Question 2: Causes of concrete floor cracks ... There are many reasons for concrete cracks, but they can be summarized as follows:
(1) concrete shrinkage
Shrinkage is a main characteristic of concrete, which has a great influence on the performance of concrete. Once the microcracks caused by shrinkage develop, they may cause cracking, deformation and even destruction of the structure.
⑵ Temperature stress
The cement in concrete releases a lot of heat in the hydration reaction, which heats the concrete and forms a certain temperature difference with the outside air temperature, thus generating temperature stress. Its size is related to the temperature difference, and directly affects the concrete cracking and crack width. (3) insufficient reinforcement
From practice, it is observed that there are more cracks in concrete structures with large reinforcement spacing and small reinforcement ratio, and there are more cracks in unreinforced concrete than in reinforced concrete.
(4) Concrete materials and mix proportion
Improper mix design directly affects the tensile strength of concrete and is the cause of concrete cracking. Improper mixing ratio refers to excessive cement dosage, high water-cement ratio, improper sand content, poor aggregate type, improper selection of additives, etc. These factors are interrelated.
Relevant test data show that when the water consumption is constant, the shrinkage of concrete increases by 5% for every increase of cement consumption 10%; When the dosage of cement is constant, the strength of concrete decreases by 20% and the bonding force between concrete and reinforcement decreases by 10% with each increase of water consumption.
5] Curing conditions
Maintenance is an important means to make concrete harden normally. Curing conditions have an important influence on the appearance of cracks. Under the standard curing conditions, the concrete will harden normally and will not crack, but it is only suitable for making test blocks or prefabricated members in factories, and it is impossible for on-site construction. However, it must be noted that the closer the field concrete curing is to the standard conditions, the less likely the concrete will crack.
[6] Construction quality
In concrete pouring construction, uneven vibration, vibration leakage or excessive vibration will cause concrete segregation and poor compactness, which will reduce the overall strength of the structure. When the air bubbles in concrete cannot be completely eliminated, air bubble cracks will appear on the surface of steel bars, which will reduce the bonding force between concrete and steel bars. If the steel bar is subjected to excessive vibration, the cement slurry will be dense around the steel bar, which will also greatly reduce the bonding force.
These factors will make concrete shrink greatly, which will lead to the rapid expansion of micro-cracks in concrete and the formation of macro-cracks.
Question 3: Causes and treatment measures of building cracks.
1. Set temperature expansion joints to prevent vertical cracks in the wall. When the total length of the building is more than 50m, expansion joints should be set at places where temperature change and material shrinkage may cause stress concentration and wall cracking is likely. For the cast-in-place reinforced concrete roof, a concrete post-pouring belt shall be set every 15 ~ 18m.
2.
Reasonable setting of settlement joint. Settlement joints should be set in the complex parts of the building, especially in the parts with large height difference. The settlement joint should be separated from the foundation, the joint width should not be less than 10cm, and it should be three joints and one joint. For the settlement cracks that have already appeared
Cracks, when the settlement develops slowly and tends to weaken, repair the wall after the cracks stabilize; When the settlement develops rapidly and tends to accelerate, temporary support measures should be taken immediately to reduce the foundation load and repair the wall after strengthening the foundation.
Body. Using 3: 7 lime-soil cushion to prevent frost heaving cracks. 3 ∶7 lime-soil has high density, low water content and good elasticity, and it is not easy to cause frost heaving, so it is best to choose the cushion under the foundation.
3: 7 lime soil cushion.
3. Actively prevent structural cracks. For the wall with cracks due to large load, small masonry section size and insufficient bearing capacity, it can be appropriately increased without destroying the main facade.
Large section size to improve its bearing capacity; For the wall with cracks due to excessive load and low masonry strength, the method of reducing the dead weight and service load of the upper structure can be adopted, or reinforced concrete can be added to the top masonry.
Concrete beam, bearing the upper load.
