Materials selection for concrete overlays : the final report Page: 54
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lanes without a decrease in traffic [57]. It has heavy traffic and the District asked the researchers
to conduct the survey after 10 pm, so that the traffic would be less intense when lane closures
were made.
The original pavement section consisted of 8-in.-thick CRCP built in 1965; a feasibility
study recommended that this section could be rehabilitated with a BCO in 1993 [57]. In June and
July of 1996, a segment about three-quarters of a mile long in each direction was overlaid with a
BCO.
The overlay was planned as an expedited BCO [58, 59], which means that mixture
proportioning for paving methods were designed to reduce the normal time between placement
and opening the lanes to traffic. With this, the overall cost of the project would have been
reduced and the burden to the public originated by lane closures and detours would have been
diminished.
However, in spite of the planning and research invested in the project, problems with the
concrete mixture resulted in the delamination of most of the eastbound and some of the
westbound BCO. Shortly after construction, some delaminations were identified during the
extraction of core samples from the pavement. Coring and seismic tests confirmed the severity
and extension of the delaminations. The comprehensive investigation that followed these events
identified the high amount of water lost by the concrete before the curing compound was applied
as a major cause of the debonding problem. A number of factors contributed to these unusual
moisture losses from the concrete: the delay in applying the curing compound in conjunction
with very high evaporation rates and inadequate surface preparation (not wetting the substrate to
SSD).. Additionally the mixture had low water content (w/c < 0.30) to begin with, because of
the higher early strength requirement of an expedited BCO, and this resulted in a stiff mixture.
Then, the surface of the existing pavement slab was not dampened before placing the overlay,
which caused moisture losses through the bottom of the slab. (To prevent water loss into the
substrate, the substrate surface should have been prepared by spraying water on it before pouring
the concrete [45]. Additionally, the cement properties changed from the time when the mixture
design was originally approved to the time of placing the overlay. All these issues affecting
critical water content combined to create a very stiff, very strong overlay with very little
adhesion to its substrate.
The BCO had to be repaired by means of injecting epoxy into spaced holes in order to
bond or re-bond the delaminated overlay to the substrate. The repair work took three weeks to
complete, and Falling Weight Deflectometer (FWD) tests confirmed the success of the remedy.
However, subsequent pullout tests indicated that some areas if the interface were not completely
filled with epoxy, and some locations had deflections higher than expected, so it was considered
at the time that those areas were still debonded [45]. These observations from shortly after the
epoxy repairs were completed support the results observed in this survey.
3.14.1 Condition Survey
For the visual inspection of the overlay in both traffic directions transverse cracks were
counted, as were distress locations and types and patches. Lane closures allowed the researchers
to walk on the outside lanes in each direction, while conducting sounding tests to detect
delaminations. The survey of the westbound direction started at 10:30 pm, and the eastbound
direction started at 12:45 am. The results are summarized in Tables 3.12 and 3.13, and some
representative photographs of the survey are included in Appendix A-9.54
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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/m1/74/: accessed July 17, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu.; crediting UNT Libraries Government Documents Department.