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.

Off-system concrete bridges with no design plans are currently a concern in New Mexico as many exist throughout the State. Load rating these bridges is problematic since the design documentation is limited or missing, thus creating uncertainties regarding the safe load limits. Two prestressed concrete bridges, a double T-beam and box beam bridge, were evaluated using advanced analyses and experimental methods (including load testing and rebar scanning). Both bridges have damaged and/or missing shear keys between the adjacent beams and thus, the load distribution path was uncertain. The bridges were evaluated based on the following procedures: estimating the material properties from past specifications and amount of prestressing steel via Magnel diagrams; verifying the steel estimate with a rebar scanner; load testing based on strain measurements; and rating the bridges using the load test results. Rating factors were determined for legal loads based on serviceability (i.e., concrete cracking) and strength (i.e., shear or flexural capacity). The serviceability ratings from load testing and strength ratings from analytical software were compared to determine the final load ratings and need for posting the bridges.

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.

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.

This paper presents the current practice of bridge load testing in Germany. At first a brief summary of the historical value and meaning of load testing bridges at commissioning is given and also accidental over-load tests are described. While load testing of brides as part of the commissioning is common in many European countries today, in Germany load testing is only permitted in exceptional cases and to evaluate damage or questioned existing bridges. While proof loading is rare, short and long-time test under serviceability loads are a common engineering task in Germany. Especially the calibration and verification of numerical models by load testing shows a high potential for the future. As the infrastructure gets older and the traffic loads increase a high demand for experimental evaluation methods becomes more important and request further research work to answer open questions. The four given examples of load tests demonstrate possible applications and different tasks for load testing of concrete bridges.