4. Strictly control the concrete construction mixture ratio. According to the concrete strength grade and quality inspection requirements and the workability of concrete, the mix proportion is determined. Strictly control the use of water ash and cement
Quantity. Choose well-graded stone, reduce void ratio and sand ratio, so as to reduce shrinkage and improve concrete crack resistance. Prevent dry cement from being coated on the concrete surface. After the concrete floor is poured, the surface scraping should be limited to a minimum.
And to some extent, the early curing of concrete has been strengthened. After the floor slab pouring is completed, the floor slab surface should be covered with materials in time, insulated and carefully maintained to prevent strong wind and hot sun exposure. Strengthen the repair of floor cracks. When cracks appear in the floor, the area is relatively large.
When it is large, carry out static load test on the floor to check its structural safety, and add a layer of steel mesh on the floor when necessary to improve the integrity of the floor; When there is a long and penetrating dangerous structural crack, the crack width is greater than
0.3mm, reinforced with structural adhesive flat steel, and joints between plates are filled with high-pressure glue.
Types of common cracks in building construction
There are two main types of common cracks in building construction. First, according to its different shapes, it can be divided into oblique cracks, horizontal cracks and vertical cracks; Second, according to different parts, it can be divided into wall cracks and floor cracks.
There are four main reasons for wall cracks:
one
It is the temperature difference deformation that causes the cracks in the wall. General materials have the characteristics of thermal expansion and cold contraction. Reinforced concrete roof slab and wall material are two different materials. When the building structure is deformed due to the change of environmental temperature, steel
Reinforced concrete roof, floor slab, ring beam and adjacent brick wall have different thermal expansion and cold contraction, and there is great temperature deformation. The distribution of this deformation difference is small in the middle and large at both ends. Due to the mutual restriction of deformation difference, temperature stress will inevitably occur.
Destroy the building structure.
Second, the uneven settlement of foundation leads to wall cracking. In the process of building foundation leveling, it is generally necessary to go through the process of high excavation and low filling. So there will be different degrees of land after the construction is completed.
Foundation settlement. If the foundation settlement is uneven, the large settlement part and the small settlement part will produce relative displacement, which will cause shear and tension on the wall. When this additional internal force exceeds the tensile and shear strength of the wall itself, it will occur.
Cracks occur, and these cracks will increase with the increase of uneven settlement of foundation.
Third, the frost heaving of foundation soil and the frost heaving of roof parapet cause wall cracks. When the temperature drops below 0℃, the water on the surface of the formation
Minutes to freeze; However, when the temperature of the upper layer of local foundation soil drops below 0℃, the water in the frost heave soil begins to freeze, and the water in the lower layer keeps pouring into the upper layer under the action of capillaries, and the upper layer keeps freezing to form ice crystals.
However, the frost heave stress caused by expansion uplift can be as high as 2×1.06mp.
Answer: Because of the different water content of the foundation, the frost heave stress formed by different foundations is also different. Similar uneven settlement of foundation will cause wall cracking. In addition, the roof drainage is unfavorable, leakage occurs, and the parapet coping cracks.
And so on will also cause the wall to crack. Fourth, the structural defects of houses can also lead to cracks in the walls. Due to the omission in structural load calculation and unreasonable structure, the unreasonable structure itself causes cracks in the wall; Wall > >
Question 4: What is the problem of large-area cracks in cast-in-place slabs? At present, in reinforced concrete civil buildings, the phenomenon of deformation and cracks in cast-in-place concrete floor slab is more common, which has become a hot issue of commercial housing quality disputes and complaints. Not only does it affect the use function and damage the appearance, but it also destroys the whole structure, reduces its stiffness, causes corrosion of steel bars, and affects durability and strength.
Based on the concrete engineering practice and long-term comparative observation in the laboratory, this paper discusses the causes of cracks in cast-in-place concrete floor slab and the construction control measures. ! The reason of concrete floor crack! .
1. 1 material factors
(1) cement varieties. The shrinkage value of different kinds of cement depends on the content of C3A, SO3, gypsum and the fineness of cement. Generally speaking, the cement with high C3A content and fine fineness has large shrinkage. Cement with insufficient gypsum content has great shrinkage, while SO3 content has significant influence on soil shrinkage in the mixed zone.
