Title: Microscopic Features of Cracked and Uncracked Concrete Railway Sleepers

Author(s): A. Shayan and G. W. Quick

Publication: Materials Journal

Volume: 89

Issue: 4

Appears on pages(s): 348-361

Keywords: aggregates; alkali-aggregate reactions; cracking (fracturing); ettringite; precast concrete; railroad ties; Materials Research

DOI: 10.14359/2560

Date: 7/1/1992

Abstract:
Precast prestressed concrete sleepers showing parallel longitudinal cracking on the top surface with some degree of map cracking at their ends, and those without cracking, were examined to determine the cause of cracking. This is the first in a series of papers dealing with this problem. Concrete specimens taken from sleepers were examined by petrographic microscope and scanning electron microscope equipped with an energy-dispersive x-ray analyzer. X-ray diffraction analysis of the matrix was also done on sections of the sleepers. This paper describes the various microstructural features of the sleepers and attempts to relate them to the cracking problem. Fine internal microcracking existed in all sleepers. It is concluded that the observed external cracking probably occurred as a result of loading of the sleepers with a matrix sufficiently weakened by alkali-aggregate reaction (AAR). Once this cracking occurred, access of moisture to the interior of the sleeper exacerbated the AAR problem and created additional stress in the concrete by forming ettringite in cracks already present around the aggregate particles and in the mortar phase.