Papers by Author: Di Tao Niu

Abstract: Durable life of concrete structures under the chloride environment depends on the permeability of chloride ion, and the chloride ion diffusion coefficient is the main indicator that reflects the permeability of chloride ion. Based on the Fick’s second law, a multi-factor model of the chloride ion diffusion is established. In this model, the influences of temperature, humidity, age of the concrete, fly ash and carbonation are taken into consideration. And the model is verified by engineering test data from the concrete structure under the wet and dry areas of marine. The results show that the calculated and measured values of the chloride ion content agree well, the actual situation of the project could be basically reflected. The multi-factor model of the chloride ion diffusion is practical.

Abstract: In order to investigate the carbonation resistance of shotcrete and the mechanical properties after carbonation, the accelerated carbonation test was carried out. The results indicate that the carbonation resistance of shotcrete is superior to that of normal concrete. With the increasing of carbonation depth, compressive strength and splitting tensile strength of shotcrete grew rapidly. The admixing of steel fiber can further improve the carbonation resistance, reduce the carbonation rate, and increase the splitting tensile strength of shotcrete greatly. Besides, based on analyzing the effects of construction technology and steel fiber of concrete for the carbonation resistance, a carbonation depth model for shotcrete was established. Key words: shotcrete; carbonation; steel fiber; mechanical properties

Abstract: With repeated action of freeze-thaw cycles, chloride ion diffusion characteristics changed obviously in concrete structure. In the study, the chloride ion diffusion coefficient and the surface chloride ion concentration were discussed especially with the trends. Based on these facts, the chloride diffusion model and the improved model of concrete was also discussed based on its time-dependent characteristics, which indicated that the freeze-thaw damage of concrete would have a greater impact to the chloride transport course in the concrete, and so on the service life of concrete structures would be affected.

Abstract: The change of steel mechanical properties is because of steel corrosion seriously. An important factor of evaluating the performance of active steel structure is the degradation of steel mechanics performance. Corroded steel mechanics performance degradation accords with four parameters Weibull distribution through analysis the rapid steel corrosion experiment data. The each Weibull distribution parameters of the degradation of the steel corrosion rate, yield strength and ultimate strength are solved through Matlab programming. Weibull distribution simulation results and experimental data agree well. These models can predict the degradation of steel mechanics performance and can help people to evaluate the corrosion steel structure bearing performance to reinforce the steel structure.

Abstract: Energy dissipation reduction design can achieve the performance-based seismic design. By contrasting the results of the frame with the SMA dampers and the original structure, the results prove that the SMA damper can reduce the seismic response effectively and achieve the important performance objective “no damage under moderate earthquake and maintainable under major earthquake”.

Abstract: The study of cover cracking pattern can provide scientific basis for the durability assessment and maintanence of concrete structures. In this paper, corrosion induced cover cracking pattern was investigated by accelerated corrosion test. The cover thickness c, bar diameter d and bar spacing s were considered in the test. Test results showed that the cracking pattern of cover concrete mainly included four typies, that is longitudinal cracks including wedge and vertical, parallel cracks, spalling and delamination. When the bar spacing was small enough, the delamination of cover concrete occurred which had nothing to do with the ratio c/d. When c/d≤1, the corrosion of side bar mainly induced the vertical cracks and the corrosion of corner bar induced both vertical and wedge cracks. While c/d>1, the corrosion of corner bar mainly induced wedge cracks.

Abstract: Based on the fast freeze-thaw test in 3.5% NaCl solution, the frost resistance of steel fiber reinforced concrete (SFRC) was studied in this paper. On the basis of scanning electron microscope (SEM) and mercury intrusion method, the microstructure and pore structure of SFRC was analysed. The reinforced mechanism of SFRC under the cooperation of freeze-thaw and NaCl solution was discussed. The test results show that adding appropriate amount of steel fibers into concrete can reduce the pore porosity and improve the compactness of concrete. The effects of steel fiber with proper volume fraction can inhibit the peeling of the concrete and reduce its damage rate. The volume of steel fiber on the frost-resisting property of SFRC is obvious.

Abstract: Durability of concrete structure due to carbonation problem has attracted worldwide attention. By studying on the performance of steel fiber reinforced concrete carbonation, simulations CO₂ corrosion environment was simulated. The impacts of the change law of carbonation depth of steel fiber reinforced concrete with water cement ratio (0.35,0.45,0.55), as well as steel fiber content (0%, 0.5%, 1.0%, 1.5% , 2.0%), pouring surface and age, on concrete carbonation depth were studied. Steel fiber reinforced concrete carbonation performance improvement role was analyzed. Thus, providing a experimental basis that can be used in the design and analysis of the durability of steel fiber reinforced concrete structure.

Abstract: In recent years, the concrete material subject to chloride corrosion which impacts greatly on the durability of the building project in the marine or coastal environment has been reported repeatedly. With the problems becoming more and more serious, much attention has been paid on it gradually. To improve the durability of the concrete structure in the marine environment, the author tried to add the different content of steel fiber into the concrete and research the permeability of the concrete in the chloride environment. Four different volumes of steel fiber were selected to prepare the concrete. The dates of the proportion are as follows, 1)0.5%; 2)1%; 3)1.5%; 4)2%. Leave the steel fiber reinforced concrete and plain concrete to soak in the sodium chloride solution which mass percent is 3.5%. The soak time is 30, 60, 90,120, 180 days separately. Among this, the test items include the penetration depth of chloride, the chloride ion content and the splitting strength after soaking period. As a result of the test, it is demonstrated that chloride penetration-resistant of steel fiber reinforced concrete is better than plain concrete. The best performance is obtained when the content of steel fiber is 1.5%. The results will be very useful to the concrete professional dealing with chloride-ion penetration. In conclusion, it is convinced that the application of this result in marine engineering will bring well economic and social environmental benefits for the society.

Abstract: A simplified integrating model is developed for soil-pile dynamic interaction system under seismic loading. Different from previous work, this simplified model constructs the whole system stiffness matrix by combining the stiffness matrix of pile and near region soil system according to the volumetric ratio of pile group and soil system. This simplified model can avoid deviation brought by previous soil-pile surface model and can felicitously simulate the co-working mechanism between soil and piles. The feasibility of the proposed approach is assessed by comparing with in-site observation result of real project.