(2) Diversity of mixed materials. Its type, content and specific surface area are the main factors affecting the drying shrinkage of cement. The specific surface area of fly ash is the smallest, and the drying shrinkage of concrete decreases with the increase of fly ash content.
(3) Aggregate varieties. The shrinkage of concrete decreases with the whitening of aggregate content and the increase of elastic modulus of aggregate, and increases with the increase of clay content in aggregate.
(4) Concrete mix proportion. Under the condition of certain raw materials, the mixing ratio of concrete has great influence on shrinkage, including unit water consumption, unit cement consumption, water cement ratio, sand ratio and mortar ratio.
The shrinkage of concrete mainly depends on the unit water consumption and cement consumption, and the influence of water consumption is greater than that of cement consumption. Under a certain water consumption, the shrinkage of concrete increases with the increase of cement consumption, and the area increases slightly; At a certain water-cement ratio, the shrinkage of concrete increases obviously with the increase of water-cement ratio; Under the same mix proportion, the drying shrinkage of concrete increases with the increase of sand ratio, but the increase is small.
(5) Types and dosage factors of additives. Adding chemical additives will increase the shrinkage of concrete to varying degrees. Adding water reducer is used to improve the workability of concrete. When the slump increases, the shrinkage of concrete mixed with water reducer is slightly larger than that of concrete without water reducer. When water reducer is used to reduce water, improve strength or save cement, the shrinkage of concrete mixed with water reducer is close to or smaller than that of concrete without water reducer.
The shrinkage of concrete mixed with calcium chloride early strength agent is obviously higher than that of concrete without calcium chloride, and it increases exponentially with the increase of calcium chloride content. However, the shrinkage of concrete mixed with triethanolamine and sodium chloride is larger than that without triethanolamine, but the increase is smaller than that mixed with calcium chloride early strength agent.
1.2 architectural factors
(l) Preparation and pouring of concrete
(1) The uneven mixing of additives leads to a large loss of additives, which can not give full play to its role. (2) The concrete mixing time is insufficient. 3 thick. The temperature of fine aggregate and mixed water bin is too high, which makes the pouring temperature too high. (4) The mixing and transportation time is too long, which causes the concrete mixture to segregate, secrete water and sink. (5) Pumping concrete, due to high liquidity requirements, the amount of cement and water is too large. ⑤ Unreasonable pouring sequence leads to "cold joint" in construction or improper treatment of construction joints. (3) Too fast pouring speed, insufficient tamping or excessive vibration cause concrete segregation and bleeding, forming a mortar layer with more cement content on the surface. (3) The reinforced concrete is disturbed before the final setting. (3) In the process of concrete pouring, the negative reinforcement of the floor is not well protected, which reduces the effective height of the section. ④ The concrete cover is too thin or the aggregate of the cover is too little.
(2) Formwork construction factors
(1) Due to insufficient stiffness of floor formwork support, poor stiffness of beam-slab support or excessive deflection of formwork, the subsidence deformation of formwork support is too large. (2) During the construction process, excessive vibration will produce multiple instantaneous relative displacements at the parts where the support stiffness changes. (3) The concrete will be dismantled, frozen or vibrated prematurely before hardening. (4) the template slurry leakage, water seepage.
(3) concrete curing factors
(1) The curing is not timely, which leads to premature dehydration in the early stage of concrete curing, resulting in concrete drying up. (2) Insufficient post-curing will aggravate the carbonation of concrete and cause carbonation shrinkage. (3) Early concrete curing is frozen.
(4) After the floor construction is completed, at the initial stage of concrete final setting, construction machines and materials are concentrated, or the next working procedure is constructed too early, resulting in large construction load and vibration and cracks.
1.3 surrounding medium factors
① The lower the relative humidity of air, the greater the shrinkage of concrete. ② The shrinkage of concrete increases with the increase of air temperature. (3) Long-term exposure to wind and sun will also increase the shrinkage of concrete.
