1 project overview
The scale of Hangzhou Bay Bridge attracts worldwide attention and requires high quality. The design base period is 100 year. As the foundation of the bridge, the construction quality of the pile cap project will directly affect the service life of the bridge. The concrete of the bearing platform is marine durable concrete, which is mass concrete. Crack control of pile caps is a major issue in technical quality work. In order to eliminate and control the cracks in the bearing platform, we mainly take comprehensive technical measures from the following aspects, and achieved good results.
Design and Optimization of Concrete Mix Proportion
This project adopts marine durable concrete, and the bearing platform is in the fluctuation zone of water level. The design strength grade of concrete is C40, and the chloride ion diffusion coefficient is ≤2.5× 10- 12m2/s, so it is required to add rust inhibitor to concrete. The pier is located in the splash zone, the concrete strength grade is C40, and the chloride ion diffusion coefficient is ≤1.5×10-12m2/s.
(1) Selection of raw materials. According to the requirements of "Special Technical Specification for Hangzhou Bay Bridge", we preliminarily determined the raw materials used and worked out the proportion by consulting relevant materials, consulting experts and investigating the output and technical indicators of various raw materials in this area. In this project, Anhui Ningguo Conch P.Ⅱ 42.5 cement is used, the coarse aggregate is 5 ~ 25mm continuously graded crushed stone from Qingzhi Stone Field in Beilun, Ningbo, and the fine aggregate is medium sand from Minjiang River. By trial mixing concrete, the performance of concrete is compared. The water reducer is Shanghai Meister (carboxylic acid water reducer, water reducing rate ≥25%), the rust inhibitor is calcium nitrite produced by Shanxi Kaike, and the mineral powder is Shanghai Baotian and Anhui.
(2) Concrete mix design. In order to eliminate and control the temperature cracks in concrete, the reduction of hydration heat of concrete was studied in depth and a series of experiments were carried out.
By comparing different raw materials and different mixture ratios, the mixture ratio with smaller temperature peak caused by hydration heat is selected, so as to control the development of casing cracks. On the premise of meeting the requirements of concrete strength and chloride diffusion coefficient, it is decided to add fly ash into the original cementitious material and appropriately reduce the amount of cement in the cementitious material, thus effectively reducing the hydration heat of concrete.
After repeated tests, the dosage of cementing material was adjusted from 435 kg to 405 kg. See table 1 for the optimized concrete mixture ratio that is widely used in practice.
In the actual construction, according to the characteristics of different seasons, the mixture ratio is appropriately adjusted. The optimization of concrete mix proportion, the addition of fly ash and the reduction of cementitious materials play an important role in controlling cracks in mass concrete.
3. Improve the crack resistance of the concrete box itself.
The construction of pile caps by using concrete box technology is also an important link to prevent the concrete box from cracking after concrete pouring of pile caps, improve the quality of concrete boxes and enhance their own anti-cracking ability. We focus on strengthening the quality control of concrete box girder prefabrication from the following aspects.
(1) Control the pouring quality of casing concrete, especially the treatment of high-performance concrete laitance at the top of casing, and strengthen secondary vibration.
(2) Strengthen the maintenance of concrete box. Before form removal, the top of the concrete box should be stored with water for curing, and after form removal, it should continue to be watered for curing to prevent dry-wet cycle. The maintenance time shall not be less than 65,438+04 days.
(3) Strengthen the roughening treatment of the inner wall of casing, especially the roughening of reserved notches, to ensure the good combination of new and old concrete.
(4) Strengthen the control of welding steel structure on the embedded iron parts on the inner wall of casing. After the casing meets the curing period, the welding time should be controlled, and the welding operation should not be carried out continuously at the same position during welding, so as to avoid the local temperature of the embedded iron parts being too high due to welding and adversely affecting the local concrete of the casing.
(5) The notch is reserved at the top of the concrete box, so that the impermeability of new and old concrete construction joints can be strengthened structurally.
(6) Adjust the box reinforcement, increase the number of stirrups, and adjust the original stirrup φ 25 @ 150 to φ 20 @ 75, so that the reinforcement ratio is increased by 28%, and the vertical crack resistance of the box is improved.
