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Bestämning av utvecklad tryckhållfasthet för platta betongkonstruktioner i ett tidigt skede

Stenberg, Daniel LU (2023) VBMM01 20231
Division of Building Materials
Abstract
Concrete is a building material that is normally cast according recipes, where a larger proportion of cement in the mixture results in a faster strength development and increased ultimate strength. The compressive strength, which can be considered as the most important property of concrete, is usually measured by a standardized procedure, where a test specimen is subjected to a progressively increasing load until it breaks. The most common is that the concrete's final strength is measured after 28 days, but the concrete's strength development continues as long as there is access to both unhydrated cement and water. Initially, this process is strongly dependent on the temperature of the concrete, where a higher temperature results in faster... (More)
Concrete is a building material that is normally cast according recipes, where a larger proportion of cement in the mixture results in a faster strength development and increased ultimate strength. The compressive strength, which can be considered as the most important property of concrete, is usually measured by a standardized procedure, where a test specimen is subjected to a progressively increasing load until it breaks. The most common is that the concrete's final strength is measured after 28 days, but the concrete's strength development continues as long as there is access to both unhydrated cement and water. Initially, this process is strongly dependent on the temperature of the concrete, where a higher temperature results in faster hardening, but gradually becomes more diffusion dependent. After 28 days, the reaction is so slow that the concrete is normally considered to have reached its final strength.
When casting constructions where demolding strength needs to be taken into account, the concrete's current strenght must be determined before the mold can be removed. This can be done in several ways. A common method is to cast test specimens that are kept in the same environment as the structure, and test them regularly until the desired strength is achieved. A potential problem with this method is that the strength development in the test specimen is different than in larger elements due to different ratios between the bodies' volume and area. This means that the heat developed in the concrete when the cement hydrates is transferred to the surroundings faster in a test specimen compared to a full-sized element. This results in the strength developing more slowly in the specimen.
In a factory environment, it is common for the concrete recipe to be adapted so that the demoulding strength can be achieved after a desired, relatively short time. This often results in more cement being used than is necessary for the desired final strength. In this degree project, temperature and strength development has been studied at an early stage for full-sized flat concrete elements in a concrete industry and compared with the corresponding standardized cubic test specimens. The results show a substantial difference in developed strength, which should mean that the cement content can ultimately be reduced. (Less)
Popular Abstract (Swedish)
De standardiserade mätmetoder som används för att bestämma betongs tryckhållfasthet är anpassade för mätning efter relativt lång tid, men ifall det är av intresse att bestämma betongens hållfasthet kort tid efter gjutning är dessa metoder inte helt rättvisande.
Cementbaserad betong är ett av världens vanligaste byggmaterial. Den har en god förmåga att motstå tryckspänningar och är inte speciellt känslig för fukt, vilket gör den väl lämpad att användas i många olika sammanhang. Världen över används årligen ca 570 kg cement per person, vilket ger upphov till ca 8% av de totala utsläppen av växthusgaser. För att minska utsläppen av växthusgaser, finns det ett stort intresse att minska mängden cement som används.
Under de senaste... (More)
De standardiserade mätmetoder som används för att bestämma betongs tryckhållfasthet är anpassade för mätning efter relativt lång tid, men ifall det är av intresse att bestämma betongens hållfasthet kort tid efter gjutning är dessa metoder inte helt rättvisande.
Cementbaserad betong är ett av världens vanligaste byggmaterial. Den har en god förmåga att motstå tryckspänningar och är inte speciellt känslig för fukt, vilket gör den väl lämpad att användas i många olika sammanhang. Världen över används årligen ca 570 kg cement per person, vilket ger upphov till ca 8% av de totala utsläppen av växthusgaser. För att minska utsläppen av växthusgaser, finns det ett stort intresse att minska mängden cement som används.
