Bonding of overlays to ultra-high performance concrete
Date
2022
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Publisher
University of Delaware
Abstract
This thesis studies the bond performance of overlays to ultra-high performance concrete (UHPC) within the context of Delaware Department of Transportation (DelDOT) bridge applications. Polyester polymer concrete (PPC), latex modified concrete (LMC), and modified class D concrete (MCD) are applied as overlays on UHPC components to protect the bridge deck from deicing salts and provide a smooth riding surface. These overlays differ in composition, bonding agents used, mechanical and physical properties. The difference in properties between these materials results in distinct bond performance to the UHPC substrate. It is critical for an overlay to develop good bonding with the substrate to maximize overlay durability and minimize maintenance. ☐ UHPC is a cementitious composite material with optimized particle packing, internal steel fiber reinforcement, and a low water-to-cement ratio, resulting in superior mechanical properties. UHPC has high compressive strength (at least 22 ksi) and sustained post-cracking tensile strength (of at least 0.72 ksi). Compared to conventional and high-performance concretes, UHPC is nearly impermeable (owing to its discontinuous pore structure), resulting in considerably improved durability. Transportation agencies are using UHPC in bridges for link slabs, connections between precast components, and entirely UHPC structural members. To ensure adequate bonding of overlays to the UHPC substrate, a minimum pull-off bond strength of 250 psi and 200 psi is recommended for PPC and LMC overlays by AASHTO T-34 and ACI 548.4M-11, respectively. In addition, DelDOT specifies a minimum pull-off bond strength of 250 psi for PPC and MCD overlays; the DelDOT specification does not have an explicit bond strength requirement for LMC. ☐ Current literature (Haber et al., 2017) discusses the bond performance of overlays bonded to conventional concrete substrates. However, when it comes to UHPC substrate, the literature is limited to investigating LMC and UHPC as overlay while considering only scarification and hydrodemolition as surface preparation methods. The lack of knowledge regarding the bond performance of other overlays (PPC and MCD) on UHPC as well as the influence of additional surface preparation methods was addressed in this work. The study evaluated the effects of cording depth, overlay age and substrate preparation method, specifically, grinding and sandblasting (GSB), hydrodemolition (HD), surface retarder (SR), and control “non-prepared” (NP) surface on the bond strength. Furthermore, the efficiency of different roughness measurement methods in quantifying the roughness of prepared UHPC was investigated. The three roughness measuring techniques were the ICRI concrete surface profile (ICRI CSP) chips, sand patching, and surface profile gauge. ☐ The sensitivity study looking at the effects of coring depth on bond strength concluded that the coring depth of 0.5 in. was the most appropriate for pull-off bond tests on UHPC. Results indicated that surface prepared by GSB performs better than NP substrate surface. However, HD and SR increased surface tortuosity beyond GSB and exposed steel fibers, which further promoted mechanical interlocking across the UHPC-overlay interface. In addition, this study evaluated the effect of each overlay age (7, 14, 28, and 56 days) to show the gain of pull-off bond strength. For MCD overlays, the analysis of the substrate hygric state’s influence (dry versus saturated-surface-dry) on the bond performance showed no distinction due to UHPC being nearly impermeable. Finally, in terms of roughness measuring methods, the surface profile gauge (micrometer) was deemed to be more effective in quantifying the roughness of prepared UHPC surfaces compared to sandpacthing that was only applicable to NP and GSB surfaces. ICRI CSP chips were not used to qualitatively assess the texture of the UHPC substrate because they were created for normal concrete applications which has significantly different microstructure compared to UHPC.
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Keywords
Bridge deck, Latex modified concrete, Polyester polymer concrete