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Fire Spalling of Concrete: Theoretical and Experimental Studies
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Fire spalling of concrete is not a new phenomenon. To some degree there has always been a risk during rapid heating of concrete. Therefore, to a certain degree the effect of fire spalling is included in the bank of data from fire tests and fires on which our understanding of the fire resistance of concrete is based. However, the development and modern use of more dense concrete mixes have produced cases of very severe fire spalling which have increased the urgency to understand this phenomenon. In this context, the use of an addition of polypropylene (PP) fibres to the mix to limit the amount of spalling has been one topic of interest for this thesis.

During fire tests on a post-tensioned concrete structure made of spalling sensitive concrete, it has been shown that substantially lower amounts of PP fibres than 2 kg/m3, which is recommended in the Eurocode (1992-1-2:2004), can be used with successful results.

As part of this study, another important aspect has emerged, i.e. the impact the test method used can have on the fire spalling depths observed in concrete specimens. This has been known for many years but is seldom discussed in the scientific literature. In this thesis it has been shown that results from tests on unloaded cubes do not necessarily correspond to results seen on larger loaded slabs. In the results presented, none of the tested cubes spalled whereas some of the large slabs spalled to the degree that the reinforcement became fire exposed. Further, the difference in spalling depths between small and large post-tensioned slabs was shown to be substantial; although in general the ranking in severity from least to greatest spalling correlated between these two specimen sizes. The correlation to larger specimens was much vaguer in the case when the small slabs were not loaded in compression as there sometimes was no spalling in the small slabs.

From time to time the randomness of the fire spalling of concrete has been mentioned. To investigate this further, an analysis of 110 fire tests performed on small slab type specimens was performed. This analyse showed that the spalling behaviour had a good repeatability between two identical tests, which proved that the so called “random factor” relating to spalling depth was low for the chosen data set. It was also possible to make a multiple least squares fit of test parameters that could be used to predict the spalling behaviour which also underlines that a substantial stochastic factor was not present.

Regarding the influence of different factors, the results compiled on the influence of ageing show that for three of the tested Self-Compacting Concrete (SCC) mixes, the amount of spalling was reduced with age whereas for the fourth mix (which included the highest amount of limestone filler, 140 kg/m3) the spalling was not reduced with age. In this test series no systematic influence of the intensity of the fire, between standard fire exposure and the more severe hydrocarbon fire, on the spalling depth was detected for this type of specimen. The only major difference was that spalling started earlier during the more severe fire exposure.

Pressure measurements conducted as part of the work within this thesis, supported by results from the literature, indicate that there is no relationship between pressure rise due to moisture and fire spalling. Based on this and the fact that the spalling event in many cases happens at relatively low temperatures where the saturation vapour pressure is low two alternative factors to explain the function of PP fibres have been presented: (i) PP fibres reduce the moisture content in the critical zone close to the heated surface which affects the mechanical properties advantageously, and (ii) PP fibres amplify moisture movement leading to larger drying creep and shrinkage which locally relax the thermal stresses.

To  investigate  the  influence  of  the  presence  of  moisture  on  the  compressive  strength, specimens were tested after being boiled for varying periods of time in a water bath. The study showed a remarkable reduction of strength due to boiling of the mortar specimens. After boiling mortar in a water bath for 3, 10 or 20 minutes, i.e. approximately the same time span as the initiation of fire spalling during fully developed fires, the strength was only 64% of the corresponding value for a dry specimen. As no strength change was detected between the specimens  boiled  3, 10 or 20 minutes,  and that the corresponding  saturation  pressure  for steam at 100ºC is negligible compared with the tensile strength of concrete, it was concluded that pore pressure is not a significant  contributor  to the measured reduction in strength.  It appears that the presence of moisture itself rather than an increased pressure is the most important factor reducing strength. This is a clear indication that moisture plays a key role in the fire spalling of concrete but in a different way from previously assumed.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , xvi, 134 p.
Series
Trita-BKN. Bulletin, ISSN 1103-4270 ; 117
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-128378OAI: oai:DiVA.org:kth-128378DiVA: diva2:647411
Public defence
2013-09-20, Kollegiesalen, Brinellvägen 8, entréplan. KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20130911

Available from: 2013-09-11 Created: 2013-09-11 Last updated: 2013-09-11Bibliographically approved
List of papers
1. Experimental study of the influence of polypropylene fibres on material propertiesand fire spalling of concrete
Open this publication in new window or tab >>Experimental study of the influence of polypropylene fibres on material propertiesand fire spalling of concrete
2007 (English)In: Proceedings from the fib Task Group 4.3 workshop “Fire Design of Concrete Structures – From Materials Modelling to Structural Performance”, 2007, 177-188 p.Conference paper, Published paper (Refereed)
Abstract [en]

A study of the fire spalling behaviour of concrete has been performed. It has been shown thatpolypropylene fibres (PP-fibres) reduce fire spalling of the investigated concretes. From thematerial testing part of this project it was revealed that PP-fibres in concrete modify the dryingbehaviour and the capillary saturation close to the surface of the concrete, and introduce aplateau in the free thermal strain curve.

