In this edition Pablo Conde shares his expertise on radon prevention and Miguel San Millan presents a real-life situation in an industrial building.
Radon gas is a noble gas that is created from the radioactive decay of naturally occurring uranium and is present in soils, rocks, water and even some building materials.
This gas emanates from the substrate on which the buildings are supported and pass-through materials and joints until it reaches the different interior spaces where there is air. In these spaces it disintegrates, emitting radioactive particles which, when inhaled, are deposited in respiratory cells, causing DNA mutations and generating lung cancer.
To achieve the objective of ensuring that occupants are not exposed to radon concentrations that pose a health risk, the European Commission sets an annual average concentration value of 200 Bq/m3.
The first step in choosing the most appropriate solution is to measure the radon concentration. Depending on the result, the applicable regulations and the characteristics of the building, we will decide which solution or combination of several solutions is the most appropriate.
Building insulation solutions:
- Installation of a continuous barrier of sheeting type or of similar effectiveness over the entire surface of the enclosure in contact with the ground.
- Sealing of critical points of the envelope that present any discontinuity such as cracks, crevices, joints, expansion joints, etc.
- Use of watertight doors and grilles that limit the passage of radon from non-habitable spaces to the rest of the building.
- Inducing overpressure on the space to be protected by introducing air inside through a mechanical ventilation system.
Solutions to reduce radon before it enters the spaces to be protected:
- Installation of a ventilation system in the containment space, which can be an air chamber (e.g., sanitary chamber) or a non-habitable room (e.g., garage) located between the ground and the spaces to be protected in the building.
- Depressurization of the ground through a specific system that extracts the gases from the ground and expels them to the outside.
Radon reduction solutions after radon penetration into the spaces to be protected:
- Improvement of the natural or mechanical ventilation of habitable spaces to promote the expulsion of radon to the outside.
Next, we will analyze the case of a Cold Storage Warehouse in Burgos in which TOP has carried out the Project Management in complete mission.
Initially, a report was prepared based on the solution A) described above, to determine the need to place anti-radon sheeting under the entire floor slab of the future industrial building located in Villalonquejar, Burgos. The building has two differentiated areas: one for refrigeration (positive cold) and a smaller one for freezing (negative cold).
According to the CTE – DB HS6, in its appendix B, the municipality of Burgos is considered in Zone 1, in terms of its impact by radon gas emanations, so one of the two solutions A or B described below must be implemented.
Except in the freezer area, where a ventilated chamber will be proposed, which will also thermally insulate the floor, in the rest of the building solution A was initially prescribed: installation of anti-radon film.
Following this first report, a second document was prepared for the justification of compliance of protection against radiation exposure n.
For its drafting as a starting concept, it is understood that the protection barrier will be any element that limits the passage of gases coming from the ground and whose effectiveness can be demonstrated by complying with the characteristics related to continuity and durability according to CTE DB HS6.
The proposed barrier will therefore have a thickness and diffusion coefficient such that the expected radon exhalation through it is less than the limiting exhalation (Elim).
In addition, the limiting Exhalation value corresponding to the volume and ventilation flow rate of the room was calculated.
Later, it was compared that this value was above the radon exhalation predicted for the next two scenarios:
- Protection barrier by means of a reinforced concrete slab.
- Protection barrier by through of double low density polyethene film.
The following values and conditions were used to calculate the Limit Exhalation (Elim):
Currently, there is empirical knowledge that concrete can function as a barrier to radon due to its low diffusion coefficient in certain dosages, obtaining values ranging from 0.0005×10-8 to 130×10 -8 m2/s. Therefore, given the radon diffusion coefficient of concrete and with the intention of optimizing the use of anti-radon membrane, the concrete slab is proposed as a protective barrier for the spaces in the building.
The values obtained in previous studies1 on the effective radon diffusion coefficient in concrete are reported in the literature where a concrete diffusion coefficient of the order of 10-8 m2/s is considered.
According to solution A1 of the Radon Remediation Guide, concrete walls and floors are understood as effective radon protection barriers for cases in which the average radon measurement does not exceed 600 Bq/m3. According to the Nuclear Safety Council’s Map of Radon Potential in Spain, the municipality is in an area with lower levels in any case.
Subsequently, the calculation of the predicted radon exhalation (E) for a 20-cm thick reinforced concrete slab was performed and it was verified that the condition E < Elim was met, i.e., that the predicted radon exhalation is less than the limiting exhalation.
A double PROFILM sheet, a 200 μm thick low density polyethene film, was installed under the sill. The first sheet with overlaps of 30 cm and the second sheet is perpendicular to the first with the same overlaps. Both together have a thickness of 0.4 mm.
By comparison with other films of similar characteristics shown in the following table, a radon diffusion coefficient of the order of 10-11 m2 /s was estimated.
And then, it was verified that the protective barrier is composed of double PROFILM foil with a total thickness of 0.4 mm met the condition E < Elim, i.e., the expected radon exhalation was less than the limiting exhalation.
With this final study, we can conclude that the designed screed solution has a double function, enclosure of the envelope and radon protection barrier since the expected exhalation does not exceed the limit exhalation calculated for the volume and ventilation flow of the room.
As an additional measure, a second sheet of polyethene film was installed under the slab. Therefore, it was validated the consideration of the screed without concrete joints as a barrier taking into account that for all the singular points such as: construction joints, contours with panel, pillars, manholes, duct entries in the screed, a certified anti-radon film was installed with the standard width of 4, paying special attention to the sealing of the film in the singular points between pieces.
By Pablo CONDE and Miguel San Millan (TOP ES)
