Field Testing of an Ultra-High Performance Concrete Overlay

CHAPTER 6. SUMMARY AND CONCLUSIONS

The first U.S. deployment of UHPC as a bridge deck overlay was completed in May 2016 on a reinforced concrete slab bridge located in Brandon, IA, in Buchanan County. A few months after installing the UHPC overlay, a field inspection of the bridge concluded that there could be some potential locations of deck delamination. However, it was not known whether delamination, if actually present, occurred at the interface between the UHPC overlay and substrate concrete, within the existing concrete deck, or within the UHPC overlay itself. The bond between the UHPC overlay and substrate concrete needed assessment. In November 2016, researchers from FHWA’s TFHRC conducted a field study to evaluate the bond between the UHPC overlay and the substrate concrete bridge deck.

Based on observations and data collected, it can be concluded that the bond between the UHPC overlay and the existing concrete bridge deck was intact. Mechanical testing verified that the locations suspected of having good UHPC-concrete bond were able to carry relatively high tensile stresses without bond failure. It can also be concluded that good bond was achieved even at locations where deck concrete was not roughened (scarified) prior to placement of the UHPC overlay; however, this was dependent on the surface quality of the substrate concrete and is not a recommended practice. The two test locations suspected of delamination were indeed found to have delaminated concrete. However, delamination was pre-existing within the deck concrete and was likely present prior to placement the of the UHPC overlay.

In all cases, visual inspection of the UHPC-concrete interface indicated that the interface between the UHPC overlay and deck concrete appeared intact. This was further investigated through microstructural analysis using a scanning electron microscope. Microstructural analysis revealed a high density of UHPC in direct contact with the concrete substrate surface in all cases studied. This high density was caused by the high content of hydration products from the UHPC and low porosity adjacent to the interface. The consequence of this was a high degree of direct contact between the UHPC and the concrete surface, which translated into high tensile strength of the interface.

Microstructural analysis also revealed an absence of steel fibers near the UHPC-concrete interface in all cases studied. That is, no steel fibers were observed on the UHPC paste band in direct contact with the concrete surface. Thus, fibers did not contribute directly to adhesion of the paste to the surface of concrete.

Comparison of phase distribution evolution at the interface revealed no significant difference in the trends of porosity, aggregate, and hydrated/unhydrated particles at the studied micro-scale between scarified and nonscarified concrete surface. However, two small differences were observed between scarified and nonscarified concrete: (1) features disrupting the contact between UHPC and concrete such as air void and accumulated debris were found on the interface of the two specimens where the concrete was scarified; and (2) the extent of the wall effect was higher in those UHPC overlays placed on top of a scarified concrete surface in comparison to those placed over a nonscarified surface. Neither of these observed differences had any effect on the tensile strength of the interface bond.