Effect of Petrogenesis on the Suitability of Some Pelitic Rocks as Construction Aggregates in the Tropics

Ten rock samples collected from five rock quarry units of Albian Asu-River Group (southeastern Nigeria) were studied megascopically and subjected to XRD to assess their petrography. The samples were subjected to degradability test to simulate their resistance to repeated wetting and drying common in tropical regions while nine of them, pelitic in composition, were further subjected to abrasion test to determine their abrasion value (LAAV) and impact test to determine their impact value (AIV). The field and petrographic studies showed that rocks of varying petrogenetic origins notably; hydrothermally altered pelitic rocks and volcanic bombs, pyroclastic rock, pelitic argillites and hornfels occur in the quarry units. XRD revealed that none of the rock types has significant amount of siliceous minerals implying that none is susceptible to alkali-aggregate reaction. Results of degradability test showed that the two pelitic argillites and one of the four hydrothermally altered pelitic rocks, having percentage mass loss ranging from 1.28 to 26.76 %, showed significant deterioration implying that the three rocks are not suitable for construction of structures like embankment and unpaved roads in tropical regions because of their petrogenesis and mineralogy. Results of the LAAV and AIV tests ranged from 9.40 to 14.00 and 19.00 to 24.00 respectively indicating that all the rocks are suitable for construction of all pavements sections. In general, all these results show that mechanical degradation of rocks is not only dependent on petrography but also on petrogenesis.