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Time dependent material properties of shotcrete for hard rock tunnelling
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis different mechanical properties for shotcrete (sprayed concrete) such as compression strength, bond strength, bending tensile strength, elastic modulus, free and restrained shrinkage as a function of its age was investigated. One of the main issues was to investigate the difference between ordinary cast concrete and shotcrete. Reliable material data for young and hardening shotcrete is scarce which in the past have made such comparisons difficult. Also, less accurate data representative for cast concrete has often been used in numerical modelling and design analyses. The focus of the project has particularly been on the properties bond strength and restrained shrinkage for which two new testing methods has been developed and evaluated. Microstructural studies have also been performed as a complement to the bond strength testing.

The bond to rock is one of the most important properties for shotcrete used as rock reinforcement. During the very first time after spraying the physical properties and the bond to the rock depend on the set accelerator and the micro structure that is formed. The investigation of early age bond strength of shotcrete is of great importance both from a production perspective and a safety perspective. The newly developed method was tested and evaluated and proved that it can be used for bond strength testing already from a couple of hours after shotcreting. The bond, or adhesion, depends on several factors such as texture of the rock, the type of accelerator, application technique, etc. In this work the development of the microstructure in the interfacial transition zone (ITZ) and strength of the bond was investigated. The results show that the bond strength is related to the hydration process, i.e. the strength gain of the shotcrete. The early development of the ITZ was here studied using a scanning electron microscope (SEM) making it possible to observe changes over time, before and after proper cement hydration.

Restrained shrinkage cracking of shotcrete, especially in the case of shotcrete sprayed on soft drains that are parts of a tunnel lining not continuously bonded to the rock, can be detrimental for the sustainability of an infrastructure tunnel system. Maintenance and repair costs can be high over time. It is shown that the developed test method realistically captures the behaviour of shotcrete drains on hard rock in situ. The method can be used in the evaluation of different technical solutions for avoiding or minimizing shrinkage cracks in shotcreted soft drains. It can also be used to assess the performance of shotcrete fully bonded to a rock surface, with respect to the ability to prevent cracking or to distribute possible shrinkage damage into several fine cracks instead of one wide.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , xiv, 79 p.
Series
TRITA-BKN. Bulletin, ISSN 1103-4270 ; 124
Keyword [en]
Shotcrete, rock, granite, bond strength, compressive strength, pullout testing, failure modes, interfacial transition zone, micro structure, ettringite, set accelerator, shrinkage, drains, glass fibres, laboratory testing, tunnels & tunnelling
National Category
Construction Management
Identifiers
URN: urn:nbn:se:kth:diva-145678OAI: oai:DiVA.org:kth-145678DiVA: diva2:719560
Public defence
2014-06-12, Sal F3, Sing-Sing, Lindstedtsvägen, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20140526

Available from: 2014-05-26 Created: 2014-05-26 Last updated: 2014-05-26Bibliographically approved
List of papers
1. Laboratory testing of early age bond strength between concrete for shotcrete use and rock
Open this publication in new window or tab >>Laboratory testing of early age bond strength between concrete for shotcrete use and rock
2013 (English)In: Nordic Concrete Research, ISSN 0800-6377, Vol. 47, 81-100 p.Article in journal (Refereed) Published
Abstract [en]

Shotcrete(sprayed concrete) is, together with rock bolts, the most important material used for reinforcement in hard rock tunnelling. Sprayed concrete differs from ordinary concrete through the application technique and the addition of accelerators which give immediate stiffening. The bond between sprayed concrete and rock is one of the most important properties in the quality assessment of shotcreted concrete. During the very early age after spraying the physical properties of the concrete and the bond to the rockdepend on the accelerator and the micro structure that is formed. In this work a laboratory test method for measuring early bond strength for very young shotcrete is presented.

Keyword
Shotcrete, sprayable, rock, bond strength, testing
National Category
Construction Management
Identifiers
urn:nbn:se:kth:diva-145676 (URN)
Note

QC 20140526

Available from: 2014-05-26 Created: 2014-05-26 Last updated: 2017-12-05Bibliographically approved
2. Laboratory testing of early age bond strength of shotcrete on hard rock
Open this publication in new window or tab >>Laboratory testing of early age bond strength of shotcrete on hard rock
2014 (English)In: Tunnelling and Underground Space Technology, ISSN 0886-7798, E-ISSN 1878-4364, Vol. 41, no 1, 113-119 p.Article in journal (Refereed) Published
Abstract [en]

This study investigates early age bond strength of shotcrete (sprayed concrete), in the case of shotcrete sprayed on hard rock. Shotcrete differs from ordinary, cast concrete through the application technique and the addition of set accelerators which give immediate stiffening. The bond between shotcrete and rock is one of the most important properties. During the very first time after spraying the physical properties and the bond to the rock depend on the set accelerator and the micro structure that is formed. In this work a laboratory test method for measuring early bond strength for very young or early age shotcrete is presented. The newly developed method was tested and evaluated and proved that it can be used for bond strength testing already from a couple of hours after shotcreting.

