Självkompakterande betong. : Användningsteknik

• During an earlier performed R&D project on self-compacting concrete, SCC, reported in [1], lack of knowledge was revealed on a number of issues such as for example how form pressure develops relative to different concrete mix designs, casting rates and form types. Also surface damages at the level of the interface between concrete layers was discovered. These questions, along with the experience gained through the construction of civil engineering structures using SCC between the years 1997 and 2000, has been the basis for the formation of the present R&D project. _x000D_ _x000D_ The aims of this project have been to study the thixotropic structural build-up of SCC at rest and to develop new test methods for the quantification of this structural build-up. Both existing as well as newly developed methods was to be investigated. _x000D_ _x000D_ It was also included in the plan to perform full-scale test where the form pressure should be monitored at the same time as the structural build-up was measured using the developed test methods. The goal was to find relationships between the development of form pressures and the variables stated in the project plan and to correlate them to the thixotropic structural build-up. _x000D_ _x000D_ The project started with a literature study on "Thixotropy of cement based materials" which resulted poorly, at least on the topic structural build-up at rest. However, a relatively great deal of information on thixotropy of cement paste was found but very little on the same subject for concrete. Nothing at all was found on the connection between thixotropy and form pressure. _x000D_ _x000D_ To characterise the thixotropic properties of SCC it is necessary to develop new test methods, or at least to modify existing ones. Four methods have been evaluated in the project: undisturbed slump-flow, tixometer, Rap-Act Plasticity meter and a BML viscometer where the measuring sequence was modified. All these methods have shown the ability to verify some kind of structural build up of SCC at rest. _x000D_ _x000D_ A laboratory investigation has been carried out where the mentioned methods were used for characterisation of the thixotropic properties of six different concrete mix designs. The concrete mixes were designed to have different particle concentration within their paste-phase. All methods were capable to register different structural build-ups in correlation with the varied particle concentrations. Unfortunately it was obvious during the laboratory investigation, and throughout of the whole project, that all tested concrete’s showed very short open times compared to those who were obtained in 1998 when three whole bridges was cast using only SCC. I can therefore be stated that the structural build-up of the concrete mixes tested within the framework of this project has not been of thixotropic nature but instead, of an irreversible kind. In other words: the definition of thixotropy has not been fulfilled. _x000D_ _x000D_ In an extension of the laboratory investigations, financed by Cementa AB, a new combination of constituent materials were tested, i.e. superplasticizer and filler material. However, no significant change in open time could be traced from these tests. _x000D_ _x000D_ The formwork used at the full-scale castings on Betongindustris ready mix concrete plant in Tumba south of Stockholm showed to be statically well defined and therefore easy to evaluate by means of form pressures. _x000D_ _x000D_ Totally eight full-scale trials have been performed. Water to cement ratio, W/C, was varied between 0.40 and 0.45 in the two different mix designs used. Casting rates was varied in three levels: approximately 1.0 m/h, 1.4 m/h and 2.2 m/h. Form surfaces consisted of board on one side and of form plywood on the other. Each half area of the form surfaces was oiled while the other was kept untreated. _x000D_ _x000D_ Measured form pressures showed strong correlation with casting rates. But also an increased concrete flowability and shorter time between finished mixing and casting resulted in a relatively higher form pressure. The conventionally vibrated concrete resulted in a form pressure lower than the calculated according to [24] and also lower than the one obtained when casting with the corresponding SCC at the same rate. _x000D_ _x000D_ The overall conclusions that can be drawn from this project are the following: _x000D_ _x000D_ 1) Form pressures obtained when casting with SCC is dominated by chosen casting rates in that an increased rate results in a higher pressure and vice versa. The results also indicate that concrete age and flowability have a clear influence on the form pressure. A shorter duration between mixing and casting increases the form pressure relative to a longer duration and stiffer concrete. _x000D_ 2) The formwork used showed to be statically well defined. _x000D_ _x000D_ 3) All developed methods can register the concrete structural build-up at rest. _x000D_ _x000D_ 4) A throughout problem has been short open time of the concrete tested within the project. This stiffening has not been of thixotropic nature but to a major part irreversible._x000D_

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Materialteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Materials Engineering (hsv//eng)

Nyckelord

BML-viscometer

formtryck

ostörda flytsättmått

partikelkoncentration

rapidmätare

självkompakterande betong

stighastighet

strukturuppbyggnad

tixometer

tixotropi

Publikations- och innehållstyp

ref (ämneskategori)

rap (ämneskategori)