Bridge Girders

Through collaboration among Iowa Department of Transportation (DOT), ISU, Federal Highway Administration, and Iowa Highway Research Board (IHRB), the State of Iowa has led implementation of two types of bridge girders. The design of the UHPC girder, which led to the first UHPC bridge in the U.S. in Wapello County, Iowa, This bridge girder used a slightly modified standard Iowa DOT section, but with an increased amount of prestressing, ultimately increasing the girder span. The original bulb tee C-beam is 1.14 m deep and uses 22, 15.2-mm strands to span 24.6 m. The UHPC girder was 1.07 m deep and designed with 49 strands to span 33.8 m. Increased girder span that allowed replacement of the original two-span bridge with a single span bridge. The combination of significantly higher prestressing than in a comparable normal concrete girder and relatively high tensile strength of UHPC also help eliminate the transverse reinforcement in the girder. This uniquely-shaped girder from the second generation, was first implemented at a bridge site in Iowa. This new shape brings new challenges due to the integrated deck, thin web and flange sections, and increased span length. But field tests confirmed that its span length can be increased, and that live load distribution factors can be reduced so that the pi-girders can be used more optimal than in this particular project. As with other normal concrete and HPC girders, the design of both girder types described above should satisfy the flexure and shear design, along with deflection and fatigue criteria.

Bridge Decks

The main advantage of using UHPC in bridge decks is that it prevents early deterioration of deck resulting from cracking that allows penetration of chloride especially during wintry months. Use of UHPC successfully in the entire bridge deck was accomplished by developing a full-depth precast UHPC waffle deck system as part of the Highways for LIFE program of the Federal Highway Administration (FHWA).

To make the UHPC waffle deck panels fully composite with the girders, three types of connections were used, namely;

  1. A pocket connection
  2. A longitudinal connection
  3. A transverse connection

Normal concrete deck having a broom finish surface and 3-mm or greater thick rough surface created by form liners were found to be adequate to produce satisfactory normal concrete-UHPC composite decks. As with the normal concrete structures, the fatigue characteristics can be defined using S-N curves, where S and N correspond to alternating stress and number of load cycles to failure, respectively. The mean stress will also play a part in the fatigue resistance of the UHPC member. Other equally important design calculations include selecting the interface roughness, punching shear, and anchorage and splicing of reinforcement within connections made from insitu UHPC.

Bridge Piles

With an objective of increasing the durability of the foundation, a tapered, H-shaped, precast, prestressed UHPC pile was developed at ISU as a means for increasing the longevity of bridge foundations.The full-scale vertical and lateral load tests on UHPC piles in the laboratory and field revealed several benefits of the UHPC pile including significantly reduced risk of damage during driving, drivability with a greater range of hammers and strokes, and the possibility of using existing equipment for pile handling and driving. To increase the application of this pile in the field, a welded splice connection that can be referred to as a “dry connection” was developed. Steel attachments used at the ends of UHPC piles requiring welding between two H-shaped steel plates facilitate the splicing of piles. Field experience has shown that UHPC piles can be driven in stiff soil without using any cushion on top of the pile.

Seismic Columns

To explore the use of UHPC in seismic regions for both bridges and buildings, three UHPC columns were designed and subjected to cyclic lateral load at ISU. Under compression loading, the UHPC reaches its ultimate strength almost in a linear fashion. The strain capacity corresponding to the ultimate compression strength is about 0.0032, which is only about 10 to 15% of the strain capacity of confined normal concrete that forms the basis for ductile seismic design adopted in current practice. Development of large compressive strains within normal concrete occurs in a nonlinear fashion, which is typically limited to the column ends known as the plastic hinge zones. With mild steel reinforcement in these regions also experiencing highly nonlinear strains, the formation of dependable plastic hinge zones enables the columns to undergo large lateral displacements, experience little or no reduction in the lateral force resistance, and dissipate a significant portion of seismic energy imparted to the structure through hysteretic action.

Wind Turbine Towers

The wind energy industry has extensively used steel tubular towers over the past several years for supporting utility scale wind turbines. The commonly used hub height today is 80 m as this height facilitates the steel tower to be transported in three segments.

The new tower concept—termed Hexcrete—comprised of six exterior post-tensioned columns along with panels that span the distance between two adjacent columns. The panels are primarily used as bracing elements between columns so that all six columns act as a composite system to resist the loads at the operational and extreme limit states. Both the columns and panels extend the entire height of the turbine and are segmented into sizes that allow for easy transportation. Once on site, the pieces may be erected using two possible construction sequences. The first consists of connecting the columns and panels together in multiple segments on the ground similar to the ones. These segments would then be stacked and fastened together by running vertical post-tensioning through the columns . At higher elevations, the number of strands required to provide the tower with sufficient load capacity is reduced. For this reason, two of the radial ducts within each column, shown in Figure 2, will be terminated at 33.5 m and 67.0 m in a 100-m tall Hexcrete tower. The center duct will extend from the foundation to the top of the tower. The second construction sequence requires each of the columns be erected to the first post-tensioning cut-off elevation. The columns are then secured by stressing each tendon, after which the panels are placed and connected.


With the introduction of the mixing UHPC in the field, use of this material in bridge connections has been growing at a rapid pace.The width of these connections is relatively small because the connections rely on anchoring the connection reinforcing bars within a short distance. In addition, the length of the connection could be reduced if, for example, the connection is between a girder and bridge deck where the connection reinforcement is extended into a pocket in the deck. The first example used UHPC connection between UHPC precast waffle deck panels and concrete girders, and used all three types of connections including the pocket connection. The second example is from New York where full depth precast concrete panels were connected using UHPC.