Lec 15 Highway Engineering - Asphalt Paving Mixtures

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Highway Engineering - Asphalt Paving Mixtures


<ul><li><p> Lecture 15 175 </p><p>Highway Eng. Asphalt Paving Mixtures 14 15 </p><p>Dr. Firas Asad </p><p>In this lecture; </p><p>- Constitutes </p><p>- Types </p><p>- Desirable Properties </p><p>- Marshall Mix Design Method </p><p>- Volumetric Properties </p><p> Asphalt Paving Mixtures </p><p> Information listed in this lecture is mainly taken from Standard Specifications for Engineering Materials and Methods of Sampling and Testing (AASHTO, 2013), A Manual for Design of Hot Mix Asphalt with Commentary (NCHRP, Report 673, 2011), Traffic and Highway Engineering (Garber, 2009), Handbook of Highway Engineering (Fwa, 2006), http://www.pavementinteractive.org (Accessed on 2015) and Highways (OFlaherty, 2007). </p><p>A- Constituents of a mix Coarse aggregates: Offer compressive and shear strength and shows good interlocking properties. E.g. Crushed stone. </p><p>Fine aggregates: Fills the voids in the coarse aggregate and stiffens the binder. E.g. Sand, Rock dust </p><p>Filler: Fills the voids, stiffens the binder and offers permeability. E.g. Rock dust, cement, lime. </p><p>Binder: Fills the voids, cause particle adhesion and gluing (cementing) and offers impermeability. E.g. Bitumen (Asphalt) and Tar. </p><p>Asphalt Concrete</p><p> (Also known as hot mix asphalt, plant mix, bituminous mix, bituminous concrete) is a combination of two primary ingredients - aggregates (90 95)% by weight of total mixture and asphalt cement (5-10)% asphalt cement. </p></li><li><p> Lecture 15 176 </p><p>Highway Eng. Asphalt Paving Mixtures 14 15 </p><p>Dr. Firas Asad </p><p>B- Types of Asphalt Paving Mixtures 1- Hot-Mix, Hot-Laid Asphalt Mixture </p><p>Hot-mix, hot-laid asphalt mixture is produced by properly blending asphalt cement, </p><p>coarse aggregate, fine aggregate, and filler (dust) at temperatures ranging from </p><p>about 80 to 160oC depending on the type of asphalt cement used. Suitable types of </p><p>asphalt materials are those with penetration grades of 60 to 70, 85 to 100, 120 to </p><p>150, and 200 to 300. </p><p>Hot-mix, hot-laid asphalt mixture normally is used for high-type pavement </p><p>construction, and the mixture can be described as open graded, coarse-graded, </p><p>dense-graded, or fine-graded. </p><p>2- Hot-Mix, Cold-Laid Asphalt Mixture </p><p>Asphalt mixtures in this category are manufactured hot and then shipped and laid </p><p>immediately or can be stockpiled for use at a future date. Thus, they are suitable for </p><p>small jobs when it is uneconomical to set up a plant. </p><p>In this type of asphalt concrete high-penetration asphalt normally is used. The most </p><p>suitable asphalt cements is AC 200-300 penetration grade. </p><p>3- Cold-Mix, Cold-Laid Asphalt Mixture </p><p>Emulsified asphalts and low-viscosity cutback asphalts are used to produce cold-mix </p><p>asphalt mixtures. They also can be used immediately after production or stockpiled </p><p>for use at a later date. The production process is similar to that of the hot-mix </p><p>asphalts, except that the mixing is done at normal temperatures. </p><p>Examples of the types and grades of asphalt material used are MS and SS. </p></li><li><p> Lecture 15 177 </p><p>Highway Eng. Asphalt Paving Mixtures 14 15 </p><p>Dr. Firas Asad </p><p>C- Desirable Properties of Asphalt Mixtures </p><p>1- Stability </p><p>Stability is the resistance of pavement to permanent deformation caused by </p><p>sustained or moving load. It is provided by two factors: </p><p>a- Interlocking and internal friction - depend on: </p><p>1- Size and shape of aggregate; </p><p>2- Surface area and roughness; </p><p>3- Applied load. </p><p>b- Cohesion resistance - depends on: </p><p>1-rate of load application </p><p>2- loaded area; </p><p>3- viscosity of the binder. </p><p>4- Temperature. </p><p>2 - Durability </p><p>It is the ability to resist the factors of weathering; water(stripping), air(oxidation), </p><p>temperature, and abrasive action from traffic (disintegration/cracking). </p><p>Percentage of binder controls on disintegration and resistance to cracking where </p><p>brittle mixes cracks under the effect of moving loads. The brittleness is mainly </p><p>caused by: (a) low ductility asphalt and (b) low content of binder. Maximum binder </p><p>content increases durability because thick binder films do not age and harden as </p><p>rapidly as thin ones do. </p><p>Temperature causes thermal cracking at low temperature and tenderness at high </p><p>temperature. </p></li><li><p> Lecture 15 178 </p><p>Highway Eng. Asphalt Paving Mixtures 14 15 </p><p>Dr. Firas Asad </p><p>3. Flexibility </p><p>It is the ability of the pavement to bend repeatedly due to traffic loads without </p><p>cracking (fatigue resistance). To obtain the desired flexibility : </p><p>1- Using appropriate content and type of asphalt; </p><p>2- Using well graded aggregate; </p><p>3- Selected temperature of mixing and compaction. </p><p>4. Skid resistance </p><p>It depends on the roughness of surface textured aggregate , hardness of aggregate </p><p>and less asphalt content. Best skid resistance is obtained with rough-textured </p><p>aggregate in a relatively open-graded mixture. </p><p>5. Workability </p><p>Workability describes the ease with which a paving mixture can be placed and </p><p>compacted. Mixtures with good workability are easy to place and compact; those </p><p>with poor workability are difficult to place and compact. </p><p>Workability can be improved by changing </p><p>a- mix design parameters </p><p>b- aggregate source, and/or </p><p>c- gradation. </p></li><li><p> Lecture 15 179 </p><p>Highway Eng. Asphalt Paving Mixtures 14 15 </p><p>Dr. Firas Asad </p><p>D- Design of Hot-Mix Asphalt Concrete </p><p>The objective of mix designs is to adequately select and mix certain proportions of </p><p>asphalt binders and aggregates to produce HMA that performs well both </p><p>functionally and structurally . Regarding performance, the HMAC should be </p><p>durable and be able to resist pavement distresses (such as permanent deformation, </p><p>load-induced fatigue, thermal fatigue, low temperature cracking, and moisture-</p><p>induced damage). From a construction standpoint, the mix should be workable </p><p>enough to place and compact with reasonable effort. Additionally, surface courses </p><p>should provide sufficient skid resistance for safety considerations. </p><p>There are several mix design methods for Hot-Mix Asphalt, Hveem (1920s), Marshall </p><p>(1930s), and Superpave (1990s) are the most common methods. In this course, only </p><p>Marshall method will be covered. </p><p>Marshall Mix Design Method </p><p>The Marshall method of mix design is intended both for </p><p>laboratory design and field control of bituminous hot-mix </p><p>dense-graded paving mixtures. Originally, it was developed by the </p><p>bituminous engineer Bruce Marshall of the Mississippi State Highway Department. </p><p>The US Army Corps of Engineers enhanced the Marshalls approach and it was then </p><p>subsequently formalized as ASTM D 1559 and AASHTO T 245 Standard Method of </p><p>Test for Resistance to Plastic Flow of Asphalt Mixtures Using Marshall Apparatus. </p></li><li><p> Lecture 15 180 </p><p>Highway Eng. Asphalt Paving Mixtures 14 15 </p><p>Dr. Firas Asad </p><p>The Marshall method uses standard cylindrical test specimens that are 64 mm </p><p>(2.5in) high by 102 mm (4in) diameter. These are prepared using a specified </p><p>procedure for heating, mixing and compacting the bitumenaggregate mixture. The </p><p>two principal features of the Marshall method of mix design are a density voids </p><p>analysis and a stability-flow test of the compacted test specimens. The stability of </p><p>the test specimens is the maximum load