Study On The Effect Of Wood Fiber On The Basic Mechanical Properties And Durability Of UHTCC

Ultra-high toughness cementitious composites(UHTCC),with multiple crack and strain hardening characteristics under tensile and bending loads,have excellent mechanical properties and durability.But their high cost limits their large-scale commercial use.As a natural plant fiber,wood fiber has the advantages of large specific surface area,small specific gravity,high specific strength and low cost.Although wood fiber havs the potential to be used as reinforcements for cementitious composites,its difficult adhesion to the cement matrix and high water absorption properties also limit its widespread application in cement-based materials.In this paper,wood fiber was used to replace part of PVA fiber to improve the economy of UHTCC.The fluidity,tensile,flexural,and compressive properties of this new material were studied through table skipping tests,axial tensile tests,three point bending tests,and axial compression tests,and its durability was also studied through drying shrinkage tests and accelerated aging tests.The main work and conclusions of this article are as follows:(1)The silane coupling agent was grafted onto the surface of wood fibers using silane modification method,and confirmed by SEM,EDS,and FTIR techniques.(2)The fluidity and apparent density of UHTCC doped with wood fibers and PVA fibers were measured,and it was found that the addition of wood fibers decreased the fluidity and apparent density of UHTCC,which was negatively correlated with the amount of wood fibers.However,silane modified wood fibers weaken the water absorption characteristics of wood fibers,so the decrease in the fluidity and apparent density of UHTCC caused by silane modified wood fibers at the same amount was smaller than that of unmodified wood fibers.Based on XCT technology,it was found that adding wood fibers would increase the porosity of UHTCC and coarsen the pores.However,compared to unmodified wood fibers,silane modified wood fibers had a smaller increase in porosity and fewer pores coarsened.(3)The effects of wood fiber content and PVA fiber content on the basic mechanical properties of hybrid fiber UHTCC were studied,respectively.It was found that UHTCC prepared using silane modified wood fibers instead of partial PVA fibers still exhibited strain hardening and multiple crack characteristics under tensile and bending load.Among them,UHTCC mixed with 2% PVA fiber and 2% modified wood fiber has the better mechanical performance,with the ultimate tensile strain reaching 3.67% and tensile strength reaching 5.23 MPa,which is improved compared to ordinary UHTCC without wood fiber.In addition,by adding 1.5% PVA fiber and 2% modified wood fiber,the ultimate tensile strain of UHTCC can reach 2.23%,and the tensile strength can reach 3.78 MPa,which has better mechanical properties and economy.Based on SEM and EDS techniques,it was found that unmodified wood fibers could not form a good bonding with the substrate,and wood fibers absorbing water caused PVA fibers to also not form a good bonding with the substrate.Silane modified wood fibers can form a close bonding with the matrix and play a reinforcing role.The surface of PVA fibers is also coated with significantly more hydration products than the group doped with unmodified wood fibers.(4)The effect of wood fiber content on the dry shrinkage properties of UHTCC was studied.It was found that wood fiber could reduce the dry shrinkage deformation of UHTCC,and it was positively correlated with the content.However,the fiber agglomeration phenomenon and high porosity at high content reduce the reduction of dry shrinkage deformation.The decrease of PVA fiber content has little effect on the dry shrinkage deformation of UHTCC.(5)The effect of wood fiber on the aging resistance of UHTCC was studied by high temperature water bath accelerated aging test.As the curing age increases,the flexural strength of UHTCC increases and the ductility decreases,but it still maintains the characteristics of strain hardening and multi crack cracking.However,adding 2% silane modified wood fiber will deteriorate the aging resistance of UHTCC to some extent.When the PVA fiber content is 0.5%and 1%,the flexural strength and retention rate of UHTCC decrease significantly with the increase of age;However,when the PVA fiber content is 1.5% and 2%,the decrease in the flexural strength and retention rate of UHTCC is not significant due to the main reinforcing effect of PVA fiber.Among them,UHTCC mixed with 1.5% PVA fiber and 2% silane modified wood fiber has a flexural strength of 13.82 MPa and an ultimate deflection of 0.78 mm after 56 days of high-temperature water bath curing,and its flexural performance is still much stronger than ordinary concrete.In summary,silane modified wood fibers have the potential to serve as UHTCC reinforcements,and UHTCC mixed with PVA fibers can still exhibit excellent mechanical properties under tensile and bending loads.Among them,the UHTCC prepared by combining1.5% PVA fiber and 2% silane modified wood fiber has good mechanical properties,durability,and economy,and has high commercial value.