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Frost-induced deterioration of concrete in hydraulic structures : Interactions between water absorption, leaching and frost action

ROSENQVIST, MARTIN LU (2016)
Abstract (Swedish)
Frostnedbrytning av betong kan få allvarliga konsekvenser för en konstruktions säkerhet, beständighet och funktionsduglighet eftersom betongens hållfasthet försämras. Frostskador uppstår vanligtvis i konstruktioner med hög fuktbelastning och som periodvis utsätts för frostangrepp. Olika typer av betongskador, vilka kan misstänkas vara frostrelaterade, har påträffats i svenska vattenkraftsanläggningar. Följaktligen har frågor väckts kring betongens beständighet i vattenbyggnader i kalla klimat. Vattenbyggnader är ett samlingsnamn för konstruktioner i anslutning till vatten - vattenkraftstationer, dammar, slussar, kanaler, hamnar och bropelare.
I föreliggande avhandling har olika typer av misstänkta frostrelaterade skador i svenska... (More)
Frostnedbrytning av betong kan få allvarliga konsekvenser för en konstruktions säkerhet, beständighet och funktionsduglighet eftersom betongens hållfasthet försämras. Frostskador uppstår vanligtvis i konstruktioner med hög fuktbelastning och som periodvis utsätts för frostangrepp. Olika typer av betongskador, vilka kan misstänkas vara frostrelaterade, har påträffats i svenska vattenkraftsanläggningar. Följaktligen har frågor väckts kring betongens beständighet i vattenbyggnader i kalla klimat. Vattenbyggnader är ett samlingsnamn för konstruktioner i anslutning till vatten - vattenkraftstationer, dammar, slussar, kanaler, hamnar och bropelare.
I föreliggande avhandling har olika typer av misstänkta frostrelaterade skador i svenska vattenkraftverk identifierats och kategoriserats. De bakomliggande orsakerna har undersökts experimentellt genom laboratorie- och fältstudier. Kunskap om betongens nedbrytning är viktig i syfte att säkerställa och möjliggöra lång livslängd för vattenbyggnader. Erhållna resultat kan användas som underlag dels vid reparation av befintliga konstruktioner samt dels vid om- och nybyggnationer. Erhållna slutsatser från de olika studierna bedöms även vara tillämpbara för vattenbyggnader i andra länder med liknande förhållanden.
Miljöförhållandena vid de flesta vattenbyggnader i Sverige kan antas vara hårda. Många vattenbyggnader står i kontakt med vatten året om, samt utsätts för frostangrepp vintertid. Vattnet i svenska sjöar och vattendrag kan generellt klassas som mjukt till följd av de geologiska förutsättningarna i Sverige. Dessutom kan is och övrigt drivgods orsaka nötningsrelaterade skador på vissa konstruktioner.
Successiv nedbrytning av betongens yta längs vattenlinjen sker på de flesta vattenbyggnader. Skadorna påminner om avskalning orsakad tösalter trots att konstruktionerna står i kontakt med sötvatten. Nedbrytningen av betongytan leder till friläggandet av grövre ballastkorn, samt i förlängningen även armeringsjärn.
Genom bestämning av cementpastans kemiska samansättning har det visats att långvarig kontakt med mjukt vatten orsakar urlakning av det yttre betongskiktet. Urlakningen leder i sin tur till att ytans beständighet gentemot frost och nötning försämras. Även betong som anses vara frostbeständig uppvisar frostskador om ytan urlakas innan frostangreppet inträffar. Att olika nedbrytningsmekanismer kan förstärka varandras effekter bekräftades experimentellt genom att samverkan mellan urlakning, frysning och nötning orsakade mer omfattande skador på betongens yta än vad de enskilda mekanismerna gjorde tillsammans.
Den successiva nedbrytningen av betongytan längs vattenlinjen börjar med att ytskiktet urlakas under vår, sommar och höst. Betongens yta blir därmed känslig för frostangrepp och ytskiktet skadas under den följande vintern. På våren faller det skadade skiktet bort. Därefter börjar processen om på nytt.
