Wadi gravel – a new concrete aggregate in Qatar: Part 1 – investigation, processing and trials

Abstract

In hot desert regions, attention is turning to local sources of aggregate that were previously regarded as ‘too difficult' to exploit. This paper describes an investigation of such a deposit in Qatar. The material, known as Wadi gravel, is a gravelly sand of Tertiary age cemented by gypsum. A field survey found the gravel content to be in the range of 10 to 20%. As-dug samples had excessively high sulfate content, which could not be removed with conventional aggregate operations. However, intensive treatment in a local sand-processing plant using multi-stage crushing, screening and washing reduced the sulfate to acceptable levels. Petrographic analysis was carried out to identify rock types potentially susceptible to Alkali Silica Reaction (ASR). Trial concrete mixes using the processed Wadi gravel were found to produce satisfactory C40 and C60 concrete. Full-scale building trials were constructed to demonstrate the practicality of using the gravel and assess in-service performance. The smooth surface and rounded particles of Wadi gravel improved the concrete workability and strength, and durability-related properties were similar to concrete made with imported gabbro. Strata similar to Wadi gravel occur widely in hot desert regions and could be useful sources of coarse and fine aggregate for concrete.

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

“…The gravel content of the deposits is generally fairly low, in the range of 10% to 20% and consists of a range of rock types, including igneous and metamorphic rocks from the Arabian Shield and limestone and other sedimentary rocks from the Arabian Shelf, thought to have been transported by large river systems. Similar deposits are found widely in Eastern KSA and other areas in the region [7]. These sand and gravel deposits are generally known as "Wadi gravel".…”

“…It is important to stress that if Wadi gravel is used as aggregate in concrete, it must be produced under a quality control system that ensures it is subject to rigorous processing to remove the gypsum-bound material and reduce the sulfate content to acceptable levels, as described in Hassan et al [7]. If excessive quantities of gypsum-bound deposits are somehow included in the concrete, the resulting expansive reactions will likely result in poor quality concrete, which will deteriorate rapidly and bring the material into disrepute.…”

“…The Wadi gravel was obtained from the Mekaines site, in the south west of Qatar, approximately 50 km from Doha, and processed using the multistage treatments described in [7]. The imported gabbro aggregate was supplied by Gulf Rocks Co. in the UAE.…”

“…The additional beams were partially buried within the fill material around the buildings, Figure 3, and were later used for coring and testing the different concrete mixtures at different ages. The purpose was to represent aggressive ground conditions and excessive heat near the ground surface [7]. Construction took place in the summer of 2016 (June/July), with an average air temperature in the range of 40°C to 50°C.…”

“…Such long transport distances for bulk materials such as aggregates, however, add considerably to the cost and the carbon footprint of the resulting concrete [6]. Research was therefore carried out to determine if it would be possible to process local Wadi gravel to reduce the sulfate content to an acceptable level and produce aggregate that met the requirements of the QCS 2014 for coarse aggregate in concrete [7]. The project found that satisfactory aggregate could be produced by multistage processing using advanced crushing and washing techniques.…”