Despite the use of blended cements in the construction of Port Bouvard and Koombana bridges, they have exhibited high permeability which is causing concern to Main Roads Western Australia. A microstructural study was conducted to help clarify the reasons for the high permeability of the concretes. Considerable microcracking has been observed in the concrete, both at the cement/aggregate interfacial zones and in the paste. In some interfacial zones, a feature of high water to cement ratio (w/c) is evidenced by high local porosity and well crystallised hydration products. The defects in the concrete may result in a high permeability. The reasons for the internal microcracking in the concretes are not immediately apparent. However, petrographic examination has established that the granitic aggregate used in these concretes is potentially reactive in the presence of alkalies. SEM and EDX analysis have confirmed the presence of alkali-rich gels near some aggregate particles. SEM examination has also shown bands of calcium sulfate crystals, amorphous ettringite and other sulfate-bearing phases in the surface zone of the concrete, indicating that some localised type of sulfate attack might have occurred. It is noted that the concretes are made with slag cement (35/65 BFS), and that the Port Bouvard bridge concrete had (in addition) 8 percent silica fume in the binder. It is very likely that without these mineral components the risk of cracking due to alkali-aggregate reaction (AAR) would have been very high. (A)
Samenvatting