Instrumentation and field testing procedures for testing integral abutments on prestressed concrete piles

Since the 1960's the Tennessee Department of Transportation has designed and constructed over 1000 jointless bridges throughout the state. An item of particular interest to TDOT concerning jointless bridges is their response to thermal lateral loading. More specifically, the behavior of the pile supporting the integral abutment is of particular interest. The University of Tennessee was initially contracted by TDOT to investigate the response of steel piles supporting integral abutments exposed to thermal lateral loading. After this research was complete, TDOT decided to repeat the tests using prestressed concrete piles. The testing of the steel and concrete piles became known as Phase I and Phase II, respectively. Full scale lateral load tests were performed on 14 in. x 14 in. x 40 ft. prestressed concrete piles supporting concrete abutments. However, instrumentation and field testing procedures needed to be established before testing began. Much of the instrumentation and many of the field testing procedures used in Phase I of the research were integrated into Phase II. Instrumentation of the concrete pile was accomplished with two types of gages, a foil strain gage and a DEMEC gage which also measures strain. Both were expoxied to the section of the pile extending from the ground to the pile/abutment interface. Each instrument was verified for accuracy in the laboratory before field testing. In addition to the strain gages, LVDT's were used to measure displacement of the pile and abutment. All of the instrumentation except the DEMEC gages were wired into the MEGADAC. The DEMEC gage required manual readings. The Optim MEGADAC in conjunction with the Test Control Software (TCS) used in Phase I was integrated into Phase II. The MEGADAC converts the signals coming from the instrumentation into useful data and then communicates with TCS which collects and organizes the information for analysis. Since the piles were not instrumented along their length (They were in Phase I), there was significantly less instrumentation and therefore less data. Preliminary data analysis uncovered some areas that needed improvement. Most importantly, the hold-down apparatus was unbalanced and caused a large moment even at rest. Analysis also verified the proper functioning of the foil and DEMEC gages. However, since the system was statically determinate and the loads were known, statics provided the easiest method for calculating moments.