A novel fiber optic water sensor based on the evanescent field-fiber loop ringdown (EF-FLRD) technique is described. After removal of the plastic jacket of a section of single mode fiber, followed by chemical etching of the fiber cladding partially, this section of bare fiber (sensor head) was placed inside a concrete mixture. Presence of water in the concrete changes the optical refractive index of the medium that is the interface between the bare etched fiber surface and the concrete, and a change in ringdown time is recorded due to a change in EF scattering loss occurred in the sensor head. Continuous monitoring of ringdown time, the water sensors read diligent data about variations of the water inside the concrete after tens of ml water is poured on the surface of a concrete bar. Eight EF-FLRD water sensor units were constructed and the sensors were embedded in different concrete bars to detect presence of water inside the concrete. The EF-FLRD water sensors show instant response. The demonstrated detection sensitivity is 10 ml water when it is poured to the surface of a concrete bar with approximate dimensions of 30 cm x 5 cm x 5 cm and the signal-to-noise ratio is 86, corresponding to a detection sensitivity limit of 0.12 ml. In addition to fast response, high sensitivity, additional unique advantage of the EF-FLRD water sensor over its counterparts, such as polymer-coated FBG water sensors, is high reversibility. The EF-FLRD water sensors embedded in concrete demonstrate to be highly reversible when tested in water-dry-water-dry duty cycles for as long as 43 h. Free of using temperature sensitive optical components in the sensor head, the EF-FLRD water sensor is immune to temperature variations in the concrete. This new type of fiber optic water sensor will have broad applications in long-term monitoring of presence/absence of water inside concrete as well as in concrete property characterization. (C) 2012 Elsevier B.V. All rights reserved.