I. Preface
In recent years, our institute has undertaken more than ten large-span prestressed concrete bridge construction stress monitoring tasks. There are various bridge types such as cable-stayed bridges, continuous girders and rigid structures. While completing the tasks specified in the contract, a lot of tests and discussions have been done to improve the quality of testing. According to the observation of concrete bridge construction stress, it is found that there are still some problems that are not well solved, summarize the lessons learned in time, and constantly improve the quality of testing, which is the need of the current development of bridge construction.
With the development of the bridge business, prestressed concrete bridges have been widely used, different bridge type of prestressed concrete bridges in the construction, span gradually increased, the structure is also to the development of thin-walled lightweight, cantilever construction insecurity factors increase. In order to avoid uncontrolled installation stress, there are more bridges to monitor the construction, and the construction stress test also put forward higher requirements. Now the construction stress testing technology is far from keeping up with the needs of bridge engineering development.
This paper summarizes the construction stress test of large-span prestressed concrete bridges over the years, and talks about the specific methods to improve the test accuracy and ensure the test quality on the basis of the existing problems of the current test.
Two, improve the quality of the test to see the discussion
Concrete is a non-homogeneous material, in addition to deformation of force, there are a variety of factors also cause deformation. At present, there is no perfect method for direct testing of concrete stress, is still measured through the strain and then converted, due to a variety of deformation of the mixing, making the test work becomes very complex.
Temperature on the bridge structure is very obvious can be divided into two aspects, one is the structure of the deformation with the temperature change, that is, thermal expansion and contraction; in addition by the sunshine and other reasons caused by the structure of the temperature difference, the temperature difference on the structure of the impact of the structure is large, but also very complex.
Humidity on the concrete structure is also more obvious, recently used pumping concrete, due to the increase in the amount of water and cement, in the shrinkage of deformation also become larger.
The volume change of concrete itself is caused by the internal hydration of cementitious materials deformation, the structure has an impact, but not too obvious.
The creep is one of the characteristics of concrete, in the case of constant load, the deformation is increasing. It has a significant and complex effect on structural forces.
About concrete deformation can be summarized into two categories: one is the deformation of concrete force, including load elastic deformation, creep deformation and deformation caused by temperature difference; the other is the volume deformation, refers to the concrete temperature deformation, humidity deformation and its own volume deformation, etc.. Can be expressed in the following formula: ε total = ε force ε volume = (ε Ho load ε creep ε temperature difference) (ε temperature ε humidity ε since)
Construction stress test is to observe the part of the elastic strain caused by the load under different working conditions. The amount of load is measured, and the stress σ load = E ε load is converted using the following formula where E is the modulus of elasticity.
And the strain measurement includes the above two types of deformation. In order to get a more accurate load strain, it has been studied for many years how to separate the other strains from the measured values.
1. Volume deformation compensation method test
The monitoring of hydroelectric dams has been using double-walled cylinder stress-free gauge, which consists of a double-layer tin cylinder and soft material filled in the middle, which eliminates the load-induced deformation. The measured value is only the concrete volume deformation, which is used to compensate for the dam concrete volume deformation.
In the 70's in jiujiang Yangtze river bridge sinkhole test had used this stress-free meter. In the sinkhole has not been sunk when observed on the stress gauge found that the temperature and strain gauge is not consistent. Humidity difference is even greater, when the well wall concrete in the shrinkage of 200 to 300 microstrain, and no stress gauge only tens of microstrain, sinking wells sinking, due to the expansion of the well wall immersed in water, the gap is gradually reduced.
It is analyzed due to the concrete in the Jane surrounded by iron and soft materials. It affects heat conduction and moisture emanation, making the concrete inside and outside the box differ in both temperature and humidity.
