## 1 Introduction  
Many important large infrastructure components are created by pouring a liquid material (e.g., cement or a polymer) into a mold and simply letting the material harden or cure. Examples may be bridges, wind turbine blades or electrical transformers. Volume shrinkage of such materials during the curing process is a well-known phenomenon. Good measurements of the shrinkage are essential for understanding and modeling the materials' behavior. For example, the durability of concrete structures is dependent on the formation of cracks, which develop due to such volume reductions during the hardening process [1, 2]. A variety of techniques exist to measure the volume shrinkage of a material while it is hardening. This introductory overview concentrates on cementitious and polymer materials, but the methods should be applicable to many other material classes. The average shrinkage can be determined by simply measuring the outer dimensions or the buoyancy of a sample [3, 4]. Rheometry [5, 6], pycnometry [7, 8], dilatometry [9, 10, 11] or thermo-mechanical analysis [12, 13] are other, more complicated methods to measure the average shrinkage of a material. Another method is the tracking of the position of embedded objects by various means [14, 15]. However,the strain information these methods yield is limited. The samples as a whole are the subjects of the measurement, and thus, the average strain is determined. In addition, often, the sample size that can be used in such experiments is rather small. This may reflect an unrealistic picture of hardening processes in real-world technical applications, since the production and dissipation of heat or the diffusion of water out of a structure are size and probably geometry dependent [16; 17]. Digital image correlation tracks surface structures and allows one to determine two-dimensional local shrinkage on the surface of a sample [18; 19]. While this technique yields much more detailed information than the above methods, it can only be applied to rather small samples, with sizes of some centimeters. The biggest disadvantage of all these techniques is that they are incapable of measuring the strain inside the structure. Especially for large structures, the knowledge about internal strain gradients may be of high importance to determine overall structural parameters. Traditional strain gauges may be placed directly into the still liquid material to measure internal strain [20; 21; 22]. However, this method yields just punctiform strain information, at the location of the strain gauge. Another method to measure internal strains is the use of optical Fibers with Bragg Gratings (FBGs) [23; 24; 25; 26]. Optical fibers are resistant to most environments, which predestine them for embedding in many kinds of materials. They are easy to apply and have good resolution. Strains are measured by sending laser light through the fiber. One frequency of the light is reflected at the Bragg grating, and the reflected light is recorded. When the component to which the fiber is attached gets strained, the fiber strains as well. This causes the Bragg grating to change length, and a different frequency is reflected. By comparing the recorded frequencies of the reflected light of the initial condition and the loaded condition, the strain at the Bragg grating can be calculated, and a measurement is done. However, as for strain gauges, strain concentrations or generally strain gradients cannot be measured well with Bragg gratings. The possibility to measure strain gradients over the length of an FBG has been reported [27; 28; 29; 30]. However, the length of such gratings is rather short, less then 25 mm in the cited works, and many FBGs have to be applied to cover the strain gradient in large structures. Measuring strain with optical fibers using an Optical Backscatter Reflectometer (OBR) has removed the above-mentioned problems. The OBR method can use regular communication fibers.