International Concrete Abstracts Portal

Load testing of concrete bridges is a practice with a long history. Historically, and particularly before the unification of design and construction practices through codes, load testing was performed to show the travelling public that a newly built bridge was safe for use. Nowadays, with the aging infrastructure and increasing loads in developed countries, load testing is performed mostly for existing structures either as diagnostic or proof tests. For newly built bridges, diagnostic load testing may be required as a verification of design assumptions, particularly for atypical bridge materials, designs, or geometries. For existing bridges, diagnostic load testing may be used to improve analysis assumptions such as composite action between girders and deck, and contribution of parapets and other nonstructural members to stiffness. Proof load testing may be used to demonstrate that a structure can carry a given load when there are doubts with regard to the effect of material degradation, or when sufficient information about the structure is lacking to carry out an analytical assessment.

Recurrent service problems and uncertainties in load distribution have been frequently reported by Departments of Transportation for skewed bridges. Service problems, such as deck cracking or excessive bridge racking can lead to bridge deterioration, and indicate the need of a better understanding of the structural response of high skew bridges to service loading. This paper presents the instrumentation and load testing of a three-span, medium span length, prestressed concrete bridge with 64° of skew to understand service, analysis and design problems associated with skew. The instrumentation plan for the bridge was developed based on service problems observed in concrete bridges with high skew such as deck cracking and displacements, as reported by the literature and by regular bridge inspections. Complete understanding of skew related responses required both short-term testing and long-term load monitoring. Structural responses of the key areas of the bridge to live and temperature loads and shrinkage were measured. The effects of certain bridge details on live load distribution were determined using finite element models validated through short-term load testing data. The evolution and magnitude of bearing movements and deck strains were captured for long periods of opposite thermal tendencies.

A large subset of the Dutch bridge stock consists of reinforced concrete slab bridges, for which assessment often results in low ratings. To prioritize the efforts of the bridge owner, more suitable assessment methods for slab bridges are necessary. Research efforts over the past years resulted in the development of several methods, at levels requiring increasing costs, time, and effort for increasing accuracy. The last option, when an analytical assessment is not possible due to uncertainties, is to use proof load testing to evaluate the bridge directly. To develop recommendations for the proof load testing of reinforced concrete slab bridges for the Netherlands, different methods are combined: pilot proof load tests on bridges with and without material damage, a collapse test, tests on beams taken from an existing bridge and new beams with similar dimensions cast in the laboratory, and an extensive literature review. The result of this study is a set of recommendations that describe how to prepare and execute a proof load test, and how to analyze the results. This paper summarizes the research program about proof load testing from the Netherlands and gives an overview of the currently developed recommendations and topics for further research.

Load testing and structural monitoring facilitated the passage of several super-heavy permit loads at the Burns Harbor access bridge near Portage, IN. Twenty super-heavy permit loads, with gross vehicle weights reaching 848 kips (3770 kN), were required to cross the bridge, which was the only feasible route out of the port. Preliminary load ratings were acceptable due to three factors; the specialized transport’s large footprint effectively distributed load, the bridge was designed for Michigan Truck Trains, and the bridge was assumed to be in good condition. The last condition came into question due to significant cracks throughout the prestressed concrete girders caused by delayed ettringite formation (DEF). While DEF cracks were a function of improper curing and not related to live-load effects, the Indiana Department of Transportation (INDOT) was concerned that repeated heavy loads would negatively influence cracks and the bridge’s overall long-term performance. Due to the cargo’s importance to the local community and lack of an alternate route, INDOT allowed use of the bridge after load tests proved that the transports would not cause damage or reduce the bridge’s service life. Structural monitoring performed during the entire transport period verified structural performance was not diminished during the numerous crossings.

Nowadays, finite element analyses provide information about the performance of a structure, but they are more or less simplified. Therefore, load tests are the only way to find the “real” behavior of an existing bridge subjected to the rating process. In this paper, the state-of-the-art concerning load tests of concrete road bridges is presented, and the problems of the execution of such tests are specified. It is pointed out that only load tests accompanied with current finite element analyses may result in a proper assessment of the level of safety of the bridge. The authors’ procedure of complex assessment of such bridges combines in-situ examination of the structure, load testing, and finite element modeling. The paper discusses the following topics: aims and fundamentals of static diagnostic and proof load tests; the load application method according to different codes and specifications; the basis for proper assessment of the target load: reliability index, partial factors approach, global rating factor approach; establishing the load allowable on the bridge, based on the applied proof load; and the proposed procedure of assessment of existing concrete road bridges by load testing.