analysis of microcracking formation during basic …

ANALYSIS OF MICROCRACKING FORMATION DURING BASIC AND DRYING CREEP IN CEMENTITIOUS MATERIALS

Aliaksandra Tsitova (1,2)*, Fabien Bernachy-Barbe (1), Benoît Bary (1) and François Hild (2)

(1) DEN-Service d’Etude du Comportement des Radionucléides (SECR), CEA, UniversitéParis-Saclay, 91191 Gif-sur-Yvette, France

(2) LMT (ENS Paris-Saclay, CNRS, Université Paris Saclay), 61 avenue du Président Wilson,94230 Cachan, France

* Main author. E-mail: [email protected]

Abstract The safety of double wall Concrete Containment Buildings (CCBs) in the French nuclear

fleet primarily depends on the level of prestress applied to concrete. The delayed strains in concrete induced by creep and shrinkage cause a loss of prestress in the inner wall that may increase the risk of potential leaks in accidental conditions. The creep rate in concrete depends on multiple factors among which microcracking is of major significance. In this paper, an experimental approach is being developed for the qualitative and quantitative characterization of creep/damage coupling. The creep behaviour in compression of matured cement paste and mortar is characterised at the macroscale for different hygro-mechanical loadings. A non-destructive Impulse Excitation Technique (IET) is applied for damage evaluation after creep tests by measuring the degradation of the dynamic elastic modulus. The comparison of creep compliances allows creep and shrinkage mechanisms along with microcracking formation to be assessed in the matrix and at matrix-inclusion interfaces. The acquired data will then help to gain insight into the general coupling mechanisms between drying, creep and damage in cement-based materials.

Keywords: basic creep, drying creep, microcracking, mortar, cement paste.

1. INTRODUCTIONThe Concrete Containment Buildings (CCBs) are commonly called the third containment

barrier for French reactors as they must provide confinement of radioactive species in the event of failure of fuel rod cladding (first barrier) and reactor coolant system (second barrier).

The double wall CCB is a widespread design of containment vessels in the French nuclear fleet (i.e., 24 out of 58 nuclear reactors). This construction ensures the confinement function with the prestressed concrete inner wall.

4th International RILEM conference on Microstructure Related Durability of Cementitious Composites (Microdurability2020)

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The inner wall of CCBs undergoes accelerated creep and shrinkage kinetics due to the particular environmental conditions (heated air of the reactor, absence of rehydration from outside). The consequence of creep and shrinkage is an increased risk of potential leaks under proof-testing or accidental high internal pressure [1]. As the safety of double wall CCBs primarily depends on the prestress applied to concrete, the prediction of delayed strains and subsequent mechanical property degradation is of major interest.

The creep rate in concrete depends on multiple factors among which microcracking is of major importance [2]. In uniaxial compression, the creep strain is linearly related to stresses at loadings under about 40% of compressive strength [3]. At higher stress levels, creep strains become nonlinear with more pronounced nonlinearity at higher stresses. It is considered that most of nonlinear creep can be attributed to microcracking. To investigate the role of microcracking in cement-based materials, compressive creep tests are conducted on mature cement paste and mortar specimens at different hygro-mechanical loadings.

2. EXPERIMENTAL PROGRAM

2.1 Materials Formulations for cement and mortar are derived from the VeRCoRs concrete formulation [4]

that is used for laboratory tests. Mortar and cement paste were mixed with W/C=0.525. The material compositions are given in Table 1.

Table 1: Material constituents

Constituents Proportion, kg Density, kg/m3 Mortar: Cement CEM I 52.5 N CE CP2 NF Gaurain 320 3100 Sand 0/4 REC LGP1 thresholded to 2 mm 681 2600 Sikaplast Techno 80 2.6 1060 Addition water 170 1000 Total water 172 1000 Cement paste: Cement CEM I 52.5 N CE CP2 NF Gaurain 320 3100 Total water 168 1000

Mortar and cement paste cylinders are molded in vertical plastic tubes 30 mm in diameter. Specimens are demolded after 24 hours and stored at 100% relative humidity. The initial height of cast specimens is ca. 100 mm. Before testing, the upper and lower parts of cylinders are cut with a wire saw to eliminate volumes with non-homogeneous properties (due to varying W/C ratio). The final dimensions are Ø 30 × h 60-65mm. Boundary faces are rectified to ensure orthogonality and planar surfaces for compression testing. Caution is exercised during specimen preparation to protect them from drying and cracking by rewetting their surfaces and wrapping with a waterproof coating.

The minimum age of specimens at the start of all tests is 90 days. Carrying out the test on the matured material allows the ageing effects on viscous behavior to be neglected.


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2.2 Experimental set-up The experimental campaign aims to investigate and decouple the influence of several factors

on the creep behavior:

• Loading level,

• Heterogeneity (effect of aggregates),

• Drying (Pickett’s effect).The loading levels are defined as the ratio of compressive strength evaluated at 90 days on

similar specimens (geometry, fabrication and preparation protocol). Two levels of loadings are chosen, namely, 30% 𝑓𝑓𝑐𝑐90 corresponding to the linear creep range for concretes, and 60% 𝑓𝑓𝑐𝑐90

corresponding to the nonlinear range (Table 2).

Table 2: Loading levels

Material 𝑓𝑓𝑐𝑐90, MPa Loading level, MPa

Cement paste 32 0.3𝑓𝑓𝑐𝑐90 = 9.6

0.6𝑓𝑓𝑐𝑐90 = 19.2

Mortar 45 0.3𝑓𝑓𝑐𝑐90 = 13.4

0.6𝑓𝑓𝑐𝑐90 = 26.8

Axial and lateral strains are measured via HBM® foil prewired strain gauges. The strain gauges are glued with X60 HBM® adhesive that provides a good adhesion for strain measurements. The longitudinal strain is measured by linear strain gauges with a 20 mm measuring grid and lateral strain is measured by linear strain gauges with 10 mm grid. Loadings are applied by a hydraulic system where the pressure is controlled by a dead weight. Before load application, specimens are preloaded with ca. 1kN. Then, the main loading is applied instantaneously.

Basic creep tests Basic creep is the delayed strain occurring under a sustained load without moisture exchange

with the environment. Therefore, several actions are taken to prevent specimens from drying during the test. First, specimens are sealed with two layers of adhesive aluminum foil. Second, loading cells are placed in isolated chambers where the elevated relative humidity is controlled by a saturated saline solution. The saturated saline solution of barium dichloride (BaCl2) is used to obtain RH = 90-95%. Performing tests at 100% RH is avoided because of water condensation that may interfere with the normal operation of strain gauges.

Drying creep tests Drying creep is the delayed strain occurring under simultaneous drying and sustained load.

Two specimens were subjected to simultaneous loading and drying at two different stress levels and one specimen was subjected only to drying to measure the drying shrinkage strains.

The tests were carried out in a ventilated climate-controlled chamber with controlled temperature and relative humidity (T = 20.3 ± 0.2°C and RH = 70.7 ± 0.5%. Ventilation in the


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chamber ensures permanent humidity profiles on the surfaces of specimens for a stable drying process.

3. EXPERIMENTAL RESULTS AND DISCUSSION

3.1 Basic creep of cement paste Uniaxial compliances for low and medium loading levels are presented in Figure 1a). First,

different dependences of short-term creep and medium-term creep on stress levels are noticed. The specimen at higher stress level exhibits increased creep growth only during the short-term phase.

a) b)

Figure 1: Basic creep of cement paste a) uniaxial compliance b) transverse compliance

For the presented experimental results, it is assumed that most of the damage is occurring during a short time after load application as delayed propagation of the microcracks formed during the loading step. The propagation of microcracks after load application in concrete during stress relaxation was demonstrated by Acoustic Emission (AE) [5]. After approximately one day after load application, the creep rate of both specimens follows the same trend. Consequently, no pronounced nonlinearity manifested itself between low and medium stresses at long-term.

