The Twenty-Fourth KKCNN Symposium on Civil Engineering December 14-16, 2011, Hyogo, Japan
Estimation of Temperature Effects on Autogenous Shrinkage of Concrete by a New Prediction Model
*Inyeop Chu
1, Seung Hee Kwon
2and Jin-Keun Kim
31, 3
Department of Civil and Environmental Engineering, KAIST, Daejeon, 305-701, KOREA
2
Department of Civil and Environmental Engineering, Myounghi Univ., Yongin, 449-728, KOREA [email protected], [email protected], [email protected]
ABSTRACT
To accurately estimate the temperature effects on autogenous shrinkage (AS) of concrete, a new prediction model based on time-temperature dependent activation energy is proposed in this study. For this purpose, a series of AS tests are performed considering different water to cementitious materials ratios and curing temperatures. Results revealed that the temperature dependency of AS cannot be described solely by a simple time shift of existing maturity methods. Therefore, to overcome the shortcomings of the maturity methods, a new prediction model based on time-temperature dependent activation energy is proposed. Finally, reasonably good agreement is observed between experimental and predicted AS, thus indicating that the proposed model can be successfully used to estimate the time-temperature effects in developments of AS.
INTRODUCTION
Autogenous shrinkage is defined as the bulk concrete volume change from the hydration of cement of a system not subjected to external forces. In most concrete members, overall shrinkage is affected more by drying than by autogenous volume changes. However, autogenous shrinkage, significant primarily in concretes with a low water-cementitious material ratio (w/cm), has received more attention in recent years due to the increasing use of high-performance concretes (HPCs).
Autogenous shrinkage depends strongly on the imposed temperature. Usually, the effects of temperature on the development of AS of concrete are modified by a simple shrinkage-maturity curve of by using the concept of effective age proposed by the CEB-FIP model code. However, several researchers reported that the traditional maturity concept based on single activation energy may not be applicable to predict AS of hardening cement paste. More importantly, the use of a single maturity function for all properties of one concrete mix is likely to be inappropriate, as satisfactory relationships between hydration kinetics, microstructure, and property development warranting this assumption have yet to established..
EXPERIMENTS of AUTOGENOUS SHRINKAGE Experimental Variables
As tabulated in Table 1, different w/cm of concrete and curing temperatures are adopted in this study as experimental variables. This is to ensure the repeatability of experimental data and to produce a data inventory of temperature effects on AS of concrete for establishing a time-temperature development prediction model of AS. As shown in Table 1, for each mix proportion, AS is investigated considering three different isothermal temperatures (20, 30, and 40oC) with an exception for concrete with a w/cm of 0.35. For concrete with a w/cm of 0.35, an additional test of AS under variable curing temperature is employed along with an experiment to determine the coefficient of thermal expansion (CTE) of concrete over time. This is done to investigate the applicability of the prediction model under variable curing temperature.
Experiment Methods
For each mixture proportion (Table 1), six identical 100mm×100mm×400mm prism specimens were cast. The specimens were then divided into three groups of two specimens each, to test all groups simultaneously but under distinct isothermal temperatures (20, 30, and 40 oC). In each specimen, the corresponding isothermal temperature was strictly maintained by air cooling (Amin, et al. 2010) starting from casting of specimen until completing the test. In all the tests, concrete was cast into
1 Graduate Student
2 Assistant Professor
3 Professor
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molds immediately after mixing, followed by sealing of the top surface of the specimens with subsequent layers of polyester film and adhesive aluminum tape to ensure perfect sealing. In order to ensure the reliability of test results, types of measuring instruments were used, a linearly variable displacement transducer (LVDT), and embedment strain gages. A thermocouple was also placed at the center of each specimen along with the embedment strain gage to monitor the temperature history. Schematic diagram of the AS apparatus is shown in Fig 1.
Table 1. Mixture proportions of concrete and corresponding curing temperatures
Mix ID Curing temperature, (oC) w/cm s/a Unit weight (kg/m3) AdC (%)
W C SFa S Gb
C30/7SF 20,30,40 0.30 0.40 163 505 38 665 1005 1.2
C35/7SF 20,30,40 0.35 0.40 175 465 35 665 1007 1.0
C40/5SF 20,30,40 0.40 0.39 170 404 21 671 1064 0.8
C40 20,30,40 0.40 0.39 170 425 - 671 1064 0.6
a 7% for C30/7SF and C35/7SF, while 5% for C40/5SF of total weight of cementitious materials
b Maximum aggregate size of 20mm
c Superplasticizer (ASTM Type-F high range water-reducing admixture), % of total cementitious materials
Experimental Results
Fig. 2 (left) shows experimental results of autogenous shrinkage (C35/7SF) and Fig. 2 (right) shows the test results corresponding to equivalent ages predicted by the maturity method (CEB-FIP model code 1990). The test results revealed that the temperature dependency of AS may not be described solely by the simple time shift of the existing maturity method, as was also found in many previous researches. From Fig. 2 (right), it can be seen that a significant error exists at early ages, possibly due to inherent limitations of maturity. For instant, maturity was developed based on single activation energy while each property of a given concrete mix may have different temperature sensitivity factors at different stages of hardening. Moreover, the existence of a cross-over effect of AS may have affected its application to the prediction of later-age AS. Consequently, the use of a single maturity function for all properties of a single concrete mix is likely to be inappropriate, as satisfactory relationships between hydration kinetics, microstructure, and property development that warrant this assumption have yet to established.
