Derived from | Peew construction
1. Basis for compilation
"Unified Standard for Quality Acceptance of Construction Projects" - GB50300-2013
Code for Quality Acceptance of Concrete Structure Engineering - GB50204-2015
Code for Quality Acceptance of Underground Waterproofing Engineering - GB50208-2011
"Winter Construction Regulations for Construction Projects" - JGJ104-2011
Technical Regulations for Concrete Structures of High-rise Buildings - JGJ3-2010
Technical specification for the construction of large-volume concrete for block foundations - YBJ224-91
Technical Regulations for Pumping Concrete Construction - JGJ/T10-2011
Code for Design of Concrete Structures—GB50010-2010
2. Scope of application
- This construction technology standard is applicable to the construction of large-volume concrete in construction projects. It is not suitable for large-volume concrete construction with ambient temperature higher than 80°C or corrosive media that poses a hazard to concrete.
- The large-volume concrete involved in this construction technology standard usually refers to the concrete with the minimum size of the concrete structure equal to or greater than 1m, or it is expected that the corresponding technical measures will not be taken to reduce the temperature of the concrete, and the cracks will be caused by the heat of cement hydration.
- The content of the construction technology standard only designs the special requirements of large-volume concrete construction, and the general and general requirements in concrete construction can be referred to the "Construction Technology Standard for Concrete Structures".
3. Installation process
Process
4. Construction methods and technical measures
4.1 Construction conditions
- The construction plan of large-volume concrete has been prepared, and the construction process determined by the construction plan, the division of the flow operation section, the pouring procedure and method, the concrete transportation and distribution method, method and quality standard, and the safety construction have been disclosed.
- The supply of concrete has been implemented, and the corresponding supporting equipment, measuring instruments, and various raw material supply channels required have been implemented and have entered the site.
- The construction road, construction site, water, electricity, and lighting have been laid, and the construction scaffolding and safety protection have been completed.
- The concrete pump and pump pipe have been laid and commissioned.
- The reinforcement, formwork and embedded parts have been installed, and the inspection and quality acceptance have been completed, the elevation line has been inspected, and the preparation required for expansion joints, settlement joints, post-pouring belts or reinforcing belt supports has been completed.
- The various cooling and anti-cracking measures that should be taken in the construction have been prepared in place, and the temperature measuring components or temperature measuring buried pipes have been prepared and qualified.
- The mechanical equipment, instruments, tools, and tools required for construction have been prepared.
- Liaison, the chain of command is ready.
- Professionals in place. The personnel who need to hold the certificate have been trained and have complete certificates.
- It has coordinated with the community, urban management, transportation, and environmental supervision departments and has gone through the necessary procedures.
4.2 Large-volume concrete anti-cracking measures
- The selection of medium and low heat cement, the addition of fly ash, and the addition of high-efficiency retarding superplasticizer can delay the release rate of hydration heat and reduce the heat peak.
- Incorporate an appropriate amount of U-shaped concrete expansion agent, prevent or reduce concrete shrinkage cracking, and make concrete densify, make concrete impermeability improve. Under the condition of satisfying concrete pumping, try to choose stones with larger particle size and good gradation; Try to reduce the sand rate, generally it should be controlled between 42-45%.
- The cooling water pipe is embedded in the foundation to cool down by circulating low-temperature water.
- Control the machine temperature and pouring temperature of concrete, and use cold aggregate and cold water to mix concrete without freezing in winter. If the temperature is high in summer, the sand and gravel should be insulated, and the sand and gravel yard should be equipped with a simple shading device, and cold water should be sprayed on the aggregate if necessary.
4.3 Mixing and transportation of large-volume concrete
- Concrete mixing should be strictly measured according to the mix ratio, and the vehicle is required to be weighed; Charging sequence: gravel→ cement → sand; If there are additives, they should be added together with cement; The powder-like admixture is added together with the cement, and the liquid is added at the same time as the water. In order to make the concrete mix evenly, the mixing time should not be less than 90 seconds, and when the construction is done in winter or when additives are added, it should be extended by 30 seconds.
- After the concrete self-mixer is discharged, it should be transported to the pouring site in time; In the process of transportation, to prevent the "segregation" of concrete, cement slurry loss, slump changes and initial setting and other phenomena, if there is a occurrence, it should be immediately reported to the technical department to take measures. The continuous time from the discharge of concrete from the mixer to the completion of pouring does not exceed the time specified in the Code. Concrete mixer tanker or loader is used for horizontal transportation of concrete, and concrete pump truck is used for vertical transportation.
