This paper presents a description and comparison of several experimentaltechniques used to determine the effective prestressing force in pretensionedprestressed concrete girders. The effective prestressing force was determined by threemethods: (1) using vibrating wire strain gages that were embedded in the girders duringfabrication; (2) load testing the girders to determine flexural cracking and crack re-opening loads and then back calculating the losses; and (3) exposing a length ofstrand, attaching resistance strain gauges on the strands, and flame-cutting theinstrumented strands.Several instruments were used to determine the flexural crack initiation and crack re-opening loads. These included crack detection gages, concrete surface strain gauges,and LVDTs, as well as visual observations. Use of data from the strain gauges placed atthe bottom surface of the girders was determined to be the most effective way ofdetecting flexural crack initiation and re-opening. Cracking loads determined fromvisual observation were significantly larger than those determined from the straingauge data.The back-calculated prestress losses determined from the measured flexural crackinitiation and re-opening loads of the girders were significantly larger than thosedetermined from other experimental methods and those predicted by the PCICommittee and AASHTO LRFD methods. Losses determined by the other experimentalmethods and the predictions correlated more closely with those back-calculated usingvisually-observed cracking loads.These results indicate that prediction of losses based on the measured flexuralcracking and crack re-opening loads using the basic theory of mechanics results in anoverestimation of prestress losses. Consequently, girders may undergo flexuralcracking and crack re-opening at lower loads than predicted using the basic theory ofmechanics.

This paper relates the historical development and use of single strandunbonded post-tensioning tendons in parking structures constructed in deicing saltregions in North America since the early 1960’s. These parking structures wereconstructed in southeast Canada and the northeastern and midwestern United Statesusing wire and strand tendons as the primary flexural reinforcing and were exposed todeicing salts during the winter. Parking structures exposed to ocean salts have notbeen reviewed. Most of the following discussion, however, will also apply to them.

A survey of the first uses of precast, prestressed or post-tensioned foldedplates roofs show that this structural system start in the 1950’s and end in the 1970’s Ahistoric perspective of this structural system will be presented starting in the 1950’s andending in the 1970’s. While prestressed concrete was making a break-through in theUnited States with the Walnut Lane Bridge in Philadelphia, PA, Europe was usingprestressed concrete in various types of structural systems. One of these systems wasthe folded plate roof system. Approximately a decade later in the United States, theCloverleaf Lanes Bowling Alley in Dade County, Florida used the same application inwhich the corrugated slab spanned 120 feet and extending transversely 286 feet. In1962, this system of folded plate shells went from long span structures to being used inroof structures for dormitories at Washington State University. The main beams for thedormitories were pretensioned in a factory, while the secondary beams were post-tensioned on site. During the 1970’s, precast folded plate structures were beingconstructed throughout the United States for various types of buildings, one notablestructure is the Hangar for Allegheny Airlines at Logan Airport, Boston, Massachusetts.Then they disappeared in the United States. Some Possible reasons why folded plateconstruction stopped are numerous but the two main factors are the architecturalsolution, and the lack of understanding of folded plate structures on the part ofengineers and architects.

In the early 1940’s Prof. G. Magnel performed extensive researchprogrammes on real scale prestressed concrete beams at Ghent University (Belgium) inorder to elaborate design methods for this new material. He also developed his ownanchorage system which was used until the mid 60’s in Belgium. He gave manylectures in several countries in which he explained in a simple way the principles ofprestressed concrete. He was also instrumental in the design of the first prestressedconcrete bridge in the USA, the Walnut Lane Bridge in Philadelphia and he was theauthor of the first English textbook on prestressed concrete. He designed one of thefirst PC railway bridges in Europe and the first statically indeterminate PC bridge in theworld. In the 1950’s many engineers from abroad spent some time in Magnels lab inGhent to perform research and to get acquainted with practical realizations.

This paper provides an overview of the ACI 440.4R-04 document on“Prestressing Concrete with FRP Tendons” reported by ACI Committee 440 on “Fiber-Reinforced Polymer Reinforcements”. The document is one of the Emerging TechnologySeries published by ACI. The paper outlines the content of the document and thephilosophy of applying FRP technology as opposed to conventional steel forprestressing. The document offers general information on the history and use of FRP forprestressing applications, and a description of the unique material properties of FRP. Italso focuses on the current state of design, development, and research needed tocharacterize and ensure the performance of FRP as prestressing tendons in concretestructures. The proposed guidelines are based on knowledge gained from worldwideexperimental research, analytical work, and field applications of FRPs used asprestressed tendon. Current developments in the document include a basicunderstanding of flexural and axial prestressed members with FRP tendons, FRP shearreinforcement, bond of FRP tendons, and unbonded or external FRP tendons forprestressing applications. The document concludes with a list of research needs.