Resistance to fatigue of self-healed concrete based on encapsulated polymer precursors

Moving cracks are often present in concrete structures and in those circumstances any self-healing technique for concrete must satisfy specific performance requirements, to guarantee its increased durability. These requirements include the capability of withstanding multiple cycles of crack movement without failing to keep healed cracks sealed. This paper shows early results from a testing protocol suggested by the authors to assess the performance of polymers as healing materials for moving cracks. Ultrasound (US) shear waves were used for continuous monitoring of small prismatic mortar specimens containing a single healed crack under a cyclic load. The maximum amplitude of US waves transmitted across healed cracks was correlated to the area effectively healed and the magnitude of crack movement. A decreasing trend of the maximum amplitude during cyclic loading was observed for strain levels on the polymer corresponding to 70% of its strain limit, but soundness at lower strain levels was confirmed after 300 cycles

Translucent self-healing cementitious materials using glass fibers and superabsorbent polymers

info:eu-repo/semantics/publishe

Determination of the degree of reaction of fly ash in blended cement pastes

This paper gives a review over methods to determine the degree of reaction for supplementary cementitious materials (SCMs) with focus on Portland cement - fly ash blends only and summarizes and highlights the most important findings which are detailed in a parallel paper published in Materials and Structures. Determination of the extent of the reaction of SCMs in mixtures is complicated for several reasons: (1) the physical presence of SCMs affects the rate and extent of the reaction of the ground clinker component – the so called “filler effect”; (2) SCMs are usually amorphous with complex and varied mineralogy which make them difficult to quantify by many classical techniques such as X-ray diffraction; (3) the rate of reaction of SCMs in a cement blend may be quite different from its rate of reaction in systems containing simply alkali or lime. From this review it is clear that measuring the degree of reaction of SCMs remains challenging. Nevertheless progress has been made in recent years to offer alternatives to the traditional selective dissolution methods. Unfortunately some of these – image analysis and EDS mapping in the scanning electron microscope, and NMR - depend on access to expensive equipment and are time consuming. With regard to fly ashes, NMR seems to be reliable but limited to fly ash with low iron content. New methods with quantitative EDS mapping to segment fly ash particles from the hydrated matrix and to follow the reaction of glass groups of disparate composition separately look very promising, but time consuming. Sources with a high proportion of fine particles will have higher errors due to lower limit of resolution (1-2 μm). Whereas for SCMs which react relatively fast (e.g. slag, calcined clay) the methods based on calorimetry and chemical shrinkage seem promising on a comparative basis, the very low reaction degree of fly ashes before 28 days means that the calorimetry method is not practical. There is a lack of data to assess the usefulness of long term chemical shrinkage measurements. The possibility to quantify the amorphous phase by XRD is promising as this is a widely available and rapid technique which can at the same time give a wealth of additional information on the phases formed. However, the different reaction rates of different glasses in compositionally heterogeneous fly ashes will need to be accounted for and may strongly reduce the accuracy of the profile decomposition method. This paper is the work of working group 2 of the RILEM TC 238-SCM “Hydration and microstructure of concrete with supplementary cementitious materials”

180^{180}Ta production in the classical s-process

The production and survival of the quasistable isomer $^{180}$Ta during the stellar nucleosynthesis has remained a matter of discussion for years. A careful analysis of the available experimental data and theoretical calculations enabled us to reproduce the observed solar abundance of $^{180}$Ta in the classical s-process ($kT=28$ keV -- 33 keV).Comment: 4 pages, 4 figure

Fracture energy of coarse recycled aggregate concrete using the wedge splitting test method: influence of water-reducing admixtures

The aim of this study is to evaluate the effect of the replacement levels of coarse natural aggregates with recycled aggregates and water-reducing admixtures on the fracture energy of concrete. Four mixes with 0, 20, 50 and 100% replacement ratios are produced per concrete family: without admixture, with plasticizer and with superplasticizer. The experimental fracture energy is tested using the wedge splitting test method on notched specimens at 28 days. The results prove that the incorporation of up to 20% coarse recycled aggregates led to improved energy absorption capacity of concrete mixes with water-reducing admixtures, reaching 1.5% for concrete with normal plasticizer and 7.0% for concrete with superplasticizer. Furthermore, the compressive strength, slump, and fresh density are tested in order to evaluate the effect of water-reducing admixtures on recycled aggregate concrete with different ratios of coarse natural aggregate replacement, allowing to conclude that the use of plasticizers and superplasticizers improves the behaviour of recycled aggregate concrete for all these properties

Bio-based pH-responsive superabsorbent polymers for self-healing cracks in concrete

Cracks endanger the durability of concrete. Introducing a superabsorbent polymer (SAP) during concrete mixing can create a self-sealing and -healing construction. SAPs are able to take up aqueous solutions up to several hundred times their own weight. Bio-based SAPs starting from polysaccharides have gained increasing interest in recent years due to their biocompatibility, non-toxicity and low price. The use of pH-responsive SAPs can also be extremely useful as they should only swell more upon crack formation and less during mixing of the SAPs in the concrete. The present work describes the development and the characterization of SAPs based on methacrylated polysaccharides (alginate and chitosan) combined with pH-responsive monomers dimethylaminoethyl methacrylate (DMAEMA) and dimethylaminopropyl methacrylamide (DMAPMA). The materials exhibited a high moisture uptake capacity up to 120% of their original weight with a negligible hysteresis. The pHresponsive swelling behavior was studied in aqueous and cement filtrate solutions with a varying pH. Chitosan combined with DMAEMA or DMAPMA showed the targeted pHresponsive swelling. Chitosan combined with DMAPMA also showed a limited compression strength reduction and a promising self-sealing and -healing behavior and could thus be considered as a very interesting future solution to seal and heal cracks in concrete