Wadi gravel – a new concrete aggregate in Qatar: Part 2 –Alkali aggregate reactivity

Abstract

Deposits of Wadi gravel are available in many parts of the Gulf region, but not widely utilized as aggregate for concrete, mainly due to the possibility of internal sulfate attack, plus the perceived risk of alkali aggregate reactivity (AAR). This paper describes the investigations for AAR of the Wadi gravel in this case, as part of the wider study described in Part 1 of this paper.Wadi gravel from the Mekaines site in Qatar was subjected to petrographic analysis, plus the gel-pat and accelerated mortar-bar test methods. The AAR potential was found to be low to normal. The accelerated mortar-bar test exhibited ‘innocuous’ behaviour after 14 days of immersion in alkali solution. When separately testing the constituent rock types of the Wadi gravel, limestone and quartz returned innocuous results, while rhyolite, granite and quartzite returned potentially alkali silica reactive (ASR) results and some reaction was confirmed using post-expansion petrographic examination.Wadi gravel was classified as potentially reactive in the RILEM AAR-4.1 accelerated concrete prism test, but of ‘low reactivity’ in the BS 812-123 test over the longer period of 12 months. Overcoming the potential problems of gypsum content and AAR successfully provides a valuable local resource of Wadi gravel aggregate for concrete.

Assessment of AAR and Sulfate Attack on Wadi Gravel Concrete after 4 Years in Service

“…Three mortar mixtures were tested to include 100% gabbro, 50% Wadi gravel and 100% Wadi gravel. Results of the accelerated mortarbar mixtures are presented elsewhere [9].…”

“…Storage conditions varied between the two tests. RILEM AAR-4.1 concrete prisms were exposed to 60 °C temperatures at as close as possible to 100 % relative humidity (RH), whereas BS 812-123 concrete prisms were exposed to humidity >96 % RH at 38 °C [9].…”

“…The petrographic examinations of the processed Wadi gravel identified five rock constituent types, which together accounted for at least 95 % of the Wadi gravel. The gypsum-bound deposits (GBD), which accounted for about 20% of the raw Wadi gravel [9], were reduced to less than 1% by the intensive multi-stage processing. When the GBD are excluded, about half the gravel consists of limestone.…”

“…Details of the expansion results are given elsewhere [9]. The 100% gabbro and 50% Wadi gravel mortars exhibited relatively low expansion values and fell into the innocuous category of ASTM C1260 after 14 days immersion (16 days age).…”

“…A further concern with using gravel containing a number of different rock types as aggregate in concrete is the possibility of Alkali-Silica Reaction (ASR), which could lead to cracking and disruption of the concrete in the long term. A detailed laboratory investigation into this aspect was carried out [9], including accelerated laboratory tests using a variety of techniques. The tests were carried out using the same mixtures used for the laboratory trials and concluded that the Wadi gravel could be safely used as coarse aggregate in structural concrete either as a 50:50 blend with imported gabbro or with the use of cement replacement materials such as ground granulated blastfurnace slag (ggbs) or coal fly ash (FA).…”