Betydelsen av betongens koldioxidupptag ur ett livscykelperspektiv

I denna artikel beskriver Ingemar L\uf6fgren, FoU\ua0chef C-lab, Thomas Concrete Group hur betong\ua0tar upp koldioxid ur luften och hur det ber\ue4knas.\ua0H\ue4r visas ocks\ue5 n\ue5gra exempel f\uf6r olika\ua0konstruktioner d\ue4r koldioxidupptaget anges i kg\ua0koldioxid per kvadratmeter exponerad yta. Koldioxidupptaget\ua0f\uf6r ett \uader familjshus redovisas\ua0och exempel ges p\ue5 hur upptaget kan \uf6kas\ua0genom alternativa l\uf6sningar. Redovisade resultat\ua0och nomenklatur \ue4r baserad p\ue5 metodik\ua0enligt den europeiska standarden EN 155804

Ballastless Track – Minimizing the Climate Impact

Railway transportation is becoming increasingly important for transport of passengers and goods in Sweden, Europe and many parts of the world. Ballastless (slab) railway systems are increasingly in use; however, their construction is known to cause a substantial climate impact. The objective of this study was to investigate possible methods to reduce greenhouse gas (GHG) emissions of slab tracks and to provide required knowledge to identify the methods with high potential for further development. The approach adopted in this study consists of two steps. First, a comprehensive literature study was carried out, including a survey of existing methods for reducing GHG emissions for slab tracks, and of those which require further research. These methods are presented and assessed with respect to criteria related to potential benefit, possibility to use in large volumes, quality assurance and cost.In the second step, recommendations are made on which of the different methods of reducing GHG emissions are suitable to further develop in future projects. Two uncertainties identified for all methods are related to quantification of potential benefits and the associated costs. Nonetheless, structural optimization of slab tracks is found to have potential to reduce the climate impact quite substantially, with the smallest risks associated. The most promising methods for structural optimization includes: geometry optimization to focus on the use of material where it is structurally most effective; stiffness optimization to reduce the energy consumption of trains; prestressing of concrete to minimize crack width; and employing steel fiber reinforced concrete to control cracks and reduce the use of traditional reinforcement. Three solutions combining these methods in different ways are suggested for future studies. Furthermore, methods related to the use of alternative binders & materials are also recommended to reduce the climate impact; however, it is noted that such methods in general exhibit larger uncertainties than structural optimization. Of the alternatives focusing on alternative binders & materials, the following were evaluated to be most promising: textile reinforcement, other cement types (e.g. CSA, BCSA & BYF cements) as well as optimized mix design of concrete. It is to be noted that the three suggested solutions based on structural optimization can also benefit from the use of alternative binders & materials. To sum up, combination of several methods is required to minimize the environmental impact, as in the suggested solutions. The needs for future investigation for each solution are also identified in the report. The project contributes to the overall goal of increasing consideration for the environment and climate by providing knowledge and road map on how GHG emissions can be reduced for slab tracks

Experimental study of time-dependent properties of a low-pH concrete for deposition tunnels

The Swedish Nuclear Fuel and Waste Management Company developed a method for the final disposal of canisters for spent nuclear fuel in tunnels at depths of about 500 meters. The concept for closure of the deposition tunnels is based on a bentonite seal supported by a spherical concrete dome structure. In order to fulfil the requirements specific to the repository concept, a special mix of low-pH self-compacting concrete was developed. A series of large-scale castings and laboratory tests were conducted to gain experience on this low-pH concrete mix, in conjunction with the full-scale demonstration test of an unreinforced concrete dome plug in the underground hard rock laboratory in \uc4sp\uf6, Sweden. The laboratory tests aimed at studying the creep properties under high sustained compressive stresses of the low-pH concrete mix, its shrinkage properties and the properties of the rock-concrete interface. This paper provides an overview of these tests and analyses the latest results of the recently completed creep tests, which include 6 years of measurements. These results allow to improve understanding of the structural behaviour of the concrete plug and to assess the effects of the very high pressure acting on the plug on its deformations, cracking and water tightness

