Why should we control the pouring speed of mass concrete?

Modern architecture often involves mass concrete construction, such as high-rise building foundation, large equipment foundation, water conservancy dam and so on. Its main feature is its large volume, and the minimum size of a general entity is greater than or equal to1m. Its surface coefficient is relatively small, the hydration heat of cement is relatively concentrated, and the internal temperature rises rapidly. When the temperature difference between the inside and outside of concrete is large, it will cause temperature cracks in concrete and affect the safety and normal use of the structure. Therefore, it must be fundamentally analyzed to ensure the construction quality. Thick structure, large amount of concrete, complex engineering conditions (generally underground cast-in-place reinforced concrete structure), high construction technical requirements and high hydration heat of cement (expected to exceed 25 degrees) are easy to cause temperature deformation of the structure. In addition to the minimum cross section and internal and external temperature of mass concrete, the plane size is also limited. Because the plane size is too large, the temperature force generated by the constraint is also large. If the temperature control measures are improper and the temperature stress exceeds the tensile limit of concrete, cracks will easily occur.

Mass concrete is a common problem in building construction. In order to help the construction technicians in the project department understand the crack prevention and temperature control of mass concrete and strengthen the communication in construction technology, I have consulted some related books according to my own knowledge. In the form of question and answer, the article first asks questions, and then answers them with popular language and scientific principles. The answer to the question also pays attention to technical essentials and practice, mainly starting from reality and paying attention to practicality. The problems are often encountered in actual construction, with the purpose of letting the construction technicians of the project department know not only how to control the mass quality, but also why to control the temperature and crack.

If you don't know about mass concrete crack prevention, temperature control and other issues, you can browse with questions to find answers and increase your knowledge, which is believed to be helpful to improve your practical working ability.

1, the definition of mass concrete

Mass concrete refers to the concrete structure with the smallest cross-sectional size greater than 1m, and its size is so large that corresponding technical measures must be taken to properly handle the temperature difference, reasonably solve the temperature stress and control the cracks. Cracks in mass concrete in concrete structures can be divided into three types according to depth: through cracks, deep cracks and surface cracks. Through cracks develop from concrete surface cracks to deep cracks, and finally form through cracks. It cuts off the cross section of the structure, which may destroy the integrity and stability of the structure, and its harm is more serious; However, it is also harmful for deep cracks to cut off the structural section; Surface cracks are generally less harmful. However, the appearance of cracks does not absolutely affect the structural safety, and it has a maximum allowable value. The maximum crack width of general components under normal indoor environment is ≤ 0.3 mm; The maximum crack width of components in outdoor or indoor high humidity environment is ≤ 0.2 mm For underground or semi-underground structures, concrete cracks mainly affect their waterproof performance. Generally, when the crack width is 0. 1 ~ 0.2 mm, although there is slight water seepage in the early stage, after a period of time, the crack can heal itself. If it exceeds 0.2 ~ 0.3 mm, the leakage water will increase rapidly with the increase of crack width. Therefore, underground engineering should try to avoid cracks with a total section of more than 0.3 mm. If this kind of crack occurs, it will greatly affect the use of the structure, and chemical grouting must be carried out to strengthen it.

On the one hand, the temperature cracks in mass concrete construction stage are internal factors of concrete: due to the temperature difference between inside and outside; On the other hand, it is the external factors of concrete: the external constraints of structure and the constraints between concrete particles prevent the shrinkage and deformation of concrete. The compressive strength of concrete is large, but the tensile force is small, so once the temperature stress exceeds the tensile strength that concrete can bear, cracks will appear. The width of this crack is within the allowable range, which generally does not affect the strength of the structure, but it does affect the durability of the structure, which must be paid attention to and controlled.

The main causes of cracks are as follows:

1, hydration heat of cement

Cement will release a certain amount of heat in the hydration process, while the mass concrete structure has a thick cross section and a relatively small surface area coefficient, so the heat generated by cement is not easy to be lost when it gathers inside the structure. In this way, the hydration heat inside the concrete can not be released in time, so that the more it accumulates, the greater the temperature difference between inside and outside. The hydration heat of cement released by concrete per unit time is related to the amount and variety of cement per unit volume of concrete, and increases with the age of concrete. Because the surface of concrete structure can naturally dissipate heat, in fact, the highest internal temperature mostly occurs in the first 3 ~ 5 days after pouring.

