EARLY AGE CREEP AND STRESS RELAXATION OF CONCRETE CONTAINING BLENDED CEMENTS

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1 EARLY AGE CREEP AND STRESS RELAXATION OF CONCRETE CONTAINING BLENDED CEMENTS Ivindra Pane and Will Hansen Department of Civil and Environmental Engineering, University of Michigan, USA Abstract The main objective is to investigate key properties that influence the stress development in concrete at early ages and the effect of using blended cements. Mineral additives and amount by weight of total binder used in the blended cements are fly ash (25%), ground granulated blast furnace slag (25%), and silica fume (1%). The properties investigated include tensile creep, elastic modulus, split tensile strength, and autogenous shrinkage. The relaxation modulus used for stress prediction was obtained from the creep data fitted using a log-power creep function. These findings show that tensile creep and stress relaxation are important properties of Portland cement concrete. These properties however are reduced in concretes containing blended cements. Blended cements affect the early age strength and elastic modulus moderately but significantly alter the autogenous deformation. Water/cement ratio (w/c), type and dosage of mineral additives were found to influence the magnitude of autogenous deformation. This deformation was found to be significant in low water-cement ratio concretes and should be included in early age stress calculations. 1. Introduction During early ages (up to 7 days), concrete undergoes significant development in mechanical properties, heat of hydration and autogenous deformation due to 277

2 cement hydration. Concrete members in the field are often restrained from undergoing early age deformation. Internal stresses are therefore developed. Concrete properties such as creep and associated relaxation are important in assessing the risk of cracking during the first few days after placement. Methods of calculating the stress history in early age concrete and predictions of the cracking tendency have been proposed (Emborg, 1998). Several key properties were identified including: creep, strength, elastic modulus, degree of hydration, autogenous shrinkage, and coefficient of thermal dilation (CTD). These properties depend on age and degree of hydration. Effects of blended cements have been recognized primarily for long term concrete performance (Decter et.al., 1989). However, their effects on early age stress development in hydrating concrete and on the early age properties still need to be investigated. The present work investigates the effects of blended cements on the early age stress development and on the development of mechanical properties on concrete. An essential feature of this work is the tension creep of concrete. In addition, the autogenous component of deformation is also included in the stress calculation. 2. Research Significance The primary interest in the early age stress analysis is the development of tensile stress in concrete during early ages associated with restraint to dimensional changes from heat of hydration, cooling, and autogenous shrinkage. It is therefore very logical to use the concrete creep compliance obtained for tensile loading. The added complexity in performing tensile creep test makes it unattractive. In this investigation, tensile creep data is obtained and used for predicting the stress development. It is significant since there are not many tensile creep data available in literature (Brooks et al. 1991; Brooks, and Neville, 1977; and Illston, 1965). In addition, creep data for concrete containing blended cements will be a valuable contribution to the existing creep data base and can be used to better understand the creep mechanism of concrete. 3. Experiments Concrete properties determined experimentally are creep, compressive and split tensile strengths, elastic modulus, and autogenous shrinkage. Information on concrete mix variables is summarized in Table 1. Two water/binder ratio (w/b) are chosen to represent two typical w/b used for pavement (w/b=.45) and high strength (w/b=.35) concrete. Percentages of additive represent the amount of 278

