Lec 15 Highway Engineering - Asphalt Paving Mixtures

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Highway Engineering - Asphalt Paving Mixtures

Text of Lec 15 Highway Engineering - Asphalt Paving Mixtures

  • Lecture 15 175

    Highway Eng. Asphalt Paving Mixtures 14 15

    Dr. Firas Asad

    In this lecture;

    - Constitutes

    - Types

    - Desirable Properties

    - Marshall Mix Design Method

    - Volumetric Properties

    Asphalt Paving Mixtures

    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).

    A- Constituents of a mix Coarse aggregates: Offer compressive and shear strength and shows good interlocking properties. E.g. Crushed stone.

    Fine aggregates: Fills the voids in the coarse aggregate and stiffens the binder. E.g. Sand, Rock dust

    Filler: Fills the voids, stiffens the binder and offers permeability. E.g. Rock dust, cement, lime.

    Binder: Fills the voids, cause particle adhesion and gluing (cementing) and offers impermeability. E.g. Bitumen (Asphalt) and Tar.

    Asphalt Concrete

    (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.

  • Lecture 15 176

    Highway Eng. Asphalt Paving Mixtures 14 15

    Dr. Firas Asad

    B- Types of Asphalt Paving Mixtures 1- Hot-Mix, Hot-Laid Asphalt Mixture

    Hot-mix, hot-laid asphalt mixture is produced by properly blending asphalt cement,

    coarse aggregate, fine aggregate, and filler (dust) at temperatures ranging from

    about 80 to 160oC depending on the type of asphalt cement used. Suitable types of

    asphalt materials are those with penetration grades of 60 to 70, 85 to 100, 120 to

    150, and 200 to 300.

    Hot-mix, hot-laid asphalt mixture normally is used for high-type pavement

    construction, and the mixture can be described as open graded, coarse-graded,

    dense-graded, or fine-graded.

    2- Hot-Mix, Cold-Laid Asphalt Mixture

    Asphalt mixtures in this category are manufactured hot and then shipped and laid

    immediately or can be stockpiled for use at a future date. Thus, they are suitable for

    small jobs when it is uneconomical to set up a plant.

    In this type of asphalt concrete high-penetration asphalt normally is used. The most

    suitable asphalt cements is AC 200-300 penetration grade.

    3- Cold-Mix, Cold-Laid Asphalt Mixture

    Emulsified asphalts and low-viscosity cutback asphalts are used to produce cold-mix

    asphalt mixtures. They also can be used immediately after production or stockpiled

    for use at a later date. The production process is similar to that of the hot-mix

    asphalts, except that the mixing is done at normal temperatures.

    Examples of the types and grades of asphalt material used are MS and SS.

  • Lecture 15 177

    Highway Eng. Asphalt Paving Mixtures 14 15

    Dr. Firas Asad

    C- Desirable Properties of Asphalt Mixtures

    1- Stability

    Stability is the resistance of pavement to permanent deformation caused by

    sustained or moving load. It is provided by two factors:

    a- Interlocking and internal friction - depend on:

    1- Size and shape of aggregate;

    2- Surface area and roughness;

    3- Applied load.

    b- Cohesion resistance - depends on:

    1-rate of load application

    2- loaded area;

    3- viscosity of the binder.

    4- Temperature.

    2 - Durability

    It is the ability to resist the factors of weathering; water(stripping), air(oxidation),

    temperature, and abrasive action from traffic (disintegration/cracking).

    Percentage of binder controls on disintegration and resistance to cracking where

    brittle mixes cracks under the effect of moving loads. The brittleness is mainly

    caused by: (a) low ductility asphalt and (b) low content of binder. Maximum binder

    content increases durability because thick binder films do not age and harden as

    rapidly as thin ones do.

    Temperature causes thermal cracking at low temperature and tenderness at high

    temperature.

  • Lecture 15 178

    Highway Eng. Asphalt Paving Mixtures 14 15

    Dr. Firas Asad

    3. Flexibility

    It is the ability of the pavement to bend repeatedly due to traffic loads without

    cracking (fatigue resistance). To obtain the desired flexibility :

    1- Using appropriate content and type of asphalt;

    2- Using well graded aggregate;

    3- Selected temperature of mixing and compaction.