Fig. 2. Comparison of experimental results of autogenous shrinkage with respect to actual test age(left) and equivalent age(right) calculated by CEB-FIP Model
Fig.1. Schematic diagram for measuring autogenous shrinkage in concrete
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NEW PREDICTION MODEL
Application of prediction model to isothermal curing temperature
Due to the inherent limitations of the maturity method, as discussed in the preceding section, the authors propose a new prediction model to estimate AS with time-temperature development. A prediction model to estimate the development of other mechanical properties of concrete considering temperature and aging effects was proposed by Kim et al.(Cement and Concrete Research, 2009). This model reduces the shortcomings of previous models and reasonably estimates the time-temperature development of mechanical properties of concrete. It is expected that the model can be reasonably implemented to estimate the effect of time-temperature development of AS of concrete provided that a different function of temperature sensitivity for AS is incorporated. The model can be expressed as follows:
(1)
The experimental results of Fig. 2 (left) were analyzed by Eq. (1), and Fig. 3 presents a comparison between predicted results by Eq. (1) and experimental results. As shown in Fig. 3, the prediction model gives a reasonably good estimation of the AS of concrete. Moreover, Fig. 4 clearly show that the difference between the calculated and experimental values is not large and that the prediction model properly estimates the relative AS in an error range of ±10%
Application of prediction model to variable curing temperature
Previous experimental investigations were based on isothermal curing temperatures. Almost all structural members are, however, subjected to temperature variations, either by the hydration of cement in mass concrete or by changes of the external environment with aging. Therefore, investigating the validity of the proposed model for AS under variable temperature is on paramount importance.
In Eq. (1) Ru is a function of both aging and temperature in variable curing temperature. Thus, in variable curing temperature condition, it is necessary to modify Eq. (1) to estimate AS under variable curing temperature as follows:
(2)
In Fig. 5, the scattered points and thick solid line denote the experimental results and predicted AS curve of concrete, respectively. All other curves on Fig. 5 are the predicted AS curves of concrete cured at various isothermal temperatures. Fig. 5 reflect that the new prediction model properly estimates the AS of concrete cured at variable curing temperature bott at early-age and at later ages.
Fig. 3. Experimental and calculated relative AS Fig. 4. Comparison of experimental and calculated results
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Fig. 5. Comparison of experimental results and predicted curve of autogenous shrinkage of concrete (C35/7SF) under variable curing temperature (a) up to 3-months, (b) up to 1-week
CONCLUSIONS
In this study, effects of temperature on autogenous shrinkage of concrete were experimentally obtained. Evaluation of a previous maturity method based on single activation energy revealed to be inappropriate to predict time-temperature development of autogenous shrinkage of concrete. It normally underestimates or overestimates the temperature effects on the development of autogenous shrinkage, both at early ages and at later ages. In order to circumvent these shortcomings of maturity method, a new prediction model of autogenous shrinkage based on the time-temperature dependency of apparent activation energy has been proposed. The validity of the new prediction model was investigated through a comparison between experimental results and prediction curves.
ACKNOWLEDGEMENTS
This study was part of a research project supported by a grant (Code# ’09 R&D A01) from Cutting-edge Urban Development Program funded by Ministry of Land, Transport and Maritime Affairs of Korean government. And this work (20101610004J) was also partly supported by the Nuclear Research & Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy. The authors wish to express their gratitude for the financial support that has made this study possible
REFERENCES
CEB-FIP (1990) model code CEB-FIP 1990
J.K. Kim, S.H. Han, K.M. Lee, “Estimation of compressive strength by a new apparent activation energy function”, Cement and Concrete Research, Vol.31, No.2, pp.217-225, 2001
M.N. Amin, J.S. Kim, T.T. Dat, J.K. Kim, “Improving test methods to measure early age autogenous shrinkage in concrete based on air cooling”, IES Journal Part A: Civil & Structural Engineering, Vol.3, No.4, pp.244-256, 2010
J.K. Kim, S.H. Han, S.K. Park, “Effect of temperature and aging on the mechanical properties of concrete: Part II. Prediction model”, Cement and Concrete Research, Vol.32, No.2, pp.1087-1094, 2002
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