1. High-efficiency concrete High-efficiency concrete is concrete with high quality and durability, which has four characteristics, namely:
High strength. The standard strength value of 15mm cube is greater than 60MPa;; High durability. Good compactness, good impermeability, strong carbonation resistance, high corrosion resistance and long service life than the design reference period;
High volume stability. The creep deformation and shrinkage deformation of concrete are small; High technology. Generally speaking, high performance concrete can better meet the functional requirements and construction technology requirements of the structure, thus prolonging the service life of the concrete structure and reducing the project cost.
Second, the application and development of high-efficiency concrete
For many years, 40 ~ 50MPa concrete has been used in the field of prestressed concrete bridges, but in actual use, the actual strength in 28 days often exceeds 50MPa. Because high-efficiency concrete has higher strength, higher elastic modulus, higher tensile strength, smaller creep and better durability, compared with ordinary concrete beams, high-strength concrete beams have smaller initial deflection, higher allowable tensile strength, smaller prestress loss, smaller camber change and longer service life. In recent years, countries all over the world have taken great interest in high-efficiency concrete and started to apply it to bridge practice.
High efficiency concrete has been used in bridges for nearly ten years in the world. Typical engineering examples are the continuous steel bridge with a span of 220 meters in Norway and the large belt bridge in Denmark. High-strength concrete with strength of 50 ~ 60 N/mm2 is used for the bridge tower, prestressed main girder and prestressed tunnel section. In Canada, more than 65,438+000 bridges have adopted high-efficiency concrete. For example, Northumberland Strait Bridge, which connects Prince Edward Island and New Brands Wake, is 8 miles long and uses high-strength concrete, with a main span of 250m. 1993. The expressway Administration of the United States began to study the high-efficiency prestressed concrete bridge. The SanAngelo Bridge in Texas is one of the research projects. The special feature of this bridge is that the high-efficiency concrete beam with the strength of 70 ~ 100 N/mm2 and the conventional concrete beam with the strength of 35 ~ 40 N/mm2 are used on the east and west sides respectively, so as to compare the cost of different concretes. In contrast, for a beam with a conventional concrete span of 37m, the span can reach 44m when using high-efficiency concrete. Because of the high strength of high-efficiency concrete, the span can be increased or the beam height can be reduced under the condition of constant span. In addition, high-efficiency concrete has strong impermeability, so the penetration of chloride ions can be reduced by half, thus improving the durability of the structure. The application effect of high efficiency concrete in bridge structure is obvious.
The Federal Highway Administration (FHWA) has optimized the design of ordinary prestressed concrete beams. Based on the analysis of the changes in the interests of fund users, it is suggested that T-beam should be adopted as the national standard of prestressed beam.
The first part aims at the beam with a span less than 27.4m, and the control condition of this kind of beam is the initial prestress in the prestress stage. Because of the long time of dead load in prestress stage, it is of no practical significance to adopt high arch concrete for this span. The second part has a span of 27.4 ~ 30.5 m, concrete strength of 465,438+0 ~ 55 MPa, span of 27.4 ~ 33.5 m, and concrete strength ≥55MPa. Due to the use of high-strength concrete, the spacing between beams can be increased. In this range, there is a balance between the savings brought by increasing the beam spacing and the increase in unit deck cost. The third part is about the situation that the span is greater than 30.5m, the concrete strength is 4 1 ~ 55MPa, the span is greater than 33.5m, and the concrete strength is greater than 55MPa. This range represents the best benefit of high strength concrete in the analysis section.
① With the increase of beam concrete strength, the optimal cost curve shifts to the right. This means that the span of the beam increases without increasing the unit cost; (2) When the strength of beam concrete exceeds 69MPa, the benefit will be reduced. When high-strength concrete is used in small span, there is no obvious benefit.
At present, high-efficiency concrete has made new development. The footbridge is made of concrete with the strength of 120 N/mm2, and mechanical tamping is not required. In Canada, a footbridge was built with fiber reinforced concrete with the strength of 200 N/mm2.
In China, in recent five years, high-efficiency concrete (≥C60 grade) has been widely used in bridge engineering. Some highway bridges on the Yangtze River and the Yellow River have successfully adopted No.60 concrete. The scientific research of high-efficiency concrete has achieved gratifying results. For example, Highway Bureau of Jiaozuo City, Henan Province and Haiwei Engineering Consulting Co., Ltd. conducted in-depth research on the application of high-efficiency prestressed high-strength concrete in bridge engineering.
Compared with ordinary PC hollow slab, high performance concrete hollow slab can save more than 35% concrete, more than 0/5% steel hinge line and 20% material cost within the span range of 16 ~ 30m. Therefore, using high-performance concrete wells to optimize the design of hollow slabs widely used in highway bridge engineering has considerable economic benefits.
Third, restrictive conditions In order to play the role of high-efficiency concrete, it is necessary to set up strong prestress.
Therefore, the steel strand with high strength and low relaxation must be selected. Using 2070MPa(300 grade) steel strand is more advantageous than using 1860MPa(270 grade) steel strand.
The section size is mainly the size selection of the lower flange (horseshoe shape). Increasing the size of the lower flange can accommodate more steel tendons and increase the volume and corresponding weight of concrete, but when the strength of concrete exceeds 55MPa, it will show a better cost-benefit ratio.
The specification, spacing and strength of steel strand have certain influence on the benefit of high strength concrete. Generally speaking, the larger the diameter of steel strand (15.2mm) and the smaller the spacing between steel strands, the more obvious the benefits will be.
Due to the limitation of traffic conditions, the cost will increase when the beam span is large.
Four. conclusion and suggestion
(1) The section height of high-efficiency concrete beam and slab can be reduced, so the project investment is less, which is of great significance for the new construction and reconstruction of bridges.
(2) The use of high-efficiency concrete is very beneficial to protect natural resources, reduce environmental pollution and protect the environment because it reduces the amount of materials.
(3) In the near future, we should concentrate on producing 60MPa strength concrete. For the existing section of steel strand with 270-class diameter 12.7mm and 50mm protective layer, 70MP4 concrete can increase the beam-slab span and make the structure more economical. For long-span beams, at least the highway department should adopt the compressive strength of 50MPa. In fact, many precast beams have reached this level.
(4) Before the concrete with strength greater than 70MPa is successfully applied, the load transfer length, deflection, lateral stability and dynamic characteristics, prestress loss, shear strength, design life and bridge deck system of the beam should be studied.
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