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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.
Water-repellent renders were studied in order to evaluate their suitability for restoration applications. Water repellent admixtures, such as powdered siloxane-based products and metal soaps (zinc and calcium stearates), were mixed with a pozzolana-lime binder similar to historical binders with hydraulic properties. The chemical-physical and structural properties, the effectiveness and the durability of the water-repellent mortars in different environmental conditions were studied. The influence of the water repellent admixtures was evaluated by FT-IR analyses, by testing the mechanical properties and the behaviour in presence of water. The durability of the water-repellent mortars was evaluated after the exposure to artificial weathering (UV-light and water) and to immersion/drying cycles in saturated sodium sulphate solution. The nature of the water-repellent admixtures influenced both the hydration reactions and the chemical-physical properties of the mortars resulting in different resistance to the weathering and to salt crystallization.
Concrete is by far the most common building material in the world. Due to its good durability it is widely used in building envelopes. One major part of the embodied energy in concrete comes from the production of Portland cement that consumes around 3,500 MJ energy per each ton of cement. Globally 2.6 billion tons of Portland cement is produced and the consumption is increasing. Since concrete will be used as main building material also in future construction, it is reasonable to develop concrete building components with lower embodied energy.Using the example of an ultra-high performance concrete (UHPC) façade element, it is demonstrated how the embodied energy can be reduced by application of hydrothermal curing.
Ingress of moisture and harmful ions (e.g. Chloride, sulfate) into cementitious materials is one of the major factors in defining their durability. Modifications of building materials by targeted deposition of surface functional agents that make the surface hydro- and/or oleophobic aim to minimize moisture and ion ingress, and thus extend Service life of the cement based structures. Water repellent and easy-to-clean coatings for inorganic Substrates have gained strong attention during the last few years and various formulations based on silicones or alkylpolysiloxanes have been developed. Cementitious building materials are highly complex Systems with many components and changing properties in the course of a building’s Service life. The interactions between the Chemical agents and the cement based materials depend on many factors. The Chemical properties of the material Substrate appear to be the most important but in the context of their influence on the functionality of silanes and/or siloxanes these characteristics have not been well understood yet. The aim of this study was to evaluate the effect of different blended cements on water repellent agents’ performance. Two organosilicon compounds
were applied on fresh blended cements containing limestone (L), fly ash (F), slag (S) and trass (T), and investigated in terms of their functionality.
The surface properties of functionalized blended cements were studied based on wettability, i.e. contact angle measurements, before and after exposure to artificial and natural weathering. The first results indicate that slag and trass more distinctly affect the water repellent Surface performance after aging.
Both in developed and developing countries, modern building materials tend to be preferred to traditional earthen construction. Reasons include low durability, inadequate performance under seismic loading and, in developing countries, a wish to replace what is perceived as 'poor' with what is perceived as 'rich'. In an age when building permissions and construction standards are a must, particularly in seismic areas, even when an owner is willing to build in earth, construction approval needs to be granted: we rely on values and standards to build, but the necessary data on material properties and structural performance of earthen building techniques is scarce if compared to the abundance of data for other materials (clay brick masonry, concrete, steel) available to the engineer. At the same time, traditional builders' skills, knowledge and confidence in earthen building techniques are decreasing if not disappearing. A wallette testing campaign was thus carried out with the aim of filling this knowledge gap. Prior to the wallette campaign, material properties, including composition and physical-mechanical parameters, were determined. Compression and diagonal compression (shear) tests were then performed, and a basic analysis of the mechanical behaviour of structural elements built in cob is provided in relation to earth block (adobe) masonry and rammed earth elements. Cob, shown to have low compressive resistance, has a relatively ductile post-peak behaviour if compared to earth block masonry specimens which, as expected, show a marked brittle behaviour. In terms of shear strength, cob performs relatively well in view of its low compressive strength. The study is part of our work within the framework of the ongoing project NIKER funded by the European Commission dealing with improving the structural performance of Cultural Heritage assets in order to limit earthquake hazards.