Nonlinear Creep Model Of Early-Age Concrete For Compression

Concrete creep causes effects, such as the stress redistribution in cross-section, extra deflection in structures. Creep of concrete at early age is greater than that of hardened concrete and its effects become more pronounced. For some composite structures, tall buildings or substructures, it is a common case that concrete is loaded at early age. In that case, as concrete is rather weak due to low hydration at early age and stress ratio is relatively high, concrete creep at early age does not satisfy the superposition principle, presenting nonlinear characteristics.The mechanism of nonlinear creep of concrete at early age is unclear and early-age creep models seldom concerned the nonlinearity. From the present study, nonlinear creep in hardened concrete under high stress levels is associated with the micro-cracks in concrete. However, there is almost no investigation about nonlinear creep of concrete at early age. For creep models, the present models are suitable for the concrete which is loaded at 7th day or later after pouring, these models cannot predict the creep of early-age concrete accurately.The main work of this study is presented as follows:1) Different existing early-age creep testing results and nonlinear creep test results were summarized. The features of nonlinear creep of early-age concrete were analyzed. Based on the visco-elastic-plastic theory, rheological elements and the adaptability to different components of early-age concrete creep were discussed, and a nonlinear creep model based on connected rheological elements for early-age concrete was proposed.2) Early-age concrete creep test was carried out. In this test, we focused on creep tests of concrete specimens loaded at 2 days. Test specimens were divided into two groups, and each group included 4 specimens which were loaded at different stress levels in order to get nonlinear creep data, two groups of test specimens were unloaded at 10 days and 25 days after loading, respectively for measuring creep recovery deformation.3) Specimens loaded at stress levels of 0.38 and 0.74 were tested by acoustic emission (AE). Test data show that the higher stress level, the more number of AE hits, and AE hits are less and less as time goes by. Comparing with creep test data, it can be found that the development of AE hits and the nonlinear creep incremental deformation is highly related, and the relationship between creep damage and nonlinear creep increment is derived. Through the analysis of characteristic parameters of AE signals, this paper argues that nonlinear creep of early-age concrete creates tension micro-cracks, but almost no shear micro-cracks.4) The parameters in the proposed model is regressed according to the test data. Results of the proposed creep model and existing creep models were compared. It can be concluded that B3 model is the most precise model in the selected creep models for early-age creep, and the errors are within 20%; MC2010 model can be used in linear stage of early-age creep, but not suitable for nonlinear creep; ACI 209R model, and GL2000 model have relative large deviation for predicting early-age creep. Besides, the data predicted by the proposed model is less than experimental data of existing literatures and the errors are within 20%.