| Nowadays,ultra high performance concrete(UHPC)has become one of the important development directions of concrete.Due to its excellent strength and durability,it has been used in various high-rise and long-span buildings.In order to ensure its full performance,it is necessary to add fibers.At this stage,the most widely used fiber is copper plated steel fiber.However,the cost of UHPC remains high due to the extensive use of steel fibers,and the high performance of UHPC required by engineering depends on steel fibers.In this paper,hollow stainless steel fibers are used instead of copper plated steel fibers to study the effects of cavitation structures in steel fibers and stainless steel on the performance of UHPC.The main research work is as follows: The influence of hollow stainless steel fibers on the mechanical properties of UHPC was analyzed by making rubber sand test blocks and conducting flexural and compressive strength tests;The influence of hollow stainless steel fibers on the durability of UHPC was studied through dry-wet cycle tests,salt spray tests,and longterm immersion tests;The distribution of hollow steel fibers in UHPC was measured using three-dimensional CT;The results show that:(1)With the increase of the content and aspect ratio of hollow stainless steel fibers,the mechanical properties of UHPC gradually increase,especially the increase in flexural strength is extremely significant.However,there is a critical point in the content and aspect ratio of steel fibers,which will form a agglomeration phenomenon once exceeded,leading to a decline in mechanical properties;(2)Compared to copper plated steel fibers and solid stainless steel fibers,hollow stainless steel fibers have a more significant effect on improving the mechanical properties of UHPC.When the volume fraction of steel fiber is 2.0% and the aspect ratio is 75,the flexural strength of UHPC doped with hollow stainless steel fiber is higher than that of copper plated steel fiber and solid stainless steel fiber by more than20.0%;(3)Hollow stainless steel fibers can greatly improve the durability of UHPC.After the completion of the dry-wet cycle experiment(150 d),the UHPC performance of copper plated steel fibers decreased by 28.1%,while that of hollow stainless steel fibers decreased by only 6.0%.At the beginning of the salt spray experiment,the flexural strength of UHPC doped with hollow stainless steel fibers was 18.4% higher than that of copper plated steel fibers.By the end of the experiment(180 d),the gap between the two increased to 34.0%.In the long-term immersion experiment using 10%Na Cl solution,the performance difference between the two increased from 23.7% at the beginning to 51.4%(18 months);(4)In alkaline environments,the copper plating layer on the surface of copper plated steel fibers will automatically fall off.And in the alkaline environment of concrete,it may accelerate the corrosion rate of copper plated steel fibers,especially where the steel fibers are interconnected;(5)The 3D CT results show that the distribution of hollow steel fibers in UHPC is more uniform and the interconnection probability is lower.For a 2.0% content of 0.4 mm steel fibers,the connection probability of solid steel fibers is 71.4%,while the connection probability of hollow steel fibers is only 46.7%.Moreover,cavitation in steel fibers can reduce the quality of a single steel fiber and reduce the settlement phenomenon caused in manufacturing engineering;(6)The electrochemical corrosion resistance of UHPC doped with hollow steel fibers is even better.At a 2.0% content,the volume resistance of UHPC with 0.3 mm hollow steel fiber is 66.5% higher than that with solid steel fiber.This is because the medium cavitation structure has a smaller current carrying capacity when conducting electricity,and the interconnection probability of hollow steel fibers in UHPC is lower. |
PDF Full Text Request