Concrete quality control and hidden crack prevention measures?
1 overview
There are 8 bridge projects, with 4029.54 linear meters and 33062m2 of concrete. The piers are all constructed with mass concrete. In the early stage of pier construction, in order to prevent or avoid harmful cracks, Qi Xin, a manager and a technician, worked together to tackle key problems, and implemented them from raw materials, construction technology, process control, post-maintenance and other aspects, which greatly improved the appearance quality of the pier and achieved the expected goal.
Two Common Cracks in Mass Concrete
2. 1 internal reference materials
Gel shrinkage, drying shrinkage, carbonization shrinkage and cracks caused by material technology.
2.2 Temperature difference crack
Cement hydration heat and environmental temperature change, temperature difference cracks belong to indirect cracks, which are exactly the same as shrinkage cracks in essence. The location, state, development and influence of cracks are basically the same, except that the former shows the reduction (contraction) of its own volume, while the latter is the disharmony between volume expansion and contraction caused by heat and temperature difference, which is very similar to shrinkage cracks.
2.3 Settlement cracks
Cracks caused by uneven settlement are the result of natural factors and human action, the difference and uneven distribution of soil layers, and the difference of compression deformation caused by gravity of structures, that is, uneven settlement.
3 crack generation mechanism
3. 1 shrinkage crack
The concrete used in a project is C35, and the main admixtures are: fly ash, water reducing agent and air entraining agent. The shrinkage of concrete material is as follows: 1) Gel shrinkage. Gelling shrinkage refers to the volume reduction of tricalcium aluminate colloid due to crystallization and solidification during the solidification of cement colloid. The macro effect is the "shrinkage" of concrete materials, which is the shrinkage of materials independent of external conditions, resulting in the overall volume reduction of materials in the whole concrete structure. 2) Dry shrinkage. Dry shrinkage refers to the phenomenon that unhydrated free water escapes through bleeding after concrete mixture is poured and vibrated, forming bleeding capillaries. The water in the pipe hole causes the surface of the surface concrete to shrink due to the surface tension. 3) It is very important to strictly control the silt content of coarse and fine aggregates. The coarse and fine aggregates are prone to cracking when the mud content is too large, the shrinkage increases and the tensile strength decreases.
3.2 the influence of material technology
1). In recent years, the cement properties, concrete composition and construction technology in China have changed. The fineness of cement particles decreases, the activity of cementitious materials increases, the coarse aggregate in concrete decreases, and the content of sand and powder increases. Due to the rapid hardening, high strength and high heat, the shrinkage deformation of concrete materials generally increases. 2) The influence of construction technology. In the process of construction, concrete segregation is caused by improper technology. Pay attention to the uneven thickness of aggregate caused by the height of pouring surface (below 2m) and the over-dense reinforcement, which destroys the continuous gradation of concrete, and cracks are easy to occur in the material gathering area, because the shrinkage rate of cement slurry is much greater than that of concrete. There are fly ash mortar, cement slurry and bubble accumulation areas in concrete, so there are areas with weak crack resistance, which lead to cracks.
3.3 Temperature difference crack
Temperature difference crack is an indirect crack, which is exactly the same as shrinkage crack in nature. The position, state, development and influence of cracks are basically the same, except that the former shows the reduction (contraction) of its own volume, while the latter is the disharmony of volume expansion and contraction caused by heat and temperature difference, which is very similar to shrinkage cracks. The most common is the hydration heat of cement and the change of environmental temperature, such as the hydration heat of mud: in the process of hydration heat of mass concrete, due to the large cross section of concrete structure and poor thermal conductivity of the structure, this heat is not easy to dissipate, and the difference between internal temperature and external temperature is formed in concrete structure. (Concrete has the property of thermal expansion and cold contraction, and the linear expansion coefficient ac is about 100uξ. If the Ec value is much greater than the Ftk value, cracks will occur), and the shrinkage stress difference caused by temperature difference will cause concrete cracking.
3.4 External ambient temperature
The outdoor temperature is directly related to the concrete pouring temperature. In the process of concrete structure construction, the higher the outside temperature, the higher the pouring temperature of concrete. With the completion of pouring, the external temperature will gradually decrease, but the internal temperature cannot be lost, which will cause the temperature difference between inside and outside. The internal and external temperature difference caused by formwork removal or low temperature in the morning and evening will increase with the increase of temperature difference, and cracks will also occur when the stress intensity exceeds the elastic strength of concrete.
4. Analysis of concrete crack factors.
Climatic characteristics of a project: annual average temperature 10.5 degrees/℃, extreme maximum temperature 4 1. 1 degrees/℃, extreme minimum temperature -23.3 degrees/℃, average temperature of Leng Yue over the years-10.8 degrees/℃, large temperature difference between morning and evening, and light breeze in spring and autumn. If the concrete pouring process is not strictly controlled, the bridge project will last 16 months, and small cracks will appear on the surface of individual piers. Through summary and elimination, cracks are divided into two categories: shrinkage cracks and temperature difference cracks. Under the condition of unchanged materials, mix proportion and construction technology, the causes of cracks in solid engineering are climate, hydration heat, temperature difference, construction technology and post-maintenance. So as to strengthen the control and avoid the occurrence of cracks.
