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Saltfrostskadors tidsberoende vid låg temperatur på betong med tillsatsmaterial

Sjöbeck, Henrik LU (2015) In TVBM VBM820 20151
Division of Building Materials
Abstract
Concrete is sometimes damaged by freezing. The damage increases if salt also is present. The damage consists of the removal of small flakes on the surface of the concrete, the inner structure may however be completely unaffected. This surface damage may increase the degree of saturation and ingress of e.g. chloride ions, which may lead to inner frost damage and corrosion on the reinforcing steel. It is also an aesthetical inconvenience.
The purpose of this master thesis is to investigate how the salt-frost damage on concrete with different binder combinations is affected by different frost cycles. Another purpose is to analyze the test method, which is made in a slightly different way than what is normally done.
Although a lot of... (More)
Concrete is sometimes damaged by freezing. The damage increases if salt also is present. The damage consists of the removal of small flakes on the surface of the concrete, the inner structure may however be completely unaffected. This surface damage may increase the degree of saturation and ingress of e.g. chloride ions, which may lead to inner frost damage and corrosion on the reinforcing steel. It is also an aesthetical inconvenience.
The purpose of this master thesis is to investigate how the salt-frost damage on concrete with different binder combinations is affected by different frost cycles. Another purpose is to analyze the test method, which is made in a slightly different way than what is normally done.
Although a lot of research has been done, there are still a lot of different theories on the characteristics of the salt scaling mechanisms. Some say that the mechanisms are similar to those of inner frost damage, while others say that it’s a completely different mechanism which is caused by tension created on the surface of the concrete when the frozen solute shrinks. A better understanding of the damage mechanisms would lead to an opportunity to design a durable material and increase the service life of the structure.
Three different combinations of binders are tested in this report. One with 100 % Portland cement, one with 65 % Portland cement and 35 % fly ash and a third mixture with 65 % Portland cement and 35 % slag. All concretes are exposed to different frost cycles in two rounds, every cycle is 24 hours long. All the cycles had the same freezing rate. The time the specimens and the solute was subjected to a temperature below -21°C and above 0°C varied between 2-12 h.
There is a common test method in Sweden for salt-frost durability called ”The Borås method” or SS 137244. The tests in this report are similar to those in the Borås method, except that the test rig is upside down. In this study the surface of the sample is submerged into a can of solute, while in the Borås method the solute is poured on top of the surface of the sample. The frost cycle in the Borås method is also a bit different from the frost cycles in this report.
The results in round one and round two points in two different directions partly. The specimens seem to have been affected by how much water they absorbed. The specimens that have absorbed the most water have also been damaged the most by scaling. In the first round it is clear that the specimens subjected to short time at freezing temperature is damaged less than the specimens in the reference cycle. The measurements also indicate that the samples in the reference cycle have absorbed more water. In the second round the results cannot be interpreted in the same way. It is not possible to decide whether the samples in the reference cycle or the samples in the long freezing cycle have scaled the most. Neither does the amount of absorbed water vary between the two cycles. This suggests that the time at freezing temperature does matter up to a certain duration, but passed this duration the concrete is not further damaged.
Which mechanism causing salt-frost scaling cannot be shown in this study. An increased time at the lowest temperature seems to increase the amount of scaling, up to a certain minimum time where further freezing does not result in worse damage. More studies are needed to further investigate the mechanisms causing salt-frost damage. (Less)
Popular Abstract (Swedish)
Betong skadas ibland av frysning. I miljöer där dessutom salt förekommer blir det särskilt svårt för betongen att inte skadas. Skadorna yttrar sig som avskalningar på betongens yta, men den inre strukturen kan vara opåverkad. Utöver den estetiska oangelägenheten kan dessa ytliga avskalningar även bidra till påskyndade korrosionskador och inre frostskador.
Syftet med detta examensarbete är att undersöka hur saltfrostbeständigheten för betong med olika bindemedelssammansättningar påverkas av olika fryscykler. Ett annat syfte är att analysera provningsmetoden som är lite annorlunda än vad som är standard för att kvantifiera saltfrostbeständighet.
Det råder ingen enighet kring vilka skademekanismer som bidrar till saltfrostskador i betong.... (More)
Betong skadas ibland av frysning. I miljöer där dessutom salt förekommer blir det särskilt svårt för betongen att inte skadas. Skadorna yttrar sig som avskalningar på betongens yta, men den inre strukturen kan vara opåverkad. Utöver den estetiska oangelägenheten kan dessa ytliga avskalningar även bidra till påskyndade korrosionskador och inre frostskador.
Syftet med detta examensarbete är att undersöka hur saltfrostbeständigheten för betong med olika bindemedelssammansättningar påverkas av olika fryscykler. Ett annat syfte är att analysera provningsmetoden som är lite annorlunda än vad som är standard för att kvantifiera saltfrostbeständighet.
Det råder ingen enighet kring vilka skademekanismer som bidrar till saltfrostskador i betong. En del teorier menar att skademekanismerna är liknande de för inre frostsprängning, medan andra menar att saltfrostavskalningar orsakas av en helt annan mekanism som beror på ytliga spänningar i betongen då den frusna saltlaken krymper. Det är viktigt att få mer kunskap om vilka skademekanismerna är samt hur betong påverkas av olika miljöer. Då ges en bättre möjlighet att designa materialet för att göra det mer beständigt och öka konstruktionens livslängd.