(7) Strictly control the protective layer thickness of steel bars in the control box. According to relevant data, the influence range of reinforced concrete cracks is six times the diameter of reinforced concrete. After adjustment, the box reinforcement is φ 20, the protective layer is 75mm, and the reinforced concrete reinforcement spacing is 75mm, so the construction is strictly controlled to ensure the quality of reinforced concrete. 4. On-site technical measures for concrete construction of pile cap structure
(1) Strengthen maintenance. Considering that the cementitious material of marine durable concrete takes a long time to hydrate after adding a lot of mineral powder and fly ash, and the high-efficiency water reducer with water reduction rate as high as 25% ~ 30% is adopted, the water-cement ratio of concrete itself is small. In order to meet the requirements of secondary hydration of concrete, it is necessary to strengthen the curing of concrete and control the initial curing time of concrete. According to the adiabatic heating experiment, the concrete temperature reaches its peak around 24 hours after pouring, during which the concrete temperature rises rapidly. After the initial setting of concrete, the bottom concrete shall be stored and maintained, and the top concrete shall be covered with geotextile, and the storage time shall not be less than 14 days.
(2) Cooling of circulating water. In order to reduce the temperature rise caused by hydration heat in concrete and the temperature difference between inside and outside concrete, cooling circulating water is used. The concrete of the bearing platform is poured in two layers. The first layer of concrete 1.3m thick, and the second layer of concrete 1.7m thick, with the spacing of 90cm and 75cm. The circulating water pipe adopts 33.5mm thin-walled steel pipe. Before the final set of concrete, the water shall be cooled, and the cooling time of circulating water shall not be less than 7 days. Adjust the direction of circulating water every 12 hour to facilitate the uniform distribution of hydration heat in concrete.
In order to improve the cooling effect of cooling water circulation, the diameter of circulating water pipes is changed from 33.5mm to 48mm, and the arrangement of circulating water pipes is adjusted, so that the distance between the upper water pipes and the concrete surface is adjusted to 50cm, and the distance between water pipes is gradually adjusted from 90cm to 60cm, so as to increase the density of circulating water pipes in the central area of concrete and further strengthen the internal temperature distribution of concrete.
(3) Mixing concrete with cold water. A water cooler was installed on Yonghe concrete mixer, which can reduce the mixing water temperature by 20℃. The mixed water in the engine room is cooled by the chiller and stored in the water tank. Concrete is mixed with cold water to reduce the temperature of concrete leaving the machine and entering the mold. In the process of construction, the temperature of concrete entering the mold is measured and controlled below 28℃.
(4) The concrete of pile cap structure is poured twice. The diameter of the cap structure is 12m and the height is 3m. For the cap constructed by concrete box technology, the structural concrete is poured twice, the first time 1.3m, and the second time 1.7m, which reduces the volume of the concrete poured once, and is beneficial to reducing the temperature gradient and peak value of the hydration heat of concrete.
(5) Ensure the pouring quality of concrete. When pouring concrete, the layers should be poured with ash, the layer thickness should be reduced as much as possible, the concrete pouring time should be appropriately prolonged, the concrete distribution position should be reasonably controlled, and the ash should be poured gradually from the periphery to the middle, and the concrete should be vibrated and compacted in layers to ensure the concrete pouring quality.
5 the effect achieved
As a result of a series of technical measures to prevent cracks in the construction of mass concrete of pile caps, with the continuous improvement and perfection of construction technology, the quality of concrete of pile caps is ensured, and the cracks in concrete boxes are effectively controlled, with very satisfactory results. After pouring the first layer of structural concrete, there is no crack in the cap constructed with concrete box. After pouring the structural concrete in the second step, there are only a few caps, and a few vertical cracks with the length of 10 cm ~ 30 cm and the width not exceeding 0. 1mm appear below the top surface of the box-shaped external surface, which belongs to the category of harmless cracks. The concrete of 37 pile caps constructed by steel box technology was poured at one time, and the amount of concrete poured at one time reached 380m3, and no cracks appeared.
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