Innovative Cross-section Shapes for Built-up CFS Columns. Experimental Investigation

Innovative cross-section shapes for built-up columns from cold-formed steel (CFS) profiles are evaluated experimentally. The goal is to obtain highly stable members with reduced sensitivity to bucking effects and initial imperfections, and therefore, higher strength-to-weight ratios. The columns have been designed following the principles of the direct strength method [1], with the ambition to exclude (or reduce) various buckling effects from the column response to compressive loads. Cross-section proportions and bolt spacing have been adapted, in order to interfere with the distortion of individual profiles and the overall buckling of the columns. Experiments show that, through proper design and insight into the behaviour of such members, columns with substantially increased overall capacity can be obtained. The good agreement between predicted and measured ultimate loads also indicates that such built-up assemblies could be integrated into everyday construction practice

Investigation on the mechanisms govergning the robustness of self-compacting concrete at paste level

In spite of the many advantages, the use of self-compacting concrete (SCC) is currently widely limited to application in precast factories and situations in which external vibration would cause large difficulties. One of the main limitations is the higher sensitivity to small variations in mix proportions, material characteristics and procedures, also referred to as the lower robustness of SCC compared to vibrated concrete. This paper investigates the mechanisms governing the robustness at paste level. Phenomenological aspects are examined for a series of paste mixtures varying in water film thickness and superplasticizer-to-powder ratio. The impact of small variations in the water content on the early-age structural buildup and the robustness of the paste rheology is investigated using rotational and oscillating rheometry

Effect of Accelerated Carbonation on AOD Stainless Steel Slag for Its Valorisation as a CO2-sequestering Construction Material

Non-stabilized Argon Oxygen Decarburisation (AODNS) slag in powdered form was examined for its carbon dioxide sequestration capacity and for its potential utilization in the fabrication of high value building materials. The curing of the sample was carried out in two accelerated carbonation environments: i) in a carbonation chamber, maintained at atmospheric pressure, 22 °C, 5 vol.% CO2 and 80% RH; and ii) in a carbonation reactor, where the CO2 partial pressure (pCO2) and temperature could be further increased. In the carbonation chamber, an average compressive strength of over 20 MPa, on a 64 cm3 cubic specimen, was obtained after one week of curing, which is sufficient for many construction applications. Further carbonation resulted in a linear increase of strength up ~30 MPa after three weeks. The CO2 uptake followed a similar trend, reaching a maximum of 4.3 wt.%. In the reactor, the compressive strength improved with an increase in pCO2 up to 8 bar, temperature up to 80 °C, and duration up to 15 h where the maximum CO2 uptake was 8.1 wt%. The reduction in porosity in the carbonated specimens was approximately in line with the strength gain in the samples. Phase analysis by X-ray powder diffraction and inspection by scanning electron microscopy showed the precipitation of calcite and formation of significant amounts of amorphous material after carbonation. Infrared spectroscopy also pointed to the presence of aragonite and vaterite. In the carbonation chamber, the calcite morphology was uniform throughout the specimen. In the reactor, however, the calcite crystals near the outer edges of the cubes had different morphology than those near the core. Carbonation of the slag resulted in the reduction of basicity by up to one pH unit, and contributed to controlling the leaching of several heavy metals and metalloids

The influence of fillers on the properties of self-compacting concrete in fresh and hardened state

This paper represents the results of an investigation of the suitability of different fillers to be used for SCC. An experimental test program has been executed with 12 different SCC mixtures, each mixture containing another type of filler. The other mix parameters (water-to-cement ratio, fine and coarse aggregates content) were the same for all mixtures. Firstly, concrete mixtures were made to examine the influence of the same volume of filler on the mix proportion. Secondly, all mixtures that could not be defined as SCC (according to workability) were adapted. The different mixtures were investigated with respect to workability (slump flow, V-funnel and U-flow), compressive strength, shrinkage, water absorption and freeze-thaw resistance (with and without de-icing salts). Each filler has been characterised by different tests: water demand (beta_p), Blaine, activity index, particle size distribution and microscopic visualisation using a scanning electron microscope (SEM). An effort has been made to link the measured properties of the SCC with the characterising parameters of the fillers.status: publishe

Chloride penetration and carbonation in self-compacting concrete

In this research program, both the steady-state and the non-steady-state migration test are used to determine the chloride diffusion coefficient D (m²/s) of 8 different self-compacting concrete mixes and 1 reference, traditionally vibrated, concrete mix. Simultaneously, the carbonation behaviour of those mixes was investigated. Therefore, the carbonation depth was measured at regular points in time according to NT BUILD 357 (1989) and a carbonation constant A (mm/√year) was deduced. Concerning the chloride diffusion coefficient, test results revealed that the determination of the steady-state migration coefficient according to NT BUILD 355 (1997) is far from easy and question marks could be placed beside the corresponding diffusion coefficient but an explanation for the observed anomalies could not be found yet. The non-steady-state diffusion coefficient according to NT BUILD 492 (1999) was used in order to rank the different concrete mixtures. The carbonation constant could best be measured using an inflated CO2-concentration, resulting in a more linear behaviour of the carbonation depth in function of time. Besides, using this carbonation constant, results reveal that the concrete mixtures could be ranked in the same way as they were by the non-steady-state chloride diffusion coefficient.status: publishe

The influence of filler characteristics on the workability of self-compacting concrete

Self-compacting concrete (SCC) is considered more and more as the “concrete of the future”. Its high workability not only improves the labour conditions and the speed of working, also the uniformity and quality of the end product is increased. To reach this high workability, the amount of coarse aggregates has to be reduced and replaced by fine material. Since cement is quite expensive and a too high amount of cement causes a strong hydration reaction, with possible problems of shrinkage cracks, fillers (i.e. mineral additions) are added. This paper presents the results of an investigation on the suitability of different fillers to be used for SCC. An experimental test program has been executed with 12 different SCC mixtures, each mix with another type of filler. The other mix parameters (water-to-cement ratio, fine and coarse aggregates content) were the same for all mixes. The different mixes were investigated with respect to workability (slump-flow, V-funnel and U-test), compressive strength, shrinkage and freeze-thaw resistance. These properties were sometimes very different if only another type of filler was used. Each filler has been characterised with different tests: water demand (beta_p), Blaine, activity index, particle size distribution (by means of laser diffraction) as well as the roundness, sphericality and surface structure of the filler particles which were observed through a scanning electron microscope (SEM). An effort has been made to link the measured properties of the SCC mixtures with the characterising parameters of the fillers. It was found that the microscopic filler particle properties (roundness, sphericality and surface structure) play a determining role.status: publishe

Invloed van vulstoffen op hydratatie en eigenschappen van SCC

De in België courante opvattingen betreffende het mengselontwerp van zelfverdichtend beton (SCC) leiden tot het gebruik van grotere hoeveelheden vulstoffen in een zelfverdichtende betonsamenstelling in vergelijking met een traditioneel verdicht betonmengsel (TC). Een belangrijk onderdeel van het fundamenteel onderzoek betreffende SCC spitst zich dan ook toe op de invloed van verschillende types vulstoffen op eigenschappen van het uiteindelijke betonmengsel zoals eigenschappen van het verse mengsel, hydratatie, duurzaamheidsaspecten en tijdsafhankelijke fenomenen.status: publishe