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Shear behaviour of reinforced concrete members without stirrups and subjectedto fatigue loads

Juan Manuel GALLEGO

Research assistantTechnical University ofMadrid (UPM)Madrid, Spainmgallego@caminos.upm.es

Carlos ZANUY

Assistant ProfessorTechnical University ofMadrid (UPM)Madrid, Spainczs@caminos.upm.es

Luis ALBAJAR

ProfessorTechnical University ofMadrid (UPM)Madrid, Spainlalbajar@caminos.upm.es

Summary

The shear fatigue behaviour of reinforced concrete elements without stirrups is a complex processthat involves a large number of variables. Because of its brittle nature, a better understanding of

shear fatigue is convenient, so that practical recommendations with mechanical basis can be proposed. Experimental evidence indicates that shear fatigue failure starts with the formation of adiagonal crack from the inclination of a flexural crack in one of the shear spans. This diagonal crackthen propagates with load cycles, both upwards to the load application point and downwards to thesupport. Final shear fatigue failure takes place by the destruction of the compression zone when itsdepth is too small to resist the compression force acting on it. In addition, it is also possible anunstable propagation of the diagonal crack as soon as it forms. A critical point of the process istherefore the formation of the diagonal crack. In this paper, the capabilities of a predictive model toestimate the number of cycles to diagonal cracking are explored. A comparison is carried out withfirst experimental results on haunched reinforced concrete beams without stirrups, which are typicalof cantilever slabs of bridge decks.

Keywords: Beams; fatigue; reinforced concrete; shear strength; bridge deck slabs.

1.  Introduction

Concrete structures may suffer fatigue when they are subjected to cyclic loads. Despite of the factthat typical fatigue failure of reinforced concrete elements is due to the brittle fracture of thelongitudinal reinforcement, experimental works have shown that shear fatigue failure is also

 possible [1]-[3]. Previous experimental works carried out in reinforced concrete elements withoutstirrups have shown that different fatigue failure mode may be developed according to the type,

 position and magnitude of applied loads, geometry, reinforcement ratio and material properties oftested specimens. In general, the shear fatigue failure appears as a consequence of the formation and

 progressive development of a diagonal crack from an existing flexural crack. After diagonalcracking, the crack propagates into the compression zone until failure takes place due to thedestruction of the compression zone when its depth is too small to resist the applied force acting on

it (failure mode referred to as ‘shear-compression failure’). The propagation of the diagonal crackcan also be instantaneous, leading to an even more brittle failure mode (referred to as ‘diagonal-cracking failure’). Accordingly, a critical aspect of the shear fatigue process is the formation of adiagonal crack. Therefore, the availability of a predictive tool for diagonal cracking seems veryinteresting.

A predictive model is proposed in the paper to estimate the number of cycles to diagonal cracking.The model has been based on the stress state analysis of a flexural crack supposed to be the one thatinclines and propagates during the shear fatigue process. The model has been used to understand thefirst fatigue tests carried out in reinforced concrete haunched beams without shear reinforcement.The specimens of the experimental campaign have been designed to represent the lateral cantileversof bridge deck slabs that are usually built without shear reinforcement.

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2.  Shear fatigue of RC haunched beams without stirrups

A specific experimental campaign is being developed by the Structural Engineering Group of UPMto study fatigue behaviour of cantilever slabs of bridge decks. It is noted that this is the first timethat an experimental campaign on haunched specimens is carried out. The results of the firstexperimental results are presented and analyzed in the full paper. As an example, specimen FT-3a

(with a maximum applied load of 58% of the static strength obtained in a control specimen) showedshear fatigue failure after 24,152 load cycles. The crack pattern at failure can be observed inFig.1(a). In this test, the section from which a diagonal crack formed was at 0.44 m from thesupport.

The proposed model can be used to analyze the experimental result of test FT-3a. The results of thisanalysis are represented in Fig.1(b), where the abscissa axis represents the distance of each cross-section of one half of the specimen to the support ( x = 0.00 m corresponds to the support, while x =1.20 m represents the midspan). With the proposed model, both the number of cycles to diagonalcracking and the fatigue strength of the steel reinforcement have been estimated at all sections ofthe specimen. The results are represented in the two curves of Fig.1(b).

According to the model, the number of cycles to diagonal cracking is smaller than the number ofcycles to fatigue fracture of the reinforcement. Furthermore, the section with the smallest fatiguestrength corresponds very well with the section from which the diagonal crack forms in the test(also represented in Fig.1(b)). It is important to note that both the experiment and the modelindicate that the critical shear crack does not appear at midspan, as usual for constant depthelements. Instead, the critical section shifts to the support.

The experimental and theoretical study presented by the authors has confirmed that shear fatiguefailure can occur in reinforced concrete haunched specimens without stirrups for moderatemaximum load levels, even before the typically reported fatigue failure due to fracture of the steelreinforcement.

(a) (b)

 Fig.1. Test FT-3a: (a) Crack pattern after shear fatigue failure; (b) Comparison between predicted

and experimental number of cycles to failure. 

References

[1]  ZANUY C., Análisis Seccional de Elementos de Hormigón Armado sometidos a Fatiga,Doctoral Thesis, Technical University of Madrid, Madrid, Spain, 2008, 251 p.

[2] 

CHANG T S., KESLER C E., Static and Fatigue Strength in Shear of Beams with TensileReinforcement, ACI Journal , Vol. 54, No. 6, 1958, pp. 1033-1057.

[3]  CHANG T S., KESLER C E., Fatigue Behaviour of Reinforced Concrete Beams, ACI Journal , Vol. 55, No. 8, 1958, pp. 245-254.