2, the outside temperature changes

In the construction stage of mass concrete, its pouring temperature changes with the change of external temperature. In particular, the sudden drop in temperature will greatly increase the temperature difference between the inside and outside of concrete, which is extremely unfavorable to mass concrete. Temperature stress is caused by temperature deformation caused by temperature difference; The greater the temperature difference, the greater the temperature stress. At the same time, under the condition of high temperature, mass concrete is not easy to dissipate heat, and the highest temperature in concrete can generally reach 60 ~ 65℃ for a long time. Therefore, temperature control measures should be taken to prevent temperature stress caused by temperature difference between inside and outside of concrete.

3. Concrete shrinkage

About 20% water in concrete is necessary for cement hardening, while about 80% water will evaporate. The evaporation of excess water will cause the shrinkage of concrete volume. The main reason of concrete shrinkage is that internal water evaporation leads to concrete shrinkage. If concrete is saturated with water after shrinkage, it can resume expansion and almost reach its original volume. Dry-wet alternation will cause the alternating change of concrete volume, which is very unfavorable to concrete.

The shrinkage of concrete is mainly influenced by cement varieties, concrete mixture ratio, admixture and admixture varieties, construction technology (especially curing conditions).

The following points should be paid attention to when selecting raw materials for mass concrete:

1, coarse aggregate shall be graded continuously, and fine aggregate shall be medium sand.

2, admixture appropriate uses retarder, water reducing agent; Admixtures should be fly ash and slag powder.

3. On the premise of ensuring the strength and slump of mass concrete, the content of admixture and aggregate should be increased to reduce the cement consumption of unilateral concrete.

4. Cement with low hydration heat and long setting time should be selected as far as possible, and medium heat portland cement, low heat slag portland cement, dam cement, slag portland cement, fly ash portland cement and pozzolanic portland cement are preferred.

However, slag cement with low hydration heat has more water precipitation than other cements, and a large amount of water is precipitated on the surface of pouring layer. This bleeding phenomenon not only affects the construction speed, but also affects the construction quality. The change of water-cement ratio of concrete is because the separated water gathers between the upper and lower pouring layers, and the excavated water takes away part of mortar, forming an interlayer with high water content, which destroys the cohesion and integrity of concrete. The amount of concrete bleeding is related to water consumption, which is large. And it is related to temperature. With the increase of temperature, the time for complete precipitation of water is shortened. In addition, it is also related to the composition and fineness of cement. Therefore, when choosing slag cement, we should try our best to choose the variety of bleeding water, and add water reducer to concrete to reduce water consumption. During construction, the separated water should be discharged in time or a certain amount of hard concrete should be mixed at the separated water and poured evenly, and then a layer of concrete should be poured after vibrating with a vibrator.

On the surface, the difference between mass concrete and ordinary concrete lies in its thickness, but the essential difference lies in the fact that the internal heat of mass concrete is not as fast as the surface heat due to the hydration of water in concrete, which leads to excessive temperature difference between inside and outside, and the resulting temperature stress may crack the concrete. Therefore, to judge whether it belongs to mass concrete, we should not only consider the thickness, but also consider the cement variety, strength grade, cement dosage per cubic meter and other factors. The more accurate method is to judge by calculating the difference between the concrete temperature increment caused by cement hydration heat and the ambient temperature. Generally speaking, when the temperature difference is less than 25℃, the temperature stress will be less than the tensile strength of concrete itself and will not cause concrete cracking. When the temperature difference is greater than 25℃, the temperature stress may be greater than the tensile strength of concrete itself, leading to concrete cracking. At this time, it can be judged that the concrete belongs to mass concrete. (Excerpted from Technical Specification for Waterproofing of Underground Engineering GB50 108-200 1)

The box foundation or raft foundation of high-rise buildings has a thick reinforced concrete floor, and the pile foundation of high-rise buildings often has a thick cap. These foundation slabs and caps belong to mass reinforced concrete structures. There are also some common thick and large structure transfer stories and girders that also belong to mass reinforced concrete structures.

