The Important Function of Cover Thickness in Reinforced Concrete Members
Steel bars and concrete have become an inseparable whole in construction engineering. From the analysis of the physical and mechanical properties of the materials, the elastic modulus of the two is relatively close, and there is good cohesion. Steel bars have strong tensile, Compressive strength, while concrete only has a high compressive strength, so reinforced concrete can share the external loads borne by structural members.
For the section design of the stressed member, the farther the tensioned steel bar is from the compression zone, the greater the external bending moment that the steel bar per unit area can bear, so the higher the efficiency of the steel bar. However, since the main component of steel is iron , iron is easily oxidized at room temperature. The steel bar is wrapped in the concrete member to form a passive protective film, which prolongs the service life of the steel bar. If the thickness of the reinforcement protective layer is too small , on the one hand , it is easy to cause the reinforcement to be exposed or the concrete on the surface will peel off when the reinforcement is stressed; on the other hand , the concrete on the surface will gradually carbonize as time goes by. The concrete outside the steel bar loses its protective effect , which leads to corrosion of the steel bar, reduction of the cross section , loss of cohesion between the steel bar and the concrete , damage to the integrity of the component, and in severe cases, damage to the entire structural system; if the protective layer of the steel bar is too large, The external bending moment that the steel bar per unit area can bear becomes smaller , and the efficiency of is reduced. The light one reduces the bearing capacity of the beam, and the heavy one will cause major accidents.
In order to achieve the purpose of ensuring the structural durability of the building and saving costs , through design calculations, scientifically selecting the appropriate thickness of the protective layer for on-site construction can realize the role of steel and concrete components.
Detection method of protective layer thickness in reinforced concrete members
The thickness of the protective layer is one of the physical inspection items in the "Code for Construction Quality Acceptance of Concrete Structure Engineering" (GB 50204-2002), and it needs to be measured on site after the reinforced concrete members are formed. The author's unit uses the local damage method and electromagnetic induction method to detect the thickness of the protective layer, and implements the "Technical Regulations for the Detection of Reinforcement in Concrete" (GJG/T152-2008) during the detection process.
1. Local damage detection method
Chisel the concrete cover until the rebar is exposed , then measure directly from the concrete surface to the outer edge of the rebar. This method is the most direct and accurate, can meet the precision required by the specification , and can be used as a means of final ruling when there is a dispute .
2. Electromagnetic induction method
1) Equipment and principle. Using the Swiss Hilti steel bar scanner , the signal transmitter is used to generate an electromagnetic field, which excites the steel bar generate an induced current, and the induced current of the steel bar stimulates an electromagnetic field, which is received and identified by the receiving device. According to the strength of the received electromagnetic field , the steel bar location , depth and bar diameter.
2) Preparation before testing. The instrument must be calibrated to pass the test, and the surface should be treated flat to avoid electromagnetic interference sources (such as motors, electric welding machines, metal embedded parts, etc.).
3) Survey line layout. Determine the arrangement direction (direction) of the steel bars to be tested, then arrange a measuring line perpendicular to the direction of the steel bars to be tested, and scan the steel bars to be tested along the line to determine the position of the steel bars and the thickness of the concrete cover. Less than one steel bar to be tested. When there are steel bars parallel to the measuring line, in order to avoid the influence of these steel bars and improve the test accuracy, the positions of these steel bars should be scanned first , and then the steel bars scanned.
4) Several methods to improve test accuracy. ①Preset the steel bar diameter; ②Select the appropriate protective layer thickness gear ; ③Reset the probe (remove the residual magnetism on the probe ) ; ④Control the scanning speed to (20cm/s is appropriate) ; ⑤Avoid the interference of irrelevant steel bars ; ⑥When When the protective layer is very small (such as less than 10mm), it is better to add a non-magnetic pad before testing.
5) Evaluation of test results. The concrete thickness detection of steel bars belongs to the counting sampling test. The qualification determination is to calculate the percentage of qualified points and detection points, so as to determine whether the unit project is qualified . Regarding whether a single point is qualified , the "Code for Acceptance of Construction Quality of Concrete Structure Engineering" (GB50204-2002) stipulates that the allowable deviation of beams and columns is +10mm, -7mm; the allowable deviation of walls and slabs is +8mm, -5mm; and not The maximum deviation of qualified points shall not exceed the times of the allowable deviation. For the qualification judgment of a unit project, it is required that when the qualified point rate of all steel bar cover thickness inspections accounts for more than 90%, the inspection result is qualified ; when the qualified point rate is between In the above, the test result is qualified ; otherwise , the test result is unqualified.
For more original content, please read the original text of "Discussion on the Thickness of Steel Bar Cover".