5 crack prevention measures
5. 1 Selection and proportion of concrete materials
1) Selection of concrete material cement: P. O42.5 factory; Fine aggregate: medium sand in area II of sand yard, fineness modulus 2.6; Coarse aggregate: 5-3 1.5mm crushed stone. 5~ 10mm, 10~20mm, 16 ~ 3 1.5 mm three-level matching; Admixture: Shanxi Jiawei JW- 1 1 retarded carboxylic acid superplasticizer, dosage1.0%; Fly ash: Shanxi Hejin Co., Ltd., Grade II, with a dosage of 20%; Mixed water: drinking water; Water-binder ratio: under the premise of ensuring the design strength grade of concrete, the water-binder ratio of 0.43 is adopted. 2) Selection of mix proportion.
5.2 Inspection and protection of materials
1) The incoming materials shall be inspected by vehicle, and the silt content of sand shall be strictly controlled not to exceed1%; The silt content of stone shall not exceed 65438 0%; 2) Dust-proof sealed greenhouse shall be used for the aggregate entering the site to avoid sun exposure, temperature rise and sundries pollution; 3) Two silos are set for cement to be used in turn, so as to avoid that the temperature of new cement is too high and the temperature of concrete entering the mold is raised; 4) Before the opening, test the moisture content of sand and gravel, convert the construction mixture ratio in time, issue the construction ingredient list, and produce in strict accordance with the material consumption shown in the ingredient list.
5.3 Mixing and Transportation of Concrete
1) Mixing and feeding sequence of concrete materials: sand+cement+fly ash+mineral powder → water and additives → coarse aggregate → mixing → discharging; 2) Control the total mixing time of each plate, with 2 minutes in summer (excluding feeding and concrete delivery) and 3 minutes in winter (excluding feeding and concrete delivery); 3) Install an automatic alarm system, share information with the supervisor, and strictly control the material errors in the concrete mixing process, such as cementing materials (cement, mineral admixture, etc.). ) 1%, admixture 1%, coarse aggregate 2%; Mixed water1%; 4) Check the cement inlet temperature by special personnel, and the highest inlet temperature in summer shall not exceed 60℃; 5) Construction in summer: assign special personnel to measure the temperature of concrete leaving the machine and entering the mould, and the temperature of concrete shall not exceed 30℃. In summer, the construction should avoid the high temperature period (12- 15) and be carried out at low temperature or at night. Winter construction: the coarse and fine aggregates are heated by floor heating, and the mixing water is heated to 50-60℃ for mixing. 5.4 Process control 1) Strictly control the free falling height of pouring concrete within 2m, and use string drums and chutes for the parts with falling height exceeding 2m; 2) When pouring, the thickness of concrete pouring layer should be controlled at 300mm as far as possible, and vibrated by layers to make the hydration heat of concrete dissipate as soon as possible; 3) Designate a special person to monitor the vibration, arrange the plum blossom evenly, insert it quickly and pull it slowly, and use a 70mm internal vibrator to vibrate. The vibrator shall keep a clear distance of about 10cm from the side formwork, and insert the lower concrete about 5cm at a time for 20 s to 30 s. Observing that the surface is flooded, there is no obvious collapse on the concrete surface. At the same time, ensure the continuous pouring of concrete and avoid the long pause time of concrete pouring. 5.5 Mold entry temperature and mold exit temperature control 1) Control the mold entry temperature of concrete, the local temperature is within the range of 30℃/℃, and the highest temperature inside the concrete is 60℃; 2) Control the difference between the internal temperature and the surface temperature of concrete during formwork removal to be less than 20℃. In case of high temperature and windy weather, the formwork removal time shall be extended. 5.6 Concrete curing 1) After concrete pouring is completed for 3-4h, the temperature shall be measured by special personnel to ensure that the temperature difference between the concrete core and the surface, and between the surface and the environment is less than 20℃. After form removal, plastic film should be used in time, the surface should be wrapped with cotton blanket, and special personnel should spray water for maintenance according to the temperature change. 2) According to the weather change, the temperature of concrete curing water should be adjusted and maintained at any time according to the ambient temperature. The temperature difference should not be too large, and it should always be kept at 8- 10 temperature /℃( 15 temperature/℃
6 conclusion
By taking technical measures to prevent cracks in mass concrete construction, in the construction process, according to the specific conditions such as climate, materials, additives, etc., the concrete pouring construction and maintenance are standardized, and the concrete components of the undertaken project have achieved good quality effects.
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