Due to the temperature difference caused by the hydration heat of mass concrete, temperature difference stress is formed, which is easy to produce surface cracks and generally has no influence on the normal use of the structure. However, the standard that cracks are not allowed in mass concrete is widely used in engineering practice, which leads to complicated maintenance and temperature control measures and a lot of extra expenses.

Pouring temperature refers to the temperature of concrete after transportation and vibration. Code for Construction and Acceptance of Concrete Structure Engineering (GB 50204-92) stipulates that the pouring temperature should not exceed 28℃. This provision does not take into account the local differences in China, such as Shanghai, Nanjing, Wuhan and other high-temperature seasons in southern China. If special measures are not taken, it will be difficult to meet this requirement, and if measures are taken, the cost will be great. So is it necessary to crack when the pouring temperature exceeds 28℃? The pouring temperature of some projects in Changzhou, Jiangsu Province reached 35℃. Due to the effective heat preservation and cooling measures, there is no temperature difference crack. Nanjing. The pouring temperature of some mass concrete projects in Shanghai, Wuhan and other places exceeds 28℃, and some projects reach 465,438 0℃, so there are no problems that endanger the structural safety and affect the use function. Therefore, in the Code for Acceptance of Construction Quality of Concrete Structure Engineering (GB50204—2002), there is no restriction that the pouring temperature should not exceed 28℃.

Conducive to controlling pouring temperature. To reduce the pouring temperature, we must start with reducing the outlet temperature of concrete, with the aim of reducing the total temperature appreciation of mass concrete and reducing the temperature difference between the inside and outside of the structure. The most effective way to reduce the temperature of concrete is to reduce the temperature of stone. Due to the high temperature in summer, in order to prevent direct sunlight, commercial concrete suppliers can be required to set up simple sunshade devices in the sand and gravel yard, and spray water mist or rinse the aggregates with water before use if necessary. In the aspect of controlling the concrete pouring temperature, by calculating the concrete quantity, the construction process and mechanical configuration are reasonably arranged, and the pouring time is adjusted to be mainly at night and less during the day, so as to avoid affecting the quality due to exposure.

Temperature change of mass concrete: firstly, it is a heating process, and then it slowly cools down after reaching the highest point, and the speed of heating up is faster than that of cooling down.

So when does mass concrete reach the highest point? Mainly depends on the mix proportion, geometric size, site conditions and other factors. According to engineering statistics, the highest point appears in 3 ~ 4 days after general mass concrete pouring.

The national code has the aforementioned provisions for temperature control, but there is no clear requirement for cooling rate. If the temperature difference between the inner and outer surfaces of mass concrete is too large, it will cause surface cracks; Then the cooling speed is too fast, which will cause the penetration of cold shrinkage joint, which is absolutely not allowed. Theoretically, the allowable temperature difference of any material is related to the limit value of the material. For mass concrete, if the temperature drops too fast, although the temperature difference between the inner and outer surfaces is still controlled within the specification requirements, cracks will appear theoretically when the temperature difference stress reaches the ultimate tensile strength of concrete due to the excessive internal temperature difference, and the cracks will appear inside the mass concrete. If the difference is too large, there will be through cracks, which will affect the use of the structure. Therefore, the cooling rate is directly related to the development of internal tensile stress of mass concrete. So, what is the cooling rate? Theoretically, the temperature difference stress must be less than the ultimate tensile strength of concrete at the same time. At present, some projects adopt the cooling rate of 2 ~ 3℃/d, and no through cracks are found in the tracking. However, for most construction units, due to the lack of comprehensive and reliable data, for safety reasons, ≤ 1 ~ 1.5℃/d is still adopted. Concrete curing can follow the principle of "large cooling range in the early stage and small cooling range in the later stage". Because the concrete is in the heating stage at the initial curing stage, the elastic modulus and temperature stress are small, while the tensile strength increases rapidly. On the basis of ensuring the wet surface of concrete, cover it as little as possible to fully dissipate heat, thus reducing the temperature of concrete, that is, the cooling rate of concrete in the early curing stage can be slightly higher. In the later period of curing, the concrete is in the cooling stage, the elastic modulus increases rapidly, and the temperature stress is greater. Therefore, it is necessary to strengthen heat preservation and control the cooling rate.

