Researchers prepare for a two-truck live load test in order toreach one of the objectives of the Montana highway project, whichis to compare load-carrying mechanisms of three different bridgedecks near Saco.
Amidst a vast expanse of farming and range lands, bridgedesigners from the Montana Department of Transportation(MDT)recognized a unique opportunity to evaluate highway bridgedeck design. Near the small town of Saco, Montana, three bridgeswere constructed less than one mile apart along the same highwaycorridor, Montana State Route 243. Consequently, each bridgewill experience identical demands from weather, traffic, and wintermaintenance. Each bridge has three 15 m spans, incorporatinga rein-forced concrete deck-on-pre stressed-stringer design withidentical global dimensions and substructure components. Theonly difference among them is the composition of the rein-forcedconcrete deck. The "Conventional" deck represents thestandard deck used in Montana – a conventional concrete with astandard rebar pattern. The "Empirical" deck also usesconventional concrete, but employs approximately one-third theamount of reinforcing steel. The "HPC" deck uses a standardrebar pat-tern with a high-strength concrete. MDT contracted withThe Western Transportation Institute (WTI) at Montana StateUniversity to design and install an instrumentation system, acquiredata from this system and perform subsequent analysis to evaluatethe performance of the three bridge decks.
Spread Spectrum radios and an Internet link transmit data frombridges to MSU.
Instrumentation
During the planning phases of the project, finite elementanalysis was done on the three deck configurations to identifycritical locations in the bridge decks for placing straingages. Based on this analysis, the instrumentation isprimarily concentrated in a single driving lane of one 15 m span ofeach three-span bridge (approx.1/6th of the bridge). Thestrain gages were placed in each bridge deck prior to casting theconcrete. The gages were positioned to monitor bothlongitudinal and transverse strains at three different depthsthrough the thickness of the deck. Three different strain gagetechnologies were used in each bridge: 35 Vishay foil strain gagesbonded to reinforcing steel, 7Vishay embedment-type strain gagessuspended in the concrete and 16 Geokon vibrating wire strain gagessuspended in the concrete. Additionally, 16 temperatures arerecorded via thermistors internal to the vibrating wire straingages. To monitor ambient conditions, a weather station waserected at the Saco Public School, located approximately one milefrom the bridge sites. Temperature, barometric pressure, windspeed/direction, and relative humidity values are measured at15minute intervals and posted to the Internet for public viewingathttp://wtigis.coe.montana.edu/saco/Saco_Current.htm
Typical strain output for a series of gages over the bent duringa single-truck, live load test.
Data Acquisition
All strain data is collected and stored using a single CR5000Measurement and Control System mounted under each bridge. Thebonded and embedded gages require Wheatstone Bridge arrangementsdesigned and fabricated by WTI. Corresponding voltages are routedthrough a single AM16/32 multiplexer. Vibrating wire strains andtemperatures are read using a single AVW1 Vibrating Wire Interfacecoupled with an AM16/32multiplexer. All gages are read onceevery hour. Data from each bridge is periodically transmittedto WTI through the Internet via a network of RF400Spread Spectrumradios based at the Saco School. Weather data is monitoredusing a CR10X Measurement and Control System and transmitteddirectly to WTI via the Internet.
Long-Term Evaluation
One of the primary objectives of the Saco Bridge Deck Evaluationis the comparative analysis of long-term bridge deckperformance. This objective will be accomplished throughqualitative and quantitative analyses of bridge deck conditionsover a two-year period. Ideally, the project will be extendedfor several years to more fully evaluate relative deck performancethroughout their service lives. Qualitative analyses includecrack mapping and surveying operations. Quantitative analyses aremostly focused on how strains in the bridge decks change whenexposed to changing weather conditions and vehicle loadevents. In addition to hourly monitoring of deck conditions,a subset of the strain gages remains continually active to capture"large vehicle events." When calibrated against portableweigh-in-motion (WIM) traffic records, large event data will offeran estimate of the cumulative demands placed on the bridges fromtraffic loading.
Live Load Testing
A second objective of this project is to compare the loadcarrying mechanisms of the three bridge decks. This objectiveis being realized by a series of live load tests. Beforebeing opened to traffic, heavily loaded vehicles were driven slowlyalong the full length of each bridge along several differentlongitudinal paths. During each test event, 41 channels ofrebar-bonded and concrete-embedment strain gage data wererecorded. To accommodate the number of sensors in each bridgeand a rapid data acquisition rate, all three CR5000s weresimultaneously used during live load testing. Longitudinalpositioning of the truck during the test was recorded via ahand-held electronic button, engaged as the test truck traveledevery two meters along the deck. In addition to the localdeck strains recorded by the internal strain gages, globalbehaviors were monitored using gages temporarily affixed to thebottom of the stringers, supplied by Bridge Diagnostics, Inc.(BDI). A majority of the tests were conducted using onetandem-axle dump truck. To simulate "worst case" behavior,two vehicles were driven side-by-side along the length of thebridge. High-speed tests were also con-ducted using a singletruck traversing the bridge at 60 mph. A second live loadtestis scheduled to take place after the bridge shave been inservice for two years.
Outcome
At present, comparative investigation of bridge deck behaviorwith this level of detail and control of variables has beenlimited. This study presents a unique opportunity to developa better under-standing of local reinforced concrete bridge deckbehaviors and load paths through the deck and substructure.Researchers and departments of transportation alike will benefitfrom a heightened understanding of bridge deck performance undervehicle loading and varying, long-term weather conditions.Application of this knowledge will lead to better planning forinfrastructure health as well as improved bridge deck design.