Based on the engineering example, this paper analyzes the causes and mechanism of cracks in mass concrete, and according to the causes of cracks, puts forward a set of effective measures to reduce cracks, which has certain practical significance.
Keywords concrete crack causes and measures
The main feature of mass concrete is large section construction and thick construction volume. The resulting problem is that the heat released by cement hydration gradually increases the internal temperature of concrete, and the generated heat is not easy to export, resulting in a large temperature difference between inside and outside. In addition, the early tensile strength and elastic modulus of concrete are low, which leads to concrete cracking and affects the engineering quality. First of all, the existence of cracks on the concrete surface will affect the aesthetics of components; Secondly, surface cracks also have a certain impact on the durability of concrete. Because the cracks are directly embedded in the structure, the thickness of the concrete protective layer will be reduced, leaving hidden dangers for the corrosion of steel bars. In addition, the secondary treatment of cracks in the construction process consumes manpower and material resources. It is precisely because of the above reasons that the problem of cracks in mass concrete has attracted more attention in engineering, and it is difficult to avoid concrete cracks, which makes the prevention and control of cracks consume a lot of manpower and financial resources. This kind of problem happened in the residential building and business building project of Beijing Plate Making Factory which I participated in. In the process of project implementation, through continuous monitoring, analysis, research and testing, I finally have a certain understanding of the causes of cracks, and have done some research and summary on the methods and measures to avoid cracks, which are hereby put forward and discussed with my colleagues.
1 project overview
The residential building and business building of Beijing Plate Making Factory are residential and commercial office buildings, and the structural form is high-rise reinforced concrete frame shear wall structure. The foundation of this project is a raft foundation, and the settlement joint in the middle divides the residential building and the business building into two independent parts. Raft of residential building, foundation bottom elevation -8.5m, raft thickness 1m, raft length 44.7m, width 28.45m. 1m thick slab concrete quantity 15438+08438+0m3. The elevation of the bottom of the raft in the commercial building is -5.3m, the thickness of the raft is 2m, the length of the raft is 26.05m, the width is 25.85m, and the concrete volume is about 1200m3. The concrete is C30. The difference of raft bottom elevation between residential building and commercial building is 3.2m. According to the order from deep to shallow, the foundation concrete of residential building below 0.00 shall be constructed first, and then the raft concrete of commercial building shall be constructed. See table 1 for the selection of concrete mixture ratio in this project.
Table 1 concrete mixture ratio
Strength grade, water cement ratio, sand ratio (%), raw material consumption per unit area (kg/m3), hydration heat (KJ)
Cement, sand and fly ash UEA BD-1115238
c30p 8 0.45 40 330 673 1030 132 205 36 9.8
The project started in April 2002. During the mass concrete construction of raft foundation slab in residential buildings and commercial buildings, three serious concrete cracks appeared. They are the construction stages between Axis ③ and Axis ④ of residential buildings and Axis ② and Axis ④ of commercial buildings.
2 Cause analysis
According to the location of several cracks, construction operation methods, construction environment, materials used and the implementation effect of corresponding improvement measures, several special meetings were organized on site to analyze and study the causes of cracks. Cause analysis includes:
2. 1 Cause analysis of surface cracks
The phenomenon that the volume of concrete expands (or contracts) with the increase (or decrease) of temperature is called temperature deformation. Mass concrete engineering has a large cross section and a large amount of cement. After pouring, the cement releases a lot of hydration heat, and the temperature of concrete rises. Due to the poor thermal conductivity of concrete, the volume is too large and the relative heat dissipation is small, so the hydration heat in concrete does not accumulate, while the external heat dissipation is fast, and the internal temperature rise produces compressive stress and the surface produces tensile stress [1]. When the temperature gradient is large enough and the surface tensile stress σ(t) exceeds the ultimate tensile strength of concrete, cracks will occur on the concrete surface.
2.2 Cause analysis of through cracks
With the end of cement hydration reaction and the continuous heat dissipation of concrete, mass concrete transits from heating stage to cooling stage. As the temperature decreases, the volume decreases. Because the internal heat of concrete radiates outward through the surface, the temperature field distribution of concrete is still in the state of high central temperature and low surface temperature in the cooling stage, so the cooling degree of the central part of concrete is different from that of the surface part, which produces greater internal constraints in concrete, and at the same time, the foundation and boundary conditions also produce greater external constraints on shrinkage concrete. Internal and external constraints make shrinkage concrete produce tensile stress. With the increase of age, the strength, elastic modulus and creep effect of mass concrete increase. Therefore, the tensile stress caused by cooling shrinkage is large, which not only offsets the compressive stress caused by heating, but also forms a high tensile stress in concrete, thus causing the through cracks of mass concrete [2].
