The building area of a power plant's civil engineering is 12,000 square meters, of which the volume of chimney concrete is about 3600 cubic meters, the volume of turbine foundation concrete is about 1800 cubic meters, and the volume of other equipment foundation concrete is quite large. The design strength grade is C30. In the construction of mass concrete, the project department should pay close attention to the prevention of temperature and dry shrinkage cracks. First, the quality and mixing of materials should be controlled. The proportion of concrete should be controlled at all stages of construction. The main measures are as follows:
1. Selection of Material
In this project, 325 slag Portland Cement with low hydration heat is used. With the permission of pumping, 5.0-31.5 mm gravel with larger particle size is selected. The sand is medium sand with mud content less than 3%. In order to meet the requirement of workability, slow down the early hydration heat of cement, delay and reduce the peak temperature, the high effective retarding water reducer is adopted, requiring the initial setting time of concrete to be 12 hours. By adding micro-expansive agent in concrete, the concrete is restrained by steel bar and prestressing stress is produced by the expansive effect of micro-expansive agent, which can counteract the tension stress produced in the cooling process of concrete and control the crack of concrete structure. The general expansion rate of micro-expansive agent or composite expansive material is between 0.03% and 0.05%.
II. Design of Optimum Mix Ratio of Concrete Material
The concrete material is tested and the best mix ratio is achieved. (1) On the premise of obtaining the design units and meeting the construction load requirements, the late strength of concrete for 60 or 90 days should be used to reduce the amount of cement and the generation of hydration heat as far as possible in the design of concrete mix ratio; (2) The mix ratio of concrete should not be too small, and the cement content should be more than 320 kg/m3, the water-cement ratio should be less than 0.5, and the sand ratio should be controlled at 35%-45%. The slump is 100-140 mm. It is strictly forbidden to add water at random to increase slump.
3. Concrete pouring
The gradient is 1:8, the thickness of each layer is 400 mm, and each pouring point is equipped with three plug-in vibrators. When tamping the upper concrete, it is necessary to insert about 50mm into the lower concrete. When pouring, the surface bleeding of concrete should be eliminated in time. Drainage holes should be set on the bottom cushion of side formwork to drain water to drainage ditches and catchment wells. In order to prevent shrinkage cracks on concrete surface due to excessive mortar, a layer of clean stone should be added on the surface of concrete after pouring, and the number of pressing surfaces should be increased. Secondary vibration is used to increase the surface compactness of concrete and reduce possible cracks. In order to reduce the temperature of concrete, the temperature of concrete should be controlled at about 15 C.
IV. CONCRETE MAINTENANCE
After concreting, leveling according to the elevation and scraping with wooden bars; rolling with iron clinker twice before initial setting, then rubbing with wooden plaster; calculating and determining the curing thickness after surface hardening, laying a layer of plastic cloth close to concrete to prevent water loss on the surface of concrete; after calculation, covering with thermal insulation materials should be required, and controlling the temperature difference between inside and outside of concrete at 25 degrees Celsius. Finally, a plastic sheet is covered to insulate the concrete from wind and control the temperature gradient of the concrete at about 1.5 C.
V. Demolition of Concrete Formwork
The formwork and insulation layer can be removed only when the difference between the internal temperature and the minimum outdoor temperature of concrete is less than 25 C. When the temperature difference between the concrete and the outside is greater than 25 C, the surface of the concrete should be temporarily covered with thermal insulation material to make it cool slowly.
Control of Cracks in Construction Stage
1. Cracks are produced by controlling temperature. During the hardening process of cement in concrete, a large amount of hydration heat is released, which obviously increases the internal temperature of concrete and can cause temperature cracks in concrete. The thicker the concrete is, the more difficult it is to dissipate the heat inside, and the temperature cracks will become larger gradually, even through the cracks. The curing temperature of mass concrete in the first three days should be lower, and the temperature of different stages should be well controlled.
2. Controlling dry shrinkage cracks. Early maintenance of concrete after pouring. When concrete is initially cured, the amount of water increases gradually with the increase of concrete strength; when concrete is finally cured, the surface should be maintained by water storage, which takes about 3 days, and the watering maintenance still needs 14 days. In addition, it is necessary to rub the wool and press it in time.
3. Quality control of mass concrete raw materials. (1) Reduce the water consumption of concrete, strictly control the water cement ratio, or add water reducing agent to keep the water cement ratio unchanged to meet its fluidity and water retention, thus avoiding the formation of stratified shrinkage cracks. (2) Quality control of aggregates, fine aggregates as far as possible with clean medium-coarse sand. The coarse aggregate is composed of stones with 5-40mm particle size distribution and the mud content is controlled below 1.5%, which can reduce the hydration heat and reduce the shrinkage deformation.
4. Use of admixtures and additives. Fly ash can improve workability and reliability of concrete, save cement and reduce hydration heat. Adding appropriate amount of wood-calcium water reducer can also reduce the mixing water and save cement, thus reducing the heat of hydration; adding expansion agent can prevent the initial cracks of concrete.
5. Remedial measures for cracks. Mass concrete cracks should be remedied in time to prevent cracks from gradually expanding and penetrating. Carefully chisel the natural breaking surface of concrete, chisel loose concrete, expose 50% of coarse aggregate, blow the surface with air bag, then rinse with water, and re-pour concrete on it.
6. Concrete quality control measures. (1) Coarse aggregate gravel and pebble can be graded continuously. Pumping concrete is usually used in mass concrete (the diameter of pump pipe is 150 mm). The maximum particle size of gravel is 40 mm and that of pebble is 50 mm. (2) Fine aggregate should be medium sand or coarse sand, and its strength modulus should be 2.70-3.10; (3) the amount of fly ash mixed in concrete should be 15-20% of the amount of cement. It can replace 10%~15% of cement; (4) It is appropriate to mix a proper amount of retarding water reducer in concrete. The initial setting time of concrete should be controlled at 8 hours and the final setting time should be 12 hours.
7. Control the outlet temperature and pouring temperature of concrete. (1) Control the outlet temperature. The temperature of stone and water is the most important factor affecting concrete outlet temperature, followed by sand temperature and cement temperature. When the temperature is high, shade should be set up and cold water should be sprayed to cool the sand and gravel pile. Mixing water can be added with ice, so that the water temperature is controlled at 5 C, and the outlet temperature of concrete should be controlled at 18 - 20 C. (2) The pouring temperature should be controlled. The construction time should be adjusted to choose low temperature and night construction as far as possible. Considering that the cooling loss has a greater impact on the pouring process, it is necessary to speed up transportation and shorten the pouring time.