Mitigation of cement Alkali Silica Reactivity in Hydropower Concrete Structures

Abstract

The research focuses on the mitigation of cement alkalis and siliceous of aggregates reactivity (ASR) influence in hydropower concrete structures and takes Merowe Dam Project (MDP) and Dam Complex of Upper Atbara Project (DCUAP) as case study. The research aims to find out methodology, for mitigating alkali silica reaction that may occur in hydropower concrete structures and to compare, the laboratory testing results obtained, with the international standard requirements for compliance. The results of the cement material; OPC and GGBS used in both projects, Merowe Dam Project (MDP) and Dam Complex of Upper Atbara (DCUAP), were of low level alkali content, less than 0.6% of Na2O equivalent. The results of crushed and natural aggregates; combined with moderate and high levels of alkali content cement for expansion test, were of low level reactive aggregates. Based on the laboratory testing results obtained from petrographic examination, chemical tests, mortar bar tests and accelerated mortar bar tests concerning alkali silica reaction for fine and coarse aggregates used in concrete and mortar production for construction of MDP & DCUAP projects, all the results fulfilled and verified the standard requirements and limits stated for mitigating alkali content in the cement material and the siliceous found in the aggregates materials, therefore, the concrete and mortar produced from such materials were mitigated from deleterious expansion and damage due to alkali silica reaction. Using of supplementary cementitious material (SCM) Fly Ash to replace 25% of the Cement used in concrete mixes not only for minimizing risk of any probable Alkali-Silica Reactivity mitigation reason, but also for reducing heat of hydration, as a filler to reduce seepage, to increase workability of concrete and as a benefit by reducing the cost of concrete production. It is obvious, from the testing results obtained; to mitigate alkali silica reactivity; there are two options: combine innocuous aggregate with even high level alkali content cement will fulfill the standard requirements for mitigating ASR or combine reactive natural sand with low level alkali content, also will mitigate ASR. The recommendations of this research for future study had been stated as to: establish visual inspection system for existing concrete structures of dams, bridges, water tanks and similar concrete structures which were exposed to wetting environment, establish service record for aggregates quarries which have been used in concrete production for dams, bridges and similar concrete constructed from the same aggregate quarries and further study of local natural pozzolan from Buyda desert central volcanic field.