AAR-5: Rapid preliminary screening test for carbonate aggregates

The ultra-accelerated mortar-bar test (AAR-2) has been widely and successfully used as a screening test to identify aggregates that are potentially alkali-reactive. However, there are exceptions; some aggregates that, though alkali-reactive according to both field experience and concrete prism test results, do not show as such in the mortar-bar test. Trials carried out by members of RILEM TC 191-ARP have established that these aggregates include carbonate materials. It has been found that any potential reactivity of such carbonate aggregates will be identified if 4/8 mm aggregate is used for making the bars instead of 0/4 mm aggregate. The development of this method (AAR-5), which is sometimes termed the ‘concrete microbar’ test, and the international trials organised by RILEM TC 191-ARP are described in [1]. The concrete microbar method is particularly beneficial when the aggregate contains both reactive silica and a kind of dolomite that is prone to dedolomitization. In the mortarbar test most of the (fine) aggregate will dedolomitize throughout, whilst in the concrete microbars only the outer shells of the (coarser) aggregate is dedolomitized [2]. This difference seems to explain why, though alkali-silica gel develops in both the tests, the aggregate develops more pressure and expansion in AAR-5 than in AAR-2. It is therefore recommended that, if dolomitic carbonate aggregates are being assessed, both the AAR-2 and the AAR-5 tests should be carried out. If the concrete microbars expand The text presented hereunder is a general draft for general consideration. Comments should be sent to the TC Chairman Dr. Philip J. Nixon, Building Research Establishment, Garston WD2 7JR Watford, United Kingdom; Tel: +44 923 664 239; Fax: +44 923 664 786; Email: nixonp@bre.co.uk, by 30 April 2006.