treatments. They exhbit smaller deflections when compared
to other prestressed variety and in fact are seen to be more
beneficial when being used in te case of service loads such as
bridges.
Prestressed Concrete One Way Slabs:
One way slabs are usually supported over the width of the
entire slabs. In order to achieve this the design of such
prestressed slabs should include for beams,piers and
abutments as the case may be. The slab might be a continous
one or might be angular for the prestressing force [12]. Mild
steeel reinforcements would also be used, or the slabs might
use continous force over several supports.Most experimental
invetsigations on these forms of slabs seems to indicate that
these slab designs are very safe within some prescribed limits
and also support load factor above the theoreticallimits [17].
Prestressed Concrete Simple Flat and Continous Slabs:
A simple slap is usually supported by means of C columns
netowrk. These are beams that are prestressed in
perpendicular and are also used to resist forces in these
directions. Slab is ususally proportioned using tendons
housed in the context of column strips and the proportioning
is also seen to be based on the total moment involved.
On the other hand a prestressed concrete continous slab
will be involved with using continous over supports that
would be in the form of a continous beam and would also use
principle direction arrnagement of the slabs such that they do
not conflict one another. The slabs in general are seen to be
used with column and/ or middle strips.
The continous flat slabs are the ones that have been used
extensively in the United States since 1955 [9].There are
typically different adanatages towards using the prestressed
concrete floor slabs and there different varieties and the types
are usually vaired based on the construction section, the span
and more [8] [15]
5. Case Study: Data Analysis and
Discussion
The case study that has been chosen for analysis here is
that of the replacement deck construction that took place in
the years of 1999-2000 in Illinois. The deck replacement was
to be done by using a precast, prestressed concrete (PPC)
deck planks. These decks were seen to be part of an extensive
bridge project that was carried out by the Illinois
Transportation Department [9]. After preliminary analysis on
the types of bridges that would require reinfrocement, it was
decided that three bridges fit the category. These three
bridges were then fitted with pre cast prestressed PPC deck
planks. With a thickness of approximae 3.0 or 3.5 inches (this
is in comparison with the 2.5 inch plank that was used
earlier), these planks are seen to be constructed with a
minimum maturity of about 60 days (the minimum amount of
days for maturity is calculated from the time the deck pour is
iniitated) [9]. The purpose of this case study is to report and
critically investiagte on the techniques that have been used in
the deck plank construction, the pre installations, the early
performance evaluation and more.
Process:
The PPC Deck plan bridges for the project modifications
were seen to be manufactured in strict adherence to
theIllinois Department of Transportation Standard
Specifications. The prestressed planks were seen to be placed
in as headers and were also reinforced with wire meshing [9].
A vibratory screed was used to level off theplanks and the
planks poured in between 10AM to 2PM were then stropped
only after 6AM to 10PM when the release strenght was
achieved for the planks. Steamcuring was allowedfor some of
the pours and this was allowed for a time period of upto 42
hours after the pour.
A prelimianry field inspection was carried out in order to
udnerstand how the deck planks thickeness could affect the
durability and also on how concrete build up could lead to
the planks to be wider than the projected estimate. Planks
were also surveyed to ensure that the vertical and the
horizontal locaitons were even for the strands. Deck planks
had to be rehjected based on the excessive cracks that they
showed initially. Some of the planks also had strand
slippacge and others had other miscellaneous defects. Of the
384 planks that was manufactured, a total of 23 percent were
rejected on an average whihc is about 90 out of the 384
planks. In the case of the 3.5 inches planks it was observed
that there was a total rejection of 26 planks and that is an
approximate 8 percent rejected and of the 3 inch planks there
were a total of 95 planks rejected whihc was the highest
percetage rejection at 28 percent.
Solution:
Based on the evaluation and the rejected planks it was
decided that an open evaluation of rejected planks be done.
Load tests were then conducted, it was estimated that the
previously rejected planks were to be selected in the form of
similarities in cracking patterns. They were cut flush at the
ends and then a simulation of bearing structure was carried
out. Deflection was measured an approximate of four times
in these tests. In addition a visual inspection was also carried
out, based on the same recommendations were made.
Discussion:
Some key elements of discussion that were driven into this
state are that the load testing method must be employed when
using deck plank bridges. The load testing method helps in
weeding out the deformed or the damaged deck planks. In
addition to this it is also recommended that the PPC deck
plank fabrication makes use of pre installation of the leveling
screws, this will ensure that the deck planks are aligned
properly and hence do not contribute to additional stress in
the construction. There is polystyrene used in deck plank
constructions. It was seen that this could well be an issue and
hence when it is necessary to use polystyrene then it has to be
extruded polystyrene only and should also be of a grade type
of 578 Type IV or higher.
6. Conclusion and Recommendations
Based on the case study it was observed by the researchers