According to the arrangement of the school, I went to the xxx project department of Wuhan XX Construction Company for construction practice on XX, XX, xxxx, which was a good opportunity for me to understand the difference between the construction site and theory and practice.

1. Project introduction

This project is an apartment building developed by Wuhan xxxx State-owned Investment Company. The contractor is Wuhan xxx Construction Company, which is Building 5 and Building 6 and Golf Fitness Building respectively. The foundation shall be borne by xxx Fourth Pile Foundation Company. Design by Beijing xxxx Design Institute. Frame shear wall structure is adopted, and the columns are anisotropic columns. Covering an area of 13000 square meters, it is a commercial and residential building, consisting of three buildings and a six-story cast-in-place reinforced concrete frame-shear structure.

Second, the internship content

1: Woodworking

1) Types and manufacturing methods of templates;

2) Quality standards for formwork installation of various structures;

3) Quality standard for formwork installation of cast-in-place structure;

4) Time and sequence of dismantling the cast-in-place structure template;

5) Precautions for template removal;

6) Methods and requirements for template cleaning, stacking and maintenance;

2. Reinforcement worker

1) Types and appearance characteristics of reinforcement;

2) Welding methods and quality requirements of steel bars;

3) Method and technology of cold working of steel bar;

4) Binding method and quality requirements of reinforcement;

5) lap length requirements for steel binding;

6) Control method of protective layer thickness of various components;

7) Master the recording methods and main contents of concealed works;

3. Concrete workers

1) Types, specifications and mixing principles of mixers;

2) Type and scope of application of vibrator;

3) Conversion of construction mixture ratio and mark content;

4) Design and treatment of construction joints;

5) Concrete curing methods and requirements;

6) Causes of concrete surface defects and preventive treatment methods;

7) Quality inspection content of concrete engineering;

Three points of harvest and experience

First of all, internship is a familiar and unfamiliar word to me, because I have experienced many internships in my ten-year student career, but this time it is so different. He will comprehensively test my abilities 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 to practice. It is related to whether I can succeed in this challenging society in the future, and it is also the key to building confidence, so my investment in it is also 100%! The intense one-month internship life is over, and I still have a lot to gain during this one-month period. It is necessary to sum up after the internship. First of all, through more than one month's internship and practice, I have learned a lot of practical knowledge. The so-called practice is the only criterion for testing truth.

Passing by the roadside station, I observed the whole construction process closely and learned a lot of practical and concrete construction knowledge, which I seldom contact and pay attention to at school, but it is also very important basic knowledge. For example, the cause and treatment of concrete cracks is a very complicated problem. Let me talk about my views:

1 Causes of cracks

There are many reasons for concrete cracks, mainly the change of temperature and humidity, brittleness and inhomogeneity of concrete, unreasonable structure, unqualified raw materials (such as alkali-aggregate reaction), template deformation, uneven settlement of foundation and so on. In the process of concrete hardening, cement releases a lot of hydration heat, the internal temperature rises continuously, and the surface produces tensile stress. In the later cooling process, due to the constraint of foundation or old concrete, tensile stress will appear in concrete. The decrease of air temperature will also produce great tensile stress on the concrete surface. When these tensile stresses exceed the crack resistance of concrete, cracks will appear. The internal humidity of many concrete changes little or slowly, but the surface humidity may change greatly or greatly. Such as poor maintenance, sometimes dry and sometimes wet, surface shrinkage and deformation are constrained by internal concrete, which often leads to cracks. Concrete is a brittle material,

The tensile strength is about110 of the compressive strength. The ultimate tensile deformation under short-term loading is only (0.6 ~ 1.0) × 104, and that under long-term loading is only (1.2 ~ 2.0) × 14. In reinforced concrete, tensile stress is mainly borne by steel bars, while concrete only bears compressive stress. If the edge of plain concrete or reinforced concrete has tensile stress, it must be borne by the concrete itself. In general design, no tensile stress or only a small tensile stress is needed. However, when the concrete cools from the highest temperature to a stable temperature during operation, it often produces considerable tensile stress inside the concrete. Sometimes the temperature stress will exceed the stress caused by other external loads, so it is extremely important to master the variation law of temperature stress for reasonable structural design and construction.

2 temperature stress analysis

According to the formation process of temperature stress, it can be divided into the following three stages:

(1) Early stage: Generally, it takes about 30 days from the beginning of concrete pouring to the end of cement exothermic. The two characteristics of this stage are that the cement releases a large amount of hydration heat, and the elastic modulus changes dramatically when setting. Due to the change of elastic modulus, residual stress is formed in concrete during this period.

(2) Mid-term: the period from the end of the exothermic action of cement to the cooling of concrete to a stable temperature. During this period, the temperature stress is mainly caused by the cooling of concrete and the change of external temperature, and these stresses are superimposed with the residual stress formed in the early stage. During this period, the elastic modulus of condensation does not change much.

