Construction Social Practice Report

According to the school arrangement, I went to the construction department of xx construction department on June 25, xx. This is a good opportunity for me to understand the construction site, let me further understand the difference between theory and practice.
One: project introduction

The project is a single apartment building developed by Wuhan Donghu Happy State Enterprise Investment Co., Ltd. The contractor is xx construction company, which is the fifth and sixth buildings, and the golf fitness building. The foundation is built by the fourth company. Designed by Beijing Weston Design Institute. The frame shear wall structure is adopted, and the column is a heterosexual column. The commercial and residential building consisting of 3 buildings with an area of ​​13,000 square meters has a six-story frame-cut structure of cast-in-place reinforced concrete.

Second practice content

1: Woodworking
1) Types of templates and methods of making them;
2) Quality standards for the installation of various structural templates;
3) Quality standards for the installation of cast-in-place structural formwork;
4) Time and sequence of removal of cast-in-place structural formwork;
5) Precautions for template removal;
6) Methods and requirements for cleaning, stacking and repairing the formwork;
2: Steel bar
1) the type and shape characteristics of the steel bars;
2) Welding method and quality requirements of steel bars;
3) Methods and techniques for cold working of steel bars;
4) Methods and quality requirements for the binding of steel bars;
5) the length of the lap joint of the steel bar;
6) A method for controlling the thickness of various component protective layers;
7) Master the hidden project recording method and main content;
3: Concrete clay
1) The type, specification and mixing principle of the mixer;
2) The type of vibrator, the scope of application;
3) Conversion of construction mix ratio and contents of signage;
4) The construction of the construction joint and its treatment method;
5) Maintenance methods and requirements for concrete;
6) Causes and prevention methods of surface defects of concrete;
7) Quality inspection content of concrete engineering;
Three gains and experience First of all, practice is a familiar and unfamiliar word for me, because I have experienced a lot of practice in my student life for more than ten years, but this time it is so different. He will comprehensively test my ability in all aspects: study, life, psychology, body, thought and so on. It is like a touchstone to test whether I can apply the theoretical knowledge I have learned in practice. It is also the key to building confidence in my ability to be successful in this challenging society. Therefore, my investment in it is also 100%! The tense month of practical life is over, and I have had a lot of gains in this month. After the practice is over, it is necessary to sum up it. First of all, through practice for more than a month, I learned a lot of practical knowledge through practice. The so-called practice is the only criterion for testing the truth. Through the side station, I have observed the construction process of the whole house from a close distance, and learned a lot of concrete construction knowledge that is very applicable. This knowledge is often my little contact at school, very few. Attention, but it is a very basic knowledge.
For example, the cause of the crack in the concrete and the location is a very complicated problem, then let me talk about my opinion:
1 Causes of cracks

There are many reasons for cracks in concrete, mainly temperature and humidity changes, brittleness and unevenness of concrete, unreasonable structure, unqualified raw materials, template deformation, uneven foundation settlement.
During the hardening of the concrete, the cement releases a large amount of hydration heat, and the internal temperature rises continuously, causing tensile stress on the surface.
In the later stage of cooling, due to the constraints of the foundation or old coagulation, tensile stress will appear inside the concrete. The lowering of the temperature also causes a large tensile stress on the concrete surface. When these tensile stresses exceed the crack resistance of the concrete, cracks may occur. Many concretes have little or no change in internal humidity, but surface moisture may vary greatly or change drastically. Such as poor maintenance, wet when dry, surface shrinkage deformation is constrained by internal concrete, and often lead to crack concrete is a brittle material, tensile strength is about 1/10 of compressive strength, the limit pull in short-term loading The extension deformation is only ×104, and the limit deformation at the time of long-term loading is only ×104. The tensile strength is the same in the same concrete due to uneven raw materials, unstable water-cement ratio, and segregation during transportation and pouring. It is also uneven, and there are many weak parts that have low tensile strength and are prone to cracks. In reinforced concrete, tensile stress is mainly borne by steel, and concrete is only subjected to compressive stress. If the tensile stress is present in the edge of the concrete or in the concrete reinforced concrete, it must be borne by the concrete itself. In general design, no tensile stress or only a small tensile stress is required. However, during construction, the concrete is cooled from the highest temperature to a stable temperature during the operation period, and tends to cause considerable tensile stress inside the concrete. Sometimes the temperature stress can exceed the stress caused by other external loads, so mastering the variation of temperature stress is extremely important for reasonable structural design and construction.

