Advancing bridge performance with UHPC

by Elaina Adams | January 1, 2013 12:46 pm

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By Raymond Krisciunas, P.Eng., Peter J. Seibert, M.Sc., MBA, P.Eng., and Philip D. Murray, M.Eng., P.Eng.
Over the past several years, ultra-high-performance concrete (UHPC) field-cast joint connections have become a common bridge solution in Ontario and gained acceptance in various states, such as New York, Iowa, Montana, and Oregon. However, the Ministry of Transportation Ontario (MTO) recently enabled this material to be used in new ways, relying on its combination of superior properties—including improved strength, durability, and bond development—to overcome significant bridge design obstacles. (For past Construction Canada articles on UHPC, see “Whitemans Creek Bridge” by Vic H. Perry, Wade F. Young, and Brent I. Archibald (July 2012), “An Ultra-high-performance Upgrade” by Gaston Doiron and Kelly A. Henry (December 2011), “Precast Solution for Performance Cladding: Ultra-high-performance Concrete for B.C. Building” by Don Zakariasen and Peter Seibert [September 2010], and “Restoring the Rialto: Specialized Precast Cladding Revitalizes an Old Hotel,” by Perry and Lisa M. Birnie [Sept. 2009]. Visit www.constructioncanada.net[2] and select “Archives.”)

Currently, MTO is embarking on an aggressive four-laning project in Northwestern Ontario that involves construction of about 30 km (19 mi) of highway and up to 30 structures in an area with extremely rugged terrain, ranging from deep swamps to massive rock cuts.

As part of this project, a new ‘parclo’ (partial cloverleaf) interchange was required at the intersection of Highway 11/17 and Hodder Avenue in Thunder Bay. The structure, founded on a combination of hard till and bedrock, spans over a total of six lanes. (The authors would like to thank the Hodder Avenue Underpass project team, which includes the Ministry of Transportation Ontario and Hatch Mott MacDonald, for their collaborative partnership throughout the design and construction of this project).

The need for a striking design
Since the interchange is the first structure drivers encounter when approaching the city of Thunder Bay, it was desirable to elevate the appearance from the somewhat utilitarian style frequently encountered in urban highway settings to a clean, slender, and open design. Given the location and exposure, it was also desirable to use precast elements throughout. Consequently, a two-span concrete box girder configuration was the most practical and cost-effective solution—however, these structures are typically rather bland and can appear bulky.

To counter this challenge, a design concept was developed whereby the pier cap beam could be incorporated into the superstructure, appearing to be integral with the box girders while providing a frame that seems to go directly into the superstructure. This appearance is achievable with the use of conventional post-tensioned cast-in-place superstructures, but extremely difficult to achieve using pre-fabricated elements.

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Other challenges included a relatively shallow superstructure in which to incorporate the main pier cap/cross beam and analytical complexity of two-way behaviour by virtue of the continuous box girders intersecting the transverse cross beam. In the final configuration, the pier cap/cross beam had to become integral with the box girders in the longitudinal direction and provide a continuous span between the piers and cantilever over the ends (Figure 1).

The initial design, using 60-MPa (8700-psi) concrete, necessitated no fewer than four pier columns to support the superstructure. Visualizations using this configuration demonstrated the structure still appeared bulky because the superstructure and pier cap depths could not be increased without dramatically affecting the rest of the bridge. An alternative was required.

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Finding success
The main pier cap was then re-designed using prestressed UHPC with a compressive strength of up to 200 MPa (29,000 psi)—this resulted in one less pier column (three, in total) and a more open appearance (Figure 2). This was the first time precast UHPC was used in this type of application in North America. The pier cap was actually an inverted T-shape that provided ledges on which the box girders could sit. It spanned continuously over the three pier columns, and was also cantilevered at the ends. For this reason, considerable prestressing was required in the member’s top and bottom.

Due to the amount of prestressed strand and other perforations and embedments, the precaster used building information modelling (BIM) to identify conflicts between the beam and box girders. It was then determined once the T-beam was released, it would significantly deflect under the prestress effects and would not sit flush on the seats at the tops of pier columns. This problem was overcome by simply anchoring and jacking to straighten the member and then filling gaps with high-performance epoxy grout.

Casting the UHPC T-section was relatively simple, using a steel form in the exact shape of the T-beam section as it was installed in the field. The UHPC material was placed from the top of the stem and gradually flowed to the top of the T-section (Figure 3). No vibration was applied.

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UHPC was also used to produce the shell pier columns—members that would be exposed to winter de-icing salt spray, which has been known to cause considerable damage to similar reinforced concrete elements on other structures. Since UHPC is extremely low in porosity and essentially immune to water penetration, it was believed its incorporation into the pier columns would result in far more durable elements. A customized UHPC shell was then designed to act as a non-structural form which could be filled with reinforced concrete. In addition to durability, the UHPC shell pier columns were shaped with flared tops for esthetic appeal (Figure 4).

