大体积混泥土水化热与裂缝

1、Hydration heat of mass concrete in the bridge and cracksIn recent years, large-scale bridge construction increasing number requires some suspension bridge anchorages and bridge pile cap due to the construction, infrastructure using the geometric dimensions of the design, the concrete construction of

2、 its huge volume of $ 10,000 to tens of thousands of cubic meters, compared with reinforced concrete, its structure is thick, concrete volume, complex engineering conditions (usually cast mostly underground or semi-underground building) construction technology requires high heat of cement hydration

3、is easy to make the structure of temperature and shrinkage deformation. Quality problems in the bridge construction process, there have been many bridges in mass concrete works, the normal use and durability of these problems will project a different degree of harm. l.The main reason a large volume

4、of concrete cracksThe occurrence of cracks of large concrete structures is caused by a variety of factors, the main factors for various types of cracks are as follows:hydration heat of cement hydration process in the evolution of considerable heat, and are concentrated around 7d after pouring, the g

5、eneral per gram of cement can release about 500J of heat, if the cement content 350kg / m 550kg/m3 to calculated per cubic meter of concrete will release 17500KJ 27500, so that the internal temperature of concrete increased (up to about 70 C or even higher). Especially in terms of mass concrete, thi

6、s phenomenon is more serious because of the different thermal conditions inside the concrete and the surface of the concrete center temperature is high, the temperature gradient is formed, the compressive stresses inside the concrete, the surface tensile stress, when the pull stress exceeds the ulti

7、mate tensile strength of concrete when the concrete surface will crack.Shrinkage of concrete induration in the air volume decrease phenomenon is known as the shrinkage of concrete. The concrete in this case without outside spontaneous deformation when subject to external constraints (support conditi

8、ons, steel, etc.), tensile stresses in the concrete, making concrete cracking. Cause the cracks of the concrete plastic shrinkage, drying shrinkage and temperature contraction of three. Mainly early in the hardened cement paste volume changes in the hydrationsolidification knot hard, post-shrinkage

9、deformation is caused by the evaporation of free water inside the concrete.the outside temperature and humidity changes of large concrete structures during the construction period, the outside temperature changes to prevent the generation of cracks in mass concrete plays a big impact. The internal t

10、emperature of the concrete is superimposed by the pouring temperature, the adiabatic temperature of the cement heat of hydration and structure of the cooling temperature, temperature and composition. Placing temperature has a direct relationship with the outside temperature, outside temperature the

11、higher the pouring temperature of the concrete will be the higher; if the outside temperature to reduce the large volume of concrete will increase the temperature difference between inside and outside the gradient. If the outside temperature is decreased too fast, will cause great thermal stress, is

12、 extremely prone to lead to concrete cracking. In addition outside the humidity has a great influence on the concrete cracks, the outside of the lower humidity will accelerate the drying shrinkage of concrete, can lead to concrete cracks.Other factors affecting the structure-based differential settl

13、ement will produce cracks, such cracks as foundation settlement and continue to increase, to be the foundation sinking stable, will not change. Super load or meet the design loading contained prematurely cause structural cracks, the crack is called a load crack.Concrete mix than bad will cause the c

14、oncrete plastic settlement cracks, usually the mix proportion of concrete, coarse aggregate gradation not continuous, the number is not enough, the rate of sand and water-cement ratio caused cracks.2. Mass concrete construction quality control measuresvolume of concrete mix designThe selection of ra

15、w materials to cement a direct impact on the amount of heat of hydration and the concrete temperature and students, and mass concrete should be used in the low heat of hydration of cement, such as low heat slag Portland cement, moderate heat Portland cement and to minimize the amount of cement. The

16、fine aggregate should be sand in Zone 2, because the use of sand with fine sand to reduce the amount of water and cement. Pumpable coarse aggregate, the choice ofparticle size 5-20mm graded stone to reduce shrinkage deformation. Admixture, the application of fly ash technology. Concrete mixed with f

17、ly ash not only can save cement, lower heat of hydration, increase concrete workability, and can significantly improve the late strength of concrete, bed temperature peak time.Admixture using. Using the reducing agent, such as retarding superplasticizer; the use of expanders, such as extensive use o

18、f U-type expansive agent anhydrous sulfur calcium aluminate or aluminum sulfate. The tests showed that the added expanders after the expansion of the concrete stress in the concrete, can offset part of the shrinkage stress of the concrete, so corresponding increase in the concrete cracking strength.

19、The temperature control measures and construction site controlThe temperature forecast. With the temperature ratio and the construction of climatic conditions and a variety of conservation programs, according to the site concrete using computer simulation technology to the temperature field and the

20、temperature difference between the computer simulation of dynamic prediction of concrete construction period, provide the structure of the temperature distribution along the thickness direction and with the age of concrete change situation, to develop the concrete during the construction period with

21、out the effect of temperature crack temperature standards, optimizing selection for heat conservation.pouring concrete program. The delay measures of the difference in temperature gradient and cooling gradient, before pouring the detailed calculation block, hierarchical placement order, the flow, ca

22、sting thickness, width, length, before and after pouring the lap time; control of the concrete into the temperature and strengthen vibrator strict control of the vibration time, distance and insertion depth to ensure that the vibration compacting, to prevent leakage of vibration and excessive vibrat

23、ion to ensure a uniform compacting concrete; do a good job-site coordination of organization and management, adequate human and material resources to ensure that the construction went according to plan , to ensure the supply of concrete to ensure that do not stay cold joints; thick mass concrete sur

24、face after pouring the slurry to make the necessary processing, with the wood long scratch feet Calibrating generalafter pouring 3 4h preliminary, before the initial setting iron drum RCC 2 times, then wood trowel rub compaction, in order to control surface cracking; the concreting After take immedi

25、ate and effective insulation measures in accordance with regulations covering conservation.The temperature monitoring. Set in the concrete external temperature measuring point, set the insulation material temperature measuring point and the conservation of water temperature measuring point-site temp

26、erature monitoring data collected and analyzed with the data collection instrument automatically. The temperature of each measuring point, the temperature difference between the value of the location of the center points and the surface points, as a basis for adjust the temperature control measures

27、to prevent the concrete temperature cracks.To reflect the temperature effect can be buried in a small number of concrete strain gauge temperature stress testing, strain gauge layout along the horizontal direction, the direction of the stress components of the level of detection.with water cooling. T

28、hin-walled steel pipe with some concrete pouring stratification did not cooling water pipes, cooling water pipe to test the water before use to prevent pipeline leaks and obstruction, according to the concrete internal temperature monitoring, water flow and temperature control of cooling water pipes

29、.The structural design of mass concrete to take on the crack control measuresdesign and reasonable structure, can reduce the quantities of works to reduce heat of hydration. Such as according to the characteristics of the suspension bridge anchorage force, design hollowed out non-critical mechanical

30、 part of the concrete volume earthworks pressure re-program, and to reduce the volume of concrete structures.full use of concrete unconfined conditions in the pit, mixing in concrete micro-expanders, and the formation of certain pre-pressure in the pit constraints, the compensation of the internal t

31、emperature of concrete shrinkage tensile stress generated in order to effectively avoid concrete cracks.large mass concrete, the construction cycle is generally longer, in accordance with the structure by the force of circumstances may reasonably determineconcrete assessment of the acceptance of age

32、, breaking the normal standard of 28 days, assess the acceptance age instead of 60d or more days to assess the acceptance age take full account of the late strength of the concrete, thereby reducing the design label, and to reduce the cement content of concrete to reduce heat of hydration for the purpose.to reduce the boundaries of the constraint will produce thermal stress, to improve the structural design of boundary constraints, if there