Confinement effect of different arrangements of transverse reinforcement on axially loaded concrete columns: An experimental study

Dario De Domenico,Devid Falliano,Giuseppe Ricciardi

doi:10.1515/jmbm-2019-0003

Dario De Domenico, Devid Falliano + Show 1 more

Open Access

https://doi.org/10.1515/jmbm-2019-0003

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Abstract

AbstractConcrete behaves as a brittle material due to its low inherent tensile strength, but it can also exhibit a markedly ductile behavior when coupled with transverse reinforcement like steel stirrups. The role of stirrups is to enhance confinement effect, to restrain the lateral expansion of concrete, thus modifying the concrete stress-strain constitutive law and enabling higher compression strains and higher ductility. This paper focuses on the confinement effect induced by different arrangements of transverse reinforcement on axially loaded concrete columns. An experimental campaign has been carried out, comprising 18 concrete columns with two different mechanical strengths and reinforced with three different layouts of stirrups, namely typical closed square hoops, closed stirrups with additional cross ties, and a novel type of stirrups involving rectangular hoops with additional restraint plates, the latter offering an enhanced diffused confinement action and limiting extensive spalling of the cover concrete. Formation and propagation of longitudinal micro-cracks are reduced with the novel type of diffused stirrups and a moderate-to-high increase of ductility is observed. However, the beneficial effects induced by diffused stirrups are more pronounced in medium-strength concrete and almost negligible in low-strength concrete that collapses due to a brittle cracking failure without involving the confinement action of the transverse reinforcement.

Highlights

Concrete behaves as a brittle material due to its low inherent tensile strength, but it can exhibit a markedly ductile behavior when coupled with transverse reinforcement like steel stirrups
A novel type of transverse reinforcement has been designed that is able to extend the e ectiveness of the con nement concrete strength, three columns have been reinforced with ordinary square hoops (SH class), three columns have been reinforced through closed stirrups with additional cross ties (CT class) and nally the remaining three columns have been reinforced with a novel type of stirrups involving rectangular hoops with additional restraint plates (RP class)
With regard to the speci c measurements, it is worth pointing out the following experimental observations: for low-strength concrete, all the 9 specimens failed in a consistent manner, and the test was successfully carried out; for the SH1 specimen of the medium-strength concrete columns, the axial strain recording was a ected by a premature detachment of two strain gauges, and only one strain gauge, partially detached, was considered; the RP2 specimen of the mediumstrength concrete columns failed prematurely, probably due to some defect in the preparation of the sample

Summary

Introduction

Abstract: Concrete behaves as a brittle material due to its low inherent tensile strength, but it can exhibit a markedly ductile behavior when coupled with transverse reinforcement like steel stirrups. Of particular relevance to the present paper, ductility and plastic deformations occurring in concrete structures should be guaranteed up to certain load levels well beyond the elastic limit load. In this regard, uncon ned concrete has a mainly brittle behavior: when subject to compression loads, high lateral strains arise due to formation and propagation of microcracks. This, in turn, implies that instability in the compression zone as well as instability of the compressed longitudinal steel bars may occur All these circumstances make the plain (uncon ned) concrete a material that is less suitable for complying with the capacity design principles. This physical circumstance has justi ed the use of plasticity-based approaches for modelling the mechanical behavior of con ned concrete [6], and the use of limit analysis theory for the determination of the load-bearing capacity of concrete structural elements and structures, see

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Preliminary remarks on con ned concrete

Experimental campaign

Experimental results

Concluding remarks