as a treatment use Methods 1 or 2 for ordinary production mix for the prevention of moisture damage and is design.
Hydrated lime is typically added to the prepared samples prior to heating and 3. Hydrated lime may be added as either a Many different types of additives may be added dry component or as slurry.
Each method has been to a batched specimen. The reclaimed asphalt, recycled shingles, lime, ground method of adding hydrated lime to the mixture tire rubber or a number of other specialty products. See chapter 9 for more details into the final mixture must consider the on methods to incorporate hydrated lime in field- temperature to which the aggregate will be heated.
If scooping from buckets, design process, many of which are proprietary round-bottomed scoops can be used. The supplier guidelines for use and Placing each aggregate fraction in the batching incorporation must be followed very carefully pan in distinct piles will aid in batching as shown unless otherwise specified by the owner or agency.
If the batch weight is accidentally exceeded for any particular aggregate, it is easier 3. If scooping from a pan, flat-bottomed aggregate fraction when they are all piled together. For example, basing a mix design 4. Using too many binder increments 4. If the material source is unknown to the designer, Introduction use five trial binder contents.
If the designer is Laboratory asphalt mixture testing is primarily familiar with the material source, the number of conducted to determine two fundamental asphalt trials may be reduced to no less than three. These values are utilized provide a distinction between points, but close in calculating volumetric properties discussed in enough together to avoid lengthy interpolation.
Laboratory binder percentage points is 0. However, asphalt mix specimens are also prepared for further if the designer is unfamiliar with aggregates being mixture testing such as moisture sensitivity or used, or if other materials or additives are used other recommended or specified performance such as RAP, RAS, lime, WMA, etc.
The specific gravity of aggregates, binders at smaller increments over a wider range may and asphalt mixtures are determined using be needed. The ultimate goal is to prepare batches that bracket the anticipated design binder content. The designer should evaluate the aggregate type, the 4. This method a fine aggregate gradation will require more still works well for unmodified asphalt binders. Additionally, high compaction temperatures In the end, the designer should never extrapolate lead to potential problems in obtaining accurate a higher or lower design binder content from density data due to low mix stiffness, excess outside the range of trial points.
This frequency is the asphalt binder is determined at two test then input into an equation to calculate mixing and temperatures, establishing a relationship between compaction temperatures.
The DSR Steady Shear Flow Procedure uses Compaction temperatures are determined a shear stress sweep from 50 to Pa at a where the viscosity-temperature line crosses the minimum of three test temperatures to determine compaction viscosity range of 0.
This viscosity is then Mixing temperatures are determined where the plotted as a function of temperature, from which viscosity-temperature line crosses the mixing the mixing and compaction temperatures are derived. Users should refer to other existing practices or supplier recommendations to determine appropriate mixing and compaction temperatures for GTR-modified asphalt binders. Laboratory mixing and compaction temperatures are intended for determining design volumetric properties of the asphalt mixture and are not intended to represent field mixing and compaction temperatures at the project level.
Small stainless steel pitchers from to have a valid comparison of their volumetric commercial restaurant supply stores work properties. In an asphalt mix facility, the mixing very well. Regardless of the selected contents. Mixing is typically done with either a planetary mixer see Figure 4. To avoid excessive aging of the binder, do not allow it to stay at the mixing temperature for much over the time needed to bring it to temperature and complete the mixing operation.
Keep enough molds in the oven to rotate their use, always keeping a hot mold available. R 30 specifies a 2-hour conditioning period prior to When you are ready to mix, place the mixing laboratory compaction. Pour the characteristics can impact both Gmb and Gmm heated aggregate batch into the bowl and verify values. The maximum theoretical specific gravity the required weight see Figure 4. Form a crater test procedure, AASHTO T , requires a in the center of the aggregate to receive the binder mixture conditioning period of at least 2 hours.
In an effort to provide the most accurate mixture Add the correct number of grams of binder, dipping design results, the Asphalt Institute recommends out any excess binder with a folded paper dipper. Aggregate batches. When transferring the mixture from the sources that have high water absorption values mixing bowl to the conditioning pans, make sure to above 2.
