<P>A few years ago, visual inspection of an 18-year-old bridge by ARRB in Australia identified considerable cracking in the precast, prestressed deck planks as well as in the cast in situ deck overlay.Laboratory examination indicated that the deck planks and the deck overlay were suffering from a strong case of alkali-aggregate reaction (AAR). Testing of concrete cores drilled from the planks indicated that the concrete component had lost around 30-50% of its compressive strength and elastic modulus.</P>
<P>Many similar bridges have exhibited the same problem. For the bridge in question, the deck was replaced after 14 years, and the affected planks stored on site. Assessment of the mechanical properties and load capacity of the planks was essential for sound decision making regarding rehabilitation of the other affected bridges.Full scale testing of three planks was undertaken in collaboration with Monash University, to examine the overall effects of the AAR-cracking.</P>
<P>Durability properties of the concrete were determined from residual strength and expansion properties of concrete, petrographic examination, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis. Residual alkali content, cement content, compressive and splitting tensile strength, elastic modulus, residual expansion and maximum expansion potential were also determined. Coarse aggregate was separated for testing, as well as accelerated testing of the planks expansion.</P>
<P>A strength assessment was conducted to determine a load rating for the planks. This assessment included ultimate strength, ductility and deflection.The SEM/EDX and petrographic examination identified two reaction mechanisms in the planks, being AAR and delayed ettringite formation (DEF). The latter occurs at high curing temperatures. The severity of the AAR in some of the planks was evident from the extent of the reaction products seen in the concrete.</P>