In 2010, a similar application at another Northwestern Ontario structure over the Wabigoon River near Vermilion Bay (shown on this magazine’s cover), was highly successful. For this bridge structure, the much shorter and circular UHPC shell pier column was rather simple.

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UHPC pier column shells
The 8.35 m (27 ft, 4¾ in.) height and complex shape of the precast UHPC pier column shells proved to be challenging. The columns were cast on an angle, using an inner and outer steel formwork. The inner steel mould was self-collapsing to allow for initial shrinkage of the UHPC while the precast element was hardening. The transition of the octagon shape (from the constant cross-section to the flared cross-section) created challenges for:
• form manufacturing;
• casting of the UHPC;
• initial shrinkage allowances; and
• stripping of the UHPC precast element.

Due to the column’s height, the form was positioned on an approximate 15-degree angle; the material was placed slowly from the top of the column through an inlet funnel. When casting a long, thin and slender UHPC element, the pouring process has to be carefully monitored to ensure proper fibre dispersion throughout the plastic concrete.

Preferential fibre orientations occur due to the plastic material’s flow characteristics, which can be used as an advantage in areas where tensile forces in the finished precast element are expected. When UHPC is placed too fluid, the steel fibres of the plastic material can settle out, resulting in a potential weak plane in the hardened and finished precast element. Therefore, proper flow characteristics and placement methods of the UHPC must be carefully developed and controlled during the manufacturing process. (For more, see “Equipment and Production Techniques with UHPC,” by Vic H. Perry and Peter J. Seibert, in the April 2011 issue of CPi Magazine, published by Concrete Plants International).

The local geometry of the UHPC shell pier column form meant several casting challenges for the precaster. The tight radius corners at the intersection of the octagonal planes created local flow resistance and subsequent irregular fibre dispersion. (When casting thin, tall, slender, and complex precast elements, a prototype cast should be first completed to validate the form design and casting techniques). As mentioned, the height of the form made it necessary to cast the element at an angle because of height clearance within the precast plant. This created additional fibre dispersion issues at the flow planes where the UHPC met from opposite sides of the form, adjacent to the fill inlet funnel.

Since the shell pier form was intended as a non-structural element, minor fibre irregularities would not be a problem; however, if the internal form release is not completed at the proper time during the setting, the UHPC may crack during early setting, due to restrained shrinkage, low early strength, and irregular fibre dispersion.

To erect the UHPC shell pier columns, the original forms were shipped to the site. These were wrapped around the elements before concrete infill placement. A robust steel support structure was necessary to support the UHPC shell pier columns and T-beam until the concrete box girders had been placed and the entire superstructure was made composite with the pier cap.

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Field-cast UHPC connections
The ‘topless’ box girder superstructure has become fairly common in Northwestern Ontario. In this design, precast concrete box girders (Figure 5) are joined together using UHPC field-cast joints acting in pure shear. No topping slab is placed over top of the structure and asphalt is placed directly on the box girders. This is made possible through a combination of high performance materials such as glass-fibre-reinforced polymer (GFRP) reinforcing and UHPC field-cast joints, in combination with carefully designed details, to result in extremely durable connections between the elements (Figure 6).

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This Thunder Bay bridge is one of the best UHPC applications whereby precast elements, such as box girders or deck panels, are connected using a field-cast UHPC solution. Historically, precast elements are preferred for bridges due to their design efficiency, fast installation, and low initial costs. However, the standard grouting material in the joints is often the first part of the bridge to fail due to continuous highway impacts that cause the joints to leak.

The use of transverse post-tensioning (P/T) or a concrete topping slab across the joints ensures adequate shear transfer between girders, such that the deck effectively remains structurally monolithic under the constant pounding of truck wheel loads and seasonal conditions. While these construction techniques can resolve the majority of the performance issues, they are not without potential problems. For example, they:
• are expensive;
• add manufacturing complexity for the precaster;
• do not allow for installation flexibility in the field;
• have potential for corrosion; and
• can be challenging for bridges with varying cross-fall.

Further, the analysis is usually complex when determining the correct post-tensioning forces and corresponding creep losses. Potential long-term corrosion of the strands could also affect the durability of the structure.

In lieu of the concrete topping slab or transverse post-tensioning, UHPC is increasingly specified in North American bridge projects for cast-in-field connections between precast elements such as:
• box-girders;
• bulb-tee girders;
• reinforced bridge deck panels;
• curb connections;
• negative moment connections over piers; and
• bridge expansion joints.