- Concrete pumping must ensure that the concrete pump can work continuously, if the failure and downtime exceeds 45 minutes or the concrete has appeared "segregation", the remaining concrete in the pipe should be immediately washed with pressurized water or other methods.
4.4 In-field transportation and distribution of concrete
- The hopper of the ground pump must be equipped with a vibrating screen with a hole diameter of 50×50mm to prevent individual large particles of aggregate from flowing into the pump pipe, and the upper surface of the concrete in the hopper should be about 200mm away from the upper mouth to prevent air from being pumped in.
- Before pumping concrete, the clean water in the receiving hopper is pumped out of the pipeline to wet and clean the pipeline, and then the pure cement slurry or 1:1-1:2 cement mortar is pressed into the pipeline and then pumped into the concrete.
- The speed should be slow when the concrete is pressurized, and when the concrete is sent out of the end of the pipe, the speed can be gradually increased, and the speed can be transferred to continuous pumping at normal speed. When the operation is not normal, the pumping speed can be slowed down, and the suction reciprocating push is carried out several times to prevent the pipe from being blocked.
- Strengthen the inspection of concrete pumps and pipelines, and stop the pump in time to eliminate faults if abnormal sound or pump pipe beating is found.
- When pumping concrete into the mold, the concrete outlet at the end of the pump pipe should be more than 500mm away from the formwork, and the end hose should move evenly during pouring, so that each layer of distribution is uniform, and it will not be poured in piles. At the reserved concave template or embedded parts, the material should be evenly distributed along its periphery.
- When the pump pipe is inclined downward to convey concrete, a horizontal piping equivalent to 5 times the drop length should be installed at the lower end of the lower inclined pipe, and an exhaust piston should be installed at the upper end of the inclined pipe when the inclination with the upper horizontal line is greater than 7o. If the horizontal piping of the downward inclined pipe cannot be set according to the above required length due to the limited construction length, the elbow or hose can be used instead, but the converted length should still meet the requirement of 5 times the drop. Only hose cloth is allowed, and vibrators are not allowed to push away concrete.
- Pipes should be laid along the ground, and 50×100mm square timbers should be padded at both ends of each section of pipes for disassembly and assembly. When conveying at an inclined downward slope, a ramp with a width of not less than 1m should be erected, scaffolding boards should be laid on it, and square wood should be supported at both ends of the pipe. The pump pipe should not be laid directly on the formwork, rebar, but on a stool or a temporary shelf.
- The pumping stop time should not be more than 60min, if it exceeds 60min, the pipe should be pigged.
- In the middle of summer construction, the pump pipe should be covered with thermal insulation.
4.5 Setting of the temperature measuring hole of the temperature measuring element of the cooling pipeline
- Principle of layout of temperature measurement point:
a) the center of the planar shape;
b) The side of the center and the corner where heat is easily dissipated.
c) The main wind direction.
In short, the location of the temperature measurement point should be selected in the place where the temperature change is large, the heat is easy to dissipate, the ambient temperature is greatly affected, the adiabatic temperature rise is the largest, and the shrinkage tensile stress is the largest.
- Vertical arrangement of temperature measurement points:
Each plane position is provided with a group of 3, respectively arranged in the upper, middle and lower positions of the concrete, and the upper and lower measuring points are located at 10 cm on the surface of the concrete. In addition, one temperature measuring point is buried in the air and the insulation layer to measure the ambient temperature and the temperature in the insulation layer. The curing of large-volume concrete is generally not less than 7 days, and the curing cycle is determined according to the temperature change of the concrete in the center of the slab and the strength of the concrete test block cured under the same conditions.
- The curing of concrete should adopt technical measures of heat preservation, moisturizing and slow cooling:
When the pouring thickness is greater than 3m, it is required to consider setting cooling water circulation cooling measures inside the large-volume concrete, setting up cooling water pipes, and controlling the temperature of the concrete center and the surface through temperature detection or the temperature of the concrete inside and the cooling water within 25 °C.
4.6 Concrete pouring
There are three types of pouring methods for large-volume concrete: (1) comprehensive layering method, (2) segmented layering method, and (3) inclined plane layering method.
- Inclined Layering Method:
Concrete pouring adopts the method of "sectional fixed-point, step-by-step advancement, one slope, one time to the top" - the pouring method of natural flow to form slope concrete, which can better adapt to the pumping process, improve pumping efficiency, simplify the water drainage treatment of concrete, and ensure that the upper and lower layers of concrete do not exceed the initial setting time, and be completed continuously at one time. When the slope angle of the large slope of the concrete is close to the end formwork, the pouring direction of the concrete is changed, that is, it is poured back from the top.
