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The susceptibility of different aggregate types used in concrete to ASR is strongly associated with their petrography. The aim of this study is to assess the influence of the aggregate size on the ASR potential, in particular applied to the sand fractions in the size range of 0.125-4 mm. The investigation concept is based on the hypothesis that at a specific particle size reactive aggregates could change their characteristic influence in mitigating ASR instead of promoting it. For the study of the size dependent behaviour mortars are prepared, considering aggregates of different reactivity, including very reactive, slow-late and quasi non-reactive types. The analysis is based on a stepwise substitution concept, in which mortars with a constant overall grain size distribution are compared, while only one size fraction of the non-reactive base aggregate is substituted by a reactive aggregate type of this specific size fraction. For the expansion experiments an accelerated mortar bar test (80°C, immersed in NaOH solution) was applied. The results show that the slow-late reactive greywacke yielded a strong reduction of the expansion below an aggregate size in the range of 0.5-1 mm. On the other side with the very reactive borosilicate glass, the expansions increased steadily with decreasing size. To validate if the acquired expansions were caused by ASR, thin sections were analysed. Mainly, higher expansion values were consistent with extensively more amounts of ASR indicators.
In recent years the German motorway network has seen an increase in the occurrence of damage to concrete road surfaces which can be attributed to the alkali-silica reaction (ASR). In view of the often drastically reduced life expectancy of road surfaces due to ASR, research activity in this field has notably increased. Alongside preventative measures in concrete technology, the main research focus up to now has been the development of performance-oriented testing procedures for ASR prevention. This included more specifically the accelerated simulation of climatic effects and external alkali penetration on road surfaces. The effects of mechanical pre-damage resulting from cyclic traffic loading and climatic impact had previously not been taken into consideration. Since 2011, the five-partner research group 1498 sponsored by the German Research Foundation (DFG) has been pursuing research on how simultaneous cyclic loading and external alkali penetration impacts destructive ASR in road surface concretes. The depiction of the myriad degradation and transport processes necessary for an understanding of these effects requires close interaction between experiments and their multi-scale modelling. This paper aims to focus on the aforementioned experiments by means of innovative testing techniques. The research is founded on a series of cyclic fatigue tests performed on large-format beams, both with and without previous application of a sodium chloride (NaCl) solution, with simultaneous tracking of crack development. Subsequently, smaller test specimens were extracted from the pre-damaged beams for further experiments. These included the spatial visualization and quantification of fatigue-induced cracks using micro X-ray 3D-computed tomography (3D-CT). Additionally, the effects of fatigue-induced cracks on alkali transport were investigated using Laser-Induced Breakdown Spectroscopy (LIBS). Subsequent storage of the small-format test specimens, with and without cyclic pre-damage, in an ASR-conducive environment then provided initial findings on the influence of fatigue-induced predamage on the ASR.
Die vorstehend dargestellten Untersuchungen lassen sich wie folgt zusammenfassen:
• Mit dem für die mechanische Vorschädigung entwickelten Versuchsstand und gewählten Prüfregime können die für die ermüdungsinduzierte Schadensevolution typische stabilisierende und stabile Phase in der auf Zug beanspruchten Betonrandzone gut abgebildet werden. So wurde dort zum Beispiel, ausgehend von dem hier gewählten praxisnahen Belastungsregime, mit Ultraschall in der Ruhephase durch das Trockenschwingen ein maximaler Abfall des relativen dynamischen E-Moduls von ca. 8 - 10 % ermittelt.
• Mit aufbauenden kapillaren Aufsaugversuchen wurde gezeigt, dass die erzeugte Betondegradation zu einer moderaten Erhöhung der Aufnahme des Prüfmediums führt.
• Zur Bewertung der Auswirkungen einer alleinigen Vorschädigung (Trockenschwingen) auf die schädigende AKR mittels Performance-Prüfung ist die Klimawechsellagerung aufgrund der einseitigen Tausalzbeaufschlagung am besten geeignet. Die hier ermittelten Dehnungsverläufe lassen auf einen moderaten Einfluss der mechanischen Vorschädigung auf die AKR schließen.
• Die zusätzlich im Kontext der Nasschwingversuche durchgeführten 60°C Betonversuche zeigen, dass die zyklische Beanspruchung vor und nach dem Aufbringen der Tausalzlösung die schädigende AKR gleichfalls moderat erhöht.