Figure 4. The spread it to an even thickness between 25 and 50 purpose of the conditioning for volumetric millimeters in depth. Place the mixture and pan, mix design is to allow for binder absorption as shown in Figure 4. Note that testing is designed to simulate the plant-mixing the conditioning time may need to be increased to be and construction effects on the mixture.
The more representative of field conditions when higher- long-term conditioning for mixture mechanical absorptive aggregates more than 2 percent are property testing is designed to simulate the aging used, subject to agency approval.
The mixture is now by local agency specification; and conditioned for further testing. Refer to chapter 3 for the preparation 4. Experience chapters 6, 7, 8 for each mix design method. In has shown that pavements that maintain an air addition, vibratory and rolling wheel compactors void level of around 4 percent provide the best that are typically used for mixture performance long-term performance in the field.
As a result, testing are covered in chapter The designer should understand since the s, the Superpave gyratory compactor that different compaction methods affect the SGC was developed under the SHRP program aggregate orientation and density profiles within in the late s and early s as part of the the specimen. Therefore, different types not Superpave mix design system. Predecessors to meaning different models within the same type of the SGC included the Texas Gyratory Testing compaction methods should not be used for mix Machine, the French Laboratoire Central des Ponts comparison.
The SGC was designed with mm the physical requirements of the project 6 inch molds to be capable of accommodating specifications; large aggregates. The concepts of the Hveem method of 4.
The test procedures mold assembly. The compaction equipment is and their application have been developed portable, simple to use and has been used with through extensive research and correlation reasonable success for many years, although many studies on asphalt highway pavements. They designers do not believe this method accurately are applicable to dense-graded paving mixtures represents field compaction.
The U. All specific gravity computations involve absorption less than or equal to 2. The water absorption can be determined as The volume includes the effective volume of the follows: aggregate, the volume of the binder and the volume of the air voids within a compacted specimen. Like the CoreLok method, the paraffin-coated method is intended to be used for compacted mixture specimens with water absorption infiltration greater than 2.
It seals the asphalt surface in a similar manner as the vacuum-sealed bags in the previous method. The CoreLok test is conducted by the water- displacement method using compacted specimens vacuum-sealed in special puncture-resistant bags. However, a problem Figure 4. The problem is greatly exacerbated in open-graded mixtures. With most conventional in the water bath directly below the scale not mixes, the surface tension of the water keeps it from shown and determine its mass under water.
The flowing out of the internal voids and the voids are last step is to determine the mass of the saturated therefore included in the volume measurement. The saturated surface- With larger void spaces or internal void spaces dry SSD mass is obtained by quickly blotting the interconnected with the surface of the specimen, the sample so that the surface is not shiny see Figure mass of the water overcomes the surface tension and 4.
The bulk specific gravity is the mass of the flows out of the specimen while drying the surface, sample divided by the mass volume of water it resulting in a non-saturated sample which causes displaces.
To overcome this problem, the CoreLok vacuum- A sealing device see Figure 4. In this case, the mass includes both the mass of the aggregate and the mass of the binder. The volume includes only the effective volume of the aggregate and the volume of the binder. If Gmb and Gmm samples Figure 4. Therefore, Gmm must always be sheets the specimen with, called Parafilm.
The a larger number than Gmb. Theoretically, if a Gmb Asphalt Institute recommends that samples be sample could be compacted until 0 percent air voids dipped in hot paraffin. The loose mix is warmed and separated into loose, individually coated aggregates. As Pb increases, more lubricity is absolute pressure gauge reading is used to determine added to the mixture which allows the specimen the proper vacuum adjustment.
Once the proper to compact and slightly reduce the volume, while almost absolute, This provides gentle also increasing as the binder fills the voids within agitation to help in the removal of any air between the compacted aggregate structure. The slight particles. The agitation ensures that the air in the reduction in volume in combination with the mixture is as close as possible to zero. The theoretical increasing mass causes the specific gravity density maximum specific gravity is calculated using the of the compacted sample to increase.
As the voids equation for the specific procedure utilized. Gmm become filled with binder, the volume of the sample is the mass of the coated aggregate divided by the begins to increase. This increasing volume is due volume of coated aggregate. Air voids are calculated entirely to the additional binder being added which from the bulk and maximum specific gravities Gmb begins to reduce the overall specific gravity of the and Gmm.
