What are the key processes of large volume concrete construction

1) Pouring of concrete: In order to ensure that there is no segregation when pouring concrete, the free height of concrete when dumping from a height should not exceed 2 meters, and when it is greater than 2 meters, tools such as string drums and chutes are often used to shorten the falling height. The integrity of the large-volume concrete foundation is required to be high, the concrete should be poured continuously, and the raft is divided into four pouring layers by using the method of large-scale inclined plane layered step-by-step, and the thickness of each layer is controlled at about 500mm, so as to reduce the temperature difference inside the concrete.

Before the poured lower concrete has set, the pouring of the upper concrete is started until the pouring is completed. When pouring concrete, it is necessary to ensure that the shear wall and frame column reinforcement left on the raft are firmly fixed, not deformed and not displaced.

2) Concrete vibration: the concrete is vibrated by a plug-in vibrating rod, and the insertion points are evenly arranged with fast insertion and slow pulling, moving point by point, sequentially, and the moving spacing is not greater than the times of the radius of the vibrating rod, generally not more than 30cm. When vibrating the upper layer of concrete, 5-10cm of the unsolidified lower layer of concrete should be inserted to eliminate the joints between the surface layer concrete and ensure the compactness of concrete pouring.

3) Surface treatment of concrete: the cement slurry on the surface of the large-volume pumped concrete is thicker, and it should be carefully treated after the concrete is poured, and after about 4-5 hours, it is scraped flat with a long scraper according to the elevation, and polished and compacted.

4) curing time and method of concrete: in order to ensure that the newly poured concrete is hardened by appropriate conditions, to prevent cracks due to dry shrinkage in the early stage, after the pouring of large-volume concrete, under the condition that the natural temperature is higher than 5, cover the concrete with straw bags, sacks, etc., and water it frequently, so as to be able to maintain sufficient moist state as appropriate. Under general climatic conditions (15) or more, in the first 3 days after pouring, watering every 2 hours during the day, watering at least 2 times at night, in the subsequent maintenance, watering at least 4 times per day and night, in dry climatic conditions, the number of watering should be increased appropriately, and the watering and curing time is generally appropriate to reach about 60% of the standard intensity.

5) Temperature control: To prevent cracks and reduce temperature stress, you can start from two aspects: controlling temperature or improving constraints. Temperature control measures, such as improving aggregate gradation and mixing to reduce the amount of cement in concrete; Layered multiple pouring, using the pouring layer to dissipate heat; According to the test report, determine a reasonable time for demoulding and carry out surface insulation when the temperature drops sharply, so as to avoid a sharp temperature gradient on the concrete surface.

6) Temperature difference measurement: in order to facilitate the grasp of the change law of cement hydration heating and cooling in the concrete in the hardening process of large-volume concrete and the temperature difference between inside and outside, so as not to cause cracks, and provide a basis for the adjustment of maintenance measures, the thermometer is used to monitor the temperature of large-volume concrete during construction. Before the initial setting of concrete, start after pouring 6 hours, measure the temperature every 2 hours, after 6 consecutive days, change to 6 hours to measure the temperature once, and measure the atmospheric temperature at 2 o'clock in the afternoon every day, in the measurement process, when it is found that the temperature difference between inside and outside is more than 25, the insulation should be strengthened or the insulation material should be properly removed, when the temperature difference is less than 25, the temperature measurement can be stopped.

Crack control method for bulk concrete.

Due to the high temperature rise, large cooling amplitude and speed block of large-volume concrete, the bulk of concrete produces larger.

Temperature and shrinkage stress are the main causes of cracks in concrete. Calculation and analysis should be carried out before construction, and measures should be taken to control it.

temperature cracks. 1 Measures to control internal confinement temperature cracks.

1) Control the temperature difference between the inside and outside of the concrete, the temperature difference between the surface and the outside, and prevent the concrete surface from cooling sharply, and use the concrete table.

surface insulation measures or water storage and maintenance measures;

2) Strengthen concrete curing, strictly control the heating rate of concrete, so that the temperature difference of concrete surface coverage is less than 8-10 °C.

2 Measures to control external confinement temperature cracks.

1) From the aspects of controlling the temperature of concrete out of the machine, temperature rise, reducing temperature difference, etc., as well as improving the construction operation process and reform.

2) Use low-heat cement, such as slag Portland cement is preferred; To use the late strength of concrete, use R60 or R90

Replace R28 as design strength; Mixed with a certain proportion of fly ash, high-efficiency superplasticizer or retarder, etc.;

3) Mixed with expansion agent, in the first 14 days of wet curing, the concrete volume slightly expands to compensate for the early water loss of concrete.

shrinkage cracks due to shrinkage;

4) Improve aggregate gradation, such as large-volume foundation concrete can be mixed with 15% stone;

5) Use mixed water and ice to reduce the water temperature, spray sunscreen or cool water to cool the sand and gravel aggregate, and store bulk cement in advance.

