MONITORING OF THE HARDENING OF CONCRETE/MORTAR USING PLASTIC OPTICAL FIBER SENSORS: FUNDAMENTAL EXPERIMENT AND DATA INTERPRETATION

In the New Austrian Tunneling Method (NATM) of tunnel design and construction, it is important to know the actual compressive strength of lining concrete at early ages, which determines the time of demold-ing and its future durability. The degree of hydration reaction of in-place concrete is regarded as a useful reference to estimate the development of the strength of concrete. Based on the concept of On-Site Visualization, this paper presents a new approach to monitoring the hardening progress of fresh concrete/mortar using plastic optical fiber (POF) sensors. The principle of sensing is illustrated conceptually. The fundamental experiments were carried out to investigate the change in the visible light intensity reflected from the sensing plane of POF sensor during the hydration of concrete/mortar at early ages. The typical influence factors were also studied. The hydration process of concrete/mortar at early ages was well observed by the POF sensors. The direction of POF sensor installation had a significant effect on the pattern of light intensity change. When the sensor was installed pointing downwards, the light intensity dropped at first but increased rapidly in a few hours, which indicated the phase evolution when the free water disappeared in the hardening concrete/mortar. While the sensor was set up pointing upwards, the light intensity dropped in the first stage and then grew up slowly with time. Furthermore, the hydration process of concrete was characterized by introducing the nominal refractive index (NRI), which could be calculated from the measured light intensity. Thus, the hydration of concrete could be divided into three stages approximately according to the curve of NRI of concrete (when the sensor was installed pointing upwards), which coincided with the common knowledge of the concrete hydration process. Finally, the future work is discussed briefly.