This is how engineers test concrete structures for strength | Science| In-depth reporting on science and technology | DW | 16.08.2018
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This is how engineers test concrete structures for strength

Not all damage to a load-bearing structure is immediately apparent. So how do civil engineers find out that a bridge or a large hall is no longer safe? Many clues form a picture.

Deutschland Symbolbild Reparaturarbeiten Brücken | Rader Hochbrücke der Autobahn A7 (picture-alliance/dpa/G. Wendt)

If damage is found, brackets must temporarily reinforce the structure — until the bridge is rebuilt

Concrete structures such as bridges or large halls have to withstand a lot: Ever heavier trucks thunder across the streets, factory buildings have to carry the weight of huge machines. The floors of dance halls have to withstand hundreds or even thousands of people jumping rhythmically at the same time. The weather also damages the buildings.

Reinforced concrete or prestressed concrete is actually quite stable and can carry heavy loads. But there are certain influences that destroy this stability.

Water, acid, rust and loads

This includes water in particular if it penetrates the building and corrodes the steel reinforcements that give the concrete its strength. It is even worse when road salt or other aggressive chemicals are added because the reinforcements rust through much faster.

Acids attack not only the metal, but also the concrete itself. The lime-containing compounds of the cement dissolve — the concrete leaches out and becomes brittle. Even rainwater can trigger something like this, especially if the concrete is rough and the surface is fissured, which allows the penetration of water.

Extreme physical stresses that cause the structure of the concrete to crumble are also a great danger. These can be vibrations, large masses that affect the structure, such as mountains of snow and ice on roofs or recurrent vibrations caused by trucks on bridges. 

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Visual inspection alone is not enough

During an inspection, engineers first take a thorough look at the building from the outside: Are there obvious water marks? Have stalactites formed under the building? This would mean that water has penetrated the concrete for a long time and rinsed out lime. Is there spalling of the concrete? Are there any reinforcement parts that are visibly rusted? Is the surface covered with algae or mosses?

Then the engineers have to find out where the reinforcements are located. Old construction plans — if available — are useful for this purpose. Next, magnetic-inductive measuring devices are used — similar to metal detectors that DIY enthusiasts use to search for cables and pipes in the wall or that treasure hunters use to search for old coins. The devices can detect metals about ten centimeters deep in the concrete. Deeper steel reinforcements can also be located with radar units. Those can also detect water accumulations.

Taking samples from the building

Engineers need to know where the reinforcements are before drilling a core as a sample. They do not want to hit the steels during drilling. The drill cores can later be tested in the laboratory for fracture and compressive strength.

The state of corrosion of the steel reinforcement in a building can first be estimated non-destructively. For this purpose, the method of potential field measurement is used. It is based on the fact that the steel wires of the reinforcement behave similar to a battery when it corrodes, for example, through penetrating salt water.

One part of the reinforcement then automatically becomes the anode, another the cathode. When engineers place a measuring device on the concrete floor and move it over its entire surface, they can measure an electric field. Where a strong anodic potential becomes visible, the reinforcement is likely to corrode deep in the concrete. The engineers then have to examine these points in more detail. 

Beton gießen (picture alliance)

The concrete should protect the iron reinforcement from water and air. This is only possible if it is firm and its surface is fine.

For this purpose, they can also open up the concrete and inspect the reinforcing steels on a trial basis or remove them. However, this is only possible if a structural engineer has ensured that the stability of the building is not endangered by the removal. These pieces, about 35 centimeters long, are sent to the lab. Experts then determine how much tensile force they can still withstand before they tear. For example, it can be determined whether the metal has already become unstable due to hairline cracks.

Are the tension wires already torn?

Reinforcements play a particularly important supporting role in prestressed concrete structures. The tension wires ensure that long bridge sections remain stable in themselves.

The engineers use a similar procedure to find out whether such tension wires are broken: They take advantage of the fact that every wire acts like a rod magnet and measure its magnetic field with devices that they move over the surface. There where a magnetic field ends and a new one begins with different polarity is definitely a break in the steel. 

With a hammer against the wall

Not only the reinforcement, but also the concrete is first tested without damaging it. The most common and universal method is to measure the compressive strength of the concrete using a rebound hammer.

This is a spring-driven bolt that hits the surface of the concrete at a defined speed. Then it rebounds more or less strongly. The strength of the rebound shows how much energy the concrete absorbs from the impact.

Litmus test: What is the pH value of the concrete?

In addition to its physical strength, good concrete must also be chemically stable enough to protect the reinforcing steel it contains. When concrete comes into contact with water, it reacts with carbon dioxide from the air. This results in a so-called carbonization of the concrete.

For the concrete itself, this would not be a problem, because it makes it even stronger than before. But the armoring irons suffer from it: They rust faster. The degree of carbonization is determined by spraying on an indicator solution of phenolphtalein — similar to the litmus pH test strip from chemistry classes. Of course, the building inspectors can also do this in the laboratory.

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