**Md Rashadul Islam \*, Sylvester A. Kalevela and Shelby K. Nesselhauf**

Department of Engineering Technology, Colorado State University, Pueblo, CO 81001, USA; sylvester.kalevela@csupueblo.edu (S.A.K.); sk.nesselhauf@pack.csupueblo.edu (S.K.N.)

**\*** Correspondence: md.islam@csupueblo.edu; Tel.: +1-719-549-2612

Received: 25 April 2019; Accepted: 4 June 2019; Published: 7 June 2019

**Abstract:** In the design of pavement infrastructure, the flow number is used to determine the suitability of a hot-mix asphalt mixture (HMA) to resist permanent deformation when used in flexible pavement. This study investigates the sensitivity of the flow numbers to the mix factors of eleven categories of HMAs used in flexible pavements. A total of 105 specimens were studied for these eleven categories of HMAs. For each category of asphalt mixture, the variations in flow number for different contractors, binder types, effective binder contents, air voids, voids in mineral aggregates, voids filled with asphalt, and asphalt contents were assessed statistically. The results show that the flow numbers for different types of HMA used in Colorado vary from 47 to 2272. The same mix may have statistically different flow numbers, regardless of the contractor. The flow number increases with increasing effective binder content, air voids, voids in mineral aggregates, voids filled with asphalt, and asphalt content in the study range of these parameters.

**Keywords:** hot-mix asphalt; flow number; effective binder content; air voids; voids in mineral aggregates; voids filled with asphalt; asphalt content