The transverse compliance reveals creep linearity as a function of stress for both short-term and long-term creep in the stress range up to 60%fc (Figure 1b)). Starting from a certain instant, both curves deviate from the initial trend. The transverse compliance of the 30% loaded specimen decreases down to a constant value while the compliance of the 60% loaded specimen exhibits an increasing rate. In the case of the 30% loaded specimen, the decrease of lateral creep rate may indicate drying or autogenous shrinkage. For the 60% loaded specimen, the increase of creep rate may be an indicator of damage propagation in the specimen. It was confirmed on a tomographic scan that displayed a network of angled cracks that could be the result of shear sliding in the cement paste.

3.2 Basic creep of mortar Unlike the case of basic creep in cement paste, uniaxial compliance of basic creep of mortar

(Figure 2a)) displays a non-linearity dependent on stress level between 30% and 60% of failure


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loading. As it is assumed that microcracks growth in the cement paste matrix at long-term does not affect significantly the creep kinetics, nonlinearity in mortar is assumed to be influenced by microcracking on the aggregate-matrix interfaces. It was proposed, based on AE monitoring of creep tests on mortar and concrete that most of the detected acoustic events corresponded to cracking of Interface Transition Zones (ITZ) [6]. Despite the nonlinearity of axial creep strains, lateral creep strains show the same trend for low and medium levels (Figure 2b)).

a) b)

Figure 2: Basic creep of mortar, a) uniaxial compliance, b) transverse compliance

3.3 Drying creep of cement paste Uniaxial compliances of cement paste in drying and basic creep are presented in Figure 3a).

The Pickett’s effect is fully manifesting itself after approximately one day after load application when the total creep rate is increasing fast. Unlike the case of basic creep, total creep strains are nonlinear wrt. the stress level in the range from low to medium stresses. Therefore, the Pickett’s effect may be influenced by the stress level.

a) b)

Figure 3: a) Uniaxial compliance of drying and basic creep of cement paste, b) effective creep Poisson’s ratio of drying creep

Further, it is observed that the total creep rate of the specimen loaded at 60% fc starts decreasing ca. 40 days after load application. The specimen loaded at 30% fc exhibits a similar behavior after ca. 130 days. From this statement, it can be assumed that under higher


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compressive loadings the drying kinetics is faster than at lower loadings. The calculation of lateral creep strains gives abnormally low values of the effective creep Poisson’s ratio. One likely explanation is that the specimens under compressive loading exhibit elevated drying shrinkage strains.

A comparison of strains of loaded specimens and the companion (unloaded) specimen reveals that the total creep strain at 30% fc is of the same order of magnitude as the drying shrinkage strain (Figure 4a)). Their relationship is almost linear with a slope close to 1 (Figure 4b)). For medium stress level, this dependence is nonlinear during the drying phase but then it converges to a linear trend (Figure 4b)).

a) b)

Figure 4: a) Total creep and drying shrinkage strains, b) total creep as a function of drying shrinkage

The Impulse Excitation Technique (IET) was applied to measure the dynamic modulus of elasticity before and after drying creep tests. Relationships between dynamic modulus and strength were observed by several authors [7], but the exact correlation between static and dynamic properties has not yet been established. Therefore, IET is used not to measure the exact static properties, but to observe the variation of stiffness before and after creep tests.

a) b)

Figure 5 : Evolution of parameters before and after drying creep tests. a) Natural frequency, b) dynamic modulus of elasticity


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The natural frequencies of flexural vibration modes are measured with the GrindoSonic® device. The dynamic modulus of elasticity in flexure is calculated according to an ASTM standard [8] as a function of specimen dimensions, mass and Poisson’s ratio assuming that the material is homogeneous, isotropic and perfectly elastic.