The ratio of these two specific gravities is compacted specimen. As the Pb increases the percent stone A common source of error with this test is that Ps decreases. Since there is no compaction or air technicians do not calibrate verify the mass of the voids involved with the measurement of Gmm, the vacuum container filled with water often enough.
This makes the Gmm property are refilled periodically, sometimes from different very sensitive to binder content. This also shows sources. Because it only takes a few minutes to the importance of obtaining representative samples calibrate, more consistent results will be generated of mix when conducting Gmm testing.
If a sample if the vacuum containers are calibrated daily or is segregated and is too coarse, the Pb will be even before each test. If the segregated sample is too fine compared 4. The volumetric 5. Volumetric properties are often specified design elements of the total mix, the aggregate 5. The relationship mixture. Specific gravity is 5. If any two of the three most often used. Within this range, 4 percent air are quantities, G, m, or v, are known through testing, voids is typically considered the best initial estimate the third can be easily calculated.
The principal for a design that balances the desired performance of using specific gravity is essential in that actual properties. Slight refinements are then considered laboratory measurements are by mass, yet mixture in the analysis of the mix test results.
Usage of specific The volumetric properties of a compacted paving gravity principles enables us to readily calculate mass mixture are important criteria by which the quality or volumetric data based on the available data. This water stone representation, shown in Figure 5. It breaks down the components into air, effective asphalt nonabsorbed , absorbed asphalt and aggregate. The standard some combination. Figure 5. Because volume is in the denominator of the ratio of the mass of a unit volume of binder the specific gravity equation, the smallest volume to the mass of the same volume of water.
Binder necessarily results in the largest specific gravity. Since the volumes can only be the same if there is zero aggregate absorption, the following inequality always 5. Gmb is applicable the impermeable void volumes and the water- to any laboratory- or field-compacted specimen permeable voids not filled with absorbed asphalt including cores, beams, slabs, etc. The size of the test sample is of the total mix volume.
Many agencies refer specified and determined by the nominal maximum to this percentage by the term Va because it is a aggregate size.
This procedure requires that the dry percentage by volume instead of a percentage by aggregate be saturated to determine the volume of mass. However, the identical term Va is also used the aggregate plus the water-permeable voids. The dry aggregate is again saturated expressed as a percentage of the total aggregate to account for the volume of the aggregate plus the mass. Note that the procedure allows saturation by the addition of 6 percent moisture as an alternative to total submersion.
This 5. If the designer is using aggregates with a of aggregate high water absorption percent , the Asphalt It is recommended that the bulk dry specific Institute recommends total submersion. Some stockpiles saturated surface-dry SSD condition as specified will be essentially coarse retained on the No.
It and fine portions. However, the apparent Eq. This can be done for filler only, as the amount of mineral filler added is typically small and the difference between Gsb and Gsa is relatively small. Agency approval would be necessary for this substitution. Asphalt Binder 1. This method is necessary calculated as follows: because the criteria being averaged involve a ratio.
It is not uncommon for aggregates by weight, with no supplemental ratio to absorb a binder amount equal to 40—80 percent involved. Assuming a blend of three very dissimilar In order to determine these values for the materials, Figure 5. If Gsb testing was conducted on 5. If individual Gsb samples were determined two additional aggregate properties, the apparent for the coarse and fine fractions of any individual specific gravity Gsa and the water absorption of stockpile, then the equation shown in section 5.
These calculations are not required to can be used to determine the Gsa and absorption determine mixture volumetric properties; however, values for the stockpile. The absorption of the aggregate indicates known, they will need to be combined into one several characteristics of the final mixture. Highly value for the blend. The Gsa volume is less than the volume used 5. The Asphalt Institute considers the asphalt absorption 5. The added to the mixture exceeds the absorption value absorptiveness of aggregate is of significant of the aggregate.
This position allows the designer to interest to the mixture designer and specifier. The Gmm values for the remaining trial quality along with increased binder demand. The binder contents, or at any binder content, can then be binder absorption is typically 40—80 percent of the calculated by computing an effective specific gravity water absorption rate.
The water absorption rate is of the aggregate. Gse is a constant that can be used to calculated by the following equation as outlined in back-calculate Gmm at any asphalt binder content. Gmm can be visually with asphalt determined from the phase diagram in Figure 5.
