Development of techniques for in-situ stress assessment in concrete structures

Abstract

Reinforced and prestressed concrete are widely used for the construction of major civil engineering structures. Concrete structures undergo distress with time due to environmental and other unfavorable operating conditions. To assess the safety and serviceability of the distressed concrete structure and to take decision on the possible repair measures, it is necessary to reliably estimate the existing level of stress. Determination of in-situ stress on the concrete surface is one way to assess the prestressing force available in the prestressed concrete members. Assessing the existing level of stress in prestressed concrete structures in service is fairly a difficult task and theengineers are often faced with lack of actual design / construction details and environmental service conditions. In-situ stress determination is based on the measurement of strain release due to local elastic stress relief, caused by core drilling and creation of stress-free boundaries.Three techniques, namely concrete core-drilling technique, concrete core trepanning technique and concrete core-drilling strain gage technique (CDSG) were developed to evaluate the in-situ stress under uniaxial and biaxial stress conditions. Concrete core-drilling technique is an experimental technique for assessment of in-situ stress under uniaxial stress condition. This technique wasdeveloped by using a special arrangement of electrical resistance strain gages suitably placed along radial- and tangential- directions of the intended core. These gages are connected through a Wheatstone bridge circuit, in full bridge configuration, to magnify the response of the measured strain. A 50mm diameter core is drilled on the structure up to 50mm depth and the strain perturbations around the core are measured to evaluate the existing stress. Laboratory studies were conducted to evaluate the reliability of this concrete core drilling technique. Calibration constants evaluated based on experiments are compared with the values evaluated using numerical analysis.