resistance, in newtons, that the standard </p><p>test specimen will develop at 60C. The flow value is the total movement or </p><p>displacement, in units of 0.25 mm, occurring in the specimen between no load and </p><p>the point of maximum load during the stability test. </p><p>Outline of method </p><p>Step 1. Aggregate Evaluation </p><p>Procedure </p><p>The aggregates proposed for use in the mix design should be have the desired </p><p>properties and satisfy the required specifications (See Lecture L13). If the aggregates </p><p>meet the requirements above, the aggregate specific gravity, absorption, and </p><p>gradation are determined. These will be used for determining the volumetric </p><p>properties of the mix. Last, the individual aggregate gradations are combined in </p><p>different proportions to develop trial blends. </p></li><li><p> Lecture 15 181 </p><p>Highway Eng. Asphalt Paving Mixtures 14 15 </p><p>Dr. Firas Asad </p><p>Step 2. Asphalt Cement Evaluation </p><p>The asphalt cement used in the mix design should satisfy the specifications when </p><p>running the required tests (See Lecture L14). After that, determine the mixing and </p><p>compaction temperatures based on viscosities of 170 20 and 280 30 centistokes </p><p>(cSt), respectively. </p><p>Step 3. Preparation of Marshall Specimens </p><p>Prepare the Marshall specimens in accordance to the requirements set in AASHTO </p><p>R-12. Compact three replicate specimens at five percentages of asphalt contents </p><p>(4.0, 4.5, 5.0, 5.5, 6.0 % by weight of total mix). The asphalt contents should be </p><p>selected with two asphalt contents falling above and below the optimum asphalt </p><p>content. These specimens are prepared using molds of dimension (4*2.5) and </p><p>using a hummer of 10Ib (4.5 kN) weight with 18 (457 mm) drop distance to </p><p>compact the sample with No. of blows as shown in the table below. </p><p>Three loose specimens should also be prepared for determining the maximum </p><p>theoretical specific gravity near the optimum asphalt content. Determine the bulk </p><p>specific gravity of the compacted specimens and maximum theoretical specific </p><p>gravity of the loose mix using AASHTO T166 (ASTM D2726) Standard Method of </p><p>Test for Bulk Specific Gravity of Compacted Asphalt Mixtures Using Saturated </p><p>Surface-Dry Specimens and ASHTO T209 Standard Method of Test for Theoretical </p><p>Maximum Specific Gravity and Density of Bituminous Paving Mixtures, respectively. </p><p>Step 4. Marshall Stability and Flow </p><p>Determine the Marshall stability and flow using ASTM D1559. Stability is defined as </p><p>the maximum load carried by a compacted specimen tested at 60oC at a loading rate </p><p>of 2 in./min. The two primary factors in determining the stability are the angle of </p><p>internal friction of the aggregate and the viscosity of the asphalt cement. Therefore </p><p>mixes with angular aggregates will have a higher stability than mixes with rounded </p></li><li><p> Lecture 15 182 </p><p>Highway Eng. Asphalt Paving Mixtures 14 15 </p><p>Dr. Firas Asad </p><p>aggregates. Similarly, mixes with more viscous asphalt cements will have a higher </p><p>stability than mixes with less viscous asphalt cements. The primary use of the </p><p>Marshall stability is to evaluate the effect of asphalt cement in the Marshall mix </p><p>design procedure. However, increasing the Marshall stability in the laboratory does </p><p>not automatically translate to increased stability of mixes in the field. </p><p>Flow is the vertical deformation of the sample at failure. High flow values typically </p><p>indicate a plastic mix that could be susceptible to permanent deformation. Low flow </p><p>values may indicate low air voids that may lead to premature cracking. </p><p>Step 5. Density and Void Analysis </p><p>Using the bulk specific gravity and maximum theoretical specific gravity test results </p><p>and the relevant