I. Overview of the Project
The project is a modern building integrating business and public apartments, with underground 12 floors and a total construction area of over 9,000 square meters. The structure type is frame-supported shear wall structure. The basement of this project is fire pool, water pump room, power distribution room and generator room. The first to third floors are mainly commercial and office buildings, and the fourth floor is apartments. In this project, pile foundation and raft foundation are poured by vibrating immersed tube. The designed bottom elevation of pile cap is -4.5m, the thickness of foundation slab is 500, C40 impervious concrete is adopted, and the impervious grade is S8. The concrete volume of the whole foundation slab is about 1 000m3. The concrete pouring time of foundation slab is planned to be one calendar day.
Second, the construction preparation work
The technical requirements of mass concrete construction are relatively high, especially in construction, to prevent temperature stress cracks caused by temperature difference caused by cement hydration heat. Therefore, we must make full preparations from the aspects of material selection and technical measures to ensure the smooth construction of mass concrete for foundation slab.
1, material selection
(1) Cement: Considering the high hydration heat of ordinary cement, especially when it is applied to mass concrete, a large amount of hydration heat of cement is not easy to dissipate. If the internal temperature of concrete is too high, there will be a large temperature difference with the concrete surface, which will cause compressive stress inside the concrete and tensile stress on the surface. When the surface tensile stress exceeds the tensile strength of early concrete, temperature cracks will occur. Therefore, the low hydration heat slag portland cement with the number of 525# was determined. By adding appropriate admixture, the performance of concrete can be improved and the impermeability of concrete can be improved.
(2) Coarse aggregate: gravel with a particle size of 5-25mm and a silt content of not more than 1. Concrete made of stone with large particle size and good gradation has good workability and high compressive strength, and can reduce water consumption and cement consumption at the same time, thus reducing the hydration heat of cement and the temperature rise of concrete.
(3) Fine aggregate: medium sand, mountain sand (45%)+ artificial sand (55%), with average particle size greater than 0.5mm and silt content less than 5. Compared with concrete mixed with fine sand, concrete mixed with medium-coarse sand with larger average particle size can reduce water consumption by about 10%, and correspondingly reduce cement consumption, thus reducing the hydration heat of cement, concrete temperature rise and concrete shrinkage.
(4) Fly ash: As the pouring method of concrete is pumping, in order to improve the workability of concrete and facilitate pumping, consider adding an appropriate amount of fly ash. According to the specification requirements, when slag portland cement is used to mix mass fly ash concrete, the maximum limit of fly ash replacing cement is 25%. Fly ash is beneficial to the hydration heat and improves the workability of concrete, but the early ultimate tensile value of concrete mixed with fly ash decreases, which is not conducive to the impermeability and crack resistance of concrete. Therefore, the content of fly ash is controlled within 10, and the external mixing method is adopted, that is, the amount of cement in the mixture ratio is not reduced. Calculate the dosage of fly ash per cubic meter of concrete according to the requirements of mixture ratio.
(5) Admixtures: There are no specific requirements for the design. Through analysis and comparison and other previous engineering experience, it is determined that the concrete adopts "Shanfeng brand" (water reducer), with 2kg per cubic meter of concrete. Water reducing agent can reduce the peak value of hydration heat, compensate the shrinkage of concrete and improve the crack resistance of concrete.
2, concrete mixture ratio
(1) The concrete adopts the commercial concrete supplied by our mixing station, and the concrete mixing station is required to carry out concrete trial mixing in advance according to the technical requirements put forward on site.
(2) The concrete mixture ratio shall be determined through trial matching. This design is based on the relevant technical requirements of the current national Code for Construction and Acceptance of Concrete Structure Engineering, Code for Design of Mix Proportion of Ordinary Concrete and Technical Code for Application of Fly Ash Concrete.
(3) When fly ash is added, only the same volume of sand is deducted from the sand. In addition, the supply of cement should be considered to meet the requirements of construction.
3. Site preparation
(1) Foundation slab reinforcement, column steel dowel and wall steel dowel shall be completed in sections as soon as possible, and concealed works shall be accepted.
(2) The pits and ponding pits on the foundation slab are closed by combined steel formwork, and the non-module parts are closed by wooden formwork.
