Durham School of Architectural Engineering and Construction


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A Dissertation Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Construction Engineering. Under the Supervision of Professors: George Morcous and Maher Tadros Lincoln, Nebraska: December, 2008.
Copyright © 2008 Amin K. Akhnoukh. December, 2008


Demand continues to increase for bridges with long spans and shallow depths. Due to safety concerns, four-span overpasses are being replaced with two span overpasses to avoid placement of piers near the highway shoulders. In the meantime, the bridge profile is restricted due to existing businesses nearby. Thus, nearly the same superstructure depth must be used for double the span length. This dissertation focuses on topics aiming at providing precast prestressed concrete girders with the shallowest possible depth for a given span. It forms parts of larger projects conducted by the University of Nebraska for the Nebraska Department of Roads and for the Wire Reinforcement Institute.

Specifically, the following issues were researched:
(1) Use of 0.7 in. diameter Grade 270 ksi strands for pretensioning of precast concrete girders at a strand spacing of 2 inches by 2 inches. This arrangement gives nearly 190 percent of the prestressing with 0.5 in. diameter strands and nearly 135 percent with 0.6 in. strands. The research focuses on the required confinement steel to allow determination of transfer and development lengths according to current procedures in the AASHTO LRFD Bridge Design Specifications for smaller strands.
(2) Develop a self consolidating concrete (SCC) mix, using Nebraska aggregates that will allow for a specified design strength at service of 15 ksi and a minimum strength at one day of 10 ksi, representing the demand at the time of release of the prestress to the concrete member. Prior to this study, standard concrete strength prevailing in Nebraska has been 8 ksi at service and 6.5 ksi at release. It was the goal of the research to keep the cost of materials as low as possible but not exceeding $250 per cubic yard, compared to the proprietary mixes that cost approximately four times this amount.
(3) Use of 80 ksi welded wire reinforcement (WWR) as the auxiliary reinforcement for shear, web end splitting and flange confinement. This would result in higher quality product, less reinforcement congestion, about 25 percent savings in the steel materials, and considerable savings in girder fabrication costs.

A combination of theoretical and experimental work has resulted in the following findings:
(1) A shear friction model can be used to estimate the required amount of confinement of the bottom flange.
(2) A reasonable reinforcement detail is needed, even with very heavily prestressed NU I girder bottom flange, to allow use of the current methods of estimating strands transfer and development lengths.
(3) Two SCC mixes with materials costs less that $200 dollars per cubic yard and with the required strengths were able to be developed. The mixes exhibited excellent flowability and predictable engineering properties.
(4) Grade 80 WWR was successfully used. Its shear resistance was theoretically predictable. It produced higher capacity than the Ultra High Performance steel fiber concrete demonstrated by the Federal Highway Administration, with much lower costs and conventionally predicable design strength.

Advisors: George Morcous, Maher Tadros