Construction and Building Materials 18 (2004) 145–154 0950-0618/04/$ - see front matter  2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2003.10.002 Continuous monitoring of setting and hardening of mortar and concrete H.W. Reinhardt, C.U. Grosse* Institute of Construction Materials, University of Stuttgart, Stuttgart, Germany Received 15 September 2003; received in revised form 12 October 2003; accepted 12 October 2003 Abstract Two main properties of cementitious mortar and concrete are most important, i.e. rheology and setting and hardening. Rheology determines workability and compaction ability. Setting determines the end of workability and hardening is responsible for strength gain and stiffness development. Rheology and setting are measured in practice by rather conventional methods, which do not lead to a continuous monitoring of properties as a function of age. A testing device was developed which utilizes the velocity of ultrasound (US)-waves in order to continuously monitor the setting and hardening of cementitious materials. Two development steps are emphasised, i.e. the design of the container and the exact determination of the transmitted US-pulse. It was shown that the method leads to very reproducible results. While the beginning of setting can be determined from the velocity vs. age of mortar curve by a mathematical procedure, the final setting is still due to empirical experience. The method is adjusted to concrete and can also be used for other materials like gypsum, lime, starch and other stiffening materials. It can be used for quality control in production of admixtures and new binders and for control of constancy of concrete production.  2003 Elsevier Ltd. All rights reserved. Keywords: Mortar; Concrete; Setting; Hardening; Monitoring; Ultrasonic 1. Introduction RILEM as the worldwide operating organization for research and testing of building and construction mate- rials and structures has set up a Technical Committee on ‘Advanced testing of cement based materials during setting and hardening’ (TC 185-ATC) which deals with methods detecting changes in the electrical and physical properties of early age concrete. The committee is preparing a state-of-the-art report on electrical methods, nuclear magnetic resonance, wave propagation, the maturity method and acoustic emission. The purpose of this paper is not to describe all those methods since other contributions in this volume will deal with some of them, but rather to focus on the wave propagation methods. In the following, the reflexion method is explained and the transmission method is tackled in more detail. *Corresponding author. Tel.: q49-711-685-3323; fax: q49-711- 685-7681. E-mail address: grasse@iwb.uni-stuttgart.de (C.U. Grosse). 2. Wave propagation methods 2.1. Reflexion method The reflexion method uses a pulse which is reflected at the boundary of a known material and the material to be investigated. If a pulse travels through a medium and encounters a boundary with different acoustic impe- dance it is partly reflected and partly transmitted. Assume Z is the impedance of the material with the 1 incident wave and Z is the impedance of the connected 2 material then the reflexion ratio is given by A Z yZ r 1 2 rs s (1) A Z qZ i 1 2 with A , amplitude of reflected wave and A , amplitude r i of incident wave. The impedance is ZsEyv (2) with E, Young’s modulus and v, stress wave velocity in the respective medium. In the 1D case if the cross-