(3) copy the elevation of the upper surface of the foundation slab on the column and wall reinforcement, and clearly mark it for leveling when pouring concrete.
(4) When pouring concrete, the embedded temperature measuring tube, plastic film and straw mat for thermal insulation should be prepared in advance.
(5) The Project Manager Department shall contact the construction unit to ensure the construction electricity consumption, concrete vibrating and construction lighting.
(6) Management personnel, construction personnel, logistics personnel, security personnel, etc. Should be scheduled day and night, stick to their posts, their respective responsibilities, to ensure the smooth progress of continuous concrete pouring.
Three, mass concrete temperature and temperature stress calculation (attached)
According to the requirements of the owner and design, the temperature of the foundation slab concrete is detected; Generally, the peak temperature rise at the center of concrete foundation slab is slightly less than the adiabatic temperature rise. Generally, it is produced about 3 days after concrete pouring, and then it tends to be stable and does not heat up, and gradually begins to cool down. According to the specification, temperature control measures should be taken for mass concrete curing according to climatic conditions, and the surface and internal temperatures of poured concrete should be measured as needed to control the temperature difference within the design requirements; When the design has no specific requirements, the temperature difference should not exceed 25 degrees; There is no specific requirement for the design of this project, that is, it shall be implemented according to the specifications. The surface temperature can be controlled by adjusting the thickness of the insulating layer.
Fourth, mass concrete construction
1, division of construction section and pouring sequence
Because the foundation slab is small in size and 500mm thick, it is a natural construction section. The pouring sequence of concrete is from axis A to axis E, from 1 to axis 27. Brick walls with a thickness of 240 are built around the outer side of the foundation slab, and then cement mortar is used for leveling layer, and waterproof coating is applied. The waterproof layer of 85 1 coating is coated with 1:3 cement mortar for 3 days, and external formwork is made. The reserved foundation pit and water accumulation pit on the foundation slab are closed by composite steel formwork, and the parts with inappropriate modulus are closed by wooden formwork.
2, steel bar
Steel processing is carried out in the steel yard on site, the main reinforcement of concealed beam is connected by flash butt welding, and the reinforcement of bottom plate is connected by cold lap joint. After the foundation slab reinforcement construction is completed, the column and wall reinforcement construction shall be carried out, and the column and wall reinforcement shall ensure the accurate position. After the construction of foundation slab reinforcement and column and wall reinforcement is completed, the concealed works shall be accepted, and concrete can be poured only after they are qualified.
3, concrete pouring
(1) The concrete is made of commercial concrete, which is transported to the site by concrete truck and poured by two concrete delivery pumps.
(2) When pouring concrete, the pouring process of "fixed points in different areas, one slope, progressive advance and reaching the top at one time" should be adopted. The length of the placing boom of the reinforced concrete pump truck shall determine the pouring area, and each pump truck shall be responsible for the concrete pouring in this area. When pouring, it should be carried out in one place until the design elevation is reached, and the concrete flows forward in a fan shape, and then it is poured continuously on its slope and pushed forward in sequence. This pouring method can better adapt to the pumping process, that is, each truck of concrete is poured on the slope formed by the previous truck of concrete to ensure that the pouring interval of each layer of concrete does not exceed the specified time. At the same time, it can solve the problem of frequent movement of pump pipes, and it is also convenient to cover and keep warm the pouring parts.
Concrete pouring shall be carried out continuously, and the intermittent time shall not exceed 6h. Under special circumstances, if the concrete cannot be continuously poured for 4h, emergency measures should be taken. That is, insert 12 short steel dowel on the surface of the poured concrete, with the length of 1m and the spacing of 50mm, which is arranged in a plum blossom shape. At the same time, the concrete surface is covered with plastic film and straw mat for thermal insulation. To ensure that the concrete surface will not freeze.
(3) When pouring concrete, arrange 3~4 vibrators at the ash outlet of each pump truck. Because the slump of concrete is relatively large, it can flow obliquely in the bottom plate with a thickness of 1.5 meters, which is about outside of 1 meter. Two vibrators are mainly responsible for vibrating the lower slope, and the other vibrator 1~2 is mainly responsible for vibrating the top concrete.
