Prestressed cored slabs have been used in North Carolina since 1969 for bridges with span lengths of 40 ft to 70 ft. Cored slabs in coastal regions are showing signs of extensive corrosion of reinforcing steel after less than 40 years in service. Extensive patching, rust stains, and widely varying visual conditions has led to uncertainty in how to accurately load rate these prestressed cored slabs. In order to develop guidelines to accurately assess their flexural strength, an experimental program was implemented including field inspection of cored slabs on two in-service coastal bridges, followed by selection of 12 slabs and their transportation to the NCSU Constructed Facilities Laboratory for non-destructive testing and flexural testing to failure.
Field inspection included recording the visual condition of the slabs, sounding for delamination, and concrete resistivity testing. Laboratory testing included obtaining concrete cores and samples of prestressing strands, more detailed concrete resistivity testing, half-cell potential testing, and finally flexural testing to failure and demolition of the slabs to examine the extent of corrosion.
Test results were used to develop recommendations and guidelines for uniformly interpreting slab conditions based on their visual condition, sounding for regions of delaminated concrete and patching, and records of the depth of spalling. The depth of spalling and presence of longitudinal rust stains were found to be key indicators of corrosion of prestressing strands. Corrosion was limited to the bottom layer of prestressing strands in most slabs, and strands were observed to generally corrode in groups of adjacent strands. The presence of spalling alone does not necessarily indicate loss of strands, nor does the area of a slab that is spalled. Due to the severe and extensive corrosion, concrete resistivity and half-cell potential testing did not seem to provide additional information to the visual inspection.