Keyword
Fire spalling, polypropylene fibres, mechanical properties, pressure measurements
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-128358 (URN)
Conference
Workshop “Fire Design of Concrete Structures – From Materials Modelling to Structural Performance” Coimbra, Portugal, November 8-9, 2007
Note

QC 20130911

Available from: 2013-09-11 Created: 2013-09-11 Last updated: 2013-09-11Bibliographically approved
2. The Influence of Pressure in the Pore System on Fire Spalling of Concrete
Open this publication in new window or tab >>The Influence of Pressure in the Pore System on Fire Spalling of Concrete
2010 (English)In: Fire technology, ISSN 0015-2684, E-ISSN 1572-8099, Vol. 46, no 1, 217-230 p.Article in journal (Refereed) Published
Abstract [en]

Fire spalling of concrete is often attributed to the pressure in the pore system. Recent results from fire tests on Self Compacting Concrete (SCC) have shown that the connection between pressure in the capillary pore system and fire spalling of concrete is not obvious [Jansson (2006) Proceedings from the 4th international workshop, Aveiro, Portugal, pp 747-756]. This issue has been investigated by performing pressure measurements on SCC and a traditional vibrated concrete designed for tunnel construction. In the tests conducted on SCC data from exposure to the hydrocarbon (HC) fire curve are presented. In the tests conducted on vibrated concrete, measurements were performed in concrete exposed to the standard fire curve (EN1363-1) and the Rijkswaterstaat (RWS) curve. The highest pressures in the two test series were measured in the concretes that did not exhibit spalling during fire. One conclusion from the tests is that pressure in the capillary system is not the driving force for spalling during fire exposure. However, pressure is involved in the redistribution of moisture during fire exposure. A new theory is proposed to explain the spalling reducing function of PP-fibres based on the presence and movement of moisture.

Keyword
concrete, fire, spalling, pressure, polypropylene fibres
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-128364 (URN)10.1007/s10694-009-0093-9 (DOI)000273032700014 ()
Conference
5th International Conference on SiF, 2008, Singapore, SINGAPORE
Note

QC 20130911

Available from: 2013-09-11 Created: 2013-09-11 Last updated: 2017-12-06Bibliographically approved
3. Factors influencing fire spalling of self compacting concrete
Open this publication in new window or tab >>Factors influencing fire spalling of self compacting concrete
2013 (English)In: Materials and Structures, ISSN 1359-5997, E-ISSN 1871-6873, Vol. 46, no 10, 1683-1694 p.Article in journal (Refereed) Published
Abstract [en]

The influence of age on the fire spalling behaviour of high performance concrete is not known in detail. The present study shows that in three out of four mixes of self compacting concretes tested, the fire spalling tendency decreases at higher age whereas for the fourth mix the spalling did not decrease. The fourth mix, with the highest limestone content, spalled more after 4 years storage than the other mixes despite the fact that the moisture content was in the same region as the other mixes tested. Important findings are also that an addition of polypropylene fibres could expunge the spalling behaviour and that the specimen size and application of compressive load on the test specimens used in this type of studies influence the spalling behaviour strongly.

Keyword
Age effect, Fire, Self compacting concrete, Spalling
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-128368 (URN)10.1617/s11527-012-0007-z (DOI)000323883200006 ()2-s2.0-84883766821 (Scopus ID)
Note

QC 20140617

Available from: 2013-09-11 Created: 2013-09-11 Last updated: 2017-12-06Bibliographically approved
4. Reduction of fire spalling of concrete with small doses of polypropylene fibers
Open this publication in new window or tab >>Reduction of fire spalling of concrete with small doses of polypropylene fibers
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The addition of polypropylene fibers has been shown to reduce the fire spalling propensity of concrete. This has been proven in many fire tests; but, very few large scale tests exist showing the function of polypropylene fibers at low dosages. In this study results on polypropylene fibers included in w/c 0.40 mixes typical used in tunnel construction that is spalling sensitive without fibers shows that an amount of only 0.6 kg/m3 (1.01 lb/yd3) has a significant effect on the spalling propensity and that even lower amounts reduce the spalling although they do not eliminate it entirely. During one of the fire tests on large slabs loaded in compression, unloaded cubes of the same mixes were also included in the furnace. None of the small specimens spalled whereas some of the corresponding large slabs spalled beyond the layer of reinforcement. This showed that the cube test method is not conservative.

Keyword
concrete, fire, spalling, polypropylene fibers, admixture, tunnel concrete, fire, spalling, polypropylene fibers, admixture, tunnel
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-128370 (URN)
Note

QS 2013

Available from: 2013-09-11 Created: 2013-09-11 Last updated: 2013-09-11Bibliographically approved
5. Fire spalling of concrete
Open this publication in new window or tab >>Fire spalling of concrete
(English)Manuscript (preprint) (Other academic)
Abstract [en]

When occurring fire spalling of concrete is a phenomenon that reduces the fire resistance of fire exposed members by loss of cross section. The reduction of strength by the presence of moisture is a main component in the fire spalling phenomena during fire exposure from one side. Without including this effect the function of an addition of polypropylene fibres could not be fully explained. In many experiments the crack leading to the formation of a flaking off occurs at a temperature substantially lower than the temperatures where the pressure from saturated steam is high enough to be the main driving force for the phenomenon. Also no simple correlation between measured pore pressures and the fire spalling phenomenon has been found. The present study made by testing the compressive strength of small mortar specimens shows that a specimen boiled for 10 minutes has only 64% of the strength of a dry specimen. This result highlights the substantial strength reducing effect of the presence of moisture at elevated temperatures independent of the pore pressure. The presence of polypropylene fibres reduces the moisture content around the critical zone.

Keyword
fire spalling, concrete, polypropylene fibres
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-128373 (URN)
Note

QS 2013

Available from: 2013-09-11 Created: 2013-09-11 Last updated: 2013-09-11Bibliographically approved

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