Employing UHPC technology and eliminating P/T for precast elements enable the designers to simplify the precast panel fabrication and installation processes. This simplified design provides the owner with improved tolerances, reduced risk, increased speed of construction, overall construction cost savings, and a more durable, longer-lasting bridge deck solution.

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When field-casting UHPC, materials are either batched in dual onsite mixers or ready-mix concrete trucks. For the Thunder Bay project, all premix materials were shipped in 1116-kg (2460-lb) sacks and mixed in two 0.5-m3 (0.65-cy) high-shear onsite mixers (Figure 7). UHPC requires significant energy to mix and to lubricate all its raw material constituents (i.e. cement, silica sand, ground quartz, and silica fume) within the powder matrix. Higher energy completes the mixing process more quickly, resulting in a self-levelling UHPC.

After batching, the fluid UHPC was placed into all the joints using buggies and chutes. To control hydrostatic pressures throughout all connections, the joints were isolated to ensure complete filling. When placing the self-levelling UHPC material into joints, it is important to take advantage of its fluid characteristics. When discharged from the concrete bucket into the joints, UHPC spreads itself throughout (Figure 8).

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Moving the discharge point so it always stays behind the ‘leading edge’ of the flow, the joint can be filled in one continuous motion. Due to its self-levelling characteristics, UHPC was placed at the lowest end of the joint using a top form to contain the concrete and moving toward the high end of the joint where a ‘chimney’ was placed. (This chimney ensures proper hydrostatic pressure within the joint and prevents any low spots.)

To avoid any dehydration of the UHPC, all joints were immediately covered with resin-coated plywood (top form). After the initial hardened strength of the UHPC was reached, the joints could be ground flush to the precast elements if necessary.

To ensure proper quality of all field-cast UHPC material, the flow characteristics of each batch were tested using a modified version of ASTM C1437, Standard Test Method for Flow of Hydraulic Cement Mortar. This was completed immediately after mixing to ensure the consistency within all batches. Compressive strengths were also validated daily.

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Design of the longitudinal moment connection was extremely complicated as it entailed joining the box girders not only to each other, but also to the transverse UHPC T-beam supporting them (Figure 9). The design had to accommodate the precast elements into a detail that was typically cast-in-place. By using UHPC and prestressing the concrete, the T-header beam could be designed in precast and match the depth of the box girders.
To connect the precast elements and provide a monolithic structure, UHPC was used to fill the joint in the field. The production of these monolithic elements was a major contributor toward optimizing the bridge’s overall efficiency.

As the inverted T-beam was designed only for the loads as a composite member, shoring towers were constructed to take the dead load of the girders during erection and until the UPHC had been placed and cured in the field-cast joint (Figure 10).

Conclusion
The Hodder Avenue Underpass is a ‘leading-edge’ project and the first bridge structure in North America to incorporate an ultra-high-performance concrete pier cap, shell-pier column, and field-cast connections, together with precast HPC box girders, parapet walls, and approach slabs (Figure 11). The project demonstrates when UHPC is used for both precast elements and field-cast connections that the result can be durable and esthetically pleasing.

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By employing the material’s combination of superior properties, bridge know-how is advanced, construction speed is accelerated, and long-term durability improved, with additional benefits that include increased structural capacity, lowered maintenance, reduced joint size and complexity, improved deck continuity, extended life, and an elimination of post-tensioning. The result is a resilient bridge structure built to last.

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Raymond Krisciunas, P.Eng., is the head structural engineer for the Ministry of Transportation Ontario (MTO) Northwestern Region. He received his bachelor of engineering science degree from the University of Western Ontario in 1978, and joined MTO in 1985. Based in Thunder Bay, Ont., Krisciunas can be reached at ray.krisciunas@ontario.ca[14].

Peter J. Seibert is the technical director for Ductal, Lafarge’s ultra-high-performance concrete (UHPC). Based in Toronto, he is responsible for the manufacturing, supply chain, quality control, and technical aspects of Ductal in North America. Seibert obtained his bachelor of civil engineering from the University of Toronto in 1996, a master’s from Queen’s University in 1998, and an MBA from Wilfrid Laurier University in 2003. He can be contacted via e-mail at peter.seibert@lafarge-na.com[15].

Philip D. Murray, M.Eng., P.Eng., is a vice-president at Hatch Mott MacDonald Ltd. Based in Mississauga, Ont., he is the firm’s subdivision manager of provincial infrastructure (including their Thunder Bay office). He obtained his bachelor’s degree in civil engineering from Queen’s University in 1994, and a master’s in engineering from the University of Toronto in 2000. Murray can be reached at philip.murray@hatchmott.com[16].

Source URL: https://www.constructioncanada.net/advancing-bridge-performance-with-uhpc/

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