1. Potential reduction of concrete deterioration through controlled DEF in hydrated concreteSamo Lubej, Milan Radosavljević, 2013, original scientific article Abstract: Delayed ettringite formation (DEF) is a chemical reaction with proven damaging effects on hydrated concrete. Ettringite crystals can cause cracks and their widening due to pressure on cracked walls caused by the positive volume difference in the reaction. Concrete may show improvements in strength at early ages but further growth of cracks causes widening and spreading through the concrete structure. In this study, finely dispersed crystallization nuclei achieved by adding air-entraining agent (AEA) and short vibration of specimens is presented as the main prerequisite for reducing DEF-induced deterioration of hydrated concrete. The study presents the method and mechanism for obtaining the required nucleation. Controlling long-term DEF by providing AEA-induced crystallisation nuclei, prevented excessive and rapid initial strength improvements, and resulted in a slight increase of compressive strength of fine grained concrete with only marginally lower density.
Keywords: delayed ettringite formation, aerated concrete, strength improvement
Published in DKUM: 10.07.2015; Views: 1514; Downloads: 31
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2. Hardening of concrete with a planned delayed ettringite formationSamo Lubej, Andrej Ivanič, Milan Radosavljević, 2009, published scientific conference contribution Abstract: Delayed ettringite formation (DEF) in cementitious materials is widely considered as a harmful chemical reaction that causes extensive damages in hardened concrete. However, preventative measures and possible improvements in general are not extensively studied and require further attention. In this study was presented an investigation into a type of controlled DEF in places of finely dispersed crystallisation nuclei and provide evidence that the process may improve compressive strength of cementitious materials. The Alkali-Silica Reaction (ASR) in hydrated concrete was achieved with the addition of fly ash and was further accelerated with the Dugganćs test. Achieved strengths and monitoring of microstructure development conducted with electronic microscopy revealed that growth of ettringite crystals in the nuclei led to harmless internal compressive stresses, expansion of hydrated concrete and overall strengthening of the concrete matrix.
Keywords: delayed ettringite formation, concrete, hardening
Published in DKUM: 31.05.2012; Views: 1803; Downloads: 45
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3. Bond improvement in cementitious mortars reinforced with continuous carbon filamentsAndrej Ivanič, Samo Lubej, Milan Radosavljević, 2009, published scientific conference contribution Abstract: Among various parameters that influence the properties of the composite, the bond between the cementitious matrix and the fibres is the most important one. This paper investigates the influence of admixtures - silica fume, superplasticizer and air entraining agent - on the improvement of the bond between bundled carbon filaments and cementitious mortars. An overall microstructural analysis has shown that the addition of silica fume strengthen the interfacial transition zone by both the reduction of its porosity and the consumption of CH crystals. The addition of a superplasticizer increased the workability of fresh mortars, but the bond improvement was insignificant. The application of the air entraining agent caused the formation of air bubbles inside the mortar matrix. Air bubbles formed in the vicinity of the multifilament yarn were found to provide mechanical anchoring for fibres leading to a significant increase in interface friction. Fibre-bundle pullout tests were carried out to confirm the effectiveness of the admixtures to improve the interface bonding. Based on the resulting load-displacement curves, air entraining agent was found to be most effective in enhancing the bonding characteristics of the cementitious composite.
Keywords: carbon fibers, cementitious composites, bond properties
Published in DKUM: 31.05.2012; Views: 1621; Downloads: 42
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