Materials selection for concrete overlays : the final report Page: 15
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of nails is at about 6-in. from the edge or joint, with spacing between nails of 15 to 30 in.
Smaller nail spacing results in a higher number of cracks of smaller width.
2.3.9 Incidental Materials [4]
It is not practical to install dowel bars, tie bars, or keyway in thin concrete overlays
because of the lack of concrete cover. Field evaluation has indicated that the load transfer
provided by aggregate interlock is generally high because of the joint spacing and the support
provided by the asphalt layer. [1, 23] However, dowel bars, tie bars, and key ways play an
important role in improving load transfer efficiently in UBCO applications where aggregate
interlock alone is not enough. Unlike whitetoppings or BCOs, UBCOs offer thicker layers of
concrete so that these materials can stay safely embedded.
Dowel Bars
Typically, billet steel, grade 60 bars that conform to ASTM A615 - 09b "Standard
Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement" or
AASHTO M31 are used. Sometimes the sizes are reduced to accommodate thinner concrete
overlays. The recommended number and spacing of dowels is the same as those for new
pavements. In general, a uniform 12-in. spacing is recommended, but non-uniform spacing has
also been used successfully. In the non-uniform dowel spacing design, the dowels are
concentrated in the wheel paths [4, 32].
Tie Bars
Typically, billet steel, grade 40 bars that meet ASTM A615 or AASHTO M31
specifications are used.
Joint Sealant Materials
If used, joint sealant materials are (1) hot-poured rubberized materials conforming to
ASTM D660- 93 "Standard Test Method for Evaluating Degree of Checking of Exterior Paints,"
or per normal design, (2) silicone materials conforming to a governing state specification, or (3)
reformed compression seals conforming to ASTM D2628 - 91 "Standard Specification for
Preformed Polychloroprene Elastomeric Joint Seals for Concrete Pavements," or a governing
state specification. When small panel sizes are constructed, sealant is often not used.
2.3.10 Separation Layer Materials for UBCOs
A separation layer allows the existing pavement and the new concrete overlay to act
independently. It also prevents distresses from reflecting into the concrete overlay. Typically, 1-
to 2-in.-thick asphalt concrete has been widely used for the purpose and has been proven
effective. Materials such as polyethylene, roofing paper, and curing compound do not work.
Most failures in unbonded concrete overlays are due to the use of inadequate separation layers or
insufficient overlay thickness.
Thin separation layers (such as sheathings) must be avoided because they are more likely
to permit reflective cracking from the existing pavement. Thicker separation layers can prevent
reflective cracks from occurring [44]. Figure 2.6 shows how a smooth slip plane can prevent
reflective cracks from occurring.15
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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/m1/35/: accessed July 17, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu.; crediting UNT Libraries Government Documents Department.