- Segmented hierarchical method:
When the concrete is poured in layered sections, the pouring height of each section should be determined according to the structural characteristics and the density of the steel bar, and the general layered height is 1.25 times the radius of the vibrator, and the maximum shall not exceed 500mm. When pouring concrete, strictly control the ash thickness and concrete vibration time, and the pouring is divided into several units, and the interval between each pouring unit does not exceed 3 hours.
4.7 Large-volume concrete vibrating and water drainage treatment
- Each layer of concrete poured should be vibrated evenly in time to ensure the compactness of the concrete. The concrete vibrating adopts the slurry method to ensure that the upper and lower concrete stubble parts are well combined, prevent vibration leakage, and ensure that the concrete is compact. When vibrating the upper layer, the lower layer should be inserted about 50 mm to eliminate the joints between the two layers. The spacing of the plate vibrator should be able to ensure the coverage of the plate of the vibrator to compact the perimeter of the vibrating part.
- Before the initial setting of concrete, two vibrations are given in an appropriate time, which can eliminate the moisture and voids generated by the concrete in the lower part of the coarse aggregate and horizontal steel bar due to water leakage, and improve the grip and wrapping force between the concrete and the steel bar. The interval between two vibrations should be controlled at about 2 hours.
- Concrete should be poured continuously, if intermittent is needed under special circumstances, its intermittent time should be shortened as much as possible, and the next layer of concrete should be poured before the previous layer of concrete is solidified. The maximum intermittent time depends on the type of cement and the solidification condition of the concrete, and generally more than 2 hours should be treated as a "construction joint".
- Construction joint treatment: the strength of the concrete is not less than 1.2Mpa before the lower layer of concrete can be poured; Before continuing to pour concrete, the concrete surface at the interface should be chiseled, remove the floating stone, and after rinsing with clean water, pour the high-grade cement mortar again, and then continue to pour concrete and vibrate compactly, make the new and old concrete closely combined.
- Water drainage treatment of concrete: when the inclined plane layered method pours concrete by pumping, in the process of pouring and vibrating, the upwelling water and the floating slurry will be concentrated under the slope surface along the slope, and the drainage hole should be left at the suitable part of the side formwork, so that a large amount of water is discharged smoothly. When taking the comprehensive layering method, each layer is poured, the water must be gradually rushed forward, the drainage hole is set up at the template to make the water drain out or the water is discharged to the construction joint, and the water pump is set up to pump the water away, and the whole layer is poured completely.
4.8 Surface treatment of concrete
- Handler
- Surface treatment
a) According to the treatment procedure, the concrete surface is treated, and after plastering treatment, it is covered with plastic film and watered for curing.
b) When there is water leakage on the surface of concrete, it should be guided to be discharged in time. When the surface slurry is thick, before the initial setting of the concrete, the stone slurry with a particle size of 20-40mm should be evenly dispensed on the concrete surface, and gently patted flat with a trowel. If there are plastic shrinkage cracks on the hardened surface of concrete, cement slurry can be poured and scraped flat.
c) When the construction area is large, the surface treatment can be carried out in sections.
d) Construction under windy days or scorching sun above level 4 shall have sunshade and wind blocking measures.
4.8 Curing of concrete
- Concrete side steel and wood formwork should be provided with insulation layer in any season construction. When the brick side formwork is used in the foundation pit, the backfill should be completed before the concrete is poured.
- Curing of concrete
a) Water storage and curing concrete:
1) The concrete surface is covered with plastic film after the initial setting, and water is injected after the final setting, and the water storage depth is not less than 80mm.
2) When the temperature difference between the surface temperature of the concrete and the curing water exceeds 20 °C, hot water should be injected to reduce the temperature difference to about 10 °C.
3) During construction in the non-high-temperature rainy season, rainproof measures should be taken in advance to prevent heavy rain and reduce the temperature of the maintenance water.
b) Thermal storage curing concrete:
1) When the cooling and mixing concrete is used in midsummer, the concrete should be covered with plastic film and thermal insulation measures should be taken immediately after the final setting.
2) During the construction at room temperature, the concrete should be covered with plastic film and watered immediately after the final setting, and when the measured internal temperature difference of the concrete or the internal and external temperature difference exceeds 20 °C, thermal insulation measures should be taken.
