ASERTIVIDAD VS. PASIVIDAD Y AGRESIVIDAD

The surface strength is one of the most important material properties concerning existing concrete structures. It can deliver qualified information about the quality of the border zone of the concrete structure.

Impact Receiver 1

I

Receiver 2

Figure 3.19 Schematic drawing of how to measure the surface velocity with impact echo.

How to assess the status of a structure 53

Figure  3.20 illustrates the difference between the surface strength of concrete and the adhesion strength of a surface protection system or a mortar- based coating on top of con-crete or masonry. Generally speaking, if the rupture occurs in the substrate, the adhesion strength of the coating to the concrete is greater than the surface strength of the substrate (concrete), and in this case the test delivers the surface and not the adhesion strength. Despite the different types of strength, the test itself is conducted equally.

Figure 3.21 illustrates the basic test procedure according to (DIN) EN 1542:1999, which regulates the determination of the surface strength of concrete. This standard is very com-prehensive and covers all possible cases, which can occur while investigating an exist-ing structure.

The test is usually conducted on site or in the laboratory on smaller samples in the fol-lowing manner:

• Drilling of a cylindrical nut with a diameter of 50 mm and a depth of at least 10 mm into the substrate

• Applying a steel die with a reactive adhesive, e.g., (sand- filled) epoxy resin

• Cleaning of the nut so that no adhesive is in the groove and falsifies the result

• Adapting of the test rig to the steel die and performing an uniaxial tension test with a load rate of 100 N/ s (concrete surfaces or stiff surface coatings) or 300 N/ s (elastic surface coatings)

• Calculating of the tensile strength as well as judging the type of rupture in percentiles of the entire cross section of the die

Determination

of the surface strength Determination

of the adhesion strength

Figure 3.20 Left: Determination of the surface strength. Right: Determination of the adhesion strength.

Figure 3.21 Determining the surface strength according to (DIN)  EN  1542:1999. (From Momber, A.W., Schulz, R.-R., Handbuch der Oberflächenbearbeitung Beton, Basel: Birkhäuser, 2006.)

The judging of the rupture percentiles is usually done based on experience of the test personnel and does not include complex measurements. The type of rupture refers to the location of rupture within the material, e.g.:

• X% substrate

• X% cohesion in the surface coating (if more than one layer is applied on the surface, the type of layer has to be indicated)

• X% adhesion between the surface coating and the substrate

In order to avoid any influences due to the location, the rules given in Figure 3.22 should be observed. (DIN) EN 1542:1999 defines 50 mm as the diameter of the steel die; thus, all mentioned dimensions in this standard are only valid for this diameter. If the diameter of the steel die has to be adapted, the given rules enable us to calculate individual dimensions.

Besides the location of the test areas, the steel dies that are used to adapt the test rig onto the surface of the structure should have certain dimensions. These dimensions are required

Depth of specimen or building member ≥ 2 . d_s

Distance of groove from edge of specimen ≥ d_s

Distance between each test area ³ d_s (if no groove is present

distance ≥ 1.5 . d_s)

(if no groove is present distance ≥ 1.5 . d_s)

Figure 3.22 Requirements for the selection of the position of test areas. (From Momber, A.W., Schulz, R.-R., Handbuch der Oberflächenbearbeitung Beton, Basel: Birkhäuser, 2006.)

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so that the die does not wrap due to the test load. Additionally, it is also required to drill at least 10 mm into the substrate. The drilling ensures that the tension is applied unidirection-ally in the surface, and the rupture only occurs in a defined area.

The surface strength of a concrete structure as well as the adhesion strength (Figure 3.23) is determined in different states of repair and retrofitting (see Chapter 7):

1. Determination of the current condition

2. After the preparation of the surface and before the application of any surface protec-tion system

3. After application of any surface protection system 3.2.3 Concrete cover

3.2.3.1 General

Detailed information about the location and type of the embedded reinforcement is a key parameter regarding any analysis of the load- bearing behaviour. Regarding corrosion pro-tection, the location of the reinforcement is also a key parameter, which has to be known because adequate cover depths are essential to ensure a sufficient durability of a construc-tion. The minimum cover depths are regulated by Eurocode EC 2 and (DIN) EN 206-1:2001, depending on the exposure of the construction.

Usually engineers need to know the concrete cover as well as the type of reinforcement of an entire structure and not only at a limited number of spots. This can only be achieved by nondestructive methods. Depending on the intention of the investigation, these methods also have to be calibrated by local inspections.

Figure 3.24 shows an example of a basement garage in which the concrete cover varies between 0 and 30 mm, and thus is too low in some areas for this specific exposure. These areas can only be detected by an extensive investigation and not by a local analysis of ran-domly selected points.

Figure 3.23 Mobile test rig used to determine the surface as well as the adhesion strength.

Generally steel reinforcement can be detected nondestructively by using magnetic or inductive methods. As shown in Section 3.2.2.2, GPR can also be used to detect the rein-forcement, but due to the complexity of GPR measurements, this method is not commonly used to detect steel rebars.