Sources of bridge deterioration are described including design and construction deficiencies, env... more Sources of bridge deterioration are described including design and construction deficiencies, environmental effects, and changes in use. The Delaware environment is described in detail, as well as Delaware bridges and the rating system, and bridge evaluations. Prestressed concrete bridges are discussed, including air entrainment and moisture barriers, bearing pads, void construction, and crack formation. Prestressed concrete beams are grouped in 2 categories: those properly detailed which have virtually no deterioration after more than 30 years; and those with beams that contain design or cosntruction deficiencies. The most notable deficiencies are omission of air entrainment to the concrete mix, the use of excessively stiff bearing pads, and failure to properly open beam drains. The sources of deterioration have been addressed through continuous design development and specification upgrade.
Journal of Composites for Construction, Feb 1, 1997
The performance of glued-laminated (glulam) timber beams may be improved by the addition of high-... more The performance of glued-laminated (glulam) timber beams may be improved by the addition of high-performance fibers between the laminations. The contribution of the fibers is further enhanced if they are pretensioned prior to incorporation into the glued-laminated beam. This research indicates that both strength and stiffness are increased by using small volumes of pretensioned Kevlar yarns. The research explores the bond, development, and adhesive performance associated with the prestressing operations.
The primary motivation in prestressing concrete is to offset tensile stresses that develop in fle... more The primary motivation in prestressing concrete is to offset tensile stresses that develop in flexural members. Consequently, using prestressed concrete in members that are largely loaded in axial compression or loaded in tension might appear contradictory. Furthermore, concrete’s low tensile strength makes its use as a tension member immediately doubtful. Nonetheless, there are cases where axially loaded prestressed concrete contributes to the engineer’s design options. Applications include ring girders for shells and foundations, circular tanks, columns, and piles.
Continuity provides an economic benefit with shallow sections and longer spans possible. Continui... more Continuity provides an economic benefit with shallow sections and longer spans possible. Continuity also inherently improves safety due to the redundancy provided by the continuity. This comes at a price of increased calculation and detailing complexity, increased construction complexity, and increased propensity for cracking and other distress due to the effect of restrained creep and shrinkage.
Prestressing a concrete member effectively applies a substantial axial force to the member that i... more Prestressing a concrete member effectively applies a substantial axial force to the member that is in place for its entire service life. In both pretensioned and posttensioned methods of prestressing, this prestressing force begins to decline immediately upon its application and continues to decline throughout its service life. This reduction in force is referred to as partial prestress loss and is addressed as part of the design of a prestressed member. Partial prestress losses, typically referred to as "prestress losses," are divided into two broad categories: initial and time-dependent effects. Initial losses occur during stressing operation and include anchor seating, elastic shortening, and friction between prestressing steel and post-tensioning ducts or tendon deviators and harped pretensioned strands. Long-term losses occur because of viscoelastic material effects and include concrete shrinkage, creep, and tendon relaxation. This chapter covers the causes of prestress losses as well as techniques for estimating those losses. Numerous methods are available for estimating losses. This is likely due, at least in part, to the difficulty in accurately predicting losses. ACI 423-10 (2016) devotes an entire chapter to the variability of loss calculations and the reader is referred there for more detail. In summary, the variability in concrete mechanical properties, curing conditions, and exposure to environment are the primary causes of this difficulty. While there are many different approaches for determining initial and time-dependent effects on prestressed concrete, the most effective method is to construct the element and measure the losses in the field. Corrections are done in the field for friction losses on occasion; other corrections are not common except for research. In many cases, a high level of accuracy is not needed to ensure suitable strength and serviceability. Consequently, the design engineer estimates the losses and bases the design on this estimate.
Experimental confirmation tests were conducted to evaluate mechanical and epoxy socketed anchors ... more Experimental confirmation tests were conducted to evaluate mechanical and epoxy socketed anchors for fiber reinforced plastic (FRP) prestressing tendons. The studies found that a parabolically varying profile provides superior performance compared to a conventional linear conic anchor. It was also found that an anchor with a bond release agent on the surface between the socket and the resin plug results in a lower peak shear stress compared to a bonded anchor. The combination of a parabolic anchor and a bond release agent permits use of a wider range of resins as socketing agents is less sensitive to construction tolerances. Mechanical anchors were much more variable in their ability to hold load and were seldom capable of developing the full tensile capacity of the tendons.
