The Influence of Fines Content on the Properties of Aggregate for Subbase Course Material for Highway Construction

Abstract

This study focuses on the granular material for subbase layer with different percentage of fines content (material finer than 0.075 mm). The experimental investigation conducted to determine the physical and mechanical properties of natural granular material and granular material with different fines material. The laboratory tests were carried out using percentage of fines 5%, 10% and 15% by weight adding to the granular material class B according to Iraqi specification (SCRB) which the specification limit the range of fines between 5-15%. RL CBR test was used to study the mechanical properties; accumulative permanent deformation, recoverable deformation and equivalent elastic modulus. The equivalent elastic modulus and accumulative deformation characteristics were determined in relation to fines content, stress and number of load repetitions. The results of the tests showed that the liquid limit and plasticity index increased by increasing fines content material. In general, the optimum moisture content increase, when adding fines material, this increase depend on the type of fines where highest optimum moisture content be at granular material with clayey fines and less value at granular material with silty fines. The maximum dry density increase as adding 5% fines material to the natural granular material for subbase and then the value dropped with increase in fines material for three type of granular material. Fines material produced an increase in strength (CBR) of granular subbase material at adding 5%, but continues adding, this caused decrease in value. The results showed the RL CBR test was useful technique to evaluate the effect of fines material, moisture content and the stress on the equivalent elastic modulus and on the development of deformation. The equivalent modulus has a maximum value when adding 5% of fines material in case of plastic fines, but in case of silty fines the increase continues until adding 10% fines material.Adding this material also led to change in recoverable deformation, where adding 5% fines made the recoverable deformation at a maximum value. Based on CBR test values and relationship with fines material best percentage of fines material ranging between (5 – 9 %) this convenient for dry areas but the percentage change for rainy areas are preferable to range between (5 – 8 %). The types of fines material (clay or silt) also have a major influence on the properties of granular subbase material.