Abandon the usual

June 20, 2005

Bridges that are built faster, need less maintenance and last longer; these benefits and more can be realized with ultra-high-performance concrete (UHPC), the next generation of high-performance concrete.

Bridges that are built faster, need less maintenance and last longer; these benefits and more can be realized with ultra-high-performance concrete (UHPC), the next generation of high-performance concrete.

UHPC, which has been used in highway and pedestrian bridges in Europe, Asia and Australia, is now starting to gain attention in the U.S. For the past two years, the Federal Highway Administration’s (FHWA) Turner-Fairbank Highway Research Center in McLean, Va., has been evaluating the viability of this new technology and assisting states willing to consider UHPC for use in bridges.

UHPC is a steel-fiber-reinforced, reactive-powder concrete capable of compressive strengths of more than twice that of any concrete used to date in U.S. bridge construction. However, strength is not the only unique characteristic that makes UHPC an attractive material for transportation structures. The durability qualities of UHPC are just as impressive. Testing has shown that UHPC is nearly impermeable to chloride ion diffusion and carbonation penetration. UHPC is immune to freeze-thaw damage and abrasion, and once thermally treated or cured, this material exhibits negligible creep or shrinkage.

In a structural form, UHPC requires only a small fraction of the time required to fabricate conventional concrete or high-performance concrete. The steel fibers in the UHPC matrix eliminate the need for any mild reinforcing steel. That is, neither temperature and shrinkage reinforcement nor shear stirrups are fabricated into prestressed UHPC structural elements, so significant material, labor and time are eliminated from the UHPC fabrication process. After a one-day initial set the unique matrix of UHPC only requires a two-day curing regime before components can be placed in service. This is an order of magnitude better than the 28-56-day curing regime required for conventional concrete or high-performance concrete.

UHPC contains many of the same constituent materials as other concretes but in altered proportions. Table 1 provides the composition of the UHPC studied in this research. The manufacturer recommended a superplasticizer called Glenium 3000 NS for this mix, and the recommended accelerator was Rheocrete CNI, both products of Master Builders Inc. The 0.2-mm-diam. by 13-mm-long carbon-steel fibers were added at a concentration of 2% by volume.

This concrete contains a large amount of cement and cementitious materials, a large amount of superplasticizer and a very small amount of water. The water-to-cementitious materials (such as cement and silica fume) ratio is 0.16.

The curing of UHPC can have a significant effect on its final properties. The manufacturer-recommended curing is a steam curing (or thermal treatment) of 194°F at 95% relative humidity for 48 hours beginning soon after stripping the molds. Once the curing process has occurred, the material matrix is stabilized and final properties are achieved.

As a part of the program at Turner-Fairbank, a full suite of material characterization tests was conducted to provide a baseline understanding of the behaviors of UHPC. A brief summary of some of the results is presented in Table 2. The testing included multiple post-set curing regimes, but only results for the steam-cured specimens are shown in the table.

Bridge scene investigations

The strength properties of UHPC are quite impressive, including a compressive strength of 28 ksi and a cracking tensile strength of 1.8 ksi. The durability behaviors exhibited by this material are more subtle but possibly more important than the strength behaviors. When tested according to the standard chloride ion, scaling and freeze-thaw tests this concrete exhibits results that are on the extremely durable end of expected concrete behaviors. The rapid chloride ion penetrability results for this steel-fiber-reinforced concrete are so low as to be near the measurement limit of the testing apparatus.

The long-term stability of the concrete matrix also is quite impressive. After steam treatment, the concrete exhibits negligible shrinkage and a creep coefficient a order of magnitude lower than that of normal concrete. However, early-age behaviors are not as stable, and shrinkage from casting until the conclusion of steam curing can be up to 850 microstrain. This shrinkage can occur very quickly as the material is setting, and great care must be taken during the casting process so as not to restrain this shrinkage.

The characteristics of UHPC are particularly well suited to strength and durability challenges that are placed on highway bridges. Specifically, the compressive strength allows for high levels of prestressing, and the tensile strengths allow for reduction or elimination of secondary (in other words, shear, shrinkage and temperature) steel reinforcement. Also, the durability qualities ensure a long-lasting structure with little need for maintenance. The latter is especially important for bridge decks, which endure mechanical and environmental abuse and often must be replaced well before the design life of the entire bridge has been reached.