Sources of bridge deterioration are described including design and construction deficiencies, env... more Sources of bridge deterioration are described including design and construction deficiencies, environmental effects, and changes in use. The Delaware environment is described in detail, as well as Delaware bridges and the rating system, and bridge evaluations. Prestressed concrete bridges are discussed, including air entrainment and moisture barriers, bearing pads, void construction, and crack formation. Prestressed concrete beams are grouped in 2 categories: those properly detailed which have virtually no deterioration after more than 30 years; and those with beams that contain design or cosntruction deficiencies. The most notable deficiencies are omission of air entrainment to the concrete mix, the use of excessively stiff bearing pads, and failure to properly open beam drains. The sources of deterioration have been addressed through continuous design development and specification upgrade.
Journal of Composites for Construction, Feb 1, 1997
The performance of glued-laminated (glulam) timber beams may be improved by the addition of high-... more The performance of glued-laminated (glulam) timber beams may be improved by the addition of high-performance fibers between the laminations. The contribution of the fibers is further enhanced if they are pretensioned prior to incorporation into the glued-laminated beam. This research indicates that both strength and stiffness are increased by using small volumes of pretensioned Kevlar yarns. The research explores the bond, development, and adhesive performance associated with the prestressing operations.
The primary motivation in prestressing concrete is to offset tensile stresses that develop in fle... more The primary motivation in prestressing concrete is to offset tensile stresses that develop in flexural members. Consequently, using prestressed concrete in members that are largely loaded in axial compression or loaded in tension might appear contradictory. Furthermore, concrete’s low tensile strength makes its use as a tension member immediately doubtful. Nonetheless, there are cases where axially loaded prestressed concrete contributes to the engineer’s design options. Applications include ring girders for shells and foundations, circular tanks, columns, and piles.
Continuity provides an economic benefit with shallow sections and longer spans possible. Continui... more Continuity provides an economic benefit with shallow sections and longer spans possible. Continuity also inherently improves safety due to the redundancy provided by the continuity. This comes at a price of increased calculation and detailing complexity, increased construction complexity, and increased propensity for cracking and other distress due to the effect of restrained creep and shrinkage.
Prestressing a concrete member effectively applies a substantial axial force to the member that i... more Prestressing a concrete member effectively applies a substantial axial force to the member that is in place for its entire service life. In both pretensioned and posttensioned methods of prestressing, this prestressing force begins to decline immediately upon its application and continues to decline throughout its service life. This reduction in force is referred to as partial prestress loss and is addressed as part of the design of a prestressed member. Partial prestress losses, typically referred to as "prestress losses," are divided into two broad categories: initial and time-dependent effects. Initial losses occur during stressing operation and include anchor seating, elastic shortening, and friction between prestressing steel and post-tensioning ducts or tendon deviators and harped pretensioned strands. Long-term losses occur because of viscoelastic material effects and include concrete shrinkage, creep, and tendon relaxation. This chapter covers the causes of prestress losses as well as techniques for estimating those losses. Numerous methods are available for estimating losses. This is likely due, at least in part, to the difficulty in accurately predicting losses. ACI 423-10 (2016) devotes an entire chapter to the variability of loss calculations and the reader is referred there for more detail. In summary, the variability in concrete mechanical properties, curing conditions, and exposure to environment are the primary causes of this difficulty. While there are many different approaches for determining initial and time-dependent effects on prestressed concrete, the most effective method is to construct the element and measure the losses in the field. Corrections are done in the field for friction losses on occasion; other corrections are not common except for research. In many cases, a high level of accuracy is not needed to ensure suitable strength and serviceability. Consequently, the design engineer estimates the losses and bases the design on this estimate.
Experimental confirmation tests were conducted to evaluate mechanical and epoxy socketed anchors ... more Experimental confirmation tests were conducted to evaluate mechanical and epoxy socketed anchors for fiber reinforced plastic (FRP) prestressing tendons. The studies found that a parabolically varying profile provides superior performance compared to a conventional linear conic anchor. It was also found that an anchor with a bond release agent on the surface between the socket and the resin plug results in a lower peak shear stress compared to a bonded anchor. The combination of a parabolic anchor and a bond release agent permits use of a wider range of resins as socketing agents is less sensitive to construction tolerances. Mechanical anchors were much more variable in their ability to hold load and were seldom capable of developing the full tensile capacity of the tendons.
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