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Control of Cracking due to Imposed Strains in Concrete Structures.

Pettersson, Dan LU (2000)
Abstract (Swedish)
Popular Abstract in Swedish

Sprickkontroll av betongkonstruktioner är ett viktigt men ofta styvmoderligt behandlat område. Detta gäller speciellt för konstruktioner utsatta för tvångsbelastning, t ex temperaturbelastning eller krympning av betongen i kombination av yttre tvång. Syftet med sprickkontroll kan antingen vara att undvika sprickor eller att begränsa sprickbredden. Syftet med denna forskning är att öka kunskapen inom dessa områden.



Ett förslag till rekommendationer för att förhindra sprickor i plattor och fundament på friktionsunderlag presenteras. Förslaget bygger på att resultat från Finita Element analyser jämförts med förenklade beräkningsmetoder. Friktionsförsök har gjorts för att bättre... (More)
Popular Abstract in Swedish

Sprickkontroll av betongkonstruktioner är ett viktigt men ofta styvmoderligt behandlat område. Detta gäller speciellt för konstruktioner utsatta för tvångsbelastning, t ex temperaturbelastning eller krympning av betongen i kombination av yttre tvång. Syftet med sprickkontroll kan antingen vara att undvika sprickor eller att begränsa sprickbredden. Syftet med denna forskning är att öka kunskapen inom dessa områden.



Ett förslag till rekommendationer för att förhindra sprickor i plattor och fundament på friktionsunderlag presenteras. Förslaget bygger på att resultat från Finita Element analyser jämförts med förenklade beräkningsmetoder. Friktionsförsök har gjorts för att bättre kunna beskriva tvånget i FE-beräkningarna. Förenklade temperaturbelastningar används som last. Temperaturbelastningar med gradient över tjockleken ger högre spänningar än en jämn temperaturbelastning. Förenklade metoder ger ganska bra uppskattning av spänningarna för tunna och slanka konstruktioner.



En teknik för att modellera sprickutveckling i armerade betongkonstruktioner redovisas. En tvådimensionell FE-metod används där fjädrar beskriver armeringens funktion i sprickor. Styvheten hos de aktuella fjädrarna beräknas på basis av vidhäftningsegenskaperna hos armeringen. Analyser visar att betongens mjuknande har underordnad betydelse för sprickor i konstruktionen. Föreskrivna temperaturpåverkningar används som last och beräkningarna utförs i steg där fjädrar införs efterhand för att simulera uppsprickningen. Metoden är utvecklad för en vägg fast inspänd i basen, men kan anpassas till andra konstruktioner och randvillkor.



Den tvådimensionell modellen ger endast ungefär halva sprickbredden jämfört med en motsvarande modell av en stång fast i båda ändarna. Armeringen har också sprickbreddsbegränsande effekt för låga armeringsmängder. Anledningen till detta är att tvånget längs basen effektivt fördelar sprickorna. Andra typer av konstruktioner med fullt tvång i längsled fungerar på samma sätt och får ungefär samma sprickbredd. Detta gäller t ex tresidigt inspända väggar, tunneltak och väggar på plattor. Sprickbredden minskar markant om friktionsunderlag finns under vägg på platta i de fall plattan är förhindrad att spricka. Allmänt gäller att ökad draghållfasthet för betongen ger ökad sprickbredd och att geometrin har underordnad betydelse. (Less)
Abstract
This thesis is concerned with crack control due to imposed deformations in concrete structures. The objective of crack control may either be to prevent cracking or to limit the crack widths. The aim here is to increase the knowledge in both fields for some common structures. Five papers are included and the three last concern crack development.



Paper I: Friction tests are performed for slabs cast on ground to study the horizontal external restraint. It is shown that the load-slip relations are non-linear and depends on the ground materials and the ground pressure.



Paper II: Recommendations for design are proposed to prevent cracking due to imposed deformations in slabs and foundations resting on a... (More)
This thesis is concerned with crack control due to imposed deformations in concrete structures. The objective of crack control may either be to prevent cracking or to limit the crack widths. The aim here is to increase the knowledge in both fields for some common structures. Five papers are included and the three last concern crack development.



Paper I: Friction tests are performed for slabs cast on ground to study the horizontal external restraint. It is shown that the load-slip relations are non-linear and depends on the ground materials and the ground pressure.



Paper II: Recommendations for design are proposed to prevent cracking due to imposed deformations in slabs and foundations resting on a frictional surface. The recommendations are based on comparisons of Finite Element results and results from simplified theoretical methods. Restraint stresses are calculated with a two-dimensional FE-model with non-linear contact elements, taking into account both horizontal and vertical movements. Imposed strains with a gradient over the thickness give restraint stresses significantly higher than uniformly imposed strains. The simplified methods give rather good estimations for thin and slender structures.



Paper III: A technique for modelling of crack development in reinforced concrete structures exposed to imposed deformations is proposed. A two-dimensional FE-method is used with closing forces in cracks concentrated to spring elements. The spring stiffness is estimated from bond stress - slip relations for reinforcement. Tensile softening of concrete can be neglected. Simplified temperature changes are used as load and the calculations are performed stepwise with opening of nodes and implementation of spring elements. The FE-method is established for a wall fully restrained at the base, but it can also be adapted to other structures and restraints.



