Influence of the composition of high-strength concrete and mortar on the compressive fatigue behaviour

The results of compressive fatigue investigations on four high-strength concretes and their corresponding mortars are presented. The influences of coarse aggregates generally, the substitution of basalt coarse aggregate by granite, the addition of silica fume and the variation of the water to cement (w/c) ratio are investigated systematically. The numbers of cycles to failure, the developments of strain, stiffness, dissipated energy and acoustic emission hits are focused on in the analyses. The results clearly show that coarse aggregates can influence the fatigue behaviour of concretes in a negative way at higher stress levels and in a positive way at lower stress levels compared to mortars. The granite coarse aggregate decreases the adverse effect at higher stress levels due to its lower modulus of elasticity compared to that of the basalt aggregate. Silica fume improves the fatigue behaviour of concrete and mortar strongly. The increase of the w/c ratio and, thus, the increase in porosity reduces the fatigue resistance of concrete and mortar significantly, due to the weakening of the mortar matrix and of the interfacial transition zone. The results demonstrate the interaction of the coarse aggregates and the mortar matrix with their specific properties, which leads to a certain fatigue behaviour. The acoustic emission gives additional valuable strain-independent information of the damage processes occurring, possibly also on micro- and nanoscales

Influence of the specimen production and preparation on the compressive strength and the fatigue resistance of HPC and UHPC

The results of tests under monotonically increasing load and cyclic compression load are often analysed by means of probabilistic methods. Although there is a considerable scattering in the results, especially in the number of cycles to failure, the cause of these cannot be completely explained. The imperfections of the specimens tested are among the causes of this scattering mentioned in the literature. Based on a round robin test the influence of HPC and UHPC production and specimen preparation techniques on the mean values of the compressive strengths, number of cycles to failure and data scattering have been evaluated. The main findings of the study are that the production techniques have an influence on the compressive strength, however, do not affect the mean number of cycles to failure. Moreover, the accurate preparation of the specimens has a positive influence on the compressive strength and the scattering of the results of both compression and cyclic load tests. The mean number of cycles to failure of HPC specimens is not influenced by the preparation techniques, whereas the polishing technique may have a positive influence on the mean number of cycles to failure of UHPC specimens. © 2021, The Author(s)

Compressive Fatigue Behaviour of High-Strength Concrete and Mortar: Experimental Investigations and Computational Modelling

A high-strength concrete and mortar subjected to compressive fatigue loading were compar-atively investigated using experimental and computational techniques. The focus of the investigations was on the influence of the coarse aggregate in high-strength concrete. Accordingly, the fatigue behaviour was analysed experimentally using the macroscopic damage indicators strain, stiffness and acoustic emission hits. The results clearly show differences in the fatigue behaviour between the concrete and the mortar, especially at the lower stress level investigated. The basalt coarse aggregate here improves the fatigue behaviour of the concrete. Indication of a negative effect can be seen at the higher stress level. A finite element approach with a gradient-enhanced equivalent strain-based damage model combined with a fatigue model was used for the computational simulation of the fatigue behaviour. The damage model includes a differentiation between tension and compression. The fatigue model follows the assumption of the reduction in the material strength based on the accu-mulated gradient-enhanced equivalent strains. A random distribution of spherically shaped basalt aggregates following a given particle size distribution curve is used for the simulation of concrete. The comparison of the experimentally and computationally determined strain developments of the concrete and mortar shows very good agreement

