Bart Craeye

Proof of Concept for Temperature and Strain Measurements With Fiber Bragg Gratings Embedded in Supercontainers Designed for Nuclear Waste Storage

The Belgian Agency for Radioactive Waste and Enriched Fissile Materials (ONDRAF/NIRAS) is developing a conceptual design for a supercontainer made of concrete and carbon steel for the post-conditioning of high-level nuclear waste prior to disposal in a geological repository site. This supercontainer should be instrumented to follow the temperature and strain evolution over time during and after the fabrication process. This paper shows that it is possible to embed bare fiber Bragg gratings in the supercontainer concrete to monitor in situ temperature and strain. The reason for choosing bare fibers, despite their known fragile nature, was to optimize the contact between the fibers and the concrete. Due to the simultaneous temperature and strain sensitivity of the fiber Bragg gratings, some sensors were also installed in a protective shielding consisting of plastic or aluminum to isolate them from effects of stress. This makes it also possible to extract both temperature and strain from the measured Bragg wavelength shifts. Moreover, the protective shielding also provides mechanical protection to avoid fiber breakage during concrete casting. The preliminary results of temperature closely agree with measurements obtained from thermocouples located near the fibers for validation purposes. Finally, the use of shielding in some of the fibers allowed to estimate strain levels in the concrete.

Experimental evaluation of mitigation of autogenous shrinkage by means of a vertical dilatometer for concrete

High performance concrete, with low w/b-factors, can suffer from early age cracking due to autogenous shrinkage. This can lead to a relevant reduction of the service life of concrete structures. In order to study autogenous shrinkage of hardening concrete, a new dilatometer has been developed. Within this newly developed dilatometer, the fresh concrete is cast into a vertical Teflon tube. After compaction on a vibrating table, the concrete sample is placed into the measuring frame, inside a climate room. Linear vertical shrinkage measurements can start immediately after casting, by means of LVDT-transducers. Temperature measurements are realized in parallel, in order to take into account the effect of the heat of hydration on the autogenous shrinkage results. With this dilatometer, the mitigation of autogenous shrinkage by means of saturated light- weight aggregate particles and super absorbing polymers has been studied experimentally. The application of theoretical formulas, predicting the required content of internal curing water in order to compensate for the autogenous shrinkage has been evaluated. The application of saturated light-weight aggregates and super absorbing polymers proves to be a good technique in order to reduce autogenous shrinkage.

Effect of super absorbing polymers on the freeze–thaw resistance of coloured concrete roads

This research project aims to improve the durability and resistance to freeze–thaw cycles of coloured concrete roads. The negative effect of the red pigment on the freeze–thaw resistance of concrete is studied and possible improvement actions are being considered and evaluated, for example, by adding super absorbing polymers (SAPs) to the fresh mix. SAPs can absorb water as much as 50 times of its own weight. The polymers also have the ability to gradually release the absorbed water during the self-desiccation processes in concrete during hardening and hydration, reducing the amount of desiccation cracks in the concrete. As a result, small cavities are created in the concrete due to the donation of absorbed water to the hydration process. These small pores can serve as pressure vessels to counteract the expansion of freezing (expanding) water inside saturated concrete during freezing. By means of a laboratory investigation the influence of the red pigment on the scaling and degradation due to freeze–thaw cycles is investigated by means of compressive strength tests, splitting tensile strength test, ultrasonic pulse velocity tests, weight loss measurements, porosity measurements, visual inspections and scanning electron microscopy (analysis). Secondly, the effect of different amounts and different types of SAPs added to the fresh mix is studied and evaluated by means of the previously mentioned testing equipment. An improvement of the freeze–thaw resistance of coloured road concrete is obtained by adding SAPs. By adding 0.26 wt% dry SAPs (wt% of weight of added SAPs relative to cement weight) to the fresh reference mix, the scaling and the internal damage after 30 freeze–thaw cycles is significantly reduced by 54% and 71%, respectively.

Half-scale test: an important step to demonstrate the feasibility of the Belgian supercontainer concept for disposal of HLW

The paper presents results of a half-scale test performed by ESV EURIDICE, an Economic Interest Grouping between the Belgian Nuclear Research Centre (SCK.CEN) and the Belgian Agency for Radioactive Waste and Enriched Fissile Materials (ONDRAF/NIRAS). The primary objective of the test was to assess the feasibility of constructing the Supercontainer and to provide experimental data to validate modelling calculations obtained using the finite element program HEAT/MLS. The test focused on the early-age behaviour of the concrete matrix materials and the practical aspects of construction. Generally, the results obtained from the half-scale test confirm that it is feasible to construct the Supercontainer with currently available techniques. The results also validate scoping calculations obtained earlier with the finite element model. These finding contribute an important step to demonstrate the feasibility to construct the Supercontainer and to validate the Belgian Supercontainer concept proposed by ONDRAF/NIRAS for disposal of high level waste (HLW) in Belgium.

Chapter 2 - Mechanical properties

The State-of-the-Art Report of RILEM Technical Committee 228-MPS on Mechanical properties of Self-Compacting Concrete (SCC) summarizes and extensive body of information related to mechanical properties and mechanical behaviour of SCC. Due attention is given to the fact that the composition of SCC varies significantly. A wide range of mechanical properties are considered, including compressive strength, stress-strain relationship, tensile and flexural strengths, modulus of elasticity, shear strength, effect of elevated temperature, such as fire spalling and residual properties after fire, in-situ properties, creep, shrinkage, bond properties, and structural behaviour. A chapter on fibre-reinforced SCC is included, as well as a chapter on specialty SCC, such as light-weight SCC, heavy-weight SCC, preplaced aggregate SCC, special fibre reinforced SCC, and underwater concrete.

Evaluation and Practical Implementation of Probabilistic Calculations of Chloride Penetration for Belgian Concretes in the Framework of a Durable Design

In order to overcome the problems of the current deemed-to-satisfy approach with respect to a durable concrete design, an adaption is required. Limiting values derived from measured material properties can be used as reference values. In case of chloride induced corrosion, the chloride migration coefficient is the most appropriate variable. Based on a full probabilistic design the maximum allowed chloride migration coefficient for a particular concrete type is determined in order to withstand the conditions of the exposure class. The results are given as tabulated values in function of the exposure class, the concrete cover and the initial chloride content. By utilizing the PSF-approach design charts have been derived. Values for the maximum chloride content at the depth of the reinforcing steel in function of material properties and exposure class can be determined for critical environmental conditions and a design service life of 50 years.