Experimental study of time-dependent properties of a low-pH concrete for deposition tunnels

The Swedish Nuclear Fuel and Waste Management Company developed a method for the final disposal of canisters for spent nuclear fuel in tunnels at depths of about 500 meters. The concept for closure of the deposition tunnels is based on a bentonite seal supported by a spherical concrete dome structure. In order to fulfil the requirements specific to the repository concept, a special mix of low-pH self-compacting concrete was developed. A series of large-scale castings and laboratory tests were conducted to gain experience on this low-pH concrete mix, in conjunction with the full-scale demonstration test of an unreinforced concrete dome plug in the underground hard rock laboratory in \uc4sp\uf6, Sweden. The laboratory tests aimed at studying the creep properties under high sustained compressive stresses of the low-pH concrete mix, its shrinkage properties and the properties of the rock-concrete interface. This paper provides an overview of these tests and analyses the latest results of the recently completed creep tests, which include 6 years of measurements. These results allow to improve understanding of the structural behaviour of the concrete plug and to assess the effects of the very high pressure acting on the plug on its deformations, cracking and water tightness

Analysis of the flexural response of hybrid reinforced concrete beams with localized reinforcement corrosion

This paper presents a modeling approach to analyze the flexural response of hybrid reinforced concrete beams with localized corrosion. A new mechanical model based on extensive uniaxial testing is proposed to describe the stress–strain relationship of corroded bars with a single pit. The proposed mechanical model is then incorporated into a sectional analysis to determine the moment curvature relationship of hybrid reinforced concrete sections with pitting corrosion. The actual crack pattern is used to divide a beam into discrete hinge elements which are then combined to compute the load–deflection response of statically determinate beams. The modeling approach is evaluated with available experimental data showing good predictive capabilities. A parametric study revealed the importance of the interaction between the tensile reinforcement ratio and the concrete postcracking residual stress. Furthermore, the deformation capacity of reinforcement bars with pitting corrosion levels beyond 0.25 was shown to have a dominant effect on the ultimate deflection of hybrid reinforced concrete beams

Comparison of the service life, life-cycle costs and assessment of hybrid and traditional reinforced concrete through a case study of bridge edge beams in Sweden

The edge beams of reinforced concrete bridges with de-icing salts sprayed experience extensive corrosion damage. The average service life of edge beams needing replacement in Sweden has been reported as only 45 years, causing great economic loss to both owners and users. Hence, finding a durable solution for edge beams would benefit society. Hybrid reinforced concrete structures, produced by adding a low-to-moderate fibre content into traditional reinforced concrete, can effectively limit the service crack width and improve resistance to chloride-induced corrosion damage. In this paper, different alternatives of hybrid and traditional reinforced edge beams were designed for a case study. The service life of the alternatives was compared by conducting chloride diffusion calculations and by applying a corrosion-induced cracking model. The economic and environmental (indicated by greenhouse gas emissions) benefits of using hybrid reinforced edge beams were assessed by life-cycle cost analysis and life-cycle assessment. The results showed that the service life of edge beams made of hybrid reinforced concrete can be prolonged by over 58%, thereby enabling a significant reduction in the total life-cycle costs and annual total greenhouse gas emissions

Correlation between concrete cracks and corrosion characteristics of steel reinforcement in pre-cracked plain and fibre-reinforced concrete beams

This paper presents results on corrosion characteristics of 66 rebars extracted from un- and pre-cracked plain concrete and fibre-reinforced concrete (FRC) beams suffering from corrosion for more than 3 years. The influences of fibre reinforcement, flexural cracks, corrosion-induced cracks and loading condition on the maximum local corrosion level (defined as the maximum cross-sectional area loss percentage) and pit morphology were examined. With 3D-scanning, the corrosion characteristics were analysed, and pit types were classified based on the maximum local corrosion level and geometric parameters of pits. Corrosion pits were observed near some flexural cracks, while the bars at other cracks were free from corrosion. Most rebars in FRC had less maximum local corrosion level than those in plain concrete under the same loading condition and maximum flexural crack width. However, the maximum local corrosion level was not dependent on the maximum flexural crack width (0.1 and 0.4 mm). Longitudinal cracks (corrosion-induced cracks) aggravated the total steel loss and changed the pit morphology by promoting the pit length development. However, longitudinal cracks did not always form, even with severe pitting corrosion. A hypothesis about the time-dependent interplay between transverse and longitudinal cracks and corrosion development was proposed. Further studies on predicting the pitting corrosion evolution and experimental work on specimens exposed for longer periods are needed to understand and quantify the long-term durability of concrete structures reinforced with both conventional reinforcing bars and fibres

