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.

Keywords:

• microsilica
• dispersion
• agglomerates
• ASTM C1240
• ASTM C430

Acknowledgements

This investigation was sponsored by the ODOT and by the Federal Highway Administration (FHWA) as Ohio State Job No. 14800(0); Contract No. 11340, under project ‘Fineness of Densified Microsilica and Dispersion in Concrete Mixes’. The Principal Investigators were Drs Anastasios M. Ioannides and Richard A. Miller, Department of Civil and Environmental Engineering, University of Cincinnati. The ODOT Technical Monitor was Mr Bryan Struble; the Administrator for the Office of Research and Development at ODOT was Ms Monique Evans; and the FHWA liaison in Columbus, OH, was Mr Herman Rodrigo. The assistance, cooperation and friendship of these individuals was a major contributor to the success of the study, and their support is gratefully acknowledged. Special thanks are also extended to Messrs Lloyd Welker and Tim Jones of ODOT. The sand and both kinds of coarse aggregates were supplied free of charge by Martin Marietta Materials, through Mr Jim Martin. The cement was donated by CEMEX, through Mr Steve Reibold, and the microsilica by Elkem Materials, through Mr Tony N. Kojundic. The MB-AE 90 air entrainer and the Rheobuild 1000 plasticiser were supplied at no cost by Master Builders, Inc., through Mr Greg Wirthlin. The authors also acknowledge the contributions to the project of graduate students Kristy Walsh and Jeff Mills.