In derzeit laufenden ergänzenden Untersuchungen wird der separate Einfluss der ermüdungsinduzierten Rissbildung auf die schädigende AKR an homogen, stärker geschädigten Prüfkörpern mit einem hohen inneren AKR-Schädigungspotenzial im 60°C Betonversuch grundhaft analysiert. Zusätzlich wird gegenwärtig der Einfluss der degradierten Betonrandzone auf die Dehnungsverteilung an großformatigen Prüfkörpern
über die Höhe im 40°C Betonversuch vertiefend für die makroskopische Modellierung untersucht.
The accelerated concrete prism test (ACPT-60 °C) facilitates testing of alkali-silica reactivity of aggregates within five months. Manual measurement is usually conducted to determine expansion of test specimen every 28 days. This conventional procedure gives only limited insight to expansion behaviour of ASR-affected prisms. Due to the large time intervals, expansion exceeding the critical value of 0.3 mm/m is likely to be noticed late. Continuous recording resolves this problem and delivers beneficial information on the shape of the expansion curve.
Displacement transducers integrated into a special testing equipment developed at BAM put continuous measurement into practice. Additionally, ultrasonic velocity and acoustic emissions are measured in-situ and provide deeper insights into hydration and crack formation processes during the test.
Aiming for an automated testing method close to the German guideline, the following technical and methodical challenges arise. Changing of the vertical orientation of the prisms with every manual measurement as standardised cannot be implemented. Furthermore, preliminary results already proved true that cooling and heating cycles, as necessarily associated with each manual measurement, can evoke additional expansion. Therefore, interrupted and continuously measured expansions reveal systematic differences, indicating the necessity to adjust the threshold expansion value for automated testing.
Results presented in the paper: Different types of reactive aggregate have been tested applying continuous and manual expansion measurement to the concrete specimens. Development of continuously measured ultrasonic velocity and acoustic emissions of the aggregates tested diverge significantly while the expansion curves vary in shape and maximum strain. Prisms produced from a slowly reacting aggregate do not exhibit noticeable crack formation. It shows that comprehensive measuring helps describing and distinguishing ASR-damage mechanisms.
Models of ASR-induced expansion, available in the open literature, so far are validated with discontinuous data hardly tracing the modelled curve. Continuous measurement improves validation and further model development.
If the amount of sulfate within the mix design for concrete is not balanced, ettringite formation which first controls solidification continues while the concrete is hardening. Ettringite forms by reaction of sulfate contained in the cement or in some admixture with calcium aluminate (C3A) as cement component. The “late primary ettringite formation” promotes an increase in volume within the hardened concrete leading to cracks. To analyze the effect of substituting part of ordinary Portland cement (OPC) by a treated brown coal fly ash mortar bars were tested according to ASTM C452. This test method was originally developed to extrapolate from the internal to external sulfate attack and is based on the idea that by testing a concrete mixture containing sulfate the process of deterioration is accelerated because it does not have to diffuse into the concrete before reaction first. In addition to the samples prescribed in ASTM C452 further samples in the size of the German SVA procedure were tested also designed for accessing the external sulfate resistance. The results show that while the replacement of 25 % of cement by brown coal fly ash leads to length changes around the limit defined by ASTM, the substitution of 50 % cement exceeded the limit by a multiple. The progress of expansion is the very similar for the two geometries tested. In-situ XRD measurements confirmed that while for pastes made of OPC the formation of ettringite is completed after 20 hours, this is not true for the mixtures containing the brown coal fly ash.
In a different study the effect of hydrothermal treatment on the phase composition of ultra-high-performance concrete was analyzed. The results show that the sulfate and aluminate resulting from the decomposition of ettringite are bound into new phases.
If this binding is permanent this might allow the larger use of sulfate bearing raw materials.
The advantages of treating prefabricated components made of Ultra-High-Performance Concrete (UCPC) at 90 °C are largely recognized, while hydrothermal treatment at 185 °C and the corresponding saturation pressure of 1.1 MPa, in contrast, is not applied in building practice so far and was studied only by a few researchers.
In the research presented, the parameters pre-storage-time before treatment and dwell time in the autoclave were systematically varied. The results illustrate in which way compressive strength increases with the duration of treatment. Already samples which were only heated up and immediately cooled down show an increase in strength compared to the 28-day reference of about 10 %. After 20 h the maximum increase of about 25 % is reached. Interestingly the compressive strength of samples treated very long hardly decreases. It can be therefore concluded that for the composition investigated the achievable mechanical strength is not severely sensitive regarding the duration of the treatment.