Two examples and experimental analysis
2. 1 example of floor crack
Through the observation of a large number of engineering examples, the author found that most of the cracks in the concrete floor are along the board surface. & gt
Question 5: The floor is cracked. Please explain the cause of this crack. This floor is the garage floor, with 50 vertical and horizontal cracks: mainly vertical and horizontal cracks. For example, a large area of cast-in-place reinforced concrete floor of a teaching building has longitudinal and transverse cracks, the width of which varies from 0.1-0.3 mm.
Surface crack: this kind of crack is mainly manifested in the cracks produced in the construction process, which is easy to control and handle. For example, in a project under construction, due to the large slab area, concrete was poured at night, and people were sent to pour it the next morning, but it was poured in front and killed in the back. At noon, turtle cracks appeared on the board, which could be recognized by the naked eye. Second, the cause of concrete floor cracks 1. Influence of concrete composition materials
Influence of cement: The shrinkage value of cement generally depends on C3A, SO3, gypsum content and cement fineness. That is to say, the cement with high C3A content and fine fineness shrinks greatly. Cement with insufficient gypsum content shrinks greatly, while SO3 content has a significant effect on concrete shrinkage.
Influence of aggregate: the shrinkage of concrete decreases with the increase of aggregate content, decreases with the increase of elastic modulus of aggregate, and increases with the increase of clay content in aggregate. In addition, in ready-mixed concrete, unreasonable aggregate gradation is also the main factor causing concrete cracks.
Influence of concrete mixture ratio: including unit water consumption, unit cement consumption, water cement ratio, sand ratio and mortar ratio. The shrinkage of concrete mainly depends on the unit water consumption and cement consumption, and the influence of water consumption is greater than that of cement consumption. Under a certain water consumption, the drying shrinkage of concrete increases with the increase of cement consumption, but the increase rate is small; Under a certain lime-ash ratio, the drying shrinkage of concrete increases obviously with the increase of water-cement ratio; Under the same mix proportion, the drying shrinkage of concrete increases with the increase of sand ratio, but the increase is small.
Influence of types and contents of admixtures: Adding chemical admixtures will increase the shrinkage of concrete to varying degrees. Adding water reducer is used to improve the workability of concrete. When the slump increases, the shrinkage of concrete with water reducer is slightly larger than that of concrete without water reducer. When water reducer is used to reduce water, improve strength or save cement, the shrinkage of concrete mixed with water reducer is close to or smaller than that of concrete without water reducer.
Construction reason
The change of water-cement ratio has obvious influence on the strength of concrete, which is basically the superposition of the changes of water and cement. Therefore, the measurement changes of water, cement and additives will directly affect the strength of concrete. For large fluidity concrete, its plastic shrinkage value is 200×l0-4, and for medium fluidity concrete, its plastic shrinkage value is about× l0-4.
Question 6: Preventive measures for floor cracks. Prevention of concrete cracks
After the completion of concrete pouring, the early tensile strength is low, and it will swell and shrink due to the influence of humidity, chemical reaction and temperature. At this time, cracks often appear in slurry concrete. Therefore, to eliminate cracks, we must start with eliminating or reducing the shrinkage of concrete.
The drying shrinkage of cement concrete is affected by the following main factors: unit water consumption of concrete, aggregate composition and duration of initial wet curing. Other factors such as stone particle size, mixture ratio and slurry content of the mixture also affect the drying shrinkage, and the total water demand of the mixture is the main factor affecting the drying shrinkage. The variety and fineness of cement also have a certain influence on the drying shrinkage of concrete. The drying shrinkage of cement with pozzolan as admixture varies with the type of pozzolan. Some pozzolans increase drying shrinkage, while others decrease drying shrinkage. Pozzolan, which increases cement water demand, usually increases drying shrinkage. Fly ash can generally reduce drying shrinkage. Cement is fine in fineness and usually shrinks greatly. In order to reduce the drying shrinkage of cement concrete and avoid cracks, the following measures can be taken:
1. Reduce the water consumption of concrete; 2. Reduce the content of stones appropriately and choose the types of stones; The shrinkage of different stones is different, and the order of shrinkage is sandstone > gravel > basalt > limestone > granite > quartzite.