Real-time monitoring the electrical properties of pastes to map the hydration induced microstructure change in cement-based materials

The effect of the supplementary materials (SCMs) on the moisture content and ion diffusivity at different hydration time is important for the service life modelling of modern concrete. This study designed a simple but valid method to monitor the microstructure change in pastes during hydration. A procedure easy to implement was proposed to detect the water content in pastes. The electrical conductivity of pore solution was evaluated by the evaporable water content in pastes and chemical composition in the binders. Results show that the electrical properties of pastes (conductivity, formation factor and its growth rate) can effectively indicate the hydration reactivity of binder, pore connectivity and volume of pore solution in the hardened pastes. The effect of waterbinder ratio and SCMs on the structure of pastes are effectively indexed by the formation factor which is the conductivity of pore solution divided by that of paste. The inflection point of average growth rate of formation factor is a good index for the final setting of pastes. The relation between volume of evaporable water and formation factor is well demonstrated by the extended percolation theory. The real-time monitored electrical conductivity and formation factor of pastes can be used to calculate the chloride migration coefficient in hardened cement pastes

Moisture and ion transport properties in blended pastes and their relation to the refined pore structure

This paper presents a study of the\ua0moisture transport\ua0properties in blended pastes measured by a new procedure and setup. The dependence of moisture transport coefficient on relative humidity (RH) is confirmed. The differences in the moisture dependency may be due to discrepancies in the critical RH for the\ua0percolation\ua0of liquid. Fly ash and slag increase the percentage of mesopores or “ink-bottle” pores with a\ua0mesoscale\ua0neck and they strongly reduce the pore connectivity in pastes. These effects cause the evident reduction in the moisture and chloride diffusivity. The additional replacement with\ua0limestone filler\ua0has little effect on the pore connectivity. The formation factor controls the moisture transport at the high RH interval, but the volume of small pores (middle capillary and mesopores) is the major determinant at a low RH interval. The relationship between water-binder ratio,\ua0pore structure\ua0and moisture transport or chloride migration coefficient is discussed

Assessment of cast-in-situ FRC linings for high-speed railway tunnels with respect to fatigue and cracking

This report presents the results of a pre-study aimed at investigating a number of critical aspects identified for the execution of a fibre reinforced concrete lining in relation to the construction of high-speed railways in Sweden. The work carried out focused primarily on the study of fatigue performance of FRC and on the structural response of the tunnel lining under the actions of various loads including air pressure and suction, shrinkage and temperature changes.In the first part of the report a literature review was carried out to examine the state-of-the-art of fatigue of FRC and derive some qualitative relations to evaluate the risk of fatigue failure due to cyclic loading of the tunnel lining (106 cycles). Subsequently, the tunnel lining was modelled using the commercial finite element software DIANA and non-linear analyses were carried out to determine the risk of cracking and the potential crack widths for a number of load cases.The main results of this pre-study showed that fibres can provide a small enhancement of the fatigue performance of FRC compared to plain concrete for moderate fibre dosages of up to 1.0% vol. Nevertheless, the effect of reverse loading and existing cracks on the fatigue behaviour of FRC are largely unexplored and require further investigation. It was also found that cyclic loading due to the air pressure and suction from passing trains produced a maximum stress variation of about 10% of the concrete tensile strength, thus posing no risk for fatigue failure provided the concrete lining is uncracked. Conversely, non-uniform shrinkage and temperature gradients were able to induce cracking. In cracked conditions, the air pressure and suction were sufficient to produce excessively large cracks, thereby posing a risk for the fatigue life and durability of the tunnel