Analyse des Ermüdungsverhaltens von Beton anhand der Dehnungsentwicklung

Die Weiterentwicklungen in der Betontechnologie führten in den letzten Jahrzehnten zu Hochleistungsbetonen mit immer höheren Festigkeiten. Der Ermüdungsnachweis wurde jedoch kaum weiterentwickelt und beinhaltet immer noch sehr grobe Herangehensweisen bei der Berücksichtigung des Materialwiderstands von Beton. Für eine grundlegende Weiterentwicklung dieses Nachweises fehlt noch das notwendige Wissen zu den Mechanismen der Betonermüdung. Das Ziel dieser Arbeit war es daher, grundlegende Erkenntnisse zum Ermüdungsverhalten hochfester Betone bei unterschiedlichen zyklischen Beanspruchungen zu ermitteln und hierdurch zu einem besseren Verständnis der Mechanismen der Betonermüdung beizutragen. In der vorliegenden Arbeit wurde das Ermüdungsverhalten eines hochfesten Betons bei Druckschwellbeanspruchung anhand der Dehnungs- und Steifigkeitsentwicklungen untersucht. Betrachtet wurden dabei die Einflüsse der bezogenen Oberspannung, der Belastungsfrequenz und der Wellenform. Zusätzlich wurden, ausgehend von in der Literatur dokumentierten Ansätzen, Versuche bei monoton steigender Beanspruchung und Dauerstandbeanspruchung vergleichend durchgeführt. Die Dehnungs- und Steifigkeitsentwicklungen werden durch die untersuchten Belastungsparameter der Ermüdungsbeanspruchung eindeutig beeinflusst. Charakteristische Zusammenhänge zwischen der Beeinflussung einzelner Kenngrößen der Dehnungs- und Steifigkeitsentwicklung und der Beeinflussung der Bruchlastwechselzahlen wurden aufgezeigt. Anhand der Dehnungen und Steifigkeiten an den Phasenübergängen konnten Hinweise auf beanspru-chungsartabhängige Gefügezustände abgeleitet werden. Die vergleichende Auswertung des Dehnungsverhaltens bei monoton steigender Beanspruchung, Ermüdungsbeanspruchung und Dauerstandbeanspruchung zeigte, dass das Ermüdungsverhalten von Beton nicht adäquat in Anlehnung an andere Beanspruchungsarten beschrieben werden kann. Die Untersuchungsergebnisse wurden in eine Modellvorstellung übertragen, die zur Beurteilung der baustofflichen Phänomene bei zyklischen Beanspruchungen geeignet ist. Dabei wurde die Hypothese aufgestellt, dass sich unterschiedlich ausgeprägte Kleinst-Gefügeveränderungen beanspruchungsabhängig einstellen, die die Entstehung und Ausbreitung von Mikrorissen beeinflussen. Die detaillierte Untersuchung der Dehnungs- und Steifigkeitsentwicklungen führte zu neuen und tiefergehenden Erkenntnissen und sollte ergänzt durch die Betrachtungen von Gefügezuständen zukünftig weiterverfolgt werden.The developments in concrete technology in recent decades have led to high-performance concretes with ever increasing strength. However, the fatigue design aspects over the same period have hardly been developed and still involve very rough estimations in their consideration of the resistance of concrete. The fundamental lack of understanding of the mechanisms of concrete fatigue is the main inhibiting factor in further advancing fatigue design. This paper, therefore, aims at gaining fundamental knowledge concerning the fatigue behaviour of high-performance concretes subjected to various fatigue loadings, thereby contributing to the general understanding of their fatigue mechanisms. To do this, the behaviour of a high-performance concrete under pure compressive fatigue loading was examined at a fundamental level on the basis of the development of strain and stiffness. The investigation involved the analysis of three influences on the fatigue behaviour of concrete – the maximum stress level, the loading frequency and the waveform. In addition, tests with monotonically increasing loads and with sustained loads were performed in a comparative manner following approaches documented in the literature. The development of strain and stiffness are measurably affected by the load parameters studied. Characteristic relationships between the influence on the individual parameters of developments of strain and stiffness and on changes in the numbers of cycles to failure were developed. Indications of load type-dependent states of the material could be derived on the basis of different strain fractions of the total strain at the transition of stages. A comparative evaluation of the strain development in static short-term loading, fatigue loading and static long-term loading showed that the fatigue behaviour of concrete cannot be adequately described based on the strain behaviour of these other two types of loading. The findings of the investigations were transferred to a model concept which is applicable to the assessment of material-specific phenomena related to fatigue loading and other types of loading. This enabled the derivation of a hypothesis that tiny structural changes, which are variously pronounced depending on the type of loading, affect the formation and expansion of microcracks as ambient conditions. The detailed investigation of concrete fatigue on the basis of the developments of strain and stiffness led to new and deeper knowledge and should be further pursued in future research in the subject area by additionally investigating different states of microstructure

