Soaking Effects on Strength Characteristics of Crushed Gravel and Limestone Unbound Aggregates

Strength characteristics of unbound aggregate materials critically affect base and sub-base thickness designs of construction working platforms and flexible pavements. Unsoaked California Bearing Ratio (CBR) is commonly used by state transportation agencies for quality control and design. However, depending on the amount of fines content (i.e., passing No. 200 sieve), the strength characteristics of unbound aggregate layers may severely be affected after soaking. This study investigates any correlations that may exist between soaked and unsoaked strengths of crushed gravel and limestone aggregates commonly used in the State of Illinois. A test matrix was established to consider the effects of varying fines content at 5% and 12%, plasticity index (PI) at 5% and 9%, and dust ratio (DR) at 0.4, 0.6, and 1.0 on aggregate strength. DR is the ratio of percent passing No. 200 sieve to percent passing No. 40 sieve. Most of the aggregate strength characterizations with 5% fines content were not sensitive to soaking in terms of CBR index. However, significant reductions in soaked and unsoaked CBR values were observed when fines content increased from 5% to 12%. The crushed limestone aggregate strengths were more affected by soaking than the crushed gravel aggregates. A prediction model was developed to predict soaked CBR values from unsoaked CBR tests considering material type, fines content, PI and DR.