Obviously, the concrete will shrink during the hardening process, why does the concrete feel expande

Theoretically, the concrete will shrink during the hardening process, and cracks will occur if it shrinks, so under normal circumstances, the concrete should limit the shrinkage of the concrete, and add an appropriate amount of expansion agent to cause the concrete to expand slightly.

1. When the concrete is solidified, the cement will produce a large amount of hydration heat, and the hydration heat generated in the middle of the concrete component may cause cracks in the concrete component once it cannot be dissipated. The curing of concrete is to reduce the temperature and reduce the effect of heat of hydration.

2 The main factors affecting the strength of concrete are: whether the mix ratio is reasonably designed, the temperature and humidity when pouring, whether the raw materials are qualified, and whether the stability of cement is qualified.

The shrinkage phenomenon inherent in concrete itself is an important cause of macroscopic cracks or leakage in structures. Theoretically speaking, the addition of bulking agent is an important technical means to solve this problem. However, due to the different types and qualities of expansion agents, concrete mixtures, structural forms, environmental conditions, and other factors, expansion agents sometimes fail to achieve the desired purpose, or even have counterproductive results [1].

If we do not distinguish the use environment of the expansion agent or do not take supporting technical measures in the use of the expansion agent, and hold the idea of "just mixing" and sit and watch the cracking, there will inevitably be various controversies on the application of the expansion agent in the current project. The use of bulking agents is conditional! In particular, modern concrete, which is mainly characterized by the use of high-efficiency superplasticizers and mineral admixtures [2], has made the conditions for the application of expansion agents more complex and demanding due to the differences in the properties of raw materials and the diversification and complexity of the design of mix ratios.

The fundamental reason for the concrete expansion and collision of the free code mold is that the formwork reinforcement and fastening measures are not effective, the stiffness of the mold board itself is insufficient or the formwork is damaged;

Of course, there are many other inducements, such as the impact force caused by the pouring height and angle when pouring concrete.

Too large, too concentrated, formwork support.

The surface is unstable and depressed, and the formwork is impacted during the pouring process, etc.;

Basically, the concrete expansion mold does not have much impact on the integrity and stability of the structure itself, mainly the impact on the appearance, of course, for the engineering parts with strict size requirements, the size deviation due to the expansion mold will also affect the use function.

There are not many specific treatment methods, nothing more than grinding or chiseling and then repairing, generally few because of the expansion mold and the concrete structure is completely chiseled and redone

1. First of all, the concrete of the expansion part to be repaired is chiseled, so that the small hammer is fine chisel to avoid damaging the structural reinforcement. It is required that the surface of the concrete after chiseling can achieve the effect of cutting fake stones, and there are no loose stones and concrete particles.

2. Brush the old fingers of the socks with a brush and rinse them with water to make them free of loose stones and dust.

3. Paint a layer of cement slurry with the same cement for interface treatment on the repair site, so that the old and new concrete can be well combined.

4. After the final setting of concrete, strengthen the sprinkling and careful maintenance to avoid cracking at the junction of the old and new concrete.

Expansion mold part treatment plan: after the template is dismantled, it is found that the concrete has the expansion of the mold, the technical quality personnel should promptly notify the supervision engineer to the site to check, the supervision engineer to check the completion of the operation personnel to the expansion of the mold part of the concrete chisel, the chisel shall not damage the structural reinforcement, after the completion of the chisel, the project department technical personnel inspection qualified, notify the supervision engineer acceptance, after the acceptance of the acceptance of the clear water will be watered and moistened with the chisel part, with the same cement configuration 1:2 1:

The cement mortar is debugged and compared before repairing, and the cement mortar is put into the barrel and stirred evenly after debugging, and the chisel surface is scraped and calendered with a scraper according to the method of scraping putty by the painter, and then cured in accordance with the concrete curing method.

If there is a large area of the expansion mold, affecting the size of the structure, then only smashed and re-poured, if it is only a small part, then it can be appropriately removed and repaired with points.

The treatment method of expansion mold when pouring concrete is:

1. For the two horizontal bars of the shear wall that have been cut, after the concrete is chiseled (that is, after the cross-sectional size meets the design requirements), lap welding is taken for double-sided welding, and the welding length is not less than 10d, and the steel bar is tied firmly.

2. The concrete here is chiseled to slightly smaller than the design section size, so that the re-welded horizontal steel bar and the vertical steel bar can be re-tied firmly, and the surrounding hairs are chiseled to make it fully combined with the concrete of the shear wall.

