Microwaves - their application in demoliton

04 December 2013

Microwaves are everywhere around us. They are simply a frequency band of electromagnetic radiation, comparable to any other radio frequency. In addition to heating food, microwaves are used to transfer information. Also, many radars operate on this frequency. The same frequency, 2,45 Mhz, that is used in an microwave oven is used for wifi applications. The reason for this is simple: this frequency is meant for household use and thus is less regulated than many others.

Microwaves are generated with a relatively simple device called a magnetron. It is well-known basic technology, as evidenced by how little a 1 kW microwave oven costs today.

How can the waves be used in heating? The purpose is to create a reaction with water, or to be more exact, water molecules. Low power levels, like those used with wifi, do not really create heat. In addition, the waves are not focused and the radiation spreads out in every direction.

Considerable power is naturally required for heating applications. Of equal importance is that the radiation is directed to a limited area only. In practice this is done with a control device, a kind of steel tube.

It is clear that cats should not be microwaved. Does this mean the waves are harmful to us? There is natural radiation around us all the time, again at very low power levels, and it is impossible for human beings to notice this, just as any radio frequency is impossible to feel. The situation is very different with microwaves at high power levels. Most important is the density of power - how much power (kW) we have per area (square meter). The damage that can be inflicted on human organs comes from heat generation (because the organs are mostly water). In small quantities, the human body can handle the heat simply by carrying it away with blood. Human eyes are different because they are one of the most sensitive parts of the body and the risk of permanent damage is truly present. There is therefore sufficient reason to be very careful. Health and safety standards concerning effects of microwaves on the human body have been published around the world.

Radiation modes

When talking about radiation, we have to make a clear difference between ionising and non-ionising radiation. Electromagnetic radiation does not ionise, meaning it has no effect on the structure of a molecule. In practice this means that no permanent change in material is caused. However, radioactive radiation is ionising in nature and must be treated totally differently.

What would microwave radiation do to a block of concrete? If we could create fractures in the block without touching it, the whole demolition industry would be changed. Even if the concrete was not broken up but just weakened, for example through the creation of microscopic cracks, the productivity of traditional demolition tools would be multiplied. The fact that the behaviour of microwaves is very well known and the components are relatively cheap makes this concept a truly interesting one.

The Tampere University of Technology in Finland has carried out research on this topic. The basic theory involved the creation of a high temperature spot that would generate heat expansion in a concrete block that in turn would create circular tension around the heated area. And we know that concrete can not stand tension too well.

Initial calculations were carried out that supported the theory. There were enough reasons to start laboratory testing. In the test, concrete blocks removed from a building were microwaved. The device had two magnetrons, each of 6 kW power and the radiation was focused as accurately as possible.

In the tests it was identified how important moisture is if successful results are to be achieved. The moisture must be deep in the material, with surface wetting alone being insufficient. Another challenge was that high moisture levels causes a high power density on the surface. When the material is dry, the radiation penetrates deeper but the power density is lower.

In action

Microwaves move in principle like waves on water. We know how waves reflect in a swimming pool when conditions are suitable, and microwaves can also be reflected, depending on the material. The problem here is that a magnetron can be damaged by such reflected radiation, even though it had just emitted that very radiation. Steel causes a much stronger reflection than concrete. In suitable circumstances, reinforcement bars may carry the waves further and when the power level is sufficiently high, there may be sparking around the steel.

Not all concrete is the same. Cement may be similar, but the sand and gravel used can consist of different minerals and this can change the behaviour of the microwave radiation on the surface of the concrete.

How the radiation is absorbed in the object depends on a number of factors, with the presence of rebar being one of the most important ones. The above mentioned type of concrete is another. The positioning of the magnetron in relation to the object block is also important, especially the distance between the block and the magnetron and the direction of the radiation.

In jobsite conditions it is hard to get perfect positioning. If safety issues are in control, there still is the reduced efficiency. This creates an issue with power. We want high temperature in a small area, this means we have to bring more power in the material than what leaks out. The target is massive and the required power is high. Poor efficiency makes the power requirement even higher.

To make the situation even more complicated, the heat vapourises the moisture and this makes the heated area expand faster.

The test proved clearly that concrete strength was reduced after microwaving and visible cracks were also created. The calculations were correct and the theory was proven correct. The disappointment was that the reaction in concrete was slow. Heat should be generated faster. From the test results it was estimated that in job site conditions, the power would have to be in the order of hundreds of kilowatts and that naturally there would be practical difficulties encountered as a result. For example, radars can handle that power but there the circumstances are very different. The final conclusion of the study was that the theory works as predicted but a practical solution is too inefficient.

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