Good morning, everyone!
Today I would like to speak about the progress in the use of UHPC in the US, focused mainly in the most dynamic application in the last decade: The use in bridges to increase their durability and minimize their maintenance costs.
This type of application was generalized previously in Switzerland with the same goal, increasing the durability of the structures against freeze-thaw cycles and melting salts (in  are described more than 250 applications done in the country since 2003). In the rest of Europe the use of UHPC is still very focused on aesthetic applications with few exceptions, as the light and durable balconies from the Danish company Hi-Con or the floating structures used in aquaculture and floating solar energy of the Spanish precaster PREFFOR.
The history of UHPC in US bridges starts in 2001, when the Federal Highway Administration (FHWA) started a research and innovation program under their initiative The bridge of the future to understand what the optimum applications of UHPC in bridges were. The largest compilation of results of this research can be found in the document “Ultra-High Performance Concrete: A State-of-the-Art Report for the Bridge Community” (Publication nº FHWA-HRT-13-060, June 2013).
Currently, there are many researches being carried out, and the FHWA promotes the use of UHPC and highlights its advantages, as can be seen in this video elaborated by the FHWA to explain its potential in infrastructures. This link of the administration shows an interactive map with the bridges where UHPC has been used. The colors indicated the year of commissioning (it is updated until 2020).
Until 2020, UHPC has been used in approximately 350 structures. Currently, this number is experiencing an exponential growth, and a significant part are UHPC overlays. This is one of the four types of applications that has been done in the US bridges, and which are described in the following lines:
1. Connections between prefabricated bridge elements
The UHPC application that has extended more (possibly because it is less complex than the rest) is the execution of connections between prefabricated UHPC elements. The advantage of using UHPC is that UHPC provides very high fatigue strength, very good abrasion and impact strength, high corrosion strength, and allows having shorter anchoring lengths due to the high bonding strength of the UHPC matrix. The technical note “Design and Construction of field-cast UHPC Connections” (FHWA Publication No: FHWA-HRT-14-084) explains the application in detail, with examples of their use in several bridges.
2. UHPC structural elements (precast girders, piles, etc.)
In the US, there have been only few cases where the whole structural element is made of UHPC. They have concentrated in Iowa, Florida, and Virginia (some of them can be seen in the bottom figures, extracted from https://highways.dot.gov). Three may be the causes for which this application has not progressed significantly: 1) The option of repairing with UHPC (overlay, etc.), when viable, requires much lower investment. 2) There is not yet a national UHPC code in the US, which implies that the engineer is more limited to design a complete structure, 3) As the technology has not extended widely in the country, there is not a learning curve that allows engineers to easily find the optimum designs, so it is more difficult that the UHPC solutions can compete.
UHPC bridge deck overlays
This is the application, which is currently experiencing a higher growth, despite that until 2020 it was used in no more than 20 bridges. It consists in the rehabilitation of bridges through a thin UHPC layer (overlay) over the existing deck, so that the renovated structure is durable and has minimum maintenance costs. These layers have a thickness between 25 and 50 mm, and it is the same type of solution that has been used in Switzerland since 2003 to minimize the maintenance of structures in very cold regions.
Technical note “Ultra-High Performance Concrete for Bridge Deck Overlays” (FHWA Publication No: FHWA-HRT-17-097) explains the application in detail. Bottom figures show different stages of the execution process carried out in Iowa and Delaware between 2016 and 2017 respectively.
Preservation and repair applications (excluding overlays)
In some bridges UHPC has been used for localized reparations. This video from the FHWA briefly summarizes some of the reparations that can be faced with UHPC in bridges. These proposals are implemented because the investment is much more reduced than carrying out a complete replacement of the infrastructure.
Some of the most common reparations implemented in the US have been in the edges of beams, expansion joints, construction joints, reparation of piles/columns or their jacketing with UHPC, and substitution of bridge joints by localized UHPC slabs.
State of the codes
Despite of the noticeable progress of the use of UHPC in the US in the last years, there is not yet a national code. Currently, transport infrastructures as bridges are designed according to the AASHTO LRFD Bridge Design Specification (AASHTO: American Association of State Highway and Transportation Officials), while buildings are designed according to the ACI 318. While ACI has just started to consider the development of a UHPC design code, AASHTO has progressed significantly in this direction. The working group of the FHWA (this link provides a list with its publications), leaded by Dr. Benjamin Graybeal, has elaborated a draft of a structural design guide with UHPC, which is being evaluated by the AASHTO for its potential implementation as design guide at the end of 2023.
Besides, with the support of the FHWA, the ASTM (American Society for Testing and Material) published a new standard for the fabrication and testing of UHPC elements (ASTM C1856/C1856M, 2017), covering fiber reinforced cementitious materials with at least 120 MPa and aggregate size below 5 mm.
Moreover, in 2012 the American Concrete Institute (ACI) started to work on the Committee ACI 239-UHPC to define this material. On 2018 the Institute published document ACI-239R-18 “Ultra-High Performance Concrete: An Emerging Technology Report”, with information and recommendations to promote its use in North America.
UHPC Market trends in the USA
Considering the significant increase of companies dedicated to the installation of UHPC overlays on bridges, the market is understanding the excellent relation between their cost and the value obtained. This is favoring the market growth and the democratization of the UHPC technology. There are three main reasons that explain this progress:
- Time has passed from the first application (2006 in the US), and this is the real proof of durability of the UHPC solutions. As happened in Spain with the floating UHPC farms, the evidence of real UHPC structures is providing progressively to the market the trust that was required.
- FHWA is spreading and informing about the UHPC benefits and the economic savings that it can generate at medium and long term.
- The recent increase of the steel costs invites finding in UHPC a more economic and durable alternative.
However, the construction of prefabricated UHPC structures for bridges is still in a more embryonic stage. The reason is not that they are not competitive at medium/long term (or even since the first day), but that local companies do not have yet enough experience in these types of elements, so it is difficult to meet an optimized solution that competes with the alternatives, which have a very mature industrial production.
With a significant Support from the FHWA (Federal Highway Administration), the use of UHPC solutions in road and railway bridges in the US is increasing, particularly in the case of UHPC overlays over existing decks and in the case of connections between UHPC elements. In a lesser extent, precast UHPC bridges have been installed (mainly in Iowa), but these structures require the progressive acquirement of expertise by the engineers and a national guide, which is expected for the end of 2023.
The growth in number of applications shows that knowing UHPC technology will be a must in the future in this sector in the US, as the trend is to minimize the maintenance costs of the structures. In the majority of cases UHPC will not be the most used material, but it will be strategically used to add a lot of value with limited investments, as successfully done in Switzerland since 2003.
 Bertola, Numa & Schiltz, Philippe & Denarié, Emmanuel & Brühwiler, Eugen. (2021). A Review of the Use of UHPFRC in Bridge Rehabilitation and New Construction in Switzerland. Frontiers in Built Environment. 7. 769686. 10.3389/fbuil.2021.769686.