Bond characterization of UHPC overlays for concrete bridge decks: Laboratory and field testing Zachary B. Haber a,⇑ , Jose F. Munoz b , Igor De la Varga b , Benjamin A. Graybeal a a Federal Highway Administration, Turner-Fairbank Highway Research Center, 6300 Georgetown Pike, McLean, VA 22101, USA b SES Group & Associates LLC, Turner-Fairbank Highway Research Center, 6300 Georgetown Pike, McLean, VA 22101, USA highlights  UHPC overlays have low porosity at the UHPC-concrete interface.  UHPC overlays are impacted by the substrate condition and overlay consolidation  UHPC overlays bond well to hydrodemolished and diamond ground concrete surfaces. article info Article history: Received 6 July 2018 Received in revised form 20 September 2018 Accepted 24 September 2018 Keywords: Ultra-high performance concrete Bridge overlay Delamination Bond Scarification Hydrodemolition Interface microstructural analysis abstract This paper aims to characterize the bond behavior and performance of UHPC-class materials as overlays on bridge deck concrete substrates. The study includes laboratory testing, field testing, and microstruc- tural analysis using scanning electron microscopy (SEM). The laboratory portion examines the direct ten- sion bond strength between UHPC and substrate concrete with different substrate surface preparations; namely, hydrodemolition and scarification. The field testing examines the tensile bond strength of the first UHPC bridge deck overlay in the United States. Lastly, microstructural analysis is used to investigate consolidation of the overlays, the substrate surface condition, and the hydration products and porosity at the interface between the overlay and substrate. The paper discusses correlation between bond strength and consolidation, mechanical interlock, chemical bond, and substrate surface condition and preparation. Published by Elsevier Ltd. 1. Introduction 1.1. Background Maintenance and rehabilitation of highway bridge decks is a continual challenge for bridge owners. Common repair and reha- bilitation measures include: sealing of existing cracks and areas of spalled concrete; water-proofing; providing additional protec- tion to corroded or damaged reinforcement; providing a smoother riding surface for increased user comfort; replacing deteriorated cover concrete; and structural strengthening. Traditionally, con- ventional cement- or asphalt-based overlays have been economical and constructible options to achieve these objectives. However, many transportation agencies recognize a limited service life extension using traditional rehabilitation methods and materials as noted in the report by Krauss et al. [1]. Thus, there is a need for resilient and durable overlay solutions for aging reinforced con- crete bridge decks. Ultra-high performance concrete (UHPC) is emerging as an innovative solution to a variety of bridge design, construction and rehabilitation problems. Some highway bridge applications of UHPC have included 100% UHPC structural elements such as prestressed girders and piles [2–4], structural patching and bridge element repair [5], jackets for columns [6], and field-cast connec- tions between prefabricated bridge elements [7–11]. One emerging application of UHPC in the highway bridge sector is thin, bonded overlays for bridge deck rehabilitation. As an overlay, UHPC can provide structural strengthening [12], protection from chloride penetration [13], and protection from water ingression [13,14]. These objectives can be achieved with a 25-mm to 51-mm thick layer of UHPC, which minimizes both the material volume and the additional dead load on the bridge structure. Furthermore, UHPC has also been shown to have good resistance to abrasion. https://doi.org/10.1016/j.conbuildmat.2018.09.167 0950-0618/Published by Elsevier Ltd. ⇑ Corresponding author. E-mail address: zachary.haber@dot.gov (Z.B. Haber). Construction and Building Materials 190 (2018) 1056–1068 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat