CEOS.FR EXPERIMENTS FOR CRACK CONTROL OF CONCRETE AT EARLY AGE

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1 CEOS.FR EXPERIMENTS FOR CRACK CONTROL OF CONCRETE AT EARLY AGE Jean-Michel Torrenti (1), Laurie Buffo-Lacarrière (2), Francis Barré (3) on behalf of CEOS.FR Program Partners ( (1) UPE, IFSTTAR, France (2) LMDC, INSA Toulouse, France (3) Coyne et Bellier, France Abstract Within the CEOS.FR national research project, several experiments on massive concrete structures were conducted in order to improve the knowledge on the cracking phenomenon and, at last, to provide the designers with reliable crack design codes, able to give a reasonable appreciation of crack width and spacing for theses structures as well as for classical ones. In this paper, experiments where deformations at early age are restrained are presented. An example of the evolution of temperature, strains and forces for the RG8bis experiment is given. A first analysis of the cracking process shows that the cracks could appear for stresses below the tensile strength. Résumé Dans le cadre du projet national CEOS.FR, plusieurs expérimentations sur des structures massives ont été effectuées dans le but d accroître nos connaissances sur le phénomène de fissuration et pour fournir des données permettant d améliorer les codes afin qu ils prédisent des ouvertures et des espacements de fissures pour ces structures comme pour les structures plus classiques. Ici, nous présentons les résultats obtenus dans les essais pour lesquels la déformation au jeune âge est gênée. Un exemple d évolution des températures, des déformations et des forces mesurées dans l expérience RG8bis est donné. Une première analyse montre que la fissuration apparaît pour des contraintes plus faibles que la résistance en traction du béton. 1. INTRODUCTION THE CEOS.FR PROJECT In several applications, including nuclear ones, concrete structures (like nuclear vessels or radioactive waste storage) have to ensure various functions and feature specific performances (like confinement) aside from their structural resistance and durability. Confinement (water or air tightness) and durability of reinforced concrete structures are affected by cracking. To the current practice and as regards crack control, structural design is based either on formula like 3

2 in design codes CEB model Code 78, fib CEB model Code 90, EC 2-1-1, SIA or ACI 318, or on detailing procedures. Experience has shown that this approach is not accurate and sometimes wrong, for thick slabs, walls and other massive structures. Therefore, the French concrete construction community has initiated the CEOS.FR national research project which gathered more than 50 research organizations and companies, including infrastructure owners, construction companies, engineering companies, private or public research centers, with the support of French Ministry of Sustainable Development (MEDTL). The aim of CEOS.FR is to improve the knowledge on the cracking phenomenon and, at last, to provide the designers with reliable crack design codes, able to give a reasonable appreciation of crack width and spacing, applicable to a greater number of structures. Granted with a total budget of 7 million Euros, the project is intended to last four years, starting from 2008 till It has been organized on a cross theme approach. Three types of situations involving cracking have been identified: static and monotonous loading, cyclic or seismic loading and thermo-hydro-mechanic (THM) loading (restrained shrinkage). For all these loadings, structural analysis and design, scientific modelling and experimental testing are performed [1]. This paper presents the THM experimental parts and its first results. 2. DESCRIPTION OF THE EXPERIMENTATION Restrained shrinkage testing bodies are I-shaped, with a central part which has a crosssection of 0.80 m x0.50 m (figure 1, 2 and 3). Two largely dimensioned steel struts (diameter equal to 32.3 cm and thickness of 4 cm) are placed laterally between the two transverse heads to prevent almost any shrinkage. They are equipped with strain gauges to know the level of restraint and to obtain the forces generated in the struts. Figure 1: I-Shaped Restrained Shrinkage Testing Body Three testing bodies were realized: RG8bis, the reference one, RG9, with a reduced reinforcement and RG10, with an increased cover. The concrete used in the RG structures is a C50/60 concrete casted with a CEM I 52.5N cement. The formulation is given in the table 1. 4

3 During the first hours, formworks will be thermally insulated to maximize thermal and shrinkage phenomena. The formworks were removed 46h after casting. Table 1: concrete mix design Constituents Quantities (kg/m 3 ) CEMI 52,5N CE CP2 NF Couvrot 400 Sand 0/4 GSM LGP 785 Gravel 4/20 GSM LGP 980 Superplastifiant Axim Total water 185 Figure 2: view of the specimen. All specimens were fully instrumented, externally and internally: - 9 points for internal temperature measurement - 24 vibrating cord sensors for local internal or external deformation measurement - 3 internal optical fiber sensors - 6 electrical strain gauges placed on reinforcement bars. The external temperature, the relative humidity and the solar radiation were also measured during the test. 5

