Influence of hydrophobisation on surface free energy of hybrid fiber reinforced ultra-high performance concrete Danuta Barnat-Hunek a , Piotr Smarzewski b,⇑ a Lublin University of Technology, Faculty of Civil Engineering and Architecture, Department of Construction, Nadbystrzycka St. 40, 20-618 Lublin, Poland b Lublin University of Technology, Faculty of Civil Engineering and Architecture, Department of Structural Engineering, Nadbystrzycka St. 40, 20-618 Lublin, Poland highlights  The content of fibers increases adhesion of the hydrophobic agents on the ultra-high performance concrete (UHPC) surface.  The highest hydrophobisation efficiency of the FRUHPC was obtained by a alkyl–alkoxy-silanes.  The SFE is nonlinear dependent on the UHPC absorptivity.  The strong correlations between the SFE values and the contact angle were formulated.  The Neumann and Owens–Wendt methods gave the most reliable SFE results. article info Article history: Received 8 July 2015 Received in revised form 19 October 2015 Accepted 2 November 2015 Available online 13 November 2015 Keywords: Surface free energy Hydrophobisation Ultra-high performance concrete Steel fiber Polypropylene fiber abstract The aim of the research presented in the paper was to evaluate the feasibility of using hydrophobic preparations based on organosilicon compounds for protection treatment on the hybrid fiber reinforced ultra-high performance concrete (FRUHPC) surface. Three polisiloxanes agents were deposited onto the seven types of concretes, with a steel-polypropylene fiber content ranging from 0% to 1%. In this inves- tigation, the surface free energy (SFE) of the coatings was calculated using the Owens–Wendt, Neumann, Wu, and Fowkes methods. In order to examine the contact angle, three measuring liquids were used: distilled water, glycerine and diiodomethane. It has been shown that the Neumann method with one polar and the Owens–Wendt method with a pair of polar liquids gave the most reliable results. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Ultra-high performance concrete (UHPC) is a type of structural concrete with a compressive strength above 120 MPa and very high durability. The material is characterized by high strength, low absorbability, low water permeability and high freeze resis- tance, which results in high durability [1]. Fibers are added to the matrix as reinforcement to control cracking and to improve the general ductility of the material [2]. Some properties of con- crete can be improved by polypropylene or steel fibers. Usually, in order to improve the mechanical and physical properties, espe- cially tensile and flexural strength as well as long-term concrete shrinkage, steel fibers are used. On the other side, polypropylene fibers do not corrode, are thermally stable, chemically inert and very stable in the alkaline environment of concrete [3,4]. Moreover, the polypropylene has a hydrophobic surface (does not absorb water) and it does not interfere in the concrete hydration reaction [5]. UHPC are often exposed to aggressive impacts of the environment and therefore they must have a high resistance to chemical corrosion, frost corrosion, weathering, impact of aggres- sive water and many other corrosive agents. One of the methods used to protect the concrete surface from corrosion caused by moisture is hydrophobisation [6]. Organosilicon compounds – siloxanes or methyl silicone resins [7,8] are mostly used as concrete hydrophobising agents. The wettability of concrete by means of liquids which contain corrosive components is of great importance in practice. It may indicate the adhesive properties of concrete, as well as protective coatings applied to its surface. Some treatments previously reported in literature for fibers in concrete applications include plasma based treatment [9], acid and other chemical treatments [3,10], are able to modify the fiber and concrete, changing the roughness, and polarity of the surface [11]. http://dx.doi.org/10.1016/j.conbuildmat.2015.11.008 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: p.smarzewski@pollub.pl (P. Smarzewski). Construction and Building Materials 102 (2016) 367–377 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat