Application of Large Prestress Strands in Precast/Prestressed Concrete Bridges

The objective of this research is to investigate the advantage of using large-diameter 0.7-inch (18 mm) strands in pretention applications. Large-diameter strands are advantageous in bridge construction due to the increased girders capacity required to sustain exponential increase in vehicle numbers, sizes, and weights. In this research, flexure capacity of girders fabricated using 0.7-inch (18 mm) diameter strands will be calculated and compared to bridge capacities constructed using smaller strands. Finally, two similar bridge sections will be designed using 0.6-inch (15 mm) and 0.7-inch (18 mm) diameter strands to quantify the structural advantages of increased strand diameter. The research findings showed that a smaller number of girders is required for bridge construction when larger strands are used. Four girders are required to design the bridge panel using high performance concrete and large diameter strands, as compared to 6 girders required when regular concrete mix designs and normal size strands are used. The advantages of large strands and high-performance concrete mixes include expedited construction, reduced project dead loads, and reduced demand for labor and equipment. Thus, large strands can partially contribute to the improvement of bridge conditions, minimize construction cost, and increase construction site safety

Developing high performance concrete for precast/prestressed concrete industry

High performance concrete (HPC) is a new class of concrete that has superior characteristics compared to conventional concrete. Despite of its superior characteristics, HPC is not widely used in local and international markets due to its high constituent materials cost. This paper presents the research done to develop economic HPC mixes using local materials and conventional mixing and curing techniques. HPC characteristics were attained using supplementary cementitious materials as silica fume and class C fly in partial replacement of Portland cement. Superplasticizers were used to maintain a high flowing ability using a low water-to-powder ratio. Concrete mixes were produced using a high energy mixer to maintain sufficient mix consistency. As a result, concrete mixes with 24 -h compressive strength of 70 MPa and 28-day strength of 105 MPa were produced. Concrete samples tested for expansion using accelerated mortar bar test (AMBT) showed that developed concrete is not susceptible to alkali-silica reaction. Improved characteristics can be used in improving the performance of concrete construction projects, reduce required maintenance, and increase construction projects life-span

Development of High Performance Precast/Prestressed Bridge Girders

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 Tadro

Improving Concrete Infrastructure Project Conditions by Mitigating Alkali–Silica Reactivity of Fine Aggregates

Alkali–silica reactivity (ASR) is one of multiple reactions responsible for premature loss in concrete infrastructure service life. ASR results in the formation of expansive, white-colored gel-like material which results in internal stresses within hardened concrete. ASR-induced stresses result in concrete cracking, spalling, and increased reinforcement steel corrosion rates. The main objective of this research is to improve the conditions of concrete infrastructure projects by mitigating ASR’s damaging effect. The expansion of accelerated mortar bars poured using fine aggregates collected from different sources is measured versus time to evaluate the aggregates’ reactivity. Different percentages of supplementary cementitious materials (SCMs), including class C fly ash and microsilica, were used in remixing mortar bars to evaluate the efficiency of different types of SCMs in mitigating mortar bar expansion. The research findings showed that SCMs can mitigate ASR, thus decreasing mortar bar expansion. The efficiency of SCMs in ASR mitigation is highly dependent on the incorporated SCM percentage and particle fineness. Silica fume, having the smallest particle size, displayed higher rates of ASR mitigation, followed by fly ash. The outcomes of this research will assist design engineers in avoiding future losses due to ASR cracking in concrete infrastructure projects, and reduce the excessive need for maintenance, repair, and replacement activities

Developing high performance concrete for precast/prestressed concrete industry

High performance concrete (HPC) is a new class of concrete that has superior characteristics compared to conventional concrete. Despite of its superior characteristics, HPC is not widely used in local and international markets due to its high constituent materials cost.This paper presents the research done to develop economic HPC mixes using local materials and conventional mixing and curing techniques. HPC characteristics were attained using supplementary cementitious materials as silica fume and class C fly in partial replacement of Portland cement. Superplasticizers were used to maintain a high flowing ability using a low water-to-powder ratio. Concrete mixes were produced using a high energy mixer to maintain sufficient mix consistency. As a result, concrete mixes with 24-¯-h compressive strength of 70-¯MPa and 28-day strength of 105-¯MPa were produced. Concrete samples tested for expansion using accelerated mortar bar test (AMBT) showed that developed concrete is not susceptible to alkali-silica reaction. Improved characteristics can be used in improving the performance of concrete construction projects, reduce required maintenance, and increase construction projects life-span

Developing high performance concrete for precast/prestressed concrete industry

High performance concrete (HPC) is a new class of concrete that has superior characteristics compared to conventional concrete. Despite of its superior characteristics , HPC is not widely used in local and international markets due to its high constituent materials cost. This paper presents the research done to develop economic HPC mixes using local materials and conventional mixing and curing techniques. HPC characteristics were attained using supplementary cementitious materials as silica fume and class C fly in partial replacement of Portland cement. Superplasticizers were used to maintain a high flowing ability using a low water-to-powder ratio. Concrete mixes were produced using a high energy mixer to maintain sufficient mix consistency. As a result , concrete mixes with 24€¯-h compressive strength of 70€¯MPa and 28-day strength of 105€¯MPa were produced. Concrete samples tested for expansion using accelerated mortar bar test (AMBT) showed that developed concrete is not susceptible to alkali-silica reaction. Improved characteristics can be used in improving the performance of concrete construction projects , reduce required maintenance , and increase construction projects life-span