Basic process of cast-in-place concrete preparation The concrete needed at the site is supplied centrally by the local concrete mixing station. When it is necessary to set up a mixing station on site due to long distance or large consumption, it is usually appropriate to use assembled production equipment to facilitate dismantling and transshipment. The loading, unloading, weighing, mixing and discharging of concrete materials need linkage automatic control, which tends to industrial production. There are two ways to prepare concrete: ① using a fixed mixer, loading concrete into a transport vehicle, and transporting it to the pouring position after unloading; (2) The required proportion of concrete materials (including admixtures) shall be installed on a car equipped with a mixer, and the whole mixing process shall be carried out on the mixer, and the starting and ending time of mixing shall be determined according to the transportation distance. A small amount of concrete can be transported horizontally at close range, and generally it can be transported by light motor dump trucks or small-capacity trolleys; A lot of concrete must be transported by concrete mixer. After the concrete is transported to the construction site, it can be transported vertically by various lifting booms and cranes according to the site conditions, or it can be put into buckets and hoisted to the pouring position by cranes. For a large number of concrete projects that need continuous pouring, pumping should be adopted; In some cases, belt conveyors can also be used to transport concrete. No matter what method is adopted, it is necessary to ensure that the concrete does not delaminate and segregate, maintain the required water-cement ratio, and control the transportation time so that the concrete can reach the pouring site and have the required slump before the initial setting. When conveying concrete with belt conveyor, measures should be taken to prevent delamination or segregation.
Process inspection: Before pouring concrete into the formwork, the steel bars and formwork shall be thoroughly inspected. Steel grade, diameter, quantity, shape, position, binding firmness, protective layer treatment, embedded parts, etc. of steel bars should be inspected one by one, and the inspection records of concealed works should be made to ensure that they are not displaced or loosened during concrete pouring and vibrating. Formwork is the main tool to ensure the shape, size and position of each part of concrete structure. Must have sufficient strength, stiffness and stability, can bear the weight of concrete and the impact exerted during pouring, and sew tightly to prevent slurry leakage, and it is easy to disassemble and assemble and facilitate turnover. Cast-in-place concrete formwork should be designed and inspected, and must be firmly supported to prevent deformation and displacement.
Pouring In order to make the concrete reach the basic requirements of uniformity, compactness and no deformation during molding, and maintain the good integrity of the concrete structure, the concrete should be evenly and sequentially spread over the pouring parts of the structure, especially at the corners and steel bars, and should be vibrated while filling, and the operation should be as continuous as possible. If the construction must be stopped, and the pouring cannot be continued before the initial setting of the first batch of poured concrete, the construction joint can be left at the part of the structure with the least stress or no shear, and perpendicular to the axis (such as beam and column) or surface (such as wall and slab) of the member, and the pouring can not be continued until the compressive strength of the first batch of poured concrete reaches above 65438±0.2 MPa, so as to avoid damaging the poured part due to vibration.
When pouring concrete, it should be carried out in stages and layers according to the nature, characteristics and site conditions of the structure. The thickness of each layer shall not exceed the working capacity and action range of the vibrator, so that the new and old layers can be bonded to each other. When pouring columns, walls, beams, plates and the whole structure, after the previous layer of concrete is poured, the sinking process is allowed, but no construction joints are allowed. In order to prevent the newly poured concrete from delamination and segregation, resulting in honeycomb and pockmarked surface after hardening, the free falling height of concrete should be limited, and buckets, chutes and hoses should be used for slow introduction when it is too high. In order to make the concrete mixture evenly fill all parts of the structure and eliminate the bubbles in the concrete to make it dense, it is necessary to tamp the concrete with a vibrator. However, excessive vibration is not desirable, because excessive vibration will make the stones sink and the cement slurry float and form separation.
The pouring sequence of concrete is usually as follows: when pouring foundation, it is advisable to advance from low to high by stages; When pouring the frame, it should be pushed from both ends to the center, or from the center to both ends at the same time; When pouring the arch shell, it should be pushed symmetrically from the periphery to the top to ensure the shape and integrity of the structure.
The purpose of curing is to protect the concrete properly in the hardening process, achieve the required strength within a specified time, reduce shrinkage and prevent cracking or freezing. Due to the limited site construction conditions, natural curing can be adopted in normal temperature season. In order to avoid the rapid evaporation of water after concrete molding, ensure the normal hydration process of cement and prevent concrete cracking, cover it for a certain period of time after pouring and water it in time to keep it wet. Used for large-area exposed structures, such as roads, squares, airstrips, floors, floors, roofs, etc. And the structure that still needs watering and curing after formwork removal, or the solution prepared by chemical substances such as latex and resin solvent can be sprayed to form a plastic film on the surface of concrete to prevent water evaporation, thus achieving the purpose of curing. In hot and cold weather conditions, other protective measures must be set up.
