Mechanical Properties,Early Age Shrinkage And Cracking Behavior Of Multi-scale Fiber Reinforced Concrete With Steel Fiber,Sisal Fiber And Nanofibrillated Cellulose(NFC)

When the concrete structure is subject to external restraint,due to its own shrinkage,the concrete will crack,which seriously affects the mechanical properties and durability of the concrete structure.In order to solve this problem,fiber materials such as steel fiber and PVA fiber are often used to limit the shrinkage cracking of concrete at home and abroad.In addition,due to the multi-scale characteristics of the failure of concrete materials,a large number of studies on the use of hybrid fibers with different scales to limit cracking of concrete structure at different scales have also been studied.However,most of the current studies are aimed at the impact of the mixing of the same or different kinds of macro fibers and the mixing of macro fibers and micro fibers on the relevant properties of concrete.There are few reports on the hybrid of nano-scale fibers and macro scale fibers.Considering the multi-scale reinforcing effect and economical efficiency,this paper proposes the multi-scalesteel fiber-green renewable sisal fiber-nanofibrillated cellulose(NFC)hybrid reinforced concrete materials.The basic mechanical properties,early-age shrinkage and cracking were studied by experimental investigation,numerical simulation and analysis.The specific research contents and conclusions are as follows:(1)The effects of steel fiber,sisal fiber and nanofibrillated cellulose on the basic working properties and mechanical properties(compressive strength,splitting tensile strength and flexural strength)of twenty-two concrete mixtures were studied.The results show that both steel fiber and sisal fiber will lead to the reduction of concrete fluidity,and nanofibrillated cellulose improves the working performance of concrete.With the increase of the content of steel fiber,the compressive strength of concrete gradually increases,the splitting tensile strength and flexural strength first increase and then decrease,and approach the peak at 1.5%.While the basic mechanical properties of concrete are not significantly improved after the single addition of sisal fiber.When the content of steel fiber is less than1.5%,the compressive strength of concrete is improved after the volume of sisal fiber replaces steel fiber.The addition of 0.15% nanofibrillated cellulose,steel fiber and sisal fiber shows an good synergy effect of multi-scale strengthening the mechanical properties of concrete.(2)DIGIMAT is used to simulate the progressive damage model and constitutive relationship of fiber reinforced concrete,and coupled with ABAQUS to establish the finite element model of concrete mechanical properties test.The final simulation value is in good agreement with the test value.The progressive damage model and constitutive relationship can be used to reasonably predict the mechanical properties of fiber reinforced concrete with different fiber contents.(3)The early age shrinkage of twenty-two concrete mixtures from 7 h to 7 d and the early age cracking of fourteen concrete mixtures were tested.The results show that both steel fiber and sisal fiber can inhibit the early age shrinkage and early cracking of concrete.At the same time,the content of sisal fiber is less and the effect is better.Compared with the single addition of steel fiber,the shrinkage strain of concrete is significantly reduced after using sisal fiber to partly replace steel fiber.Meanwhile,it is observed that there is no crack in concrete slab test specimens for 24 hours.Last but not least,with the increase of nanofibrillated cellulose content,the shrinkage strain value of concrete mixtures first decreases and then increases,and the performance is the best when the content of NFC reaches 0.15%.In all,the hybrid multi-scale steel fiber-sisal fiber-0.15% nanofibrillated cellulosecould control early age shrinkage and improve the early cracking of concrete significantly.