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Reducing Radon Gas Emissions in Concrete

Time: Fri 2022-12-02 10.00

Location: Kollegiesalen, Brinellvägen 8, Stockholm

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Language: English

Subject area: Civil and Architectural Engineering, Concrete Structures

Doctoral student: Magnus Döse , Betongbyggnad

Opponent: Professor Franz Josef Maringer, BOKU - University of Natural Resources and Life Sciences, Vienna, Austria

Supervisor: Professor Johan Silfwerbrand, Bro- och stålbyggnad, Betongbyggnad

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QC 20221108


Several compulsory regulations and recommendations regarding ionising radiation for building products have been introduced in recent years. Furthermore, industry-affiliated aggregate and concrete companies strive to implement environmental goals that should be fulfilled regarding building materials. In Sweden, a certification system for high-level environmental quality assurance has beendesigned by the Green Building Council (Miljöbyggnad 3.0). The certification system is used in procurements of house constructions to achieve a high standard and good quality environment for habitants inside new buildings. One of these environmental goals, as part of this certification system,concerns acceptable levels of radon within the indoor environment. In recent decades it has become increasingly common for the concrete industry to use combinations of different Supplementary Cementitious Materials (SCMs) in concrete to reduce the carbon dioxide emissions of cement production. Additions of SCMs and different admixtures can also improve the properties of concrete, such as increased strength and durability. However, knowledge of ionising radiation and radon is still limited. How do SCMs and hydrophobic admixtures contribute regarding properties such as radon gas exhalation from concrete? Are there any advantages? Disadvantages? Can one make use of specific properties in specific indoor climate environments? What is the effect of moisture?The main part of the thesis embraces these concerns. Twelve different concrete recipes were cast to investigate the 222Rn radon exhalation rate of these concrete mixtures in relation to their relative humidity (RH). Ten recipes consisted of different mixes of binders and hydrophobic admixtures containing crushed rock with a slightly enhanced 226Ra-activity concentration (Bq/kg), while two other recipes included crushed rock with low levels of radioactivity. As a reference cement, a CEM I, 52.5 Rproduced by Cementa AB/HeidelbergCement Group (Skövde cement factory) was used. The concretes´ composition had a water binder ratio (w/b) of 0.55.For radon gas analysis and radon diffusion measurements, a method using the decay rate of alpha energies from 222Rn and 218Po was employed. The amount of decay per unit time was calibrated in relation to a well-defined radon gas level. The readings or output from the radon gas monitor were then displayed as 222Rn content in air in the unit Bq/m3. Diffusion measurements included an instrument named RAD 7 from Durridge, Inc. The instrument´s measuring technique uses a solid-statedetector.The results imply that SCMs and hydrophobic admixtures (liquid) have a moderate to fairly large impact on the radon exhalation rate, with a humidity of 75 % and 60 %. The largest impact at a relative humidity of 75 % is shown by micro-silica (SF-30), which reduces the radon exhalation rate by up to 57 %. However, at a relative humidity of 45 %, the radon exhalation of the reference concrete is in line with most other concrete mixes regarding their radon exhalation rates. One needs to separate between radon gas exhalation and radon gas diffusion, although they both affect the radon rate within a building. In the study, the radon gas exhalation rate generally decreased with decreasing relative humidity. The radon gas diffusion, however, increased in general as the relative humidity decreased. Also, the natural process of carbonation affects the radon exhalation rate. The study performed as part of the thesis, relating to carbonation and its influence, generated different results depending on the concrete recipe, but can be summarised as follows: (i) concrete with only CEM I or CEM I combined with an hydrophobic admixture indicated a reduced radon exhalation rate; and (ii) for a concrete recipe containing CEM I as a binder combined with slag or fly ash, the radon exhalation rate increased.Another study, as part of the thesis, embraced induced cracks and their influence upon the radon exhalation rate. The study showed that the influence of cracks can be quite large. In two cases an increase of 200-250 % was calculated compared to the radon exhalation rate of the same concrete without cracks. In the other cases, the increase was proportional to the increase of the concrete surface.iiiSeveral factors influence the final rate of radon being exhaled from a building material. The radon exhalation in the examined building materials can also be addressed as the production rate of radon (exhalation of radon per unit volume) for the investigated concrete mixes. The production rate is mainly governed by the emanation coefficient, the content of radium in the materials and the material´s density (volume and mass) as well as the radon decay constant. Since the investigated concrete mixes have a similar density and radium content, these variables are of less importance when assessing the differences between the concrete mixes’ exhalation rates. Consequently, the influence of the radon emanation becomes a major parameter when comparing the different concrete mixes. The radon emanation has in the ongoing assessments been demonstrated to show substantial variation due to the influence of the relative humidity. Initially, in a water filled system (100 % RH), the water acts as a barrier, and radon are accumulated in the pores (e.g., the recoil theorem). When the moisture level decreases, the initially high radon levels in the pore system are enabled to diffuse into the free air. The initially high concentration of water molecules also acts as a carrier for some of the radon atoms. This promotes the successive reduction of the relative humidity in the concrete samples, and the number of radon atoms that reach the concrete surface is also diminished, which consequently reduces the radon exhalation rate. In other words, the most important factor for differences in the radon exhalation rate can be dedicated to its radon emanation, which refers to the number of radon atoms being released from the material itself into the free air. Therefore, the tightness of the concrete, or its permeability, is very important. This is in part reflected in the diffusion coefficient or radon diffusion length being assessed for the different concrete samples.That the radon gas diffusion increases with a lower relative humidity in the concrete is reasonable since the diffusion rate in water is markedly lower than in air. The diffusion rates in the investigated concrete samples have a subordinate role, however, when one evaluates the final exhalation rate. The high radon exhalation rate in this study is foremost due to (i) the material´s high radium content and (ii) higher emanation coefficient at higher relative humidities. It is of importance to note that the materials´ slightly elevated radium content has a large influence on the high radon exhalation. Comparing concrete recipe C with a recipe replaced with a low radioactive content aggregate (i.e., a low amount of radium), the production rate is very limited, which means a low radon exhalation rate, even though a moderate emanation coefficient can be shown. Conclusively, this implies that the relation between the relative humidity (RH), radon concentration and diffusion within a concrete wall, ceiling or floor is a complicated interaction. In practice, the influence of relative humidity is the dominant factor for the final radon exhalation rate from a building material. Consequently, the radon exhalation rate, in general, decreases over time as the concrete driesout and the relative humidity decreases. Some essential conclusions derived from the thesis are that: SCMs and hydrophobic admixtures can effectively reduce the radon gas exhalation rate, specifically at higher relative humidity levels;fractures in concrete may generate substantial radon concentration increases; and, depending on the choice of binders, the carbonation of concrete may have a positive or negative effect on the radon exhalation rate