How To Find Rebar In Concrete & Locate,Position Distribution

Definition of reinforced concrete cover

It refers to the minimum distance between the concrete surface and the outer edge of the nominal diameter of the outermost reinforcement in the concrete to protect the reinforcement from direct exposure. The minimum thickness of the protective layer is to meet the durability requirements of the concrete structure.

What is reinforced concrete cover used for

A reinforced concrete cover is a concrete layer with a certain thickness wrapped outside the reinforcement. It can ensure structural stress performance, safety and durability, fire resistance, etc.

Protect the reinforcement from corrosion 

Many factors affect the durability of reinforced concrete structures, such as chloride ion erosion, freeze-thaw damage, uncompacted concrete, concrete carbonization, aggregate reaction and so on.

After the reinforcement is rusted, the volume of rust expands, resulting in the cracking of the concrete protective layer and the infiltration of moisture, which will accelerate the reinforcement corrosion and shorten the reinforcement rust and damage to buildings.

So, we should ensure that the thickness of the protective layer is within the design range, protect the reinforcement from corrosion to the greatest extent, delay the time when the concrete carbonization depth reaches the reinforcement surface, and ensure the structure’s life. Some buildings in a harsh environment, such as a corrosive gas environment, should adopt a particular reinforcement protective layer of the concrete structure to ensure the durability of the building structure.

Related:

How to measure reinforcement corrosion

Protect the reinforcement from high temperature 

There is a required thickness for a protective layer, which can protect the reinforcement from collapse due to the structure’s rapid loss of bearing capacity under high-temperature conditions or in case of fire.

Ensure the mechanical performance of the structure

The concrete and reinforcement is the primary condition to ensure structural members’ bearing capacity and structural performance. Concrete is a brittle material with good compressive properties, and steel bar is a ductile material with good tensile properties. Combining these two materials ensures buildings or structures with compression, bending, shear and torsion properties.

This influential work depends on the sufficient gripping force between concrete and reinforcement. The gripping force is composed of adhesion, friction and mechanical bite force, which is directly related to the bearing capacity of reinforced concrete structures. It is required that the protective layer must have a certain thickness to ensure the binding force between concrete and reinforcement.

If the thickness of the protective layer is too small, it will decrease the gripping force between the concrete and the reinforcement. Therefore, the minimum size of the thickness of the concrete cover must not be less than one diameter of the stressed reinforcement.

Common reinforcement specifications and cover thickness

The nominal diameters of common reinforcement are 6, 8, 10, 12, 14, 16, 18, 20, 22, 25, 28, 32, 36 and 40mm.

Detection of reinforcement in concrete

Electromagnetic induction method: a single or multiple coil probe generates an electromagnetic field. When the steel bar or other metal object is located in the electromagnetic field, it will cause a change in the magnetic line.

The interference produced by metal will change the electromagnetic field intensity distribution, which is measured by the probe and displayed by the instrument. If the size and material of the tested reinforcement are correctly calibrated, it can be used to find the position, diameter of reinforcement and thickness of the concrete cover.

In general, the integrated reinforcement scanner adopts multiple coils, which can reach the real-time positioning of reinforcement. In comparison, a single reinforcement scanner adopts a single coil, which can not realize the real-time positioning of reinforcement.

Before measuring, we should reset the steel bar scanner, and the probe should be away from metal objects. When measuring the thickness of the protective reinforcement layer by electromagnetic induction method, it needs to predict the diameter of the measured reinforcement to achieve accurate measurement.

They are large and small, with two types of steel bar scanners in range. In the process of large range, the coil current is large, the sensitivity is high, so the measurable depth is larger, but the test accuracy worsens. In the process of small range, the coil is an ideal working mode, and the measurable depth is small, but the accuracy is high.

During the actual test on-site, the design value of the thickness of the protective layer is commonly less than 60mm, and we can choose a small range and ensure the accuracy. If the protective layer thickness is more than 60mm or we cannot measure the reinforcement in a small range, We should switch to the large range instead of the small range.

Diameter estimation: The instrument based on electromagnetic induction can accurately measure the thickness of the protective reinforcement layer only through the determined reinforcement diameter. Similarly, If you want to know the accurate reinforcement diameter only through the determined protective layer thickness.

However, it is difficult to predict the exact thickness of the protective layer in the application, so we can only estimate the diameter.

