SEI was contracted to perform GPR investigations of outdoor balconies at a residential condominium building to verify the depth of structural steel rebar within the concrete balconies.  Balcony construction consisted of a concrete pad reinforced with structural steel with wrought iron railing, exposed on all sides, and overlooking views of the city and the intracoastal waterway.  Concrete spalling and exposed steel was observed at a few of the balcony levels, resulting in concerns of the residents as to the structural integrity of the balconies.  A total of 321 individual balconies were scanned, including 14 penthouse locations.

The purpose of the GPR survey was to verify the depth of the rebar steel in the concrete for each balcony from the floor surface and outer edge in reference to building code specifications that required a minimum of one (1) inch of concrete coverage. 

A high frequency, high resolution 1,500 Megahertz (MHz) GPR antenna and a SIR-3000 digital control unit were employed to map the structural steel.  This equipment is ideal for identifying rebar, tendons, conduit, mesh, and voids within concrete.  The SIR-3000 control unit allows the technician to perform highly detailed analysis of the location, depth, and response of the radar signal to the target structure.  The equipment is manufactured by GSSI of New Salem, New Hampshire.  Distance control of the GPR transects was accomplished by use of a calibrated survey wheel attached to the GPR antenna. 

Depth control (to top of rebar) was accomplished by calibrating the dielectric constant (and thus the radar wave velocity) of the concrete to correlate with verified rebar depths from concrete boring data.  Scanning of the balcony edges was challenging and required the use of ladders and safety harnesses to secure personnel and equipment.  Each balcony was scanned from the bottom, top and outer sides to ensure complete imaging of the structural steel orientations.  

The GPR survey was performed with minimal intrusion to the residents and non-destructive testing of the balcony structures resulting in a cost-effective investigation with highly detailed analysis of the concrete structural steel, in accordance with the project goals.  SEI was able to accurately map the depths to steel reinforcement structures of the condominium balconies, providing the client with information to verify building code compliance.