Microwave-assisted selective demolition of concrete - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 118

Compression Tension

(a) (b)

Figure 3. 31 (a) Concrete is strong in compression but weak in tension. (b) Typical steel
reinforcement bars in RC structural elements.

Understanding the effect the embedded reinforcing bars have on the dis-
tribution of microwave power in reinforced concrete is crucial to achieve a
realistic estimate of the thermal stresses and pore water pressure developed
during the microwave selective demolition process. Contrary to concrete,
which absorbs the microwave energy partially, the steel rebars within con-
crete reflect the majority of the incident microwave power. As a result of
this reflection back toward the microwave-exposed surface of concrete, the
microwave field intensity within the cover concrete above the reinforcing
bars near the surface is increased (Figure 3.32). This is a positive phenom-
enon if the aim of the selective concrete demolition technique is to remove
the surface layer of the cover concrete in applications such as decommis-
sioning of nuclear power plants and can result in increased performance,
translating into cost and energy savings provided the desired depth of sur-
face removal is less than the thickness of the concrete cover.

Figure 3.32 Reinforcing bars in concrete reflect the microwave power back toward the
microwave-exposed surface of concrete, increasing the amount of the dis-
sipated microwave power within the cover zone.