Materials and Structures

, 23:358 | Cite as

Strategic reinforcement for controlling volume-change cracking in base-restrained concrete walls

  • Ghazi F. Kheder
  • Sarab A. Fadhil
Article

Abstract

In this work the control of volume-change cracking in concrete walls with continuous base restrains was considered. Investigators and design codes have proposed several different procedures for the calculation of the amount and distribution of steel reinforcement required for the control of this type of cracking. All these methods provide amounts of reinforcement which are in excess to that really required. This is either due to simplifying the design procedure or neglecting the effect of variable restraint in the wall. Since crack formation and its width depends on the amount of total restrained movement in th concrete member, steel reinforcement is necessary only in positions in which wide cracks are expected to form. Using finite-element analysis to obtain the distribution of restraint in the wall, degree-of-restraint contour diagrams in walls with different length/height ratios of 1, 2, 3, 4, 6 and 8 were prepared. These diagrams can be used to determine the amount and distribution of steel reinsforcement in positions in which it is really effective in controlling cracking. At positions of low restraint in the walls in which no or only narrow cracks will form, only minimum reinforcement will be provided.

Keywords

Crack Width Steel Reinforcement Concrete Wall Crack Spacing Wall Height 

Notation

A

Effective tension area of concrete surrounding the reinforcing bars and having the same centroid as that of reinforcement, divided by the number of bars

Ab

Area of reinforcing bar

Af

Cross-sectional area of restraining foundation

Aw

Cross-sectional area of concrete wall

Bh

Width of wall cross-section at levelh above the base

C

Linear thermal coefficient of expansion of concrete

d

Reinforcing bar diameter

dc

Thickness of concrete cover measured from concrete surface at which cracks are being considered to the centre of the nearest reinforcing bar

Ec,Es

Modulii of elasticity of concrete and steel, respectively

Ef,Ew

Modulii of elasticity of the restraining foundation and the wall, respectively

esh,eth

Shrinkage and thermal strains, respectively

eult

Elastic tensile strain capacity of concrete

fb

Average bond strength between concrete and reinforcing steel adjacent to the crack

fs

Calculated steel stress at the crack

ft

Tensile strength of concrete

fy

Yield strength of reinforcing steel

H

Wall height

L

Wall length

NH

Number of reinforcing bars spaced along the H face or faces

n

Ratio of modulus of elasticity of steel to that of concrete

p

Steel ratio (%)

pcrit

Critical steel ratio (%)

pr

Reduced steel ratio (%)

R

Degree of restraint

Save,Smax,Smin

Average, maximum and minimum crack spacing, respectively

Te

Effective temperature change in member including an equivalent temperature change for drying shrinkage

Wmax

Maximum crack width

Résumé

On a examiné comment contrôler la fissuration due aux variations volumétriques de parois en béton encastrées par la base. Différentes méthodes ont été proposées par les chercheurs et dans les codes pour calculer le pourcentage et la répartition de l'armature d'acier nécessaire pour contrôler le type de fissuration. Toutes ces méthodes indiquent des taux d'armature excessifs par rapport à la réalité, soit parce qu'elles simplifient la méthode de calcul, soit parce qu'elles négligent l'effet d'encastrement variable dans le mur.

Etant donné que la formation de la fissure et sa largeur dépendent du degré auquel l'encastrement empêche tout mouvement, l'armature d'acier s'impose uniquement dans les cas où de larges fissures risquent de se produire.

En utilisant l'analyse des éléments finis pour obtenir la distribution de l'action d'encastrement dans le mur, on a préparé des diagrammes de profils d'encastrement variable avec différents rapports L/H (1, 2, 3, 4, 6 et 8). On s'est servi de ces diagrammes pour évaluer la quantité et la réparatition d'acier aux endroits où elles limitent efficacement la fissuration. Dans les cas de faible encastrement, là où aucune fissure ou seulement des fissures étroites peuvent se produire, on utilisera une armature minimale.

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Copyright information

© RILEM 1990

Authors and Affiliations

  • Ghazi F. Kheder
    • 1
  • Sarab A. Fadhil
    • 1
  1. 1.University of MustansiriyaBaghdadIraq

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