Study On Application Of High Performance Concrete In The Passenger-dedicated Railway Line

It is necessary to use high performance concrete in the high-speed railway construction project for the purpose of 100 years service life and maximum sociall benefits. This paper takes the Tianxingzhou Road/Rail Yangtze River Bridge project as the background and investingates the mass concrete of pile cap and HPC of box girder. The method of mix optimization design was studied. The mix design of HPC was optimized with an intelligent and visualized optimization technology for passenger-dedicated railway line. The electric flux, chloride diffusion coefficient experiment as well as results of MIP analysis were used to evaluate the pore structure of concrete. The impermeability and durability of concrete were studied. The effect of micro-cracks and pore structure change to the impermeability after being subject to compressive loads was discussed. The results were showed as following.(1) The high quality mineral admixtures (fly ash, slag) can refine the pore size of concrete by a secondary hydration reaction and a micro-aggregate effect. The most probable pore size of concrete with 30% fly ash (slag) are 79.9% (66.4%) of that of concrete without fly ash (slag), respectively. So, it can be concluded that the mineral admixtures make the structure of hydration product more compact, which decreases the permeability of concrete.(2) After being subject to compressive loads under 85%P, the electric flux of concrete was not changed significantly. After being subject to compressive loads with 90%P, the electric flux of concrete was increased, and was about 2.2-3.3 times of that before loading. It is shown that, after being subject to compressive loads, there were some micro-cracks that can not be cicatrized. So, it can increase the permeability of concrete, and provides diffusion channels for corrosive substances, which is the main factor that accelerates concrete deterioration.(3) Entraining air in concrete (5.3% by volume) increased the total porosity and decreased its compressive strength. But, it optimized pore size distribution, and clogged up the continuous pores, which was beneficial to decrease the permeability of concrete. Entraining excessive air in concrete (8.9% by volume) can decrease both the compressive strength and impermeability of concrete. Especially after being subject to compressive loads, the occlusive pore was destroyed; some capillary pores run through again, which increased permeability much more.(4) The highest temperature rise and its time occurred in mass concrete were simulated and forecasted with finite element software Midas Civil for mass concrete of pile cap in road/rail bridge. Effective temperature control and crack prevention measures in time were taken. The mix proportion of C50 HPC for box girder was designed with an intelligent and visualized optimization technology as following: cement: fly ash: slag: sand: coarse aggregate: water: admixture = 300: 120: 42: 692: 1178: 146: 2.31; the compressive strength at 28d was 63.2 MPa; the electric flux at 56d was 435 C; chloride diffusion coefficient at 56d was 12.6×10^-14m²/s. Both the compressive strength and the durability of concrete at late age were improved with the optimized mix design.