Performance Of Self-Healing Cementitious Composites Using Aligned Tubular Healing Fiber

The cracking of concrete is a critical issue that threatens the safety and serviceability of infrastructures.Knowing that concrete is one of the most used construction materials,it is also one of the most susceptible to cracks due to its low tensile strength and brittleness.Repairing cracks in structures usually requires regular maintenance which makes infrastructures unavailable for a certain period,consumes materials,and often requires reaching inaccessible places such as underground structural elements or radioactive waste disposal facilities.Self-healing is a solution that was implemented in construction,allowing concrete structures to self-heal cracks through a natural or artificial mechanism without human intervention.Autonomic self-healing by a chemical method was investigated in its feasibility and was indicated as one of the most efficient self-healing methods in cementitious composite structures,due to its ability to heal a certain category of cracks-width.However,by using randomly distributed containers in a structure matrix,some containers are hardly reached when a crack occurs,which limits the effectiveness of the healing process.In this investigation,soft tubular healing fibers made of polyvinylidene fluoride resin(PVDF)were aligned parallel to intended tensile stress in a cementitious composite to study the influence of container orientation on the healing ability.First,the tensile strength and the bonding strength of the PVDF healing fiber with a cementitious matrix were investigated,which have shown that the container is both resilient and able to break when a crack occurs in the structure matrix.The good bonding strength between the healing fiber and the cementitious matrix also helps to prevent sliding between both components when a crack occurs.Secondly,the method of aligning healing fibers in a cementitious fresh mixture was investigated.Since steel fibers can be dragged by a magnetic field,the healing fibers were combined with varying amounts of steel fibers to create hybrid healing fibers.The steel fibers were then aligned using a magnetic field and as the consequence,the healing fibers were also aligned due to the drag of the steel fibers.Two types of hybrid healing fibers(AHFCC-1SF and AHFCC-2SF,an aligned healing fiber combined with one and two steel fibers,respectively)were made and tested.And then the alignment efficiency of the hybrid healing fibers was tested in a fresh mixture with varying workability,to determine the optimal requirements for an efficient alignment process.It was revealed that the alignment method requires combining two steel fibers with each healing fiber and minimum mixture workability,to allow an effective alignment of the hybrid healing fibers in a fresh mixture.The obtained requirements for the alignment process were then used to make the specimens for the healing activation process.Then,the healing ability of the cementitious composites with aligned and random healing fibers inside,was investigated.For that purpose,the healing process was activated by splitting each type of specimen(AHFCC and RHFCC,aligned and random healing fiber cementitious composite),which respectively obtained 270 μm and 220 μm as crack width,then they were cured for 7 additional days for self-healing.The permeability of both specimen types was tested after the healing process.The composites containing aligned healing fibers(AHFCC)with a higher crack width,exhibited a lower permeability than that containing the randomly distributed fibers(RHFCC)with a lower crack width.Moreover,the healing ability was also investigated through both composites’ mechanical performance before and after healing,using the splitting tensile test.After healing,compared to the random healing fibers(RHFCC),the aligned healing fibers(AHFCC)have recovered much of the splitting tensile strength before healing.Additionally,the compressive strength of cementitious composites containing healing fibers was investigated according to different volume fractions of healing fiber(0 %,0.12 %,and 0.18 %).According to the test,although the alignment of healing fibers improves the healing capacity in cementitious materials,it’s advised to use a low ratio of healing fiber in the composites to limit the decreasing effect on its mechanical performance.The research results have shown that aligning a tubular PVDF healing fiber parallel to intended splitting tensile stress(perpendicular to the potential cracking surface)in a cementitious composite,significantly improved the healing capacity in this material even with higher induced crack width in AHFCC compared to RHFCC with a lower crack width.It was revealed that the combination of steel fibers with the healing fibers allowed both fibers being effectively dragged by a magnetic field in a fresh mixture using suitable workability.The healing fiber alignment greatly increased the fiber amount across the induced cracksection during the healing activation,by which the composite recovered better its initial permeability and mechanical properties.The obtained experimental results indicate the alignment of tubular containers in a matrix as a means to effectively improve the healing ability in cementitious structures.