We present a novel concept of quasi-distributed optical fibre extensometers for embedment into concrete, with optimized, continuous transfer of the strain field to the fibre, and fully corrected from thermal variations. These sensors have been developed for structural health monitoring applications. They are composed of a combination of optical cavities and Bragg gratings cascaded along a single fibre. The cavities, which are the parts sensitive to the concrete strain, are formed by partially reflecting elements inserted into the fibre. Their length, which is also the measurement basis, can range from 10 cm to several metres. Several cavities can be cascaded along a single fibre, allowing quasi-distributed strains measurements. Bragg Gratings are inserted along the same fibre, close to the cavities, and are used to measure the temperature locally. Both types of sensors are read by a fibre optic low coherence interferometer featuring a temporal delay line. The interferometer is used in an original way to measure simultaneously the length variations of the cavities and the wavelength shifts of the Bragg gratings. In this paper we present the design of the sensor, and in particular the study of the composite packaging whose shape and mechanical properties have been optimized by finite elements modelling to minimize the intrusion effect and ensure a continuous transfer of the strain field when embedded into concrete. We develop the optical reading method, presenting the theory of the sensor interrogation, or how to get the strain and temperature information. We describe also the instrumentation. Finally we present some laboratory experiments that show very good agreement between standard sensors and OFS, and an example of implementation into a bridge near Angouleme, France.