Preparation And Properties Of High Performance Concrete Under The Plateau Environment With Low Temperature

Under the strategic background of western development strategy and 'the belt and road initiative',the development in plateau areas is always of significance and difficulties.Developing the highway can improve the developmental environment and regional accessibility,promote the integration of western economy and optimize the resource allocation in Tibetan plateau area,so as to booster the rapid economical development in western area.The low temperature and the great temperature difference in Tibetan plateau area have an adverse effect on the concrete working performance,mechanical properties and durability.Therefore,by experiments,this paper studies the preparing techniques of high performance in plateau environment and the promoting techniques of concrete properties,and the high performance concrete applicable for the plateau environment is prepared based on the Yellow River extra-long bridge projects in Gama Yangqu and Tangnag,which has a practical significance and strategic value for the bridge construction in Tibetan plateau area.Investigation of the environment,concrete,raw materials as well as the typical damages of bridges in Qinghai region were conducted.Qinghai region has environmental characteristics of low temperature,great temperature difference,dry and windy weather and long time of sunshine;the concrete in Qinghai region has a high water cement ratio,more rough sand,high consumption of cementitious materials and low utilization ratio of mineral admixtures;the typical bridge damages include cracks and water percolation in the superstructure,and cracks in the substructure which have an adverse impression of the quality.Based on different parts of the highway bridges,two series of high performance concrete C30 and C50 were prepared to study the effect those parameters which includes water cement ratio,sand rate,content of flyash have on the concrete,in the meanwhile,according to the local design,construction requirement as well as the raw material situation,the duration property is promoted by focusing on the aggregate gradation design,the double mixing technology of admixture and high performance reducing agent as well as the air entraining technology,and the integrated design method HPC ratio focused on duration property was proposed.This paper also studies the frost resistance property and microstructure damage mechanism of high performance concrete under freeze-thaw environment.The results indicate that when the air content of C30 concrete is 4.5%?6.0%,C50 is 5.5%?6.0%,the pore diameter range is from 40 to 280?m,the air bubble spacing coefficient is 120?150?m,the frost resistance are all greater than 300 freeze-thaw cycles.The frost resistance property can be promoted significantly by properly increasing the air content,decreasing the pore size and the air bubble spacing coefficient.The C30 concrete used in bridges applicable for low temperature is designed,the results indicate that under the negative temperature and variable temperature curing condition,the strength of concrete mixed with antifreeze agent was obviously higher than those without antifreeze agent,but under the standard curing condition,the concrete mixed with antifreeze agent has a high early strength,while the later strength grew much slowly.The mechanical properties of low temperature concrete can be improved efficiently through decreasing water cement ratio,controlling the flyash content,heating the raw materials beforehand and storing heat after casting.The ring shrinkage strain rules of C50 concrete under variable temperature are studied,the results indicate that there are mainly three stages in the concrete strain curve under normal temperature:the stage of stress concentration and stress buffer and stress equilibrium stage.The early crack resistance of concrete and shrinkage performance has a direct relationship,circle constraint effect on the early concrete strain change is bigger,the easier the concrete cracking.Variable temperature condition aggravated the shrinkage deformation of concrete,especially in the case of positive and negative temperature,therefore,it is of great necessity to take curing measurement as well as optimize the mix ratio to decrease the possibility of concrete cracking.