Causes of cracks:

(1) hydration heat of cement

Cement produces a lot of heat during hydration, which is the main heat source of mass concrete. Due to the large cross-section thickness of mass concrete, the hydration heat is not easy to be lost when it gathers inside the structure, which makes the internal temperature of concrete rise. The highest temperature in concrete mostly occurs in 3 ~ 5 days after pouring. When the temperature difference between the inside and outside of concrete is too large, temperature stress and temperature deformation will occur. The temperature stress is proportional to the temperature difference, and the greater the temperature difference, the greater the temperature stress. When the tensile strength of concrete is not enough to resist temperature stress, temperature cracks begin to appear. This is the main reason why mass concrete is prone to cracks.

(2) Constraints

When mass reinforced concrete is poured together with the foundation, the expansion deformation caused by early temperature rise is constrained by the lower foundation, forming compressive stress. Because the elastic modulus of concrete is small, the creep and stress relaxation are large, and the connection between concrete and foundation is not firm, so the compressive stress is small. However, when the temperature drops, it will produce greater tensile stress. If the tensile strength of concrete is exceeded, vertical cracks will appear in concrete.

(3) the change of outside temperature

In the process of mass concrete construction, the change of external temperature has a great influence on the cracking of mass concrete. The internal temperature of concrete is the superposition of pouring temperature, adiabatic temperature of cement hydration heat and concrete heat dissipation temperature. The higher the outside temperature, the higher the pouring temperature of concrete. The external temperature drop, especially the sudden drop, greatly increases the temperature gradient in the outer and inner layers of concrete, resulting in temperature difference stress and cracks in mass concrete. Therefore, controlling the temperature difference between the concrete surface temperature and the outside air temperature is also an important link to prevent cracks.

(4) Shrinkage and deformation of concrete

In the mixing water of concrete, only about 20% water is necessary for cement hydration, and the remaining 80% will be evaporated. The evaporation of excess water in concrete is one of the main reasons for the volume shrinkage of concrete. This shrinkage deformation is not affected by constraints. If there are constraints, shrinkage stress will occur and cracks will appear. In the construction of mass concrete, the hydration heat of cement should be reduced as much as possible and the peak time of heat release should be postponed. For example, the 60-day concrete strength is used as the design strength (which must be recognized by the design unit) to reduce the amount of cement; Adding fly ash can replace part of cement, which can not only reduce the dosage of cement, but also delay the appearance of exothermic peak because of the slow hydration reaction of fly ash. Adding additives can also reduce the dosage of cement and water and delay the appearance of exothermic peak; In summer construction, measures such as mixing with ice water, shading the sand yard and spraying cold water on the concrete conveying pipeline can reduce the temperature of concrete leaving the machine and entering the mold. These measures can reduce the temperature stress in the process of concrete hardening. Second, carry out heat preservation and moisture preservation curing, and the curing time shall not be less than 14d, so that the temperature difference stress generated in the hardening process of concrete is less than the tensile strength of concrete itself, thus avoiding harmful cracks in concrete. Thirdly, the concrete is poured in layers and sections, and the hydration heat of concrete is dissipated as soon as possible through layered vibrating compaction. You can also use the method of secondary vibration to increase the compactness of concrete and improve the crack resistance, so that the upper and lower layers of concrete can be well combined before initial setting. Fourth, do a good job of temperature measurement, control the temperature change in concrete at any time, and adjust the heat preservation and maintenance measures in time so that the difference between the central temperature and the surface temperature of concrete, and the difference between the surface temperature of concrete and the atmospheric temperature does not exceed 25℃. How to deal with the temperature measuring hole of the foundation slab after measuring the temperature? After measuring the temperature, every hole is a weak part, and it is easy to leak out from the hole if it is not handled well. Therefore, every hole must be carefully filled with waterproof materials such as plugging agent or waterproof treasure. Conditions for removing insulation layer and end time of temperature measurement: When the temperature of concrete drops, the difference between the central temperature of concrete and the surface temperature is less than 20℃, and the difference between the surface temperature and the atmospheric temperature is less than 20℃, the insulation layer should be removed layer by layer.

