I. Introduction
1824 after the appearance of portland cement, humans began to use concrete to build buildings. Then reinforced concrete and prestressed reinforced concrete appeared in 1850 and 1928 respectively, and concrete was widely used. In the 1920s, with the relative maturity of structural calculation theory and construction technology, reinforced concrete structures began to be adopted on a large scale, and their application fields became wider and wider. At present, it is the largest and most widely used building material in the world.
Concrete is an excellent building material, but it is widely used because of its poor bending, tensile and impact toughness. Therefore, it is considered whether short and fine fibers with high tensile strength and good toughness can be added to concrete to improve its properties.
190 1 year, porter published the first paper on steel fiber reinforced concrete. 19 1 1 year, Graham of the United States proposed adding steel fibers to ordinary reinforced concrete. In the forties, due to the need of military engineering, Britain, the United States, France, Germany, Japan and other countries have successively conducted research and published some patents, but the progress is not great, because these studies and patents almost failed to explain the reinforcement mechanism of steel fiber to concrete. Fiber reinforced concrete really entered the practical research in the early 1960s. 1963 Romualai published a research report on the crack development mechanism of steel fiber confined concrete, which really brought this research into a new development period.
Second, the reinforcement mechanism of steel fiber reinforced concrete
There are two main theories to study the reinforcement mechanism of steel fiber reinforced concrete: composite mechanics theory and fiber spacing theory. These two theories explain the reinforcement effect of steel fiber on concrete from different angles, and the results are consistent.
(A) mechanical theory of composite materials
Based on the theory of composite mechanics, steel fiber reinforced concrete is regarded as a fiber reinforced system. The stress, elastic modulus and strength of steel fiber reinforced concrete are derived by using the mixing principle, and the fiber direction coefficient is introduced, taking into account the ratio of effective fiber volume ratio in the tensile stress direction and the uneven distribution of discontinuous short fiber stress along the fiber length.
(B) fiber spacing theory
Fiber spacing theory explains the inhibition of steel fiber on the occurrence and development of cracks according to the principle of linear elastic fracture mechanics. The theory holds that in order to enhance the tensile strength of concrete, a brittle material with internal defects, it is necessary to reduce the size of internal defects as much as possible and reduce the stress field intensity factor at the crack tip. For brittle materials such as concrete, because there are weak links in the interface area between cement slurry and fine aggregate, and between mortar and coarse aggregate, although all components have high tensile strength, concrete generally breaks and its macro tensile strength is very low. The addition of steel fiber can span both sides of the crack, so that the bond stress between steel fiber and concrete on both sides of the crack can inhibit the crack.
Third, the application of steel fiber reinforced concrete
(1) Water conservancy and hydropower projects
20ZLB-70 axial flow pump is widely used in agricultural pumping stations at present. 22 o'clock (pipe diameter 55cm, wall thickness 3cm) This pump type adopts steel fiber concrete pump pipe to solve the problems existing in other pump pipes in engineering cost, construction period, management and maintenance. The main technical standards adopted by Jiangsu Siyang Concrete Products Factory for steel fiber reinforced concrete pump pipes are: when the applied lift reaches 7.5m, the working pressure in the pipe reaches 0.075MPa, the indoor inspection pressure reaches 0. 15MPa, and there is no leakage at 0. 1MPa. The safety factor value is 3. The proportion of concrete used is cement, yellow sand, crushed stone and water = L: 2.06: 1. 12: 0.5. The volume content of steel fiber is 1.5%, and the fiber length-diameter ratio is 60- 100. Regarding the use value of steel fiber reinforced concrete pump pipe, the factory compares this kind of pump pipe with the same type of cast iron pump pipe, steel plate pump pipe, self-stressing cement pump pipe and reinforced concrete pump pipe, and finds that the steel plate pipe and cast iron pipe use the largest amount of steel, followed by reinforced concrete pipe and prestressed reinforced concrete pipe. Steel mesh cement pipe and self-stressing pipe are smaller; The newly developed steel fiber pipe has only 8kg of steel, which is the least. In terms of production management, steel plate pipes and cast iron pipes are easy to rust, joint bolts and water-stop fillers are easy to corrode, and maintenance costs are high. Reinforced concrete pipes and prestressed reinforced concrete pipes have small maintenance costs, but they are bulky and inconvenient to transport and install. Although the steel wire mesh cement pipe and self-stressing cement pipe use less steel and their own weight than the former, they need special production technology and equipment. Steel fiber reinforced concrete pipes can make up for the above six kinds of pipes. In terms of pump pipe performance, all kinds of pump pipes can meet the strength requirements.
(2) Bridge engineering
Chongqing Jiaotong University and other units have analyzed the dynamic performance of steel fiber reinforced concrete ribbed arch bridge, and successfully designed a ribbed arch bridge with a clear distance of 60m and an arch ring height of 1.55m by partially adopting steel fiber reinforced concrete. After the bridge is completed, the natural frequency, mode and impact performance of the bridge are tested. The results show that the steel fiber reinforced concrete ribbed arch bridge is not only low in cost, but also obviously smaller than ordinary concrete ribbed arch bridge under earthquake action.
(3) Housing projects
Joints are the force transfer hubs of the beams and columns of the frame, and they are also the weak links of the frame. Several large earthquakes at home and abroad show that many reinforced concrete frame joints are damaged to varying degrees under earthquake action, and the seismic problem of joints has attracted the attention of engineering circles. According to the traditional method, in order to improve the seismic strength and ductility of reinforced concrete joints, it is necessary to configure many and dense stirrups at the joints, and the construction of stirrups at the joints is difficult. Too dense reinforcement at joints also affects the pouring quality of concrete. Replacing part of stirrups with steel fiber reinforcement at frame joints can effectively solve this problem.
It was first put forward by Professor Fan Chengmou of Harbin Jianzhu University and applied to engineering after laboratory tests. The earliest applications were the office building of Jilin Radio Station1661(1988 May) and the experimental building of Heihe Construction Committee (1989 May). The annual temperature difference and the temperature difference between day and night in the construction sites of the above two projects are relatively large. In order to separate the waterproof layer from the leveling layer and reduce the influence of shrinkage and temperature stress, a certain amount of expansive agent is mixed into steel fiber concrete, and good results are obtained. The material consumption per cubic meter of steel fiber concrete is: cement: sand: crushed stone: steel fiber: water: water reducer: expansion agent = 450: 720: 720: 72:198: 4.5: 63. The water reducing agent is a high efficiency water reducing agent made in Shanghai, and the water reducing rate is 15%. The expansion agent is Hefei product, and its free expansion value is less than 0. 1%. The rigid waterproof roof is designed in grids, each grid is 3×6m. There is a warehouse-dividing seam between each warehouse and the surrounding walls. The joints of compartments are filled with PVC waterproof ointment. The batten size is 20×30mm, and it will be taken out after 24 hours of construction. The thickness of waterproof layer is 40mm.
The application fields of steel fiber reinforced concrete are very extensive, so I won't list them here.
To sum up, due to a series of outstanding advantages and huge technical development potential, steel fiber reinforced concrete can be predicted to achieve greater technological progress and broad application prospects in the next 2 1 century.
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