Tests on reinforced concrete slabs with pre-stressing and with transverse reinforcement under impact loading

An abstract of the technical paper presented to:
20th International Conference on Structural Mechanics in Reactor Technology (SMiRT 20)
Espoo, Finland
August 9–14, 2009

Prepared by:
Nebojsa Orbovica, Medhat Elgoharyb, Namho Leeb, Andrei Blahoianua
a) Canadian Nuclear Safety Commission (CNSC) Ottawa, Ontario, Canada
b) AECL-EACL, Mississauga, Ontario, Canada

Abstract

Introduction

Current empirical formulae for design of concrete elements under impact loading are based on tests carried out on reinforced concrete elements with longitudinal reinforcement and they are applicable only to such elements (References 1, 2). The influence of pre-stressing or transverse reinforcement is not taken into account. However, the code provisions (References 3, 4) for punching resistance of concrete elements under conventional loadings take into account beneficial effect of these parameters. In addition, the design acceptance criteria according to these formulae are based on the damage of a concrete element. More precisely, they are based on the visual damage of the rear side of the concrete element and formulated as scabbing of the concrete surface or missile perforation through the element. There is no quantification of the damage in terms of width and depth of the scabbed area or in terms of deformation of the element.

Aim of the work

The aim of this test campaign (Reference 5) carried out at VTT testing facility in Espoo, Finland, was to assess the influence of pre-stressing and transverse reinforcement, separately and combined, on punching resistance of concrete elements under impact loading. Six tests presented in this paper were carried out on low aspect ratio concrete slabs (l/h< 10; l is the slab span, h is the slab thickness) under medium velocity (around 100 m/s) hard missile impact. The variables were: the level of pre-stressing introduced using threaded bars and the transverse reinforcement in form of T-headed bars. The goal of the test campaign is to compare the level of damage of concrete elements keeping the same missile parameters (mass, stiffness, velocity, diameter and shape) and varying one slab parameter in each test. The slab and the missile characteristics were chosen to obtain punching behaviour and to cause significant damage to the reference reinforce concrete specimen in order to facilitate this comparison. The reference test was carried out on a reinforced concrete slab with longitudinal reinforcement only. In two following tests, in addition to the longitudinal reinforcement, pre-stressing was introduced using threaded bars. The slabs were pre-stressed introduced in both longitudinal directions. The level of pre-stressing was 5 MPa in one slab and 10 MPa in the other. In three following tests the same slab design was used with additional transverse reinforcement in form of T-headed: one slab was reinforced concrete slab with longitudinal and transverse reinforcement and two remaining slabs with mentioned pre-stressing levels and transverse reinforcement. Two last tests were performed to asses the combined effect of pre-stressing and transverse reinforcement.

Essential results

The results are presented in terms of scabbed concrete area and permanent deflection for each tested slab. Each test was compared to the reference test in reinforced concrete. The slab and the missile characteristics were chosen to cause significant damage to the reference reinforce concrete specimen in order to facilitate comparisons. It was observed that the punching resistance of pre-stressed concrete specimens (without transverse reinforcement) was lower than the reference reinforced concrete specimen, which is not consistent with code provisions for conventional loadings. However, significant difference in damage was not observed between two tested levels of pre-stressing (5MPa and 10 MPa). After the test results, the transverse reinforcement, in form of T-headed bars, increases punching resistance of concrete elements under impact loading. Therefore, the results are consistent with code provisions for punching resistance under conventional loadings. The improvement can be seen in both measured parameters: the scabbed area and the permanent deflection. The transverse reinforcement combined with pre-stressing significantly improves punching resistance of concrete elements for tested missile parameters. However, it was observed that combination of these two parameters modifies the failure mode. A punching cone, which is a current failure mode for target (concrete slab) and missile characteristics used in this campaign, is reduced to a punching cylinder with a diameter comparable to the missile diameter. Further tests are needed to evaluate ultimate resistance (close to complete perforation) of slabs with pre-stressing and transverse reinforcement.

Conclusions

The tests performed on concrete slabs under impact loading showed that introducing pre-stressing in slabs with longitudinal reinforcement does not increase (even decrease) their punching capacity. Introducing transverse reinforcement in form of T-headed bars have beneficial effect related to the punching resistance and the transverse reinforcement combined with pre-stressing increase significantly the punching resistance for given slab and missile characteristics. Further tests are needed to asses the benefits of these two parameters related to ultimate resistance of concrete slabs under impact loading.

References

1) George E. Sliter, Assessment of Empirical Concrete Impact Formulas, ASCE, Journal of the Structural Division, May 1980.

2) Eric Buzaud, Christian Cazaubon, Daniele Chauvel, Assessment of empirical formulae for local response of concrete structures to hard projectile impact, CONSEC'07, Tours, France.

3) CSA-A23.3-04, “Design of Concrete Structures”, Canadian Standards Association, 2004.

4) ACI 318-05, “Building Code Requirements for Structural Concrete and Commentary”, American Concrete Institute, 2005.

5) DOHA, VTT Database, Impact Project, 2008.

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