Crack Behaviour Under Vehicular Loading Along Mbagathi Road in Nairobi County
Nairobi County has nearly 100% road network as bituminous roads and only 4km road network (Mbagathi Road) is concrete road. This road was a pilot project that was implemented by Kenyan Government between the years 2005 and 2007 at a total construction cost of over 400 million Kenya Shillings. The design was done based on South Africa design procedure on a 210 mm thick Plain Jointed Concrete Pavement (PJCP). Performance of Plain Jointed Concrete Pavement (PJCP) depends on aggregate interlock, which is the mechanism by which loads are transferred from one slab to the adjacent one. This study attempted to use the information on load transfer mechanics in order to quantify the effect of aggregate properties to crack size as well as the propagation of any cracks formed in service. Modern methods applied during the design of concrete pavement do not consider fracture mechanics into consideration. These make scheduling of repair and other maintenance works difficult. This research was undertaken in two different ways namely experimental and computer simulation. The experimental approach entailed measuring widths and depths of cracks existing in Mbagathi road, and also involved setting up concrete beams in the laboratory and loading them to failure with predetermined loads and measuring sizes of resulting cracks. On the other hand, the computational approach was verified by experimental results entailed simulating crack widths and depths with recent devised fracture models. Cracks were experimentally noticed to occur due to loading the pavement with vehicular loads bigger than safe load determined as 430.4kN. Cracks generated had crack width equivalent to the measured flexural deflection and for an ideal concrete pavement crack width and crack depth exhibited an exponential relationship. Experimental set-up yielded a deflection load curve that showed a quintic relationship, the maximum deflection recorded was 0.35mm. It was found that vehicular loads induce stresses in concrete pavements which exceed permissible tensile stresses, hence a better understanding of the resultant cracks, as well as their propagation. Aggregate properties found to have an effect to crack width and depth were water to cement ratio, hardness and surface texture. This research will lead to an improvement of the design of concrete pavements in Kenyan environment.
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