Prestressed concrete

Four concrete piles prestressed with Carbon Fiber Reinforced Plastics were cast, in which two piles were fabricated with CFRP transverse reinforcement. The remaining two were provided with transverse steel spiral reinforcement. The piles were designed according to Florida Department of Transportation (FDOT) guidelines. The Pile Driving Analyzer (PDA) was chosen as the primary data acquisition system for the pile driving due to its mobility, reliability and robustness based on the high frequency excitation. The Pre-driving analysis consisted of several stages. The estimated static bearing capacity of the experimental piles was first calculated followed by SPT sampling at the pile driving site to obtain the soil conditions. The percent skin and toe friction, ultimate capacities, driving system parameters, maximum displacements, energy, integrity, tensions and static capacity were determined prior to pile driving. The piles were then driven and the data from the pile driving compared with the analysis.

Experiments were conducted to evaluate occurrence of any deterioration of prestressing steel tendon to concrete bond as a consequence of cathodic polarization. Pretensionned concrete specimens were cathodically polarized with current densities ranging from 50 to 5000 m^2 of steel while exposed to a constant flow of natural sea water. The concrete and steel dimensional changes were monitored by strain gages mounted on the tendons and embedded in the concrete. Contractions of the steel of 25 to 50 percent of the initial tensioning were recorded after 17 to 36 MC/m^2 were transferred to the tendons on specimens polarized at the highest currents. This corresponds to 54 to 114 years of polarization at 10mA/m^2 if bond loss was solely dependent on the charge transfer. It was noticed that the smaller the current, the more the charge that was transferred before steel contraction began. These results imply that cathodic polarization should impose no threat to the prestressing steel-to-concrete bond on typical structures over their expected lifetime.

The flexural behavior of rectangular concrete beams strengthened with externally bonded Carbon Fiber Reinforced Plastic (CFRP) laminates was studied by varying the number of plates bonded to their bottom tensile face. The increases in strength and stiffness of the beams provided by the bonded plates, over control beam without CFRP plates, were evaluated. Failure loads of the beams were determined by the ACI strain compatibility method using a FORTRAN software developed for this purpose. The predicted collapsed load agrees reasonably well with the actual failure load. Precracked solid and voided slab bridge models retrofitted with varying number of CFRP laminates were used to evaluate their contribution to the flexural resistance. The increases in strength and stiffness of the retrofitted slabs were based on the deflections, strains and crack patterns at ultimate load. Theoretical analyses to predict the load-deflection behavior of the precracked sections were performed using PCFRAME software. The predicted values agree reasonably well with the experimental results.

The acoustic emission (AE) testing reported herein was
conducted on a large size prestressed concrete slab placed
in contact with sea water in a state of electrically induced
accelerated corrosion. AE signals were monitored and
successfully analyzed in an attempt to evaluate the severity
of the deformation process in the concrete as a result of
the corrosion induced cracking. Several features of the AE
data which were sensitive to the process were plotted to
show the different levels of the cracking due to the
corrosion. These were amplitude, counts and energy
distributions, and event distribution with time. A location
test was employed to find the source of the activities. The
results of the amplitude distributions were found to have
similar characteristics to those obtained from the
reinforced concrete AE experiments performed at Florida
Atlantic University (1,2). These tests can be easily
applied to a field location for an early detection of the
deformation process in the concrete structures.