After a successful installation and very satisfactory results obtained by the SOFO system from the Rio Puerco bridge monitoring project, the Highway Department mandated once again the New State Mexico University to perform a structural monitoring, this time on a more important interstate highway bridge I-10, close to Las Cruces, New Mexico.
The I-10 bridge consists of five spans, and each span of six lines of pre-fabricated pre-stressed squeezed girders, with a cast-on-site superstructure. The girders cross-section has a “U” shape with wings. The girders were cast over pre-stressed strands in a prefabrication plant, and then steam cured over two days. After the cure was finished the strands were cut and the pre-stressing force brought in to the girders. The girders were stored for two months and then transported on-site where they were put in place and covered by cast-on-site superstructure slab.
Objectifs de contrôle:
The aim of monitoring is to increase the knowledge of the structural behaviour, to verify hypothesis and to insure safety. The monitoring parameters are average strain, average shear strain, average curvature, deformed shape and pre-stress losses. For this purpose it was decided to equip all the girders of the fifth span, laying on abutment, with sensors in different configurations.
First two girders were fully equipped with sensors. Crossed sensors necessary to evaluate the average shear strain were installed in the cross-section closest to abutment. Three non-coplanar sensors were installed in five cross-sections over girders in order to monitor both horizontal and vertical deformed shape. Finally the thermocouples were installed in three cross-sections in order to measure the temperature variations necessary to separate thermally generated strain from structural strain. Other girders were equipped with fewer sensors that are used as control and redundancy. Data analysis is performed using the SOFO SPADS software.
The sensors were embedded in the girders during the fabrication. Thus they provided for full-life measurements of girders, including the very early age and pre-stressing. The system is fully centralized, and measurements are performed automatically from a control room built on-site.
Measurements started immediately after the pouring. In this way the early and very early age deformation were recorded during the first three days. The deformation is later recorded during the prestress phase, after each strand was cut. Thus, real initial strain state of girders was stored. Followed the period of continuous monitoring before transportation on-site, during transportation and during the pouring of the deck. In present, long-term monitoring is carried on. The results helped comparison with different theoretical models and confirmed very good condition of the bridge after construction.
In addition concrete mechanical properties were caclulated. Empirical equations originally created for regular strength concrete are not necessarily applicable to high performance concrete and the ACI 363 revised equation for HPC and the PCI Design Handbook equation for HPC were found to be the most accurate to the true value. The coefficient of thermal expansion for the HPC study bridge fell within the accepted range for regular strength concrete, supporting its use for HPC.