This is because the percentage of aggregate, which has a higher specific gravity, necessarily decreases for a unit volume with an increase in the percentage of binder, which has a lower specific gravity. Therefore, the VMA can be Keep in mind that this manual defines Pa as the calculated by subtracting the volume of the aggregate percentage of air voids by volume and Va as the determined by its bulk specific gravity from the bulk measured volume of air voids.
They consist of the volume of the compacted paving mixture. VMA can be visually determined from the The property Pa can be visually determined from phase diagram in Figure 5. A reasonable rule of thumb says that for each 1. The equations shown above are for analyzing Using the data in Table 5.
Table 5. Therefore, the percentage of binder of the aggregate structure to consolidate. Small changes in binder content, at or near Since the target air voids Pa typically remains the the design binder content, typically will same, the VMA must increase to allow sufficient have minimal effect on the compacted VMA room for the additional asphalt binder.
In fact, anything that impacts the ability of the compactor to consolidate the mixture in the mold will affect the resulting VMA. Some of the more notable factors are discussed below. Large variations from the design this section will impact the final VMA result. The Bailey method is viscosity will increase. This increasing mixture an excellent tool that will predict the change in viscosity will increase the resistance to VMA in response to gradation changes, with compaction in the mold and in the field, thus all of the other factors remaining constant.
Detailed information on the Bailey method is Many warm mix additives are available that available at www. This technology is discussed in chapter More cubical or angular materials The voids filled with asphalt VFA is the will increase the resistance to compaction.
Aggregate strength is critical tends to increase as the mix becomes finer and since a weak aggregate can degrade or break gains more total aggregate surface area.
The VFA down during compaction, thus changing the can be calculated with either of the following gradation and greatly impacting VMA. A Using the data in Table 5. It is very difficult to predict the Binder absorption Note that Pbe is expressed as a percentage The percent binder absorption Pba is the of the total mix mass. It is assumed that the amount of content, because Pba is a percentage of the total binder absorbed into the aggregate is a constant aggregate and Pbe is a percentage of the total mix.
Note that if the absorption was mass of the total mix are so close in magnitude calculated based on the total mass of the mix, the that in a practical sense, when calculated to the percent absorption would change based on the nearest 0.
It can be calculated as diagram in Figure 5. The typical allowable range for this property is 0. The goal is to 0. A low P0. Mixes tend to stiffen as in Figure 5. A mix with a high P0. One exhibit a multitude of small stress cracks during might expect the VMA to remain constant with the compaction process, called check-cracking.
This varying asphalt content, thinking that the air voids property is usually calculated for dense-graded would simply be displaced by asphalt cement. In mixes only. With the increase in P0. Limit 12 4 4. Therefore, an additional process for determining aggregate up to a point, the bulk density of the mix increases structures that will meet VMA specifications. The and pushed apart by the lower specific gravity three levels of compaction of the Marshall mix material asphalt cement.
As hand, increasing side of this VMA curve, even shown in Figure 5. Any amount of additional shifts. If a mix is designed slightly to the left of compaction from traffic leads to inadequate minimum VMA at a compaction level of 50 blows room for asphalt expansion, loss of aggregate- and the pavement actually endures heavier traffic to-aggregate contact and eventually, rutting and than expected closer to a blow design level , shoving in high-traffic areas.
At the higher traffic level, this curve is very flat, meaning that the compacted mix will be susceptible to rutting. In the normal range of direction. If a mix, designed at a compaction level asphalt contents, compactability is influenced more of 75 blows as shown in Figure 5. However, at some point, placed in a pavement with much lower volumes the quantity of asphalt will become critical to the of traffic, then the final percentage of air voids behavior of the mix, and the effect of asphalt will Pa will be considerably higher than planned.
This dominate as the VMA increases drastically. This Specifically, the aggregate grading should be condition may also lead to stripping as discussed modified to provide additional VMA; suggestions in chapter 9. The design asphalt For this reason, it is important that the content should not be selected at the extremes of compactive effort used to simulate the design the acceptable range, even if the minimum criteria traffic expected in the pavement be selected are met. On the left-hand side, the mix would be accordingly in the laboratory.
