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Durability of fire exposed concrete: Experimental Studies Focusing on Sti„ffness & Transport Properties
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures. SP Technical Research Institute of Sweden.
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Road and rail tunnels are important parts of the modern infrastructure. High strength concrete (HSC) is commonly used for tunnels and other civil engineering structures, since it allows high load carrying capacity and long service life. In general, Swedish road and rail tunnels should be designed for a service life of 120 years. However, HSC has shown to be sensitive to severe fires in the moist tunnel environment, i.e., fire spalling may occur. Extensive research shows that addition of polypropylene (PP) fibres in the fresh concrete mix significantly reduces the risk of fire spalling. The durability of a concrete structure is mainly governed by the transport properties. Further, experimental studies aimed at understanding the protective mechanism of PP fibres indicate that fluid transport increases in connection with the melting temperature of such fibres. This might reduce the durability of fire exposed concrete with addition of PP fibres. This study aims to investigate whether the use of PP fibres has any significant effect on the durability of moderate fire exposed concrete structures.

The experimental study focused on transport properties related to durability and stiffness reduction of fire exposed civil engineering concrete with and without addition of PP fibres. The study consists of three parts; (i) unilateral fire exposure in accordance with the standard time-temperature curve (Std) and a slow heating curve (SH), (ii) uniformly heating of non-restrained samples to 250oC, and (iii) moderate unilateral fire exposure, 350oC, of restrained samples. Changes in material properties caused by the fire exposure were studied by means of ultrasonic pulse velocity, full field-strain measurements during uniaxial compression core tests, polarization and fluorescence microscopy (PFM), water absorption and non-steady state chloride migration.

The study shows that fire exposure influences different properties of importance for load carrying capacity and durability. To get a clear image of the fire damage one has to combine different test methods during damage assessments. Transport properties of concrete both with and without addition of PP fibres were considerably affected even at moderate fire exposure. Hence, the service life might be reduced. All series with addition of PP fibres exhibited higher water absorption compared to the series without PP fibres. The practical importance of this might, however, be small since also the water absorption of concrete without PP fibres was considerably affected for the fire scenarios considered in this study. Behind the fire exposed surface, i.e., between 30 and 60 mm, no change in water absorption was observed for concrete without PP fibres. However, higher water absorption of the series with addition of PP fibres was observed.

Indicative fire tests aimed to evaluate the resistance to fire spalling during a subsequent severe fire was also conducted. The concretes with addition of PP fibres showed no signs of fire spalling, while progressive spalling was observed for the concrete without PP fibres.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , viii, 50 p.
Series
TRITA-BKN. Bulletin, ISSN 1103-4270 ; 133
Keyword [en]
Concrete, Fire, Polypropylene fibres, Durability, Tunnel, Damage assessment, Ultrasonic pulse velocity, Water absorption, Chloride migration, DIC
National Category
Civil Engineering
Research subject
Civil and Architectural Engineering
Identifiers
URN: urn:nbn:se:kth:diva-168386OAI: oai:DiVA.org:kth-168386DiVA: diva2:816099
Presentation
2015-06-08, L1, Drottning Kristinas väg 30, Stockholm, 13:30 (Swedish)
Opponent
Supervisors
Note

QC 20150603

Available from: 2015-06-03 Created: 2015-06-02 Last updated: 2015-06-03Bibliographically approved
List of papers
1. Assessment of fire exposed concrete with full-field strain determination
Open this publication in new window or tab >>Assessment of fire exposed concrete with full-field strain determination
2011 (English)In: 2nd International RILEM Workshop on Concrete Spalling due to Fire Exposure, 2011, 337-344 p.Conference paper, Published paper (Refereed)
Abstract [en]

A concrete structure exposed to fire deteriorates when the temperature increase in the structure. An experimental study has been performed in order to evaluate the degree of degradation of concrete exposed to two different fire scenarios. As the thermal diffusivity of concrete is low, high thermal gradient is induced in the cross-section. This causes inhomogeneous mechanical properties of a concrete structure. In traditionally core testing of the elastic modulus and compressive the material is assumed to be homogeneous, this is not the case when concrete has been exposed to a real fire. By using an optical full-field strain measuring device the mechanical response at different depth, from the fire exposed surface, can be studied. In this study a typical concrete mix for civil engineer applications were used. In addition a similar concrete mix with reduced aggregate size was tested. The test samples were exposed to the standard fire curve ISO 834-1 or a temperature rise of 10 °C/min. In addition, Ultrasonic Pulse Velocity measurement and PFM Microscopy were conducted in order obtain a reliable picture of the residual mechanical properties and the durability.

