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Borderline aggregates are those that do not meet all of the requirements for satisfactory aggregate but may be suitable for applications where inadequacies of the aggregate will not have a significant adverse effect on the life or performance of the concrete. The borderline quality is determined usually by standard acceptance tests using generally agreed upon acceptance limits, and by standard and specific tests with specially set acceptance limits.
A thorough knowledge of both the aggregate characteristics and the specific properties needed in the concrete is essential to properly assess a borderline aggregate for any application. Petrographic examinations help to identify the deficiencies. In many applications, when comparing the properties of borderline aggregates with requirements of the job, it may be helpful to consider the strength and durability factors separately.
Methods which help to ensure safe use of borderline aggregates and the principal beneficiation methods which can be used to improve the quality of aggregates are listed and examples are given.
aggregates, borderline, concretes, durability, petrographic examination, amelioration, standard tests
Research petrographer, Toronto, Ontario