Exploration Of Several Problems About Creep Of Modern Concrete

Creep is an important inherent time-dependent property of concrete and it has a great effect on the concrete long-term stress and deformation. Despite major successes in the study of concrete creep were obtained in the previous century, modern concrete materials which are characteristic of higher strength, common use of mix of mineral admixture and chemical admixtures, application of pumping technology in construction and application of various structural forms have brought new problems for further study. Researchers in structure engineering have attached more importance to basic research on creep of modern concrete. In this paper, several problems about creep of modern concrete are discussed and these problems include:(l)To meet the development of modern concrete structures, how to establish a set of convenient and practical prediction formulae with reasonable accuracy;(2) To meet the need of experimental research on creep of modern concrete, how to improve the traditional creep test system in our country;(3) How to calculate thermal creep stress filed of early-age concrete considering temperature effect by ANSYS software. On the basis of the discussion about the problems mentioned above, the main work done and some conclusions obtained in this paper are as follows:1. Referring to a lot of literatures and some related models overseas, a convenient and practical prediction model which can predict creep coefficients with certain accuracy is proposed on the basis of GL2000 model. Moreover, based on the prediction model about creep coefficients, a formula which can estimate specific creep is proposed with the compound shape law. To evaluate the accuracy of the prediction model proposed in this paper, GL2000 model and CEB-FIP 1990 model, these models are compared based on the 8 series of domestic creep experimental data. Various statistical methods are used to determine the order of accuracy of each model. The result shows that the practical prediction model proposed in this paper performs best and satisfies engineering requirement.2. With the help of advanced auto-control technology and reasonable devices used for loading and measuring, the traditional creep test system in our country is improved. A detailed and practical scheme about full-graded concrete creep experiment is studied out. Test method for the behavior of concrete stress relaxation is analyzed theoretically. Then, the equipment, apparatus and work flow chart are determined suitably. A lever loading device which can carry out concrete tensile creep experiment is developed and manufactured. Early-age concrete tensile creep experiment is carried out. Some experiment results are as follows: (1)Early-age tensile modulus is higher than compressive modulus. The earlier the age of concrete is, the larger the range of increase is.(2)Due to creep, concrete ultimate tensilestrength reduces.(3) When the water-cement ratio and cement content of concrete are the same, mix of pumping agent into concrete makes early-age tensile creep higher .The earlier the loaded-age of concrete is, the larger the range of increase is. The tests show that the lever loading device is of good performance and can be refered to other users. 3. Based on the theory of equivalent age and with the help of APDL offered by ANSYS software, the research on thermal creep stress calculation of early-age concrete considering temperature effect on the modulus and creep is carried out.The thermal creep stress of an early-age concrete block is calculated. The result shows that due to the temperature effect, the maximum principal thermal tensile stress and the crack risk at the surface center node are reduced.The maximum principal thermal compressive stress at interior center node increases and when it gradually changes to the principal thermal tensile stress, the crack risk at the interior center node increases. The higher the initial temperature of fresh concrete is, the higher the maximum principal thermal tensile stress and the crack risk at the surface center node are. The higher the initial temperature of fresh concrete is, the larger the maximum principal thermal compressive stress at interior center node is. The environment temperature has a great effect on the thermal creep stress and the crack risk at the surface center node. On the contrary, the environment temperature has a little effect on the thermal creep stress at interior center node.