International Concrete Abstracts Portal

Due to increased traffic loads and changes in the code provisions many highway bridges in Germany exhibit deficits in shear capacity according to current codes. The majority of these bridges’ structures comprises continuous concrete beams whose calculatory shear capacity is often exceeded by now. However, the actual shear capacity of prestressed concrete continuous beams is usually underestimated since the design procedures have been derived on the basis of single-span beam tests and neglect significant shear transfer mechanisms. In order to extend the service life of existing bridges, the reserves in the design procedures can be partially taken advantage of by the application of refined design approaches. For this reason, five shear tests on prestressed concrete continuous beams have been performed at the Institute of Structural Concrete of RWTH Aachen University in Germany. Within these tests, the influence of cross-section type (rectangular and I-shaped cross-section), load distribution (concentrated and distributed loads) and the shear reinforcement ratio are investigated. In this paper, the test results of three beams under concentrated loads will be presented.

This paper presents an on-going research program to develop an effective strengthening method using post-tensioned near-surface-mounted (NSM) carbon fiber reinforced polymer (CFPP) composites for constructed bridges girders. Various technical aspects associated with strengthened girders are examined through computational modeling, laboratory experiments (small- and full-scale tests), and a field project that is the world’s first site application of post-tensioned NSM CFRP. The flexural behavior of the bridge girders is improved by strengthening (that is, cracking, yield, and ultimate loads, as well as serviceability) relative to unstrengthened control girders, and the post-tensioned NSM CFRP should cover at least 60% of the girder length. The influence of CFRP post-tensioning on the girder concrete adjacent to the anchorage exponentially decays and becomes negligible beyond a distance of 800 mm (31 in.), irrespective of girder size. The presence of initial damage in the girder does not affect the efficiency of the strengthening system until failure occurs. The site application is dedicated to upgrading the design live load capacity of a 56-year old bridge in South Korea from 318 kN (72 kips) to 424 kN (95 kips). Step-by-step procedures are detailed for the technology transfer. Long-term performance monitoring for this upgraded bridge is underway and corresponding results will be reported when sufficient data are available.

The span length of precast prestressed concrete girder bridges is typically limited to 140–160 ft (43–49 m) due to handling and transportation restrictions on individual girder segments. Span lengths may be doubled by splicing individual girder segments within the spans to form a continuous bridge. A design for a three-span continuous prototype bridge with a 240 ft (73 m) main span and 190 ft (58 m) end spans using modified Tx70 precast concrete girders has been developed. A full-scale experimental study investigated the performance of the prototype bridge details in the splice region under service and ultimate loads. The tested splice connection details were selected to represent critical design parameters. The splice connections performed well under service level loads. However, the lack of continuity of the pretensioning through the splice connection region had a significant impact on the behavior at higher loads approaching ultimate conditions. Moderate ductility was observed for positive bending with low ductility for negative moment. Ideally, spliced connections should be located in regions of low moment demands, away from the peak positive or negative moments. Improved connection behavior at ultimate conditions is expected through enhanced connection details, and several detailing suggestions are discussed.

The title of this paper borrows from the 1981 book by Harold Kushner entitled “When Bad Things Happen to Good People”. In his book, Kushner attempts to explain why a universe created by a deity who is of a good and loving nature still holds so much pain and suffering for good people. In the context of this paper, the title is meant to be an epigraph that suggests that although a building may be meeting its intended structural purpose, bad things, at least as they are perceived, can happen during design, construction and service of the building that bring its safety into question. One of the main circumstances that can bring into question the integrity of an unbonded post-tensioned building is corrosion of the strands and anchorage components. This paper will highlight the unnecessary demise of a modern high-rise post-tensioned structure due to corrosion, and contrast that outcome to several existing unbonded post-tensioned buildings that experienced corrosion and were successfully repaired and continue to function.

Un-bonded post-tensioned slabs were developed and principally flourished in the USA since the mid 1950’s. The continuous un-bonded one-way post-tensioned slabs became popular due to their predominant use in parking structures all over the country which is true even now. Two-way un-bonded post-tensioned slabs, mainly flat-plates and flat-slabs gained popularity since mid 1960’s. The use of a Banded System of placement of un-bonded post-tensioning tendons, introduced in the early 1970’s, made flat-plate and flat-slabs more competitive. Flat-plate/slab and shear-wall system became and remains very popular for mid-rise and hi-rise buildings. A brief discussion on the development of un-bonded post-tensioned slabs and its relevance in current design and construction is made in part-1 of this paper. Many of these slabs are now between 30 to 60 years old. These structures need routine repairing and retrofitting work. Existing methods are labor intensive and expensive. An alternative method could be repair work with composite materials. Use of composites (mainly CFRP) as a repair material for concrete structures is becoming very common. Most of the repair procedures are based on researches with reinforced concrete specimens and in some cases with pre-tensioned specimens. Research work using un-bonded post-tensioned specimens, especially two-way slabs is practically non-existent. A testing program was developed with the goal of finding the cracking and ultimate strength behavior of un-bonded post-tensioned slabs (before and after repair with CFRP) with different boundary conditions. A total of six slabs were tested. In the first phase two two-way simply supported un-bonded post-tensioned slabs were tested. In these tests CFRP repair configurations were varied. In one case CFRP was placed across the cracks in another case it was orthogonal (parallel to the edges). In the second phase, four more slabs were tested (three one-way slabs and one two-way slab). One one-way slab had two ends fixed, another one had one end fixed and one end simply supported and the third one had both ends simply supported. The CFRP placement configuration in these three slabs varied. CFRP was placed across the cracks in the supports and mid spans. The fourth slab was a two-way slab with simple support. In this case the CFRP repair configuration was similar to the first slab (but CFRP had 2 inches/5 cm overlaps). Sketches of different cracking patterns and CFRP configurations are shown inside. The repair and testing of slabs is discussed in part-2 of this paper.