Fracture Properties Of Coconut Fiber Magnesium Phosphate Cement

Magnesium Phosphate Cement(MPC)has the characteristics of fast setting,early strength,good volume stability and excellent durability,and has wide application potential in the quick repair and reinforcement.However,the application of MPC is limited by its brittleness and easy cracking.In the early stage of basic mechanics research^[1-3],author discovered that the Coir Fiber(CF)incorporated into MPC could improve the brittleness and cracking of MPC.However,in order to ensure the reasonable application of CF-MPC in practical engineering,it is necessary to avoid the unstable crack propagation.The mechanical properties of CF-MPC cannot be limited to the traditional strength test and theory.Therefore,it is of great engineering value to ensure that CF-MPC is in a stable state during the service period and does not occur crack instability and propagation.In this paper,the effect of CF-MPC interface bonding properties and fracture properties was studied by macro mechanical test,and the fiber action mechanism was explored by micro test.The main contents are as follows:(1)The effects of different pretreatment methods(water immersion temperature and water immersion time)on mechanical properties of CF were studied by fiber axial pull-out test.on the.The results show that the tensile strength and elastic modulus of pretreated CF increase first and then decrease with the increase of immersion temperature and immersion time.However,the elongation at break of CF decreased with the increase of immersion temperature and immersion time.In addition,the research combined with micro test methods(SEM,EDS,etc.)further analyzed and clarified the influence mechanism of CF chemical composition changes on the mechanical properties of CF.(2)The effects of interfacial bonding properties of CF-MPC with different pretreatment methods(water immersion temperature and water immersion time)and fiber embedded length on the were studied by fiber pull-out test.It was found that the interfacial bonding capacity of CF-MPC increased with the increase of immersion temperature,but the effect of water immersion time on the interfacial bonding capacity of CF-MPC was not obvious.The interface failure energy of CF-MPC first increases and then decreases with the increase of immersion temperature,but the effect of immersion time on the interface failure energy of CF-MPC is not obvious.Finally,the micro morphology of CF surface under different pretreatment methods was studied by micro test,and the influence mechanism of the change of CF surface morphology on the interfacial bonding properties was analyzed.Finally,the correlation of immersion temperature,immersion time and embedding length on the interfacial bond strength is analyzed by Spearman correlation coefficient.Additionally,the calculation model of CF-MPC interfacial bond strength was established by SPSS.(3)The effect of volume fraction and the length of fiber on the fracture properties of CF-MPC was investigated by three-point bending test of notched beam.The results show that when the fiber length is short,the fracture capacity of CF-MPC shows an obvious upward trend with the increase of the volume fraction of fiber,but the fracture capacity of CF-MPC decreases when the volume fraction of CF-MPC exceeds 3%.The fracture energy of CF-MPC increases with the increase of the volume fraction and the length of CF.It is also found that the change rule of the initial fracture toughness of CF-MPC specimens shows a similar trend to fracture capacity of CF-MPC.Moreover,the initial fracture toughness of CF-MPC specimens reaches the maximum when the fiber length is 20 mm and the volume fraction is 4%.However,the initial fracture toughness of CF-MPC specimens decreases after the volume fraction of CF-MPC reaches 3%due to the addition of excessively long fibers(25mm and 30mm).The unstable fracture toughness and fiber bridging toughness of CF-MPC increase with the increase of volume fraction and length of CF.Finally,the correlation of fiber volume fraction and fiber length with fracture energy and fracture toughness gain ratio was analyzed by Spearman correlation coefficient,and the calculation model of CF-MPC fracture energy and fracture toughness gain ratio was established by SPSS.