Construction and supervision of mass concrete in the cap of Xihoumen Bridge in Zhoushan.

Xihoumen Bridge is the fourth bridge in Zhoushan Continental Link Island Project, with its northern end connected with Cizi Island and its southern end connected with Jintang Island. Crossing Xihoumen Waterway, it is a long-span suspension bridge with a main span of 1650m ... The concrete plane size of the north and south caps is 16.8-22.8m and the height is 7m. The concrete volume of a single pile cap is about 2643m3, and the design strength grade of concrete is C30. Pile foundation is adopted for the north and south caps, and there are12Ф 2.8m rock-socketed piles at the bottom of the caps.

Due to the effect of hydration heat, mass concrete will go through three stages after pouring, namely heating period, cooling period and stable period. At this stage, the volume of concrete will also expand and contract. If the volume change of each concrete is restrained, temperature stress will occur. If the stress exceeds the crack resistance of concrete, concrete will crack.

In order to prevent temperature cracks in mass concrete, we should mainly start from two aspects: first, improve the crack resistance of concrete material itself; The second is to reduce the influence of external force, temperature and constraints on the structure.

The main difficulty in mass concrete construction is how to control the hydration heat and avoid concrete cracking or excessive temperature stress. At present, the commonly used methods include optimizing the mixture ratio, adjusting the temperature of concrete materials entering the mold, adjusting the internal temperature of concrete, reasonably dividing the pouring height and pouring sequence, and strengthening concrete maintenance.

2. Optimization of concrete mix proportion and selection of raw materials

In order to make mass concrete have low hydration heat, good pumpability, good volume stability, excellent corrosion resistance and crack resistance, the concrete is prepared as follows:

2. 1 cement: Huaxin 32.5 slag portland cement and Nanjing Shuanghou 32.5 slag portland cement are selected. According to the test results, the fineness, standard consistency, setting time, stability and mortar strength of cement meet the specification requirements.

2.2 Fly ash: I-class fly ash from Jianbi Power Plant is selected, and its quality inspection index meets the specification requirements.

2.3 Admixture: JM-2 retarding and efficient precipitant from Jiangsu Academy of Building Research is selected, and its quality inspection index meets the specification requirements.

2.4 Sand: Fujian Minjiang sand is selected, and its performance inspection index meets the specification requirements.

2.5 Stone: Zhenhai Dadong Stone Field stone is selected, and its physical and chemical performance inspection indexes meet the requirements of the specification.

2.6 Water: tap water.

2.7 The water-binder ratio of concrete is 0.4 14. After many trial mixes and parallel tests in the supervision laboratory, it is determined that the dosage of C30 pumping concrete per cubic meter is 259kg of cement, 759kg of medium sand, 0/069 kg of crushed stone/KLOC-0, 0/53.2 kg of water/KLOC-0 and 0/65438 of fly ash. The slump is 80 ~185 mm.

3. Simulation calculation of temperature stress of mass concrete.

The key to mass concrete construction is to control the appearance of temperature cracks. In order to check whether the temperature stress caused by concrete temperature difference and concrete shrinkage exceeds the ultimate tensile strength of concrete at that time, the theoretical calculation of crack prevention is carried out:

3. 1 numerical model

The numerical models of adiabatic temperature rise, elastic modulus and creep used in the calculation are as follows:

3. 1. 1 adiabatic temperature rise

The adiabatic temperature rise formula adopts hyperbolic function.

3. 1.2 elastic modulus

3. 1.3 Creep

3.2 concrete material parameters

The elastic modulus, splitting tensile strength, adiabatic temperature rise and linear expansion coefficient of pile cap concrete are determined according to experience.

3.3 Other calculation conditions

U The concrete plane size of a single pile cap is16.8-22.8m, and the height is 7m. It is poured in four layers, with the thickness of 1.5m, 1.5m and 2.5m respectively.

U cap is constrained by12Ф 2.8m pile foundation, and the elastic modulus of foundation is 2.4- 104MPa during calculation.

U When calculating, consider the thermal insulation of concrete surface, and consider that the side is covered with a layer of polyethylene coiled material and a layer of colored strip cloth, and covered with two layers of sacks.

U Consider the cooling effect of cooling water pipe in calculation. Five layers of cooling water pipes are laid on the concrete of the bearing platform, and the horizontal spacing of the cooling water pipes is 0.9m The hydration heat temperature of C30 bearing platform concrete rises to 32℃.

U see the tender documents for temperature data, and the average wind speed is considered as 6m/s. ..

U Consider the influence of creep on concrete stress in calculation.

3.4 Calculation results

The pile cap is poured in four times, and the principal tensile stress in concrete at each age is less than the splitting tensile strength of concrete, and the safety factor of concrete cracking is K≥ 1.3, which meets the requirements.

4. Temperature control standard

The principle of concrete temperature control is: 1) try to reduce the temperature rise of concrete and delay the occurrence of the highest temperature; 2) reduce the cooling speed; 3) Reduce the temperature difference between concrete center and surface, between new and old concrete, and control the temperature difference between concrete surface and air temperature. The method and system of temperature control should be determined according to the temperature (season), internal temperature of concrete, structural size, constraint conditions, concrete mix proportion and other specific conditions. According to the actual situation of this project, control the following temperature control standards:

◆ The highest concrete pouring temperature in summer is ≤ 30℃;

◆ Maximum hydration heat temperature rise of concrete: C30 concrete of pile cap ≤ 29℃;

◆ Maximum temperature difference between inner surface and adjacent blocks: CAP ≤ 25℃;

◆ The temperature difference between concrete surface and air temperature in winter is ≥20℃, and the temperature and condensation of curing water on concrete surface.

Soil surface temperature difference ≤ 65438 05℃;

U the maximum allowable cooling rate of concrete is ≤ 2.0℃/d.

5. Architecture

5. 1 concrete pouring temperature control

Reducing the pouring temperature of concrete is very important to control concrete cracks. For the same concrete, the temperature rise when the mold temperature is high is much greater than that when the mold temperature is low. The temperature of concrete entering the mould should be adjusted according to the air temperature. The hot climate shall not exceed 30℃, and it shall not be lower than 5℃ in winter. Before concrete pouring, the pouring temperature can be estimated by measuring the temperatures of cement, fly ash, sand, stone and water. If the pouring temperature is not within the control requirements, phased measures should be taken.

5. 1. 1 Measures to reduce the storage temperature of concrete in summer are:

1) The cement should be fully cooled before use to ensure that the cement temperature is ≤50℃ during construction.

2) Build a sunshade, pile up high aggregate, take material from the bottom, and spray aggregate with water.

3) Avoid direct sunlight on the formwork and fresh concrete, and the temperature of the formwork and reinforcement before entering the formwork and attaching.

The recent local temperature does not exceed 40℃. Therefore, the construction period should be arranged reasonably and night pouring should be adopted as far as possible.

4) When the pouring temperature exceeds 30℃, water should be mixed with ice.

5) When the temperature is higher than the warehousing temperature, the transportation and warehousing speed should be accelerated to reduce the temperature rise of concrete during transportation and pouring. Concrete conveying pipes should be covered with straw bags and watered frequently.

6) In the concrete heating stage, in order to reduce the maximum temperature rise, the formwork and concrete surface should be cooled, such as sprinkling water to cool down and avoiding sun exposure.

5. 1.2 winter construction

If the daily average temperature is lower than 5℃, in order to prevent concrete from freezing, measures such as stirring water heating and heat preservation during transportation can be taken.

5.2 Control the intermittent and layered thickness of concrete pouring.

The concrete pouring interval of each layer should be controlled at about 5-7 days, and the longest time should not exceed 10 day. In order to reduce the constraint of old concrete, thin layer, short interval and continuous construction are needed. If the interval time is longer for some reason, the thickness of the upper concrete should be adjusted according to the actual situation and on the basis of full checking calculation.

According to the construction requirements, the concrete of the bearing platform is planned to be poured in four layers, and the pouring thickness is 1.5m, 1.5m, 1.5m and 2.5m respectively. In order to reduce the foundation constraint and consider the structural characteristics, the layered thickness is from thin to thick, and the schematic diagram of layered thickness is shown in Figure 5- 1.

5.3 Burying and control of cooling water pipes

5.3. 1 water pipe position

According to the characteristics of internal temperature distribution in concrete, one layer of cooling water pipes is laid below 2m, two layers of cooling water pipes are laid above 2m, and five layers of cooling water pipes are laid altogether. The cooling water pipes are φ40-2mm welded steel pipes with a horizontal spacing of 0.9m The maximum length of each cooling water pipe is 150-200m. The inlet and outlet of cooling water pipes are arranged centrally, which is convenient for unified management.

