Mechanical performance of rca-steel slag mixtures as base and subbase layers

Abstract

This study aims to evaluate the capability of recycled materials and industrial by-products, which are Recycled Concrete Aggregate (RCA), Electric Arc Furnace (EAF) slag, Ladle Furnace Slag (LFS), and their binary mixtures, to be used in road applications as sustainable alternatives to natural aggregates. The increase of natural source consumption, resource limitation, and requirement of infrastructure development make using alternative materials more significant. Although in the previous studies, RCA and steel slags were investigated separately, using them in a combination and finding the optimum mixture ratio is emphasized more in this study. It aims to show the performance of these materials in combinations and improve them in this way and reduce the challenges and issues. Each material has some suitable properties, a weak point, and limits. These differences are the reason to study the mixtures to evaluate what limits are eliminated and what positive points are improved. To study over these features, the mixtures with different ratios (25%-75%, 50%-50%, and 75%-25%) for each system (RCA-EAF and RCA-LFS). Then all the samples were prepared, including the pure materials and mixtures, and the experimental program was performed. These experiments were sieve analysis, specific gravity, standard compaction, California bearing ratio in both dry and soaked conditions, direct shear test, and swelling potential tests. In RCA- EAF mixtures, the synergetic behavior is observable considerably. Even though EAF slag singly has a high strength, its volumetric instability is high, and this issue prevents this material from being used in road projects. However, adding RCA to it reduces the swelling potential and stabilizes the mechanical behavior. The 25% ratio has positive effects approximately and changed the swelling potential from 0.82% to 0.52%, but more balanced ratios (50% RCA and 75% RCA) provide the allowed limit efficiently. There are optimum points in each test, such as the standard compaction test (E75-R25) with the highest maximum dry density. In the CBR test, E75-R25 has the best CBR value. As well, E50-R50 has close results to the optimum point with less swelling potential. The direct shear test result shows high performance in the mixtures. In the RCA-LFS mixtures, the same pattern was observed with different details. LFS singly has an average strength with a fine structure. By adding RCA to LFS, the maximum dry density increases, and at an optimum point (L75-R25), the CBR value increases. The direct shear test shows that in LFS mixtures the main parameters, such as cohesion and internal friction angle, improved obviously. LFS didn't show a swelling potential in this study and didn't pass the limit despite repeating the test three times. The general results show that the mixtures of RCA-EAF and RCA-LFS not only make up the limits of pure materials, but also they form improved behaviors that are not seen in the materials individually. Some mixtures have higher density, better shear stress, higher CBR value, more suitable bearing capacity, and less swelling potential. These performances are compatible with the geotechnical standards, and they meet the requirements to be used in the road construction projects, such as base layer, subbase layer, and embankments. As a result, this thesis demonstrates that designing and using the RCA, EAF slag, and LFS in optimum combinations may be utilized in road layers efficiently. These mixtures not only have great performance, but also, by using only the waste materials and reducing natural aggregates, they have a magnificent role in environmental concerns and the circular economy.