[Abstract] In the construction, the quality of concrete pouring is difficult to control, especially in our country, the operation of concrete pouring mostly depends on manual work, and the degree of mechanization and standardization of construction is not high, which seriously affects the quality and construction progress of the project. With the acceleration of international integration and the improvement of national building quality standards, the quality control of concrete pouring needs to be strengthened urgently in industrial and civil buildings, bridges and water conservancy projects. This paper focuses on the treatment of cracks in concrete pouring and other matters needing attention.
[Key words] Construction technology for quality control of concrete pouring
Chinese Library Classification Number: TU375 Document Identification Number: A Article Number:
Text:
1 Preparations for Concrete Casting
1) The concrete should be cleaned up before pouring, and the concrete pouring should not be allowed until the pouring surface is qualified.
2) In order to ensure the good combination of new and old concrete construction joints, before pouring the first layer of concrete, the cement mortar with the same label as concrete should be laid 2-3 cm. The area of the mortar should be suitable for the strength of concrete pouring, and the thickness of the mortar should be uniform to avoid the phenomenon of excessive thickness or thin.
3) The concrete pouring should adopt the method of flat paving or step construction. Rolling pouring method is strictly prohibited. It should be carried out according to a certain thickness, sequence, direction and layers. The pouring layer should be smooth, and the wall should be evenly raised symmetrically when pouring. The pouring thickness should be 30-50 cm.
4) Concrete pouring should be done first and then vibrated. Vibration is strictly forbidden to replace liquidation. Vibration time is based on the fact that the coarse aggregate of concrete no longer sinks significantly and begins to flooding. Bubbles in concrete are vibrated out to avoid short or long vibration time, resulting in undervibration, leakage and overvibration. Vibration should be pulled out slowly after the completion of vibration, and the speed is strictly prohibited. The vibration radius of concrete should not exceed 1.5 times of the effective radius of the vibrator. The vibrator should be inserted into the lower concrete about 5 cm, and should not be too deep, so as to avoid overvibration of the lower concrete.
5) During the concrete pouring period, if there is more surface overflow, it should be removed in time, and measures should be taken to reduce the overflow. It is strictly forbidden to open holes in the formwork to drive water, so as not to take away the mortar.
6) In the process of concrete pouring, especially in the roof pouring, the displacement and deformation observation points should be set up, the template deviation should be observed regularly by special persons, and the template should be inspected and strengthened by special persons.
7) In order to reduce the segregation of concrete caused by transportation after pouring, the preparation of concrete should be as close as possible to the final place of pouring. It should be layered so that each layer can be evenly compacted. The time interval between layers should be limited to ensure that the newly poured concrete is well integrated with the front pouring.
8) When concrete is poured in one time, it should be removed immediately by hand or machine vibrator, otherwise aggregate segregation will occur. Generally speaking, vibrators should not be concentrated on the concrete previously raised.
2. Concrete pouring technology
Concrete structure requires high integrity, and construction joints are usually not allowed. In order to ensure the integrity of the structure and the continuity of construction, layered pouring should ensure that the upper concrete is poured before the initial setting of the lower concrete. Therefore, it is necessary to ensure the coordination of concrete mixing, transportation, pouring, vibration and tamping processes. On this basis, according to the structure size, steel density and other specific conditions, the following pouring schemes are selected:
2.1 Comprehensive Stratification
In the whole structure of layered concrete, to complete the first layer of all pouring, before the pile solidification back to pour the second layer, so layer by layer, until the completion of pouring. With this scheme, the structural plane should not be too large as far as possible, and the construction should start from the short side and pour along the long side. If necessary, it can also be carried out from the middle to both ends or from both ends to the middle at the same time.
2.2 Segmentation and Layering
When the overall layering scheme is adopted, the pouring strength is very high, and the concrete mixer and conveyor can not meet the construction requirements, the sectional layering scheme can be adopted. When pouring concrete, concrete is poured from the bottom, then the second layer is poured after a certain distance, so that the above layers are poured forward in turn. The sectional layered pouring scheme is suitable for structures with less thickness and larger area or length.
2.3 Surface Layering
It is suitable for the case where the length of the structure exceeds three times the thickness. When concrete is poured to the top at one time, the slope is 1:3 because the concrete naturally forms a slope. Vibration and tamping of concrete should begin at the bottom of the pouring layer and gradually move upward to ensure the quality of concrete construction. Large section of mass concrete structure is easy to cause structural cracks in concrete.
The main causes of structural cracks are temperature reduction and shrinkage. Any cooling difference includes the temperature difference and contraction equivalent caused by hydration heat, and can be divided into uniform and non-uniform cooling differences. The former produces external restraint, which becomes the main cause of penetrating cracks; the latter causes self-restraint and forms surface cracks; only by controlling these two kinds of cooling differences at the same time, can the cracks be reduced and avoided.