3 cement replacement by weight. For fly ash and slag concrete, a 25% replacement is considered moderate. A 1% replacement for silica fume concrete is considered maximum since beyond that benefits of using silica fume may be questionable. Table 1 Mix information and variables. Mix variables Water/binder:.35 and.45 Additives: 25 % fly ash (FA), 25 % slag (GGB) and 1 % silica fume (SF) Aggregate: Glacial gravel Binder content: 35 kg/m 3 Concrete strength and Young s modulus are obtained from a uniaxial test using an MTS servo-hydraulic compression machine. Specimens are tested starting from the age of hours up to 98 days. The creep compliance test set up consists of a lever-type loading apparatus equipped with a pneumatic pump that allows one to control the loading rate. To avoid eccentric loading, a guiding apparatus is added to the system. Two specimens attached to this guiding apparatus are then loaded with a stress level approximately 3% of the split tensile strength at the age of loading. Autogenous shrinkage measurement is performed on two specimens using specially design molds that provide zero displacement at one end measures the displacement using an LVDT the other end of a specimen. To avoid any thermal deformation the mold has been manufactured with channels that can flow water at constant temperature. Total early age stress development is measured uni-axially using a horizontal MTS servo-hydraulic machine, which allows the measurement to begin immediately after concrete is cast. The specimen is then subjected to a temperature history using a temperature controlled water-bath system. 4. Results It must noted that a thorough analysis requires the input from hydration data. In absence of drying shrinkage, early stresses in restrained concrete members are generated by temperature changes and autogenous deformations due to hydration. Effects of temperature are found to be significant on concrete hydration and associated mechanical properties and are not considered in this work. A more detailed work on the analysis and modeling on the subject can be found in the other work by the authors (Pane and Hansen, 2). 279

4 4.1 Strength and Young s Modulus Strength and Young s modulus data are very important for the early age stress analysis. Knowledge about the strength development with age of concrete helps us assessing the cracking tendency. Figure 1 shows how concrete Young s modulus increases with age. The change of split tensile strength with time is depicted in Figure 2. The effect of mineral additives can be seen for both properties. The general trend seems to indicate that fly ash initially reduces the early tensile strength and stiffness of concrete but ultimately produces catches up. The property development with time indicates how fast hydration proceeds during early ages as affected by the reactivity of each additive. 4.2 Tensile Creep Compliance To obtain the creep compliances of different concrete mixes tensile creep data must be extracted from deformation curves obtained from loaded specimens and free deformation curves of unloaded specimens. For most of the specimens, the loading is applied at the age of 1-day, 3-day, 7-day and 14-day. The durations for each age of loading are the same as the intervals between the ages of loading. For the stress analysis the data must be fitted with an appropriate creep compliance function. As suggested in (Bazant and Chern, 1985), an aging log power equation avoids the possibility of having a negative relaxation function OPC + 1% SF 25 OPC OPC + 25% GGBF Young's modulus (GPa) OPC + 25% FA Age (hr) Figure 1 Young s modulus development with age. 28

5 5 OPC 4 OPC + 1% SF Split tensile strength (MPa) 3 2 OPC + 25% FA 1 OPC + 25% GGBF Age (hr) Figure 2 Split tensile strength with age. The equation used to fit the data is (in 1/MPa): m3 J ( t, t' ) = 1/ E( t' ) + m1 ln(1 + m2 ( t' )( t t' ) ) (1) The parameters m 1, m 2, and m 3 in Eq.(1) are specific for each mix. The elastic compliance 1/E(t ), is obtained from inverting the curve-fitted Young s modulus data, m 2 is a function of age but m 1 and m 3 can be fixed. Figure 3 shows the curve fit (lines) and the experimental data (markers) for the control mix (plain, w/c=.45). The following age-dependent creep parameters are used to fit the data: (.1418) ( 1.4) 1/ E ( t' ) = (8.47E 5) t' m 1 = 3.33E 4 m 2 = 11.68t' m =.849 (2) 3 For all data, the obtained regression constant, R 2, that indicates the accuracy of the fit, was all above 95%. The effects of mineral additive on the creep compliance can be seen in Figure 4 (at age=3-day). Predicted curves show that creep compliance of concrete is reduced by the presence of additives except for the silica fume concrete. The creep curves are seen to deviate from each other after approximately 1 hour. It also interesting to see that addition of silica fume does not reduces creep as the other additives do. This behavior might be caused by the differences in the degree of reactivity of hydration products with different additives relative to each other and to OPC. In other words, silica fume may react faster or earlier than fly ash or blast furnace slag. However, as the time increases the rate or the slope of the creep curves of all concrete containing 281

