Reinforcing steel strains measured by Bragg grating sensors

Steel strain measurements along reinforcing bars allow an indirect determination of bond shear stresses acting between concrete and reinforcement. Traditional measuring techniques such as the application of electrical resistance strain gauges are not entirely satisfactory because they affect the bond properties. This paper describes experiments using wavelength multiplexed optical fiber Bragg grating sensors for strain measurements along 10-mm-diameter reinforcing bars embedded in reinforced concrete beams subjected to bending. The sensors were found to be capable of measuring large strains and strain gradients with high precision, without significantly affecting the bond properties. The experimental results are discussed and compared with calculations according to three bond shear stress-slip models. Recommendations for the application of optical fiber Bragg grating sensors on reinforcing bars are made and a simple method for the deflection calculation of partially cracked beams is presented, providing lower und upper bounds corresponding to maximum and minimum theoretical crack spacings, respectively.