The Study On High Performance Concrete With Ultrafine Fly Ash And Its Application

To meet the increasing demand of the pre-stressed precast concrete elements with excellent properties for high-speed railway construction, it is necessary to solve the key technology problems of the pre-stressed concrete elements during precast. The work is focused on investigating the efficiency of ultra-fine fly ash (UFA) in paste and concrete with low water to binder ratio (w/b). The properties characteristics of high performance concrete with ultra-fine fly ash (UFA-HPC) and the influences of UFA-HPC on the properties of the pre-stressed precast concrete elements for railway sleepers and bridges are symmetrically studied. The corresponding achievements and outcomes have been included in the related design specifications and guilder lines. Practical applications have brought remarkable technical and economic benefits and social effects.The main achievements and contribution of the thesis are as follows:1,Two new indices, namely relative density and minimum water-demand of paste are proposed to evaluate the filling-effect of UFA in paste and concrete with low w/b based on the experimental and theoretical analysis. Through the measurement and analysis of both hydration of the cement-fly ash system and microstructure of the hardened pastes and etc, the effects of fineness of fly ash on its physical characteristics and pozzolanic reaction are investigated. It is demonstrated that the filling-effect of UFA can greatly improve the properties of cement hydration, pore structure of cementitious system and the interface transition zone in concrete. When w/b is low, the improvement of the compressive strength of the hardened paste with UFA is higher than that of the hardened ordinary cement paste. In addition, there exist the synergic effects between UFA and high range water reducer in cementitious system.2,It is found that the high strength concrete with high volume fly ash is not adaptable to the steam curing during the experiments. To improve the adaptability of the steam cured concrete with high volume fly ash, the efficient technical treatments are to choose appropriate fineness and amount of fly ash, to combine fly ash with a certain amount of ground granulated blast-furnace slag (GGBS) and to reduce w/b. A ultrafine fly ash composite (UFAC), which can be used to prepare the steam cured concrete for the pre-stressed precast concrete element, has been successfully developed on basis of modification of gradeⅡfly ash by some technical measurements such as pulverization and combination with a certain amount of GGBS.3,The reasonable fineness range and maximum replacement level of UFA and UFAC are determined in terms of the experiments on the properties of UFA-HPC and corresponding theoretical analysis. For the production of the pre-stressed precast concrete element with C50 to C60 grade UFA-HPC, the reasonable specific area range of UFA and UFAC is about 550 m~2/kg to 700m~2/kg, while the maximum replacement of cement by UFA or UFAC is 30% or 40% respectively. Furthermore, by optimization of mix parameters of concrete, the UFA-HPC with good workability and excellent mechanical properties and durability can be prepared. As a result, the key technical problem that balances both the earlier strength of UFA-HPC required by stripping of the pre-stressed precast concrete elements and their long-term performance is solved.4,Steam cured UFA-HPC is used to fabricate the pre-stressed concrete sleepers in practice for the first time. Through the tests on these sleepers, it is proved that the mechanical properties, fatigue properties and long-term performance of the typeⅢsleepers made by UFA-HPC are much better than those sleepers with ordinary concrete. It has been shown that the properties of the sleepers also meet the requirements of related specification. The addition of UFA or UFAC can effectively improve the compactness of the steam cured concrete and reduce the damage accumulation inside sleepers. Hence, significant technical economy benefits and social impacts can be achieved by using UFA-HPC in sleeper production. 5,The creep characteristics of UFA-HPC are studied and compared with those of ordinary concrete. The effects of creep deformation of two concretes on the camber of the model of a simply supported pre-stressed concrete bridge are further investigated by both experimental and theoretical analysis. The effect of UFA on creep characteristics of concrete is analyzed and the corresponding mechanism is proposed. A mathematical model is presented to predict the long-term creep degree of UFA-HPC. The results indicates that, compared with C50 ordinary concrete, the coefficient of creep of C50 UFA-HPC is decreased by 40 %, especially for drying creep of UFA-HPC, which results in a great reduction in the sensitivity of deflection of simply supported pre-stressed concrete bridges to ambient humidity. The increment in creep deformation of concrete caused by loading at 10d age can be finished within two months. In addition, the camber caused by creep deformation of the model of a simply supported pre-stressed concrete bridge with C50 UFA-HPC can be remarkably decreased compared with that of the model of the bridge with C50 ordinary concrete. Finally, some technical treatments to control the remnant camber of simply supported pre-stressed concrete bridges for the non-ballast track caused by creep deformation are suggested.6,The corresponding achievements have been included in the four design specifications and guilder lines such as Technical Specification of Mineral Admixture Application, Guide to Design and Construction of Self-Compacting Concrete, Guide to Concrete Durability for Qinghai-Tibet Railway Frozen Plateau Area, Design Specifications of Non-Ballast Track of Passenger Dedicated line。...