Nuclear concrete structures are typically passive under normal operating conditions. They are, however, play a key role in mitigating impacts of extreme/abnormal operating and environmental events. As structures age, changes in material properties arise from continuing microstructural changes and environmental influences. Changes in environmental conditions anticipated during the original design can sometimes lead to unpredicted effects, which may jeopardize the in-tegrity of the structures.
In order to continue successful operation, continuous integrity of the safety-related concrete structures shall be ensured by controlling and mitigating aging related degradation. A key element of aging management is the systematic and rigorous assessment of structures most commonly referred to as Condition Assessment (CA). Over the years, SNC-Lavalin personnel have assessed containment structures and safety related structures of Nuclear Power Plants (NPPs), research reactor and waste storage facilities.
This paper discusses requirements and approach to aging management for existing as well as new NPP concrete structures, and presents the methodology used to perform the CA.
The effectiveness of two types of procedure for removing large air bubbles in order to increase the percentage of small air bubbles, which is envisaged to be effective for improving the self-compacting properties of fresh concrete, by the addition of an antifoaming agent during mixing has been investigated. In the method of removal using an antifoaming agent after entraining many small air bubbles by divided water mixing, in all the cases investigated, namely adding antifoaming agent at the same time as the primary water, adding antifoaming agent at the same time as the secondary water, and adding antifoaming agent finally, it was not possible to remove large diameter air bubbles, on the contrary they increased. On the other hand, in the method of removing by mixing all together and adding the antifoaming agent finally after a large quantity of air bubbles has been entrained, if the quantity of air entraining agent is increased, the percentage of small diameter bubbles is increased, and the percentage of large diameter bubbles can be reduced.
Triaxial compressive tests were conducted at different confining pressures (Pc) and water contents to understand the deformation mechanism of hardened cement paste (HCP). Results showed that the stress did not decrease up to 10% strain and macroscopic damage was not observed when Pc was higher than a certain value. Pc required for this ductile behavior differed depending on the HCP water content. Stepwise creep tests were also conducted. Most of the slopes of the obtained differential stress–strain rate curves in the double logarithmic chart were approximately three, which indicates that dislocation creep was the dominant deformation mechanism. The samples saturated with sucrose solution exhibited a more brittle behavior after the peak stress than those saturated with tap water. The different behavior in the softening region was attributed to calcium hydroxide precipitation because it was suppressed in the sucrose solution. These results indicated that the HCP deformation may be affected by dislocation, mechanical twinning, and pressure solution.
The objective of this paper is to determine the critical size of air bubbles, which harmfully affects the stability of air in mortar of self-compacting concrete (SCC). Mortar samples produced by different type of mixing procedures and mixing time with various dosage of air-entraining agent (AE) were tested. Air diameter distribution of these mortar samples was measured at fresh stage with air-void analyzer (AVA). With AVA machine, size of air bubble measured is considered as the chord length, which is assumed to be 2/3 of the diameter of air bubble (according to ASTM C 457). It was assumed that air bubbles with over the critical size were easily to escape either by collapsing or floating upward. It was found that instability in volume of air in fresh mortar of SCC was caused mainly by the existence of air bubbles with chord length of over 1000 µm and partially by 500 to 1000 µm due to unification between air bubbles with chord length of less than 1000 µm.