Similarly, the mixture too dry, prone to segregation and would probably must be constructed with appropriate compaction be too high in air voids. On the right-hand side, the effort in the field to produce adequate initial mix would be expected to rut. If the minimum VMA criteria are completely It is also important to note that the VMA criteria violated over the entire asphalt content range do not change based on the level of compaction.
Section 5. The voids 4 percent is the level desired after several purpose is to avoid less durable mixes in light years of traffic. This design level of air voids does traffic situations. VFA criteria with relatively low percent air voids This design air void range will normally be achieved less than 3. Because effort and the percent air voids after construction low air void contents can be very critical in terms of is no more than 8 percent.
Some consolidation with permanent deformation as discussed previously , traffic is expected. It has been shown that The VFA criteria provide an additional factor mixtures that ultimately consolidate to less than of safety in the design and construction process 2 percent air voids can be expected to rut and shove in terms of performance.
Since changes can occur if placed in heavy traffic locations. Several factors between the design stage and actual construction, may contribute to this occurrence, such as: an an increased margin for safety is desirable. Brittleness, premature cracking, It is not uncommon for a mixture designer to raveling and stripping are all possible under these complete a laboratory mixture design only to conditions.
The Superpave mix design process the design asphalt content to less than 0. This changed to fit the pavement type being considered process can be used to evaluate different aggregate in the design. This process can also be very 5. The process can Although VFA, VMA and Pa are all interrelated and be summarized as an analysis of multiple aggregate only two of the values are necessary to solve for combinations at one estimated design binder the other, including the VFA criteria helps prevent content during a single laboratory session.
The main effect of the VFA criteria is to structure evaluation requires the individual limit maximum levels of VMA, and subsequently, aggregate characteristics, specific gravity and maximum levels of asphalt content. These procedures are discussed content for mixes that are near the minimum in chapter 3. Trial blends are then established by VMA criteria. Typically the blended determine the volumetric properties of each trial properties are then compared to the specification blend.
The asphalt binder content for each trial criteria required for the mixture to be designed. The blend must be determined. The goal is to estimate number of material combinations can be limitless, the percent binder required for the compacted depending on the characteristics of aggregate specimen to achieve the design air void content, materials and the purpose for which a design which is 4 percent if you are conducting a standard aggregate structure is being performed. Often when Superpave mixture design.
Some designers range allowed in the contract, while maintaining the utilize existing methods such as those discussed in aggregate properties to meet the specified criteria. Others will rely on past An example plot of multiple trial gradations is experience, intuition or simply use a fixed value shown in Figure 5.
A highly accurate initial Specified aggregate properties can be estimate is not critical for this procedure, as an estimated by mathematical calculation. However, estimated design binder content will be calculated consideration should be given to actually batching based on the results obtained from the laboratory and testing for those parameters that may be specimen compacted at this initial binder content borderline or are at risk of failure during actual selected.
The binder content of mixtures generally varies After the aggregate properties have been based on the maximum aggregate size, gradation evaluated, the next step is to compact specimens to and aggregate properties.
The steps shown are complete for If the mixtures The values in Table 5. Aggregate combinations having significantly Step 5—Calculate the Correction higher Gsb may need less asphalt and those with a Factor for Gmb Nini—for gyratory lower Gsb may need more asphalt. Other tests Ht. The following step-by-step process is Ht. The basic Ht. Additional steps are required Step 6—Calculate the Gmb Nini— for that are specific to gyratory compacted mixtures.
This system was developed to give pavement The procedures and calculations for conducting a engineers and contractors the tools they needed mix design containing Recycled Asphalt Pavement to improve the performance of HMA pavements. A mixture performance and analysis system was Superpave is a system. A pavement mixture will developed utilizing the Superpave Shear Tester not perform successfully on the roadway unless SST , but due to complexities of the equipment the appropriate binder and aggregate materials are and procedure was not implemented as part of incorporated into the work.
The following summarizes the current Superpave mix 6. The The consensus aggregate properties specified in grade of asphalt binder to be incorporated into the Superpave system are the following: coarse the HMA is usually specified during the project aggregate angularity; fine aggregate angularity; design and is not a variable considered during flat and elongated particles; and clay content the mix design phase.
If the mix designer is Sand Equivalent. The criteria for these properties responsible for selecting the proper binder are based on the traffic level and position within grade, please refer to and become familiar with the pavement structure.