In order to take advantage of the host of superior material properties exhibited by UHPC, researchers at Turner-Fairbank worked with those at the Massachusetts Institute of Technology (MIT) to develop an optimized highway bridge girder. The MIT work resulted in the girder cross-section shown in Figure 1. This bulbed-double-tee prestressed girder, or ?-girder, is optimized for 70- to 100-ft spans. The prestressing arrangement shown in the figure is for a 70-ft span. This girder has a 3-in.-thick deck, a 33-in. overall depth and an 8-ft width. This bridge superstructure unit contains no mild steel, so all tensile forces are carried by the concrete matrix and the steel-fiber reinforcement.

The optimized bridge girder was designed in accordance with the AASHTO LRFD Bridge Design Specifications (2002). The design loadings included the HL-93 load configuration and an anticipated 25-lb/sq ft wearing surface. The girder is designed for no cracking at the service limit state (Service III) and for Strength I at its ultimate capacity. The formal design of the girder was completed through the use of 3-D finite-element modeling.

Intelligent design

The desire to rapidly construct UHPC bridges also guided the shape optimization done at MIT. A shape with an integrated deck was considered acceptable due to the manageable size, shape and weight of the resulting girder. This 70-ft-long, 8-ft-wide optimized girder only weighs 23 tons, thus allowing for easy transport to the bridge site. This girder also is easy to erect because of its light weight and inherent stability. The stable design of this girder allows for placement of any number of girders across the bridge width without the need for temporary bracing.

Another aspect in the design that allows for rapid construction is the connection mechanism between parallel girders. The tensile strength of the UHPC allows for a bolted connection to be used to join girders through the shear key formed by the ends of adjacent flanges. Once the bolts are in place, the bottom of this female-to-female key only needs to be filled with a foam backer rod before grout may be placed and the connection is completed. Depending on the machinery used for grout and grout placement, joining girders can be accomplished quite quickly.

Make it real

An optimized UHPC bridge of the design discussed above has been constructed at the Turner-Fairbank facility. The 16-ft-wide bridge was constructed using the rapid construction techniques previously mentioned. The erection of the two superstructure units was completed in less than one hour through the use of two 60-ton cranes. The bolted connections were subsequently completed. Bolting the single longitudinal joint between the two girders was completed within two man-hours and could easily be completed quite quickly by a multi-person crew. The grouting of the longitudinal shear key was then completed in approximately three hours. After setting of the grout, the bridge was structurally complete. This optimized structure will be studied over the next several months. Structural experiments will investigate the integrity of the longitudinal connections, transverse load distribution and overall behavior. Also, the stability of the material in the environment will be monitored.

In addition to the bridge, two other optimized UHPC ?-girders will be tested in the FHWA Structures Laboratory. They will be evaluated for flexural capacity, shear capacity and strength and fatigue life of the longitudinal connection. The UHPC studied in Turner-Fairbank’s research program has displayed an impressive set of material properties. When steam cured as recommended by the manufacturer, the user can expect to achieve a compressive strength of 28 ksi and a tensile cracking strength of at least 1 ksi. As compared to normal concretes, a steam-cured UHPC section should be effectively immune to freeze-thaw, scaling and chloride ion penetration damage.

As a result of this program, the states of Iowa and Virginia are currently working toward constructing the first public highway UHPC bridges in the U.S. The Iowa structure will be constructed in the summer of 2005 while the Virginia bridge construction will follow shortly thereafter. In both cases the bridge girders used will be of very similar design to standard prestressed concrete highway bridge girders. However, some attempts at economy will be made through the thinning of webs or truncating of compression flanges.

FHWA’s research into optimal uses of UHPC in bridges will continue focusing on achieving economy and efficiency by producing long-lasting, very durable bridges that can be fabricated and constructed quickly. Validation of the MIT-developed optimized cross section for a standard highway bridge girder will continue through the end of 2005.

About The Author: Hartmann is a research structural engineer at the Federal Highway Administration’s Turner-Fairbank Highway Research Center, McLean, Va. Graybeal is a research engineer at Turner-Fairbank.

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