Prevention and treatment of cracks in the hottest

Prevention and treatment of cracks in concrete engineering

1.1.1 causes of dry shrinkage cracks

dry shrinkage cracks often occur in a period of time after the completion of concrete curing or about a week after the completion of concrete pouring. Dry shrinkage cracks are mainly caused by different deformation results due to different degrees of water evaporation inside and outside the concrete. Affected by external conditions, the surface moisture loss of concrete is too fast and the deformation is large; The internal humidity changes little and the deformation is small. Large surface shrinkage deformation is constrained by the interior of concrete, resulting in large tensile stress and cracks. The lower the relative humidity is, the faster the drying shrinkage of cement paste is, and the easier the drying shrinkage cracks are. Most of the dry shrinkage cracks are surface parallel linear or shallow fine cracks, and the width is mostly between 0.05~0.2mm. The 863 new materials technology field office organized experts to accept the major project in Beijing. The plane parts of mass concrete are common, and the thinner beams and slabs are mostly distributed along its short direction. The dry shrinkage of concrete is mainly related to the water cement ratio of concrete, the composition of cement, the amount of cement, the nature and amount of aggregate, the amount of admixture, etc

1.1.2 preventive measures for dry shrinkage cracks

cover the protective cover

dry shrinkage cracks usually affect the impermeability of concrete, cause the corrosion of reinforcement, affect the durability of concrete, produce hydraulic splitting under the action of water pressure, affect the bearing capacity of concrete, etc. the main preventive measures are:

(1) select cement with small shrinkage, generally use medium and low heat cement and fly ash cement, and reduce the amount of cement

(2) the dry shrinkage of concrete is greatly affected by the water cement ratio. The greater the water cement ratio, the greater the dry shrinkage. Therefore, in the design of concrete mix proportion, the selection of water cement ratio should be controlled as much as possible, and an appropriate amount of water reducing agent should be added at the same time

(3) strictly control the mix proportion in concrete mixing and construction, and the water consumption of concrete must not be greater than the water consumption given in the mix proportion design

(4) strengthen the early curing of concrete and appropriately extend the curing time of concrete. During winter construction, the concrete insulation coverage time shall be appropriately extended, and the curing agent shall be applied for curing

(5) set appropriate contraction joints in the concrete structure

1.2 causes and prevention of plastic shrinkage cracks

1.2.1 causes of plastic shrinkage cracks

plastic shrinkage refers to the shrinkage caused by rapid water loss on the surface of concrete before setting. Plastic shrinkage cracks usually appear in dry and hot weather or windy weather, and most of the cracks are in a state of discontinuity with width in the middle, thin at both ends, and different lengths. Shorter cracks are generally 20~30cm long, while longer cracks can reach 2~3m and 1~5mm wide. The main reasons for this are: the concrete has almost no strength or very small strength before final setting, or when the concrete is just final setting and the strength is very small, under the influence of high temperature or large wind force, the water loss on the concrete surface is too fast, resulting in a large negative pressure in the capillary, which makes the volume of concrete shrink sharply, and at this time, the strength of the concrete can not resist its own shrinkage, so cracking occurs. The main factors affecting the plastic shrinkage cracking of concrete are water cement ratio, concrete setting time, ambient temperature, wind speed, relative humidity and so on

1.2.2 preventive measures for plastic shrinkage cracks

(1) select Portland or ordinary portland cement with small dry shrinkage value and high early strength

(2) strictly control the water cement ratio, add superplasticizer to increase the slump and workability of concrete, and reduce the amount of cement and water

(3) before pouring concrete, water the base course and formwork evenly and thoroughly

(4) cover plastic film or wet straw mats, hemp chips, etc. in time to keep the surface of concrete moist before final setting, or spray curing agent on the concrete surface for curing

(5) in high temperature and windy weather, sunshade and wind proof facilities should be set up and maintained in time

1.3 causes and prevention of settlement cracks

1.3.1 causes of settlement cracks

the occurrence of settlement cracks is caused by uneven settlement caused by uneven and soft soil of structural foundation, or inaccurate backfill, or immersion; Or it is caused by insufficient formwork stiffness, too large formwork support spacing or loose support bottom. Especially in winter, the formwork is supported on the frozen soil, and the uneven settlement occurs after the frozen soil thaws, resulting in cracks in the concrete structure. Such cracks are mostly deep or penetrating cracks, and their trend is related to the settlement. They generally develop along the direction perpendicular to the ground or at an angle of 30 ° ~45 °. Larger subsidence cracks often have a certain dislocation, and the crack width is often in direct proportion to the settlement. After the foundation deformation is stable, the settlement cracks also tend to be stable. The crack width is less affected by temperature change

1.3.2 preventive measures for settlement cracks

(1) necessary compaction and reinforcement shall be carried out for loose soil and filled foundation before the construction of superstructure

(2) ensure that the formwork has sufficient strength and stiffness, and the support is firm, and the foundation is stressed evenly

(3) prevent the foundation from being soaked by water during concrete pouring

(4) the time of formwork removal should not be too early, and attention should be paid to the order of formwork removal

(5) when erecting formwork on frozen soil, certain preventive measures should be taken