3. Don't use too fine sand and sand with too much mud;
4. Appropriate extension of moisture curing time;
5. Cut off or accurately set deformation joints in time;
6. Steam curing can reduce shrinkage cracks of concrete.
7. Careful selection of the variety and fineness of cement will affect the drying shrinkage of concrete to some extent;
8. Appropriate addition of concrete admixture;
9. Avoid sun exposure;
10. Pre-shrink before vibrating or secondary vibrating;
1 1. Seal water with film material with good sealing performance.
Question 7: What are the concrete cracks caused by construction? What is the specific reason? Architecture:
Excessive vibration in concrete construction, too dry formwork and cushion. After the concrete is poured and vibrated, the coarse aggregate sinks, squeezing out water and air, and the surface seepage water forms vertical volume shrinkage and sinking, resulting in a surface mortar layer, which has greater drying shrinkage performance than the lower concrete, and it is easy to form shrinkage cracks after the water evaporates. However, when the formwork and cushion are poured, the water is insufficient and too dry, and the formwork will absorb a lot of water, resulting in plastic shrinkage of concrete and cracks. After concrete is cast and compacted, too much plastering and calendering will make the fine aggregate of concrete float to the surface too much, forming a cement slurry layer with high water content. Calcium hydroxide in cement slurry reacts with carbon dioxide in the air to produce calcium carbonate, which will shrink the surface volume and lead to cracks on the surface of concrete slab. In the construction process, due to improper construction technology, the negative reinforcement at the bearing sinks, the protective layer is too large, the fixed bearing becomes a plastic hinge bearing, and the upper part of the plate cracks along the beam bearing; During construction, when the concrete fails to reach the specified strength, the formwork is removed prematurely, or the concrete is loaded before the final setting time, which leads to the elastic deformation of the concrete floor. When the concrete has low or no early strength, it bears bending, compression and tensile stresses, resulting in internal injuries or fractures of the floor. The uneven settlement of the floor slab on both sides of the beam will also cause the negative bending moment of the bearing and produce transverse cracks. Improper field maintenance is the main reason of concrete shrinkage and cracking. If the concrete pouring surface is not covered and watered in time, the surface moisture will evaporate quickly and shrinkage cracks will easily occur. Especially in the case of high temperature, low relative humidity and high wind speed, drying shrinkage is more likely to occur. At present, when pouring concrete in many construction sites, it is impossible to cover and maintain in time. Generally, it will not be covered until the final finishing, and many construction sites are not covered at all. Especially in summer, the temperature is very high, the water in concrete evaporates quickly, there are not enough construction workers, and the poured concrete is exposed to the scorching sun. As a result, the concrete was poured first and then cracked. The construction party only sends someone to water it every few hours. So cracks will inevitably appear.
In addition, concrete is a shrinkable material. Although the absolute value of shrinkage is not large, even a small shrinkage deformation will produce great tensile stress because of its high elastic modulus and low tensile strength. When the tensile stress exceeds its tensile strength, concrete will crack. Therefore, what we have to do is to minimize the shrinkage value of concrete and maximize the tensile strength of concrete. First, try to use less shrinkage cement, such as slag cement. The shrinkage of slag portland cement is about 25% larger than that of ordinary portland cement. Second, under the condition of satisfying the workability of construction, the water-cement ratio of concrete should be reduced as much as possible, and the dosage of cement slurry and mortar per unit volume should be reduced as much as possible. As we all know, the greater the water-cement ratio of concrete, the greater the shrinkage and the lower the tensile strength. What needs special attention is the fineness of cement and the mud content of stone. The finer the fineness of cement, the easier it is for concrete to crack. This is because: (1) the cement with large fineness is hydrated quickly and consumes a lot of water, which is easy to cause self-drying shrinkage of concrete. (2) Fine cement will make capillaries thinner, and the thinner capillaries will produce greater tension when they lose water. (3) Fine particles are easy to fully hydrate, resulting in more hydrates which are easy to dry and shrink, such as gels. The reduction of coarse particles reduces the stable volume of unhydrated particles, thus affecting the long-term performance of concrete. The higher the mud content of stones, the easier it is for concrete to crack. This is because the mud on the stone surface hinders the occlusal combination of stone and mortar, weakens the interface structure of stone, reduces the interface strength, and also reduces the strength of concrete, especially the tensile strength. Therefore, under the same shrinkage strain, concrete with high mud content in stone is more likely to crack. This article is taken from China Architectural Digest. Please indicate the source website in the form of a link: 863p/...9.