In order to further verify also produced concrete test blocks, while buried working strain gauge and no stress gauge. The difference between the two was found to occur gradually during the incubation period after the specimen was filled, and the difference became larger and larger as the age increased. At this time the test block is not stressed, the strain gauge value should be the same as the stress gauge are volume deformation, the two are inconsistent, proving that the cylinder stress-free can not play a compensatory role. Later also tried different sizes of concrete blocks, the same type of strain gage inside, as a compensation block. Compensation block and the corresponding concrete on the structure at the same time filling, the same maintenance, the test will be placed on the compensation block in the observation of the site, try to maintain the same conditions. However, due to the small size of the compensating block and the many interfaces, the heat conduction and moisture emanation were relatively fast, and later it was also found that the temperature and humidity inside the compensating block differed greatly from that of the structure. The same does not play a good role in compensation.
2. Measures to reduce the temperature difference and the effect of creep
The temperature difference of the bridge structure is mostly from sunshine, and the temperature difference can also be formed by the sudden drop of temperature. According to observation, the temperature difference between the top of the beam and the bottom of the beam caused by sunshine can reach 17 ~ 19 ℃. It can make some parts of the considerable temperature stress. It may also make the cantilever end of the construction produce large dislocation. Specifically related to the mechanical properties of the structure itself and the surrounding constraints. As the temperature field caused by insolation is itself a random variable. So the temperature difference to the structure is quite complex.
The effective way to reduce the effect of temperature difference is to measure in the morning before sunrise. The temperature difference formed by sunshine basically tends to stabilize overnight. Only the closed box inside and outside the temperature difference, but the impact is not significant. In the morning, not only the beam parts of the temperature difference is minimized, and the temperature difference of the whole bridge parts is also minimized. Therefore, the morning before sunrise measurement is to eliminate the effect of temperature difference effective method.
The creep deformation of concrete is affected by a variety of factors, which is relatively complex. Among them, the size of creep deformation is related to the length of the loading age. Reduce the impact of creep, can be used to encrypt the number of measurements, take each time before and after loading the relative value of the cumulative approach, there is a certain effect. Cantilever construction of each casting (or assembling) a beam section takes a certain amount of time, the test can be a few of the load change is relatively large process selected. Such as moving hanging baskets, tying steel, pouring concrete, tensioning prestressing, etc., before and after these processes were measured, take the difference between before and after the superimposed accumulation. This is not possible to completely separate out the influence of creep, but can always reduce some of the error.
Three, on the selection of test components
Large-span concrete bridge construction stress test has a number of conditions and certain difficulties, so the test components are also very high requirements. First, the test time is long, generally about a year, some also long, the requirement of long-term stability of the test element; Second, the load increase and decrease frequently, but each time the amount of change is not necessarily very large, so the requirement of the test element to be high sensitivity; Third, the bridge is mostly a thin-walled structure, dense reinforcement and no pre-stressing, the element is difficult to bury the component, the component volume is as small as possible, the construction of a simple, not easy to damage; Fourth, the measurement point is relatively decentralized, the wires, Cable dispersed on the bridge, due to construction interference bruises and smashes often occur, so the best components have a strong adaptability.
According to the above requirements, the previous use of differential resistance (DI a 100) strain gages, it can meet the requirements in terms of sensitivity and long-term stability. Small size, installation and burial is also relatively convenient, but it is the use of "resistance change" principle made of sensors, susceptible to temperature, in addition to the components themselves need to be corrected at any time by the impact of temperature, the cable and the connector changes to the measured value of the fluctuations caused by. Originally, the test of the concrete structure is very complex, coupled with the component itself is susceptible to external interference, more difficult to increase the test work.
After the change to vibrating string type strain gauge, it is in the long-term stability and sensitivity can also meet the requirements, in addition, it is through the vibration frequency changes in the conversion of data, from the construction of steel strings and shells are also steel, the coefficient of thermal expansion of basically the same, and therefore insensitive to temperature changes, the wire is not high requirements. After the change to vibrating string components not only organize the data is simple and trouble-free, and from the test results to see the regularity is also good.