(3) Late stage: the operation period after the concrete is completely cooled. Temperature stress is mainly caused by the change of outside air temperature, and these stresses are superimposed with the first two residual stresses.

According to the causes of temperature stress, it can be divided into two categories:

(1) autogenous stress: the temperature stress of a structure without any constraint or completely static boundary is generated due to the mutual constraint of the structure itself when the internal temperature is non-linearly distributed. For example, piers have relatively large structural dimensions. When concrete is cooled, the surface temperature is low and the internal temperature is high. Tensile stress appears on the surface and compressive stress appears in the middle.

(2) Constraint stress: All or part of the boundary of the structure is constrained by the outside world and cannot be deformed freely.

The pressure. Such as box girder roof concrete and guardrail concrete.

These two kinds of temperature stress often interact with the stress caused by concrete drying shrinkage. It is a complicated work to accurately analyze the distribution and magnitude of temperature stress according to the known temperature. In most cases, it depends on model test or numerical calculation. The creep of concrete greatly reduces the temperature stress. When calculating the temperature stress, the influence of creep must be considered, and the specific calculation is not repeated here.

3 temperature control and measures to prevent cracks

In order to prevent cracks and reduce temperature stress, we can control the temperature and improve the constraint conditions.

Measures to control the temperature are as follows:

(1) Take measures such as improving aggregate gradation, using hard concrete, mixing mixture, adding air entraining agent or plasticizer to reduce the cement content in concrete;

(2) When mixing concrete, add water or water to cool the crushed stone to reduce the pouring temperature of concrete;

(3) When pouring concrete in hot weather, reduce the pouring thickness and use the pouring layer to dissipate heat;

(4) embedding water pipes in concrete and introducing cold water for cooling;

(5) Set a reasonable time for formwork removal, and conduct surface insulation when the temperature drops suddenly to avoid a sharp temperature gradient on the concrete surface;

(6) The surface of concrete pouring blocks or thin-walled structures exposed for a long time during construction shall be insulated in cold season.

Measures;

Measures to improve the constraint conditions are:

(1) Reasonable jointing and plugging;

(2) Avoid excessive fluctuation of foundation;

(3) Reasonable arrangement of construction procedures to avoid excessive height difference and long-term exposure of the side;

In addition, it is very important to improve the performance of concrete, improve crack resistance, strengthen maintenance, prevent surface drying shrinkage, and especially ensure the quality of concrete. Special attention should be paid to avoid through cracks, which are difficult to restore structural integrity. Therefore, priority should be given to preventing through cracks in construction. In concrete construction, in order to improve the turnover rate of formwork, it is often required that the newly poured concrete be removed as soon as possible. When the concrete temperature is higher than the air temperature, the time of formwork removal should be properly considered to avoid early cracks on the concrete surface. Dismantling the formwork in the early stage of new pouring causes great surface tensile stress and "temperature shock" phenomenon. At the initial stage of concrete pouring, due to the hydration heat, the surface causes considerable tensile stress, and the surface temperature is also higher than the air temperature. At this time, when the formwork is removed, the surface temperature drops sharply, which will inevitably lead to a temperature gradient, so

Adding a tensile stress to the surface, which is superimposed with the thermal stress of hydration, causes the concrete to dry and shrink, and the tensile stress on the surface reaches a great value, which may cause cracks. However, if the surface is covered with light thermal insulation materials, such as foam sponge, in time after formwork removal, it will have obvious effect on preventing excessive tensile stress on the concrete surface. Steel bars have little effect on the temperature stress of mass concrete, because the reinforcement ratio of mass concrete is extremely low. It only affects ordinary reinforced concrete. When the temperature is not too high and the stress is lower than the yield limit, the properties of steel are stable and are not affected by stress state, time and temperature. The difference between the linear expansion coefficient of steel bar and the linear expansion coefficient of concrete is very small, and only a small internal stress is generated between them when the temperature changes. Because the elastic modulus of steel is 7~ 15 times that of concrete, when the stress of internal concrete reaches the tensile strength and cracks occur,

The stress of steel bars will not exceed 100 ~ 200 kg/cm2 ... Therefore, it is difficult to use steel bars to prevent tiny cracks in concrete. However, after reinforcement, the number of cracks in the structure generally increases, the spacing becomes smaller, and the width and depth become smaller. Moreover, if the steel bars are thin in diameter and closely spaced, the effect of improving the crack resistance of concrete is better. Thin and shallow cracks often appear on the surface of concrete and reinforced concrete structures, among which

Most of them belong to shrinkage cracks. Although this kind of crack is generally shallow, it still has some influence on the strength and durability of the structure.

