Tarif Jaber, Jaber Engineering Consulting, Inc. and Ahmad Ardani, Colorado Department of Transportation
In 2000, the Colorado Department of Transportation (CDOT) received a $700,000 award under the Innovative Bridge Research and Construction (IBRC) program to investigate new, innovative materials in the reconstruction of the I-225 and Parker Road interchange southeast of Denver. The design of the bridge deck included the development of high performance concrete (HPC) mixes, plus the use of partial depth, precast, prestressed concrete deck panels with fiber reinforced polymer (FRP) reinforcement.
Parker Road Bridge
The bridge at the I-225 interchange consists of post-tensioned, cast-in-place, reinforced concrete box girders with 5-in. (12.7-mm) thick, precast, prestressed concrete deck panels and a cast-in-place slab for a total deck thickness of 8 in. (203 mm). Under the IBRC program, part of the bridge deck was constructed using the HPC mix and deck panels with FRP reinforcement.
To validate the design, several studies were undertaken at the University of Colorado at Boulder. The studies included the development of HPC mixes; evaluation of the mechanical properties of FRP reinforcement under static and cyclic fatigue loads, after environmental preconditioning; evaluation of the load carrying capacities of full scale, precast, prestressed concrete deck panels with FRP reinforcement; and evaluation of long-term fatigue endurance of a model bridge deck simulating the Parker Road bridge.
HPC Mixes
CDOT experimented with several HPC mixes. The objective was to develop a mix that improved durability by reducing cracks from shrinkage and reducing permeability to deicing chemicals. At the same time, the mix had to meet criteria for strength and workability. This was achieved by reducing the cementitious materials content in an effort to produce a concrete with a lower modulus of elasticity and higher creep at early ages.
Seven bridge deck mixes with lower cement content, silica fume, and Class C r F fly ash were evaluated and tested for compressive strength, rapid chloride permeability, crack resistance, and drying shrinkage. As a result of the study, two mixes were adopted by CDOT — Class H for full depth concrete decks and Class HT for overlays. These are now being used in many CDOT projects. Class H was used on the Parker Road bridge.
The two selected HPC mixes have a low early strength and low heat of hydration. These characteristics allow the concrete to better accommodate volume changes and temperature variations and make it more resistant to shrinkage cracking. They also have low chloride permeability values at 56 days. Details of the study are presented in CDOT Report No. CDOT-DTD-R-2003-13 available from the National Technical Information Service.
FRP Reinforcement
In addition to the development of new and improved HPC mixes, the project investigated the use of carbon FRP (CFRP) and glass FRP (GFRP) bars as reinforcement in the precast concrete deck panels. Pretensioned CFRP bars were used in place of the conventional steel strands and GFRP bars were used instead of nonprestressed steel reinforcement. The objective was to reduce both construction and life cycle costs by reducing corrosion problems experienced with steel reinforcement. Extensive tests were performed to validate how exposure to severe environments affected the durability of the FRP reinforcement. Both CFRP and GFRP bars were subjected to freeze-thaw cycles. Loading tests were then performed to predict the effect on the tensile strength of the reinforcing bars and to establish a basis for cyclic fatigue testing procedures.
Both the tests and actual performance to date on the I-225 bridge deck indicate that the corrosion resistance, light weight, and superior tensile strength of the FRP reinforcement will prove beneficial in extending service life and lowering life cycle costs.
The Future
The project and related studies have demonstrated that new construction materials can make a major difference in the cost and effort of maintaining bridge decks and roads in severe climates. CDOT will continue to measure the benefits it receives from the combination of creative thinking and solid engineering that results from projects like this.