Combined with previous experience, through the comparison and experimental analysis of various situations, it is finally concluded that there are many reasons for cracks, in addition to load, temperature, shrinkage, uneven settlement and other reasons, there are the following reasons:
1, poor quality of raw materials: the silt content of sand and gravel is too large, and the fineness modulus of sand is too small.
2. Improper mixture ratio: the slump and unilateral cement are not properly controlled.
3. Improper construction method: negative bending moment reinforcement is trampled. Short curing time, improper secondary light exposure time and insufficient formwork support stiffness.
4. Personnel factors: the operators have poor quality awareness, low technical quality and unclear technical disclosure.
5. Environmental factors: improper control of concrete pouring time, such as sun exposure and uneven settlement.
3 mechanism analysis
3. 1 requirements for raw materials of mass concrete
3. 1. 1 cement
Low-heat cement should be used in mass concrete construction, and the hydration heat of cement is a function of its mineral composition and fineness. To reduce the hydration heat of cement, it is mainly to choose appropriate mineral composition and adjust the fineness of cement powder. Therefore, P042.5 cement produced by Liu Lihe Cement Plant is selected for this project, and its hydration heat in 4 days is 3 14KJ/Kg.
3. 1.2 Total
Aggregate with good gradation should be selected, with the particle size of coarse aggregate of 5 ~ 40 mm and fine aggregate of medium coarse sand, and the silt content of sand and gravel should be strictly controlled not to exceed 3% and 1%.
3. 1.3 active mixed materials
In this project, a certain amount of fly ash active substance is added to "decompose" the hydration heat and improve the workability of concrete. Adding fly ash can prolong the setting time of concrete and avoid the appearance of construction joints without changing the design strength. Adding fly ash can reduce the creep, shrinkage and thermal expansion coefficient of concrete, improve the anti-bleeding and segregation performance, and significantly improve the impermeability of concrete.
3. 1.4 admixture
Admixtures used in mass concrete construction are different from ordinary concrete and should have the following characteristics: 1) retarding effect: the retarding effect of admixtures can slow down the heat release rate of cement hydration, which is beneficial to heat dissipation and reduce the internal temperature rise of concrete. 2) Efficient water reduction: Efficient water reduction can greatly reduce the water consumption of concrete mixing, and the cement consumption of concrete can be greatly reduced when the water-cement ratio is basically unchanged, that is, the internal cause of hydration heat can be reduced. The water reducer BD- 1 used in this project has the dual functions of reducing water and retarding setting, prolonging the setting time of concrete, ensuring the overall pouring of concrete, prolonging the internal heat dissipation process of concrete and avoiding temperature cracks.
3.2 Mass Concrete Construction Technology
The construction technology of mass concrete is: construction preparation-pouring technology-vibrating-bleeding water treatment-surface treatment, etc. The main steps are pouring and vibrating, so the temperature of concrete entering the mold should be reduced as much as possible. If concrete is mixed with underground cold water or ice water, a sun protection shed will be set up in the gravel yard. Secondly, the integrity of mass concrete foundation requires high requirements, requiring continuous pouring of concrete at one time. Construction technology should be layered pouring, layered tamping, control the thickness and progress of pouring layer. According to the overall requirements, structural dimensions, steel bar density, concrete supply and other specific conditions, concrete pouring adopts a layered method combining plane and inclined plane. The residential building is divided into three floors, the business building is divided into five floors, and the inclined plane layered thickness is 300-450 mm When using the plug-in vibrating rod, when operating vertically and obliquely, insert it quickly and pull it slowly to avoid layering or forming a cavity, and twitch it up and down to make it vibrate evenly. When pouring in layers, the vibrating upper layer should be inserted into the lower layer by about 5cm. Vibrate at each point for 20-30 seconds until the concrete surface no longer sinks obviously, bubbles no longer appear and the mortar glows. After 20-30 minutes, vibrate again. After each vibrating period, it should be spread out immediately. After pouring, smooth it once within 10 hour. Two hours before the initial setting, plaster for the second time. Finally, thermal calculation should be carefully carried out, temperature measurement should be strengthened, and heat preservation and curing should be adopted to reduce the temperature difference between inside and outside of mass concrete. The arrangement of temperature measuring points is shown in figure 1:
Figure 1 Layout of Temperature Measuring Points
Each group of temperature measuring points is provided with three temperature measuring holes, which are arranged in a triangle to measure the temperature of the bottom, middle and surface of the bottom plate respectively, and the distance between the temperature measuring holes is 600.
