Bridge-in-a-Backpack(TM) task 4 : development of improved arch concrete mix to facilitate field use.
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Bridge-in-a-Backpack(TM) task 4 : development of improved arch concrete mix to facilitate field use.

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    • Abstract:
      This report includes fulfillment of Task 4 of a multi-task contract to further enhance concrete filled FRP tubes, or the Bridge in a Backpack. Task 4 investigates the development of improved concrete mixes for filling the FRP tubes. One critical step in the construction of bridges using the Bridge-in-a-Backpack system is filling the hollow tubes with concrete. This step represents a small part of the construction process, but mistakes here can have a tremendous impact on construction schedule and project cost. Problems arising during concrete placement can include inadequate or slow arch filling due to impeded flow and segregation of the mix due to the inclusion of large amounts of superplasticizer. This may result in voids in the arch which must be addressed after the initial filling, changing the arch strength properties and slowing construction. The current mix also uses an expansive admixture to compensate for shrinkage and cause expansion of the concrete while curing. The amount of this additive necessary to ensure adequate arch structural performance is not precisely known. Presently this additive is proportioned according to recommendations from the supplier as well as test data showing positive expansion of the concrete after final set. The objective of this task is to provide AIT and the Department of Transportation with information that will allow them to make informed decisions about arch concrete specifications and bound mix parameters to ensure that concrete arriving on site will perform as expected. The results are highly variable and inconclusive. Additional testing with a larger sample size would be recommended if additional investigation into the bending strength of CFRP tubes filled with varying concrete mixes is of interest. However, it is important to note that while this research topic was of high interest when the project initiated, significant improvements have been realized in the mix design since then. These improvements have addressed the concerns that motivated this research, and a large number of CFFT arch bridges have been successfully constructed with SCC mixes that have performed well.
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