Technical Questions

ACI Committees, Membership, and Staff have answered common questions on a variety of concrete related topics.



How can durable aggregates be ensured?

Q. How can durable aggregates be ensured?

 

A. Durable aggregates can be ensured by a knowledge of their past performance under conditions similar to those expected for the structure in question. Where only new and untried materials are available, examination by a petrographer at the sources and in the laboratory, as well as laboratory tests are justified for important structures. The ASTM C33 specification for concrete aggregates should serve as a guide to the characteristics to be investigated. Where resistance of concrete to severe freezing and thawing while saturated will be required, ASTM C666 procedure A should be used. In this test procedure, concrete specimens are subjected to rapid freezing and thawing in water (ACI 201.2R). Soundness tests outlined in ASTM C88 may sometimes be useful, but considerable uncertainty exists as to the proper specification limits (ACI 221R).

The mortar bar test given in ASTM C1260 should be used when aggregates are suspected of being susceptible to alkali silica reaction (ASR). Bars are kept in sodium hydroxide solution at 80ºC (176 ºF). The aggregate is considered nonreactive if the bar expansion is less than a certain value (ACI 221.1R). ASTM C441 is the test method that evaluates the effectiveness of a pozzolan or slag in preventing excessive expansions due to ASR. In this test, Pyrex glass is used as a standard reactive aggregate. The tested pozzolan or slag qualifies as effective if the mortar-bar expansion meets certain criteria.

While this method qualifies the type of pozzolan or slag, it does not establish minimum effective amounts unless tests are made with several levels of pozzolan or slag (ACI 221.1R). ASTM C1293 is a test involving concrete specimens that may be used to confirm the indications of ASTM C1260. Aggregate susceptibility to alkali-carbonate reaction is much less common than that for ASR. Alkali-carbonate reactive rocks have characteristic composition and texture. Such rocks should be tested for tendency to expand in the presence of alkalies (ASTM C586).

 

References: SP-1(02); E-1(16); ACI 201.2R-16; ACI 221R-96; ACI 213R-14; ASTM C33; ASTM C666; ASTM C88; ASTM C1260; ASTM C441; ASTM C1293; ASTM C586

Topics in Concrete: Aggregate; Concrete Fundamentals; Durability; Testing

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