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Form pressure generated by self-compacting concrete: influence of thixotropy and structural behaviour at rest
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Self-compacting concrete (SCC) offers rational and fast casting process since it merely has to be poured, or pumped, into the formwork without any compaction work needed. But this can be at the cost of high form pressure. However, reported results show that SCC can act thixotropically, i.e., build up a structure at rest, and this can reduce the form pressure considerably. Thus, in order to utilise the favourable possibilities to increase effectiveness without risking form collapses, the need arises for deeper and broader understanding of the mechanisms behind this thixotropic behaviour.

Methodologies have been developed for the characterisation and measurement of the structural build-up at rest, both for the fluid (micro mortar) phase and the concrete itself. Hypotheses state that thixotropic mechanisms originate within the colloidal domain and, thus, motivate studies on the fluid phase comprising this domain. The stress-strain methodology is based on the hypothesis stating that the magnitude of the structure is represented by the maximum elastic stress the fresh material can withstand before the structure breaks. An instrumented steel tube is used to simulate various casting heights and rates.

Results show that both micro mortar and SCC are thixotropic and this behaviour is influenced by every measure taken influencing the interparticle colloidal forces. The time-dependent structural build-up of SCC is a function of an irreversible structure (slump-loss) and a reversible, thixotropic structure.

There is apparently a threshold value of the structural build-up necessary to reach before obtaining any significant form pressure reduction. Housing SCC´s, with W/C = 0.58, show low degree of structural build-up and pressure decrease while civil engineering SCC´s can show the opposite, but this often at the cost of slump-loss.

Recommendations are presented and for the nearest future, suggesting a conservatism regarding design of formwork systems when SCC is used. If the behaviour of a SCC is known it should be used to optimise the formwork. If not, calculating with hydrostatic pressure should be done or the knowledge missing should be gained by using this methodology. A third option is given and this is to monitor the form pressure in real time using sensors.

Place, publisher, year, edition, pages
Stockholm: Byggvetenskap , 2006. , xiv, 91 p.
Series
Trita-BKN. Bulletin, ISSN 1103-4270 ; 85
Keyword [en]
self-compacting concrete, thixotropy, structural build-up, form pressure, rheometer, viscometer, instrumented pressure tube
National Category
Building Technologies
Identifiers
URN: urn:nbn:se:kth:diva-4144ISBN: 91-7178-464-0 (print)OAI: oai:DiVA.org:kth-4144DiVA: diva2:10913
Public defence
2006-10-27, Sal F3, KTH, Lindstedtsvägen 26, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100812Available from: 2006-10-10 Created: 2006-10-10 Last updated: 2010-09-10Bibliographically approved
List of papers
1. Mechanisms behind reduced form pressure when casting with SCC
Open this publication in new window or tab >>Mechanisms behind reduced form pressure when casting with SCC
2005 (English)In: RILEM Proceedings PRO 42 / [ed] Yu ZW; Shi C; Khayat KH; Xie YJ, 2005, Vol. 42, 589-598 p.Conference paper, Published paper (Other academic)
Abstract [en]

Among the topics within the more than 10 year experience from use of self-compacting civil engineering concrete in Sweden there is the fact that the lateral form pressure can be lower than the design values for conventional concrete. This is in total contradiction to results from different parts of the world where pressures close to or equal to hydrostatic pressure have been reported. A project has been started and partly carried out in order to investigate on fresh SCC behaviour at rest. The hypothesis is that this extremely flowing concrete can build up a structure at rest withstanding the load of the concrete layers above with limited effect on the increasing lateral form pressure. A new methodology has been developed using rheometers for both SCC and its micro mortar phase. The mechanism behind the thixotropic behaviour is to be found in the latter phase where colloidal forces dominate. In the present stage of this project it has been established that SCC indeed develops a structure at rest. The magnitude of this structure is dependant on the concrete constituents.

Series
RILEM PROCEEDINGS, 42
Keyword
Construction & Building Technology; Engineering, Civil
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-14401 (URN)10.1617/2912143624.063 (DOI)000243127200063 ()
Conference
First International Symposium on Design, Performance and Use of Self-Consolidating Concrete, SCC’2005
Note
QC 20100804Available from: 2010-08-04 Created: 2010-08-04 Last updated: 2012-02-13Bibliographically approved
2. Development of SCC static yield stress at rest and its effect on thelateral form pressure
Open this publication in new window or tab >>Development of SCC static yield stress at rest and its effect on thelateral form pressure
2005 (English)In: SCC 2005, Combining the Second North American Conference on the Design and Use of Self-Consolidating Concrete and the Fourth International RILEM Symposium on Self-Compacting Concrete, 2005, 583-589 p.Conference paper, Published paper (Refereed)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-24056 (URN)
Conference
SCC 2005, Combining the Second North American Conference on the Design and Use of Self-Consolidating Concrete and the Fourth International RILEM Symposium on Self-Compacting Concrete
Note
QC 20100812Available from: 2010-08-12 Created: 2010-08-12 Last updated: 2012-02-13Bibliographically approved
3. Form Pressures Generated by Self-Consolidating Concrete
Open this publication in new window or tab >>Form Pressures Generated by Self-Consolidating Concrete
2005 (English)In: Concrete International, ISSN 0162-4075, Vol. 27, no 10, 35-42 p.Article in journal (Refereed) Published
Abstract [en]

Since its development, it has been generally assumed that self-consolidating concrete (SCC) would generate hydrostatic formwork pressure due to the rate at which it can be cast and the long time before it starts to set. However, in 1998 when the first bridges were cast using SCC in Sweden, measured form pressures were even lower than the design values for conventional concrete. To investigate the need for special formwork design considerations when using SCC, a series of eight wall specimens were cast, and the resulting form pressures were monitored.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-24057 (URN)
Note
QC 20100812Available from: 2010-08-12 Created: 2010-08-12 Last updated: 2012-02-13Bibliographically approved
4. Time-Dependent Growth of Static and Dynamic Yield Stress of SCC
Open this publication in new window or tab >>Time-Dependent Growth of Static and Dynamic Yield Stress of SCC
2006 (English)In: Materials and Structures, ISSN 1359-5997, E-ISSN 1871-6873Article in journal (Other academic) Submitted
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-24059 (URN)
Note
QS 20120316Available from: 2010-08-12 Created: 2010-08-12 Last updated: 2012-03-16Bibliographically approved
5. SCC Structural Behaviour at Restand Its Influence on Form Pressure
Open this publication in new window or tab >>SCC Structural Behaviour at Restand Its Influence on Form Pressure
2006 (English)In: RILEM Materials and Structures Journal, ISSN 1359-5997Article in journal (Refereed) Published
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-24060 (URN)
Note
QC 20100812Available from: 2010-08-12 Created: 2010-08-12 Last updated: 2012-02-13Bibliographically approved

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