Question 8: Is floor cracking a quality problem? Housing quality problems are generally divided into main structural problems and non-main structural problems. As long as it is the main structure problem, it refers to the problems existing in the pillars and girders of the house. If there are serious quality problems in the main structure, it will affect the living safety of the house. The floor cracks you mentioned should belong to non-main structural problems, and generally will not affect the living safety. As for whether it is a qualified commercial housing, it should be determined by the housing quality inspection department.
Question 9: What about cracks in concrete? What caused it? Tianyu shield technicians can tell you from a professional point of view that there are many reasons for cracks, which are generally caused by thermal expansion and cold contraction. This kind of crack usually takes years to appear. However, if it happens within two years, it is due to the mix of materials, inadequate maintenance, premature formwork removal, foundation sinking, the use of sea sand (many houses in Hainan Province are shocking), the use of slimming steel bars (many commercial houses), the impact of external forces on the floor (lightning can also cause cracks and leaks in the floor), leading to rain leakage.
As long as there is no water leakage, the problem of cracking is not serious. In case of water leakage, Tianyu Shield reminds you to find professional materials and never use black ones. These are all used by unscrupulous merchants to cheat money. They have a characteristic, large quantity, which is often used by van guerrillas, that is, they are poured with fire or boiling. This therapy is ineffective.
Question 10: What is the reason for the cracks in the freshly poured floor concrete? This is an early plastic crack in concrete. In practical engineering, concrete cracks caused by shrinkage are the most common. Among the shrinkage types of concrete, plastic shrinkage and shrinkage (dry shrinkage) are the main reasons for the volume deformation of concrete, and autogenous shrinkage and carbonation shrinkage are also the main reasons.
Plastic shrinkage. It happens during the construction and about 4-5 hours after concrete pouring. At this time, the hydration reaction of cement is fierce, molecular chains are gradually formed, exudation and moisture evaporate rapidly, and concrete shrinks due to water loss. At the same time, the aggregate sinks due to its own weight, so the concrete has not hardened, which is called plastic shrinkage. The range of plastic shrinkage is very large, which can reach about 1%. If the aggregate is blocked by steel bars during sinking, cracks will form along the direction of steel bars. At the vertical variable cross-section of members, such as the joint of T-beam and box girder web with top plate and bottom plate, cracks will appear on the surface along the web direction due to uneven settlement before hardening. In order to reduce the plastic shrinkage of concrete, the water-cement ratio should be controlled during construction to avoid too long mixing time, too fast blanking, dense vibrating and vertical variable cross-section layered pouring.
Shrinkage shrinkage (dry shrinkage). After concrete hardens, with the gradual evaporation of surface moisture, the humidity gradually decreases and the volume of concrete decreases, which is called shrinkage (dry shrinkage). Because concrete loses water quickly on the surface and slowly on the inside, uneven shrinkage occurs with large surface shrinkage and small internal shrinkage. Surface shrinkage deformation is constrained by internal concrete, which makes the surface concrete bear tension. When the tensile force of surface concrete exceeds its tensile strength, shrinkage cracks will occur. The shrinkage of concrete after hardening is mainly shrinkage. For members with large reinforcement ratio (greater than 3%), the reinforcement has obvious restraint effect on concrete shrinkage, and cracks are prone to appear on the concrete surface.