(4) Due to the large slump of concrete, water will be generated at the lower part of the surface reinforcement, or tiny cracks will be generated in the concrete at the upper part of the surface reinforcement. In order to prevent such cracks, secondary plastering and compaction measures are taken before the initial setting of concrete and after the pre-settlement of concrete.
(5) Make three groups of test blocks on site according to each pouring 100 cubic meter (or one shift), with 1 group pressing strength for 7d and 1 group pressing strength for 28d as technical files; The strength of group L is still 14d.
(6) Waterproof concrete impermeability test block shall not be less than 2 groups per unit project as stipulated in the specification. Considering that the project is not too big, two groups of waterproof concrete impermeability test blocks are taken as required.
5, concrete temperature measurement
(1) When pouring foundation slab concrete, assign special personnel to cooperate with the embedded thermometer. The length of temperature measuring tube is divided into two specifications, and the arrangement of temperature measuring points is shown in Figure 2. The temperature measuring line shall be buried according to the temperature measuring layout. When buried, the temperature measuring tube should be firmly bound with the steel bar to avoid displacement or damage. Each group has two temperature measuring lines (that is, temperature measuring lines with different lengths), and the upper and lower ends of the lines are marked with tape, which is convenient for distinguishing the depth. The temperature measuring line shall be wrapped with plastic tape and bound firmly, and the temperature measuring end shall not be affected with damp. The position of the temperature measuring line is marked with a protective wooden frame, which is easy to find after heat preservation.
(2) Equipped with full-time temperature measuring personnel, according to two shifts. Training and technical disclosure should be given to temperature measuring personnel. Temperature measuring personnel should be serious and responsible, and measure the temperature according to the hole on time, without omission or fraud. The temperature measurement record should be filled in neatly, and the disclosure should be made during the shift change.
(3) The temperature measurement shall be carried out continuously, once at a time, and the temperature measurement and concrete strength shall reach the specified time and strength, and the temperature measurement shall be stopped with the consent of the technical department.
(4) When measuring the temperature, if the difference between the highest temperature inside the concrete and the departmental temperature reaches 25 degrees or the temperature is abnormal, the technical department and the technical director of the project shall be informed in time so as to take timely measures.
(5) Liquid crystal digital display electronic thermometer is used for temperature measurement to ensure accurate temperature measurement reading.
6, concrete curing
(1) After concrete pouring is completed, it will be covered with heat preservation immediately after secondary plastering and compaction. First, cover the concrete surface with two layers of straw mats, and then cover it with a layer of plastic film.
(2) The fresh concrete has a fast hydration speed, and can be cured after being covered with plastic film, so as to prevent the concrete surface from drying shrinkage cracks due to dehydration, and at the same time, it can avoid the straw mat from reducing its thermal insulation performance due to moisture absorption.
(3) Column and wall steel dowel is the difficulty of heat preservation, so special attention should be paid to cover tightly to prevent the temperature difference from being large or freezing.
(4) With the consent of the technical department and the technical person in charge of the project, the thermal insulation layer and plastic film can be gradually removed to make the concrete dissipate heat.
Verb (abbreviation of verb) main management measures
1, the raw materials of mixing concrete need to be inspected, and can only be used after they are qualified. At the same time, we should pay attention to the temperature of raw materials to ensure that the temperature of concrete entering the mold is basically close to the theoretical calculation.
2, in the concrete mixing station by designated personnel with admixture, dosage should be accurate.
3, the commercial concrete on the construction site should be checked by car, determine the slump and temperature of concrete, and check whether the concrete quantity is consistent. The concrete temperature should be controlled within l-l inches, and it is forbidden to temporarily add water to the concrete mixer at the construction site.
4, concrete pouring should be continuous, intermittent time shall not exceed 3~5h, at the same time, before being covered by new pouring concrete, the surface temperature of the poured concrete shall not be lower than 0.
5, the test department designated persons responsible for temperature management and maintenance, found that the problem should be promptly reported to the project technical director.
6. Before pouring concrete, the sundries in the base groove should be cleaned up.
7, strengthen the concrete block production and maintenance management, block after ripping timely number and sent to the standard room for maintenance.