Non-Contact Strain Measurements of Steel Reinforcement in Concrete Structures

Abstract

Engineers currently lack the ability to non-destructively measure through-thickness elastic and plastic strain of steel reinforcement in reinforced concrete structures. This capability would be of considerable use in assessing structures damaged by extreme events, corrosion, or slower-acting expansive reactions in concrete such as alkali-silica reaction (ASR), delayed ettringite formation (DEF) and sulfate attack. New non-contact electromagnetic sensors have been demonstrated for providing this capability on bridge girder sections. When steel is strained, the electronic properties of the metal are altered and there is a measurable change in the electromagnetic response of the metal. These changes in material properties can be used to measure strain in the steel reinforcement from outside the concrete. This paper describes the results of preliminary tests involving samples of exposed rebar, concrete-encased rebar, and a full-scale reinforced concrete beam. The rebar specimens were placed in tension while typical strain gauges and the non-contact electromagnetic stress measurements were simultaneously performed. The specimens were then loaded well in excess of the yield point, to ensure both elastic and plastic straining occurred. Electromagnetic sensors were positioned to monitor changes in strain in a stirrup and a longitudinal reinforcing bar during a flexural test of a full-scale beam that had previously undergone significant expansion from ASR and DEF. The results of these early tests indicate the eStress system can provide valuable insight into the strains in bridges and other structures, thus providing an improved method for maintenance and repair.