The demolition time of concrete formwork shall be determined according to the structure type and concrete strength. In the case that the concrete surface and edges and corners are not damaged by formwork removal, some non-bearing formwork can be removed first; Load-bearing formwork should be treated separately according to the structure category and span size. Usually, concrete can only be removed after it reaches 50 ~ 70% of the design strength. For beams, slabs and other structural flexural members with large span (≥ 8m), the bottom formwork can only be removed after the concrete reaches the design strength, especially the supporting columns of cast-in-place beams and slabs in multi-storey buildings, which can only be removed after the concrete strength is enough to support the full load of the upper columns.
Under special circumstances, cast-in-place concrete pouring mass concrete will produce a lot of hydration heat during the concrete solidification process, resulting in internal and external temperature difference, which will lead to concrete cracking. Cement with low hydration heat should be used to improve the gradation of sand and gravel and reduce the amount of cement and water. During the curing period, measures should also be taken to accelerate heat dissipation, such as embedding cooling water pipes in concrete, using circulating water to reduce the temperature of concrete, or supplementing water outside the foundation according to the site conditions. In addition, adding a proper amount of large stones into concrete and filling the cracks with concrete with high fluidity can also reduce the hydration heat of concrete and control the cracks, but the stones must be solid and clean, stacked evenly, and the concrete should be filled with cracks, vibrated and compacted to eliminate bleeding and ensure the quality.
Due to the high atmospheric temperature, the hydration heat of concrete develops rapidly and the initial temperature is high, which is easy to produce temperature cracks. Due to the rapid evaporation of water in concrete, plastic shrinkage cracks are easy to appear, which seriously affects the later strength of concrete and the bond with steel bars is also lost. Therefore, when pouring concrete in hot season, cooling measures must be taken, mainly wetting the foundation and formwork, reducing the temperature of concrete and strengthening water conservation. One of the effective measures to reduce the temperature of concrete is to reduce the temperature of mixed water. If necessary, crushed ice can be added to the mixed water to reduce the water temperature. In addition, adding retarder can delay the hardening time of concrete; Adding water reducer can make cement particles disperse evenly. These two additives contribute to the full hydration of cement and can offset the adverse effects of environmental temperature on concrete to some extent.
See winter construction for pouring concrete at low temperature.
Other construction methods of cast-in-place concrete rockfill concrete, also known as pre-filled aggregate concrete, are concrete formed by laying clean coarse aggregate in the formwork in advance and then filling cement mortar in the cracks of the coarse aggregate with pressure. It is suitable for projects that are difficult to pour underwater or by other methods and have little requirement for volume change. Coarse aggregate should be hard gravel or pebble with a particle size of not less than 3.8 cm, and the porosity should be between 38% and 48%. Mortar can be mixed with proper amount of plasticizer in ordinary cement mortar to increase its fluidity. Because the mortar is pumped, the strength and rigidity of the formwork should be high. Laying stones should limit the height of their free fall. When paving stones, mortar pressure delivery pipes and observation pipes with pre-calculated spacing should be placed. The lower end of grouting pipe is about 15cm away from the bottom surface, which can be gradually increased with the increase of grout during grouting. The observation tube is used to check the mud height of each part. According to different elevations, drill small holes in the pipe to make the slurry flow in, and place floating balls in the pipe to check the position of the slurry surface. During construction, continuous operation should be carried out from bottom to top as far as possible to avoid gaps or missing irrigation. Grouting speed should be kept stable.