1.4 causes and prevention of temperature cracks

1.4.1 causes of temperature cracks

temperature cracks often occur after concrete pouring. Affected by climate change in the hardening process, due to the large volume of concrete, a large amount of hydration heat accumulates in the concrete and is not easy to be dissipated, resulting in a sharp rise in internal temperature, while the concrete surface dissipates heat faster, forming a large internal and external temperature difference, Make the concrete surface produce a certain tensile stress (practice has proved that when the temperature difference of the concrete itself reaches 25~26 ℃, the tensile stress of about 10MPa will be generated in the concrete). When the tensile stress exceeds the tensile strength limit of concrete, cracks will appear on the concrete surface, which mostly occur in the middle and late stage of concrete construction

the trend of temperature cracks is usually irregular, and large-area structural cracks are often crisscross; For structures with large length and size, cracks are mostly parallel to the short side; Deep and penetrating temperature cracks are generally parallel or nearly parallel to the direction of the short side, and cracks appear in segments along the long side, with dense middle. The width of the crack is different, which is obviously affected by the temperature change. It is wider in winter and narrower in summer. The concrete temperature cracks caused by high temperature expansion are usually thick in the middle and thin at both ends, while the thickness change of cold shrinkage cracks is not obvious. The occurrence of such cracks will cause corrosion of reinforcement, carbonation of concrete, and reduce the freeze-thaw resistance, fatigue resistance and impermeability of concrete

1.4.2 preventive measures for temperature cracks

(1) try to choose low or medium heat cement, such as slag cement, fly ash cement, etc

(2) reduce the cement consumption and try to control the cement consumption below 450kg/m3

(3) reduce the water cement ratio, and generally control the water cement ratio of concrete below 0.6

(4) improve aggregate gradation and add fly ash or superplasticizer to reduce cement consumption and hydration heat

(5) improve the mixing and processing technology of concrete, adopt the new technology of secondary air cooling on the basis of the traditional three cooling technology, and reduce the pouring temperature of concrete

(6) add a certain amount of admixtures with water reducing, plasticizing, retarding and other functions into the concrete to improve the fluidity and water retention of the concrete mixture, reduce the heat of hydration, and delay the occurrence of the thermal peak

(7) when pouring in high temperature season, auxiliary measures such as setting up sunshades can be used to control the temperature rise of concrete and reduce the temperature of pouring concrete

(8) the construction process of mass concrete should be reasonably arranged and poured in layers and blocks to facilitate heat dissipation and reduce constraints

(9) set cooling pipes inside the mass concrete to cool with cold water or air, reduce the internal and external temperature difference of concrete, and reserve temperature contraction joints

(10) before pouring concrete, it is advisable to lay about 5mm sand cushion on bedrock or old concrete or paint with asphalt and other materials to reduce constraints

(11) after concrete pouring, cover it with wet straw curtains, hemp sheets, etc. in time, pay attention to watering and curing, and appropriately extend the curing time to ensure that the concrete surface cools slowly. In cold seasons, thermal insulation measures shall be set on the concrete surface to prevent cold wave attack

(12) a small amount of steel bars or fiber materials are configured in the concrete to control the temperature cracks of the concrete within a certain range

1.5 cracks caused by chemical reaction and prevention

(1) when the concrete is mixed, some alkaline ions will be produced, which will react with some active aggregates and absorb the water in the surrounding environment to increase the volume, resulting in crispness, expansion and cracking of the concrete. This kind of crack usually occurs during the use of concrete structures. Once it occurs, it is difficult to remedy it. Therefore, necessary preventive measures should be taken in the construction: select sand and stone aggregates with low alkali activity, low alkali cement, low alkali or alkali free admixtures, and appropriate admixtures to inhibit the alkali aggregate reaction

(2) due to poor concrete pouring and vibration or thin protective layer of reinforcement, harmful substances enter the concrete and cause corrosion of reinforcement, resulting in concrete expansion and cracking. Most of these cracks are longitudinal cracks, which appear along the position of reinforcement. The usual preventive measures include: ensuring the thickness of reinforcement protective layer, good concrete grading, dense concrete pouring and vibrating, and painting anti-corrosion paint on the surface of reinforcement

2 crack treatment

the occurrence of cracks will cause corrosion of reinforcement, accelerate the carbonation of concrete, and reduce the durability, fatigue resistance and impermeability of concrete. It should be handled in time according to the nature and specific conditions of cracks to ensure the safe use of buildings

2.1 surface repair method

surface repair method is a simple and common repair method, which is mainly applicable to the treatment of surface cracks and deep cracks that have no impact on the bearing capacity of the structure. Generally, measures such as applying cement slurry, Epoxy Mastic on the surface of cracks, painting paint, asphalt and other anti-corrosion materials on the concrete surface, and pasting glass fiber cloth on the surface of cracks are adopted

2.2 grouting and caulking method

grouting method is mainly applicable to the repair of concrete cracks that have an impact on the integrity of the structure or have anti-seepage requirements. It uses pressure equipment to press the cementitious material into the cracks of the concrete, and the cementitious material hardens and forms a whole with the concrete, so as to achieve the purpose of plugging and strengthening. Common cementing materials include cement paste, epoxy resin, methacrylate, polyurethane and other chemical materials

crack sealing method usually chisels along the crack, and fills the groove with plastic or rigid water stop materials, so as to achieve the purpose of sealing the crack with sticky materials such as powder metal or plastic. Commonly used plastic materials include PVC mastic, plastic ointment, butyl rubber, etc; The commonly used rigid water stop material is polymer cement mortar

2.3 structural reinforcement method

when cracks affect the performance of concrete structures, reinforcement method is usually adopted to deal with concrete structures. The commonly used methods are: increasing the cross-sectional area of the concrete structure, wrapping steel sections at the corners of the members, strengthening by prestressing, strengthening by pasting steel plates, strengthening by adding fulcrum, and strengthening by shotcrete

2.4 concrete replacement method