Influence of Irradiation on Concrete Strength

Mechanical properties of concrete are changing in the course of its ageing. Ageing of concrete is accelerated by climatic conditions, mechanical stress, aggressive liquid, etc. This article deals with a question of irradiation influence, which arises, for example, as a consequence of uranium 238U and 235U fission reaction in the nuclear reactor. It concerns neutrons and gamma irradiation acting on concrete constructions and accelerating their surface and internal changes. The concrete in these facilities plays not only the structural role, but also the role of shielding material that has to absorb the aforementioned types of irradiation and prevents their penetration. Of the irradiation spectrum we will observe only influence of gamma irradiation. The gamma irradiation is being absorbed at the passage through concrete, what can induce water radiolysis 1 in the material and consequently chemical reactions leading to changes of the concrete phase composition 2 and thus the hydration degree decrease. We can expect that this hydration degree decrease becomes evident by the decrease of the concrete strength. This hypothesis is being proved in this article.


Introduction
Mechanical properties of concrete are changing in the course of its ageing.Ageing of concrete is accelerated by climatic conditions, mechanical stress, aggressive liquid, etc.This article deals with a question of irradiation influence, which arises, for example, as a consequence of uranium 238 U and 235 U fission reaction in the nuclear reactor.It concerns neutrons and gamma irradiation acting on concrete constructions and accelerating their surface and internal changes.The concrete in these facilities plays not only the structural role, but also the role of shielding material that has to absorb the aforementioned types of irradiation and prevents their penetration.Of the irradiation spectrum we will observe only influence of gamma irradiation.The gamma irradiation is being absorbed at the passage through concrete, what can induce water radiolysis [1] in the material and consequently chemical reactions leading to changes of the concrete phase composition [2] and thus the hydration degree decrease.We can expect that this hydration degree decrease becomes evident by the decrease of the concrete strength.This hypothesis is being proved in this article.

Concrete composition
Samples determined for testing were made according to the formulation, which is stated in Table 1 [3].This formulation was used at construction of the containment of the nuclear power plant Temelín.Composition of cement CEM I 42,5 R Mokrá is listed in [3].Concrete beams, on which consequently the experiment took place, were made of this mixture.Volume mass of concrete 28 days old was 2358 kg×m -3 .

Method of irradiation
Irradiation was carried out in the company Bioster a.s.Veverská Bítýška, where 60 Co is used as a gamma irradiation source.The activity of source 60 Co at the beginning of experiment was A = 462 kCi (1.73×10 16 Bq) and at the end of experiment was 449 kCi (1.68×10 16 Bq).These values were necessary for calculating of total radiation dose rate which was 0.935 kGy×h -1 .Concrete beams at dimensions 0.4×0.1×0.1 m after 90 days from concreting were used for the experiment.Each beam had gamma irradiation dosimeters fixed on it.Beams were irradiated for 90 days with a dosing input of approx.0.25 kGy×h -1 , i.e. the maximum dose was 0.5 MGy for the whole period of irradiation.The difference between radiation dose rates is caused due to items,

Influence of g Irradiation on Concrete Strength
V. Sopko, K. Trtík, F. Vodák  which were placed between source of radiation and irradiated samples.Classical rectifying diodes calibrated by means of standard dosimeters were used as dosimeters [4], which error is about twenty per cent.Standard dosimeters are limited for higher doses of the gamma irradiation, therefore, the dosimeters, which we used, were calibrated for lower doses and consequently extrapolated for high irradiation doses.

Method of strength measurement
At first, the flexural strength test (three -point supporting) was applied to concrete samples.After this destructive test the compressive strength test was used on the first fragment (in accordance with ČSN 73 13 18) and the tensile strength test on the second fragment of splitting (in accordance with ČSN 73 13 17).

Conclusion
Measured values of concrete strength (Fig. 1, 2) are according to current measurments obtained by various experimentators viz [5].

Table 1 :
Aging of concrete due to gamma irradiation.Strength of concrete are in a good correspondency with already known results.Composition of 1 m 3 of concrete