Main methods of high-rise building construction
(1) Construction measurement of high-rise buildings,
1. Positioning and setting out of buildings,
Carry out according to the positioning basis and positioning conditions given by the design.
When the positioning basis is the original building (structure), you must go to the site together with the construction unit and the design unit to determine the sides, corners, center lines, elevations, etc. of the building (structure) based on the positioning. The specific location should be clearly designated and confirmed, and if necessary, photos should be taken for verification and archiving.
When the positioning basis is planning red lines, road center lines or measurement control points, after handing over the piles to the construction unit and design unit face to face on site, the coordinates and elevation values ??of each point must be calibrated. The spacing, angle and height difference should be checked on site to see if each pile position is correct. If there is any discrepancy, the construction unit should be asked to handle it properly. Before positioning high-rise buildings according to the site plane control network, the positions of the control pile points used should be calibrated to prevent misuse of pile positions that are subject to collision and settlement deformation.
2. Vertical control of high-rise buildings
When the construction of a high-rise building reaches ±0.000, as the structure rises, the axis of the first floor must be measured upward layer by layer for As the basis for laying out each layer and vertical control of the structure. Among them, the measurement of the building outline axis and the control elevator shaft axis are more important. "Technical Regulations for Concrete Structures of High-Rise Buildings" (JGJ3-2002, J186-2002) stipulates that the following axes should be projected upward: the axis of the building's outline; the axes on both sides of expansion joints and settlement joints; the axes on both sides of the elevator room and stairwell; the unit , the dividing axis of the construction flow section.
For vertical projection measurement of the axis of high-rise buildings, the following two methods are often used: external control method and internal control method; in addition, the comprehensive internal and external control method can also be used. No matter which method is used to project the axis upward, it is necessary to accurately measure the building outline and detailed axes to ±0.000 on the first floor after the foundation project is completed and the building axis control piles are calibrated according to the building site plane control network. On the plane, it serves as the basis for upward projection of the axis.
3. Overview of commonly used measuring instruments in high-rise building construction
Measuring instruments have generally gone through four generations in the past century. The first 20 to 30 years at the beginning of the 20th century was the first generation; before and after the Second World War, it was the second generation. The level was slightly tilted, and a refractive prism was installed above the level tube to improve the leveling. Accuracy, theodolite is an optical dial and centering; in the 1960s and 1970s, it was the third generation. The level on the level and the vertical plate indicator level tube of the theodolite were replaced by automatic compensation mechanisms. From then on, measuring instruments became automatically leveled; After the 1980s, the readings of levels and theodolite were electronically displayed digitally, and measuring instruments entered the era of automation, electronics and digitization.
(2) Deep foundation pit engineering
1. Concept and current situation of foundation pit engineering
Foundation pit engineering is to protect the safety of foundation pit construction and underground structures The general term for the support, foundation pit soil reinforcement, groundwater control, excavation and other projects to prevent damage to the surrounding environment and the surrounding environment, including survey, design, construction, monitoring, testing, etc.
In most cases, foundation pit projects are temporary projects and do not attract enough attention from geotechnical engineers. Therefore, there are currently many inconsistencies in concepts, theoretical systems, calculation methods, etc., and engineering design is conservative and wasteful. , There are many accidents in foundation pit projects at home and abroad. The importance and technical difficulty of foundation pit engineering have increasingly attracted people's attention.
2. Types of supporting structures
The supporting structure consists of a retaining structure and an anchor structure. When the supporting structure cannot play a water-stopping role, a water-stopping curtain can be set up at the same time or precipitation inside and outside the pit can be adopted.
Foundation pit supporting structures can be divided into the following two categories:
1) Pile and wall supporting structures:
Pile and wall supporting structures The structure often uses steel sheet piles, reinforced concrete sheet piles, column-type cast-in-place piles, underground diaphragm walls, etc. The supporting piles and walls are inserted into the soil at the bottom of the pit to a certain depth (generally inserted into the harder soil layer), and the upper part is cantilevered or equipped with an anchor system.
This type of supporting structure is widely used, has strong applicability, and is easy to control the deformation of the supporting structure. It is especially suitable for excavation of deep foundation pits with a large depth, and can adapt to various complex geological conditions. The design calculation theory is relatively mature and there is more engineering experience in various regions. It is the main form often used in foundation pit engineering.
2) Solid gravity support structure
Solid gravity support structure often uses cement-soil mixing pile retaining wall, high-pressure jet grouting pile retaining wall, soil nail wall, etc. This type of supporting structure has a large cross-section and relies on the gravity of the solid wall to retain soil. It is calculated according to the design principles of gravity retaining walls. The wall can also be designed in various forms such as lattice or ladder. There is no anchor or internal support system. Earth excavation is convenient and suitable for small foundation pit projects. When soil conditions are poor, the excavation depth of the foundation pit should not be too large. When the soil conditions are good, the use of the cement mixing process is limited. The soil nail wall structure has greater adaptability.
