A Deeper Look at Recycled Concrete (Source: The Military Engineer)
Recycling of demolished concrete for use as base material for military airfield pavements has never reached its full potential. This limitation is attributable to uncertainties in maintenance costs and long-term performance of the pavement because of potential latent properties of the recycled concrete that might have some adverse impact on the new pavement.
Yet there are powerful pressures pushing towards less use of virgin aggregate, and to find ways to use recycled materials more—creating both economic and environmental benefits.
Concerns about performance
The main concern about the performance of concrete used as base material involves cases in which the demolished pavement was actively deteriorating as a result of alkali-aggregate reaction (AAR). AAR is a phenomenon that occurs in concrete where an expansive chemical reaction develops involving the Portland cement and reactive components present in some aggregates. This expansion first develops on a microscopic scale, but eventually progresses to the point that cracking and breakdown in the concrete pavement occurs. The expansion also results in gross expansion of the entire structure, which can have damaging effects on adjacent structures. There is no practical way to stop the progression of AAR as long as the concrete is exposed to water.
AAR requires the presence of water and so usually occurs in structures in outdoor exposures. There is almost always presence of sufficient water around airfield pavements to support an active AAR condition if the properties of the cement and aggregates are suitable for the reaction to develop. This is even true in desert environments. The soil under pavements normally contains some water vapor from deeper water in the underlying geology. This vapor condenses on the underside of a pavement in sufficient quantity to support AAR.
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