Petrographic study of coarse aggregate to evaluate their susceptibility to Alkali Silica Reactivity in different rocks of District Shangla, Swat, Pakistan
Keywords:
Aggregates; Petrography; Alkali Silica Reactivity.Abstract
Detailed petrographic investigation for identifying reactive silica in the form of microcrystalline, cryptocrystalline, amorphous, polycrystalline and undulose quartz in natural aggregate are extremely vital for prognosis of Alkali Silica Reactivity (ASR) before using them in high performance concrete. In active orogenic belt hydrothermal fluids alter feldspar and plagioclase into extremely fine grained quartz and mica in the form of muscovite or sericite by sericitization while the same fluids alter biotite and garnets into chlorite by chloritization. Similarly, the intergrowth reaction between plagioclase and k-feldspar produces fine blebs of quartz by myrmekitization, which is extremely susceptible to ASR. Petrographic study indicates the occurrence of microcrystalline, polycrystalline, micro-fractured, strained and angular quartz grains plus the sillimanite protrusion in quartz make the aggregate samples extremely vulnerable to deleterious ASR. The protrusion of fibrolitic sillimanite from biotite into quartz is alarming because it creates tiny microscopic fractures, which can make the affected quartz grains extremely susceptible to ASR in the presence of alkali dissolved water. Additionally, phyllosilicates including micas and chlorite are vulnerable to expansion by absorbing water that could potentially exert pressure on concrete bounded surfaces. Initial cracks on the surface of concrete due to expansion of these layered silicate minerals can provide channel paths for more water circulation in the concrete, which can further bulge muscovite, biotite, chlorite and alkali silica gel plus enhance the alteration of alteration sensitive minerals. Therefore, this study indicates that the rocks in the investigated areas predominantly consist of multiply deformed metamorphic and igneous rocks with high alkali silica reactivity (5-50%) and have exceeded International threshold limit. It is concluded from the current work that the coarse aggregate from metamorphic and igneous rocks of Shangla district, Swat are not recommended for high performance concrete and construction related activities
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