The preface of 1
In recent years, the amount of concrete works in domestic construction projects is increasing, especially in foundation basements. At the same time, with the development of building technology and the needs of urban construction and urban environmental protection in China, ready-mixed commercial concrete has been widely used for its intensive production mode and stable and excellent product quality.
However, ready-mixed concrete must meet the requirements of strength, stiffness, integrity and durability, and also meet the requirements of actual construction on site. In the construction of ready-mixed concrete, it is necessary to meet the requirements of transportation from the ready-mixed station to the construction site and on-site pumping and pouring technology, and its slump is far greater than the traditional on-site self-mixed concrete construction technology. Therefore, it is very important to effectively prevent and control the appearance and development of concrete deformation cracks in the mass concrete construction of foundation and the concrete construction of basement exterior wall.
Combined with the basement concrete construction of the new ward building of Zhejiang Women's Hospital, this paper introduces the experience of effectively preventing the deformation and cracks of foundation mass concrete and basement exterior wall concrete through a series of measures such as controlling the mix proportion, pouring and curing.
2 engineering examples?
The new ward building project of provincial maternity hospital is located at No.2, Shi Xue Road, Hangzhou. Now, in the hospital of provincial maternity hospital, the surrounding buildings are dense and the space is narrow. There are two basements on the ground floor of the main building 15 and on the ground floor of the podium. The building area is 26570m2, with frame-shear wall structure.
2. 1 mass concrete construction of foundation slab?
This project adopts the raft foundation with pile caps and ground beams upturned. The bottom plate is 900mm thick and the platform beam is high1800 mm. The plane shape is approximately rectangular, with a length of 54m× a width of 33 m, and a post-cast strip with a width of 800mm is set between the main building and the podium. The concrete strength grade of foundation basement is C 40, the impermeability grade is S8, and the concrete of foundation slab is about 3000m3.
Because of the large amount of foundation concrete and the deep foundation pit, in order to ensure the integrity and safety of the foundation structure, considering the difficulty of construction lap and urban construction, the foundation slab is divided into two sections: A and B, with the post-pouring belt as the boundary: Section A is the podium part west of the post-pouring belt, with the concrete amount of 540m3；; Section B is the main building on the east side of post-cast strip, and the concrete quantity is 1500m3. No construction joints shall be left in each horizontal section, and casting shall be completed at one time. Vertical construction joints shall be set 500mm above the foundation lintel.
Concrete pouring should be layered, segmented, continuous and thin. Because the foundation is upturned ground beam, the bottom plate should be poured and vibrated for compaction first, and then the upturned beam should be poured 2 hours after the bottom plate is poured, so that the bottom plate concrete has a certain settlement time. After the concrete is poured to the design elevation, it shall be leveled with a long trowel, polished with wooden crab iron plate after removing residual laitance, and the concrete shall be repeatedly calendered and leveled with iron plate after water collection.
2.2 Concrete construction of basement exterior wall slab?
The basement exterior wall is 500mm thick, with a total extension of 200m. Concrete C40, impermeability grade S8. Same as the foundation construction, it is divided into two sections, A and B, which are bounded by post-cast strip. Because there is a two-story basement, there are four construction joints in the vertical direction, and water is stopped by steel plate water stop.
In order to ensure the integrity and consistency of external wall concrete pouring, and prevent cold joints from appearing in construction, independent columns and internal wallboard concrete are poured before external wall concrete pouring, so as to concentrate on continuous external wall concrete pouring.
Two concrete pump trucks are used for external wall concrete pouring, of which 1 fixed pump is parked on the north side of the foundation pit and connected by hard pipes, and the other one is placed on the south side of the foundation pit, which is dominated by mobile pumps and hoses for automobiles. Concrete pouring starts from the post-pouring belt, and the inclined plane layering method is adopted for vibrating and tamping. According to the temperature at that time and the initial setting time of concrete, the pump pipe should be adjusted in time for each pouring period, and the pump pipe should be promoted in sequence to avoid cold joints during construction.
In order to avoid shrinkage cracks (usually in the form of vertical cracks) in exterior wall concrete, it is required to set horizontal temperature steel bars on the outside of exterior wall during construction, with the spacing not greater than 150mm, and strictly control the thickness of concrete protective layer, and it is forbidden to be super thick.
According to the requirements of pumping technology, the slump of concrete during site discharge is strictly controlled at 12+2 cm. Anyone who goes beyond the scope will leave the scene and someone will be in charge of the work. It is absolutely forbidden to add water to the site.
3 Analysis of the causes of concrete cracks
3. 1 foundation mass concrete crack?
