COMPARATIVE MOISTURE RESISTANCE ANALYSIS OF QUARRY DUST AND PERIWINKLE SHELL ASH MODIFIED ASPHALT CONCRETE FOR TROPICAL FLEXIBLE PAVEMENTS

Moisture damage remains one of the leading causes of premature failure of flexible pavements in the humid tropics, where prolonged rainfall, high water tables and poor drainage expose asphalt concrete to repeated wetting. This study evaluates quarry dust (QD) and periwinkle shell ash (PSA), two locally available waste-derived materials, as modifiers of dense-graded asphalt concrete and compares their influence on compressive strength under progressive moisture submergence. Crushed granite and sand were blended with the aid of Excel Solver, and the optimum binder content of 5.5% was established by the Marshall procedure using bitumen contents of 4% to 6% and 75 blows on each face. Specimens were then produced at the optimum binder content with QD or PSA added at 2%, 4%, 6%, 8% and 10% by weight and were submerged in water for 0 to 5 days before compressive strength testing. Both modifiers raised dry compressive strength above the unmodified control value of 3.345 MPa, reaching peaks at 6% content of 5.367 MPa for QD (a 60.4% gain) and 4.428 MPa for PSA (a 32.4% gain). Beyond 6% content, strength declined for both materials. After five days of submergence the control retained 69.8% of its dry strength, whereas 8% and 10% PSA retained about 86% and 85%. QD at 6% gave the highest residual strength at five days (3.036 MPa, 30.1% above the control) but retained only 56.6% of its dry strength, and all QD mixes showed an abrupt strength drop between the third and fourth days of submergence. The results indicate that QD is the more effective strengthening modifier, while PSA at 8% to 10% offers better retention of strength in water. A modifier content of 6% QD or 8% PSA is suggested for further performance verification.