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.

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.

Current codes often underestimate the capacity of existing bridges. The purpose of the tests presented here has been to assess the real behaviour and capacity of three types of bridges in order to be able to utilize them in a more efficient way. The three studied bridges are: (1) Lautajokk – A one-span trough bridge tested in fatigue to check the shear capacity of the section between the slab and the girders; (2) Övik – A two span trough bridge strengthened with Near Surface Mounted Reinforcement (NSMR) of Carbon Fibre Reinforced Polymers (CFRP) tested in bending, shear and torsion; and (3) Kiruna – A five-span prestressed three girder bridge tested to shear-bending failures in the girders and in the slab. The failure capacities were considerably higher than what the code methods indicated. With calibrated and stepwise refined finite element models, it was possible to capture the real behaviour of the bridges. The experiences and methods may be useful in assessment and better use of other bridges.

Torsion due to superimposed loads is often ignored in prestressed concrete bridge girders because it is considered negligible compared to other forces that control the structural design. However, during load testing of prestressed concrete girder bridges, shear strains due to torsion can be on the same order of magnitude as shear strains due to the vertical shear force resultant for superimposed loads. The inability to differentiate between the two types of shear strains may lead to inaccuracy when determining the vertical shear force distribution in statically indeterminate bridge structures. Rosette strain gages need to be placed on both sides of the girder web to differentiate between torsion and vertical shear to characterize the shear distribution. The need for this instrumentation configuration likely applies to other studies in the literature that have calculated shear force through the use of rosette strain gages on only one side of prestressed concrete girder webs in bridges. This paper discusses best practices to quantify shear distribution data. The study included tests and finite element modeling of laboratory and field bridges.

Self-consolidating concrete (SCC) has emerged as an alternative to build stronger structures with longer service life. Despite the advantages of using SCC, there are some concerns related to its service performance. The effect of a smaller coarse aggregate size and larger paste content is of special interest. It is fundamental to monitor the response to service loads of infrastructure employing SCC in prestressed concrete members. Bridge A7957 was built employing normal-strength and high-strength self-consolidating concrete in its main supporting members. The diagnostic test protocol implemented in this research included static and dynamic tests and the calibration of refined finite element models simulating the static loads acting on the structure during the first series of diagnostic tests. The main objective of this study centered on (a) presenting a diagnostic test protocol using robust and reliable measurement devices (including noncontact laser technology) to record the bridge’s initial service response; and (b) obtaining the initial spans’ performance to evaluate and compare the SCC versus conventional concrete girders’ response when subjected to service loads. The initial response of the end spans (similar geometry and target compressive strength, but with girders fabricated using concrete of different rheology) was compared, and no significant difference was observed.