Construction practice, this is a good opportunity for me to understand the construction site and let me know more.
The difference between theory and reality.
1. Project introduction
This project is a single apartment building developed by a happy state-owned enterprise in East Lake, Wuhan. The contractor is Wu.
The fourth construction enterprises in Han Dynasty were Building No.5 and Building No.6, Golf Fitness Building and Foundation No.4 respectively.
Pile foundation enterprises undertake construction. Design by Beijing Weston Design Institute. Frame shear wall structure is adopted, and the columns are of opposite sex.
Column. A commercial and residential building, with an area of 65,438+03,000 square meters, consists of three buildings with six-story cast-in-place reinforced concrete frame-shear structure.
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 regulations for template cleaning, stacking and maintenance;
2. Reinforcement worker
1) Types and appearance characteristics of reinforcement;
2) Welding methods and quality regulations of steel bars;
3) Methods and techniques of cold working of steel bars;
4) Binding methods and quality regulations of steel bars;
5) the provisions of the steel binding lap length;
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) Specific maintenance methods and regulations;
6) Causes and preventive measures of concrete surface defects;
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 been a student for more than ten years.
Ya has also experienced many internships, but this time it is so different. He will comprehensively test each of us.
Local ability: study, life, psychology, body, thought, etc. Like a touchstone, check
Test whether I can apply the theoretical knowledge I have learned to practice. It is related to whether I can stand here smoothly in the future.
A challenging society is also the key to building my confidence, and so is my investment in it.
100%! A month of intense internship is over, and I still have some questions for more than a month.
The harvest is very small. It is necessary to sum up after the internship. First of all, through more than one month's internship,
I learned a lot of practical knowledge through practice. The so-called practice is the only criterion for testing truth.
After passing the side station, I observed the whole construction process at close range and learned a lot of applicable tools.
Knowledge of body structure, which I seldom touch and pay attention to at school, is ten.
Important basic knowledge.
For example, the cause and treatment of concrete cracks is a very complicated problem, so I will talk about mine.
Viewpoint:
1 crack
There are many reasons for concrete cracks, mainly the change of temperature and humidity, the brittleness of concrete and the shortage of concrete.
Uniformity, unreasonable structure, raw materials (such as alkali aggregate reaction), template deformation, foundation
Uneven settlement and so on.
During the hardening process of concrete, most of the hydration heat of cement is released, and the internal temperature keeps rising, resulting in tensile stress on the surface.
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. If it is not well maintained, sometimes it is dry and sometimes it is wet, and the surface shrinkage deformation is constrained by the internal concrete, the concrete that often leads to cracking is brittle material, the tensile strength is about110, and the ultimate tensile deformation under short-term load is only (0.6 ~ 1.0) × 104. The ultimate elongation deformation under long-term load is only (1.2 ~ 2.0) × 104. Due to uneven raw materials, unstable water-cement ratio and segregation during transportation and pouring, the tensile strength of the same concrete is uneven, and there are many weak parts with low tensile capacity, which are prone to cracks. 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, there is no tensile stress or only a small tensile stress. However, when the concrete is cooled from the highest temperature to a stable temperature during operation, it often produces great 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 completion of cement exothermic. Two characteristics of this stage, one is that most of the hydration heat is released by cement, and the other is the sharp change of elastic modulus on coagulation. Due to the change of elastic modulus, residual stress is formed in concrete during this period.
(2) Mid-term: from the time when the exothermic effect of cement is basically completed to the time when concrete cools 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 which 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. Such as piers, structural scales
When the concrete is cooled, the surface temperature is low and the internal temperature is high, and the surface tensile stress appears.
Occasionally, compressive stress occurs.
(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 stresses often work together 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 causes temperature stress.
Very large relaxation, the influence of creep must be considered when calculating the temperature stress, and the specific calculation is not here.
Detailed description.
3 temperature control and measures to prevent cracks
In order to prevent cracks and reduce temperature stress, we can start with controlling temperature and improving constraint conditions.
Measures to control the temperature are as follows:
(1) Improve aggregate gradation, use hard concrete, mix mixture, add air entraining agent or plasticizer, etc.
Measures to reduce the amount of cement 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) Provide a reasonable time for form removal, and conduct surface insulation when the temperature drops suddenly to avoid freezing of the concrete surface.
Sharp temperature gradient;
(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 deployment of construction procedures to avoid excessive height difference and long-term exposure of the side;
In addition, improve the performance of concrete, improve crack resistance, strengthen maintenance, and prevent surface drying shrinkage, especially
It is very important to ensure the quality of concrete and prevent cracks, and special attention should be paid to avoid through cracks and cracks.
At present, it is difficult to restore its structural integrity, so measures should be taken to prevent through cracks in construction.
What happens is the main thing.
In concrete construction, in order to improve the turnover rate of formwork, it is often stipulated that the newly poured concrete should be removed as soon as possible.
Go to hell. When the concrete temperature is higher than the air temperature, the time of formwork removal should be properly considered to avoid premature surface of concrete.
Periodic cracks. Dismantling the formwork in the early stage of new pouring causes great surface tensile stress and "temperature shock" phenomenon.