Therefore, when pouring large volume concrete, we must take the following appropriate measures: make concrete have greater crack resistance as far as possible, that is, high tensile strength and low linear expansion coefficient. For this reason, low-heat Portland cement is chosen to incorporate fly ash and external water reducing agent, such as air entrainment, retarding, early strength and other additives to minimize the amount of cement and water consumption per cubic meter of concrete. Generally, medium-coarse sand and large-size, well-graded stone are used. When the temperature is high, simple shading devices or straw bales can be installed on sand, stone yard and transport equipment. Thermal insulation material, concrete mixing with low temperature water or ice water; enlarging pouring surface and heat dissipation surface, reducing the thickness and pouring speed of pouring layer, if necessary, laying cooling pipe inside concrete, using circulating water to reduce concrete temperature; after pouring, sewage should be removed in time, and secondary vibration should be carried out when necessary; strengthening temperature measurement of concrete, which has been constructed for construction organizers. The timely and accurate adoption of temperature control measures in the process provides a reliable basis.
3. Treatment of concrete cracks
The cracks can be cleaned and sealed by epoxy grouting or surface brushing after drying. (2) When the cracks are large, the cracks are chiseled into eight-dimensional grooves, washed and wetted, brushed with cement slurry once, compacted and polished with 1:2 or 1:2.5 cement mortar layer by layer, or inlaid with epoxy cement after drying. (3) Cracks caused by structural deformation such as temperature, drying shrinkage and creep can be sealed by annular radon mud or anticorrosive coating or glass cloth. (4) For structural cracks affecting the requirement of waterproof and seepage control of the whole structure, cement pressure filling and chemical filling can be used to repair them according to the width and depth of the cracks, or surface sealing and grouting can be used at the same time; serious cracks which obviously reduce the stiffness and bearing capacity of the structure should be strengthened by prestressing according to the situation.
4. Points for Attention in Concrete Pouring
1) When pouring concrete into moulds, the impact formwork or steel frame should not be poured centrally. When the pouring height is greater than 2 m, string barrels and chutes should be used to cut materials. The free height of pouring from the outlet of the material pipe to the pouring layer should not be greater than 1.5 m.
2) Concrete must be poured within 5 hours (from the start), in order to prevent concrete pouring cold joints (cold joints: construction quality joints formed when the time interval between the upper and lower layers of concrete exceeds the initial setting time), twice concrete pouring time does not exceed 1.5 hours, and continuous stirring of vibrating rods is used at the junction.
3) During the pouring process, the duration of vibration should make the concrete surface produce floating slurry, no bubbles and no sinking. The vibrator inserts are arranged in a plum-blossom shape and move in a determinant order. The distance of the moving position should be no more than 40 cm, so that no vibration leaks and no vibration occurs.
4) When pouring beam-slab concrete, first pour the beam-slab concrete, starting from the beam-column joints to ensure the vibration and compaction of the beam-column joints, and then pour the high-grade concrete before pouring the low-grade concrete when pouring the concrete with different labels (strength grade difference of 10 MPa) at the beam-column joints. And always keep the pouring height of high-grade concrete higher than the surrounding low-grade concrete pouring height; reinforcement of beam-column joints and other steel bars
Vibrating rod with diameter of 30 mm is used in compact vibration tamping, and the vibration spacing is 100 mm to 200 mm, so as to avoid serious quality problems such as concrete honeycomb.
5) The concrete surface is covered and maintained in time after twice and three times of plastering. After the concrete is finally set, it should be fully wetted by spraying water, sealed and covered with plastic film, and the surface of plastic film should be inspected frequently. However, when there is no water droplet on the surface of film, water should be sprayed again. The maintenance time of the roof of underground layer 3 to underground interlayer is 7 days, and that of the roof of underground layer 1 is 14 days.
5 Summary
The workability of concrete mixtures should be controlled on the standard value. When concrete is put into storage, it is forbidden to destroy the cohesion of concrete and prevent the segregation of concrete. When concrete is put into storage, it should be evenly paved. The thickness of each layer should be controlled between 20 and 30 cm. This is conducive to vibrating tamper. When vibrating, the tamper bar should enter the concrete quickly and put forward slowly, so that the bubbles in the concrete can return evenly, and no bubbles can escape from the concrete surface. The oblique vibration method is generally adopted, which is characterized by labor-saving operation, high efficiency, fast pulping, easy to discharge air, and no serious segregation.
Reference:
[1] Ma Liangzhu. Quality Control and Quality Detection of Cast-in-situ Concrete [J]. Scientific and Technological Information Development and Economy 2003 (06)
[2] Sun Xiuai, Cheng Shuming. Construction technology of one-time integral pouring of beam-slab concrete for large-area frame structure [J]. Architectural technology 2003 (01)