The materials near the MS The not contain detailed aggregate requirements. During the development of Superpave, 6. These characteristics were source properties. A detailed discussion of variety of aggregate types available in different these properties and test procedures are included geographic areas, and are not broad, all-inclusive in chapter 3.
Regardless of the actual design life of the roadway, determine the design ESALs for 20 years and choose the appropriate Ndesign levels. Table 6. Control points have been established to combinations have the best chance of meeting define the type of mix. It is important to note that Superpave mixture requirements, especially the the recommended gradation control points are VMA requirement. The following 6. Actual Prior to the development of the Superpave mix field production testing may and should consist design procedure, laboratory compaction data of a larger nest of sieves with tighter tolerances were only available on samples compacted to the than the limits shown in the following table.
The final design density. Implementing a specification for specified levels of laboratory compaction density compliance or quality control based solely on the at three different levels of gyration based on following gradation parameters without mixture expected traffic loading as shown in Table 6. These catastrophic failure. However, significant alterations of typical virgin Mixtures that compact too quickly may be aggregate materials may be required.
The final inherently tender to compact and would therefore aggregate materials must be chosen based on be undesirable. The Nini density specifications range the blended materials meeting the gradation and from 89 to Very coarse, harsh mixes may Ndes is the number of gyrations specified to reach have densities at Nini around 82 to 83 percent of the target density of the mix and is based on the Gmm, be inherently difficult to compact and have estimated field density in the middle of its service TABLE 6.
Special-purpose roadways serving recreational sites or areas may also be applicable to this level. Applications include collector roads or access streets. Medium- 0. Applications include many two-lane, multilane, divided and partially or completely controlled access highways.
Applications include the vast majority of the U. Interstate System, both rural and urban in nature. Both methods will work if the aggregate and structure is unaltered. It is capped at 98 greatest effect on specimen compaction.
As the percent for all traffic levels. Low air voids in the field angle increases, the compaction effort increases. Originally, specimens were compacted to compaction will decrease. Relatively small changes Nmax gyrations and the density levels after Nini and in the gyratory angle can make significant Ndes gyrations were back-calculated.
The current changes in the compaction level. It is critical to practice is to compact specimens to Ndes gyrations, maintain a well-calibrated compactor to assure no measure the density at that level directly, then deviation in the internal gyration angle. Figure 6. Separate demonstrates the compaction parameters.
In specimens at the design binder content are then the fabrication of the SGC specimens, a constant compacted to Nmax gyrations and the density measured consolidation pressure is applied to the sample directly to check compliance with specifications.
To provide specimens of The number of design gyrations was established consistent density, it is very important that the by comparing lab-molded densities at different SGC maintain a constant pressure and a known gyration levels to resulting field densities. Once the constant angle of gyration during the compaction design gyration levels were established, the initial process.
Each model of SGC uses a different and maximum gyration levels were established by method of setting, inducing and maintaining the following equations: the angle of gyration. Inconsistencies in the density Table 6. This can be very troublesome when comparing mix designs prepared by different 6. Regardless of the actual design life of the roadway, determine the design ESALs for 20 years. If the aggregate gradation passes beneath the specified PCS Control Point, the dust-to-binder ratio range may be increased from 0.
Mixtures with VMA exceeding the minimum value by more than 2 percent may be prone to flushing and rutting. Unless satisfactory experience with high VMA mixtures is available, mixtures with VMA greater than 2 percent above the minimum should be avoided. TABLE 6. The mold provides confinement for the mix during compaction.
An accepted rule mold of thumb is that the mold diameter should be at least four times the maximum aggregate size of the mixture. Specimen Ave. This 1. The issue concerns properly capacity for mixing asphalt and aggregate; measuring the inside mold diameter in the area 5. Designers are urged to become familiar 7. This function is 9. The Specimen preparation mixture design will require the molding of mixture and compaction specimens at a minimum of four different binder 6.
It is recommended that specimens be molded at the anticipated design binder The following samples must be prepared to conduct content and at —0. Preheat the asphalt binder to the predetermined mixing temperature. Three aggregate sample sizes are used in The time required for this step varies depending conducting a Superpave mix design, depending on on the container size and method of heating. Care their final use. Find out more about OverDrive accounts. Irving Kett.
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