2 Analysis of temperature stress

According to the formation process of temperature stress, it can be divided into the following three stages:
Early: From the beginning of the pouring of concrete to the end of the cement heat release, generally about 30 days. Two characteristics of this stage, one is that the cement releases a large amount of hydration heat, and the other is the sharp change in the elastic modulus of the coagulation. Due to the change in the modulus of elasticity, residual stress is formed in the concrete during this period.
Medium term: from the end of the concrete heat release to the time when the concrete is cooled to a stable temperature. During this period, the temperature stress is mainly caused by the cooling of the concrete and the change of the outside air temperature. These stresses are superimposed on the residual stress formed in the early stage. The modulus of elasticity on the coagulation during this period did not change much.
Late: The period of operation after the concrete is completely cooled. The temperature stress is mainly caused by changes in the outside air temperature, and these stresses are superimposed on the residual stresses of the first two. According to the causes caused by temperature stress, it can be divided into two categories:
Self-generated stress: A structure with no constraints or completely static on the boundary. If the internal temperature is nonlinearly distributed, the temperature stress occurs due to the mutual constraint of the structures themselves. For example, the pier body of the bridge has a relatively large structural size. When the concrete is cooled, the surface temperature is low, the internal temperature is high, tensile stress is generated on the surface, and compressive stress occurs in the middle.
Constrained stress: The stress caused by the free deformation of all or part of the boundary of the structure. Such as box girder roof concrete and guardrail concrete. These two temperature stresses often interact with the stress caused by the shrinkage of the concrete.
Accurate analysis of the distribution and size of temperature stress based on known temperatures is a more complicated task. In most cases, it is necessary to rely on model tests or numerical calculations. The creep of concrete causes a considerable relaxation of temperature stress. When calculating the temperature stress, the influence of creep must be considered. The specific calculation will not be repeated here.

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3 Temperature control and measures to prevent cracks

In order to prevent cracks, reducing temperature stress can be started from two aspects: controlling temperature and improving constraints.
The measures to control the temperature are as follows:
Use improved aggregate grading, dry hard concrete, blending materials, air entraining agent or plasticizer to reduce the amount of cement in concrete;
When mixing concrete, add water or water to cool the gravel to reduce the pouring temperature of the concrete;
Reduce the thickness of the pouring when pouring concrete on hot days, and use the pouring layer to dissipate heat;
Water pipes are buried in the concrete and cooled by cold water;
Provide a reasonable time for demoulding, and carry out surface insulation when the temperature drops suddenly to avoid a sharp temperature gradient on the concrete surface;
The surface of the concrete pouring block or the thin-wall structure exposed during the long-term construction, and the insulation measures are taken during the cold season;
The measures to improve the constraints are:
Reasonably splitting the blocks;
Avoid excessive foundation fluctuations;
Reasonably arrange the construction process to avoid excessive height difference and long-term side exposure;
In addition, improving the performance of concrete, improving the crack resistance, strengthening the curing, preventing the surface from shrinking, especially ensuring the quality of the concrete is very important to prevent cracks. Special care should be taken to avoid the occurrence of through cracks, and the integrity of the structure should be restored after the occurrence. It is very difficult, so the construction should be based on the prevention of penetrating cracks. In the construction of concrete, in order to improve the turnover rate of the formwork, the newly poured concrete is often required to be demolished as early as possible. When the concrete temperature is higher than the temperature, the mold removal time should be properly considered to avoid causing early cracks on the concrete surface. The early casting of the new casting mold caused a large tensile stress on the surface, and a "temperature shock" phenomenon occurred. In the initial stage of concrete pouring, due to the emission of hydration heat, the surface causes considerable tensile stress. At this time, the surface temperature is also higher than the temperature. At this time, the template is removed, and the surface temperature drops suddenly, which inevitably causes a temperature gradient, thereby attaching a surface to the surface. The tensile stress, superimposed with the hydration thermal stress, and the dry shrinkage of the concrete, the tensile stress of the surface reaches a large value, which may cause cracks. However, if the surface is covered with a light insulation material after removing the template, For example, foam sponges have a significant effect on preventing excessive tensile stress on the concrete surface.
Reinforcement has little effect on the temperature stress of mass concrete because the mass ratio of mass concrete is extremely low. It only has an effect on general reinforced concrete. Under the condition that the temperature is not too high and the stress is lower than the yield limit, the properties of the steel are stable regardless of the stress state, time and temperature. The linear expansion coefficient of steel differs greatly from the coefficient of linear expansion of concrete, and only a small internal stress occurs between the two when the temperature changes. Since the elastic modulus of steel is 7 to 15 times that of concrete, when the internal concrete stress reaches the tensile strength and cracks, the stress of the steel will not exceed 100~200kg/cm2. Therefore, it is intended to be utilized in concrete. Reinforcing bars to prevent the appearance of small cracks is difficult. However, the cracks in the structure after reinforcement generally become a large number, a small pitch, and a small width and depth. And if the diameter of the steel bars is thin and the spacing is dense, the crack resistance of the concrete is improved.