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Investigation of non-linear drying shrinkage for end-restrained shotcrete with varying thickness
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.ORCID iD: 0000-0001-8375-581X
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.ORCID iD: 0000-0001-8336-1247
(English)In: Magazine of Concrete Research, ISSN 0024-9831, E-ISSN 1751-763XArticle in journal (Refereed) Submitted
Abstract [en]

Tunnels in hard, jointed rock is commonly reinforced with shotcrete (sprayed concrete) applied directly on the irregular rock surface. The thickness for such lining can be as low as 50 mm which result in a fast drying. The resulting shrinkage of the restrained lining is a well known phenomena that causes cracking. Installation of some drainage system also results in an end-restrained shotcrete lining which is more prone to shrinkage cracking. The drying process is a complex problem that depends on multiple factors such as cement content, porosity and conditions of ambient air, i.e. temperature, relative humidity and wind speed. In this paper, two numerical models capable of capturing the structural effects of drying shrinkage was compared. Results shows that inclusion of non-linear drying shrinkage is important to accurately describe crack initiation in a end-restrained shotcrete slab. Best fit to experimental data was obtained when the rate of drying was described as a non-linear decreasing function.

Keyword [en]
Drying shrinkage, cracking, shotcrete, varying thickness, rock support
National Category
Infrastructure Engineering
Research subject
Civil and Architectural Engineering
Identifiers
URN: urn:nbn:se:kth:diva-205309OAI: oai:DiVA.org:kth-205309DiVA: diva2:1088430
Funder
Rock Engineering Research Foundation (BeFo)
Note

QC 20170418

Available from: 2017-04-12 Created: 2017-04-12 Last updated: 2017-04-19Bibliographically approved
In thesis
1. Analyses of shotcrete stress states due to varying lining thickness and irregular rock surfaces
Open this publication in new window or tab >>Analyses of shotcrete stress states due to varying lining thickness and irregular rock surfaces
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Shotcrete is sprayed concrete applied pneumatically under high pressure and was invented in the beginning of the 1900's. This new technique decreased the construction time and since steel fibres were introduced in the shotcrete during the 1970's, shotcrete has been the primary support method for tunnels.

Tunnels excavated with the drill and blast method creates a highly irregular rock surface which results in a shotcrete lining with varying thickness. The structural behaviour as well as the loads acting on the shotcrete lining depends on the interaction between the shotcrete, rock and rock bolts. There are several parameters influencing this interaction, e.g. bond strength, the stiffness of the rock and thickness of the shotcrete. All of these parameters are difficult to predict accurately which makes the structural design of the lining to a complex problem.

This thesis present the first part of a research project with the long-term goal to improve the understanding of the structural behaviour of the shotcrete lining. To achieve this, numerical modelling have been used to study the build up of stresses and cracking of shotcrete when subjected to restrained loading caused by e.g. temperature differences and drying shrinkage. The response in the lining when subjected to a gravity load from a block has also been studied. The model is capable of describing the non-linear deformation behaviour of both plain and fibre reinforced shotcrete and uses presented in situ variations in thickness to more accurately account for the effects of expected variations in thickness. The thesis discuss and demonstrate the effect of important loads that acts on the shotcrete lining and how the irregular geometry of the rock surface in combination with the varying thickness of the shotcrete affect the development of stresses in the lining. It is also discussed how a full or partial bond failure affect the structural capacity of a shotcrete lining.  

 

Abstract [sv]

Sprutbetong är betong som appliceras pneumatiskt under högt tryckt, en metod utvecklad i början av 1900-talet. Kort därefter gjordes de första försöken att använda sprutbetong som bergförstärkning. Den här nya tekniken minskade produktionstiden och när stålfibrer introduceras under 1970-talet kunde det tunga arbetet med att placera armering minimeras. Sedan dess har sprutbetong blivit den preliminära förstärkningsmetoden, särskilt för tunnlar i hårt berg där tunna lager av sprutbetong ibland kan användas som den enda förstärkningsåtgärden. 

Tunnlar byggs normalt genom metoden "borrning-sprängning" vilket leder till att bergytan där sprutbetongen appliceras få r en oregelbunden form. Under sprutning är det svårt att fastställa den exakta tjockleken och sprutbetongen har därmed en oregelbunden tjocklek. Beroende på in situ förhållanden kan oarmerad eller fiberarmerad sprutbetong i kombination med bergbultar användas för att förstärka berget. Det strukturella beteendet och lasterna som påverkar förstärkningen beror på interaktionen mellan sprutbetong, berg och bergbultar. Denna samverkan styrs av flera parametrar som t ex; vidhäftningshållfastheten, bergets styvhet och tjockleken hos sprutbetongen. Dessa parametrar är svåra att förutsäga vilket gör dimensionering av en sprutbetongförstärkningen till ett komplext problem.

Den här uppsatsen presenterar den första delen av ett forskningsprojekt med det långsiktiga målet att öka förståelsen för det strukturella beteendet hos en sprutbetongförstärkning. För att uppnå detta har numerisk modellering använts för att studera spänningsuppbyggnaden och uppsprickningen av sprutbetong som utsätts för förhindrade rörelser orsakade av temperaturförändringar eller uttorkningskrympning. Sprutbetongens beteende när den utsätts för en blocklast har också studerats. En numeriskt modell för att analysera spänningarna i sprutbetong som tar hänsyn till tidsberoende materialegenskaper har använts. Modellen kan beskriva det icke-linjära deformationsbeteendet av oarmerad samt fiberarmerad sprutbetong och använder sig av presenterad fältdata för att beskriva de förväntade tjockleksvariationerna. Uppsatsen disskuterar och demonstrerar effekten av viktiga laster som verkar på sprutbetongförstärkningen och hur bergets oregelbundna yta i kombination med sprutbetongens varierande tjocklek påverkar spänningsuppbyggnaden i förstärkningen. Det diskuteras också hur ett fullständigt eller partiellt vidhäftningsbrott på verkar sprutbetongförstärkningens bärförmåga.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. 75 p.
Series
TRITA-BKN. Bulletin, ISSN 1103-4270 ; 150
Keyword
Shotcrete, cracking, shrinkage, block load, varying thickness, numerical simulations, fibre reinforcement
National Category
Infrastructure Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-205311 (URN)978-91-7729-368-2 (ISBN)
Presentation
2017-05-17, B21, Brinellvägen 23, Stockholm, 13:00 (English)
Opponent
Supervisors
Funder
Rock Engineering Research Foundation (BeFo)
Note

QC 20170418

Available from: 2017-04-18 Created: 2017-04-12 Last updated: 2017-04-20Bibliographically approved

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