With the development of the construction industry, the population is increasing and the available land area is getting smaller and smaller. So now countries are studying how to develop into super high-rise buildings. Super high-rise buildings have extremely high requirements for the building's self-weight, so the requirements for concrete strength are also very high. Research on ultra-high strength concrete is urgent. The use of high-strength concrete has significant potential to reduce the weight of the structure and improve the load-bearing capacity of the structure. At the same time, applying transverse prestress to the top plate of the beam body and applying vertical prestress to the web can increase the length of the flange plate, further reduce the thickness of the top plate, bottom plate, and web, and minimize the weight of the structure. However, the shrinkage of ultra-high-strength concrete during dry hard setting is too severe. Many cracks will occur during the shrinkage process. Ultra-high-strength concrete has a high density and is not suitable for conventional maintenance methods. The humidity inside it cannot be guaranteed. At the same time, the water-cement ratio of ultra-high-strength concrete is very large, and the hydration heat release phenomenon is obvious. Therefore, the study of how to reduce the autogenous shrinkage of concrete and control the crack width is now a key issue for the quality of ultra-high-strength concrete structures.
1 The harm of cracks to concrete
Due to the self-shrinkage and strict curing conditions of ultra-high-strength concrete, it is technically almost impossible to completely avoid concrete cracking. Therefore, our country stipulates that some harmless cracks are allowed to exist in concrete. Harmless cracks: stable deformation cracks and stress cracks, the maximum crack width complies with national standards and will not endanger structural safety. Harmful cracks: refers to unstable cracks or cracks whose maximum width exceeds national regulations. Hazardous cracks have a significant impact on the safety and durability of a structure. Hazards: (1) When cracks appear in concrete, if moisture penetrates into the concrete and freeze-thaw occurs, the cracks will gradually widen and the concrete will lose its bearing capacity. (2) When cracks occur, the steel bars may be exposed on the surface. After contact with the air, the corrosion of the steel bars will be accelerated. After the steel bars are corroded, the bearing capacity of the concrete will be greatly reduced. Affect the structural bearing capacity. (3) Accelerate the spalling of concrete. (4) Destroy the structural integrity and reduce structural rigidity.
2 Take the UHPC produced by our school as an example to analyze the UHPC bridge plate preparation of our school using a water-binder ratio of 0.19 to 0.2. In order to reduce manufacturing costs and ensure the quality of UHPC For strength, we try to reduce the usage of cement and silica fume and increase the usage of slag, which can reduce the production cost of UHPC to a certain extent. In terms of concrete curing, we use steam curing. The steam curing method for precast concrete components has the advantages of high early strength and high production efficiency. If ordinary concrete is steam cured, the concrete's 28-day strength will lose more than 10%, but ultra-high-strength concrete has high adaptability to steam curing. 4 hours of steam curing can reach more than 80% of the strength of standard curing for 28 days. During the production process, high-efficiency water-reducing admixture should be appropriately added, with the dose ranging from 1.6 to 1.9. Due to the particularity of UHPC, the strength increases greatly in the later period, so the professor suggested using 90d as the design standard strength. Our school’s UHPC seismic experiment also started loading about 100 days after production. During production, a standard cube specimen of 100mm × 100mm × 100mm should be produced at the same time.
3 Conclusion
There are many factors that affect the width and number of UHPC cracks, including the composition of the material, post-maintenance methods, the amount of steel fiber added and the amount of concrete added. If you want to obtain high-strength and qualified UHPC prefabricated components, every link must be strictly controlled.
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