Materials selection for concrete overlays : the final report Page: 6
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deteriorated with major surface distresses. A separation layer (typically, a 1-in-thick asphalt
layer) is used to maintain separation between concrete overlay and existing pavement.
Performance Factors
The following factors determine the performance of UBCOs:
* Effectiveness of the separation layer: An effective separation layer will act as a
shear plane that will prevent cracks from reflecting up from the existing pavement
into the overlay. In addition, the separation layer prevents bonding between the
new and the old layer allowing them to move independently.
* Effective drainage: A well-constructed drainage system will prevent the building up
of pore pressure from the traffic loads. The system serves to prolong the life of the
overlay by reducing pumping, asphalt stripping of the separation layer, faulting, and
cracking.
Common Modes of Failure
The following failure modes are commonly seen in UBCOs:
* Failure to consider at-grade and overhead structures: The elevation of the pavement
after an UBCO placement will increase, in some cases, significantly therefore, at-
grade and overhead structures should be raised, or the existing pavement should be
removed and replaced near these structures [2].
* Inadequate separation layer: The separation layer prevents reflective cracks from
occurring. If the new overlay is not structurally separated from the deteriorated
existing pavement, the movement of two structures will be similar, which can
induce heavy reflective cracking to the overlay from underneath.
* Poor drainage: The higher elevation of the pavement necessitates a change in the
drainage grade lines. Additional right-of-way may be required to provide the
proper slopes for the ditches [3].
2.3 Recommended Materials
This section discusses general recommended materials for concrete overlays. Materials
discussed have been historically effective for concrete overlay construction; therefore provide an
insight on which materials to select for the research.
2.3.1 Cement
The most commonly used cement types are Type I, Type I/II, and Type III. Type I is
usually preferred over Type III because it develops less heat of hydration, avoiding many of the
problems associated with high temperature development [4, 5].
When high early strength is desired, a Type III or more finely ground Type I cement is
used. However, the use of these cements will result in an increased heat of hydration, and
caution should be taken to reduce thermal cracking. Other characteristics to consider when
selecting cement are long-term mechanical properties, toughness, volume stability, and long life
in severe environments [4, 6].
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Kim, Dong H.; Fowler, David W.; Ferron, Raissa P.; Trevino, Manuel M. & Whitney, David P. Materials selection for concrete overlays : the final report, report, July 2012; Austin, Texas. (https://texashistory.unt.edu/ark:/67531/metapth303706/m1/26/: accessed July 17, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu.; crediting UNT Libraries Government Documents Department.