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Bond and splitting in reinforced concrete: test results on bar pull-out

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Abstract

The test results regarding the pull-out of 16 large-and small-diameter bars (db=18 mm), embedded in presplit specimens, are presented here, at the conclusion of a systematic research project on the deterioration, confinement-sensitivity and path-dependency of bond, caused by concrete splitting.

The 16 tests carried out at constant width of the splitting crack show that the role of the diameter is not negligible and that small-diameter bars have the edge in terms of bond strength and stiffness, while crack opening is always detrimental to bond efficiency.

In all cases, the bond stress-slip curves exhibit a welldefined softening branch, and the bond stress-confinement envelopes are characterized by limited cohesion, a mostly linear ascending branch and a sort of plateau.

As for bond confinement-sensitivity and path-dependency, 4 tests on medium-diameter bars were carried out at constant confinement: bond peak-strength turns out to be an almost linear function of the confinement stress, and path-dependency is very limited at high confinement levels.

The results are intended to contribute to the formulation of more reliable laws regarding the governing parameters of bond-splitting interaction, in order to quantify and control the unfavorable effects of concrete splitting.

Résumé

On présente ici les résultats d'essais d'adhérence sur 16 barres de faible et de grand diamètre (db=14 et 24 mm), et sur 4 barres de diamètre moyen (db=18 mm), les 20 barres étant enrobées dans des prismes de béton préfissuré. Ces résultats marquent l'aboutissement d'une recherche systématique sur la détérioration mécanique de l'adhérence et sur sa sensibilité au confinement après le fendage du béton, ainsi que sur les effets de l'histoire des charges. Les 16 essais, exécutés avec une ouverture de fissure constante, montrent que le rôle du diamètre n'est pas négligeable et que les diamètres réduits sont quelque peu avantageux en ce qui concerne la résistance et la rigidité de l'adhérence, alors que l'ouverture des fissures est toujours préjudiciable à l'efficacité de l'adhérence.

Dans tous les cas, les courbes contrainte/glissement laissent voir une branche d'adoucissement bien définie et les domaines contrainte d'adhérence-contrainte de confinement sont caractérisés par une cohésion réduite, une branche ascendante presque linéaire et par une sorte de plateau.

En ce qui concerne la sensibilité au confinement et la dépendance de l'histoire des charges, les résultats obtenus sur les 4 barres de diamètre moyen, soumises à une force de confinement presque constante, montrent que la résistance de l'adhérence est presque proportionnelle au niveau de confinement et que la dépendance de l'histoire des charges est très réduite lorsque le confinement est élevé.

Les résultats présentés ici ont pour objet de contribuer à la formulation de lois plus fiables en ce qui concerne le rôle des paramètres qui régissent l'interaction entre l'adhérence et le fendage du béton, afin de quantifier et de contrôler les effets défavorables du fendage du béton.

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Abbreviations

db :

bar diameter

fy (fyk):

steel (characteristic) strength in tension

fc, fct :

concrete cylindrical strength in compression and in tension

ir :

bond index of a ribbed bar

L:

bonded length of a bar

M:

bending moment at any given section of a beam

∂τ/∂δt :

bond stiffness (∂τ/∂δt) and bond-confinement coupling (∂τ/∂δt)

t:

specimen thickness

T:

pull-out force

V:

shear at any given section of a beam

w:

crack opening or width (flexural cracks)

δn :

opening or width of the preformed splitting crack

δt :

total bar slip

δ ot :

“free” bar slip (during crack opening)

δ ⋆t :

adjusted or reduced bar slip (= δt - δ ot )

σ:

nominal confinement stress (=2N/πdbL)

τ:

nominal bond stress (=T/πdbL)

av:

average (value)

exp:

experimental (value)

in:

initial (value)

max:

maximum or peak (value)

sd:

standard deviation

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Authors and Affiliations

  1. Department of Structural Engineering, Politecnico di Milano, 20133, Milan, Italy

    Pietro G. Gambarova & GianPaolo Rosati

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  1. Pietro G. Gambarova
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  2. GianPaolo Rosati
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Editorial note Dr. Pietro G. Gambarova and Dr. GianPaolo Rosati are both working at the Politecnico di Milano, a RILEM Associate Member.

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Gambarova, P.G., Rosati, G. Bond and splitting in reinforced concrete: test results on bar pull-out. Mat. Struct. 29, 267–276 (1996). https://doi.org/10.1007/BF02486361

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