Undispersed agglomerates and the strength of microsilica concrete

Abstract

Material specification C1240 of the American Society for Testing and Materials (ASTM) requires wet-sieved microsilica to pass the #325 sieve with no more than 10% retained and advises that care be exercised ‘to avoid retaining agglomerations of extremely fine material’. The Ohio Department of Transportation (ODOT) has found that densified microsilica samples that are submitted sometimes do not meet this specification when subjected to test method ASTM C430, apparently because wet-sieving is not capable of breaking the agglomerations. In this study, the possible repercussions of densification of microsilica into larger particle sizes on the mechanical and other engineering properties of the resulting concrete mix are examined. Results of microsilica tests conducted suggest that ASTM C430 is not an appropriate test for assessing the suitability of microsilica for use in concrete. Tests conducted on concrete specimens indicate that those made with undensified microsilica show higher flexural and compressive strengths than concrete made with densified microsilica and with microsilica abused by prolonged exposure to moisture, for both natural and crushed aggregates. Trends observed in almost all mixes with respect to increase in strength with age, microsilica type, aggregate type and specimen size were as expected. The strength increased with age and strength increase was more rapid in the initial ages than during later ages. Both large and small cylinders attained compressive strengths of more than 34.5 MPa after 28 days of curing. Therefore, densified microsilica concrete can be used in the construction of pavements and bridges by ODOT.