Infrasense completed a ground penetrating radar (GPR) survey to map the depth of steel reinforcing bars within the new Loon Mountain Resort bridge in Lincoln, NH. The newly constructed bridge replaces one that was compromised and partially collapsed in 2011 due to high waters of the Pemigewasset River that it spans during Tropical Storm Irene. A temporary bridge has been providing access to the resort during the construction of the new Bridge, which was nearly completed in June 2016, at the time of the survey.
The objective of the survey was to map the depth of steel reinforcing bars within the travel lanes and sidewalk to verify the proper concrete cover thickness per the NHDOT 5126.96.36.199.3.6 quality assurance specification, which requires that the rebar/concrete cover depth be verified and recorded prior to opening the bridge to traffic. GPR was the preferred method for this survey because of its ability to image accurately measure the location and depth of objects within a material (concrete) without the need for cutting, drilling, or coring.
GPR data were collected along the centerlines of the three planned travel lanes and along the centerline of the sidewalk, including the new deck area as well as the approach slabs totaling approximately 1300 linear feet across 4-alignments. The data collection was completed using a 1.5-GHz antenna and a GSSI SIR-20 GPR control unit, providing efficient and accurate results, requiring less than 2-hours to complete.