5.3.2. Use and control of cooling water pipes

1) Tap water shall be used for cooling water, and seawater shall not be used;

2) Water pressure test should be conducted for cooling water pipes before use to prevent water leakage and water plugging;

3) In order to ensure the internal cooling effect of mass concrete, the flow rate of cooling water should reach 32-40L/min, and the flow direction of cooling water should be controlled to make the cooling water flow from the high temperature area to the low temperature area of concrete;

4) In order to ensure the uniform internal cooling of mass concrete, the temperature difference between the inlet and outlet of cooling water pipes is less than 5℃.

5) After the concrete is poured to the elevation of each layer of cooling water pipes, water will be drained. When the cooling rate of each layer of concrete exceeds 2℃/d after reaching the peak, water will be drained ... In order to prevent the temperature of the lower concrete from rising after the upper concrete is poured, secondary water cooling is adopted, and the water cooling time is determined according to the temperature measurement results.

6) Control the inlet and outlet temperatures. In summer, the inlet water temperature does not exceed 25℃, and groundwater or deep reservoir water can be selected; It shall not be lower than 10℃ in winter, and it can be mixed with cooling water to raise the temperature.

In order to ensure the initial cooling effect of cooling water, the project department should coordinate with the temperature control unit, optimize the pipeline layout of cooling water pipes according to the actual situation on site, reasonably select water pumps and equip maintenance personnel to ensure the normal work of the cooling system.

5.4 Internal surface temperature difference control

For mass concrete, the temperature will continue to rise due to the exothermic hydration. If the temperature is not too low, heat dissipation should be strengthened during heating, such as sprinkling water on the template to cool down. When the concrete is in the cooling stage, it should be covered with heat preservation to reduce the cooling speed.

In the cooling stage of concrete, such as low temperature or sudden cold wave, the temperature difference between the inner and outer surfaces is more than 25℃, or the temperature is more than 20℃ lower than the surface temperature of concrete, so mass concrete must be maintained by heat preservation. The practice is as follows: the concrete surface is wrapped with a layer of colored strip cloth for insulation with hanging sacks (geotextile). When necessary, the tower should be equipped with a thermal insulation shed, and the concrete surface should be irradiated by iodine tungsten lamp and maintained with hot water. Not only the strength of concrete, but also the temperature of concrete and the temperature difference between inside and outside should be considered when concrete is dismantled, so as to avoid cracking due to too fast cooling when it suddenly comes into contact with air. In winter, the formwork removal time should be extended by not less than one week, and the formwork removal time should be selected in the period with higher temperature in a day. Cover and keep warm in time after formwork removal.

5.5 concrete curing

Concrete curing includes humidity and temperature. The crack resistance and durability of structural surface concrete depend on the temperature and humidity maintenance in the process of construction maintenance to a great extent. Because cement can only be hydrated to a certain extent to form a microstructure conducive to the strength and durability of concrete. Common problems in engineering field at present.

Insufficient wet curing has a great influence on concrete quality. The wet curing time should depend on the different components of concrete materials and specific environmental conditions. Wet curing is particularly important for concrete with low water-binder ratio and admixture. See table 5- 1 for the shortest curing period of different concretes.

This project can use cooling water pipes to save water, which can not only achieve the effect of heat preservation and moisture retention, but also reduce the waste of water resources. In summer or when the temperature is high, the wet curing of concrete surface should be strengthened, and surface water storage should be used as much as possible when conditions permit to prevent concrete from drying and cracking. When the temperature drops suddenly or is lower than 5℃, it should be cured by sprinkling water or using plastic film. While wet curing, the temperature change of concrete should be controlled. According to different seasons, comprehensive heat preservation and heat dissipation measures are taken to ensure that the temperature difference between the inner surface of concrete and the temperature difference between the air temperature and the concrete surface are within the control range.

5.6 Construction control

In order to ensure the construction quality of mass concrete and improve the uniformity and crack resistance of concrete, it is necessary to strengthen the construction control of each link. Concrete construction shall be carried out in strict accordance with Technical Specification for Construction of Highway Bridges and Culverts (JTJ04 189), and special attention shall be paid to the following aspects:

1. Before concrete is mixed into ingredients, all kinds of weighing instruments should be calibrated by the clearing and measuring department, and the weighing error meets the requirements.