(3) Mass concrete construction
1. Definition of mass concrete
As the shape of the building (structure) continues to increase, the corresponding structure The size of the components will inevitably increase. For concrete structures, when the volume or area of ??components is large, large temperature stress will be generated in the concrete structure and components. If special measures are not taken to reduce the temperature stress, concrete cracking will inevitably occur.
The occurrence of temperature cracks is not simply a matter of construction methods, but also involves many factors such as structural design, structural design, material selection, material composition, constraints and construction environment.
American ACI5.1 Introduction Definition: “Any large-volume concrete poured in situ is so large that measures must be taken to solve the problem of hydration heat and the resulting volume deformation to maximize the Reduce cracking. The definition of the Japan Architectural Society Standard (JASS5) is: "Concrete with a minimum structural cross-section size of more than 80cm, and the difference between the maximum temperature inside the concrete and the outside temperature caused by the heat of hydration is expected to exceed 25°C, is called mass concrete. ."
The definition of my country's current industry standard JGJ55-2000 "Common Concrete Mix Proportion Design Regulations": "The minimum physical size of concrete structures is equal to or greater than 1m, or it is expected that the heat of cement hydration will cause internal and external damage to the concrete. Concrete with cracks caused by excessive temperature difference"
(2) Key points in large-volume concrete construction
Due to the large cross-section and large cement consumption, the water released by cement hydration is large. The heat of transformation will produce large temperature changes. Due to the poor thermal conductivity of concrete, the external heat will dissipate quickly, while the internal heat will not dissipate easily, resulting in temperature differences and temperature stress between various parts of the concrete, resulting in temperature cracks.
Crack types:
According to the causes, they can generally be divided into cracks under load (accounting for about 10%), cracks under deformation (about 80%), and cracks under coupling action. Cracks (about 10). According to the harmfulness of cracks, they are divided into two types: harmful cracks and harmless cracks. Harmful cracks are cracks whose width has an impact on the functionality and durability of the building. Generally, cracks with a width slightly exceeding the specified 20% are considered mild harmful cracks, and cracks exceeding the specified 50% are moderate.
Generally speaking, initial cracks caused by temperature shrinkage stress do not affect the load-bearing capacity of the structure, but only Impact on durability and water resistance. For cracks that do not affect the bearing capacity of the structure, the cracks should be sealed or reinforced to prevent corrosion of steel bars, carbonization of concrete, waterproofing and anti-seepage. For underground or semi-underground structures, cracks in concrete mainly affect its waterproof performance. Generally, when the width of the crack is 0.1~0.2mm, although there is slight water seepage in the early stage, the crack can heal itself after a period of time; if it exceeds 0.2~ 0.3mm, the amount of water seepage increases in proportion to the third power of the crack width, and chemical grouting must be performed. Therefore, in underground projects, cracks exceeding 0.3mm and penetrating the entire section should be avoided as much as possible.
1. Concrete pouring and vibration
For large-volume concrete pouring of basement wall structures, in addition to general construction techniques, some technical measures should be taken to reduce the shrinkage of concrete , increasing the ultimate tensile strength, which is very effective in controlling temperature cracks.
Improving the concrete mixing process is of great significance to improving the mix ratio of concrete, reducing the heat of hydration, and increasing the ultimate tensile strength.
In order to further improve the quality of concrete, a new mixing process of secondary mortar-wrapped stone or pure slurry-wrapped stone is adopted, which can effectively prevent moisture from concentrating on the interface between stone and cement mortar, making the interface transition layer after hardening The structure is denser and the bonding is strengthened, thereby increasing the strength of concrete by about 10%, and also increasing the tensile strength and ultimate tensile value of concrete; when the strength of concrete is basically the same, the amount of cement can be reduced by about 7%.
In addition, secondary vibration of the poured concrete can eliminate the moisture and voids generated in the lower part of the coarse aggregate and horizontal steel bars due to bleeding of the concrete, and improve the holding force between the concrete and the steel bars. Prevent cracks caused by concrete settlement, reduce internal micro-cracks, increase concrete density, increase the compressive strength of concrete by 10 to 20, thereby improving crack resistance.
2. Concrete pouring temperature
After the concrete is discharged from the mixer, the temperature after transportation, pumping, pouring, vibration and other processes is called the pouring temperature of concrete. Since too high a pouring temperature will cause large dry shrinkage, the pouring temperature of concrete should be appropriately limited. Generally, it is recommended that the maximum pouring temperature of concrete should be controlled below 40°C.
3. Concrete exit temperature
In order to reduce the total temperature rise of large-volume concrete and reduce the temperature difference between the inside and outside of the structure, it is very important to control the exit temperature. Among the raw materials of concrete, stone has a smaller specific heat, but it accounts for a larger mass per cubic meter of concrete. Water has the largest specific heat, but it accounts for the smallest mass in concrete. Therefore, the temperature of gravel and water has the greatest impact on the exit temperature of concrete, followed by the temperature of sand, and the temperature of cement has the least impact. In response to the above situation, in order to reduce the exit temperature of concrete during construction, effective methods should be adopted to reduce the temperature of the stones. When the temperature is high, in order to prevent direct sunlight, a simple shading device can be set up in the sand and gravel yard. If necessary, water mist must be sprayed on the aggregate or cold water must be used to rinse the aggregate.