I en del fall har inre skador påträffats vid och strax ovanför vattenlinjen i betong tillhörande vattenkraftverk uppförda företrädesvis innan 1950-talet. Resultat från försök i laboratoriet bekräftar att inre skador kan inträffa ovanför vattenlinjen i icke frostbeständig betong. Bestämning av fuktnivåer i befintliga konstruktioner och i provkroppar efter laboratorieförsök visar däremot att risken för inre skador vid och ovanför vattenlinjen är låg i frostbeständig betong.
En annan typ av inre skador har påträffats långt under vattenlinjen i tunna och dämmande betongkonstruktioner. Skadornas art kan karaktäriseras av spjälkning av större betongsjok. Den gemensamma faktorn för konstruktionerna är att de har utsatts för långa perioder av ensidig frysning under vintern. Genomförda studier har visat att låg betongkvalitet, samt effekter av åldring och defekter från byggtiden kan möjliggöra makroskopisk islinsbildning, vilken orsakar spjälkning.
I fullt frisk betong med vattencementtal (vct) lägre än 0.9 är dock risken för makroskopisk islinsbildning låg. I betong med inre sprickbildning till följd av frostskador kan däremot spjälkning inträffa inom några dagar oberoende av betongens vct. Även i betong med inre defekter, till exempel mindre håligheter, kan spjälkning inträffa. Högre kvalitet på betongen kräver dock längre perioder av ensidig frysning för att makroskopisk islinsbildning ska orsaka skador. Eftersom ofördelaktiga temperatur- och fuktförhållanden kan existera vintertid i tunna och dämmande betongkonstruktioner bör risken för spjälkning av betong beaktas.
Genom bestämning av vattenmättnadsgrader i en 55-årig damm har det visats att höga fuktnivåer i betong kan uppkomma över tid så att även frostbeständig betong riskerar skadas vid frostangrepp. Mikrostrukturella observationer av betongprover från dammen visade även att frostbeständigheten kan försämras över tid till följd av att luftporerna i betongen fylls med ettringit och kalciumhydroxid. Trots att höga fuktnivåer uppkommer över tid kan risken för en del frostskador minimeras genom att speciellt utsatta konstruktioner skyddas från frostangrepp. (Less)
Abstract
Frost deterioration of concrete can result in severe consequences to the safety, durability and functionality of a structure since it impairs the strength of the concrete. Frost damage may occur to structures that are in contact with water and subjected to frost action. Different types of concrete damage, which are suspected to be frost related, have been observed in concrete structures in Swedish hydro power plants. Questions have consequently been raised regarding the durability of concrete in hydraulic structures in cold climates. “Hydraulic structure” is the umbrella term for structures in contact with water – hydro power stations, dams, locks, canals, harbours and bridge piles.
Different types of suspected frost related damage to... (More)
Frost deterioration of concrete can result in severe consequences to the safety, durability and functionality of a structure since it impairs the strength of the concrete. Frost damage may occur to structures that are in contact with water and subjected to frost action. Different types of concrete damage, which are suspected to be frost related, have been observed in concrete structures in Swedish hydro power plants. Questions have consequently been raised regarding the durability of concrete in hydraulic structures in cold climates. “Hydraulic structure” is the umbrella term for structures in contact with water – hydro power stations, dams, locks, canals, harbours and bridge piles.
Different types of suspected frost related damage to concrete structures in hydro power plants are identified and categorised in this thesis. The underlying causes of damage are experimentally investigated by means of laboratory and field studies. Knowledge about the deterioration of concrete is important in order to secure long service life for hydraulic structures. The obtained results can be used as input for repairs of existing structures and in the case of new builds. The results may also be applicable to hydraulic structures in other countries where similar environmental conditions are present.
The environmental exposure conditions of most Swedish hydraulic structures can be considered harsh. Most hydraulic structures are in contact with water all year round and are subjected to frost action in winter. The water in Swedish rivers is generally considered soft due to the geology of Sweden. Ice floes and driftwood may also cause abrasive wear to some structures. Progressive deterioration of the concrete surface occurs at the waterline of many hydraulic structures. Despite the fact that these structures are exposed to fresh water, the damage is similar in appearance to salt scaling of concrete. Surface deterioration results in exposure of the coarse aggregate and eventually the reinforcing steel.
