Materials selection for concrete overlays : the final report Page: 9
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asphalt substrates. Also, typically, silica fume is relatively expensive, rarely available, and
difficult to handle, so the use of silica fume is not recommended for overlays.
2.3.6 Admixtures
Typical admixtures used in concrete overlays include air entrainment, high range water
reducers, and retarders. When combinations of these admixtures are used, their combined effects
should be considered. Care must be taken to avoid any admixtures that cause unnecessary
reduction in the rate of strength gain. Trial batches should be made to evaluate the interaction
between the admixtures and other constituents. For BCO applications, preliminary bond tests
should be conducted to see if chemical admixtures affect bond strengths obtained at early ages
[5].
Air Entrainment
Air entrainment protects the hardened concrete from freeze-thaw damage and deicer
scaling. Air entrainment also helps increase the workability of fresh concrete, significantly
reducing segregation and bleeding [4].
High Range Water Reducers (HRWRs)
HRWRs can make concrete with a low water-to-cementitious materials ratio workable
enough for placement [10]. This allows for a lowering of the w/cm, while maintaining a desired
slump. This has the beneficial effect of reducing permeability. Although HRWR is not
commonly used in concrete overlay constructions, whenever fibers are used in concrete overlays
the use of HRWR is highly recommended.
2.3.7 Reinforcement
Since concrete is weak in tension, reinforcement can be added to increase the
performance of concrete overlays. The installation of reinforcement can be time consuming;
therefore, careful planning is needed to minimize time spent from using reinforcement.
Wiremesh
Based on an evaluation survey done on IH610 in Houston, welded wire mesh fabric
provided more effective restraint on concrete volume change potential than steel fibers [18]. The
increase in volume change restraint can achieve better bond between the overlay and the existing
pavement. Wire mesh is relatively easier to install than reinforcement bars; however, it still
takes careful placement and additional construction time compared to using fibers.
Reinforcement Bars [4]
When reinforcement bars are placed in concrete overlays, typically No. 5 and No. 6 bars
are used for longitudinal and transverse reinforcement. Larger bar sizes are likely to cause
segregation of the coarse aggregates and voids in the mixture near the bars.
Reinforcement Bars for BCOs
Steel bars can be placed directly over the surface of the existing pavement, rather than at
mid-depth of the overlay. The performance of the steel has been demonstrated to be the same,
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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/m1/29/: accessed July 17, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu.; crediting UNT Libraries Government Documents Department.