Abstract
Recent advances in physical understanding of creep and shrinkage of cement paste suggest a novel approach to setting and hardening processes. In high-strength concrete, due to a low water-cement ratio, self-desiccation occurs immediately after setting, and capillary pressure produces compaction of the assembly of hydrating cement grains. For higher watercement ratio, water possibly can withstand cavitation, but then the volumetric balance of hydration requires that water move through the solid skeleton. According to Darcy's law, water only can be displaced by a gradient of hydraulic pressure, as long as the water pressure remains lower than its cavitation threshold. This water pressure only can be equilibrated by a compaction of the solid matrix, with compressive contacts particularly between the C-S-H layers. In both cases, with or without liquid-gas menisci, hydration acts like a distributed pumping of water, the liquid phase is under tension, and the solid skeleton is compressed. The high affinity of C-S-H for water and the viscoplastic behavior of the cement gel can explain the evolution of the contact between two hydrating cement grains and the formation of a continuous and cohesive bridge. Swelling and shrinkage then can be analyzed as two co-existing mechanisms, whose balance is controlled by the permeability of the solid matrix.
Résumé
Les progrès accomplis récemment dans la compréhension des mécanismes de retrait et de fluage des matériaux à base de ciment apportent un nouvel éclairage sur les processus de prise et de durcissement du ciment. Dans les bétons de haute résistance, les faibles rapports eau/ciment conduisent, au cours de la prise, à une désaturation des pores qui engendre des tensions capillaries et une contrainte de compression au sein du squelette minéral. Au-dessus d'une certaine valeur du rapport eau/ciment, s'il n'y a pas cavitation, alors le déficit volumétrique de la réaction d'hydratation produit nécessairement des mouvements d'eau à travers le système granulaire, et des gradients de pression dans le liquide (loi de Darcy), donc des compressions dans l'empilement granulaire, notamment au niveau des contacts entre les grains de ciment. Même en l'absence de tout ménisque, il y a des tensions dans la phase liquide, donc des compressions dans le squelette granulaire. La compréhension que nous avons aujourd'hui du comportement viscoplastique du gel des C-S-H permet d'expliquer comment le contact entre deux grains hydratants peut se transformer en un pont continu et cohésif. Retraits et gonflements apparaissent ainsi comme deux conséquences de l'hydratation, dont la résultante est contrôlée par la perméabilité (décroissante) du squelette.
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Editorial Note Prof. Paul Acker is a RILEM Senior Member.
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Acker, P. Swelling, shrinkage and creep: a mechanical approach to cement hydration. Mat. Struct. 37, 237–243 (2004). https://doi.org/10.1007/BF02480632
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DOI: https://doi.org/10.1007/BF02480632