2 1 century, China has carried out large-scale subway construction in Beijing, Shanghai, Guangzhou, Nanjing and other big cities, and a considerable number of sections have been constructed by shield method. Shield tunnel construction technology is gradually becoming the mainstream technology in subway tunnel construction because it can minimize the impact on other facilities in the city.
At present, the shield segment is mainly reinforced concrete segment, but in practice, it is found that the general A-type reinforced concrete segment with a width of 1.2m weighs about 4t, which is easy to be damaged and cracked during transportation and installation. The damage and crack of segment have adverse effects on the safety and durability of tunnel, which is a thorny but must be solved problem in shield construction. Research and experiments show that steel fiber reinforced concrete has good mechanical properties and can solve the above problems. There are examples of steel fiber reinforced concrete shield segments abroad. However, the high price of steel fiber material is the main factor limiting its application. This paper briefly analyzes its feasibility.
Mechanical properties of 1 steel fiber reinforced concrete
Steel fiber reinforced concrete (SFRC) is a multi-directional composite building material. Compared with ordinary concrete, steel fiber reinforced concrete has the following main advantages:
① High tensile strength, bending strength and shear strength;
② Good toughness and impact resistance;
(3) Strong ability to inhibit crack propagation and large deformation energy;
④ Excellent wear resistance, good freeze-thaw resistance and heat resistance;
⑤ There is less fragmentation at the edges and corners, and it is not easy to burst.
2 advantages of steel fiber reinforced concrete shield segment
2. 1 The bearing capacity is improved.
(1) The concrete in the tensile area of reinforced concrete members loses its function after cracking, and the cracked steel fiber concrete can still bear a certain tension;
(2) Generally, the principle of structural design is to design according to the limit state method of bearing capacity and check according to the limit state method of normal use. The formula for checking the maximum crack width of steel fiber reinforced concrete structures [1] is:
Among them:
Wfmax-the maximum crack width of reinforced steel fiber reinforced concrete members calculated according to the standard combination of load effect and considering long-term effect;
WMAX-the maximum crack width of reinforced concrete members of the same grade, calculated according to the current Code for Design of Concrete Structures;
βCW—— the influence coefficient of steel fiber on the crack width of members. For eccentric compression structure, β CW = 0.35;
λ f is a characteristic parameter of steel fiber, which is related to the length-diameter ratio (L/D) and volume ratio (ρf) of steel fiber. For shield segments, ρf is generally 0.4%~0.6%.
Therefore, under the same internal force conditions, the steel content required for calculation will be significantly reduced (as can be seen from table 1), that is to say, under the same reinforcement conditions, the bearing capacity of steel fiber reinforced concrete structures will be significantly improved.
2.2 Safety and durability
For underground structures, carbonization of structures and corrosion of aggressive media are important factors affecting their durability, and electrochemical corrosion of stray current in subway tunnels is also an important factor.
The corrosion of stray current is mainly through electrochemical reaction, which produces soluble basic silicate or aluminate at the interface between steel bar and concrete, which significantly reduces the bond strength. Where the current leaves the steel bar and returns to the concrete, the steel bar is the anode and corroded. The accumulation of corrosion products on the anode produces mechanical expansion force, which causes concrete to crack. This is the same as the mechanism of concrete cracking caused by rust expansion force caused by steel corrosion in natural environment.
Therefore, the corrosion of steel bars in concrete is the most important factor leading to the durability damage of concrete structures.
The experimental study shows that when the rapid carbonation age is 28d (under normal atmospheric conditions, the natural carbonation depth of concrete with a natural storage age of 50 years is equivalent to the 28d carbonation depth calculated by the national standard method), the carbonation depth of fiber concrete is less than that of plain concrete, which is reduced by about 50%; When the steel fiber content is 30kg/m3, the cracking time of steel fiber reinforced concrete is about 65438 0.5 times that of plain concrete. When the thickness of protective layer and the diameter of steel bar are unchanged, when the content of steel fiber reaches 60kg/m3, the cracking time of protective layer is 1.7 times that of zero content, so the service life of reinforced concrete can be prolonged by adding fiber.
2.3 steel fiber reinforced concrete segment cost
2.3. 1 initial investment cost
Taking the shield segment of a bid section of Beijing subway as an example, assuming that the section reinforcement of the whole ring section is the same, a brief analysis is made. In the original design, the inner diameter Ri of the A-type reinforced concrete segment is 2.7m, the thickness is 300mm, and the width is1.2m. The upper and lower segments are respectively equipped with eight φ 18 threaded steel bars, and the reinforcement ratio ρ is 1. 13%. Using steel fiber reinforced concrete segments for design, there are two schemes: one is that the segment thickness is constant, the section reinforcement is 8 φ 14 threaded steel bar, and the steel fiber content is 30k g/m3；; The other is that the upper and lower sections are equipped with 8 φ 12 threaded steel bars, and the steel fiber content is 40kg/m3. Remember that the above three schemes are ①, ② and ③ in turn. In practice, the unit price ratio of materials is concrete (m3): steel bar (t): steel fiber (t)= 1:8:20. Because the amount of earthwork is not much different, it is not considered. Based on the concrete unit price of 350 yuan /m3, only the material cost is calculated here, and other expenses are assumed to be the same. See table 1 (per ring) for the preliminary investment estimate.
As can be seen from the above table, the total investment of steel fiber reinforced concrete segments has not increased significantly. Compared with the reinforced concrete segment, the additional investment required by schemes ② and ③ is 7.2% and 9.2% of the original investment of the reinforced concrete segment, respectively.
2.3.2 Life cycle cost
Although the initial investment cost of steel fiber reinforced concrete has increased, the leakage caused by segment damage will be greatly reduced due to the improvement of impact resistance and tensile strength of concrete, thus reducing the waterproof and plugging costs and long-term maintenance costs; At the same time, the durability of steel fiber reinforced concrete is better than that of reinforced concrete, which can prolong the service life of the project. Considering the safety of subway tunnel structure, the cost of operation and maintenance and the life cycle cost of supporting equipment, it is economically feasible to adopt steel fiber reinforced concrete segments.
2.3.3 Steel fiber price forecast
The main factor restricting the application of steel fiber is its price. At present, steel fiber is expensive and only used in some key projects or key parts of some projects in China, so the demand for steel fiber is not large, and the existing steel fiber manufacturers can meet the demand. However, the widespread use of steel fiber in China is a trend. If there is obvious competition in domestic steel fiber, the price of high-quality steel fiber will inevitably fall.
It is reported that most excellent steel fibers in China are 7000~8000 yuan/ton, and the price of Becca steel fiber is 14000~ 15000 yuan/ton, but the price of foreign Becca steel fiber is about 6000~7000 yuan/ton, which has great price elasticity. If the price of steel fiber can be reduced, the advantages of steel fiber reinforced concrete segments will be more obvious.
3 Conclusion
To sum up, as a new type of shield segment, steel fiber reinforced concrete segment has better performance than ordinary reinforced concrete segment. Considering the structural safety, operation and maintenance cost of subway tunnel and the life cycle cost of supporting facilities, it is feasible to adopt steel fiber reinforced concrete segments. On the other hand, when considering the application of steel fiber reinforced concrete, the construction department should not only calculate the one-time investment, but also consider the indirect cost saved, the excellent performance of steel fiber reinforced concrete, the lower maintenance cost and the comprehensive economic benefits obtained by prolonging the service life. Its potential value cannot be underestimated.
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