Adjustable Restraining Frames for Systematic Investigation of Cracking Risk and Crack Formation in Reinforced Concrete Under Restrained Conditions

The Institute of Structural Concrete at Graz University of Technology developed adjustable restraining frames (ARFs) in order to enable systematic investigations of the behaviour of reinforced concrete under restrained conditions. The focus was hereby set on the following aspects:holistic experimental simulation of the stress history due to imposed deformations including cracking and crack opening by time, as well asexperimental determination of cracking patterns to be expected in externally restrained and ordinarily reinforced thick members (reinforcement is located near the surface whereby the core is not affected by reinforcement; A _c ≫ A _c,eff). The results of these experiments give profound insights on the stress history, the cracking risk, the crack pattern and further crack opening to be expected due to the complex interplay of thermal, hygric and mechanical behaviour of concrete. Special attention was given to the restraining condition as well as the present reinforcement. In particular, the most important results of the whole campaign were:confirmation and additional findings regarding the effect of viscoelastic behaviour on the hardening-induced stress history,clarification of the behaviour of reinforced concrete when it comes to superimposition of hardening-induced stresses with further stresses due to imposed deformations during service life andexperimental verification of the existence and effectiveness of secondary cracks in thick members on the limitation of the crack width of the primary crack. Besides the outlined findings and their application in guidelines for crack risk assessment (ÖBV in Analytical design of watertight structures with optimized concrete (in German). Austrian society for construction technology, Vienna, Austria, 2018) as well as crack width control and jointless design (BAW in Merkblatt früher Zwang – Rissbreitenbegrenzung für frühen Zwang in massiven Wasserbauwerken. Referat Massivbau der BAW, Karlsruhe, 2010; BAW in Merkblatt Zwang bei fugenlosen Wasserbauwerken. Referat Massivbau der BAW, Karlsruhe, 2019), these experiments provide also clear and transparent data for the verification of thermo-mechanical calculation models for simulation of hardening-induced stress histories, (e.g. Schlicke in Mindestbewehrung für zwangbeanspruchten Beton. Graz University of Technology, Graz, 2014; Heinrich in effiziente Berechnung viskoelastischer Spannungen in gezwängten Bauteilen. Graz University of Technology, Graz, 2018; Jędrzejewska in Constr Build Mater 174, 2018) as well as nonlinear calculation models for simulation of secondary cracking in thick concrete members.