EARLY AGE BEHAVIOR OF MASSIVE CONCRETE ... 3 – RILEM-JCI International Workshop...

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  • CONCRACK 3 RILEM-JCI International Workshop on Crack Control of Mass Concrete and Related Issues Concerning Early-Age of Concrete Structures, 15-16 March 2012, Paris, France

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    EARLY AGE BEHAVIOR OF MASSIVE CONCRETE STRUCTURES: FROM EXPERIMENTS TO NUMERICAL SIMULATIONS

    Farid Benboudjema (1), Matthieu Briffaut (2), Adrien Hilaire (1), Jean-Michel Torrenti (3) and Georges Nahas (1,4)

    (1) LMT/ENS Cachan/CNRS UMR8535/UPMC/PRES UniverSud Paris, Cachan, France

    (2) Laboratoire Sols Solides Structures et Risques (3S-R), Universit Joseph Fourier, Grenoble, France

    (3) Universit Paris Est, IFSTTAR, Materials Department, Paris, France

    (4) Institut de radioprotection et de sret nuclaire, Fontenay-aux-Roses, France

    Abstract

    Cracking at early-age is an important problem in massive concrete structures. Indeed, autogenous and thermal shrinkage may be (partially) restrained by previous lift, due to temperature gradient ... This cracking depends highly on the concrete mix (impacting shrinkages, Youngs modulus, creep ...), the (mechanical, thermal, wind ...) boundary conditions ... It is difficult to study it with an experimental device in laboratory conditions, due to the needed massive character of the concrete structures which is needed in order to be representative. Therefore, an original device has been developed consisting of heating a brass ring, with concrete cast around. The results are then used to validate the adopted model. Creep in compression and tension, including the effect of temperature has been investigated. Some numerical simulations are performed in order to highlight the influence of creep, its interaction with cracking and boundary conditions. Finally, the model is applied to the CEOS test. Rsum

    La fissuration au jeune ge dans les structures massives en bton est un enjeu majeur. En effet, le retrait endogne et thermique peut tre (partiellement) empch par les leves prcdentes, du fait des gradients thermiques ... Cette fissuration dpend fortement de la composition du bton (impactant les retraits, le module de Young, le fluage...) les conditions aux limites (mcaniques, thermique, vent...)... Il est trs difficile de l'valuer en conditions de laboratoire, du fait du caractre massif ncessaire pour tre reprsentatif. Par consquent, un dispositif original a t dvelopp consistant chauffer un anneau en laiton, autour duquel un anneau en bton est coul. Les rsultats obtenus permettent de valider le modle dvelopp. Une tude du fluage en compression et en traction, incluant l'effet de la temprature a t mene. Des simulations numriques sont effectues afin de mettre en vidence l'influence du fluage, son interaction avec la fissuration et des conditions aux limites. Finalement, le modle est utilis pour simuler les essais raliss dans le projet CEOS.

  • CONCRACK 3 RILEM-JCI International Workshop on Crack Control of Mass Concrete and Related Issues Concerning Early-Age of Concrete Structures, 15-16 March 2012, Paris, France

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    1. INTRODUCTION At early age in massive concrete structures, cracking may occur during hardening. Indeed,

    hydration is an exothermic chemical reaction (temperature in concrete may overcome 60C [1-3]). Therefore, if autogenous and thermal strains are restrained (self restraint, construction joints), compressive stresses and then tensile stresses rise, which may reach the concrete strength and induce cracking in a real structure. For instance, Ithurralde [3] observed several crossing cracks (opening up to 0.5 mm) in a 1.2 m width concrete wall (representative of French nuclear power plant containment), cast on a concrete slab. For structures like tanks or nuclear containment vessels, this cracking may significantly increase concrete permeability and reduce tightness. For other massive structures (bridges, tunnels) serviceability may be reduced due to the penetration of aggressive species (such as carbon dioxide, sulfate and chloride ions).

