*Article* **Calibration of Steel Rings for the Measurement of Strain and Shrinkage Stress for Cement-Based Composites**

### **Adam Zieli ´nski \* and Maria Kaszy ´nska**

Faculty of Civil Engineering and Architecture, West Pomeranian University of Technology in Szczecin, al. Piastów 50, 70-311 Szczecin, Poland; maria.kaszynska@zut.edu.pl

**\*** Correspondence: adam.zielinski@zut.edu.pl

Received: 18 June 2020; Accepted: 30 June 2020; Published: 2 July 2020

**Abstract:** Concrete shrinkage is a phenomenon that results in a decrease of volume in the composite material during the curing period. The method for determining the e ffects of restrained shrinkage is described in Standard ASTM C 1581/C 1581M–09a. This article shows the calibration of measuring rings with respect to the theory of elasticity and the analysis of the relationship of steel ring deformation to high-performance concrete tensile stress as a function of time. Steel rings equipped with strain gauges are used for measurement of the strain during the compression of the samples. The strain is caused by the shrinkage of the concrete ring specimen that tightens around steel rings. The method allows registering the changes to the shrinkage process in time and evaluating the susceptibility of concrete to cracking. However, the standard does not focus on the details of the mechanical design of the test bench. To acquire accurate measurements, the test bench needs to be calibrated. Measurement errors may be caused by an improper, uneven installation of strain gauges, imprecise geometry of the steel measuring rings, or incorrect equipment settings. The calibration method makes it possible to determine the stress in a concrete sample leading to its cracking at specific deformation of the steel ring.

**Keywords:** restrained ring test; autogenous shrinkage cracking; concrete cracking test; concrete shrinkage cracking test; restrined ring calibration