Keyword
Bond strength, Compressive strength, Failure modes, Granite, Pull-out testing, Shotcrete
National Category
Construction Management
Identifiers
urn:nbn:se:kth:diva-140790 (URN)10.1016/j.tust.2013.12.002 (DOI)000334086500013 ()2-s2.0-84891653342 (Scopus ID)
Funder
Formas
Note

QC 20140203

Available from: 2014-02-03 Created: 2014-01-31 Last updated: 2017-12-06Bibliographically approved
3. Interfacial transition zone between young shotcrete and hard rock
Open this publication in new window or tab >>Interfacial transition zone between young shotcrete and hard rock
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-145679 (URN)
Note

QS 2014

Available from: 2014-05-26 Created: 2014-05-26 Last updated: 2014-05-26Bibliographically approved
4. Investigation of restrained shrinkage cracking in partially fixed shotcrete linings
Open this publication in new window or tab >>Investigation of restrained shrinkage cracking in partially fixed shotcrete linings
2014 (English)In: Tunnelling and Underground Space Technology, ISSN 0886-7798, E-ISSN 1878-4364, Vol. 42, 136-143 p.Article in journal (Refereed) Published
Abstract [en]

This study investigates shrinkage of accelerated shotcrete (sprayed concrete), especially in the case of shotcrete sprayed on drains, a part of a tunnel lining not continuously bonded to the rock. One of the goals is to find methods of avoiding shotcrete shrinkage cracks in such drain structures. If cracks yet develop the crack distribution is of great importance, i.e. several fine cracks instead of one wide. By using both steel and glass fibres this may be achieved. A newly developed test set-up for shrinking, end-restrained shotcrete slabs is also presented and evaluated. The performed tests show that the addition of very fine glass fibres could be a solution to the cracking problem. The newly developed test equipment using concrete interacting with an instrumented granite slab represents a realistic way of testing restrained shrinkage. The on-going research focuses on the optimization of the glass fibre addition and the understanding of the interaction between shrinkage and creep of shotcrete.

Keyword
Concrete testing, Concretes, Cracks, Equipment testing, Fibers, Glass fibers, Shrinkage, Substrates, Crack distribution, Cracking problem, Drains, Laboratory testing, Restrained shrinkage, Shrinkage and creep, Shrinkage cracks, Sprayed concrete, Shotcreting, cracking, creep, laboratory method, optimization, rock mechanics, shotcrete, shrinkage, steel
National Category
Construction Management
Identifiers
urn:nbn:se:kth:diva-145673 (URN)10.1016/j.tust.2014.02.011 (DOI)000336819600014 ()2-s2.0-84896386205 (Scopus ID)
Funder
Formas
Note

QC 20140526

Available from: 2014-05-26 Created: 2014-05-26 Last updated: 2017-12-05Bibliographically approved
5. Shrinkage testing of end-restrained shotcrete on granite slabs
Open this publication in new window or tab >>Shrinkage testing of end-restrained shotcrete on granite slabs
2014 (English)In: Magazine of Concrete Research, ISSN 0024-9831, E-ISSN 1751-763X, 1300348- p.Article in journal (Refereed) Published
Abstract [en]

This study investigated the shrinkage of shotcrete (sprayed concrete), especially the case of shotcrete sprayed on drains – a part of tunnel lining not continuously bonded to the rock. A newly developed method for testing the shrinkage of end-restrained shotcrete slabs is presented and evaluated. The test setup, which is designed to capture in situ behaviour, consists of shotcrete sprayed on an instrumented granite slab partly covered with a plastic sheet. The primary goal was to practically use and evaluate the test method with shotcreted test samples. Results from restrained shrinkage tests are presented along with results for free shrinkage. It is shown that the method realistically captures the behaviour of shotcrete drains on hard rock in situ. In addition, the corresponding compressive strength and flexural crack strength as functions of shotcrete age are also reported. The test method can be used for an evaluation of different solutions for avoiding shrinkage cracks in shotcreted soft drains, or in shotcrete that is fully bonded to a rock surface, with respect to preventing cracking or distributing the shrinkage strain into several fine cracks instead of one wide crack.

Keyword
cracking, fibers
National Category
Construction Management
Identifiers
urn:nbn:se:kth:diva-145674 (URN)10.1680/macr.13.00348 (DOI)000339879000001 ()2-s2.0-84906828128 (Scopus ID)
Funder
Formas
Note

QC 20140526

Available from: 2014-05-26 Created: 2014-05-26 Last updated: 2017-12-05Bibliographically approved

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