COMPACTION PROBLEMS OF THIN HMA LAYERS CONSTRUCTED ON WEAK PAVEMENTS

A Hot Mix-Asphalt (HMA) layer as the shown in the picture, having a thickness lower than 5 cm, placed over a weak pavement structure will be almost impossible to reach an optimum compaction level. In Costa Rica, LanammeUCR has recommended to the Maintenance Area of the Department of Transportation to pay this type of thin-layers based on samples on the volumetric properties go samples taken at HMA plant. Paying the material based on field air voids is a mistakes, since the weak pavement structure will not provide enough stiffness to function as a stable contraction platform, hence, the energy of the compactor will be dissipated within the weak pavement layers. Trying to apply more vibration to the compactor will break the bigger aggregate particles. In Costa Rica, layers compacted with less than 4 cm are call non-structural layers, however, there is a thin line to differentiate the behavior if the layer reaches up to 5 cm, when weaker pavements provide the base beneath the new HMA. An useful to determine the pavement stiffness is the Falling Weight Deflectometer (FWD) or the old, but still in use, Benkelman Beam. Both equipments, by means of the pavement deflection basin or the maximum deflection, provide important information on the structural condition of the pavement structure. Costa Rica has had 5 campaigns of evaluation of its paved national road network, of about 5,000.00 km. Among the evaluated parameters, FWD data is available for each meter of the national road network since 2004, therefore, the project engineer has a powerful tool to determine the pavement condition of his project and to define the strategy to apply. As we have insisted before, this type of decision requires a well prepared pavement engineer with sound knowledge to understand the pavement behavior, its design process, material properties and the expected performance. Those characteristics, plus good field experience will leave to the citizens the best project with the highest possible quality, according the available budget.