| Summary: | The extended round robin testing program (RRT + ) is used in the Working Group 1 of the COST Action TU1404 as a fundamental mechanism: i) to validate advanced, non- standardised experimental techniques for testing cement-based materials and structures, ii) to benchmark different sustainable variations of concrete mixes prepared with mineral admixtures, recycled materials and/or by-products, and iii) to obtain input data for a range of concrete properties which could serve designers and engineers to better predict lifespan, durability, and serviceability of concrete struct ures. With a total of 45 laboratories from Europe, Japan and Canada, performing over 50 test methods on the same concrete mix, it presents one of the most extensive initiatives for joint testing of cement-based materials. The RRT + is divided into two phases: the initial and main experimental phases. During the initial phase, an ordinary concrete mix is prepared using the same constituting materials and following identical preconditioning, preparation, conditioning and test procedures. Even though the framework is identical and potential external causes of deviations are limited, concrete is prepared in different laboratories and some scatter in results can be expected. This paper describes the observations during the initial experimental phase and discusses methods including statistical analysis performed to understand the scatter and results obtained
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