| Concrete is one of the most important building materials in modern construction projects,but concrete is prone to cracks under the action of tension,which causes accelerated corrosion of internal steel bars and reduced structural durability.Nowadays,fiber concrete has become one of the main ways to improve the performance of concrete,and its wide application prospects have always attracted the attention of scholars.The principle of fiber concrete reinforcement is that randomly distributed fibers can form a uniform three-dimensional grid in the concrete matrix,thereby limiting the development of cracks and greatly improving the comprehensive performance of fiber concrete.There are many types of fibers used to reinforce concrete.Compared with non-structural fibers,structural fibers mainly resist and limit macroscopic cracks that appear in components,thereby effectively enhancing the toughness of concrete after cracking.Basalt fiber is a new type of green fiber material,and its structured research and exploration is of great significance.It can effectively fill the gap in the current structural fiber market.It can make up for structural steel fibers that are easy to rust,easy to reduce fluidity,and easy to wear equipment and other defects and make up for structural synthetic fiber of insufficient strength and rigidity,thus forming complementary advantages.At the same time,through fiber morphology design,structural fibers of different shapes can also be developed to better strengthen the bonding performance between the fiber and the matrix.Based on three newly developed structural BFRP fibers and two commonly used structural fibers in the market,this article carried out research on the mechanical properties of structural chopped fiber reinforced concrete.The main research is as follows:(1)The basic mechanical properties of three structural BFRP fibers and the influence of structural chopped fibers on the compressive properties of concrete are studied.The basic mechanical performance parameters of three new BFRP fibers,including fiber tensile strength and elastic modulus,were measured through fiber tensile test.Observe the cracking failure process of the specimen through the concrete compression test,and measure the compressive strength of the specimen.The test measured the tensile strength of straight BFRP fiber>1100MPa,the elastic modulus of straight BFRP fiber >37GPa,the tensile strength of twisted BFRP fiber >1300MPa,the elastic modulus of twisted BFRP fiber >44GPa,which exceeded the structural fiber commonly used in the market at present.These two new types of BFRP fibers have good application prospects;The test results show that there are two types of failure in the structural fiber reinforced concrete compression specimens,namely conical failure and ductile failure.Except for the wave-type BFRP fiber reinforced specimens which exhibits obvious brittleness and conical failure,other types of failure Structural chopped fibers can change the failure form of concrete to a certain extent.Among the three structural BFRP fibers,twisted BFRP fibers can increase the compressive strength of concrete by about 11%,thanks to the fiber’s good bridging effect on cracks.(2)Study the interface bonding performance between structural chopped fiber and concrete matrix.Observe the fiber failure mode through single fiber pull-out test,and calculate the maximum fiber pull-out load and interface bonding strength.The test parameters include fiber type,fiber embedding depth and matrix strength.Tests have found that the type and shape of the fiber largely affect the pull-out behavior of the fiber.Different fiber pull-out curves show different characteristics.In addition to straight BFRP fibers,other types of structural fibers can have hardening characteristics during the pull-out process.This is mainly due to the special shape and rough surface that can enhance the bite force between the fiber and the matrix.The increase in the embedding depth and the strength of the matrix can improve the bonding performance between the two,because the increase in the embedding depth increases the contact area between the fiber and the matrix,and the increase in the strength of the matrix makes the cement gel at the interface more dense.Among the three structural BFRP fibers,the wave-shaped BFRP fiber has the best bonding performance with the matrix.When the embedding depth is 20 mm and the matrix strength is C40,the maximum drawing load can reach921.76 N,and the interface bonding strength is 8.2MPa.This is due to the larger diameter of the fiber and the special wave shape.Wave-type BFRP fiber should not be used in the concrete matrix with a strength grade of more than C50,because this fiber will break and fail in the C50 strength matrix.(3)Study the influence of structural chopped fibers on the bending toughness of concrete.Observe the cracking process and failure mode of the specimen through four-point bending test,record the initial crack load and ultimate load,and calculate the corresponding bending toughness evaluation index.The test parameters include fiber type,fiber length and fiber content.The test results show that the non-structural chopped fiber cannot enhance the performance of concrete after cracking,while the failure morphology of the structural chopped fiber reinforced specimens has been improved,and the ductility characteristics are obvious.This is because the structural fiber can effectively bridge the macro cracks and imit the development of cracks.Structural chopped fibers also have a certain improvement in the flexural initial cracking strength.Among them,the straight BFRP fiber with a fiber length of35 mm can increase the initial cracking strength of C40 concrete by about 21%,and the twisted BFRP fiber with a fiber length of 50 mm can increase the initial cracking strength of C40 concrete by about 24%,which exceeds 13% of the end hook type steel fiber.The structural polypropylene fiber even reduces the flexural initial cracking strength of concrete,indicating that the structural BFRP fiber is better than the current commonly used in improving the initial cracking performance.The flexural toughness of structural chopped fiber reinforced specimens generally increases with the increase of fiber length and fiber content,because the more fibers and the larger the bonding area,the better the limiting effect on cracks.Straight BFRP fiber and twisted BFRP fiber have the most obvious effect on improving the bending toughness of concrete.When the fiber length is 50 mm,the bending toughness ratio of the specimen reinforced by twisted BFRP fiber is 0.76,which is not much different from the end hook steel fiber,indicating that this fiber has a significant effect on toughening concrete. |
