Discussion on crack control measures of large volu

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Talking about the control measures of "cracks in large volume and super long reinforced concrete structures"

cracks in concrete structures are common and unavoidable quality defects, which directly affect the durability of structures. For a long time, people only require and pay attention to the strength of reinforced concrete structure quality in construction engineering, ignoring the durability of concrete, and only accept the strength grade (except for special performance concrete). With the continuous development of science and technology, the improvement of people's living standards and needs, the development of reinforced concrete technology in construction engineering, especially in recent years, large volume, super long, super high reinforced concrete structure design is becoming more and more common. The crack defects of large volume, super long and super high reinforced concrete structures should be paid attention to as a topic worthy of study and discussion to ensure the safety and appearance quality of the structure

I. crack types and morphological characteristics of reinforced concrete structures

1 Temperature difference crack: the temperature rise of cement hydration heat or the short-term sharp temperature drop outside lead to a large temperature difference inside and outside the concrete structure, generally more than 25 ℃, causing temperature stress and temperature difference cracks in the concrete

temperature difference cracks are generally vertical to the member section, with regular intervals, and some cracks will run through the entire member section

2. Shrinkage crack: volume change caused by water evaporation after concrete pouring. Due to the capillary tension generated by the analysis diagram of the travel range of the tension machine of the internal voids of concrete formed by cement hydration

shrinkage cracks are generally vertical to the member section, and the cracks are large and regular, and some cracks will also penetrate the member section

was discovered by French scientist lemoigne in 1926 3 Plastic shrinkage crack: the plastic shrinkage crack on the surface caused by the rapid evaporation of excess water on the surface of concrete before solidification

plastic shrinkage cracks appear on the surface of components, and the cracks are small and irregular

4. Plastic settlement cracks in concrete are mainly caused by poor concrete mix proportion, which is generally caused by discontinuous grading of coarse aggregate, insufficient quantity, and excessive sand ratio and water cement ratio in the design of concrete mix proportion

plastic settlement cracks are wide and deep, and generally crack along the horizontal direction

5. Alkali aggregate reaction crack: the alkali in cement and active silica in active aggregate react chemically to produce cracks

this kind of crack is thick and irregular, with white colloid on the surface

6. The uneven settlement cracks of the building foundation, which are regular and large, make many plastic people smell the "taste" of raw materials to rise in price. The cracks continue to increase with the settlement

7. Load cracks: cracks occur in the structure due to insufficient reinforcement, overload, or excessive load that does not reach the design strength

based on the above analysis of the main types of reinforced concrete structural cracks, the following four factors lead to structural cracks:

1 Structural factors: structural deformation is mainly divided into free deformation, constrained deformation and actual deformation

2. Material factors: cement variety, dosage, aggregate performance, gradation, additive selection, sand and gravel silt content, etc

3. Construction factors: pouring, vibrating, curing, concrete slump

4. Environmental factors: temperature, humidity, wind speed, etc

II. Preventive measures

the above is the superficial understanding of reinforced concrete cracks and the brief factors of the causes of cracks. In order to continuously improve the quality of concrete engineering and ensure the durability and safety of the structure, the following measures should be taken to control the cracks of super long and mass concrete structures

1. In terms of design

① for the selection of structural system, the plane shape of the structure should consider the uniformity and symmetry of stiffness as much as possible, the plane length and concave convex parts should be controlled as much as possible, and irregular structural designs such as overhanging and retracting should be designed, requiring special treatment in the design

② crack prevention measures such as post cast strip, expansion strip, expansion concrete and fiber concrete should be considered in the design of super long structure

③ for super long structures and large volume structures, various factors affecting the compensation reinforcement of cracks should be considered from the design reinforcement (generally according to the principle of fine diameter and dense reinforcement)

2. In terms of the use of concrete materials

① there are differences in the self shrinkage rate of different types of cement, such as aluminate cement and early strength cement; The self shrinkage rate of medium heat and low heat cement is small (the self shrinkage rate of slag cement is larger in the later stage, which is generally greater than that of ordinary cement after 21 days of age)

in addition, it is related to the fineness of cement. Avoid using high fineness cement, slag, silica fume, etc., especially large volume and some ultra long structures. Great attention should be paid to the selection of cement

② the selection of aggregate performance, gradation and additives, especially the aggregates that reflect alkalinity, should be prohibited

③ the selection of mineral admixture materials, such as slag, aged soil, silica fume, etc., must be determined after trial mixing and testing, and reliable data are obtained

3. Construction factors

① mix proportion design of concrete, strictly control the amount of cement per square meter of concrete, appropriately mix fly ash, and the coarse aggregate grading is continuous and reasonable to the best state, so as to reduce the volume of mortar and increase the amount of coarse bone

② add water reducing agent, retarder, micro expansion agent, cellulose, etc., control the unit water consumption of concrete, and select the best working degree

③ control the temperature of concrete into the mold, and add ice and other cooling measures for concrete mixing

④ control the pouring time and maintain continuity. The pouring thickness and vibration time during concrete pouring should be strictly controlled. Cold joints and over vibration are strictly prohibited during construction

the spring comes from our life ⑤ after the concrete is poured, the key is to maintain, first, to ensure that the concrete does not lose water early; The second is to ensure that the temperature difference inside and outside the components is not greater than 25 ℃. Generally, measures such as thermal insulation coverage, cooling circulating water, spraying and curing can be adopted for mass concrete

4. Environmental factors:

the temperature, humidity and wind speed are mainly considered, especially the environmental conditions before the initial and final setting after pouring. Some effective measures such as sunscreen, windproof and increasing humidity should be taken

after the above analysis, the following four measures should be taken to control the cracks of super long and mass concrete: 1 In terms of design, structural system selection, reinforcement, deformation, settlement and other aspects are considered; 2. The selection of materials mainly includes cement varieties, admixtures, mineral admixtures, etc; 3. In construction, the concrete coordination design, the selection and control of admixtures, and the maintenance measures; 4. Construction environment temperature, humidity, wind speed, etc

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