The temperature measuring points are arranged at the edge and middle of the bottom plate, avoiding the wall and the anti-beam, *** 10.
The thermometer hole is made of Ф 20 ~ 25 thin-walled steel pipes. Before embedding, cut the steel pipe according to the required length, weld the lower end with the steel bar head, and wrap the upper end tightly with plastic sheets. When embedded, the upper end shall be reserved with 10cm.
It is also important to strengthen the maintenance of concrete. During the construction of mass concrete, it should be covered while pouring, and watered after 5 ~ 7 days of heat preservation and curing. Because the newly poured mass concrete is in the heating stage, if it is watered or soaked, the outside of the concrete will suddenly cool, resulting in excessive temperature difference between inside and outside, which is easy to cause serious cracks. Therefore, premature watering should be avoided.
4 concrete measures to control cracks in mass concrete
Compared with ordinary concrete, mass concrete should not only meet the requirements of strength, integrity and durability of ordinary concrete, but also consider how to control the occurrence and development of temperature deformation cracks. Mass concrete should try to reduce the maximum temperature inside the concrete and the temperature difference between inside and outside, which involves the plane size of the structure, the thickness of the structure, the constraint conditions, the arrangement of steel bars, concrete materials, construction technology and maintenance conditions.
Technical measures to control cracks in mass concrete specifically include the following aspects:
4. 1 Reduce the hydration heat of cement
Choose cement with low hydration heat or medium hydration heat to prepare concrete; Make full use of the late strength of concrete and reduce the cement consumption per cubic meter of concrete; Fly ash concrete is used, the strength grade age is set at 60 days, and the corresponding water reducer is added to improve the workability. Reduce the water-cement ratio to achieve the purpose of less cement consumption and low hydration heat.
4.2 reduce the concrete into the mold temperature
Try to avoid hot weather and pour concrete; Stir with low-temperature water; Pre-cooling aggregate or covering aggregate to prevent solar heating; Adding retarding water reducer; Measure and record the temperature of concrete entering the mold.
4.3 Strengthen the temperature control in construction.
The floor of the residential building is 1.0m thick, with 1 layer of plastic sheets and two layers of flame-retardant straw bags for water storage, heat preservation and moisture retention. According to the climatic conditions at that time, cement used in concrete, concrete mixture ratio, admixture and admixture, concrete thickness, thermal insulation materials and thermal calculation of mass concrete, ensure that the temperature difference between the concrete surface and the interior exceeds 25? 0? 2C； The floor thickness of the business building is 2.0m. Due to the high temperature, the floor is thickened. Through thermal calculation, plastic sheets and straw curtains are used to store water, keep warm and moisturize. Arrange temperature measuring holes as required. Adjust the curing materials and curing coverage time according to the measured temperature. If the atmospheric temperature drops or the heat preservation is not in place, the temperature difference between inside and outside is close to or even exceeds 25? 0? 2C. Cover insulation should be strengthened; The water storage and thermal insulation curing time shall not be less than 10 day to promote the early strength growth of concrete.
4.4 In order to prevent the development of temperature cracks, anti-crack reinforcement should be added at the concrete edge of residential floor slab, φ 10 @ 360, bidirectional.
5 conclusion
By analyzing the causes and mechanism of cracks and taking some targeted measures, the cracks in mass concrete can be reduced to a reasonable range. However, due to the complex causes of cracks, it is still very difficult to completely avoid cracks, which requires a lot of research and practice. Controlling and preventing cracks in mass concrete is a systematic project, which involves many uncertain factors such as design, raw material selection, construction technology and maintenance. Problems in any link will lead to concrete cracking. Although there are many factors that affect the cracks in mass concrete, the main reason is temperature stress. As engineers and technicians, we should know the temperature change law in concrete, reasonably select cement varieties, strictly control cement dosage, add appropriate mixed materials and additives, optimize concrete mix ratio, rationally construct, monitor in real time and strengthen maintenance. Secondly, we should seriously study concrete cracks, treat them differently, take reasonable measures to deal with them, and take various effective preventive measures during construction to prevent the occurrence and development of cracks and ensure the safety of buildings and components. You can find a lot on Baidu.