Specification requirements, in strict accordance with the determined mixture ratio of construction.

2. The concrete shall be poured in layers according to the specified thickness, sequence and direction, and shall be completed before the initial setting of the lower concrete.

The thickness of the upper concrete and concrete layered distribution shall not exceed 30cm.

3, in strict accordance with the specification requirements for horizontal and vertical construction joints layered and block, along one side of the concrete surface spread crack-proof metal mesh, to prevent surface cracks.

5.7 on-site monitoring

Through a lot of work, the temperature control of mass concrete construction process will get substantial results, but on-site monitoring is still an indispensable and important link. In order to test the construction quality and temperature control effect, master the temperature control information, adjust and improve the temperature control measures in time, and realize the information construction, it is necessary to monitor the temperature and strain of concrete. Because the temperature, stress development and crack prevention of mass concrete is a very complicated problem, such as the changes of external temperature, humidity, construction conditions and raw materials. Will cause changes in temperature and pressure. Only through temperature control and stress monitoring can we know the quality and crack safety of the structure more accurately. Field monitoring mainly includes temperature and stress monitoring.

Step 6 manage

In view of mass concrete construction, the supervisor should first carefully review the construction organization design, construction scheme and temperature control scheme, check whether the pouring construction scheme and construction procedure are feasible, check and recheck the quality of engineering materials and equipment, put an end to potential quality accidents and ensure the smooth pouring of mass concrete.

6. 1 check the materials and equipment needed for mass concrete.

Due to the large amount of mass concrete materials, the supervisor must ask the construction unit to ensure that cement, sand, stone, water, fly ash and additives meet the dual requirements of quantity and quality, and then review whether the concrete mixture ratio is correct on the premise of having the test report and quality assurance certificate. Because of the construction on the island, we must also ensure the supply of fresh water.

In view of the long construction time of mass concrete and its geographical location on the island, in order to ensure continuous construction and prevent mass concrete pouring from stopping halfway due to equipment failure, standby equipment is needed. Moreover, before the start of construction, the construction unit is also required to carry out the trial operation of the main equipment, check all kinds of measuring instruments, and provide the measurement certificate of the measurement department.

6.2 Comprehensive inspection before mass concrete construction

In order to ensure the quality of the structure, before the mass concrete construction, it is necessary to comprehensively check whether the types and positions of the steel bars and their joints, templates and cooling water pipes in the pouring parts are consistent with the drawings. For embedded steel bars and embedded parts, check them one by one to prevent less or wrong burial. For reinforced joints, various forms of mechanical connection and welding are adopted. Therefore, before each layer is poured, the supervisor should conduct random inspection, and pouring can only be carried out after it is qualified.

In view of the large thickness of mass concrete, the formwork has transverse joints. In order to ensure the quality, the joints of the formwork must ensure that there is no slurry leakage, and the interior is smooth without obvious joints.

Water pressure test should also be carried out for cooling water pipes to prevent water leakage and water blockage.

6.3 In the process of mass concrete construction, the station shall supervise the whole process.

In the process of mass concrete construction, the on-site supervision engineer must conduct on-site supervision 24 hours a day. When problems are found, they should be dealt with in time. The test supervision engineer must personally supervise the concrete mixing station, check whether the production mix ratio is correct, determine whether the concrete slump is stable, check whether the concrete discharge temperature record, specimen extraction and production meet the requirements, and conduct random sampling. If quality problems are found, timely notify the rectification to eliminate quality hidden dangers.

6.4 At the end of mass concrete pouring, temperature records and curing conditions shall be checked at any time.

After mass concrete construction, we must pay attention to its temperature measurement records and concrete curing. If it is found that the temperature exceeds the design temperature, it must be handled immediately by the construction unit to prevent temperature cracks. If it is found that the maintenance is not timely or the weather may suddenly change, measures must be taken in advance to prevent the concrete from cracking.

7. concluding remarks

Using cooling technology to control the temperature of mass concrete within the design requirements can effectively prevent temperature cracks. Through the test and calculation before construction, combined with the careful organization of the construction unit and the supervision and guidance of the supervision unit, it not only creates good conditions for the mass concrete construction of the bearing platform, but also provides favorable guarantee for the quality of the mass concrete structure and valuable information for other mass concrete construction.

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