Research Article TRR JOURNAL OF THE TRANSPORTATION RESEARCH BOARD Investigation of Moisture Damage in Open Graded Asphalt Friction Course Mixtures with Basic Oxygen Furnace Steel Slag as Coarse Aggregate under Acidic and Neutral pH Environments Santanu Pathak 1 , Rajan Choudhary 1 , and Abhinay Kumar 1 Abstract Open graded asphalt friction courses (OGAFCs) are specialty asphalt mixtures used to improve skid resistance and surface drainage. OGAFCs have additional benefits of reduced splash and spray, and lower tire–pavement interaction noise. Prolonged exposure to rainwater and load transfer through stone-on-stone contact in OGAFCs demands aggregates that are strong and hydrophobic. Rainwater acidity is expected to affect the aggregate–asphalt bond and thus moisture damage performance of OGAFC. This paper investigates the effect of rainwater acidity on moisture sensitivity of OGAFC mixtures with different aggregate types (natural aggregate, basic oxygen furnace (BOF) steel slag, and combinations of both) and modified binder types. For the first time, the present research reports the moisture damage potential of BOF OGAFC mixtures under different moisture conditioning environments created by varying the pH of contact water. With different combinations of BOF slag and natural aggregates (100:0, 25:75, 50:50, 75:25, and 0:100), and binders (polymer and crumb rubber modified), OGAFC mixtures were characterized for moisture damage through tensile strength ratio, wet Cantabro abrasion loss, and modified boiling water tests. Functional aspects of OGAFC mixtures subjected to moisture conditioning under different pH environments were also evaluated through permeability testing. Results showed that an acidic environ- ment exacerbated the moisture damage, however, OGAFC mixtures containing BOF slag showed better performance than the control mixture (with natural aggregates only). Inclusion of BOF slag in OGAFC mixtures enhanced resistance to moisture damage under both pH environments. OGAFC mixes with 100% BOF slag content performed the best considering all moisture damage tests under both conditioning environments. An open graded asphalt friction course (OGAFC) is a thin overlay of a typical hot mix asphalt (HMA) with a significantly high amount of air voids, typically in the range of 18%–22% of the mix volume (1), resulting in an open graded aggregate structure. Conventional dense graded hot mix asphalt courses, in contrast, have an air void content of about 3%–5% (2). OGAFC is laid to a thickness of about 25–50 mm (1–2 in.) (3,4) over an exist- ing impermeable dense graded surface course, to facili- tate quick drainage of stormwater runoff. During a precipitation event, the stormwater traverses through the interconnected network of voids in the OGAFC overlay until it reaches a day-lighted edge along the road perimeter. The major benefits associated with OGAFCs include: high skid resistance, improved visibil- ity of pavement markings, reduced splash/spray and headlight glare, reduced potential for hydroplaning, and attenuated tire–pavement interaction noise. Massive highway infrastructural development projects are underway in India for the construction of new 1 Department of Civil Engineering, Indian Institute of Technology Guwahati, Guwahati, Assam, India Corresponding Author: Rajan Choudhary, rajandce@iitg.ac.in Transportation Research Record 0(0) 1–15 ! National Academy of Sciences: Transportation Research Board 2020 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0361198120925459 journals.sagepub.com/home/trr