The so-called "nondestructive testing" technology of concrete is to obtain physical quantities related to concrete and steel bars by using testing instruments without destroying structural members. Because these physical quantities have a good correlation with concrete quality, strength, concrete defects and the location of steel bars, the obtained physical quantities can be used to estimate the quality (strength, concrete defects) of concrete and the situation of steel bars.
First, the past concrete structure engineering quality inspection review
A few years ago, in the process of quality supervision of concrete structure engineering, simple tools, such as small hammer and tape measure, were generally used to judge the surface, while the quality of hidden parts could only be judged by data, which inevitably lost accuracy and objectivity. In particular, if the internal management of the construction unit is chaotic and the information is falsified, there will be considerable quality hidden dangers, which will often lead to cracks, sinking and even serious collapse accidents after the house is completed and accepted, causing great losses to people's lives and property.
For example, a project failed to accurately detect the strength of reinforced concrete beams and columns in the acceptance process, and the acceptance personnel only judged it as a qualified project based on their experience and the strength test report with the same conditions provided by the construction unit, resulting in cracks in the house after moving in. After experts on-site inspection of each component one by one and key local damage inspection, after comprehensive analysis, the conclusion is that the interior of concrete is not dense, and the strength of pellets is lower than the design requirements. The reinforcement project cost a lot of money, and the cost of later reinforcement far exceeded the treatment cost at that time, which caused great losses and troubles to all parties concerned.
Second, the application and effect of nondestructive testing technology in concrete structure engineering
Concrete nondestructive testing technology does not damage concrete structural members, and can obtain the information of concrete physical quantity that people need most. The test operation is simple, the test cost is low, and it is not limited by the shape and size of the structure. The test can be repeated many times, the important structural parts can be monitored for a long time, the concrete structure (or component) can be inspected (monitored), and all kinds of information can be processed in time to reduce losses and avoid accidents. Practice has proved that concrete nondestructive testing technology has strong vitality.
According to the code for design of building structures, building structures must meet the safety requirements during the design reference period. Applicability and durability requirements, in which safety determines the life of the building structure. The concrete quality of each component (beam, slab and column) in the building structure is directly related to the safety of the structure, and the protective layer of steel bars also has an important influence on the safety and durability of the building structure. In practice, quality defects and accidents related to the strength of concrete and the location of steel bars often occur.
For example, in the acceptance of a concrete sub-project of a project, in addition to checking the data, nondestructive testing equipment-digital display rebound instrument and rebar position detector are mainly used to detect the concrete strength and rebar position of the concrete structure respectively, and it is found that the concrete strength of several reinforced concrete columns and beams can not meet the design requirements (the results are consistent through core sample sampling and breakage inspection). When measuring the negative bending moment steel bar of cantilever plate with steel bar position protective layer tester, it is found that the thickness of protective layer is higher than the design value. After on-site inspection, it is found that the settlement of steel bar is large, the spacing does not meet the requirements, and the number of steel bars arranged is not enough. If these problems are not found in time, it will cause serious quality accidents. With the effective technical support of nondestructive testing equipment, it is helpful for us to accurately find and eliminate hidden dangers.
Iii. Enlightenment of nondestructive testing on exploring and studying new building technologies
In the process of nondestructive testing, we have been thinking about and studying some new construction techniques, which can eliminate the displacement of steel bars such as plates and columns during construction and the quality defects that concrete can not meet the strength design requirements in certain environments, such as wet and cold areas.
3. 1 Thickness control of reinforced protective layer of plate column
In a large number of structural tests, it is found that most of the main reinforcement protective layers of columns are unqualified, and controlling the thickness of longitudinal reinforcement protective layers has always been a difficult point in construction. We explored the method of welding tic-tac-toe reinforcement (as shown in figure 1), that is, welding tic-tac-toe reinforcement from concrete floor along the longitudinal reinforcement direction 10~20mm, aligning the tic-tac-toe reinforcement with the sideline, and coating sections with anti-rust paint at both ends to avoid damaging the main reinforcement by 20~30mm during welding, so as to support the formwork and the protective layer of the main reinforcement.
After welded TIC-tac-toe reinforcement, according to the specifications of the main reinforcement, select the plastic protective layer retaining ring, and set a retaining ring along the main reinforcement.
Clamping 500 ~ 700 mm column top retaining ring, straightening reinforcing cage, and then closing the mold. After the formwork reinforcement and correction is completed, place a special (50~ 100) main reinforcement fixture on the lower skin of the column top beam. After concrete pouring is completed, remove the fixture for the next specification and reinforced concrete. However, it should be noted that the well-shaped reinforcement cannot be welded continuously along the longitudinal reinforcement, so as to avoid the deformation and cross-section increase after the column is formed. In order to ensure the thickness of the protective layer of the negative bending moment reinforcement of the slab, the methods of padding and supporting under the horse stool walkway and negative reinforcement are mainly adopted, such as fixing with thin iron wire to avoid the rotation of negative reinforcement, which can better ensure the construction quality.
3.2 Treatment of cracks in cast-in-place concrete in wet environment (such as sewage treatment project)
For example, cracks of different sizes appear on the surface of reinforced concrete water pipes in sewage treatment projects. When measuring the thickness of reinforced protective layer on the inner wall of reinforced concrete water flow pipe with reinforced protective layer tester, it is found that the value is obviously small. These surface or shallow concrete cracks will not affect the safety of building structures, but will lead to the deterioration of frost resistance, impermeability and erosion resistance of concrete. The water in cracks in winter will accelerate the corrosion of steel bars and the frost heaving damage of surface concrete in winter, which will affect the safety and durability of the structure. However, the thickness of the protective layer on the inner wall of reinforced concrete flow pipe is obviously small, mainly because the construction party focuses on fixing the position of formwork and vibrating concrete, but ignores the thickness of the protective layer. We suggest that cement-based permeable crystalline waterproof material (SJF) containing activating ingredients should be used in the construction process, and it should be painted on the surface of concrete members, so that it can penetrate into the concrete structure and crystallize, resulting in waterproof effect. This can prevent water and air from entering concrete, eliminate internal defects of concrete and ensure structural safety. It is observed that the treatment effect is good.
Four. Concluding remarks
There are many difficulties in quality control, such as the above. In a large number of engineering inspection practices, many technical methods we have explored have been applied to engineering construction, and the damage inspection completely conforms to the Code for Acceptance of Construction Quality of Concrete Structure Engineering (JB50204-2004). Through the application of digital nondestructive testing technology in recent years, we have found many quality hidden dangers, eliminated them in time and prevented them before they happen. The objective test data provides a scientific and reliable judgment basis for building structure detection, which makes the building structure detection workers change the unscientific working mode of drawing conclusions from experience in the past. Modern and advanced digital scientific inspection management methods provide technical support for the quality of construction projects, making the quality supervision of construction projects more scientific and standardized.
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