Preparation And Mechanical Properties Of Warp-knitted Spacer Cementitious Composite

Warp-knitted spacer cementitious composite(WSCC)is a flexible three-dimensional textile reinforced composite made by filling technology of dry cementitious powder,which can be hardened and formed in one piece by watering without stirring and mold use.Compared with traditional concrete materials and slurry mixing cementitious composites,WSCC has the characteristics of energy saving,environmental protection,efficient and flexible construction,and is especially suitable for use in situations where manpower,time and equipment are limited.It has been promoted and used in civil fields such as geotechnical engineering,construction,water conservancy,agriculture,environment and transportation in China.However,as a thin-walled material,the problems of improper control of water dosage,slow setting speed and poor bending and punching performance in the early stage are common in practical applications.The theoretical research on the failure mechanism of WSCC under varying loading conditions and the internal optimization of warp-knitted spacer structure is still relatively lacking,thus limiting the expansion of application fields of WSCC.In view of the above problems,a new type of warp-knitted spacer cementitious composite with fast setting and early strength characteristics was prepared in this thesis.The effects of different water-cement ratios,loading conditions and reinforcement schemes of warp-knitted spacer structure on the mechanical properties of WSCC were systematically studied.The mechanical response behavior,damage mode and strengthening mechanism of WSCC were also revealed.It is expected to provide a theoretical basis for the development of new knitted three-dimensional fabric reinforced cementitious composites represented by WSCC,and to promote the expansion of applications of WSCC in both civil and military fields.The main contents and conclusions of the thesis are as follows.(1)The warp-knitted spacer fabric and its cementitious composite were prepared by Raschel double-needle bed warp knitting machine and concrete shaking table,etc.The effect of water-cement ratio on the structure and properties of WSCC was studied by changing the water dosage.The results show that WSCC has a great setting speed,and the mechanical properties of WSCC increase and then decrease with the increase of water-cement ratio at the early curing stage.However,the mechanical properties of samples with higher water-cement ratio tend to exceed with the increase of days.Due to the absence of stirring processes,the structure of the substrate is influenced by gravitational deposition and diffusion resistance,and insufficient watering is likely to result in poor adhesion of hydration products on the upper layer,thus weakening the bonding fastness between the matrix and fabric.An appropriate overdosage of water is considered more conducive to the long-term stability of WSCC.(2)The quasi-static and low-velocity impact mechanical properties when placed in different loading directions(warp and weft,positive and reverse)were investigated using the optimum water-cement ratio(0.42).The results show that the damage process of WSCC has great ductility.The peak load in each group depends on the strength of surface layer of fabric and has higher energy absorption along the interlacing direction of knit loop(warp direction).The bridging effect of spacer yarns under weft loading is more obvious due to the arrangement shape of spacer yarns,which leads to a lower number of cracks in the matrix along the weft direction.The asymmetric structure of surface layer of fabric has a great influence on the bursting and impact mechanical response behavior,with higher peak loads at positive placement.(3)Organic and inorganic high-performance multifilaments were used as reinforcing yarns for sewing into the mesh layer of fabric,and the yarn-lined WSCCs with different lining methods and yarn types were prepared and the mechanical optimization effect is studied.The results show that the reinforcing yarns mainly share the tensile stress of mesh layer through debonding and pulling from the matrix and its tensile fracture to play a reinforcing role,but has little effect on the overall failure mode and the toughening effect is not obvious.The strength utilization rate of full-width warp and weft lining yarns are higher than that of the partial weft lining yarns,and the lining yarns in both warp and weft directions can play a synergistic role in strengthening.The higher the tensile strength of the lining yarns,the greater the increase in ultimate load carrying capacity of WSCC.The bonding performance between the lining yarns and matrix mainly affect the initial modulus and the initial crack load of WSCC.The surface yarn-lined structure can give full play to the strength of yarn with minimal increase in yarn usage,which may have good quality improvement benefits in engineering applications.(4)Polymer melt-spun monofilaments with large diameter were used as reinforcing bars for heat setting and inserting into the interlayer of fabric,and stiffened WSCCs with different bars,arrangement and combination methods were prepared and the mechanical optimization effect is studied.The results show that the reinforcing bars have a greater influence on the overall failure mode.During the bending deformation,bars will not only have frictional crushing and debonding slip with matrix,but also produce tensile and bending resistance interlocking with spacer yarns,thus playing a role of strengthening and toughening.The stiffness and flexibility of bars has significant differences on the cracking pattern of matrix and energy absorption at each stage.The different combination methods of bars have good synergistic effect,and the vertical combination can further enhance the transverse stress transfer efficiency within the spacer layer,and the reinforcement height of bars is positively correlated with energy absorption.The interlayer stiffened structure can significantly improve the impact performance of WSCC,which may have good potential for application in protective barrier structure and secondary hazards from blast loads.