Analysis of the chemical composition of the cement paste shows that long-term exposure to soft water causes leaching of the concrete surface. Leaching leads to a decrease in the resistance against frost action and abrasion. The surface of frost resistant concrete may also suffer from frost damage if subjected to leaching prior to freezing. This thesis experimentally demonstrates that different degradation mechanisms can boost each other since the amount of superficial damage caused by the combined effects of leaching, frost action and abrasion exceeded the total amount of damage separately caused by these mechanisms.
Progressive deterioration of the concrete surface at the waterline begins with leaching during the spring, summer and autumn months. Hence, the concrete surface becomes susceptible to frost action and is damaged in winter. During the spring, the damaged surface layer falls off and the process begins again.
In some cases, internal damage has been observed at and just above the waterline of hydro power structures built prior to the 1950s. Experimental results show that internal frost damage may occur above the waterline in non-frost resistant concrete. Determination of saturation levels in concrete in existing structures and in specimens after laboratory tests showed that the risk of internal frost damage at and above the waterline is low in frost resistant concrete.
Another type of internal damage has been observed far below the waterline of thin water retaining concrete structures. The damage can be characterised by spalling of large concrete pieces. All damaged structures were subjected to long periods of unidirectional freezing in winter. The studies performed show that poor concrete quality and the effects of ageing and imperfections in the concrete may facilitate growth of macroscopic ice lenses, which causes spalling.
In mechanically sound concrete with a water to cement ratio (w/c) lower than 0.9, the risk of macroscopic ice lens growth is low. In concrete with internal cracking due to frost damage, spalling may occur within a few days regardless of the w/c-ratio. Concrete spalling may occur also in concrete with cavities and other imperfections. However, the higher the quality of concrete, the longer the freezing time required to facilitate macroscopic ice lens growth. Since unfavourable temperature and saturation conditions may exist in winter, the risk of macroscopic ice lens growth in thin water retaining concrete structures cannot be overlooked.
Determination of saturation levels in concrete from a 55-year old dam showed that high saturation levels are to be expected so that even frost resistant concrete is at risk of frost damage. Microstructural observations of concrete from the dam further showed that the frost resistance of concrete can be reduced due to the fact that air voids fill with ettringite and calcium hydroxide crystals over time. In spite of high saturation levels in the concrete, the risk of frost damage can be minimised by preventing thin water retaining structures from freezing in winter.
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Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Hooton, R.D., University of Toronto, Canada
organization
alternative title
Frostnedbrytning av betong i vattenbyggnader : Samverkan mellan vattenupptagning, urlakning och frysning
publishing date
type
Thesis
publication status
published
subject
keywords
Vattenbyggnader, Dammar, Betong, Cement, Vattenabsorption, Vattenmättnadsgrad, Nedbrytning, Frostbeständighet, Spjälkning, Makroskopisk islinsbildning, Urlakning, Synergi, Hydraulic structures, Dams, Concrete, Cement, Water absorption, Degree of saturation, Deterioration, Frost resistance, Spalling, Macroscopic ice lens growth, Leaching, Synergy
pages
74 pages
defense location
V:B, V-building, John Ericssons väg 1, Lund University, Faculty of Engineering.
defense date
2016-10-10 13:15
ISBN
978-91-7623-900-1
language
English
LU publication?