3. The upper part is slightly chiseled deep, leaving a gap for pouring concrete, which is linear and convenient for concrete pouring.

4. The concrete should be made of fine stone concrete, shrinkage compensating concrete should be used, the grade should be one grade higher than the original design grade, and the cement should be added with micro-expansion agent.

5. After chiseling, the concrete should be poured immediately after the reinforcement is tied and accepted, and the maintenance needs to be timely, and it is forbidden to dismantle the formwork in advance, and the formwork can be dismantled only after reaching the strength, and the curing time is not less than 14 days.

Definition of concrete expansion form:

In concrete construction, due to various reasons such as poor formwork reinforcement and fast concrete pouring speed, formwork deformation and other phenomena often occur, which is called concrete mold expansion.

1. Analysis of the causes of cracks Due to the different composition materials, microstructure and external influences of concrete, there are many reasons for cracks in concrete, mainly the change of temperature and humidity, the brittleness and inhomogeneity of concrete, as well as unreasonable structure, unqualified raw materials, formwork deformation, uneven settlement of foundation, etc. (1) In the process of hardening concrete, cracks are generated when the volume deformation caused by dry shrinkage is constrained, and the width of such cracks is sometimes very large, and even penetrates the entire component. (2) During the hardening of concrete, the cement produces a large amount of hydration heat, and the internal temperature continues to rise, causing tensile stress on the surface of the concrete.

In the later cooling process, due to the constraints of the foundation or the old concrete, tensile stress will occur inside the concrete. The drop in air temperature can also cause large tensile stresses on the surface of the concrete, and cracks occur when these tensile stresses exceed the concrete's ability to resist cracking. (3) In the component with large thickness, because the plastic collapse of the concrete is inhibited by the formwork or the top reinforcement, this crack caused by the plastic collapse of the concrete will occur a few hours after pouring.

4) When there is a constraint, the volume expansion and contraction of the concrete caused by the thermal expansion and contraction of the concrete, because of the limitation of the binding force, the temperature stress is generated inside, and because the tensile strength of the concrete is low, it is easy to be pulled and cracked by the tensile stress caused by temperature, resulting in temperature cracks. Cracks due to sun exposure are also the most common phenomenon in engineering. (5) After the concrete is mixed with water, the alkaline substances in the cement react with the activated silicon oxide in the active aggregate, and the precipitated colloidal alkali-silica gel absorbs water from the surrounding medium and expands, and the volume increases by three times, so that the concrete cracks and cracks.

6) The internal humidity of many concrete changes little or slowly, but the surface humidity may change greatly or change drastically, such as improper maintenance, dry and wet, and the surface dry shrinkage deformation is constrained by the internal concrete body, and cracks are often produced. (7) Cracks caused by component overload, for example: the component produces internal force bending moment under the action of the uniform load or concentrated load that exceeds the design, and the crack perpendicular to the longitudinal axis of the component appears, and the component produces oblique cracks under the action of large shear force, and extends upward and downward.

Reasons: the rigidity of the formwork is not enough, the tie rod and support are not laid, the concrete pouring speed is too fast, and the tamping time is too long. Avoid the phenomenon of bullswelling:

The first is to do a good job in formwork, and the support system and formwork should be checked for some large-volume concrete or special-shaped columns with complex appearances.

Formwork acceptance should be carried out, and now most of them are carried out at the same time as the acceptance of reinforcement, which is actually wrong. Special attention should be paid to the position of the joint, the column base, the root of the beam, and whether the middle of the beam plate is arched.

The construction process should be monitored to strengthen the monitoring, especially when the beam slab concrete is poured, personnel should be arranged to observe the lower layer. Timely reinforcement of formwork and support to avoid expansion of range.

Solution: Unfortunately, it can only be chiseled, but attention should be paid to the time and method of chiseling, and barbaric operation is strictly prohibited.

Rational seam and blocking; Avoid excessive fluctuations in the foundation; Reasonable arrangement of construction procedures to avoid excessive height difference and long-term exposure of the side; Improve the performance of concrete, improve crack resistance, strengthen curing, prevent surface drying, especially to ensure the quality of concrete to prevent cracks is very important, special attention should be paid to avoid through cracks, it is very difficult to restore the integrity of its structure after the occurrence, so the construction should be based on preventing the occurrence of penetrating cracks.

If it is only the concrete expansion mold, then as long as the expansion mold part is chiseled, the chisel surface should be lower than the original concrete pouring surface, and then the chisel surface is cleaned up, moistened with clean water, and smoothed with 1:2 or cement mortar and steel wire mesh!