| Ultra High Performance Concrete(UHPC)is widely used in the construction field due to its high strength,ductility,and durability.However,it can cause cracking and spalling at high temperatures.How to reduce this phenomenon has become a research hotspot.Therefore,the experiment is mainly based on the existing UHPC mix ratio,and the high-temperature spalling and spalling phenomena are reduced by changing the addition of different types of fibers.This article studies the compressive and flexural properties of UHPC mixed with fibers at high temperatures,and obtains the high-temperature resistant UHPC mix ratio.Based on this,the bond slip performance of high-temperature resistant UHPC with steel bars is studied.The main research content is as follows:(1)Add fibers with different dosages to UHPC and conduct experiments at six temperature levels(20℃,200℃,400℃,600℃,800℃,1000℃)to study the characteristics of the test block,compressive failure characteristics,and compressive strength.The main analysis is the impact of temperature changes and fiber content on compressive strength,and two sets of optimal mix ratios are obtained.The results show that at room temperature,the compressive strength of single doped steel fibers with high aspect ratio is greater than that of mixed PVA fibers and greater than that of mixed PP fibers;In the temperature test,the two groups of test blocks containing only PP fibers remained intact at six different temperatures,while the rest burst at around400 ℃,and the compressive strength first increased and then decreased with the increase of temperature.(2)Based on the two sets of mix ratios obtained from compressive tests,bending test blocks were made at six temperature levels,with a focus on studying the effects of different steel fiber lengths and temperatures on the bending performance when polypropylene fibers were added.The bending test results and stress-strain curves were analyzed.The experimental results show that in the flexural strength of UHPC specimens mixed with PP fibers after high temperature,the flexural strength of steel fibers with high aspect ratio alone is greater than that of mixed steel fibers;The bending strength reaches its maximum at 400℃.Before 400℃,the ultimate bending stress gradually increases with the increase of temperature,and after 400℃,it gradually decreases.(3)Pull out tests were conducted on 36 sets of specimens under two sets of high-temperature resistant mix ratios,six temperature levels,and three bond lengths(2 times the diameter of the steel bar,4 times the diameter of the steel bar,and 6 times the diameter of the steel bar).The failure morphology of the specimens was described,and the influencing factors of the change in bond stress-strain curve,ultimate bond strength,and slip amount were analyzed.The test results indicate that the failure modes in the tensile test of steel bars and high-temperature resistant UHPC can be divided into steel bar pull-out failure and steel bar pull-out failure;Under six temperature tests,only the steel bars with a bonding length of 2 times the diameter of the steel bars were pulled out,and the bonding slip curve showed a trend of first increasing and then decreasing.The peak bonding stress of the 2.5% 15 mm steel fiber group was greater than that of the 1.5% 15 mm and 1% 6mm steel fiber group,while the ultimate slip amount was smaller than that of the mixed steel fiber group.When the bonding length was 4 times or 6 times the diameter of the steel bars,the ultimate bonding stress and ultimate slip amount were both smaller than that of the mixed steel fiber group;Among the three specimens with different bonding lengths,the ultimate bonding stress gradually decreases as the bonding length increases. |
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