yes
id
dfac7b31-229a-4d1d-a02e-43258ce060f4
date added to LUP
2016-09-14 14:22:25
date last changed
2016-09-19 08:45:21
@misc{dfac7b31-229a-4d1d-a02e-43258ce060f4,
  abstract     = {Frost deterioration of concrete can result in severe consequences to the safety, durability and functionality of a structure since it impairs the strength of the concrete. Frost damage may occur to structures that are in contact with water and subjected to frost action. Different types of concrete damage, which are suspected to be frost related, have been observed in concrete structures in Swedish hydro power plants. Questions have consequently been raised regarding the durability of concrete in hydraulic structures in cold climates. “Hydraulic structure” is the umbrella term for structures in contact with water – hydro power stations, dams, locks, canals, harbours and bridge piles.<br/>Different types of suspected frost related damage to concrete structures in hydro power plants are identified and categorised in this thesis. The underlying causes of damage are experimentally investigated by means of laboratory and field studies. Knowledge about the deterioration of concrete is important in order to secure long service life for hydraulic structures. The obtained results can be used as input for repairs of existing structures and in the case of new builds. The results may also be applicable to hydraulic structures in other countries where similar environmental conditions are present.<br/>The environmental exposure conditions of most Swedish hydraulic structures can be considered harsh. Most hydraulic structures are in contact with water all year round and are subjected to frost action in winter. The water in Swedish rivers is generally considered soft due to the geology of Sweden. Ice floes and driftwood may also cause abrasive wear to some structures. Progressive deterioration of the concrete surface occurs at the waterline of many hydraulic structures. Despite the fact that these structures are exposed to fresh water, the damage is similar in appearance to salt scaling of concrete. Surface deterioration results in exposure of the coarse aggregate and eventually the reinforcing steel.<br/>Analysis of the chemical composition of the cement paste shows that long-term exposure to soft water causes leaching of the concrete surface. Leaching leads to a decrease in the resistance against frost action and abrasion. The surface of frost resistant concrete may also suffer from frost damage if subjected to leaching prior to freezing. This thesis experimentally demonstrates that different degradation mechanisms can boost each other since the amount of superficial damage caused by the combined effects of leaching, frost action and abrasion exceeded the total amount of damage separately caused by these mechanisms.<br/>Progressive deterioration of the concrete surface at the waterline begins with leaching during the spring, summer and autumn months. Hence, the concrete surface becomes susceptible to frost action and is damaged in winter. During the spring, the damaged surface layer falls off and the process begins again.<br/>In some cases, internal damage has been observed at and just above the waterline of hydro power structures built prior to the 1950s. Experimental results show that internal frost damage may occur above the waterline in non-frost resistant concrete. Determination of saturation levels in concrete in existing structures and in specimens after laboratory tests showed that the risk of internal frost damage at and above the waterline is low in frost resistant concrete.<br/>Another type of internal damage has been observed far below the waterline of thin water retaining concrete structures. The damage can be characterised by spalling of large concrete pieces. All damaged structures were subjected to long periods of unidirectional freezing in winter. The studies performed show that poor concrete quality and the effects of ageing and imperfections in the concrete may facilitate growth of macroscopic ice lenses, which causes spalling.<br/>In mechanically sound concrete with a water to cement ratio (w/c) lower than 0.9, the risk of macroscopic ice lens growth is low. In concrete with internal cracking due to frost damage, spalling may occur within a few days regardless of the w/c-ratio. Concrete spalling may occur also in concrete with cavities and other imperfections. However, the higher the quality of concrete, the longer the freezing time required to facilitate macroscopic ice lens growth. Since unfavourable temperature and saturation conditions may exist in winter, the risk of macroscopic ice lens growth in thin water retaining concrete structures cannot be overlooked.<br/>Determination of saturation levels in concrete from a 55-year old dam showed that high saturation levels are to be expected so that even frost resistant concrete is at risk of frost damage. Microstructural observations of concrete from the dam further showed that the frost resistance of concrete can be reduced due to the fact that air voids fill with ettringite and calcium hydroxide crystals over time. In spite of high saturation levels in the concrete, the risk of frost damage can be minimised by preventing thin water retaining structures from freezing in winter.<br/>},
  author       = {ROSENQVIST, MARTIN},
  isbn         = {978-91-7623-900-1},
  keyword      = {Vattenbyggnader,Dammar,Betong,Cement,Vattenabsorption,Vattenmättnadsgrad,Nedbrytning,Frostbeständighet,Spjälkning,Makroskopisk islinsbildning,Urlakning,Synergi,Hydraulic structures,Dams,Concrete,Cement,Water absorption,Degree of saturation,Deterioration,Frost resistance,Spalling,Macroscopic ice lens growth,Leaching,Synergy},
  language     = {eng},
  month        = {09},
  pages        = {74},
  title        = {Frost-induced deterioration of concrete in hydraulic structures : Interactions between water absorption, leaching and frost action},
  year         = {2016},
}