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  • Recycled HMA | Washington Asphalt Pavement Association
    predetermined percentage of RAP is added There is ample evidence that HMA which incorporates RAP performs as well as HMA without RAP The benefits of RAP use are two fold The RAP aggregate can be used in place of a portion of the virgin aggregate which lowers cost reduces waste and can lower the energy and greenhouse gas GHG footprint of the material production process The RAP asphalt binder is reheated and used in place of a portion of the virgin asphalt binder which lowers cost reduces waste and can lower the energy and greenhouse gas GHG footprint of the material production process Pavement Note on RAP In Washington State maximum limits for RAP addition are agency specific For instance The WSDOT 2010 Standard Specifications for Road Bridge and Municipal Construction M 41 10 allows contractors the option of using RAP in the amount up to 20 percent of total HMA weight For WSDOT the mix design is determined using virgin material only even though RAP will be included in the HMA production Other agencies will allow different limits depending upon their experience and desires If used RAP is most commonly added at 10 to 30 percent by weight although additions as high as 80 percent by weight have been done and additions as high as 90 to 100 percent by weight are feasible FHWA 2001 cheap essay service Other general considerations when using RAP are When heated RAP may give off gaseous hydrocarbons To minimize these emissions HMA plants generally heat RAP indirectly usually it is added after the aggregate is heated and thus heats up through contact with the already hot aggregate RAP is typically added cold and thus may require longer HMA plant heating times This can sometimes reduce plant output by as much as half This can be overcome by preheating RAP but the added energy equipment and emissions concerns often make preheating undesirable RAP usually contains between 3 and 7 percent asphalt by weight or about 10 to 20 percent asphalt by volume FHWA 2001 In general the asphalt binder in RAP will be more viscous than virgin asphalt binder due to aging effects Therefore if enough RAP is added a softer virgin asphalt binder can be used but is not required to counteract the more viscous RAP asphalt binder After milling or crushing RAP gradation is generally finer than pure virgin aggregate because of the degradation that occurs during removal and processing RAP in Cold Plant Mix Recycling Cold plant mix recycling involves mixing RAP with an asphalt emulsion or foamed asphalt at a central or mobile plant facility A rejuvenating agent can be added to improve the recycled asphalt binder viscosity and new aggregate can also be added to improve overall performance The resulting cold mix is typically used as a stabilized base course Since cold in place recycling has become more commonplace cold plant mixing has become less popular Other Recycling Options HMA can also be recycled in place via hot or

    Original URL path: http://www.asphaltwa.com/2010/09/18/pavement-types-recycled-hma/ (2016-04-26)
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  • Drainage | Washington Asphalt Pavement Association
    HMA tends to be impermeable below about 8 percent air voids therefore proper compaction practices should be followed to ensure an impermeable pavement Also minor cracks in the HMA should be promptly sealed Slope The pavement section should be sloped to allow rainwater to sheet flow quickly to the edge where it is typically collected in a curb and gutter system or a roadside ditch A generally accepted standard is a 2 percent cross slope Grade The curb and gutter or roadside ditch must be properly graded to allow flow to central collection points such as catch basins or detention ponds A generally accepted standard is a grade of 0 5 percent or more although lesser grades have been used effectively Subsurface Drainage Subsurface drainage is concerned with removing water that percolates through or is contained in the underlying subgrade This water typically the result of a high water table or exceptionally wet weather can accumulate under the pavement structure by two chief means Gravity flow Water from surrounding areas can be absorbed by the soil then flow by gravity to areas underneath the pavement structure In pavement with high air voids above 8 9 percent water can percolate down through the pavement structure itself Capillary rise Capillary rise is the rise in a liquid above the level of zero pressure due to a net upward force produced by the attraction of the water molecules to a solid surface e g soil Capillary rise can be substantial up to 20 ft or more In general the smaller the soil grain size the greater the potential for capillary rise Often capillary rise is a problem in areas of high groundwater tables Most pavements have performed adequately without considering these effects However HMA pavements can fail because of subgrade support deterioration as