But the vibrating string element is not yet a unified standard, the manufacturer is mostly a small local factory, which is the key to the treatment of the steel string. If the treatment is not in place, in the long-term test is easy to relax, resulting in zero drift, to the data analysis brings trouble. Before use, it is best to purchase six months in advance, and then observe the zero point stability. In addition to selecting a few for recalibration, calibration curve of the calibration of the manufacturer.
Currently, the vibrating string strain gage shell with steel pipe, some steel pipe wall thickness, should check the stiffness of the strain gage itself before use, such as greater than the same cross-section of the concrete stiffness, but also through the test to find out the stiffness ratio of the two on the measured value of the correction.
Vibrating string strain gage due to the structure of the relationship between the tensile stress test error, when the observation part of the tensile stress, should be changed to other test components.
Four, on the test of several issues that should be noted
(1) construction stress monitoring involves a lot of information and data. In addition to design information, construction is also a lot, such as construction technology, construction program, construction organization design and hanging basket, template data. The weight of the main construction machinery and equipment on the bridge and other construction loads. Beforehand should be carefully collected, carefully investigated.
Theoretical analysis and calculation of the structure is very necessary. Testing should be based on the need to establish their own calculation system. Prior to the observation of the stress state of the site in a variety of working conditions to analyze and calculate, so as to have a clear idea.
(2) strain gauge installation to go through the process of concrete casting, vibration and concrete hardening. Especially concrete hardening is a very complex process of change, there are hydration heat temperature rise and their own volume of contraction. Due to the temperature field and in the shrinkage changes are not uniform, often resulting in residual stress will affect the strain gauge, resulting in large fluctuations in readings and instability. In order to determine the initial value of the measurement point, the concrete should be tracked after casting observations to stabilize the measured value as the initial value. Some of the measuring points take a long time to stabilize, and with the construction of the measurement, this part of the measuring point, when the data are found to be obviously unreasonable, can be adjusted appropriately.
(3) large-span bridge construction process, it is inevitable that there will be some problems, sometimes disrupting the normal construction sequence, the construction load will change, the test should pay attention to the changes, and make good records. Many changes will directly affect the test results.
Additionally, the test found that individual measurement point abnormalities, should be checked in a timely manner, found that the interference as soon as possible to exclude. According to experience, when the test point near the parking construction equipment and materials or because of the water above, will also bring to the test results.
(4) can reflect the structure of the working characteristics of the stress of the test site is more concentrated, should be appropriate to set up more components, in order to prevent damage caused by the installation or testing process and can not measure the key data, but also can be used by different means or other types of components, testing at the same time.
(5) due to the nature of concrete variability, in the construction stress test, according to the actual situation of the site, should do some tests and observations on the nature of concrete, such as the modulus of elasticity of concrete, in the amount of shrinkage, such as the conditions can be carried out in other aspects of the observation, the use of these data on the test data analysis, may reduce some of the error.
(6) observation data in addition to timely organization of the entrusted unit, but also to carry out systematic analysis, found abnormal or excessive deviation, find out the reasons, as soon as possible to correct. From the past observation data, measured and theoretical calculations there is always a gap. Test data and the calculated value of the deviation is normal, the construction of overweight suspended casting blocks, construction load changes, the error caused by creep and test components and instrumentation errors, will give the measured value of the deviation.
(7) because of the construction stress test work is difficult, so we must organize a certain amount of experience in the field, in addition to careful testing work, but also to continue to analyze the research and explore ways to improve the accuracy of the test. From the previous test situation, the staff involved in the test of responsibility and work attitude is the key to test whether to obtain reliable data.
V. Conclusion
In summary, there are still a lot of problems without better solutions for stress monitoring of large-span prestressed concrete bridge construction. Before the scientific method is established, the accumulation of experience is very important. At present, due to the engineering development needs, is promoting this test work continuously, in this great opportunity, as long as adhere to the continuous practice, continuous analysis and summarization, continuous experimental exploration, will make the concrete bridge construction stress test work faster towards perfection.
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