Mary Lou Ralls, Texas Department of Transportation
Louetta Road Overpass near Houston, Texas, is one of the first projects in the Federal Highway Administration ( F H WA) national high performance concrete (HPC) bridge implementation program. The overpass, which consists of two parallel bridges, utilizes precast, prestressed and cast-in-place HPC in both the superstructure and substructure.
The superstructure consists of simple span pretensioned trapezoidal-shaped 54- in. (1.37-m) deep U-beams with design compressive strengths as high as 13,100 psi (90.3 MPa). This concrete strength allowed a maximum span length of 135.5 ft (41.3 m) and a maximum girder spacing of 15.8 ft (4.82 m).
The superstructure also includes precast, pretensioned concrete deck panels supported on the U-beams’ top flanges with a cast-in-place composite concrete topping. The overpass was selected as a 1 9 9 8 Precast/Prestressed Concrete Institute (PCI) Bridge Design Award winner for spans greater than 135 ft (41.1 m).
Construction of the Louetta Road Overpass provides a valuable learning experience for future bridges. Several of the lessons from the project are described below.
Optimum Concrete Compressive Strength for Beams
The use of high strength concrete allows for longer spans. However, the longer spans and the U-beam’s large cross section necessitate the use of hauling systems and erection cranes with larger capacities than typically used in Texas bridge construction. In addition, routing to the jobsite and site access are areas of concern for beams longer than about 120 ft (37 m). In view of these constraints, beam compressive strengths in the 10,000 to 13,000 psi (69 to 90 MPa) range appear to be a practical upper limit for design optimization.
Use of 0.6-in. (15.2-mm) Diameter PrestressingStrand
The majority of the Louetta Road beams use 0.6-in. (15.2-mm) diameter strands at 1.97-in. (50-mm) spacing. The 0.6-in. diameter strand is more efficient than 0.5-in. (12.7-mm) diameter strand because 40 percent more force can be provided in each strand. For the Texas designs, 0.6-in. diameter strand was required to take full advantage of concrete strengths in excess of about 10,000 psi (69 MPa).
At many beam fabrication plants, pretensioning beds are designed for 0.5-in. (12.7-mm) diameter strands. These beds may not have adequate capacity for the number of 0.6-in. (15.2-mm) diameter strands needed to utilize higher concrete compressive strengths. Modifications to existing beds or construction of new beds may be required prior to fabrication of high strength HPC beams. It is, therefore, important for designers to work with local producers concerning bed capacities. If bed capacity is not available, a combination of pretensioning and posttensioning provides another solution.
Concrete Compressive Strength for HPC Decks
Typical bridge decks in Texas use 5000 psi (34 MPa) compressive strength concrete in pretensioned concrete panels and a 4000 psi (28 MPa) cast-in-place composite topping. The Louetta Road overpass has a similar deck system, except that the precast panels use 8000 psi (55 MPa) design strength and the cast-in-place topping is 8000 psi (55 MPa) on the southbound bridge and 4000 psi (28 MPa) on the northbound bridge.
Little benefit is gained from using the higher compressive strengths in the deck. In Texas, design compressive strengths of 5000 psi (34 MPa) and higher for cast-inplace bridge decks require a significant change from typical practice. In order for the contractor to be assured of getting the higher strengths, a lower watercementitious materials ratio is used. In addition, a high-range water-reducer may be required to facilitate placing, consolidating, and finishing. The resulting mix behaves differently from typical mixes and can make construction more difficult. These low-bleedwater mixes are also more susceptible to plastic shrinkage cracking which can occur prior to the application of interim curing compound and wet cotton mats. Consequently, Texas is currently specifying 4000 psi (28 MPa) HPC for decks and substructures. In practice, the actual 28-day concrete compressive strengths may be considerably higher due to the use of pozzolans to achieve improved durability. The current durability specification for decks and substructures requires a permeability less than 2000 coulombs at 28 days by AASHTO T277 and 5 to 8 percent total air content.
Further Information
Further information about the Louetta Road Overpass is available in Proceedings of the PCI/FHWA International Symposium on High Performance Concrete (1997) available from PCI or by contacting the author at 512-465-7963 or
[email protected].