    Original URL path: http://www.asphaltwa.com/2010/09/17/design-factors-drainage/ (2016-04-26)
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  • Environment | Washington Asphalt Pavement Association
    soil saturation as ice within the soil melts WAPA Pavement Note on Frost Action Frost action is generally not a concern west of the Cascade Mountains Frost Heave Frost heaving of soil is caused by crystallization of ice within the larger soil voids and usually a subsequent extension of this ice to form continuous ice lenses layers veins or other ice masses As depicted in Figure 3 An ice lens grows and thickens in the direction of heat transfer until the water supply is depleted or until freezing conditions at the freezing interface no longer support further crystallization As the ice lens grows the overlying soil and pavement will heave up potentially resulting in a rough cracked pavement Figure 4 Frost heave occurs primarily in soils containing fine particles often termed frost susceptible soils while clean sands and gravels small amounts of fine particles are non frost susceptible NFS Thus the degree of frost susceptibility is mainly a function of the percentage of fine particles within the soil Many agencies classify materials as being frost susceptible if 10 percent or more passes a No 200 sieve or 3 percent or more passes a No 635 sieve Figure 3 Formation of Ice Lenses in a Pavement Structure Figure 4 Frost Heave Pavement Note the Casagrande Criterion In 1932 Dr Arthur Casagrande proposed the following widely known rule of thumb criterion for identifying potentially frost susceptible soils Under natural freezing conditions and with sufficient water supply one should expect considerable ice segregation in non uniform soils containing more than 3 of grains smaller than 0 02 mm and in very uniform soils containing more than 10 percent smaller than 0 02 mm No ice segregation was observed in soils containing less than 1 percent of grains smaller than 0 02 mm even if the groundwater level is as high as the frost line Note 0 02 mm No 635 sieve Application of the Casagrande criterion requires a hydrometer test of a soil suspension in water to determine the distribution of particles passing the 200 sieve and to compute the percentage of particles finer than 0 02 mm Thaw Weakening Thawing weakening occurs when the ice contained within the subgrade melts As the ice melts and turns to liquid it cannot drain out of the soil fast enough and thus the subgrade becomes substantially weaker less stiff and loses bearing capacity Therefore loading that would not normally damage a given pavement may cause significant damage during spring thaw Thawing can proceed from the top downward or from the bottom upward or both How this occurs depends mainly on the pavement surface temperature During a sudden spring thaw melting will proceed almost entirely from the surface downward This type of thawing leads to extremely poor drainage conditions The frozen soil beneath the thawed layer can trap the water released by the melting ice lenses so that lateral and surface drainage are the only paths the water can take Mitigating Frost Action Frost action mitigating

    Original URL path: http://www.asphaltwa.com/2010/09/17/design-factors-environment/ (2016-04-26)
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  • Loads | Washington Asphalt Pavement Association
    value the pavement is considered to have reached the end of its useful service life Traffic distribution On any given road one direction typically carries more loads than the other Furthermore within this one direction each lane carries a different portion of the loading The outer most lane often carries the most trucks and therefore is usually subjected to the heaviest loading Vehicle speed In general slower speeds and stop conditions allow a particular load to be applied to a given pavement area for a longer period of time resulting in greater damage If mix design or structural design have been inadequate this behavior is sometimes evident at bus stops where heavy buses stop and sit while loading unloading passengers and intersection approaches where traffic stops and waits to pass through the intersection WAPA Pavement Note on Loads The Washington State load limits are Tires is 600 lbs inch of tire width Single Axle 20 000 lbs Single axle with dual tires 500 lbs inch of tire width Tandem axle 34 000 lbs Gross vehicle weight 105 500 lbs WSDOT has a good short publication on load limits and why they exist here Load Quantification Pavement structural design requires a quantification of all expected loads a pavement will encounter over its design life This quantification is usually done in one of two ways Equivalent single axle loads ESALs This approach converts wheel loads of various magnitudes and repetitions mixed traffic to an equivalent number of standard or equivalent loads based on the amount of damage they do to the pavement The commonly used standard load is the 18 000 lb equivalent single axle load Using the ESAL method all loads including multi axle loads are converted to an equivalent number of 18 000 lb single axle loads and this number

