Ahmad Abu-Hawash and Norman McDonald, Iowa Department of Transportation
In the September/October 2003 issue of HPC Bridge Views, the Iowa Department of Transportation (DOT) discussed its strategy and performance requirements for using high performance concrete (HPC) on one of the biggest reconstruction projects in Iowa. With over 70 bridges, the reconstruction of I-235, an urban interstate through Des Moines, gave the Iowa DOT the opportunity to introduce innovative materials, such as HPC and self-consolidating concrete (SCC)—both newcomers at the time to Iowa.
HPC Development
Although HPC has been used widely in the United States since early 2000, implementing the use of HPC mixes in Des Moines was not a simple task. A group of Iowa DOT engineers from the disciplines of design, materials, and construction, along with staff from the Federal Highway Administration, collaborated on developing mix designs and construction specifications that were suitable for central Iowa. The HPC mixes had to utilize locally available aggregates and meet new design requirements in terms of strength and permeability (see HPC Bridge Views, Issue No. 29, September/October 2003).
Many challenges were encountered along the way including a lack of local supplier experience in producing high performance mixes, implementation of a new aggressive Iowa DOT policy on curing concrete within minutes of casting, and dealing with harsh winter temperatures.
Cooperation among Iowa DOT staff, contractors, and material suppliers helped overcome these challenges. The end result was improved structural concrete in terms of higher strength and lower permeability, with significant reduction in shrinkage cracking in bridge decks.
The I-235 experience has allowed the Iowa DOT to expand usage of HPC to other areas of the state. Design specifications for two new Mississippi River crossings in Dubuque and Bettendorf require the use of HPC for the bridge decks and substructure components, and precast, prestressed concrete beams on the approach spans.
HPC is also currently being used on the Council Bluffs Interstate System improvement project, which includes a new Missouri River crossing. Although the Iowa DOT has not officially adopted HPC for statewide use, many requirements associated with it are being added to the traditional mixes. This can be attributed to the successes achieved on the I-235 project. Furthermore, some changes to Iowa’s construction specifications are being introduced to take advantage of the proven practices, such as improved concrete curing.
Iowa HPC mixes have specified target values for both compressive strength and permeability, which are defined in the Iowa DOT’s “Special, Developmental or Supplemental Specifications.” The minimum 28-day compressive strength for cast-in-place concrete is set at 5000 psi (34 MPa), while rapid chloride permeability values for the deck and substructure concretes are 1500 and 2500 coulombs, respectively. Contractors have not had any problems meeting these target values.
Although the specifications give the contractors the flexibility to design their own HPC mixes, they generally choose Iowa DOT mixes proven to produce the desired characteristics.
New Family of HPC Beams
To introduce a new family of precast, prestressed HPC beams, the Iowa DOT collaborated with the prestressed concrete industry in the state. The new set of bulb-tee beams provides a competitive alternate for medium-span bridges, which are typically constructed using steel girders. These beams have proven to be a perfect fit for many two-span bridges; multiple-span, urban viaducts; and approach spans of major river crossings.
Attributes of the new beams include compliance with the AASHTO Load and Resistance Factor Design (LRFD) Specifications, permeability less than 2500 coulombs, compressive strengths up to 9000 psi (62 MPa), efficient design with span lengths up to 155 ft (47 m), beam spacings up to 9 ft 3 in. (2.8 m), and an aesthetically pleasing shape.
The new bulb-tee beam family was initially limited to two unique beam sections, designated the BTC and BTD beams, to meet the immediate I-235 project needs. It was then expanded to include shorter and longer span lengths (BTB and BTE beams). The expanded beam family now has overall depths that range from 36 to 63 in. (915 to 1600 to mm) and features the use of 0.6-in. (15-mm) diameter strands at 2 in. (50 mm) centers.
Other Applications
In addition to the medium-span HPC beams, the Iowa DOT introduced an alternative to the state’s traditional low-slump concrete deck overlay mix. The new HPC deck overlay mix, unlike the low-slump overlay mix, does not require specialized mixing and casting equipment because it can be cast using standard deck construction techniques and equipment, thus reducing cost. The biggest advantage of using HPC in deck overlays is the elimination of nuclear density testing, which involved security issues. For interstate and primary projects, prewetted burlap must be placed within 10 minutes of finishing.
Another application of HPC in Iowa has been for mass concrete, where the use of slag proved to be beneficial in controlling the heat of hydration and reducing the potential for shrinkage cracking.
Self-consolidating concrete (SCC) has proved to be an attractive use of HPC for precast applications and for casting aesthetic features and components with a complex geometry. Currently, SCC is being used for deck panels, noise walls, mechanically stabilized earth (MSE) walls, and in some precast, prestressed concrete beams.
Iowa continues to advance the use of HPC in structural applications and is leading the way in introducing the use of ultra-high performance concrete (UHPC) in precast, prestressed concrete beams for county bridges.
Further Information
For further information, contact the lead author at [email protected].