Degradation Mechanism Of Tensile Properties Of PVA-ECC In Sub-high Temperatures

PVA Fiber-Engineered Cementitious Composites(PVA-ECC)has excellent characteristics of strain hardening and multi-crack cracking.PVA Fiber has a good compatibility with cement paste,thus improving the toughness and strength of the cement matrix Composites and achieving the effect of the reinforcement.Due to high temperature and the special engineering application environment,PVA fiber reinforced cement matrix composites work under the long-term circumstance of high temperature.The decomposition temperature(200? material)and melting point(material 245?)of PVA fiber are close and PVA fiber is with low thermal stability,therefore the damage of PVA fiber in the high temperature environment has an important influence on the application of composite materials.According to the current research,the degradation analysis of the properties of PVA-ECC in the sub-high temperature environment is not much.This paper focuses on the degradation mechanism of the tensile properties of PVA-ECC in the sub-high temperature environment.The main research results and conclusions are as follows(1)Influence of sub-high temperature environment on the surface features of PVA fibers As temperature and duration increase,the fibers gradually bend,their length and diameter decrease,and their color changes from white to brown to black.This indicates that PVA fibers suffer degradation in different degrees.The mass loss of PVA fibers was slower at 50??150? and then accelerated at 200?.The mass loss reached 15%of the total fiber mass after 9 hours(2)Influence of sub-high temperature environment on the tensile properties of PVA fibers.At 50?,the tensile strength of PVA fibers changes slowly.At 100?,it shows a downward trend after 6 hours.At 150?,the deterioration begins after 3 hours.At 200?,the tensile strength decreases rapidly after 1 hour,and after 9 hours,the PVA fiber tensile strength distributes between 191?286MPa,which was only 14.4%of the strength in normal temperature.Both temperature and duration have obvious effects on the tensile properties of PVA fibers.At the same temperature,different duration has different damage degrees to PVA fibers.Even at lower temperatures(50,100?),which are far from reaching the melting or decomposition temperature of PVA fibers,the tensile strength decreases as the duration increases.(3)Influence of sub-high temperature environment on microcosmic structures of PVA fiber.The thermal movements of water and oil molecules destroy the integrity of the fiber With the increase of temperature and duration,the grain size of PVA fiber gradually becomes larger and rougher,and the crystal arrangement becomes disordered and irregular After 200? and 9h,the crystallinity decreases by about 10%compared with specimen in normal temperature,and the hydrogen bond is also seriously damaged(4)Influence of sub-high temperature environment on the appearance of PVA-ECC specimens.With the increase of temperature and duration,the color of the specimens gradually changes from gray to yellow,and the color of PVA fibers in the composite gradually changes from white to yellow,consistent with the change of PVA fibers treated in same situation uncovered.The average mass loss rate of specimens increases in a stepped manner and tends to be stable at 150?200?(5)Influence of sub-high temperature environment on the tensile properties of PVA-ECC.At no higher than 200?,the ultimate tensile strength of PVA-ECC mainly shows a trend of increasing first and then decreasing.At 200?,the ultimate tensile strength decreases rapidly.For the excellent toughness index of PVA-ECC,the limit strain increases at 50?compared with normal temperature and decreases gradually at 100?150?.At 200?,PVA-ECC loses its strain advantage,brittle failure occurs after 1 hour.After the same temperature treatment,the tensile strength of water-cooled PVA-ECC is lower than that of naturally cooled.Meanwhile,the strain was improved,which is better than that of naturally cooled(6)Influence of sub-high temperature environment on the interface of PVA-ECC.When PVA-ECC is in an appropriate sub-high temperature environment,the chemical bond and frictional stress between PVA fiber and cement matrix are improved,which makes the strain hardening stage of PVA-ECC obvious and the tensile strength rises constantly.When exposed to a higher sub-high temperature environment(200?),PVA fibers deteriorate extremely and are closely embedded with the cement base.In the process of drawing,the fibers cannot be pulled out but are severely rubbed with the cement base,then resulting in the disappearance of bridging effect and direct fracture of PVA fibers.