Fatigue-Induced Damage in High-Strength Concrete Microstructure

A high-strength concrete subjected to compressive fatigue loading with two maximum stress levels was investigated and the behaviour was evaluated using the macroscopic damage indicators, strain and acoustic emission hits (AE-hits), combined with microstructural analyses utilising light microscopy and scanning electron microscopy (SEM). A clustering technique using Gaussian mixture modelling combined with a posterior probability of 0.80 was firstly applied to the AE-hits caused by compressive fatigue loading, leading to two clusters depending on the maximum stress level. Only a few cracks were visible in the microstructure using light microscopy and SEM, even in phase III of the strain development, which is shortly before failure. However, bluish impregnated areas in the mortar matrix of higher porosity or defects, changing due to the fatigue loading, were analysed. Indications were found that the fatigue damage process is continuously ongoing on a micro- or sub-microscale throughout the mortar matrix, which is difficult to observe on a mesoscale by imaging. Furthermore, the results indicate that two different damage mechanisms take place, which are pronounced depending on the maximum stress level. This might be due to diffuse and widespread compressive damage and localised tensile damage, as the findings documented in the literature suggest

Einfluss der Gesteinskörnung auf die Ermüdungsschädigung in hochfestem Beton

In diesem Beitrag wird der Einfluss der groben Gesteinskörnungsart auf das Ermüdungsverhalten von hochfesten Betonen analysiert, beschrieben und vergleichend dargestellt. Dabei werden zwei hochfeste Betone mit Basalt und Granit als Grobzuschlag auf zwei Spannungsniveaus, Sc,max = 0,85 und Sc,max = 0,70, untersucht. Mit Hilfe der Schallemissionsanalyse und mit detaillierten Dehnungsmessungen werden die ablaufenden Schädigungsprozesse erfasst und anhand von Schädigungsindikatoren wie Steifigkeitsdegradation, dissipierte Energie je Lastwechsel und Schallemissionsaktivität ausgewertet. Die Ergebnisse zeigen dabei signifikante Unterschiede in den ablaufenden Degradationsmechanismen, aufgrund der unterschiedlichen mechanischen Eigenschaften der Gesteinskörnung auf und geben Hinweise auf generelle ermüdungsbedingte Schädigungsprozesse im Betongefüge.In this paper the influence of the coarse aggregate type on the fatigue behaviour of high-strength concrete is analyzed, described and compared comparatively. Two high-strength concretes with basalt and granite as coarse aggregate with two different stress levels, Sc,max = 0.85 and Sc,max = 0.70 were investigated. With the acoustic emission analysis and detailed strain measurements, the ongoing damage processes were determined and evaluated on the basis of damage indicators such as siffness degradation, dissipated energy per load cycle and acoustic emission activity. The results show significant differences in the ongoing damage mechanisms due to the different mechanical properties of the coarse aggregate and give indicators of general fatigue related damage processes in the concrete structure