The duration of temperature measurement is related to the thickness and importance of the structure. For large thickness (more than 2m) and important projects, the duration of temperature measurement should not be less than 15d. It is best to accumulate the 28d temperature record together with the strength of the test block for reference in temperature stress analysis. For small thickness and general engineering, the duration of temperature measurement can be 9 ~ 12d, which is too short to achieve the purpose of temperature control and monitoring.

construction technique

A, mass concrete mainly refers to the minimum geometric size of concrete structure entity is not less than 1m, or it is expected that harmful cracks will occur due to temperature change and shrinkage caused by hydration of water in concrete.

Second, the preparation of mass concrete materials shall meet the following requirements:

1. Cement with stable quality, low C3A content and relatively high C2S content, which is conducive to improving the crack resistance of concrete, should be preferred.

2. Fine aggregate should be medium sand with good gradation, and its fineness modulus should be greater than 2.3.

3. In non-pumping construction, the particle size of coarse aggregate can be appropriately increased.

4, should choose retarding superplasticizer.

Three, mass concrete mixture ratio shall meet the following requirements:

1, the design of mass concrete mixture ratio should not only meet the requirements of design strength grade, durability, impermeability and volume stability. , should also meet the requirements of the technical characteristics of mass concrete construction, and should conform to the principle of rational use of materials and reduce the adiabatic temperature rise of concrete.

2. The slump of concrete mixture on the pouring surface should not be greater than 160mm.

3, mixing water consumption should not be greater than170 kg/m/m. ..

4, the dosage of fly ash should be appropriately increased, but should not exceed 40% of the dosage of cement; The dosage of slag powder should not exceed 50% of the cement dosage, and the total amount of two additives should not be greater than 50% of the cement weight in concrete.

5. The water-binder ratio should not be greater than 0.55. 8.2.4 When required by the design, flaky (including rubble) can be filled in the concrete. Filling flaky shall meet the following requirements:

(1) Stones with a thickness of not less than 15cm can be buried, and the number of buried stones should not exceed 20% of the volume of the concrete structure.

(2) Stone materials without cracks, scale, rust, interlayer, unfired and frost resistance meeting the design requirements should be selected and cleaned.

(3) The compressive strength of stones shall not be less than 0.5 times of concrete strength grade.

(4) The stones should be evenly distributed, with a clear distance of not less than 150mm and a clear distance of not less than 250mm from the side and top surface of the structure. Stones should not touch steel bars and embedded parts.

(5) When the temperature of concrete in the tension zone is lower than 0 C, stones shall not be buried.

Four, mass concrete construction technical scheme should include the following main contents:

1. In addition to checking the strength, stiffness and stability of the formwork and supporting system of mass concrete according to the current national standards, the design of thermal insulation structure should also be combined with the maintenance method of mass concrete.

2, template and support system in the process of installation or removal, temporary fixed measures must be set up to prevent capsizing.

3. See Appendix D for the calculation of temperature stress and shrinkage stress of mass concrete structure. ..

4. Determination of temperature control index and technical measures in the construction stage.

5, raw material optimization, mix design, material preparation and transportation plan

6. Main concrete construction equipment and site general layout.

7. Temperature monitoring equipment and test arrangement.

8, concrete pouring sequence and construction progress

9, concrete thermal insulation and moisture curing method, the thickness of thermal insulation coating can be calculated according to the requirements of temperature control index, refer to the method in Appendix E. ..

10, main emergency safeguard measures.

1 1, post responsibility system and succession system, temperature measurement operation management system.

12, construction measures for special parts and special weather conditions.

5. The temperature, temperature stress and shrinkage of mass concrete structure should be calculated, the peak temperature rise of mass concrete pouring body, the temperature difference between core and surface and the control index of cooling rate should be predicted in the construction stage, and the corresponding temperature control technical measures should be formulated. The process test of the first pouring body is carried out, and the temperature monitoring of the initial construction structure is emphasized. The temperature monitoring system shall have the functions of automatic acquisition and automatic recording.