3) When the air temperature is lower than the concrete forming temperature, the concrete should be covered with plastic film and thermal insulation measures should be taken immediately after the final setting.
4) In order to keep the insulation layer dry, the upper surface of the insulation layer should be covered with an impermeable covering.
- When other operations need to be carried out during the concrete curing period, the insulation layer should be lifted, the operation should be completed as soon as possible, and the insulation layer should be restored immediately.
- Curing period of concrete: On the basis of satisfying the temperature control of concrete, the curing time of concrete is calculated from the beginning of concrete pouring, and the use of ordinary Portland cement is not less than 14d, and the use of other cement is not less than 21d, and the hot weather should be appropriately extended.
- During the curing period (including after removing the insulation layer), the concrete surface should always be kept warm and humid (there should be condensate in the plastic film), and the concrete mixed with expansion agent should be especially water-rich curing, but the temperature should not be watered when the temperature is lower than 5 °C.
4.10 Temperature measurement and temperature control
- Temperature measurement requirements
a) Temperature measurement duration: from the beginning of concrete pouring to the removal of insulation, and should not be less than 20d.
b) Temperature measurement time interval: after the concrete is poured, it should be measured no less than 4 times a day (morning, noon, evening, midnight), and the time interval must be encrypted if the temperature change is abnormal.
c) Temperature measurement point: The temperature measurement point should be placed in the representative part of the concrete structure, and the temperature measurement point should be arranged at the edge and in the middle, according to the cross arrangement, the spacing should be 3-5m, and the pouring height should be arranged in the middle of the bottom and the surface, and the measuring point should be greater than 1 meter from the edge around the bottom plate. The temperature measurement points should be numbered on the floor plan and marked on the site.
d) Make a detailed record of the temperature measurement results, and organize and draw the temperature curve. The temperature tester is responsible for timely feedback on temperature changes, and should give an early warning when the temperature difference reaches 20 °C, and an alarm when it reaches 25 °C.
- Thermometer
a) When using the building electronic thermometer for temperature measurement, the semiconductor sensor attached to the steel bar should be isolated from the steel bar, and the plug to protect the temperature measuring probe should not be polluted or soaked in water, and it should be wiped clean and kept dry before inserting the thermometer to prevent short circuit. It can also be buried in advance, and a reusable sensor can be inserted into the pipe to measure the temperature.
b) When using other thermometers, it should be operated according to the product manual.
- Temperature control
When there are no special requirements in the design, the measured temperature during the hardening period of concrete should meet the following requirements:
a) The internal temperature difference of concrete (100mm below the center and surface) is not more than 20 °C.
b) The temperature difference between the concrete surface temperature (100mm below the surface) and the temperature at 50mm outside the concrete surface is not more than 25 °C.
c)混凝土降温速度不大于 1.5 ℃ /d。
d) When the insulation layer is removed, the difference between the concrete surface and the atmospheric air temperature is not more than 20 °C.
e) When the measured temperature does not meet the above requirements, the insulation layer should be adjusted in time or other measures should be taken to meet the requirements of temperature and temperature difference.
4.11 Mould removal
- For concrete structures with foundation pits, it is advisable to return to caution as soon as possible after removing the side formwork, otherwise, if it is in the curing period, it should be maintained in the same way as the surface layer.
- Other requirements can be found in the formwork project.
4.12 Construction joints, post-pouring belts and reinforcing belts
- For large-volume concrete construction, in addition to reserving the post-pouring belt as far as possible, no longer facility joints, and in case of special circumstances, when facilities must be constructed, they should be treated according to post-pouring joints.
- Construction joints, post-pouring belts and reinforcing belts should be supported by steel plate mesh or steel wire mesh. For example, when supporting the formwork, it should be chiseled and cleaned before pouring concrete.
- The water-swelling stop used in the post-pouring joint must have slow expansion performance, and the 7D expansion rate should not be greater than 60% of the final expansion rate.
- Expanded strips should be securely placed, and self-adhesive strips should also be secured with cement nails spaced 500mm apart.
- The post-pouring belt and construction joints should be cleared of debris and wetted before concrete pouring, and then 10-20mm thick 1:1 cement mortar or interface agent should be laid after the horizontal joints are brushed and cleaned, and the concrete should be poured immediately.
- The expansion rate of the post-pouring joint and the reinforced tape concrete should be higher than the expansion rate of the bulk concrete by more than 0.02% or determined according to the design or product manual.
