Form Filling with Self-Compacting Concrete

Beton er det mest anvendte byggemateriale. Normal beton skal vibreres for at sikre omstøbning af armering og at betonen opnår de ønskede egenskaber. For ca. 20 år siden begyndte man i Japan at anvende såkaldt selv-kompakterende beton (SCC), som ikke behøver at blive vibreret. Denne beton har et stort anvendelsesmæssigt potential, både produktivitetsmæssigt, arbejdsmiljømæssigt og kvalitetsmæssigt. Bortset fra de traditionelle krav til beton stilles der til SCC krav til evnen til ved egen vægt at fylde formen og omstøbe armering samt til at forblive homogen både under støbning og hærdning. Den nuværende praksis for valg af materialer og støbemetoder er erfaringsbaseret; beregningsbaserede planlægningsmetoder forventes at kunne motivere en forøget anvendelse af SCC. Projektet har haft til formål at etablere modeller til simulering af formfyldning med SCC under hensyntagen til betonens flyde (rheologiske) egenskaber, formens og armeringens geometri samt støbeprocessen. Strategien har været at modellere beton som en homogen væske og at anvende et hastighedsbaseret kriterium for hvornår, der er risiko for, at stenene i betonen tilbageholdes ved indsnævringer (blokering). Herved begrænses beregningstiden betragteligt i forhold til simulering af beton som en komposit bestående af partikler i en matrice. Kombinerede simuleringer og eksperimentelle forsøg har vist, at det er muligt at simulere SCCs flydning i en form. Dette gælder både forme, som anvendes til kvalitetskontrol af SCC (flydesætmål og L-boks), en mindre væg (1 m høj, 0,3 m bred og 3 m lang, med og uden armering) og fem forskellige støbninger af vægge (4 m høje, 0,5 m brede og 5 m lange) på en byggeplads. Undersøgelserne har endvidere påvist eksperimentelle faktorer, som har væsentlig betydning for fx prøvningsresultater. Et hastighedsbaseret kriterium for blokering er udviklet og anvendeligheden er eftervist for et begrænset antal materialekombinationer og formgeometrier. Resultaterne fra det udførte ph.d. projekt er løbende blevet nyttiggjort i forbindelse med et igangværende dansk forskningskonsortium, SCC Konsortiet. Det forventes endvidere, at Teknologisk Institut vil kommercialisere projektets resultater. Abstract The overall subject of this project was Self-Compacting Concrete. More specifically it has been to establish a modelling approach for prediction of the form filling behaviour of SCC in a vertical formwork. Self-Compacting-Concrete (SCC) was first introduced in the 1980s. The purpose was to obtain flow properties which would make it possible to cast into a formwork without the need for subsequent compaction, e.g. by using poker vibrators. The possibility of not having to carry out vibration should encourage a wide use of SCC due to the prospects of improving the structural quality, working environment, productivity, and architectural appearance. However, especially in vertical applications there is a great unused potential. Controlling the casting process is important in many different industries such as the metal, plastic, and food industry. The casting process may have a significant influence on the finished product and the challenges vary depending on the material characteristics and the type of flow. Concrete may be regarded as a suspension defined as particles dispersed in a matrix phase. When the particles remain homogeneously distributed during form filling the important form filling characteristics comprise the form filling ability and flow patterns. The form filling ability describes the ability of the material to flow out into every corner of the formwork, and the flow patterns describe the intrinsic flow characteristics of the homogeneous flow, e.g. the direction and rate of flow at every point and time during placing, which may have a significant influence on the heterogeneous flow phenomena. The heterogeneous flow phenomena comprise so-called blocking and dynamic segregation. Blocking refers to the situation where the flow of aggregates is disturbed by their interaction with the reinforcement bars, which may result in severe accumulation of the aggregates. Dynamic segregation refers to the situation, when particles segregate during flow. Compared to blocking, dynamic segregation is not caused by particle interactions with the solid boundaries, but it gradually evolves during flow over a larger scale of time and length. Any change in the particle volume fraction affects the local effective properties of the suspension and thus poses a threat to any process relying on flow of a homogeneous material. In order to obtain a satisfactory form filling and thereby a satisfactory structural quality, it is necessary to develop theoretical tools to predict form filling with SCC. Trial and error is rarely an option, especially in-situ where the structural size and in-situ production often leave only one form filling attempt. Lack of theoretical prediction tools is one of the main reasons for the haltering use of SCC in vertical applications. A lack of prediction tools may lead to selection of concrete mix compositions and casting techniques that are not suitable for a given application. This project proposes a modelling approach within the framework of Computational Fluid Dynamics (CFD). CFD is applied to simulate the homogeneous form filling characteristics, i.e. the form filling ability and flow patterns, taking into account the rheological properties and casting technique. It is assumed that the rheological properties of SCC follow a Bingham model with a yield stress and plastic viscosity. In this way the accuracy and ability of CFD to simulate the homogeneous flow on a realistic form filling scale is utilized compared to, e.g. a discrete particle flow approach which requires a much larger computer capacity, especially in three dimensions. For the heterogeneous flow phenomena, this project focusses on the assessment of blocking, which is of special interest in relation to high quality and complicated structures with a dense reinforcement configuration. A phenomenological micro-mechanical model has been developed, which introduces a flow rate criterion below which blocking will occur. The model takes into account the flow domain dimension, particle size, rheological properties of the suspension and matrix, particle volume fraction, maximum particle volume fraction, and particle shape. Applying a flow rate criterion to the homogeneous form filling simulation provides a theoretical tool to assess parts of the formwork where there is a risk of blocking. The modelling approach has been applied to selected flow domains comprising standard test methods, the slump flow test and the L-box test, and form filling applications. The latter comprise two vertical laboratory formwork with and without reinforcement and five full scale formwork. Comparing the simulations with the experimental results, it shows that the modelling approach is applicable for simulation of the form filling ability and the flow patterns, and for identifying zones in the formwork with a risk of blocking. A similar modelling approach may also be applicable for assessment of the risk of dynamic segregation, and it is expected that a criterion can be developed based on theories for so-called shear-induced particle migration and gravity induced segregation due to differences in density. In future constructions with Self-Compacting Concrete the proposed modelling approach may be applied to optimise the rheological parameters, particle configuration, and casting technique for a given application in order to obtain the structural quality required.