Separation method of strain caused by concrete shrinkage and creep in. bridge health monitoring

Transcription

1 Applied Mechanics and Materials Submitted: ISSN: , Vols , pp Accepted: doi:1.428/ Online: Trans Tech Publications, Switzerland Separation method of strain caused by concrete shrinkage and creep in bridge health monitoring Yafeng Gong 1,a, Xiaobo Sun 2,b, Xianghui Li 2,b and Yubo Jiao 3,c 1 College of transportation, Jilin University, Changchun, China a gongyf@jlu.edu.cn, b @163.com, c jiaoyb@jlu.edu.cn (corresponding author) Keywords: Concrete, Shrinkage, Creep, Separation, Field measurement, D62 model, ACI model Abstract. The strain caused by concrete shrinkage and creep is separated through the field measurement data in this paper. Then D62 model and ACI model of finite element model is established to predict shrinkage and creep. Compared with the measured data, the feasibility and practicability of these two models can be proved. Introduction Due to the large strain caused by concrete shrinkage and creep, it is necessary to separate shrinkage and creep strain when evaluate the bridge safety. In this way the structure of the stress state can be analyzed accurately [1]. So in bridge health monitoring, it is necessary to separate strain caused by concrete shrinkage and creep, consider separately with loading strain. Separation of strain caused by concrete shrinkage and creep can be obtained by the field measurement method and the finite element method. Through the measured data, the strain caused by concrete shrinkage and creep can be separated. Then D62 prediction model based on CEB - FIP (199) model [2] and the American association of concrete ACI finite element model [3] are established respectively. The properties of concrete material are given according to the shrinkage and creep calculation theory, then the calculation can be run. Through extracting model strain data and comparing with the measured data, the feasibility and practicability of this two kinds of shrinkage and creep prediction model can be verified. Solid bridge profile Taking Dongfeng avenue interchange mainline bridge as an example, the space finite element model is established by MIDAS. Construction stage is divided according to the actual construction process. Data collection point is divided into the corresponding construction stage when the whole bridge is completed, so model data and measured data can be extracted and analyzed. The property of vibrating wire sensor that embedded into Dongfeng avenue interchange mainline bridge section is good. In this paper, the cross section of the first across is taken as an example to illustrate the influence of creep and shrinkage to strain. The sensor of first span across is embedded along the longitudinal bridge, fixed on the top and bottom edge of web rebar. Analysis of measured data Measured data is the concrete strain of measuring point in the direction of sensor. Take the whole bridge steel beam after tensioning as initial value, the dead load of the second phase and temperature, the average vehicle load influence value on the measuring point are all removed. So All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-21/2/16,11:47:26)

2 1422 Mechanical Components and Control Engineering III strain data is mainly caused by shrinkage and creep strain. Each measuring point data is shown in Fig.1( 2, 3 and 6 measuring point data is too small to analyze) : Fig.1. Shrinkage and creep strain curve of point 1, 4, 5, 7, 8 Separation of shrinkage and creep strain respectively To inspect shrinkage regularity of concrete in prestressed concrete bridges, four prism specimen are made. Strain sensor is embedded in the center of the specimen along the longitudinal. Specimen are poured and cured with the same period of girders. The data which strain sensor measured is the shrinkage strain of concrete bridge. Sensor and specimen set position is shown in Fig.2 Fig.2. Sensor and specimen set position The averaged data of this four sensors measure can be used to analyze shrinkage strain of concrete bridge. Shrinkage strain is decreased by total shrinkage and creep strain, then creep strain data can be obtained. Thus concrete shrinkage and creep separated from total strain respectively can be realized. Concrete shrinkage and creep strain variation rule with time is shown in Fig Shrinkage Fig.3. Shrinkage strain curve It can be seen from the figure, the shrinkage changing with time is previous fast, late is slowing. Concrete creep strain variation rule with time is shown in Fig.4