Also, pre-storage times before autoclaving longer than 0.5 days showed no significant impact on strength development.
The phase composition measured by XRD exhibits significant changes compared to the reference. The amount of ettringite and portlandite have vanished, the cement clinker phases are substantially decreased and hydroxylellestadite and hydrogarnet are formed as new phases. The increase in strength can be assigned to an improved pozzolanic and hydraulic reaction. Contradictory to literature, there was found no tobermorite, a mineral which is known to be susceptible for disintegration at Long autoclaving duration. This explains why no significant strength loss appeared.
By the combination of an optimized granulometry, a reduced water cement ratio and the use of superplasticizers ultra-high performance concrete achieves a compressive strength of over 150 N/mm2 and a high resistance regarding acids and water-soluble salts. In different Research projects the effect of an autoclavation on the mechanical strength and the phase composition was analyzed. In systematic studies, the concrete mix and the process parameters were varied.
The results show that autoclavation leads to an improved pozzolanic and hydraulic reaction and significantly improves the mechanical strength compared to a conventional thermal treatment. If a minimum time-span for hydration is ensured, the achievable strength level is not dependent on the prestorage time. However, the duration of the autoclaving is significant. After the Maximum strength is reached there is only a very slight decrease, even if unrealistically long autoclaving times are applied.
The low tensile strength of cement-based materials can be improved by the addition of fibres. In a joint research project, an industrial partner designed special coatings for polymer and carbon fibres by integrating surfactants and hydrophilic compounds. Aim of the developed functionalization was to ensure an even coverage of the fibre surfaces and to anchor them chemically in the cement-based matrix. Task of the BAM was to quantify the effect of the improved bond. In a first step a workable mortar adjusted to the strength of the tested fibres was developed and the fibre distribution assessed by light microscopy and computed tomography. To test the new coating for its efficiency to prevent cracking during hardening and to improve the loadbearing behaviour new test setups were developed or existing methods were adjusted.
The experiments showed that the functionalization leads to a reduction of the crack area measured after the exposure of the wet mortar to strong drying conditions in the wind channel. Regarding the Efficiency to mitigate shrinkage cracks the functionalisation turned out to be more efficient for fibres made from polyacrylonitrile (PAN) than for carbon fibres. An improvement of the tensile strength after cracking of the cementitious matrix could only be documented for the coated carbon fibres. It could be quantified, however, only in the three-point bending tests because the fibres turned out to be too brittle for the applied single fibre pull-through test.
Die geringe Zugfestigkeit von zementgebundenen Materialien kann durch die Zugabe von Fasern maßgeblich verbessert werden. Ziel eines gemeinsamen Verbundprojekts mit einem Industriepartner war die Erhöhung der Leistungsfähigkeit von kurzen Polymer- und Carbonfasern durch eine Verbesserung des Verbundes zwischen Faseroberfläche und Zementsteinmatrix.
Von der IONYS AG wurde dazu eine spezielle Funktionalisierung entwickelt, die über eine Hydrophilisierung der Faseroberfläche eine chemische Anbindung an die Zementsteinmatrix gewährleistet. Aufgabe der BAM war es, die Effizienz der neuen Beschichtung bezüglich der Erhöhung der Nachrissbiegezugfestigkeit und der Reduzierung der Schwindrissbildung während der Erhärtungsphase zu quantifizieren. Die Ergebnisse zeigen, dass die Funktionalisierung die Neigung zur Schwindrissbildung für die Carbonfasern und in noch stärkerem Maße für die Polymerfasern reduziert.
Eine Erhöhung der Nachrissbiegezugfestigkeit konnte dagegen nur für die deutlich steiferen Carbonfasern nachgewiesen werden.
Composite Facade Elements with Self-Cleaning Surface made of Ultra-High-Performance Concrete (UHPC)
(2020)
In the framework of the European project H-House various concrete façade elements were developed with the aim to ensure a long service life by combining a very durable material with self-cleaning properties. The façade elements presented are made of a shell of UHPC filled with blocks of aerated autoclaved concrete as insulating material. Self-cleaning properties were realized amongst others by imprinting a microstructure into the surface during casting. The paper focuses on selected technological aspects of the manufacturing process of prototypes which had to be performed in two concreting sections. Furthermore the challenges faced when upscaling the self-cleaning properties are addressed and the strategy to assess the self-cleaning properties by measuring the contact and the roll-off angel is presented. The results show that a successfull upscalaing process requires detailed planning and that the best results can often be achieved with a moderate work effort or material use.