A model of damage to fresh concrete in a corrosive sulphate environment was formulated to investigate how and why the strength of corroded concrete changes over time. First, a corroded concrete block was divided into ...A model of damage to fresh concrete in a corrosive sulphate environment was formulated to investigate how and why the strength of corroded concrete changes over time. First, a corroded concrete block was divided into three regions: an expanded and dense region; a crack-development region; and a noncorroded region. Second, based on the thickness of the surface corrosion layer and the rate of loss of compressive strength of the corroding region, a computational model of the concrete blocks' corrosion-resistance coefficient of compressive strength in a sulphate environment was generated. Third, experimental tests of the corrosion of concrete were conducted by immersing specimens in a corrosive medium for 270 d. A comparison of the experimental results with the computational formulae shows that the calculation results and test results are in good agreement. A parameter analysis reveals that the corrosion reaction plays a major role in the corrosion of fresh concrete containing ordinary Portland cement,but the diffusion of the corrosion medium plays a major role in the corrosion of concrete mixtures containing fly ash and sulphate-resistant cement. Fresh concrete with a high water-to-cement ratio shows high performance during the whole experiment process whereas fresh concrete with a low water-to-cement ratio shows poor performance during the late experiment period.展开更多
This study examined the impact of trade facilitation on economic development, particularly the impact of customs environment on trade flows over the period from 1995 to 2010. Five countries of the East African Communi...This study examined the impact of trade facilitation on economic development, particularly the impact of customs environment on trade flows over the period from 1995 to 2010. Five countries of the East African Community （EAC）, namely, Tanzania, Kenya, Uganda, Rwanda, and Burundi, are involved. The study employs a gravity model for estimating bilateral trade flows between the EAC partner states. The ordinary least square （OLS） technique is adopted and applied for the regression analysis by using the Stata 10.0 software. Results suggest that, the customs environment of the importer is significant and possesses a strongly positive impact on East African trade flows. Results also find that the customs environment of the exporter is insignificant, even though it shows a negative relationship with the East African trade flows, hence a negative determinant. East African countries have to improve their customs environment, especially when undertaking an importation, in order to boost the overall trade flow in the block. They should also improve other trade facilitation indicators, such as port efficiency, regulatory environment, and infrastructure. The aid for trade, in terms of technical and financial assistance, should also be enhanced for the development of infrastructure, including roads, railways, ports, bridges, and border posts.展开更多
基金Project(51078176) supported by the National Natural Science Foundation of ChinaProject(JK2010-58) supported by the Construction Science and Technology Research Project in Gansu Province,China
文摘A model of damage to fresh concrete in a corrosive sulphate environment was formulated to investigate how and why the strength of corroded concrete changes over time. First, a corroded concrete block was divided into three regions: an expanded and dense region; a crack-development region; and a noncorroded region. Second, based on the thickness of the surface corrosion layer and the rate of loss of compressive strength of the corroding region, a computational model of the concrete blocks' corrosion-resistance coefficient of compressive strength in a sulphate environment was generated. Third, experimental tests of the corrosion of concrete were conducted by immersing specimens in a corrosive medium for 270 d. A comparison of the experimental results with the computational formulae shows that the calculation results and test results are in good agreement. A parameter analysis reveals that the corrosion reaction plays a major role in the corrosion of fresh concrete containing ordinary Portland cement,but the diffusion of the corrosion medium plays a major role in the corrosion of concrete mixtures containing fly ash and sulphate-resistant cement. Fresh concrete with a high water-to-cement ratio shows high performance during the whole experiment process whereas fresh concrete with a low water-to-cement ratio shows poor performance during the late experiment period.
文摘This study examined the impact of trade facilitation on economic development, particularly the impact of customs environment on trade flows over the period from 1995 to 2010. Five countries of the East African Community （EAC）, namely, Tanzania, Kenya, Uganda, Rwanda, and Burundi, are involved. The study employs a gravity model for estimating bilateral trade flows between the EAC partner states. The ordinary least square （OLS） technique is adopted and applied for the regression analysis by using the Stata 10.0 software. Results suggest that, the customs environment of the importer is significant and possesses a strongly positive impact on East African trade flows. Results also find that the customs environment of the exporter is insignificant, even though it shows a negative relationship with the East African trade flows, hence a negative determinant. East African countries have to improve their customs environment, especially when undertaking an importation, in order to boost the overall trade flow in the block. They should also improve other trade facilitation indicators, such as port efficiency, regulatory environment, and infrastructure. The aid for trade, in terms of technical and financial assistance, should also be enhanced for the development of infrastructure, including roads, railways, ports, bridges, and border posts.