Under de senaste decennierna har byggbranschen i Sverige övergått från att gjuta på plats till att använda prefabricerade betongelement. Dessa element tillverkas ofta i en kontrollerad miljö och tillåts vanligen härda fullt ut innan de monteras på byggnaden. Av produktionstekniska skäl anpassas ofta mängden cement som används till betongen så att den snabbt ska uppnå en viss avformningshållfasthet, det vill säga en hållfasthet där gjutformen kan rivas och betongelementen kan flyttas. Detta medför att det ofta används mer cement än vad som egentligen krävs för att uppnå tillräcklig sluthållfasthet.
Cementens reaktion med vatten, sker likt många andra kemiska reaktioner snabbare vid en högre temperatur. Några timmar efter att cement kommit i kontakt med vatten startar dess huvudreaktion som ger upphov kraftig till värmeutveckling. Värmeutvecklingen avtar efter hand som vattnet binds kemiskt till cementet, och fritt vatten får det svårare att tränga in till de oreagerade delarna av cementkornen. Detta resulterar i en kortvarig topp i betongens temperaturutveckling, varefter den kyls av för att slutligen hamna i jämvikt med sin omgivning.
På grund av geometriska skillnader mellan en standardiserad provkropp och ett fullstort element har det observerats betydande skillnader i temperaturutvecklingen, där de fullstora elementen blir varmare än provkropparna. Detta borde innebära att en högre hållfasthet har utvecklats i det fullstora elementet än vad motsvarande provkropp visar. Detta examensarbete har visat att mängden cement som används i prefabricerad betong i det studerade fallet kan minskas betydligt, vilket ger en lägre miljöbelastning. (Less)
Please use this url to cite or link to this publication:
author
Stenberg, Daniel LU
supervisor
organization
course
VBMM01 20231
year
type
M3 - Professional qualifications ( - 4 Years)
subject
keywords
Betong, cement, tidig avformningshållfasthet, bestämma avformningshållfasthet, avformningshållfasthet, Concrete, early demolding strength, determine demolding strength, demolding strength
report number
5133
other publication id
LUTVDG/TVBM-23/5133-SE
language
Swedish
id
9136356
date added to LUP
2023-09-15 15:02:57
date last changed
2023-09-15 15:02:57
@misc{9136356,
  abstract     = {{Concrete is a building material that is normally cast according recipes, where a larger proportion of cement in the mixture results in a faster strength development and increased ultimate strength. The compressive strength, which can be considered as the most important property of concrete, is usually measured by a standardized procedure, where a test specimen is subjected to a progressively increasing load until it breaks. The most common is that the concrete's final strength is measured after 28 days, but the concrete's strength development continues as long as there is access to both unhydrated cement and water. Initially, this process is strongly dependent on the temperature of the concrete, where a higher temperature results in faster hardening, but gradually becomes more diffusion dependent. After 28 days, the reaction is so slow that the concrete is normally considered to have reached its final strength.
When casting constructions where demolding strength needs to be taken into account, the concrete's current strenght must be determined before the mold can be removed. This can be done in several ways. A common method is to cast test specimens that are kept in the same environment as the structure, and test them regularly until the desired strength is achieved. A potential problem with this method is that the strength development in the test specimen is different than in larger elements due to different ratios between the bodies' volume and area. This means that the heat developed in the concrete when the cement hydrates is transferred to the surroundings faster in a test specimen compared to a full-sized element. This results in the strength developing more slowly in the specimen.
In a factory environment, it is common for the concrete recipe to be adapted so that the demoulding strength can be achieved after a desired, relatively short time. This often results in more cement being used than is necessary for the desired final strength. In this degree project, temperature and strength development has been studied at an early stage for full-sized flat concrete elements in a concrete industry and compared with the corresponding standardized cubic test specimens. The results show a substantial difference in developed strength, which should mean that the cement content can ultimately be reduced.}},
  author       = {{Stenberg, Daniel}},
  language     = {{swe}},
  note         = {{Student Paper}},
  title        = {{Bestämning av utvecklad tryckhållfasthet för platta betongkonstruktioner i ett tidigt skede}},
  year         = {{2023}},
}