3 Applied Mechanics and Materials Vols Fig.4. Creep strain curve of point 1, 4, 5, 7, 8 The creep changing with time is also previous fast, late is slowing. Shrinkage strain growth speed is slower than creep relatively, more long duration. Shrinkage strain before 16 months accounts for only 69% of the total 26 months shrinkage strain. But creep strain of the measuring point 1, 4, 5, 7, 8 before 16 months respectively accounts for 97%, 83%, 79%, 85%, and 79% of the total 26 months creep strain. In addition the numerical value of shrinkage strain is smaller than creep strain. Shrinkage strain is only half of creep strain after 26 months. Predicted model analysis D62 specification prediction model based on CEB - FIP(199) and American concrete institute ACI model are used in the finite element model to predict concrete shrinkage and creep of Dongfeng avenue interchange mainline bridge. Hyperbolic power function is used in shrinkage and creep process of ACI model. Shrinkage process curve is shown in Fig.5 (D represents D62 model and A represents ACI model) Shrinkage D Shrinkage A Fig.5. Prediction of shrinkage strain curve It can be seen from the figure, in the prediction of shrinkage strain, D62 model is slower than ACI model in the early development. But the prediction of final value is bigger than ACI model. D62 model and ACI model are also used in the prediction of concrete creep. Due to the interaction effects of creep and the loss of prestress, iterative calculation method is used in the model. The model is divided into several loss of prestress calculation stage on the time span. The initial model is established to calculate the first phase loss of prestress value caused by shrinkage and creep. Then in the second phase this loss value is subtracted from the strain of prestressed steel beam, the creep and shrinkage of this phase can be calculated. In turn, the loss of prestress caused by shrinkage and creep can be obtained. The above steps are repeated in the subsequent stage. So the coupling of shrinkage and creep with the loss of prestress can be simulated. According to the simulation method, creep process curve of measuring point 3,4,5,6 calculated by D62 model and ACI model is shown in Fig.6

4 1424 Mechanical Components and Control Engineering III D 5D 3A 5A 4D 6D 4A 6A Fig.6. Creep process curve of measuring point 3,4,5,6 Measured data and prediction data contrast analysis The result of measured shrinkage strain compared with D62 model and ACI model is shown in Fig.7 Fig.7. Shrinkage strain of measured value compared with predictive value Creep strain of the point 4, 5 predicted by D62 model and ACI model is chosen in this paper. The result of predictive value compared with the measured value is shown in Fig.8 and Fig D 4A 4 Fig.8. Creep strain of measured value compared with predictive value of point D 5A 5 Fig.9. Creep strain of measured value compared with predictive value of point 5 In the early stage of shrinkage strain, the strain predicted by ACI model is larger than the measured strain. But the strain predicted by D62 model is smaller than the measured strain. So in

5 Applied Mechanics and Materials Vols the early stage ACI prediction model of concrete shrinkage strain is relatively safe. Instead, D62 prediction model of concrete shrinkage strain is relatively safe in the late. For the prediction of creep strain, the development of D62 model is slower than ACI model in the early time, but the estimate of final data is bigger than ACI model. In the most of time, both the creep strain predicted by D62 model and ACI model is larger than measured strain. Both of the two models on the prediction of creep are safe. Conclusions In the early time of concrete shrinkage, the data predicted by D62 model is less than measured data. So, concrete shrinkage cannot be predicted by D62 model safely. But the shrinkage strain predicted by ACI model is relatively close to measured data, so shrinkage strain can be predicted by this model accurately. ACI model should be adopted to predict the concrete shrinkage strain in the early time. In the late, shrinkage strain predicted by D62 model is relatively close to measured strain, but the prediction data of ACI model is less than the measured data. So D62 model should be adopted to predict concrete shrinkage strain in the late. In the early time of concrete creep, the strain predicted by D62 model is relatively close to measured strain. D62 model should be adopted to predict the concrete creep strain. In concrete pouring eight months, the strain predicted by ACI model is relatively close to measured strain. ACI model should be adopted to predict the concrete creep strain. Especially in the late period of concrete shrinkage and creep (24 months after concrete pouring), creep strain predicted by ACI model is accurate. In practice, the advantages and disadvantages of the two models should be considered respectively, more accurate predictions should be made by them. References [1] Huirong Yan, Guoxing Huang, Bingruo Yi. The creep of concrete [M]. Beijing: China railway publishing house, [2] CEB the European commission on international concrete. CEB-FIP schema specification of 199 (concrete) [S]. Beijing: China construction science research institute structure specification room :57-7. [3] ACI Committee 29 (192). The Prediction of creep and shrinkage and temperature effects in concrete structure (ACI29-82) [s]. ACI, [4] Di Hu. Prestressed concrete bridge creep effect analysis [D]. Hunan: central south university Ph.D. Thesis,

6 Mechanical Components and Control Engineering III 1.428/ Separation Method of Strain Caused by Concrete Shrinkage and Creep in Bridge Health Monitoring 1.428/