An Investigation on the Effects of Aggregates Properties on the Performance of Unbound Aggregate Base Layer

Unbound aggregates are the most common type of materials used for construction of the base layer of flexible pavements in Iran and worldwide. Due to the lack of binder, the performance of the compacted unbound aggregates is highly dependent on the properties of the gravel (particles remaining on sieve 4.75mm, sand (particles passing sieve 4.75mm and remaining on sieve 0.75mm), and fines (particles passing sieve No. 200) fraction of the mixture. As the bearing capacity and permeability are the main properties influencing the performance of the base layer; in this research, the effects of the angularity of gravel and sand, and the content and plasticity of fines on the bearing capacity and permeability of the compacted unbound aggregates have been investigated. The bearing capacity of the materials in dry and saturated conditions has been measured using CBR test, and the permeability has been measured using constant head permeability test method. The results showed that the CBR of the compacted mixture decreases with decreasing the percentage of angular sand and gravel in the mixtures. The strength of the mixture was shown to be optimum at a certain amount of fine content and decreases with increasing the plasticity of the fine fraction. It was found that the saturated CBR of the mixtures with non-plastic fines is higher than that of the dry CBR. It was also found that the angularity of the sand is more effective than the angularity of gravel on the CBR. The permeability of the mixtures is shown to increase with decreasing the angularity of the gravel and sand fraction in the mixture.