    Cracking highly depends on creep (essentially basic creep in massive structures). However, the question whether creep strains are the same in compression (such tests are classical) and in tension (difficult to perform) is not fully resolved. This literature review highlights the fact that there is no consensus in scientific community regarding basic creep in tension. Moreover, at early age, concrete structures can reach 60C and thus, an important effect of this temperature evolution is expected on the concrete behaviour and especially on the basic creep strains rate. Therefore, both compressive and tensile creep test have been performed and the effect of temperature have been studied on compressive test. The obtained results will be presented in the first part of this paper. Next, a new configuration of restrained shrinkage ring test is presented. Indeed, thermal shrinkage does not occur in such device, whereas in massive structures thermal strains restraint (due to internal restraint, i.e. temperature gradients or due to construction joints) is the main phenomena involved in cracking [2, 4]. Therefore, a device, which is an evolution of the classical restrained shrinkage ring test, has been developed to study the cracking due to restrained thermal shrinkage in laboratory conditions [17] (in order to be representative of a massive structure). The second part is devoted to modelling of early age behaviour: influence of boundary conditions and basic creep strains at early age (age effect and temperature effect), including coupling between cracking and creep, and dissymmetric effect in compression/tension. This study is based on the RG8 experiment (CEOS national project, [5]) and on a concrete mix which is representative of a nuclear power plant which are used for numerical simulations.

    2. EXPERIMENTAL RESULTS

    2.1 Creep experiment Compressive and tensile creep tests have been performed in which the loading level is

    equal to 30 % of the compressive/tensile strength at the loading age (calculated from compressive/tensile (splitting) test on 11x22 cm cylindrical specimen) to stay in primary creep domain and avoid non linearity of creep stains with regards to the load rate and/or failure due to tertiary creep. The specimens (7x7x28 cm in compression, 30x11cm cylindrical specimen in tension) are placed in a controlled room at 20C and protected from drying with a double aluminium adhesive layer. For these tests, hydraulic loading frames (calibrated with a load cell) are used in compression, whereas pneumatic loading frame (alimented by a

  • CONCRACK 3 RILEM-JCI International Workshop on Crack Control of Mass Concrete and Related Issues Concerning Early-Age of Concrete Structures, 15-16 March 2012, Paris, France

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    pneumatic compressor) is used in tension. The strains measurements are performed on 2 or 3 measurement lines, and for each loading age, at least 3 specimens are tested.

    Compressive and tensile creep stains are displayed in Figure 1. These results suggest that for the studied concrete, at early age, compressive and tensile creep strains are almost similar (the maximum difference is 20 %). As previously aforementioned in the introduction, there is still no consensus on this subject.

    Figure 1: Specific basic creep strain in compression and tension at early-age.

    In massive structures, the concrete temperature can overcome 60C. Thus, an important effect of this temperature evolution is expected on the concrete behaviour and especially on the basic creep strains rate. Compressive basic creep test are performed in a temperature controlled room with the same device presented previously. Basic creep strains of 8 specimens (submitted to three temperature histories) are compared in Figure 2:

    a constant temperature of 20C during cure and 20C during the test (20 - 20 - 20C) a constant temperature of 20C during cure and 60C during the test (20 - 60 - 60C) a constant temperature of 20C during cure and a temperature decrease from 60C to

    20C during the test (20 - 60 - 20C)

    -90

    -80

    -70

    -60

    -50

    -40

    -30

    -20

    -10

    00 1 2 3 4 5

    Basi

    c cr

    eep

    stra

    ins

    (m

    /m/M

    Pa)

    Temps (jours)

    Compressive test (CT =20C ; TT = 60C

    Compressive test (CT=20C ; TT = 20C)

    Compressive test (CT=20C; TT = 60-20C)

    Time(days)

    Compressivetest:20 20 20CCompressivetest:20 60 60C

    Compressivetest:20 60 20C

    Figure 2: Evolution of specific creep strain for 3 different temperature histories.

  • CONCRACK 3 RILEM-JCI International Workshop on Crack Control of Mass Concrete and Related Issues Concerning Early-Age of Concrete Structures, 15-16 March 2012, Paris, France

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    Figure 2 shows that the temperature effect on basic creep strains in compression is important because strains measurements obtained at 60C are close to twice of the one measured at 20C. For the test with a temperature decrease (rate of 0.6C/h), the basic creep strains are more important than for a stable temperature of 60C. This could be explained by thermal transient creep [6]. However, Bazant et al. [7] and Sabeur and Meftah [8] suggest that these strains correspond respectively to drying creep (the specimens are here protected against moisture loss) or dehydration and only exist above 100C. It needs still to be resolve, some (parasite) water loss may have occurred during our experiments.

    2.2 Thermal ring test [17]

    The ring test proposed in this study, aiming at predicting the behaviour and the cracking of concrete at early age of massive structures (like nuclear power plant containment) is a thermally controlled device. Its principle is to create the thermal strain effects by increasing the temperature of the brass ring in order to expand it and to repr