National Category
Building Technologies
Identifiers
urn:nbn:se:kth:diva-168406 (URN)
Conference
2nd International RILEM Workshop on Concrete Spalling due to Fire Exposure, 2011, Delft, the Netherlands
Note

QC 20150603

Available from: 2015-06-03 Created: 2015-06-03 Last updated: 2015-06-03Bibliographically approved
2. Assessment of fire exposed concrete structures
Open this publication in new window or tab >>Assessment of fire exposed concrete structures
2012 (English)In: Symposium 2012, 2012, 619-622 p.Conference paper, Published paper (Refereed)
Abstract [en]

Annually, several concrete structures, such as buildings, bridges, parking garages and tunnels are exposed to fires. An assessment is then necessary to decide whether the structure can be repaired or needs to be replaced. In a recent research project, recommendations for assessments of fire exposed concrete structures have been developed. The recommendations are based on a literature survey, results from an experimental study, where ultrasonic measurements, microscopy, Digital Image Correlation (DIC) measurement on loaded core samples were used and practical experience of real post-fire structural assessments. A refined assessment of the fire damage is obtained by combining these test methods.

National Category
Building Technologies
Identifiers
urn:nbn:se:kth:diva-168405 (URN)
Conference
Concrete Structures for Sustainable Community, Stockholm, Sweden, June 11-14, 2012
Note

QC 20150603

Available from: 2015-06-03 Created: 2015-06-03 Last updated: 2015-06-03Bibliographically approved
3. Capillary suction and chloride migration in fire exposed concrete with PP-fibre
Open this publication in new window or tab >>Capillary suction and chloride migration in fire exposed concrete with PP-fibre
2012 (English)In: Concrete Repair, Rehabilitation and Retrofitting III (ICCRRR), 2012, 128-129 p.Conference paper, Published paper (Refereed)
Abstract [en]

Annually, several concrete structures, such as buildings, bridges, parking garages, tunnels, etc. are exposed to fires. Many fires are small, for example single car fires in tunnels. These fires do not affect the load carrying capability of the concrete structure and thus minor or no reparations are required. In modern concrete for civil engineer applications the use of Polypropylene fibres (PP-fibre) to reduce fire spalling is growing. Some studies have been carried out which indicate that the use of PP-fibres will not affect the durability of the concrete. But in case of a fairly moderate fire exposure, a fire exposure that does not lead to structural damage, the PP-fibres can potentially lead to reduced durability. During low intensity fires or at long distances downstream a large fire in a tunnel the PP-fibres melts and form channels in the concrete. After such degradation of the PP-fibres it is plausible that accelerated damage may occur when moisture, de-icing salts and carbon dioxide can more easily penetrate the concrete. In this experimental study the chloride migration and the capillary suction are studied in moderately heated concrete containing PP fibres. The chloride migration tests were conducted with heated samples with and without PP-fibres. The capillary suction tests were even conducted with different fibre contents. As a reference the results are compared with results from unheated concrete. The aim of the project is to define whether or not measures have to be taken to repair concrete structures after small fires and at long distances downstream from large fires in tunnels. If the durability is affected the costs and consequences of not repairing and refurbishing after the fire can potentially be very high especially after a fires in very long tunnel.

National Category
Building Technologies
Identifiers
urn:nbn:se:kth:diva-168407 (URN)
Conference
3rd Internetional Conference on Concrete Repair, Rehabilitation and Retrofitting 2012, Cape Town, South Africa
Note

QC 20150603

Available from: 2015-06-03 Created: 2015-06-03 Last updated: 2015-06-03Bibliographically approved
4. Durability of fire exposed concrete cover considering non-linear thermal gradient, boundary effecs and polypropylene fibres
Open this publication in new window or tab >>Durability of fire exposed concrete cover considering non-linear thermal gradient, boundary effecs and polypropylene fibres
(English)Manuscript (preprint) (Other academic)
National Category
Building Technologies
Identifiers
urn:nbn:se:kth:diva-168408 (URN)
Note

QS 2015

Available from: 2015-06-03 Created: 2015-06-03 Last updated: 2015-06-03Bibliographically approved

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