Study On Durability Of High Performance Concerte In The Gobi Area Of Northwest China

The Gobi area in northwestern China is characterized by a climate of strong wind, great drought and acute temperature change. So far, the techniques concerning high-performance concrete for high-speed railways built elsewhere have not been well adapted to this area. With the rapid development of high-speed railway construction in this area, the durability research of high-performance concrete for this area is urgently needed. This dissertation deals with the durability problem in three aspects, that is, optimizing the application of admixtures, improving aggregates and enhancing curing techniques, which are detailed as follows, respectively.1. Mineral admixtures are much essential to achieve specified durability for high-performance concrete. Based on Aim-Goff packing model, the effect of mineral admixtures on cement-based composite cementitious systems is analyzed by using scanning electron microscopy (SEM). Based on grey theory, this dissertation discusses the effect of particle size distribution (PSD) and particle fineness of mineral admixture on macro properties of the mortars and establishes a corresponding grey incidence model. The optimal combination of mineral admixtures is studied by using grey analysis and the effect of pore structure on macro properties of the concrete is evaluated, with a grey model for the correlation between pore size distribution and concrete strength presented. A multivariate linear model for the relationship between pore structure parameters and the late strength of the concrete is developed by using regression methods.It is shown that for concrete with low requirements of both strength and durability, a single admixture of fly ash can be adopted; for concrete with high requirements of both strength and durability, a double combination of fly ash and silica fume (alternatively, mineral powder, for economic consideration) can be utilized; for concrete with very high requirements of both strength and durability, a triple combination of fly ash, silica fume and mineral powder can be implemented.2. The current material-related codes and specifications are short of clear instructions on the use of artificial aggregates, especially on the prescription of fracture surface, which shall be considered for crushed cobble. This dissertation includes a systematical study on the macro properties of crushed-cobble concrete, establishing a mathematical relationship of concrete strength and durability versus fracture surface ratio of crushed cobble, and introducing a computer model for particle size versus fracture surface ratio of crushed cobble.The analysis indicates that for the concrete with a large water-cement ratio, the fracture surface ratio should be limited to 40-60%, while for the concrete with a small one it can be 60-80%, so as to achieve necessary strength and durability; generally, cobbles of less than 20mm in size should not be used to make aggregate for concrete; for cobbles of 30-80mm in size, the fracture surface ratio can reach 40-60% after several times of crushing; for cobbles of greater than 90mm in size, the fracture surface ratio would be more than 70% once they are crushed.3. To investigate proper techniques to cure concrete in Gobi's inhospitable environment, a numerical simulation for the temperature field of concrete in piers of a bridge is performed by using MIDAS, a FEM (finite element method) based software. The analysis shows that the membrane curing can reduce the temperature difference between the inner part and the surface of the pier, make the surface and center stresses lower than the allowable tensile stress, and avoid the occurrence of temperature cracking. Corresponding laboratory tests are also performed to verify the theoretical analysis. With the Gobi environment simulated, the pore structure and distribution are tested by using mercury injection method. The results suggest that the harsh environmental factors, including strong wind, great aridity and acute temperature change as mentioned above, have a significant influence on the pore distribution in the concrete, resulting in the dramatic increase of harmful pore content, porosity, average pore size and the most probable pore size; a membrane curing based on heat and wet preservation can substantially decrease the unfavorable influences.With solid concrete piers constructed on site in the typical Gobi environment of Xinjiang section of the Second Lanzhou-Xinjiang Railway Line, the concrete temperature field is real-monitored and the concrete macro properties are measured to further verify the validity of the membrane curing technique based on heat and wet preservation in Gobi environment.