In order to ensure the quality of concrete engineering, prevent cracking and improve the durability of concrete, the correct use of additives is also one of the measures to reduce cracking. For example, the use of water reducing agent and anti-cracking agent, the author summarizes its main functions in practice as follows:

(1) There are a lot of capillary pipes in concrete. After water evaporates, capillary tension is generated in the capillary, which makes the concrete shrink and deform. Increasing the pore size of capillary can reduce the surface tension of capillary, but it will reduce the strength of concrete. This theory of surface tension was recognized internationally as early as 1960s.

(2) The water-cement ratio is an important factor affecting the shrinkage of concrete, and the water consumption of concrete can be reduced by 25% by using water reducing and crack preventing agent.

(3) Cement dosage is also an important factor of concrete shrinkage. Under the condition of maintaining the strength of concrete, the dosage of cement can be reduced by 65,438+05%, and its volume can be supplemented by increasing the dosage of aggregate.

(4) Water reducing and anti-cracking agent can improve the consistency of cement slurry, reduce concrete bleeding and reduce shrinkage and deformation.

(5) Improve the cohesive force between cement slurry and aggregate, and improve the crack resistance of concrete.

(6) Concrete is restrained to produce tensile stress when it shrinks, and cracks will occur when the tensile stress is greater than the tensile strength of concrete. Water-reducing and anti-cracking agent can effectively improve the tensile strength of concrete and greatly improve the tensile strength of concrete.

Crack resistance.

(7) Adding admixture can make concrete compact, effectively improve the carbonation resistance of concrete and reduce carbonation shrinkage.

(8) The retarding time of concrete mixed with water reducing and anti-cracking agent is appropriate, which can effectively prevent the rapid hydration and heat release of cement and avoid the increase of plastic shrinkage caused by long-term non-coagulation of cement.

(9) Concrete mixed with admixture has good workability, and its surface is easy to level off, forming a micro-membrane, reducing water evaporation and drying shrinkage. Many additives have the functions of retarding setting, increasing workability and improving plasticity. We should do more experimental comparison and research in this field in engineering practice, which may be simpler and more economical than simply improving external conditions.

4 Early curing of concrete

Practice has proved that the common cracks in concrete are mostly surface cracks with different depths, mainly due to the sudden drop of temperature in cold areas caused by temperature gradient, which is easy to form cracks. Therefore, the thermal insulation of concrete is particularly important to prevent early cracks on the surface.

From the point of view of temperature stress, thermal insulation shall meet the following requirements:

1) to prevent concrete temperature difference between inside and outside and concrete surface gradient, to prevent surface cracks.

2) In order to prevent concrete from freezing, the lowest temperature of concrete during construction should be not lower than the lowest temperature of concrete.

Stable service life and temperature.

3) Prevent the old concrete from supercooling, so as to reduce the constraint between new and old concrete. The main purpose of early curing of concrete is to maintain suitable temperature and humidity conditions, so as to achieve two effects. On the one hand, it can protect concrete from adverse temperature and humidity deformation and prevent harmful cold shrinkage and dry shrinkage. A party

The surface makes the cement hydrate smoothly to achieve the design strength and crack resistance. Appropriate temperature and humidity conditions are interrelated. Thermal insulation measures often have a moisturizing effect on blood coagulation. Theoretically, the water content of fresh concrete can completely meet the requirements of cement hydration. However, due to evaporation and other reasons, it often causes water loss, thus delaying or preventing the hydration of cement, and the surface concrete is most easily and directly affected by this adverse effect. Therefore, the first few days after concrete pouring is the key period of maintenance, and more attention should be paid in construction. Therefore, in the process of construction, we should handle these incidents carefully and deal with them according to different situations. Attention should be paid to these problems in construction, such as what kind of cement and its dosage, and early maintenance of concrete.

And template design:

(1) construction preparation

1. Basic work before formwork installation: 1) Setting-out: First, measure the axes of the side columns and walls of the building, and lead out all the articles from this axis.

Axis. During template setting-out, the center line and edge line of the template shall be popped up with chalk line according to the construction drawing, and the edge line and outer control line of the template shall be popped up for the wall template, so as to facilitate the installation and correction of the template.

2) According to the requirements of actual elevation, directly measure the horizontal elevation of the building to the installation position of the formwork with a level.

3) The bottom of the formwork shall be leveled in advance, and sundries shall be cleaned to ensure the correct position of the formwork, so as to prevent slurry leakage or root rot at the bottom of the formwork after concrete molding.

4) The foreman determines the assembly design scheme of the template in advance, and makes technical, quality and safety disclosure to the construction team.

5) Template shall be coated with release agent. There are many matters needing attention, so I won't list them here.

I firmly believe that through this period of internship, the practical experience gained will benefit me for life, and will be constantly verified in the actual work after graduation. I will continue to understand and appreciate what I have learned in my internship, and apply my theoretical knowledge and practical experience to my future practical work to fully demonstrate my personal value and life value. Strive to realize my ideal and bright future.