Water movement control across organic primers for pavements
In order to curtail vertical moisture movement through pavement layers, a layer of primer is applied on the top surface of the base. During precipitation, water flows into a porous layer and then moves laterally on top of the primer to drain out through the sides. Some of it diffuses across the primer into the base causing its stiffness to reduce hence culminating in pavement damage. In this research, water movement through organic primers was studied. The objectives were to establish the rheological characteristics of these primers with varying ratios of bitumen and diluents, assess the influence of primer composition on vertical moisture movement across them, and evaluate whether moisture movement across these primers obeys Fick‘s second law of diffusion. Two penetration grade bitumens (80/100 and 60/70) of Arabian origin were cutback using kerosene from Shell company in percent ratios of 70/30, 60/40, 55/45 and 50/50 respectively, to form eight primers. Model pavement bases were made of Crusher Run Rock (CRR) mixed with laterites in proportions of 100/0, 75/25, 50/50, 25/75 and 0/100, respectively. Model bases were compacted using one - point load CBR to maximum dry density and optimum moisture content. The specimens were then left in the laboratory for 24 hours. A primer was applied onto the base top and left to cure for 48 hours, after which a controlled amount of tap water poured onto the primed base top. Movement of water vertically across the primer was monitored after 0, 0.5, 2, 4, 8, 12, 24 and 48 hours using a moisture probe placed at the bottom of the primer. Results show that the primers under study were generally pseudo-plastic, which behaviour changed with increased shear rate. Moisture movement across the primer increases as the cutter kerosene dosage was increased from 30 to 50% owing to peptization of asphaltenes. The water flow data did not fit onto the specific solution for the partial differential equation for Fick‘s second law of diffusion implying that the phenomenon of vertical water movement across the primers was not a diffusion process. More dedicated research is needed to characterise the water movement across primers so as to devise scientific means of controlling it.