Figure 6a) shows the change of natural frequencies before and after drying creep. The frequencies of specimens subjected to drying creep do not considerably evolve, while those of the companion specimen dropped from 14 Hz to 11 Hz. The dynamic modulus of elasticity of specimens after drying creep has decreased as their masses have decreased due to drying (Figure 6b)). However, as their natural frequencies have not changed, no significant positive effect (due to cement paste consolidation) nor negative effect (due to microcracking) of creep is observed. The drastic decrease of the natural frequency of the companion specimen subjected to drying is attributed to microcracking in the vicinity of the surfaces. In further studies, the specimens will be examined and subjected to destructive tests to measure the elastic modulus and strength.

4. CONCLUSION AND PERSPECTIVESThe presented experimental results give an insight into delayed strains of homogeneous and

heterogeneous materials and the role of damage in the viscous response. Basic creep of cement paste is observed to be linear at medium term in the range of stresses up to 0.6 fc. Thus, it is assumed that there is no progressive damage propagation after the short-term. Mortar exhibits a nonlinear behavior from 0.3 fc to 0.6 fc. As the basic creep of the cement paste in this stress range is mostly linear, it is hypothesized that non-linearity of mortar is attributed to microcracking accumulation at the aggregate-matrix interfaces.

In drying creep tests, the specimen subjected to higher loading (0.6 fc) exhibits faster drying kinetics in comparison to the one subjected to low compressive stress (0.3 fc). To check the hypothesis about drying shrinkage/sustained stress coupling, periodic weighting and mass comparison of specimens subjected to drying shrinkage and to drying creep at low and high stresses are required. The low magnitude of calculated lateral creep strains indicates that simultaneous loading and drying can lead to increased drying shrinkage strains.

In a further study, tomographic investigations will be used to estimate damage in the tested specimens. Basic creep tests of mortar and cement paste stabilized at RH = 70% and drying creep of mortar at RH = 70% will be carried out to complete the necessary data for the estimation of microcracking influence on the creep kinetics.

REFERENCES [1] E. Gallitre, P. Labbe, G. Pastor, and Y. Le Pape, ‘Durée de vie des enceintes de confinement’, Revue

Générale Nucléaire, pp. 43–47, Mar. 2014.[2] Z. P. Bažant and Y. Xi, ‘Drying creep of concrete: constitutive model and new experiments

separating its mechanisms’, Materials and Structures, vol. 27, no. 1, pp. 3–14, Jan. 1994.[3] M. F. Ruiz, A. Muttoni, and P. G. Gambarova, ‘Relationship between Nonlinear Creep and

Cracking of Concrete under Uniaxial Compression’, Journal of Advanced Concrete Technology,vol. 5, no. 3, pp. 383–393, 2007.

[4] S. Huang, ‘Comportement vieillissant du béton en fluage : application au béton VeRCoRs’, PhDthesis, Université Paris-Est, 2018.


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[5] E. Denarié, C. Cécot, and C. Huet, ‘Characterization of creep and crack growth interactions in thefracture behavior of concrete’, Cement and Concrete Research, vol. 36, no. 3, pp. 571–575, Mar.2006.

[6] J. Saliba, ‘Contribution of the Acoustic Emission technique in the understanding and the modellingof the coupling between creep and damage in concrete’, Theses, Ecole Centrale de Nantes (ECN),2012.

[7] V. M. Malhotra, V. Sivasundaram, and N. J. Carino (Eds.), ‘Resonant Frequency Methods.’, inHandbook on Nondestructive Testing of Concrete, West Conshohocken: CRC Press LLC (ASTMInternational), 2004, pp. 7–1 – 7–21.

[8] ASTM E1876-15, ‘Standard Test Method for Dynamic Youngs Modulus, Shear Modulus, andPoissons Ratio by Impulse Excitation of Vibration’. ASTM International, West Conshohocken, PA,2015.


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