6 blended cements become smaller than the ordinary cement concrete. Such behavior indicates that the long-term creep of blended cement concrete is less than the ordinary one. The same trend is also observed for creep curves at ages of 1 day. This implies that concrete containing blended cements do not relax as much as ordinary cement concrete. The early age stress can be calculated using either the relaxation modulus or the creep compliance. In this work, the creep compliance, J(t,t ) is first converted to the relaxation modulus, R(t,t ), which is then used to calculate the stress. R(t,t ) is obtained by solving the following integral equation: t 1 = J ( t, t' ) dr( t' ) (3) Eq.(3) is solved numerically using the trapezoidal rule OPC, w/c=.45 t'= age of loading t'=2 h Creep compliance J (1/MPa) t'=46 h t'=11 h.2 t'=291 h t-t' (hr) Figure 3 Tensile creep compliance of plain concrete at different ages. As stated earlier, tensile creep data reported in literature are scarce. For this test to be accepted it is necessary to compare the magnitude of tensile creep obtained in this work with available data. Tensile creep per unit stress loaded at the age of 11 hours (Figure 3) is about 3x1-6 /MPa after 1 hours. This value is slightly higher than that reported in (Brooks et. al. 1991) for the same 282

7 load duration at the age of 85 hours for concrete with almost the same w/c but cured at 47 C. This discrepancy may be caused by the accelerating effect of temperature on hydration and by the fact that the specimen was tested under saturated condition. Such a condition may reduce/eliminate the autogenous deformation measured in the unloaded specimen. It is also of interest to compare tensile creep and compressive creep. However, since no compressive creep data was obtained in this study, it is preferable to refer to works that report both properties. As reported in (Brooks et al. 1991, Brooks and Neville, 1977, and Illston, 1965), creep under uniaxial tension at different ages has been found to be higher than that under uniaxial compression. The differences in magnitude have been reported to increase with the duration of loading. Since the stress-time history in concrete at early ages is predominantly tension, tensile creep data is more pertinent..1 w/c=.45 OPC + 1% SF.8 OPC Creep compliance J (1/MPa).6.4 OPC + 25% GGBF OPC + 25% FA t-t' (hr) Figure 4 Typical tensile creep compliances of concrete containing blended cements at age=3-day. 4.3 Autogenous Deformation Autogenous deformation of.45 w/c-concrete has been found to be insignificant and will not be reported here. However, for.35 w/c-concrete significant 283

8 shrinkage has been observed. The autogenous deformations during the first week are illustrated in Figure 5. Additives clearly change the autogenous deformation of concrete. Fly ash seems to reduce the shrinkage and therefore, can be beneficial. A similar finding has also been reported (Chan et.al., 1998). The use of granulated slag induces significant expansion in the beginning thus, reducing the shrinkage at later ages. However, the shrinkage after the expansion ceased seems to be comparable to that of plain concrete. The high initial expansion may be attributed to the early pozzolanic reaction of cement-slag systems. This has been confirmed by a calorimeter study (De Sachutter and Taerwe, 1995). For plain concrete, the magnitude of shrinkage strain after 7 days can be as high as 14 micron. Although it has been reported that the autogenous shrinkage is influenced by the amount and type of aggregate used (Takada et.al., 1998). The result obtained is still within the range reported in (Brooks et.al., 1998). This agreement is very important since a standard method of measuring the autogenous deformation is yet to be developed and deviations from one measuring device to the other might produce different results. The autogenous deformations shown in Figure 5 seem to decrease with time and follow a trend observed in the hydration curves. This suggests that autogenous deformation is closely related to the degree of hydration. Recently, Loukili et.al., (2) report the influence of temperature on the autogenous deformation. Since hydration is significantly influenced by the temperature history, the effect of temperature on autogenous deformation should be included. 4.4 Early Age Stress Development Typical measured stress and temperature-time histories are shown in Figure 6. The driving forces in terms of strains consist of two components: thermal and autogenous. For high w/c-concrete autogenous deformation may be neglected. In this study, a constant value of coefficient of thermal dilation (11x1-6 / C) is assumed. The measured stress in Figure 6 is well below the tensile strength and therefore, a linear aging viscoelasticity can be assumed. The stress history as affected by creep can be calculated in a straightforward way by using the superposition method (Neville et.al., 1983): 284

