Alkali silica reaction of railroad sleppers

Cause of Deterioration of Canadian Concrete Railroad Ties

Litigation 1991
An unusual type of concrete deterioration has been observed in Canadian concrete railroad ties. The ties were made of a low alkali Portland cement, coarse aggregates consisting of mainly crushed limestone from the Francon Quarry in Quebec and crushed siliceous sand. During service the ties showed cracking and a major investigation containing ordinary microscopy and SEM-EDX of rocks and concrete was carried out to establish the cause of cracking. The concrete was cracked due to alkali silica reaction. The reactive aggregate was a silica-rich dolomite rock of the coarse aggregate fraction. The reaction was, however triggered by the occasionally presence of an alkali-rich silico-carbonatite rock also present in the coarse aggregate fraction. It showed up that the silico-carbonatite rock appeared as sills through the limestone sequence of the Francon Quarry. This type of rock is in fact very rare and for mineralogists the Francon Quarry is very famous its variety of minerals. The silico-carbonatite rock contains a high amount alkali-rich minerals such as Dawsonite, NaAl(CO₃)(OH)₂. The investigation identified Dawsonite to have played a major role in the concrete deterioration. In order to test the aggregates for concrete production only the limestone rock was tested and found non-reactive. The combination of the two rock types was apparently not tested. During thin section examination of the concrete the dolomite aggregates was found heavily cracked and alkali silica gel appeared in cracks radiating out to in the ambient paste. SEM-EDX analysis showed that the reactive component of the dolomite rock was micro-crystalline quartz finely disseminated in the dolomite structure. A reaction zone consisting of calcite and gibbsite was observed around all of the silico-carbonatite aggregates in the concrete. The interior part of the silico-carbonatite rock was highly porous and by comparing the aggregate structure with the natural rock from the quarry it was showed that the high porosity was due to the decomposition of Dawsonite. This decomposition of Dawsonite resulted in an increased alkali content of the pore solution, which then triggered the alkali silica reaction in the dolomite rock. The decomposition of Dawsonite can be described by the following reaction: NaAl(CO₃)(OH)₂ + Ca(OH)₂ = CaCO₃ + Al(OH)₃ + NaOH (dawsonite) (calcite) (gibbsite)	The Francon Quarry in Canada.
	Cracked railroad ties.
	Dawsonite crystals in the SEM. The dark gray rim is low in sodium.
	Highly porous dawsonite aggregate rimmed by a zone of dense paste.
	Dolomite aggregate cracked by ASR. Gel appears in the crack.