Betong med tre olika bindemedelsammansättningar testas i denna rapport. En med 100 % anläggningscement, en med 65 % anläggningscement och 35 % masugnsslagg samt en tredje blandning med 65 % anläggningscement och 35 % flygaska. Samtliga utsätts för varierande fryscykler i två omgångar, en cykel är i samtliga fall 24 timmar. Samtliga cykler hade samma temperaturgradienter vid nedkylning och uppvärmning. Det som varierades var tiden som provet och saltlösningen hade temperatur under -21°C och över 0°C.
Det finns en vanlig metod i Sverige för att testa om saltfrostbeständigheten för betong, kallad Boråsmetoden eller SS 137244. Försöken i denna rapport liknar denna metodik, förutom att provuppställningen är upp- och nervänd. Största skillnaden är att i dessa försök doppas provet i en burk med saltlösning, medan man i Boråsmetoden häller saltlösningen ovanpå betongytan. Vid provning enligt Boråsmetodens manual finns det också en standardiserad fryscykel, som skiljer sig något från fryscyklerna i denna rapport.
Resultaten i omgång 1 och omgång 2 pekar i delvis olika riktningar. Proven tycks ha påverkats av hur mycket vatten de sugit upp, där de prover som sugit upp mest vatten är de som skadats mest av avskalningar. I omgång 1 är det tydligt att proverna utsatta för fryscykeln med den korta frystiden skadas mindre än de i referenscykeln. Mätningarna visar också att proverna i referensfrysen har sugit upp mer vatten. I omgång 2 går det däremot inte att tolka resultaten på samma sätt. Vid jämförelse av mängden avskalningar mellan cykeln med den förlängda frystiden och referenscykel går det inte att se om den ena cykeln är värre än den andra. Även vattenuppsugningen varierar oberoende av vilken cykel proven utsätts för.
Vid nedkylning till precis under -21°C pekar resultaten på att skadorna förvärras upp till en viss (okänd) minimitid. Efter temperaturen hållits precis under -21°C denna okända tid så tycks skadorna sluta öka med tiden. Det går inte att visa vilken skademekanism/skademekanismer som orsakar saltfrostskador efter dessa försök. För att få ökad kunskap om skademekanismerna krävs vidare studier. (Less)
Please use this url to cite or link to this publication:
author
Sjöbeck, Henrik LU
supervisor
organization
alternative title
The time dependency of salt-frost damage at low temperature on concrete with SCMs
course
VBM820 20151
year
type
H2 - Master's Degree (Two Years)
subject
publication/series
TVBM
report number
5101
ISSN
0348-7911
other publication id
TVBM-5101
language
Swedish
id
7454864
date added to LUP
2015-06-26 14:00:24
date last changed
2015-06-26 14:00:24
@misc{7454864,
  abstract     = {Concrete is sometimes damaged by freezing. The damage increases if salt also is present. The damage consists of the removal of small flakes on the surface of the concrete, the inner structure may however be completely unaffected. This surface damage may increase the degree of saturation and ingress of e.g. chloride ions, which may lead to inner frost damage and corrosion on the reinforcing steel. It is also an aesthetical inconvenience.
The purpose of this master thesis is to investigate how the salt-frost damage on concrete with different binder combinations is affected by different frost cycles. Another purpose is to analyze the test method, which is made in a slightly different way than what is normally done. 
Although a lot of research has been done, there are still a lot of different theories on the characteristics of the salt scaling mechanisms. Some say that the mechanisms are similar to those of inner frost damage, while others say that it’s a completely different mechanism which is caused by tension created on the surface of the concrete when the frozen solute shrinks. A better understanding of the damage mechanisms would lead to an opportunity to design a durable material and increase the service life of the structure. 
Three different combinations of binders are tested in this report. One with 100 % Portland cement, one with 65 % Portland cement and 35 % fly ash and a third mixture with 65 % Portland cement and 35 % slag. All concretes are exposed to different frost cycles in two rounds, every cycle is 24 hours long. All the cycles had the same freezing rate. The time the specimens and the solute was subjected to a temperature below -21°C and above 0°C varied between 2-12 h. 
There is a common test method in Sweden for salt-frost durability called ”The Borås method” or SS 137244. The tests in this report are similar to those in the Borås method, except that the test rig is upside down. In this study the surface of the sample is submerged into a can of solute, while in the Borås method the solute is poured on top of the surface of the sample. The frost cycle in the Borås method is also a bit different from the frost cycles in this report. 
The results in round one and round two points in two different directions partly. The specimens seem to have been affected by how much water they absorbed. The specimens that have absorbed the most water have also been damaged the most by scaling. In the first round it is clear that the specimens subjected to short time at freezing temperature is damaged less than the specimens in the reference cycle. The measurements also indicate that the samples in the reference cycle have absorbed more water. In the second round the results cannot be interpreted in the same way. It is not possible to decide whether the samples in the reference cycle or the samples in the long freezing cycle have scaled the most. Neither does the amount of absorbed water vary between the two cycles. This suggests that the time at freezing temperature does matter up to a certain duration, but passed this duration the concrete is not further damaged. 
Which mechanism causing salt-frost scaling cannot be shown in this study. An increased time at the lowest temperature seems to increase the amount of scaling, up to a certain minimum time where further freezing does not result in worse damage. More studies are needed to further investigate the mechanisms causing salt-frost damage.},
  author       = {Sjöbeck, Henrik},
  issn         = {0348-7911},
  language     = {swe},
  note         = {Student Paper},
  series       = {TVBM},
  title        = {Saltfrostskadors tidsberoende vid låg temperatur på betong med tillsatsmaterial},
  year         = {2015},
}