Six, mass concrete pouring shall meet the following requirements:

1, concrete into the mold temperature (50mm after vibrating? 100mm) depth temperature should not be higher than 28 degrees. The temperature rise of the concrete pouring body shall not exceed 45 degrees on the basis of the mold entry temperature.

2. The construction of mass concrete engineering should adopt layered continuous casting construction (Figure A) or push continuous casting construction (Figure B). Should be evenly segmented and layered according to the design size. When the cross-sectional area is less than 200m, the section should not be greater than 2 sections; When the cross-sectional area is less than 300m, the cross-section should not be greater than 3 sections, each section area should not be less than 50 m, and the concrete thickness of each section should be 1.5m? 2.0 meters. The vertical construction joints between sections should be parallel to the direction of the smaller section size of the structure. When pouring in sections, templates shall be set for vertical construction joints. Vertical construction joints of upper and lower adjacent floors should be staggered.

3. When pumping concrete, the thickness of concrete pouring layer should not be greater than 500mm. When non-pumping concrete is used, the thickness of concrete pouring layer should not be greater than 300mm.

4. In mass concrete construction, the setting of horizontal construction joints should not only meet the design requirements, but also be determined according to the requirements of temperature crack control during concrete pouring, the supply capacity of concrete, the construction of reinforcement engineering, the installation of embedded pipe fittings and other factors.

5. In the process of mass concrete pouring, measures should be taken to prevent the displacement and deformation of steel bars, positioning bars and embedded parts.

6, mass concrete pouring surface should be timely secondary plastering treatment.

Seven, mass concrete after each concrete pouring, in addition to the routine maintenance of ordinary concrete, should also be timely according to the requirements of temperature control technical measures for heat preservation and maintenance, and shall meet the following requirements:

1, and the duration of moisturizing and curing shall not be less than 28d. The heat shield should be removed gradually. When the maximum temperature difference between the surface temperature of concrete and the environment is less than 20°C, it can be completely removed.

2, in the process of moisturizing curing, should always check the integrity of the plastic film or curing agent coating, keep the concrete surface moist.

3. In the thermal insulation and curing of mass concrete, it is necessary to detect the temperature difference and cooling rate between the core and the surface of the concrete pouring body. When the measured results do not meet the requirements of temperature control index, the heat preservation and maintenance measures should be adjusted in time.

4. After mass concrete is dismantled, maintenance measures such as preventing cold current invasion, sudden cooling and severe drying should be taken.

Eight, mass concrete should be appropriately delayed ripping time, when the template as part of the thermal insulation and maintenance measures, the ripping time should be determined according to the requirements of temperature control.

Nine, mass concrete construction in hot, winter, windy or rainy and snowy weather and other special climatic conditions, effective technical measures must be taken to ensure the quality of concrete pouring and maintenance, and shall meet the following requirements:

1. When pouring mass concrete in hot season, cover the concrete raw materials to avoid sun exposure, and mix the concrete with cooling water, or cool the aggregate and add ice cubes during mixing to reduce the warehousing temperature. If necessary, cooling pipes can be buried in concrete for cooling. After concrete pouring, it should be kept moist and insulated in time to avoid the formwork and concrete from direct sunlight. When conditions permit, concrete should be poured at high temperature.

2. When pouring concrete in winter, measures such as hot water mixing and heating aggregate should be taken to improve the temperature of concrete raw materials, and the temperature of concrete entering the mold should not be lower than 5℃ ... After pouring concrete, it should be kept warm and moist in time.

3. When pouring concrete in windy weather, windproof measures should be taken on the operation surface to reduce the wind speed on the concrete surface, increase the number of plastering on the concrete surface, and cover the plastic film and thermal insulation materials in time to keep the concrete surface moist and prevent air drying.

4. It is not advisable to pour concrete in the open air in rainy and snowy days. When construction is needed, effective measures should be taken to ensure the quality of concrete. In case of sudden heavy rain or heavy snow during pouring, construction joints should be kept in reasonable parts of the structure in time to stop concrete pouring as soon as possible; Immediately cover the cast concrete that has not been hardened, and it is forbidden for rain to directly wash the newly cast concrete.