    Original URL path: http://www.asphaltwa.com/2010/09/17/design-factors-loads/ (2016-04-26)
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  • Subgrade | Washington Asphalt Pavement Association
    improved subgrade performance Removal and replacement over excavation Poor subgrade soil can simply be removed and replaced with higher quality fill Although this is simple in concept it can be expensive Additional base layers Marginally poor subgrade soils may be made acceptable by using additional base layers These layers spread pavement loads over a larger subgrade area This option is rather perilous when designing pavements for poor subgrades the temptation may be to just design a thicker section with more base material because the thicker section will satisfy most design equations However these equations are at least in part empirical and were usually not intended to be used in extreme cases In short a thick pavement structure over a poor subgrade may not make a good pavement Stabilization with a cementitious or asphaltic binder The addition of an appropriate binder such as lime portland cement or emulsified asphalt can increase subgrade stiffness and or reduce swelling tendencies Subgrade Physical Properties Subgrade materials are typically characterized by 1 their resistance to deformation under load in other words their stiffness or 2 their bearing capacity in other words their strength In general the more resistant to deformation a subgrade is the more load it can support before reaching a critical deformation value Although there are other factors involved when evaluating subgrade materials such as swell in the case of certain clays stiffness is the most common characterization There are three basic subgrade stiffness strength characterizations commonly used in the U S California bearing ratio CBR A simple test that compares the bearing capacity of a material with that of a well graded crushed stone thus a high quality crushed stone material should have a CBR 100 CBR is basically a measure of strength It is primarily intended for but not limited to evaluating the strength of cohesive materials having maximum particle sizes less than 0 75 inches AASHTO 2000 It was developed by the California Division of Highways around 1930 and was subsequently adopted by numerous states counties U S federal agencies and internationally Most agency and commercial geotechnical laboratories in the U S are equipped to perform CBR tests Resistance value R Value A test that expresses a material s resistance to deformation as a function of the ratio of transmitted lateral pressure to applied vertical pressure It is essentially a modified triaxial compression test Materials tested are assigned an R value The testing apparatus used in the R value test is called a stabilometer and is identical to the one used in Hveem HMA mix design The R Value is basically a measure of stiffness Resilient modulus M R A test used to estimate elastic modulus a material s stress strain relationship The resilient modulus test applies a repeated axial cyclic stress of fixed magnitude load duration and cyclic duration to a cylindrical test specimen While the specimen is subjected to this dynamic cyclic stress it is also subjected to a static confining stress provided by a triaxial pressure chamber

    Original URL path: http://www.asphaltwa.com/2010/09/17/design-factors-subgrade/ (2016-04-26)
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  • Superpave Method | Washington Asphalt Pavement Association
    the Superpave mix design methods Some agencies and private laboratories may use the Marshall method Each method has been proven to produce quality HMA from which long lasting pavements can be constructed As done by WSDOT the Superpave mix design method consists of three basic steps Aggregate selection Aggregate is specified in three ways First restrictions on aggregate gradation are specified by using gradation specifications Second there are requirements on aggregate angularity flat and elongated particles and clay content Third aggregate criteria which the Asphalt Institute 2001 calls source properties because they are considered to be source specific such as durability and soundness are specified Asphalt binder selection WSDOT uses the Superpave PG system for asphalt binder specification Superpave PG asphalt binders are selected based on the expected pavement temperature extremes in the area of their intended use These extremes can be calculated using software such as LTPPBind or more commonly determined based on standard PG binders that WSDOT has specified for each area of the State Sometimes these standard binder grades can be adjusted based on the anticipated traffic level type or speed Optimum asphalt binder content determination In the Superpave method this step can be broken up into

    Original URL path: http://www.asphaltwa.com/2010/09/17/superpave-method/ (2016-04-26)
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  • Pavement Structure | Washington Asphalt Pavement Association
    loading A flexible pavement structure is typically composed of several layers of material Each layer receives the loads from the above layer spreads them out then passes on these loads to the next layer below Thus the further down in the pavement structure a particular layer is the less load in terms of force per area it must carry Figure 1 Figure 1 Flexible Pavement Load Distribution Basic Structural Elements In order to take maximum advantage of this property material layers are usually arranged in order of descending load bearing capacity with the highest load bearing capacity material and most expensive on the top and the lowest load bearing capacity material and least expensive on the bottom A typical flexible pavement structure Figure 2 consists of Surface Course The layer in contact with traffic loads It provides characteristics such as friction smoothness noise control rut resistance and drainage In addition it prevents entrance of surface water into the underlying base subbase and subgrade This top structural layer of material is sometimes subdivided into two layers the wearing course top and intermediate binder course bottom Base Course The layer immediately beneath the surface course It provides additional load distribution and contributes

    Original URL path: http://www.asphaltwa.com/2010/09/17/pavement-structure/ (2016-04-26)
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  • Pavement Response | Washington Asphalt Pavement Association
    pavement material characteristics This section presents the typical stresses and deformations experienced by an HMA pavement structure under load Stress The stresses that occur in a HMA pavement under load are quite complex routine calculation of these stresses is a recent development Using a basic two dimensional layered elastic model the basic relationships between layer stiffness and stress for a two layer flexible pavement structure is shown in Figure 1

    Original URL path: http://www.asphaltwa.com/2010/09/17/pavement-response/ (2016-04-26)
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