Study On The Properties Of Steel-polypropylene Hybrid Fiber Cement Based Materials

Cement-based materials are the most widely used and widely used materials in modern civil engineering.However,due to the low tensile strength and poor crack resistance,cement-based materials are often damaged in use,especially in wet and harsh environments.Therefore,it is becoming more and more important to improve the related properties of cement-based materials in some way.Because cement-based materials are multi-scale structural material,and its failure process runs through different stages,the most effective method is to mix fibers of different types and sizes into cement-based materials.In this study,steel fiber and polypropylene fiber were added to study the influence of fiber on compressive strength,flexural strength,ratio of flexural to compressive strength,shear bond strength and water absorption of cement-based materials,and the influence of different adding methods(single and mixed),different lengths and different adding amounts on the performance was analyzed.Steel fibers with lengths of 6mm,13 mm,25mm and 30 mm were used in the test,and the volume content was 0.5%,1%,1.5% and 2%.Polypropylene fibers with lengths of 3mm,6mm,9mm and 19 mm are used,and the volume content is 0.1%,0.2%,0.3% and 0.4%.Test results show that:(1)Steel fiber can significantly improve the mechanical properties of cement-based materials.The optimal fiber content is 1%-1.5%,and the optimal fiber length is 25 mm.Compared with the blank group,the compressive strength at 7d and 28 d increased by 7.50% and 28.36%,and the flexural strength at 7d was increased by 34.6%.The shear bond strength at the age of 28 decreased gradually with the increase of steel fiber content from 1.5% to 2%.With the increase of volume content,the water absorption coefficient of cement-based materials decreases at first and then increases.When the volume content is 1%,the coefficient of cement-based materials is smaller.When steel fibers with different lengths are mixed,the test results are best by mixing the fibers with the same length of 6mm,13 mm,25mm and 30 mm,which is better than steel fiber mortar with single length in improving the mechanical properties of cement-based composites.Among them,the compressive strength of 7d and 28 d was 6.25% and 2.78% higher than that of single length fiber mortar test,respectively the 7-day and 28-day compression ratio increased by 4.14% and6.23% respectively compared with the optimal group of single steel fiber length test results.Compared with the test results of blank group,the water absorption coefficient decreased by 16.67%.(2)Polypropylene fiber can significantly improve the mechanical properties of cement-based materials,with the best volume content of 0.2%-0.4% and the best length of 9 mm.With the increase of fiber content,the compressive strength increased first and then decreased.When the fiber volume content increased to 2%,it increased by 6.7% compared with the blank group.The 7d and 28 d flexural strength reached the highest at 0.4% dosage,which increased by 34.6% and 28.9% respectively compared with the blank group.The 7d and 28 d folding-compression ratio reached the best effect with 0.4% content,which increased by 36.41% and 31.35% respectively.The shear bond strength reached a higher level with 0.4%volume content,which was 11.42% higher than that of the blank group.The water absorption coefficient of cement-based materials decreases first and then increases with the increase of volume content.When the volume content was 0.2%,the water absorption coefficient was the smallest.When polypropylene fibers of four lengths are blended,the best test results were obtained by blending fibers with the same length of 3mm,6mm,9mm and 19 mm,which was better than single length polypropylene fiber mortar in improving the mechanical properties of cement-based composites.The compressive strength of 7d and 28 d increased by5.31% and 9.33% respectively compared with the single length polypropylene fiber mortar experimental group compared with single length fiber mortar.Compared with single polypropylene fiber mortar,the flexural strength of 7d and 28 d increased by 14.3% and 9.05%,respectively.Compared with the single-length fiber mortar experimental group,the folding ratio of 7d and 28 d increased by 7.67% and2.53%.Compared with single length polypropylene fiber,the shear bond strength increased by 8.91%.(3)When steel-polypropylene fiber is mixed,the influence on the mechanical properties of cement-based materials is further improved on the basis of single fiber.The optimum mix ratio is 1.5% by volume of steel fiber and 0.2% by volume of polypropylene fiber,and the lengths of the two fibers are mixed with four lengths.After 28 days,the flexural strength and shear bond strength increased by 9.81%and 21.01%,respectively,while the improvement effect of compressive strength and compression ratio was basically the same as that of single fiber.Through microscopic analysis of fiber cement matrix,it was observed that steel fiber and polypropylene fiber can improve the properties of cement-based materials mainly by inhibiting the generation and propagation of micro-cracks.