Abstract
The effects of confining pressure are investigated for two samples of a macro-cracked concrete. Samples are first macro-cracked with a splitting tensile strength test (Brazilian) technique. Gas permeability is continually measured under increasing (or decreasing) confining pressure, whereas crack closure (or opening) is recorded with an LVDT (Linear Variable Differential Transformer) device. Despite a mechanical closure of the macro-crack, identified at around 20 MPa confining pressure, gas permeability continues to decrease as confinement is increased. This means that a model of the macro-crack by two parallel planes (using Poiseuille law) cannot be used to represent permeability variations during closure (or opening) of cracks. As a consequence, a physical model is designed in order to simulate with a better consistency the real behaviour of the macro-crack. This simple modelling allows both behaviours, mechanical and hydraulic, under confining pressure, to be simulated with the same set of parameters.
Highlights
France has 58 nuclear reactors, which supply it with around 75% of its electricity
The most recent tests have shown that some second-generation containment envelopes require reinforcement in order to improve their gas tightness
Various reinforcement scenarios are currently being studied; one of them would involve placing an external pre-stressed structure on the outer side of the inner envelope, in order to recompress it—thereby closing the cracks thought to be the cause of the most significant leaks. This kind of issue has been often studied for fragile rocks such as granite [4], soft rocks like argillite [5] and concrete [6,7] for which many experimental studies indicate that the occurrence of microor macro-cracks may induce increase in permeability by several orders of magnitude compared to that of the intact material
Summary
France has 58 nuclear reactors, which supply it with around 75% of its electricity. These reactors are approximately thirty years old, and EDF (‘Electricité de France and operator of the nuclear reactors) is considering extending their life by several tens of years. E unloading phase of the cycle remains inside this band (Figures 7 and 8) This is consistent with the mechanical crack behaviour that is reversible after the first phase 3o.f2(.lRareglaet)iocnloshsiupreb.etween Confinement and Gas Permeability. (2) FinTahlleys,eeveexnpwerhimenetnhetscsrahcokwiswmietchhcaenrictaalilnytcylotsheadt,(sfeoerncoleteaninfSreaccttiuonre3s.1s)u, tchheapsert-hose described meabilihtyervea,ritehseornelywsillilgahtllwy,awyisthbea vparleufeeoref naptiparloflxiomwateclhya1n0n−15elms,2,wanhdicthhuws irlelmnaointsbe closed by a much gnreoartmerathl alnevtheel mofatsrtirxepsesrm(4e5a–b5il0ityM, wPhaicihs iaslorfetahdeyorqdueritoef 1a0h−17igtoh1v0−a18lume2.).HAowt -the end, these ever, when the permeability of the two different samples is compared (Figure 9), a ratio of approximately one order of magnitude is found, even after mechanical closure. Even when mechanical closure is achieved, the effective permeability remains much higher than the matrix permeability of t4h.eCmraactkerBiaelhiatsveilof.ur Modelling
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