Mortars and concretes are changing its volume during setting and hardening. Many factors are influencing this mechanism:
If the concrete is still in the fluid state, volume change doesn't matter. (maybe you get, due to the shrinkage, some percent less cubic meters of concrete volume then ordered). If hardening begins, shrinkage and expansion will cause inner strains. If this strain is higher then the strength inner cracks or other may appear.
Here you may find a survey paper about measuring the early shrinkage of mortars. This presentation was given at the Third International Drymix Mortar Conference, idmmc three, in Nürnberg, Germany, 2011 and published in the Drymix Mortar Yearbook, 2011, Editor Ferdinand Leopolder, ISBN 978-3-9814004-1-0
A presentation with the title "Testing the mechanical Properties of Drymix Mortars" heldt at the Drymix Mortar Meeting 2016 gives also a survey about shrinkage measurement.
Another survey regarding the autogenous free and restrained shrinkage you may find in the Ph.D thesis of Sören Eppers: Assesing the Autogenous Shrinkage Cracking Propensity of Concrete by Means of Restrained Ring Test
The Schleibinger Bending Drain is the so called "Curling Profile Apparatus" in the new EN 13892-9:2018 "Methods of test for screed materials. Dimensional stability."
The Schleibinger Measurement Gauge is according to the new EN 12390-16:2019 Testing hardened concrete - Part 16: Determination of the shrinkage of concrete; and many others!
Schleibinger offers several instruments to measure this volume change from the early beginning with high precision.