Prepare measures to avoid the temperature of the new cement being too high to reduce the temperature of the concrete;

6) Reasonably arrange the construction process to carry out thin layer pouring and smashing, and rise evenly to facilitate heat dissipation;

7) For large-volume foundation concrete construction, cooling water pipes can be buried in the foundation to make the temperature difference between the inside and outside of the concrete less than 25°C;

8) Reasonable joints and blocks of construction, for relatively long structures should be set up post-pouring belt; on bedrock or old concrete cushions, at the table.

50 100mm sand cushion is laid on the surface to eliminate bedrock restraint and embedding;

9) Appropriate configuration of temperature reinforcement to reduce the temperature stress of concrete;

10) Strengthen the curing of concrete, appropriately extend the curing time and demoulding time, and make the concrete surface cool slowly.

Molding, casting, curing (to prevent overheating).

What problems should be paid attention to when building large-volume concrete: cold joints should not be left in concrete pouring. Ensure that the handover time of pouring is controlled before the initial setting.

Ensure that the vibration is compact, strictly control the vibration time, moving distance and insertion depth, and strictly prevent leakage vibration and over-vibration. Ensure the concrete** and make sure that there are no cold joints. Concrete vibration should be timely, and at the same time no vibration leakage, but not over-vibration to prevent segregation.

Test the temperature difference between the inside and outside of the concrete on time. Adjust the water flow rate of the cooling pipe in time according to the temperature difference regulations.

bury cooling devices, such as cooling pipes;

Pay attention to continuous and uniform pouring, and a long stop time in the middle will produce cold joints;

Pay attention to the quality of concrete, the segregated concrete can not be poured into, and the concrete with unqualified collapse should be dealt with by the test personnel;

Pay attention to the reinforcement of the formwork when pouring, if it is found to be loose and run the mold, the pouring should be stopped immediately, and the pouring should be started after the reinforcement is completed;

Pay attention to the number of tankers, don't press too many cars, don't break the car, and communicate with the dispatch in time.

Pay attention to the vibration.

Dear, hello, I have inquired for you, what need to be paid attention to when building large-volume concrete;First of all, the cement used is very important, you can use low hydration heat cement to mix concrete, you can also add a certain amount of micro-expansion agent and retarding water reducer, which can reduce the impact of large volume concrete volume shrinkage, but also reduce the phenomenon of cracking. 2.

Before the start of construction, the relevant settings should be carried out to reduce the temperature stress and reduce the temperature inside the concrete. After construction, you need to cover the surface with something to prevent the concrete from drying too quickly and reduce the possibility of cracking. During construction, cold water pipes can be pre-embedded, and the heat in the concrete can be brought out by circulating water.

Pro, precautions for large-volume concrete pouring: Before large-volume construction, various pre-construction preparations should be made, and contact the local meteorological station, power bureau, and water supply company to grasp the recent situation. Article 1 of the Technical Regulations for Concrete for Building Structures (JGJ3-2002) stipulates:

1. The difference between the internal temperature and the surface temperature of the large volume of concrete, and the difference between the outer surface of the concrete and the ambient temperature should not exceed 25 degrees Celsius; 2. Try to reduce the temperature of concrete into the mold; After the concrete is poured, it should be covered with moisturizing and thermal insulation within 12 hours; The curing period of waterproof concrete is at least 14 days; 3. Large-volume concrete must be plastered twice to reduce surface shrinkage cracks; 4. The general contractor shall inspect the cement variety, strength grade, packaging or bulk warehouse number, exit date, etc., and shall re-inspect its strength, stability, setting time, hydration heat performance indicators and other necessary performance indicators when entering the cement site. 5. The aggregates, fly ash and granulated blast furnace slag powder used should meet the current national standards. The quality and application technology of the admixtures used should be in line with the current national standards.

6. If the strength of 60d or 90d is used as the index for the large-volume concrete mix ratio, it should be used as the design basis for the concrete mix ratio; 7. The slump of the configured concrete should not be less than 160 when pouring to the working face. 8. Before the preparation of concrete, the conventional mix ratio test should be carried out, and the test of the technical parameters required for the control of cracks in large-volume concrete, such as heat of hydration, water secretion rate, pumpability, etc., should be carried out 9, and the temperature rise caused by the heat of hydration of the concrete pouring block, the internal and external temperature difference of the concrete pouring block and the cooling rate, and the key problem of its construction technology is to prevent harmful temperature cracks (including concrete shrinkage) in the concrete.