Steps

1. The thickness detection of the protective reinforcement layer

2. We choose three different positions to detect

3. Calibrate the instrument after startup

4. Keep the instrument away from the area with metal and press OK to calibrate

5. Directly enter the thickness detection interface

6. Start the calibration interface with the “up” key

7. Press OK to calibrate

8. then use the protective layer thickness measurement interface

9. Input the reinforcement diameter

10. Check the stirrup first

11. Start testing

12. When the red light is on, mark the position of reinforcement with a pen

12-1 mark the position of the other reinforcement

13. Select two stirrups and draw a straight line between them

14. Align the center line of the instrument detection coil with the straight line just drawn

15. Start

15-1 twice for each point

16.Test other two measuring points according to the above method

17. Test three group data.

Detection of reinforcement distribution and protective layer thickness in concrete

The reason why we need to determine the reinforcement distribution and protective layer thickness in concrete.

1) Estimate the position, depth and size of reinforcement in concrete.

2) When there is no information to survey the structure.

3) Avoided position before other tests.

Detection principle

1) Testing method: we can use a rebar scanner to measure the position of reinforcement and determine the thickness of the protective layer and the diameter of reinforcement.

2) Detection principle: The probe of the rebar scanner generates an electromagnetic field. When a steel bar or other metal object is located in the electromagnetic field, it will cause a change in the magnetic field line and local electromagnetic field intensity.

There is a certain corresponding relationship between the change of electromagnetic field intensity and the size of a metal object with the probe distance. If we know the rough size of reinforcement and the material, then we can measure the position, depth and size of reinforcement in concrete.

Technical requirements for steel bar scanner

The scanner includes a probe, instrument and connecting wire. At present, most of them are integrated steel bar scanners.

1) The measuring range of the protective layer of the scanner should be more than 120mm.

2) The accuracy of the scanner:

• 0~60mm，±1mm。
• 60~120mm，±3mm。
• ＞120mm，±10%

3) Suitable for measuring the size Φ 6~ Φ 50, not less than 12 grades in reinforcement diameter series.

4) Normal work in the environmental conditions with a temperature of 0 ~ 40 ℃, relative humidity ≤ 85% and no strong magnetic field interference.

Calibration of rebar scanner

1) The particular calibration block should be used to calibrate the protective layer thickness gauge. When measuring the thickness of the protective layer given by the calibration block, the measured value must be within the accuracy range.

2) The calibration should be in the environment without external magnetic field interference.

3) Before each test, we should calibrate the instrument.

Detection step

1. Reinforcement distribution in concrete

We should measure the prominent load-bearing members, the primary stress parts of load-bearing members, the parts where the reinforcement corrosion potential test results and the parts determined according to the structure and other testing needs.

2. Layout principle of measuring area

1) When testing a single component, we should evenly arrange the measuring area on the component according to the size, and the number of measuring areas on each component is not less than three.

2) Components with a single size of more than 5m can appropriately increase the number of measuring areas.

3) The measuring area should be evenly distributed, and the spacing between two adjacent areas is not less than 2m.

4) The surface of the area should be clean and flat, avoiding joints, honeycombs, pitted surfaces, embedded parts and other parts.

5) Mark the number of the area, and record the location and appearance.

6) Number and requirements of measuring points:

At least 10 measuring points for each measuring area on the component.

The distance between measuring points should be less than the sensor length of the tester.

7) For the same type of components testing, we can adapt the sampling number that should not be less than 30% of the number of similar components and not less than 3 pieces. The layout of each component area should be arranged according to the requirements of a single component.

8) The whole structure measuring can be classified according to the component type first and then tested according to the type.

3. Measure

1) It should know relevant drawings and data before testing to determine the type and diameter of reinforcement.

2) Before measuring and reading the thickness of the protective layer, It should determine the position of the reinforcement in the measuring area:

Move the sensor of the tester in parallel on the surface of the component when the instrument displays or sounds; the measured reinforcement’s position is directly below the sensor.

After finding the reinforcement position, draw the reinforcement position on the surface of the area.

3) Measurement and reading of protective layer thickness.

Place the sensor directly above the position of the reinforcement, move it slightly left and right, and read the displayed value of the instrument.

The value of each measuring point should be tested 2 ~ 3 times and get the average value with an accurate level of 1mm.

Avoid the measurement at the intersection of reinforcement.

4) For lack of data and cannot determine the diameter of reinforcement, It should measure the diameter of reinforcement first. To measure reinforcement diameter, test 5 ~ 10 times, eliminating abnormal data and calculating its average value as the final value.