1、Hydration heat of mass concrete in the bridge and cracks In 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 o
2、f 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. 1.The main reason a large volume
4、 of concrete cracks The 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: 1.1 hydration heat of cement hydration process in the evolution of considerable heat, and are concentrated around 7d after pouring
5、, the general 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 or even higher). Especially in terms of mass concrete
6、, this 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
7、 ultimate tensile strength of concrete when the concrete surface will crack. 1.2 Shrinkage of concrete indurations 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 (supp
8、ort conditions, 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 hydration solidification knot hard, pos
9、t-shrinkage deformation is caused by the evaporation of free water inside the concrete. 1.3 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 impac
10、t. The internal temperature 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
11、 temperature the 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 t
12、hermal stress, is 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. 1.4 Other factors affecting the structure-ba
13、sed differential settlement 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 th
14、an bad will cause the concrete 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 measures 2.1 volume of concrete
15、mix design 2.1.1 The selection of raw 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
16、 minimize the amount of cement. The 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 of particle size 5-20mm graded stone to reduce shrinkage deformation. Admixture, the application of fly
17、ash technology. Concrete mixed with fly 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. 2.1.2 Admixture using. Using the reducing agent, such as retarding superplasticizer； t
18、he use of expanders, such as extensive use of 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
19、 increase in the concrete cracking strength. 2.2 The temperature control measures and construction site control 2.2.1 The 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
20、 simulation technology to the temperature field and the 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
21、develop the concrete during the construction period without the effect of temperature crack temperature standards, optimizing selection for heat conservation. 2.2.2 pouring concrete program. The delay measures of the difference in temperature gradient and cooling gradient, before pouring the detaile
22、d calculation block, hierarchical placement order, the flow, casting 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 c
23、ompacting, to prevent leakage of vibration and excessive vibration 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
24、to ensure that do not stay cold joints； thick mass concrete surface after pouring the slurry to make the necessary processing, with the wood long scratch feet Calibrating general after 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 immediate and effective insulation measures in accordance with regulations covering conservation.
