Relative to the present stage of the construction project, reinforced concrete due to low cost, strong and durable and a wide range of material sources, high compressive strength of concrete, steel and other advantages of good tensile properties are widely used. Good bonding strength and similar thermal expansion coefficient is added between steel and concrete, concrete can be good protection for steel, combined with work performance and good durability, has become an important part of modern building materials. However, many reinforced concrete structures due to disease reasons, the actual service life is far less than the design life, in which the protection layer thickness deviation caused by the most common diseases.
2. Definition and function of concrete cover thickness
The thickness of concrete cover is the minimum distance between steel surface and concrete surface. It has two main functions: one is to make the stress of reinforcement and concrete bond layer between the adhesive force enough, to ensure the concrete and reinforcement work; two is to protect the steel structure in the design service life to avoid external invasion and destruction of harmful medium. Concrete protective layer must reach a certain thickness in order to play its due role. National and industry technical specifications have specified the minimum thickness of different types of reinforced concrete protection layer.
3. Influence of concrete cover thickness on reinforced concrete structure
The thickness of concrete protective layer, external harmful medium is easy to penetrate the protective layer to the steel surface caused by steel corrosion expansion, concrete cracking by reinforcement, crack or even fall off, thereby accelerating the corrosion damage. The damage rate of reinforced concrete structures caused by the insufficient thickness of concrete protective layer in coastal areas is much faster than that in inland areas. For example, the coastal area of Jiangsu Province water conservancy project of reinforced concrete structure is considerable due to lack of protective layer thickness, tend to work less than ten years, the shortest only two or three years, it has been fully reinforced corrosion layer, the surface spalling of concrete protective structure, security is greatly threatened.
It is because of the above reasons, the Ministry of transport industry standard "harbor engineering concrete structure corrosion protection technology standard" (JTJ275 - 2000) than the national standard "concrete construction quality acceptance" (GB50204-2015) of the minimum thickness of concrete protective layer put forward higher requirements.
The thicker the concrete protective layer is, the more favorable the durability of the structure is. However, the effective section size decreases and the bearing capacity decreases when the structural section size is unchanged. Therefore, in the construction process should be strictly in accordance with the requirements of design drawings of steel cage, concrete protective layer is too thin too thick will cause major accidents. "Code for acceptance of construction quality of concrete structures" (GB50204-2015) of the protective layer thickness of allowable deviation made strict rules: steel plate of the protective layer thickness of the allowable deviation of value (+8 mm, -5 mm), the allowable deviation for beam type (+10mm, -7 mm).
According to the test data for statistics, our province water conservancy project protection layer thickness all qualified reinforced concrete structure is less than 30%, the beam bottom and the bottom protection layer thickness above 80% presents negative deviation; large section height of reinforced concrete structure, the side and the top surface protection layer thickness more than 80% showed a positive deviation. This leaves hidden dangers to the safe operation of the project.
4. Concrete protective layer thickness detection
1 detection method and detection equipment
The thickness of concrete protective layer can be tested by general survey (non reinforced detector damage) and local cut (micro damage) combination method. Steel detector with electromagnetic detector and radar detector method of reinforced reinforced two steel, the former is divided into steel detector detector and application of eddy current effect of the application of electromagnetic induction.
Electromagnetic steel is usually composed of detector probe, the host and the connecting line. The probe receives host commands, generates electromagnetic fields, continuously contacts with the concrete surface and scans. When the magnetic field deformation in reinforced concrete and other metal objects in the magnetic field, interference, steel and other metal caused by electromagnetic field intensity distribution is changed, to probe and receive delivery back to the host, the host in analogue or digital mode explicit metal position. Proper calibration of the metal and concrete measured by the instrument, the main show that the steel (metal) protective layer thickness.
2 influencing factors of detection accuracy
The main factors affecting the detection accuracy of concrete cover thickness are:
(1) the density of the reinforced principle. The density of reinforced protective layer thickness determines the detection precision, when the bar spacing is 1.5 times larger than the thickness of the protective layer, the adjacent steel has no effect on the accuracy of detection of protective layer thickness.
When the reinforcement spacing is less than the thickness of the protective layer, the denser the steel bar, the smaller the indication value of the instrument. Especially reinforced with small diameter, the thickness of the protective layer in a range of Neiduogen reinforced dense array, showing the value of small instrument can reach more than 30%. But the depth direction dense arrangement steel bar has the relatively small influence to the protection layer thickness detection precision.
(2) the probe axis and axis angle steel. Whether the probe axis is parallel to the axis of the steel bar also has a great influence on the detection accuracy of the concrete protective layer thickness. When the probe axis is parallel to the axis of the steel, the indication value is true. When the probe axis and the steel axis 45 degrees oblique, the display value is greater than 10%. When the probe axis is perpendicular to the bar axis, the display value is larger than 20%.
(3) instrument parameter setting. Whether the setting of instrument parameters is accurate has great influence on the thickness detection accuracy of concrete protective layer. When the diameter of the steel bar of the instrument is equal to the actual size, the indication value of the instrument is true. Set value twice or smaller than actual size