Paper IV: The influence on crack development of reinforcement and concrete properties as well as geometry is studied for the wall fully restrained at the base. It is shown that the two-dimensional behaviour of the wall only gives about half the crack widths compared to a one dimensional bar with the same percentage of reinforcement. The reason is that the restraint along the base will effectively facilitate distribution of cracking along the wall. The crack widths are limited also at low reinforcement ratios. The crack widths will increase with tensile strength of concrete. The geometry of the wall and the bond stiffness has very little influence on the cracking widths and distribution of cracks in the wall.



Paper V: Crack development is studied for different types of structures and varying boundary conditions. It is shown that the type of restraint has significant influence on crack widths. For structures with restraint along the length direction the restraint effectively facilitates distribution of cracking, similar to a wall fully restrained at the bottom. Crack widths always become smaller with increasing reinforcement ratio, but also low reinforcement ratios reduces crack widths. Walls on slabs resting on a frictional surface give significantly lower crack widths compared to a fully restrained structure. (Less)
Please use this url to cite or link to this publication:
author
opponent
  • Professor Cederwall, Krister, Departement of Structural Engineering, Concrete Structures, Chalmers University of Technology, Göteborg.
organization
publishing date
type
Thesis
publication status
published
subject
keywords
hydraulic engineering, Civil engineering, Friction., Restraint, Shrinkage, Imposed deformations, Minimum reinforcement, Crack widths, Reinforced concrete, Crack development, Crack control, offshore technology, soil mechanics, Väg- och vattenbyggnadsteknik
pages
140 pages
publisher
Division of Structural Engineering, Lund University, P.O Box 118, SE-221 00 Lund, Sweden.,
defense location
Lund Institute of Technology, V-building, Lecture Hall V:D, John Ericssons v. 1, Lund (LTH, V-huset, sal V:D).
defense date
2000-08-25 10:15
external identifiers
  • other:ISRN: LUTVDG/TVBK - 00/1020 - SE (140p)
ISSN
0349-4969
language
English
LU publication?
yes
id
b1019b0e-79d7-4a2d-b53d-9b7789f8e817 (old id 40656)
date added to LUP
2007-08-01 10:39:31
date last changed
2016-09-19 08:44:56
@phdthesis{b1019b0e-79d7-4a2d-b53d-9b7789f8e817,
  abstract     = {This thesis is concerned with crack control due to imposed deformations in concrete structures. The objective of crack control may either be to prevent cracking or to limit the crack widths. The aim here is to increase the knowledge in both fields for some common structures. Five papers are included and the three last concern crack development.<br/><br>
<br/><br>
Paper I: Friction tests are performed for slabs cast on ground to study the horizontal external restraint. It is shown that the load-slip relations are non-linear and depends on the ground materials and the ground pressure.<br/><br>
<br/><br>
Paper II: Recommendations for design are proposed to prevent cracking due to imposed deformations in slabs and foundations resting on a frictional surface. The recommendations are based on comparisons of Finite Element results and results from simplified theoretical methods. Restraint stresses are calculated with a two-dimensional FE-model with non-linear contact elements, taking into account both horizontal and vertical movements. Imposed strains with a gradient over the thickness give restraint stresses significantly higher than uniformly imposed strains. The simplified methods give rather good estimations for thin and slender structures.<br/><br>
<br/><br>
Paper III: A technique for modelling of crack development in reinforced concrete structures exposed to imposed deformations is proposed. A two-dimensional FE-method is used with closing forces in cracks concentrated to spring elements. The spring stiffness is estimated from bond stress - slip relations for reinforcement. Tensile softening of concrete can be neglected. Simplified temperature changes are used as load and the calculations are performed stepwise with opening of nodes and implementation of spring elements. The FE-method is established for a wall fully restrained at the base, but it can also be adapted to other structures and restraints.<br/><br>
<br/><br>
Paper IV: The influence on crack development of reinforcement and concrete properties as well as geometry is studied for the wall fully restrained at the base. It is shown that the two-dimensional behaviour of the wall only gives about half the crack widths compared to a one dimensional bar with the same percentage of reinforcement. The reason is that the restraint along the base will effectively facilitate distribution of cracking along the wall. The crack widths are limited also at low reinforcement ratios. The crack widths will increase with tensile strength of concrete. The geometry of the wall and the bond stiffness has very little influence on the cracking widths and distribution of cracks in the wall.<br/><br>
<br/><br>
Paper V: Crack development is studied for different types of structures and varying boundary conditions. It is shown that the type of restraint has significant influence on crack widths. For structures with restraint along the length direction the restraint effectively facilitates distribution of cracking, similar to a wall fully restrained at the bottom. Crack widths always become smaller with increasing reinforcement ratio, but also low reinforcement ratios reduces crack widths. Walls on slabs resting on a frictional surface give significantly lower crack widths compared to a fully restrained structure.},
  author       = {Pettersson, Dan},
  issn         = {0349-4969},
  keyword      = {hydraulic engineering,Civil engineering,Friction.,Restraint,Shrinkage,Imposed deformations,Minimum reinforcement,Crack widths,Reinforced concrete,Crack development,Crack control,offshore technology,soil mechanics,Väg- och vattenbyggnadsteknik},
  language     = {eng},
  pages        = {140},
  publisher    = {Division of Structural Engineering, Lund University, P.O Box 118, SE-221 00 Lund, Sweden.,},
  school       = {Lund University},
  title        = {Control of Cracking due to Imposed Strains in Concrete Structures.},
  year         = {2000},
}