Methodology and integrity monitoring of foundation concrete piles using Bragg grating optical fibre sensors

This paper reports on the use of Bragg grating sensors for strain and temperature monitoring of reinforced concrete foundation piling. The use of optical fibre sensors on two reinforcing cages was made possible by the development of appropriate protection systems for the sensors. The cages were approximately 46 m long and 1.5 m in diameter. Sixteen Bragg grating sensors were bonded and protected by means of a carbon–fibre composite system in each reinforcing cage. The sensor protection systems were deployed successfully and all the fibre Bragg grating strain and temperature sensors survived the installation, transportation and subsequent filling of the foundation bores with concrete. The monitoring of the reinforcing cages during the pouring of the concrete into the borehole highlighted the presence of thermal tensile strains applied to the steel rebars because of the temperature of the liquid concrete. A change in the strain distribution along the whole depth of the foundation piles was observed half way through the concrete curing. A variation in the strain distribution was monitored due to the simultaneous effect of the construction of the building floors and the ground heave.