Study On Impact Resistance Of Basalt Fiber Concrete After Freeze-thaw Cycle At Ultra-low Temperature

For liquefied natural gas（LNG）storage tanks and large buildings susceptible to external loads and ultra-low temperature freeze-thaw cycles in cold regions,their structures are prone to damage and strength decline,which will lead to the reduction of building stability and safety.Therefore,in the design and preparation of building materials,auxiliary means such as adding fiber or other admixtures are often used to strengthen the mechanical properties of building structures in freezing and thawing cycles at low and ultra-low temperatures.Therefore,basalt fiber is used as improvement fiber in this thesis to study the dynamic and static mechanical properties changes and fiber strengthening mechanism of concrete after freezing and thawing cycles at low and ultra-low temperatures,which has important reference significance for architectural engineering design under extreme temperature environment.In this thesis,an electronic universal testing machine and a large-diameter split Hopkinson pressure bar with a diameter of 75mm were used to study the temperature（20℃curing,freeze-thaw cycles in the range of 20℃～-20℃and 20℃～-190℃）,the volume content of basalt fiber（0%,0.2%,0.25%,0.3%,0.35%,0.4%）and strain rate(60s^-1～80s^-1,80s^-1～100s^-1、100s^-1～120s^-1)were the main variables.The dynamic and static mechanical properties of basalt fiber reinforced concrete after ultra-low temperature freeze-thaw cycle were studied.It is found that basalt fiber can improve the splitting tensile strength,flexural strength and compressive strength of concrete at normal temperature.The influence of basalt fiber on the compressive strength of concrete is not always positive,and it will degrade when the amount of basalt fiber is too much.After freezing and thawing at low temperature,the three mechanical strengths of all test blocks were lost.The basalt fiber can significantly improve the loss rate of split tensile strength and flexural strength of concrete after the low temperature freezing-thawing cycle.The optimization effect of basalt fiber on the strength loss rate and peak strength is firstly increased and then slightly decreased with the fiber content.The general trend of the stress-strain curve of the test block with basalt fiber after freezing-thawing cycle at low or ultra-low temperature is much wider and flatter than that of the test block without fiber.The stress-strain curve in the initial stage is basically linear and then enters the yield stage（peak and stress platform will appear）.At the same temperature,the dynamic failure strength of concrete first increases and then decreases with the increase of fiber content.The failure strength of basalt fiber reinforced test block with 0.35%content at normal temperature is 60.952MPa,which is 26.18%higher than that of ordinary concrete test block,and is also the best content in this group.At low temperature,the optimal content is 0.25%,and the failure intensity of the test block is 48.868MPa,which is 10.5%higher than that of ordinary concrete under the same conditions.Basalt fiber still plays a favorable role in the failure strength of concrete under impact after ultra-low temperature freezing-thawing,and the samples with the content of 0.25%and 0.3%have better performance.At the same temperature,the impact toughness of basalt fiber reinforced concrete is better than that of ordinary concrete,but the fiber toughening effect is slightly weakened at low strain rate.With the increase of fiber content,the impact toughness of concrete increases first and then decreases,and the optimal content is 0.25%at ultra low temperature.As the temperature of freeze-thaw cycle decreases,the toughness degradation trend of fiber concrete is lower than that of ordinary concrete.Therefore,the addition of basalt fiber can effectively improve the energy absorption capacity of concrete;The difference of dynamic and static failure strength of the test block is the most obvious at room temperature,and then gradually decreases with the decrease of circulating temperature,With the increase of fiber volume content,the difference of freezing-thawing at normal temperature and low temperature first increases and then decreases slightly.