The Effect of CKD and RAP on the Mechanical Properties of Subgrade Soils

Abstract

The construction of pavement layers on subgrade soil with good characteristics decreases thethickness of these layers, which in turn lowers the cost of building and maintaining roadways.However, it is impossible to avoid constructing pavements on unsuitable subgrade due to anumber of limitations. Using conventional additives like lime and cement to improve subgradeproperties results in additional costs. As a result, utilizing by-products (cement kiln dust andreclaimed asphalt pavement) in this field has benefits for the environment, economy, andtechnology. Large amounts of cement kiln dust (CKD), a by-product material, are produced inPortland cement factories. On the other hand, large amounts of reclaimed asphalt pavement(RAP) are accumulated as a result of the rehabilitation of old roads. This paper discusses usingCKD and RAP to improve the characteristics of poor subgrade layers by conducting a series ofUnconfined Compressive Strength (UCS) and California Bearing Ratio (CBR) tests on samples ofnatural soil and soil stabilized with different percentages of CKD and RAP with different curingtimes to investigate their impacts on soil properties. The curing was carried out by wrapping thestabilized samples with several layers of nylon and then placing them in plastic bags at roomtemperature. The compaction results illustrated that the addition of CKD increases OMC anddecreases MDD, in contrast to RAP, which decreases OMC and increases MDD. The addition ofCKD and RAP led to a significant and unexpected increase in the CBR values. The results showthat the soaked and unsoaked CBR values improve from 3.4% and 12.1% for natural soil to220.1% and 211%, respectively, after adding 20% CKD and curing the samples for 28 days. Also,the addition of 25% RAP to soil-20% CKD blend increased the soaked and unsoaked CBR valuesto 251% and 215%, respectively. All the additions resulted in a significant reduction in swelling