|Abstract:||Steel furnace slag (SFS) is an industrial by-product material that can contain free calcium oxide (CaO) and free magnesium oxide (MgO), both of which can cause significant expansion when hydrated. SFS aggregates are therefore not commonly used in concrete, although SFS aggregates have been used as a high quality frictional aggregate for hot-mix asphalt (HMA) surface courses. The resultant fractionated reclaimed asphalt pavement (FRAP) when the HMA with SFS is removed has also seen little usage.
This study aims to continue the previous work by the authors that indicated that up to 50% dolomite FRAP can be used in concrete as a replacement of coarse aggregate. The objective of this work was to evaluate the effects of SFS FRAP at 20% and 50% replacements of the coarse aggregate in concrete. In addition, the chemical, mineralogical, physical, and expansive properties of three SFS FRAP sources were investigated along with investigations of three virgin SFS sources for comparison.
The chemical, mineralogical, and physical properties of the SFS FRAP and virgin SFS sources were similar to values presented in the literature. The estimated total free CaO content of the virgin SFS sources ranged from low (<0.1%) to high (3.4%), while the free CaO content of the SFS FRAP sources was estimated to be 1.0% to 1.5%. The free MgO content of the virgin SFS sources ranged from 0.2% to 2.2%. Autoclave expansion tests correlated well for the virgin SFS sources with regard to the free CaO content (i.e., high free CaO content resulted in high expansion), while the SFS FRAP with asphalt binder removed did expand, but not necessarily proportionally to the free CaO content. Autoclave expansion tests of the SFS FRAP with the asphalt binder resulted in contraction rather than expansion. The results validated the findings from other studies that have shown that SFS FRAP will not significantly expand because of the asphalt coating.
Concrete tests revealed that the strength, modulus, shrinkage, and fracture properties were similar between concretes with SFS FRAP and with dolomite FRAP. The modulus of elasticity was slightly higher for concrete with SFS FRAP compared with dolomite FRAP, possibly because of the presence of the stiffer SFS aggregate. The fracture properties were statistically similar for concrete with and without SFS FRAP aggregates. The freeze/thaw durability was reduced with higher SFS FRAP contents, possibly because of the asphalt coating on the FRAP rather than the SFS in the FRAP, because the mixes with 100% virgin SFS exhibited superior freeze/thaw durability.
Based on these findings, it is evident that SFS FRAP can retain free CaO and free MgO contents, despite years in service in a pavement layer and/or years being weathered in a stockpile. The presence of the asphalt coating hinders, but may not necessarily prevent, the hydration of the free CaO and free MgO in the SFS. Therefore, it is recommended that the SFS FRAP be tested for free CaO, free MgO, and asphalt contents and autoclave expansion potential prior to being utilized in a structural concrete layer. Immediate usage of SFS FRAP may not be detrimental for non-structural applications, such as temporary roads or shoulders. To ensure that future SFS FRAP can be used in concrete pavements, the SFS that is presently used in HMA should be weathered and have low free CaO contents and low expansion potential.