9 t σ ( t) = R( t, t') dε ( t') (4a) dε ( t) = dε autosh ( t) + CTDdT ( t) (4b) Where ε autosh (t), CTD, and T(t) are respectively, autogenous strain, coefficient of thermal dilation, and temperature of concrete. The integration of Eq. (4a) is performed using the trapezoidal rule. The comparison between the predicted and measured stress histories is shown in Figure 6 for plain concrete with w/c=.35. The inclusion of the autogenous component of deformation in the calculation improves the stress prediction moderately. However, for concrete undergoing large autogenous deformation the improvement can be very significant. Some deviation between the predicted and measured stresses may be attributed to the temperature effects mentioned earlier. 5. Conclusions Major findings: 1. During early ages concrete undergoes significant tensile creep and associated stress relaxation 2. Tensile creep is decreased in concrete containing blended cements (e.g. fly ash, granulated slag, and silica fume). 3. Autogenous deformation was found to be significant for low water cement ratio (.35) OPC and blended cements. Silica fume increased autogenous shrinkage whereas fly ash and GGBFS decreased shrinkage. 4. Stress calculation incorporating autogenous shrinkage is more accurate. 6. Acknowledgement This work was sponsored by the Advanced Cement Based Materials Center, UM project entitled Early Age Stress in Concrete Containing Blended Cements. 285

10 15 w/b=.35 C+1%SF Autogeneous shrinkage (micron strain) 1 5 OPC C+25%FA C+25%GGB References tim e (hr) Figure 5 Autogenous deformations of different concrete mixes 1. Emborg, M., Models and methods for computation of thermal stress, RILEM Rep. No.15, Prevention of Thernal Cracking in Concrete at Early Ages, E&FN Spon, Decter, M.H., Short, N.R., Page, C.L., and Higgins, D.D., Chloride ion penetration into blended cement pastes and concrete, ACI SP-114, Proc. 3 rd Int. Conf., Trondheim, pp , Brooks, J.J., Wainwright, P.J., and Al-Kaisi, A.F., "Compressive and tensile creep of heat-cured ordinary portland and slag cement concretes," Mag. Conc. Res. 43, pp.1-12, Brooks, J.J., and Neville, A.M., "A comparison of creep, elasticity and strength of concrete in tension and in compression," Mag. Conc. Res. 29, pp , Illston, J.M., "The creep of concrete under uniaxial tension," Mag. Conc. Res. 17, pp.77-84, Pane, I., and Hansen, W., Early age stress analysis of hydrating blended cement concrete, manuscript in preparation,

11 Temperature (C) Tim e (hr) 2 M easured 1.5 Stress (MPa) 1.5 Calculated with autogeneous sh. Calculated without autogeneous sh Time (hr) Figure 6 Temperature and stress histories of plain concrete (w/c=.35). 7. Bazant, Z.P., and Chern, J.C., Log double power law for creep of concrete, ACI Journal 82, pp , Chan, Y.W., Liu, C.Y., and Lu, Y.S., Effects of slag and fly ash on the autogenous shrinkage of high performance concrete, Proc. Autogenous Shrinkage of Concrete JCI (E. Tazawa, ed.), Hiroshima, pp ,

12 9. De Schutter, G. and Taerwe, L. General hydration model for portland cement and blast furnace slag cement, Cem. Conc. Res. 25, pp , Takada, K., Van Bruegel, K., Koenders, E. A., and Kaptijn, N., Experimental evaluation of autogenous shrinkage of lightweight aggregate concrete, Proc. Autogenous Shrinkage of Concrete JCI (E. Tazawa, ed.), Hiroshima, pp , Brooks, J.J., Cabrera, J.G., and Johari, M.M., Factors affecting the autogenous shrinkage of silica fume high strength concrete, Proc. Autogenous Shrinkage of Concrete JCI (E. Tazawa, ed.), Hiroshima, pp , Loukili, A., Chopin, D., Khelidj, A., and Le Touzo, J.Y., A new approach to determine autogenous shrinkage of mortar at an early age considering temperature history, Cem. Conc. Res. 3, pp , Neville, A., Dilger, W.H., and Brooks, J.J., Creep of plain and structural concrete, Construction Press, Longman Ltd., New York,