At the early stage of concrete pouring, due to the hydration heat, the surface produces great tensile stress. At this time, the surface
The temperature is also higher than the air temperature. At this point, when the template is removed, the surface temperature suddenly drops, which will inevitably cause a temperature gradient, so
Adding a tensile stress to the surface, which is superimposed with the hydration thermal stress, causes the concrete to dry and shrink, and the surface tensile stress reaches a large value, which may lead to cracks, but if the surface is covered in time after formwork removal.
Lightweight thermal insulation materials, such as foam sponge, are very important to prevent excessive tensile stress on the concrete surface.
Have a significant effect.
Steel bars have little effect on the temperature stress of mass concrete, because the reinforcement ratio of mass concrete is extremely low. IEt wuyou training network
It only affects ordinary reinforced concrete. Under the condition that 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. Linear expansion coefficient and mixing of steel
The difference of linear expansion coefficient of concrete is very small, and only a small internal stress is generated between them when the temperature changes. Due to steel
The elastic modulus is 7~ 15 times that of concrete. When the stress of internal concrete reaches the tensile strength and cracks appear,
The stress of steel bars will not exceed 100 ~ 200 kg/cm2 ... so steel bars should be used to prevent it in concrete.
The appearance of fine cracks is very difficult. However, after reinforcement, the number of cracks in the structure generally increases and the spacing becomes smaller.
The width and depth are smaller. Moreover, if the steel bars are thin in diameter and closely spaced, the crack resistance of concrete will be improved.
The effect 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 is harmful to the strength and durability of the structure.
It still has some influence.
In order to ensure the quality of concrete engineering, prevent cracking and improve the durability of concrete, it is also necessary to use additives correctly.
This is 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 most capillary pipes in concrete. After water evaporates, capillary tension is generated in capillaries, which leads to blood coagulation.
Soil shrinkage and deformation. 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 use of water reducing agent and anti-cracking agent can reduce the water consumption of concrete.
Reduce by 25%.
(3) The amount of cement is also an important factor of concrete shrinkage, and the curing of concrete mixed with water reducer and anti-cracking agent.
Under the condition of concrete strength, the amount of cement can be reduced by 65438 05%, and its volume can be supplemented by increasing the amount 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) When the concrete shrinks, it is restrained to produce tensile stress, and when the tensile stress is greater than the tensile strength of the concrete, it produces cracks.
A seam will be created. 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 admixtures can make concrete have good compactness, effectively improve the carbonation resistance of concrete and reduce.
Carbonization shrinkage.
(8) Appropriate retarding time of concrete after adding water reducing agent and anti-cracking agent can effectively prevent the rapid hydration and heat release of cement.
In fact, the increase of plastic shrinkage caused by long-term non-coagulation of cement is avoided.
(9) The concrete mixed with admixture has good workability, and the surface is easy to level off, forming a micro-membrane, reducing water evaporation, reducing
Drying shrinkage.
Many additives have the functions of retarding setting, increasing workability and improving plasticity, so more efforts should be made in engineering practice.
It may be simpler and more economical to compare and study experiments in this place than to simply improve external conditions.
4 Early curing of concrete
Practice has proved that the most common cracks in concrete are surface cracks with different depths, and the main reason is temperature.
The sudden drop of temperature in cold areas caused by temperature gradient is also easy to form cracks. Therefore, the thermal insulation of concrete is very important to prevent the surface.
Early surface cracks are particularly important.
From the point of view of temperature stress, thermal insulation should meet the following requirements: iEt Worry-Free Research Network
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 no lower than the stable temperature of concrete during use.
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 functions. One is to protect concrete from unfavorable temperature and humidity deformation and prevent harmful cold shrinkage and dry shrinkage. In one place, the hydration of cement goes smoothly to achieve the designed 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.
These problems should be paid attention to in the construction, and what kind of cement and dosage should be paid attention to in the construction.
Yes, and early curing of concrete.
And template design:
(1) construction preparation
1. Basic work before template installation:
1) Setting-out: First, measure the axes of the side columns and walls of the building, and take this axis as the starting point to lead out all the lines.
Axis. When setting out the formwork, the centerline and sideline of the formwork will be popped up with chalk line according to the construction drawing, and the wall formwork will be elastic.
Release the template sideline and outer control line to facilitate template installation and correction.
2) According to the provisions of the actual elevation, the horizontal elevation of the building will be directly measured on the formwork installation with a level.
Location.
3) The bottom of the template should be leveled in advance, and the sundries should be cleaned up to ensure the template position is correct.
After concrete molding, the bottom of the formwork leaks slurry or roots rot.
4) The foreman determines the assembly design scheme of the template in advance, and implements the technology, quality and safety of the construction team.
Full disclosure.
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 the practical experience gained through this period of internship will benefit me for life.
After that, I will continue to be verified in real work, and I will continue to understand and appreciate the knowledge I have learned in my internship.
In the future work, I will continue to apply the theoretical knowledge and practical experience I have learned to practical work.
Fully display your personal value and life value. Strive to realize my ideal and bright future.