4 3. RESULTS OF THE TEST RG8BIS We present here the experimental results of the test RG8bis. The final crack pattern due to restrained shrinkage is presented on figure 4. Three main cracks were observed respectively 75 hours, 169 hours and 242 hours after casting. Figure 3: RG8bis Testing Body Reinforcement Figure 4: crack pattern test RG8bis. Crack 1 was observed 72 hours after casting, crack hours and crack hours. 6

5 3.1 Temperature evolutions Figure 5 shows the evolution of the temperature of concrete in the center of the specimen, using three PT100 sensors situated near the top (Su), in the center (C) and near the bottom (Sl) of the specimen. The evolutions of the external temperature and the theoretical temperature that would be obtained for this structure in adiabatic conditions are also presented. The adiabatic evolution was obtained from a specific test where concrete has been placed in a 300 mm cubic container, thermally isolated. Then, it has been placed in a climatic compound which simulates concrete s temperature at the heart s concrete structure. Surface and interior temperature has been taken during 5 days in order to ensure that there is no difference between the two. The influence of the solar radiation is visible: there is a continuous difference between temperatures at the bottom and at the top of the specimen. Lateral differences were also observed. The first crack occurred after 72 hours when the main part of the cooling was done. T ( C) Tad formwork removed Su C Sl T Su T C T Sl T ext T ad Text Time from casting (h) Figure 5: evolution of internal temperatures in RG8bis test compared to the adiabatic evolution (T ad ) and to the external temperature (T ext ). 3.2 Forces in the struts Figure 6 presents the evolution of the forces in the struts. After a period of around 10 hours where the forces are very small, an evolution is observed corresponding to a rapid increase of the mechanical properties of concrete: due to the temperature increase of concrete, at the beginning the restraint generates a tensile force (positive) in the struts, then, after the removal of the formwork and the isolation and due to the effect of autogeneous shrinkage, a compressive force (negative) is measured. The occurrence of the first crack corresponds to a 7

6 compressive force around 70 tons. Investigations are done concerning this value that seems very low. It corresponds to a mean tensile stress of around 1.7 MPa in concrete. This value is below the tensile strength at an age of 75 hours (the tensile strength was larger than 3.5 MPa at this age despite conservation at 20 C conducting to a lower maturity than is the real experiment). So the first crack occurs with a mean stress lower than the tensile strength. If the evaluation of the force is correct, this could be due the effect of gradient (with local tensile stresses higher than the tensile strength), to the existence of local defects or to tertiary creep coupling creep and damage [2]. Note also, that after 3 days, the force evolves with the variation of external temperature but the maximal value is still around 70 tons. The effect of shrinkage does not generate higher forces because there is also an effect of creep of concrete. So the others cracks are generated for almost constant stresses. This could also be explained by means of superposition of stresses due to gradients of temperature or to tertiary creep [2]. F(tons) 40 formwork and isolation removed Time from casting (h) Strut 1-40 Strut Total force Figure 6: evolution of the force in the struts and of the global force. 3.3 Strains Figure 7 presents the evolution of the relative displacement of the central part of the beam obtained with the three internal optical fibers. The measurement base of each sensor is equal to mm. One can see that the restraint was not complete during the test. An interesting way to analyse the results of these sensors is to follow the derivative of the signal (figure 8). The occurrence of the fist crack is clearly visible. For the second and the third cracks, unfortunately on this test, the occurrence of the cracks is not visible. 8

7 0,4du (mm) 0,3 formwork and isolation removed 0,2 0,1 0-0,1 Time from casting (h) ,2-0,3-0,4-0,5-0,6 Figure 7: evolution of the displacement on the measurement base of the internal optical fiber sensors. The upper sensor corresponds to the blue line, the center to the black one and the lower to the green line. 0,05 displacement rate (mm/mn) 0, :00 64:48 69:36 74:24 79:12 84:00-0,025 time after casting (hours) -0,05 Figure 8: evolution of the rate of displacement of the lower internal optical fiber sensors. 4. CONCLUSIONS During the CEOS.FR project, three experiments on the effect of restrained shrinkage at early age of massive concrete structures where performed. By the means of several 9

8 measurements, the evolution of temperatures inside the structure, of strains and of the global force are known. A first analysis of the cracking process shows that the cracks could appear for stresses below the tensile strength of concrete. The exploitation of the results is still ongoing. The results will be published later and available for the scientific community. REFERENCES [1] Demilecamps L., Reliable shrinkage and crack design: CEOS.FR French national research program, experimental aspects, 3 rd fib international congress, Washington, [2] Briffaut M., Benboudjema F., Nahas G., Torrenti J.-M., Numerical analysis of the thermal active restrained shrinkage ring test to study the early age behavior of massive concrete structures, Engineering Structures, 33 (4) pp , 2011, doi: /j.engstruct