7.2 Placement Considerations

There are, of course, many considerations to take into account when placing HMA. Many are dependent upon local materials, weather, crew knowledge and training, and individual experience. This subsection presents a few of the basic considerations that apply in virtually all situations:
  • Lift thickness
    . A "lift" refers to a layer of pavement as placed by the asphalt paver. In order to avoid mat tearing (which generally shows up as a series of longitudinal streaks), a good rule-of-thumb is that the depth of the compacted lift should be at least three times the nominal maximum aggregate size (TRB, 2000). Table 6-3 provides the minimum and maximum lift thickness for every mix type.
    Table 6-3: TxDOT Lift Thickness Requirements
    HMA Mixture Type
     
    Nominal
    Maximum
    Aggregate Size
    Minimum Lift
    Thickness
    (inches)
    Maximum Lift
    Thickness
    (inches)
    Typical Location of Pavement Layer
    Dense Graded
    (Items 340/341)
    TY-A
    1-1/2”
    3.0”
    6.0”
    Base
    TY-B
    1”
    2.5”
    5.0”
    Base/Intermediate
    TY-C
    3/4”
    2.0”
    4.0”
    Intermediate/Surface
    TY-D
    1/2”
    1.5”
    3.0”
    Surface/Level-Up
    TY-F
    3/8”
    1.25”
    2.5”
    Surface/Level-Up
    PFC
    (Item 342)
    PFC-F
    or
    PFCR-F
    1/2”
    0.75”
    1.5”
    Surface
    PFC-C
    or
    PFCR-C
    3/4”
    1.0”
    2.0”
    Surface
    Superpave Mixtures
    (Item 344)
    SP-A
    1”
    3.0”
    5.0”
    Base
    SP-B
    3/4”
    2.25”
    4.0”
    Base/Intermediate
    SP-C
    1/2”
    1.5”
    3.0”
    Intermediate/Surface
    SP-D
    3/8”
    1.25”
    2.0”
    Surface/Level-Up
    SMA
    (Item 346)
    SMA-C
    3/4”
    2.25”
    4.0”
    Intermediate/Surface
    SMA-D
    1/2”
    1.5”
    3.0”
    Intermediate/Surface
    SMA-F
    3/8”
    1.25”
    2.5”
    Surface
    SMAR-C
    3/4”
    2.0”
    4.0”
    Intermediate/Surface
    SMAR-F
    3/8”
    1.5”
    3.0”
    Surface
    CAM
    a
    (SS 3000)
    CAM
    3/8”
    1.0”
    2.0”
    Base/Intermediate
    Surface
    a
    Thin Overlay Mixtures
    (Item 347)
    TOM-C
    TOM-F
    1/2”
    1/2”
    0.75”
    0.5”
    1.25”
    0.75”
    Surface
    Surface
    Thin Bonded Friction Courses
    (Item 348)
    PFC-C
    3/4”
    1.0”
    2.0”
    Surface
    PFC-F
    1/2”
    0.75”
    1.5”
    Surface
    TY-A
    3/8”
    0.5”
    0.75”
    Surface
    TY-B
    1/2”
    0.625”
    1.0”
    Surface
    TY-C
    3/4”
    0.75”
    1.25”
    Surface
    a
    Generally not recommended as a surface mixture for pavements with a posted speed limit > 45 mph.
  • Longitudinal joints
    . The interface between two adjacent and parallel HMA mats. Improperly constructed longitudinal joints can cause premature deterioration of multilane HMA pavements in the form of cracking and raveling, and potentially serve as a conduit for moisture damage to lower layers.
  • Handwork
    . HMA can be placed by hand in situations where the paver cannot place it adequately. This can often occur around utilities, around intersection corners, and in other tight spaces. Hand-placing should be minimized because it is prone to aggregate segregation and results in a slightly rough surface texture. If hand placement is necessary, the following precautions should be taken (Asphalt Institute, 2001):
    • Place the HMA in a pile far enough away from the placement area that the whole pile must be moved. If the pile is located in the placement area, its appearance, density, or aggregate distribution may be slightly different than the surrounding handworked mat.
    • Carefully deposit the material with shovels and then spread with lutes. Do not broadcast (scoop and pitch) the HMA with shovels – this is likely to cause aggregate segregation.
    • All material should be thoroughly loosened and evenly distributed. Chunks of HMA that do not easily break apart should be removed and discarded.
    • Check the handworked surface with a straightedge or template before rolling to ensure uniformity.
  • Stone Matrix Asphalt (SMA)
    . Gap-graded mixes behave differently than dense-graded mixes during placement and compaction. Experience and understanding of dense-graded mix placement should be augmented with specific training and precautions before attempting to place an SMA for the first time. SMAs are generally stickier and more difficult to work with than dense-graded mixes because (1) they have more asphalt binder, (2) the asphalt binder is modified, and (3) the binder and filler combination creates viscous mastic. Also, it is not uncommon for large amounts of mastic (the combination of asphalt binder and mineral filler) to collect on paving equipment. If not carefully monitored, this mastic will release from the equipment into the mat, leaving an over-asphalted area – commonly referred to as a "fat spot." These considerations only scratch the surface of SMA construction. A more thorough treatment can be found in:
    National Asphalt Pavement Association (NAPA) 2002.
    Designing and Constructing SMA Mixtures
     –
    State-of-the-Practice
    , Quality Improvement Series 122. National Asphalt Pavement Association. Lanham, MD.
  • Thin Overlay Mixes (TOM); Thin Bonded Friction Courses
    . These mixtures are particularly vulnerable to rapid cool-down once placed and must be compacted promptly. Compaction cessation temperature (no further densification possible) is typically reached in less than 10 minutes, even under optimal conditions. WMA additives/processes may be effective in prolonging the compaction window by a few minutes when paving at lower ambient and surface temperature conditions. Using two breakdown rollers in tandem may expedite compaction when the compaction window is limited.
  • Permeable Friction Course (PFC)
    . Similar concerns of rapid cooling in compacting permeable friction courses exist as for the thin surface mixtures cited above. In addition, the compactive effort imparted should be the minimum necessary to seat the mixture without causing excessive breakage of the aggregate. A steel-wheeled roller, operated in the static mode, should be used.
    Mat problems
    . The asphalt paver, MTV, rollers, mix design, and manufacturing introduce many variables into flexible pavement construction. A familiarity with common causes of the more typical mat problems can help improve construction quality. Some common mat problems are micro cracking (checking), fat spots, joint problems, non-uniform texture (including mechanical segregation), roller marks, shoving, surface waves, tearing (streaks), and transverse marks. Compaction can also be compromised when working with certain mixtures that are prone to a “tender zone” phenomenon where the mat will encounter excessive lateral displacement and shoving when subject to roller loads. Any number of parameters can contribute to mixture tenderness, including excessive mix temperature, rounded aggregates, excessive binder content, and excessive moisture. Overcoming this tenderness state may be as simple as waiting several minutes for the mixture to cool slightly, or as problematic as developing a new mixture design.