In the construction of mass concrete foundation, due to the different heat dissipation rates between the inside and the surface of concrete, a large temperature gradient is formed on its surface, resulting in a large surface tensile stress. At the same time, at this time, the age of concrete is very short, the tensile strength is very low, and the surface tensile stress caused by temperature difference exceeds the ultimate tensile strength of concrete at this time, which will produce surface cracks on the concrete surface. This kind of crack usually occurs on the third day after concrete pouring (heating stage).
In the cooling stage of concrete, shrinkage occurs due to gradual cooling, while in the hardening process of concrete, hydration and evaporation of mixed water in concrete and gelation of colloid promote the shrinkage of concrete during hardening. These two kinds of shrinkage will also produce great tensile stress because of the constraints of the base or the structure itself, until shrinkage cracks appear.
3.2 What are the causes of concrete cracks in basement exterior walls?
The concrete cracks in the basement exterior wall are mainly shrinkage cracks. The shrinkage of concrete in the process of cooling and hardening is constrained by the structure itself and the side wall of the foundation pit, resulting in greater tensile stress until shrinkage cracks appear.
4 the choice of concrete mixture ratio
4. 1 concrete mixture ratio?
According to the above causes of cracks in mass concrete and basement exterior wall concrete, the choice of concrete mixture ratio is very important. After repeated trial mixing with commercial concrete manufacturer (Huawei Commercial Concrete Company), the selected mixing ratio is cement 1: 3.52: 4.86: 0.44, DXH-B admixture 1.8%, UEA-H micro-expansive agent 0.23% and CF mineral powder 0.54%.
4.2 Selection of raw materials?
(1) Cement: No.525 ordinary portland cement from Qian Chao Cement Factory is selected and stored for more than 7 days after leaving the factory. It can only be used after re-inspection before use. ?
(2) Sandstone: medium sand with fineness modulus greater than 2.4, and 5 ~ 25mm gravel with good gradation. The silt content of sand and stone should be less than 65438 0%.
(3) Admixture: High-performance ground mineral powder ground by Shanghai Baosteel slag is selected, and the specific surface area is more than 400m2. ?
(4) Admixture: DXH-B high-efficiency retarding water reducer is selected as the water reducer; Micro-expansive agent is UEA-H high-efficiency concrete expansive agent produced by Beijing Zhongyan Special Engineering Materials Company.
4.3 What are the characteristics and functions of this mixture ratio?
One of the biggest characteristics of the mixture ratio selected in this project is that the cement consumption is only 220kg/m3 (generally, the cement consumption of C40 and S8 concrete is 350 ~ 400kg/m3). The maximum adiabatic temperature rise Tmax of concrete is linearly proportional to the cement content per cubic meter of concrete. In this project, the amount of cement is reduced to prevent cracks in mass concrete. On the premise of ensuring the strength of concrete, the amount of cement is reduced by adding high-performance ground mineral powder instead of part of cement. The dosage of ground mineral powder is 34% of the dosage of cementing material.
The reason why slag cement with long initial setting time and low hydration heat is selected to reduce the cement consumption is because slag cement has stronger water separation ability than ordinary portland cement, and a large amount of water is precipitated on the surface of pouring layer. The separated water gathers on the surface of the upper and lower pouring layers, which changes the water-cement ratio of concrete, forms an interlayer with high water content, hinders the bonding of the two layers of concrete, and destroys the integrity of concrete. The bleeding of this kind of concrete is in direct proportion to the water consumption.
At present, in addition to grinding mineral powder, fly ash, silica fume and zeolite powder are also used as concrete admixture. Because the durability of fly ash is not as good as that of ground mineral powder, silica fume is relatively scarce and expensive, and zeolite powder needs a lot of water, which increases the bleeding of concrete after mixing, so high-performance ground mineral powder is selected as the admixture of concrete in this project.
Because the bonding strength between cement mortar and coarse aggregate, that is, the interfacial bonding force, is one of the main factors that determine the strength of concrete. Therefore, it is very important to choose high-quality admixture with good adaptability to cement and high water reduction rate. At the same time, considering that the planned pouring time of foundation mass concrete is 65438+1early October, and the daytime temperature is still above 27℃, the initial setting time of concrete still needs to meet the construction requirements. After adding DXH-B high-efficiency retarding water reducer, the performance of concrete fully meets the design and construction requirements.
In view of the variety of UEA micro-expansive agent in the market, the fifth generation UEA high-efficiency concrete micro-expansive agent (UEA-H), a patented product of Beijing Zhongyan Special Engineering Materials Co., Ltd., was carefully selected after many studies.
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