Underwater concrete pouring, also known as conduit concrete, is a method of pouring concrete through vertical pipes and relying on its own weight. Suitable for pouring cofferdam, caisson foundation, open caisson foundation, underground continuous wall, pile foundation and other underwater or underground projects. The concrete slowly flows out from the bottom of the pipe and spreads around, which is not disturbed by the surrounding water flow and ensures the quality. When using this method, the concrete must have good workability, the sand content is between 40% and 50%, the coarse aggregate should be pebbles not more than 3.8 cm, the water-cement ratio should be controlled at about 0.44, and additives such as retarding and plasticizing can be added to the concrete. Generally, the diameter of conduit is 8 times of the largest stone particle size, and the spacing between tubes is generally 4.5 meters. During construction, in order to prevent water flow and sundries from entering the conduit, the bottom of the conduit can be blocked before the conduit is run, and the block can be washed away by the weight of the first tank of concrete, so that the concrete can be poured in place. When working in deep water, the catheter can be prevented from floating. During operation, the catheter can be filled with water, and a rubber ball close to the pipe wall is installed at the top of the catheter, then concrete is poured, and the rubber ball is pressed out along the catheter, and then pouring can be carried out. Slowly lift the pipeline with the pouring, and the lifting range is about 15 ~ 60 cm each time. When pouring, the conduit should be prevented from swinging to avoid concrete cavity. The conduit method is suitable for pouring underwater concrete. When the curing conditions are good, the 28-day strength can generally reach 28 ~ 56 MPa.
Pumping concrete is a construction method (see color map) that uses the pressure of pump to transport concrete to the pouring site. It is often used in projects with large quantities, which need continuous pouring or are difficult to transport by other methods. If the equipment conditions are superior, the pumping capacity per hour can reach 70 m3, the horizontal conveying distance can reach 300 meters and the vertical conveying distance can reach 90 meters. Commonly used pumps are plunger pump (Figure 1), air pump and extrusion pump. Pumping pipes should reduce elbows as much as possible and keep the pipe diameter unchanged. Pipe joints must be tight, easy to assemble and disassemble, and easy to clean.
Pumped concrete must have proper fluidity to avoid pipeline blockage. The suitable slump is 1 1 ~ 13cm. When the ingredients are mixed, water reducing agent, air entraining agent or mineral powder can be properly added to prevent moisture precipitation and precipitation. During construction, before the concrete is put into use, the pipeline should be lubricated with cement mortar. The feeding speed should be kept stable. After pouring is completed, the residual concrete in the pipe must be discharged and washed clean by water pressure (by sweeper).
Construction method of fine stone concrete sprayed by pressure spray gun. It is usually used for pouring thin-walled structures, such as tunnel linings, walls and ceilings, or linings of other structures and protective layers of steel structures. There are two kinds of shotcrete: dry mixing method and wet mixing method. Dry mixing method is a method of uniformly mixing cement, sand and stone in a dry state, sending compressed air into a nozzle and mixing it with pressure water for sprinkler irrigation (Figure 2). This method must be operated by skilled personnel, the water cement ratio should be small, the stones should be graded continuously, the particle size should not be too large, and the cement dosage should not be too small. Generally, concrete strength of 28 ~ 34 MPa and good cohesive force can be obtained. However, due to the high injection speed, serious dust pollution and rebound, the use is limited. Wet mixing method is to send the mixed concrete to the nozzle through grouting pump, and then spray it with compressed air. Mixing and spraying methods should be adopted in construction to reduce the change of consistency. The spraying speed of this method is low, and the initial strength of concrete is also low due to the increase of water-cement ratio, but the rebound situation is improved, the material ratio is easy to control, and the work efficiency is higher than that of dry mixing method.
Decorative concrete is a construction method that when pouring structural concrete, the external decorative surface of concrete is completed at one time by using templates and lining molds embedded with line patterns or patterns. By adopting this method, after the concrete structure is completed, the processing procedures such as plastering, facing, washing and chiseling can be omitted, which is beneficial to controlling the construction period, saving manpower and material resources and reducing the cost. The key to ensure the quality of facing concrete lies in the manufacture and use of external formwork, which requires accurate size and straight board surface. Decorative lines can be made of wood or other materials, but they must be embedded firmly, polished with putty and coated with a mixture of epoxy resin and cement to prevent leakage; Formwork with decorative plastic or rubber lining can also be used, and the release agent must be thin paint, and the colors of cement and sand should be the same to avoid damaging the appearance. During construction, slurry leakage and scaling at the joints between the upper and lower layers of concrete should be prevented. When facing concrete is used for the exterior wall of high-rise building, large formwork can be used to climb layer by layer, but its support and jacking system should be designed and calculated.
Looking forward to concrete pouring in the field, although the conditions are more limited, the construction technology is more complicated and the cost of facilities is higher, the rigidity, integrity and seismic performance of the structure are better than those of prefabricated components, which can adapt to the complex cross-sectional shape of components, buried pipelines and many holes left behind, and save the costs of steel, cement, prefabrication, transportation and hoisting of components. Therefore, it has obvious advantages in building construction. At present, the key is to take effective measures to make each process gradually finalize the design and industrialization, so as to improve its economic and technical effects.