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

Ermüdungsverhalten von hochfesten Vergussmörteln

Hochfeste Vergussmörtel werden häufig in Verbindungen der Gründungsstruktur von Windenergieanlagen eingesetzt, wo sie hochzyklischen Beanspruchungen ausgesetzt sind. Neuere Untersuchungsergebnisse deuten auf ein unterschiedliches Ermüdungsverhalten von Vergussmörteln im Vergleich zu hochfestem Beton hin. Im Rahmen dieses Beitrags wird das Ermüdungsverhalten von unterschiedlich fein zusammengesetzten Vergussmörteln und einem hochfesten Beton als Referenzbeton verglichen. Anhand der Bruchlastwechselzahlen konnte kein drastischer Unterschied im Ermüdungsverhalten zwischen Vergussmörteln und dem Referenzbeton erkannt werden. Es konnte jedoch gezeigt werden, dass die Erwärmungsrate von der Mörtelzusammensetzung und von der bezogenen Oberspannung beeinflusst wird. Eine Korrelation zwischen einer höheren Erwärmungsrate und einer größeren Dehnungszunahme der Unterdehnung in Phase II konnte erkannt werden.High-strength grouts are commonly used in the connection of wind turbines, where they are exposed to high cyclic loading. The lastest scientific results indicate difference between the fatigue behavior of high-strength grouts compared to high-strength concrete. Therefore, the fatigue behaviour of high-strength grouts with different fineness of composition and high-strength concrete as reference was investigated in this contribution. No drastic difference in the fatigue behaviour between high-strength grouts and the reference concrete can be detected on the basis of the numbers of cycles to failure. It could be shown that the heating rate is influenced by the concrete composition and by the maximum stress level. A correlation between a higher heating rate and a larger increase in strain at the minimum stress level could be detected

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

Compressive fatigue investigation on high-strength and ultra-high-strength concrete within the SPP 2020

The influence of the compressive strength of concrete on fatigue resistance has not been investigated thoroughly and contradictory results can be found in the literature. To date, the focus of concrete fatigue research has been on the determination of the numbers of cycles to failure. Concerning the fatigue behaviour of high-strength concrete (HPC) and, especially, ultra-high-strength concrete (UHPC), which is described by damage indicators such as strain and stiffness development, little knowledge is available, as well as with respect to the underlying damage mechanisms. This lack of knowledge has led to uncertainties concerning the treatment of high-strength and ultra-high-strength concretes in the fatigue design rules. This paper aims to decrease the lack of knowledge concerning the fatigue behaviour of concrete compositions characterised by a very high strength. Within the priority programme SPP 2020, one HPC and one UHPC subjected to monotonically increasing and cyclic loading were investigated comparatively in terms of their numbers of cycles to failure, as well as the damage indicators strain and stiffness. The results show that the UHPC reaches a higher stiffness and a higher ultimate strain and strength than the HPC. The fatigue investigations reveal that the UHPC can resist a higher number of cycles to failure than the HPC and the damage indicators show an improved fatigue behaviour of the UHPC compared to the HPC.German Research Foundation (DFG)Cyclic Deterioration of High-Performance Concrete in an Experimental-Virtual La

Micro- and nanostructural investigations of high and ultra-high performance concrete under fatigue

A fine-grained UHPC, both undamaged and damaged by fatigue loading, was comparatively examined by various microstructural analytical methods, to evaluate the different techniques with respect to their applicability and relevance for the investigation of fatigue damage processes. The fatigue tests were stopped at the transition from phase II to phase III of the s-shaped strain development. The cyclic compression loading was performed with a frequency of ft = 1 Hz, and a stress level between Smin = 0.05 and Smax = 0.75 (fcm = 170.2 MPa). The fatigue process under these loading conditions is dominated by alterations and damages on the nano-scale, that can be observed by transmission electron microscopy. The resulting coarsening of the pore structure was also visible with dynamic vapor sorption. Nanoindentation indicates, that changes of the HD-C-S-H-phase occur. IR spectroscopy also indicates changes of the C-S-H phase and thermal analysis changes of the water content. Dynamic mechanical analysis (DMA) gave insight into the complex Young’s modulus and Poisson’s ratio changes. The acoustic emission technique gives information on the different processes during the single phases of fatigue and reveal a very different damage behaviour of dry and moist materials. Some microcracks are visible with light microscopy. It appears, that the number of cracks after fatigue is higher than before. With X-ray computed tomography, X-ray powder diffraction, the drying behaviour, the free water uptake, the water uptake under vacuum and by mercury intrusion porosimetry no significant differences between specimens with and without fatigue loading could be observed in this examination

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