Thermal Integrity Profiler, RB13-013, 2019

(2019) Thermal Integrity Profiler, RB13-013, 2019. Transportation, Department of

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Abstract

Assessing the structural integrity of bridge foundations is critical to ensuring the reliability of the foundations and superstructure, as well as the safety of the traveling public. However, nondestructive methods currently used in practice to determine the integrity of reinforced concrete drilled shaft foundations are limited by their inability to provide full coverage of the foundation cross-section, particularly in the vital region outside of the rebar cage. The concrete cover outside the rebar cage is critical because: (1) it provides the soil-structure interface over which stresses are transferred to the soil to develop the geotechnical resistance and geotechnical capacity, (2) it offers protective cover for the rebar cage, and (3) it is the region of the cross section that resists the maximum bending stresses. Currently, the most widely used testing methods for quality assurance of drilled shafts are the cross-hole sonic logging (CSL) and gamma-gamma logging (GGL) methods. For example, the Iowa DOT requires CSL tests for all drilled shafts supporting bridges, light towers, and sign structures. However, the CSL method can only detect flaws located between pairs of source and receiver CSL pipes, and not in the critical concrete zone outside the rebar cage. CSL results are also adversely affected by de-bonding of the access pipes, which can increase travel time and thus falsely indicate shaft defects or low quality concrete. The nuclear GGL method can detect flaws outside the rebar cage, but only within a 3 to 4-inch radius around each access pipe. Therefore, neither CSL nor GGL tests enable assessment of the concrete quality over 100% of the cross-sectional areas of typical drilled shafts. The relatively newer Thermal Integrity Profiling (TIP) method was developed to overcome these limitations, by employing measurements of the heat of hydration of curing concrete. The temperature measurements can be made using either sacrificial wires of thermal sensors cast into the foundation, or an infra-red thermal probe lowered into access pipes cast into the foundation. Several previous studies have demonstrated that the technique can detect internal flaws such as soil inclusions or voids, as well as bulging, necking, and loss of concrete cover outside the rebar cage. However, the studies did not assess the accuracy of the technique for indicating the specific location, size, and general shape of the flaws.