Concrete with superpowers? Well kind of…
There are types of concrete capable of putting up a barrier to the passage of X-rays, almost like superman!
I know, now you are going to ask me..."To what extent would it shield?" Look… the name of this blog is Practically Concrete; it has just one goal: conveying simple and practical concepts about concrete… do you really think I'll give you a list of numbers and data? This is not the right place to find them. Here my aim is to opening horizons, launching ideas, explaining in simple terms things that aren't....Ok, so what about movin’ on?
Concrete with shielding power pose a valid barrier not only to X-rays but also to gamma rays and neutrons and this is why they are normally used in areas such as: Nuclear medicine plants, particle accelerators, industrial radiographic installations and so on.
Why choose concrete rather than lead, iron or other shielding materials? Obviously it is a question of costs, practicality, transportation, disposal when the structures are decommissioned (concrete with shielding power, in fact, unlike other materials, can be considered non-toxic). Not being a cladding material but a real concrete structure, if properly designed and built, it will be free of joints, ensuring optimal shielding.
What gives these “superpowers” to ordinary concrete?
The aggregates that compose it. The following are commonly used:
baryte, limonite, hematite, iron-phosphorus; additions such as fragments of iron, lead and pyrex can also be used.
It is always a rather heavy, high-density material. The table illustrates the indicative density and indicative concrete weight per m³.
“Cela va sans dire” the choice of the type of aggregate depends on its availability in the area, notwithstanding the significant higher cost of these raw materials, due not only to less availability but also to the greater difficulties in transportation (heavier weight implies trucks which runs with lesser volume loaded and therefore delivery of smaller quantities at a time). The probable distance of the quarries from the batching plants also implies scrupulous planning of logistics in order not to face a shortage of aggregates in the middle of a casting and so risking the non-completion of the work of art, which instead must necessarily be monolithic.
In my personal experience I have used barytic aggregates and I came to the conclusion that some aspects should never be underestimated:
Although heavy, the barytic aggregate, is rather fragile and can’t handle well prolonged and vigorous mixing. The risk is the crushing of the coarse component with a consequential higher water demand (very dangerous especially in relation to shrinkages which must be scrupulously avoided).
In order to achieve high strength values, therefore, one cannot rely on the aggregates, as it were, but given the kind of artefacts to be realized, it is not advisable to opt for 525 cements with an exaggerated fineness; this would increase the risk of high hydration heat with consequent cracking. Better to choose titles such as 325 and possibly pozzolanic or blast furnace.
The thicknesses of the artefacts also play a significant role in limiting shrinkage phenomena.
Being a very heavy material, once stored on the batching plant yard (if “bare”, i.e. without industrial flooring or asphalt in contact) the pile, under the effect of its own weight, will tend to sink the material ... take precautions!
As in the case of the aggregates’ delivery, so it is for the delivery of the concrete: the concrete mixer truck will have to run with a reduced load which could make it necessary to count on a greater number of vehicles available to meet the needs of the casting.
The setting must take place with scrupulous care, so as to avoid damaging the aggregates with the vibration and to segregate the mix more easily, but at the same time guarantee the total air expulsion.
The formworks then must be particularly braced and sturdy given the greater weight of the concrete. In case the concrete pump should be used, avoid pumping at too high pressures or for too long stretches.
Last but not least, concrete with shielding power IS STILL CONCRETE, therefore it must be cured with all the necessary care, indeed, if possible, even more because any shrinkage would be really harmful.