    4. Skid resistance

    It depends on the roughness of surface textured aggregate , hardness of aggregate

    and less asphalt content. Best skid resistance is obtained with rough-textured

    aggregate in a relatively open-graded mixture.

    5. Workability

    Workability describes the ease with which a paving mixture can be placed and

    compacted. Mixtures with good workability are easy to place and compact; those

    with poor workability are difficult to place and compact.

    Workability can be improved by changing

    a- mix design parameters

    b- aggregate source, and/or

    c- gradation.

  • Lecture 15 179

    Highway Eng. Asphalt Paving Mixtures 14 15

    Dr. Firas Asad

    D- Design of Hot-Mix Asphalt Concrete

    The objective of mix designs is to adequately select and mix certain proportions of

    asphalt binders and aggregates to produce HMA that performs well both

    functionally and structurally . Regarding performance, the HMAC should be

    durable and be able to resist pavement distresses (such as permanent deformation,

    load-induced fatigue, thermal fatigue, low temperature cracking, and moisture-

    induced damage). From a construction standpoint, the mix should be workable

    enough to place and compact with reasonable effort. Additionally, surface courses

    should provide sufficient skid resistance for safety considerations.

    There are several mix design methods for Hot-Mix Asphalt, Hveem (1920s), Marshall

    (1930s), and Superpave (1990s) are the most common methods. In this course, only

    Marshall method will be covered.

    Marshall Mix Design Method

    The Marshall method of mix design is intended both for

    laboratory design and field control of bituminous hot-mix

    dense-graded paving mixtures. Originally, it was developed by the

    bituminous engineer Bruce Marshall of the Mississippi State Highway Department.

    The US Army Corps of Engineers enhanced the Marshalls approach and it was then

    subsequently formalized as ASTM D 1559 and AASHTO T 245 Standard Method of

    Test for Resistance to Plastic Flow of Asphalt Mixtures Using Marshall Apparatus.

  • Lecture 15 180

    Highway Eng. Asphalt Paving Mixtures 14 15

    Dr. Firas Asad

    The Marshall method uses standard cylindrical test specimens that are 64 mm

    (2.5in) high by 102 mm (4in) diameter. These are prepared using a specified

    procedure for heating, mixing and compacting the bitumenaggregate mixture. The

    two principal features of the Marshall method of mix design are a density voids

    analysis and a stability-flow test of the compacted test specimens. The stability of

    the test specimens is the maximum load resistance, in newtons, that the standard

    test specimen will develop at 60C. The flow value is the total movement or

    displacement, in units of 0.25 mm, occurring in the specimen between no load and

    the point of maximum load during the stability test.

    Outline of method

    Step 1. Aggregate Evaluation

    Procedure

    The aggregates proposed for use in the mix design should be have the desired

    properties and satisfy the required specifications (See Lecture L13). If the aggregates

    meet the requirements above, the aggregate specific gravity, absorption, and

    gradation are determined. These will be used for determining the volumetric

    properties of the mix. Last, the individual aggregate gradations are combined in

    different proportions to develop trial blends.

  • Lecture 15 181

    Highway Eng. Asphalt Paving Mixtures 14 15

    Dr. Firas Asad

    Step 2. Asphalt Cement Evaluation

    The asphalt cement used in the mix design should satisfy the specifications when

    running the required tests (See Lecture L14). After that, determine the mixing and

    compaction temperatures based on viscosities of 170 20 and 280 30 centistokes

    (cSt), respectively.

    Step 3. Preparation of Marshall Specimens

    Prepare the Marshall specimens in accordance to the requirements set in AASHTO

    R-12. Compact three replicate specimens at five percentages of asphalt contents

    (4.0, 4.5, 5.0, 5.5, 6.0 % by weight of total mix). The asphalt contents should be

    selected with two asphalt contents falling above and below the optimum asphalt

    co