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Mechanisms of degradation of concrete by external sufate ions under laboratory and field conditions
(2011)
The durability of concrete is a major challenge for the construction, which devotes one third to one half of its annual investment to building maintenance. The lack of fieid data regarding concrete durability, especially in the case of exposure to Sulfate ions (“sulfate attack”) makes it dijficult to determine the appropriate fest methods andperformance criteria. Additionally, the increased use of sustainable blends (cement with mineral admixtures, typically slag from the iron industiy) suffers from a lack of experience regarding their long-term performance. Most results for sulfate resistance are derived from accelerated laboratoiy tests xvhere performance criteria are based only on macroscopic properties, especially expansion. To fill this gap and better widerstand the mechanisms of sulfate attack under real conditions, a parallel study of laboratoiy micro-concrete and fieid concrete samples under sulfate exposure was undertaken, focussing on microstructural changes in addition to the conventional macroscopic characterisation.
Four exposure regimes were designed in the laboratoiy: full immersion (ponding), pH-control, semi-immersion and wet/diy cycles. Pure Portland blends and slag blends witli high level of slag replacement (70 wt.-%) were investigated.
The exposure regime has been found to play a major role in the damage process. In ponding conditions, the damage process takes place in three stages characterised by a first period of induction, followed by surface damage thatfinally extends to the bulle of the material. Paradoxically, the w/c-ratio does not seem to have much impact on the ionic transport phenomena but might be more decisive in the microstructure mechanical strength against local stresses. The slag blends, considered as sulfate resistant in ponding exposure, revealed badperformances under wet/diy cycles. This beliaviour was attributed to poor proper physical resistance of the slag hydrates against diying.
The fieid concretes selected for the comparison with the laboratoiy cases were partially buried in a sulfate-enriched soil in Argentina. A pure Portland blend and a slag blends with high level of slag replacement (80 wt.-%) were investigated. The submerged part of the samples could be compared to the laboratoiy ponding exposure, wliile the upper layer of the samples subjected to weathering could be compared to the laboratoiy wet/diy cycles exposure. The fieid obsen’ations tend to confirm the laboratoiy results and validate the fest settings.
It has been underlined that a direct relationship between damage (e.g.; cracking/expansion) andphase assemblage was not evident. However, the study highlights that sulfate combination with the hydrates of the cement (e.g.; C-S-H) and with those of the slag would play a rote in the initiation of the expansion, which would be attributed to a swelling of the hydrates or to the precipitation offine ettringite after the Saturation level in sulfate of the hydrates has been reached.
Deterioration of cementitious materials by sulfate ions is a concern for concrete in contact with ground water (and in a lesser extent sea water) in many parts of Europe, and is an important issue for underground construction identified as a major area for progress in the construction industry. Sulfate resistance relates closely to testing and standardization and therefore to prescriptive approaches. The lack of feed back on the durability of structures made of blended cements makes it difficult to fit these new concretes with existing test methods and performance criteria. This paper concerns the microstructural study of field samples and structures made of blended concretes available across Europe. This concerns several materials exposed to various climate regimes, ranging from Southern to Northern Europe (Spain, Germany, United Kingdom, Denmark and Norway). Complementary techniques such as SEM, micro-XRF, XRD and PIXE are used to evaluate the microstructural performance and stability of the phase assemblage of those blended concretes in the case of sulfate (and chloride in some instances) ingress, compare to pure Portland concrete.
Traditional earthen structures of cultural value are often damaged by static or dynamic loads. This is usually manifested by the appearance of cracks. All too often these cracks are insufficiently or inappropriately repaired if at all because of lack of knowledge and/or technology. In particular, the behavior of crack repair by grouting poses a challenge in earthen materials and demands specific requirements for the grouting mortar, such as low water content, good water retention, low shrinkage, etc. If dynamic loads, e.g. induced by earthquakes, are expected, the grouting material requires additional specifications such as a compatible strength and modulus of elasticity as well as good adhesion to the earthen materials. The study presents results from the development of a grouting material based on hydraulic lime mortar suitable for the repair of cracks in a variety of earthen building techniques. The goal was to develop a material also compatible with earthen structures exposed to dynamic load. The grouting mortar was designed to be adaptable in strength properties and at the same time to have sufficient robustness for the use on the construction site. First results show a satisfactory performance of the grout concerning fresh and hardened mortar properties as well as injectability. The study is part of our work in the framework of the ongoing project NIKER, funded by the European Commission dealing with improving imovable Cultural Heritage assets against the risk of earthquakes.
Lehm ist ein seit Jahrtausenden bewährter, weit verbreiteter und heute ein moderner Baustoff. Für vordringliche Bauaufgaben in der Altbausanierung, dem energieeffizienten Neubau sowie dem Bauen in Entwicklungs- und Schwellenländern ist Lehm hervorragend geeignet. Der Baustoff Lehm ist zu einem Material für anspruchsvolle Bauvorhaben geworden. Seine Ästhetik und Ausstrahlung, seine Wirkung für Raumklima und Wohlbefinden werden von vielen Menschen geschätzt. Besonders zukunftsrelevant sind die umweltspezifischen Eigenschaften des Materials, z.B. die unerreichbar günstige Energiebilanz vieler Lehmbaustoffe. Lehmsteine werden im Neubau überwiegend für die Ausmauerung von Holzständerkonstruktionen verwendet (Bild 1). Die Anwendung für tragende Wände ist derzeit noch untergeordnet, gewinnt aber an Bedeutung.
In der Altbausanierung und Denkmalpflege wird Lehm als historisch authentischer Baustoff und aufgrund seiner bauphysikalischen Qualitäten vielfältig eingesetzt.