4. Concrete curing
After the basement exterior wall is poured, in order to reduce the temperature difference between the inside and outside during the heating stage and prevent shrinkage cracks due to dehydration on the concrete surface, the concrete should be properly moisturized. ; In order to hydrate the cement smoothly, increase the ultimate tensile strength of concrete, slow down the cooling rate of hydration heat of concrete, and prevent excessive temperature stress and temperature cracks, the moisturizing and thermal insulation maintenance of concrete should be strengthened.
In addition, it is important to take reasonable technical measures during construction, such as using formwork for maintenance and delaying the removal of formwork, which all play a big role in controlling cracks. Moisture curing is to continuously replenish moisture on the surface of the concrete after it is poured. The methods include pouring water, laying a wet sand layer, wet sacks or straw bags, etc., and it is best to cover the surface with a layer of plastic film. The longer the moisture maintenance time, the better, but considering the construction period, it is generally no less than half a month, and no less than 1 month for important structures. Within a few months after concrete is poured, even if the curing is completed, it should not be directly exposed to wind and sun for a long time. For structures such as basement walls, automatic sprinkler pipes (plastic pipes with fine holes) can also be used for automatic water supply and maintenance, and horizontal sprinkler pipes on long walls can be used to continuously spray water on the walls for a long time. The effect is: Better. If a curing agent coating is used for maintenance, attention must be paid to the quality of the curing agent and the necessary coating thickness. At the same time, certain moist curing conditions should be provided and covered with a layer of plastic film. During insulation and maintenance, 2 to 3 layers of insulation materials such as straw bags or straw mats can be used for covering and maintenance.
5. Windproof and backfilling
External climate is also one of the factors that affect the occurrence and development of concrete cracks. Among them, wind speed has a direct impact on the evaporation of water in concrete, which cannot be ignored. Basement Exterior concrete walls should seal doors and windows as much as possible to reduce convection. Soil is the best curing medium. After the basement exterior wall concrete construction is completed, it should be backfilled as soon as conditions permit. During insulation and maintenance, 2 to 3 layers of insulation materials such as straw bags or straw mats can be used for covering and maintenance.
6. Set up the post-casting belt
In the cast-in-place reinforced concrete structure, the temporary temperature and shrinkage deformation joints are set up during the construction period. The joints shall be retained to a certain extent according to the project arrangement. time, and then fill and compact it with concrete to form an overall structure without expansion joints.
The spacing between post-pouring strips is determined by the calculation of the maximum overall pouring length. Under normal circumstances, it is determined by formula (3-20), and the spacing is 20 to 30m. When using post-cast tape for segmented construction, the calculation is to divide the cooling temperature difference and shrinkage into two parts. In the first part, the structure is divided into several sections, so that it can effectively reduce the temperature and shrinkage stress; in the later stages of construction, these Several sections are cast into a whole, which continues to bear the influence of the cooling temperature difference and shrinkage of the second section. The superposition of temperature stress generated by the cooling temperature difference and shrinkage of these two parts should be less than the design tensile strength of concrete. This is the principle of using post-cast tape to control cracks and avoid permanent expansion joints. There are three types of structures for post-cast belts: flat joint type, T-shaped type, and tongue-and-groove type, as shown in the figure. The width of the post-cast strip should be considered to facilitate construction and avoid stress concentration. The width can be 700 to 1000mm. When both the ground and underground are cast-in-place reinforced concrete structures. The location of the post-cast strip should be marked in the design and should penetrate the entire structure above and below ground, but the steel bars should not be cut off. The retention time of the post-cast tape should generally not be less than 40 days. During this period, the early temperature difference and shrinkage of more than 30 degrees have been completed. Before filling the concrete, the original slurry on the entire concrete surface must be chiseled away to form a rough surface, garbage and debris must be removed, and it must be watered and soaked overnight. The filled concrete can be expanded concrete, which requires a concrete strength ratio of 5 to 10 N/mm2 and a moisture curing period of not less than 14 days.
7. Structural design
When designing the basement wall structure, attention should be paid to the importance of structural reinforcement. It has a great impact on the crack resistance of the structure. However, at present, domestic and foreign Not enough attention is paid to this. It is not advisable to use separate reinforcement for continuous slabs, but continuous reinforcement in the upper and lower layers; for floor slabs at corners, it is better to use upper and lower layers of radial reinforcement, with a diameter of 8 to 14 mm and a spacing of about 200 mm. At the same time, it should be used as much as possible. Small diameter, small spacing. Around the holes and at the corners of variable sections, temperature changes and concrete shrinkage will cause stress concentration and lead to cracks. Therefore, diagonal steel bars and steel mesh can be added around the holes; local processing can be done at the variable sections to make the section gradually Transition, and add anti-crack steel bars to prevent cracks.