equations, the volumetric properties of the mix can be determined. </p><p>This information is used in Step 6. </p><p>Step 6. Tabulating and Plotting Test Results </p><p>With the completion of Steps 4 and 5, the average (from three replicates) results </p><p>can be tabulated and plotted. The following plots can then be made to evaluate the </p><p>mix: </p><p>* Density (or Unit Weight) vs. Asphalt Content </p><p>* Marshall Stability vs. Asphalt Content </p><p>* Flow vs. Asphalt Content </p><p>* Air Voids vs. Asphalt Content </p><p>* VMA vs. Asphalt Content </p><p>* VFA vs. Asphalt Content </p></li><li><p> Lecture 15 183 </p><p>Highway Eng. Asphalt Paving Mixtures 14 15 </p><p>Dr. Firas Asad </p><p>The density</p><p>The Marshall </p><p> plot typically shows a trend of increasing density until the peak is </p><p>reached. After this peak, the density begins to decrease. </p><p>stability has a similar trend but its peak is typically at a lower asphalt </p><p>content than density. Some recycled mixes may show a decreasing stability with </p><p>increasing asphalt content with no peak. </p><p>Flow</p><p>The percent </p><p> typically increases with increasing asphalt content. </p><p>air voids should decrease and the VFA increase with increasing asphalt </p><p>content. </p><p>VMA</p><p>Step 7. Optimum Asphalt Content Determination </p><p> is another property that increases with asphalt content until it reaches its peak </p><p>and then decreases with additional increase in asphalt content. </p><p>The criteria used to select the optimum asphalt content can vary considerably </p><p>between agencies. In the Asphalt Institute method (adopted in Iraq), the OAC is the </p><p>average of the following three asphalt contents: </p><p>* Asphalt content at maximum stability </p><p>* Asphalt content at maximum density </p><p>* Asphalt content at the mid-point of specified air void content (typically at 4%) </p><p>Optimum asphalt content = 13[ AC (Max. stability) + AC (Max. Density) + AC (Mid-</p><p>range of VTM) ] </p><p>Compare the proposed optimum asphalt content against the criteria in Tables </p><p>below. The proposed optimum asphalt content is selected if it meets the criteria in </p><p>the Tables. If not, the mix should be redesigned. </p></li><li><p> Lecture 15 184 </p><p>Highway Eng. Asphalt Paving Mixtures 14 15 </p><p>Dr. Firas Asad </p><p> Following is the corresponding the Iraqi specifications (SCRB, 2007, Table R9/5). The Marshall Stability of surface course for Expressway, all bridges and the approaches to bridges for a distance of 200m on each side shall be not less than 10 KN, if directed by the Engineer. </p></li><li><p> Lecture 15 185 </p><p>Highway Eng. Asphalt Paving Mixtures 14 15 </p><p>Dr. Firas Asad </p></li><li><p> Lecture 15 186 </p><p>Highway Eng. Asphalt Paving Mixtures 14 15 </p><p>Dr. Firas Asad </p><p>D- Volumetric Properties of Asphalt Mixtures </p><p>A compacted asphalt concrete mix consists primarily of aggregate, asphalt, and air. </p><p>The volumetric properties associated with the combination of these three </p><p>components are widely used for mix design and production control. Since it is </p><p>impractical to measure the volume of constituent components within a HMA mix in </p><p>the laboratory or in the field, mass volume relationships are used to convert the </p><p>measurable masses into their corresponding volumes. </p><p>The (S) letter refers to aggregate, (m) to mixture , and (b) to binder (asphalt). </p><p>Vm: volume of mix (bulk) Mm: mass of mix </p><p>Va: volume of air Ma: mass of air assumed to be equal to zero </p><p>Vb: volume of binder ( asphalt) Mb: mass of binder </p><p>VFA: voids filled with asphalt (Vbe) Mbe: mass of effective (free) binder </p><p>Vba: volume of absorbed binder Mba: mass of absorbed binder </p><p>VMA: voids in mineral aggregate Ms: mass of aggregates </p><p>Vbe: volume of...</p></li></ul>


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