4.13 Winter construction
- When the concrete poured in winter is mixed with negative temperature composite admixture, different negative temperature admixtures should be used according to different temperature conditions. And before use, it must be subject to special tests and technical appraisal of relevant units. Before the winter construction, the winter construction plan should be formulated, and the heating, mixing, transportation, pouring and maintenance of raw materials should be calculated, and the construction should be carried out accordingly.
- Before pouring concrete, ice, snow and dirt on the formwork and steel bars should be removed. Containers used for transporting and pouring concrete should be insulated. During the transportation and pouring process, the temperature should conform to the data determined by the thermal calculation, and if it does not match, measures should be taken to adjust it. When using heat curing, the temperature before concrete curing shall not be lower than 2 ° C.
- When the monolithic structure is heated and cured, the pouring procedure and the position of the construction joint, should be able to prevent the occurrence of large temperature stress, such as when the heating temperature exceeds 40 °C, it should be determined after soliciting the opinions of the design unit. The temperature of concrete shall not exceed the provisions of the code.
- In addition to the normal number of groups of concrete test blocks, two groups should also be added to curing with the same conditions as the structure, one group is used to test the strength of concrete before freezing, and the other group is used to test the strength of transferring to room temperature curing 28d.
5. Key points of quality control
- The unit responsible for the construction of large-volume concrete should have a sound quality assurance system, a sound construction management system, and be able to implement strict quality control of the construction process.
- Applicable and optimized construction technical schemes should be prepared for large-volume concrete, and the construction technology, quality control, acceptance standards and technical measures to prevent harmful cracks for large-volume concrete should be clearly stipulated, and detailed technical disclosure should be implemented for construction operators.
- The qualifications and competencies of the personnel involved in construction management and construction operations must meet the following requirements:
a) Construction management personnel, including: professional and technical engineers, QC engineers, must have the corresponding qualification certificates;
b) Concrete workers and rebar workers should work in this position for more than three years, and have participated in the construction of important concrete structures, and have rich practical experience;
c) Welders participating in flash welding, electroslag welding or arc welding must have the corresponding qualified welder quality certificate;
d) Concrete tester should work in this position for more than three years, have professional knowledge of concrete forming and curing, and hold a concrete tester (or sampler) certificate.
- The following requirements must be met for the construction machinery and equipment, measurement and testing equipment and inspection and testing equipment entering the site:
a) The construction machinery and equipment, measurement and testing equipment and inspection and testing equipment entering the site must have the required equipment capacity and integrity.
b) Construction machinery and equipment must have the "intact equipment" logo, applicable operating procedures, equipment operation records and maintenance records, equipment operators have qualified mechanical equipment operation qualification certificates.
c) the implementation of multi-shift operation of machinery and equipment, should implement the shift system, carefully fill in the shift record; The successor can only carry out the work after checking and confirming that it is correct.
d) The measurement and testing equipment used shall be verified according to the specified verification cycle, and there shall be a verification certificate and a valid identification of the verification qualification.
- Strengthen process discipline and carry out strict and effective quality control
a) must strictly implement the technical scheme and corresponding specifications and standards for the construction of large-volume concrete, organize the construction in strict accordance with the specified construction methods and quality control requirements, and implement strict and effective monitoring, measurement and inspection tests on the construction process and products.
b) For the construction of large-volume concrete, it is advisable to use the commercial concrete provided by the regular commercial concrete batching plant, and use the concrete mixer truck to transport, and each tank of commercial concrete enters the site, the collapse inspection should be done, and the concrete quality should be accepted.
c) The laboratory responsible for the inspection and test of concrete specimens must have provincial qualifications.
d) Take effective technical measures to reduce the temperature difference between the inside and outside of large volume concrete and prevent cracking on the concrete surface. It should be done: the cooling rate of concrete is not more than 1.5 °C/d. The temperature difference between the inside and outside of concrete is not more than 20°C. The temperature difference between the concrete surface temperature and the concrete surface at 50mm outside the concrete surface is not more than 25 °C, and the difference between the concrete surface and the general air temperature is not more than 20 °C when the insulation layer is removed.
- Ensure that concrete construction has a suitable working environment, including: temperature, humidity and environmental civilization, the specified construction environment must meet the requirements of the standard specifications, to ensure that the concrete construction work is carried out normally.
- For the unqualified in the construction process, it must be reviewed and dealt with according to the regulations, the cause analysis must be carried out, the corrective action plan or rectification plan must be prepared, and the implementation effect of the corrective measures must be tracked and verified.