More than 15 years have passed since the Federal Highway Administration (FHWA) first published its state-of-the-art report on high-performance concrete (HPC). Since that time, many state departments of transportation (DOTs) have implemented HPC in their bridge decks and other transportation structures.
Because of their geographic location, climate and environmental exposure, some DOTs have well-developed programs for implementing HPC in their bridge decks while others are still in the early stages of such programs.
This article profiles the approaches that Arizona (ADOT), Colorado (CDOT) and Oregon (ODOT) are taking to develop competency in an emerging technology that will prolong the service life of bridge decks and roads across the western U.S.
Almost all HPC used on bridge decks in the U.S. includes silica fume as a mineral admixture. Silica fume improves concrete quality by reducing permeability and the rate of chloride penetration and increases concrete resistance to freeze/thaw damage. The use of silica fume allows a 4:1 reduction in total cement, which reduces paste content and the concrete’s potential for cracking.
Sky’s the limit
To confirm that HPC technology is applicable in its state, especially in areas with severe weather conditions, ADOT conducted a pilot project on the Sunshine Bridge on westbound I-40 between Holbrook and Flagstaff.
The project involved replacing the deteriorated concrete bridge deck that was built in 1968 by ADOT. It consists of a 7.5-in. concrete deck supported by a three-span, five-steel-girder system with a skew of 43°.
The pilot consisted of replacing the deteriorated deck with a cast-in-place concrete slab using HPC and low-corrosion reinforcing steel and measuring its performance.
The following factors played a significant role in the execution of the pilot:
- Tight schedule due to rail traffic volume increase in the fall;
- No feasible equipment access to the bridge deck from the railroad level; and
- Contractor needed at least 90 days to develop the HPC mix and make the necessary adjustments in construction schedules to meet project specifications; this was their first experience with HPC.
A special provisional specification was written with these performance criteria:
- Durability under freeze-thaw exposure;
- Lower permeability to salt penetration;
- Lower shrinkage potential; and
- Reduced steel corrosion.
Test results indicated that a concrete mix design with 0.41 water-to-cementitious materials ratio (w/cm) and 5% silica fume by weight of cement provided overall optimum performance against project requirements.
Concrete samples were cast at the batch plant and tested in the laboratory to measure hardened concrete properties of the HPC mix against the ADOT Class S control mix. The chloride permeability for the HPC was an average of 768 coulomb compared with 2,610 for the control mix. This represents a 70% increase in the concrete’s ability to resist chloride ion migration.
The HPC had a paste content of 23.5%, compared with 31.6% for the control mix. A lower paste content reduces the shrinkage potential. Air void systems for both mixes were sound and are expected to provide required durability under freeze-thaw conditions.
The concrete supplier made five trials batched at their Flagstaff plant, 40 miles west of the bridge location. None of the trial batches achieved the desired field properties for slump, air and w/cm ratio. It was clear that the variations in aggregate moisture and the below-SSD conditions of the aggregate caused many of the trial batches to fail.
Because of its low and variable moisture condition, the aggregate absorbed large portions of the mixing water during the initial stages of batching. This caused water demand to increase and made it difficult for the concrete mixture to achieve the required slump. Air content also was variable and unstable when slump and water demand fluctuated as a result of the aggregate moisture conditions.
ADOT, the design team, the contractor and the concrete supplier worked through a number of challenges to adjust the concrete mix, field procedures and performance criteria to achieve a successful placement. These included:
- Modify project requirement to extend acceptance age of the HPC compressive strength from 28 to 56 days;
- Increase the specified maximum concrete temperature at placement from 80° to 85°F;
- Increase fly ash content from the specified 110 to 165 lb;
- The aggregate needed to be at SSD conditions 24 hours before batching HPC according to project specification;
- Adjust silica-fume content from 25 to 30 lb;
- Perform additional batches incorporating these recommended adjustments to ensure consistent concrete production.
Actual concrete placement was on Aug. 24, 2005, and started at 2:37 a.m. and ended at 8:10 a.m. A total of 206 cu yd were placed at a rate of approximately 37 cu yd per hour.
The design team recommended that HPC be used on future bridge projects in Arizona. In the early stages of using HPC on bridge decks, cost increases can be expected as bridge contractors develop experience and knowledge in HPC technology construction. These costs will decrease as a result of competitive pricing when more HPC projects are constructed and more contractors become familiar with HPC.
Rocky Mountain try
In 2000, CDOT received a $700,000 award under the Innovative Bridge Research and Construction (IBRC) program to investigate innovative materials to reconstruct the I-225 and Parker Road interchange southeast of Denver.
The bridge consists of post-tensioned, cast-in-place, reinforced concrete box girders with 5-in.-thick precast concrete deck panels and a cast-in-place slab for a total deck thickness of 8 in. Under the IBRC program, part of the bridge deck was constructed using the HPC mix and deck panels with fiber-reinforced polymer (FRP) reinforcement.
Studies undertaken at the University of Colorado at Boulder 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.
CDOT experimented with seven HPC mix designs to develop two that improved durability by reducing cracks from shrinkage and had reduced 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 to produce a concrete with a lower modulus of elasticity and higher creep at early ages.
Two selected HPC mixes now in use have a low early strength and low heat of hydration. These characteristics allow the concrete to better accommodate volume changes and temperature variations, making it more resistant to shrinkage cracking. They also have low chloride permeability values at 56 days.
The study identified significant conclusions and recommendations to reduce concrete cracking and permeability:
- W/cm has the greatest effect on chloride permeability;
- Time to first cracking is directly and inversely related to cement content;
- Optimum silica-fume content for bridge decks is 4% by weight of cement;
- Class F fly ash is better than Class C fly ash in improving both the chloride permeability and cracking resistance; and
- Proper increase of coarse aggregate can improve chloride permeability, cracking resistance and 28-day strength of concrete.
The study also provided field practice recommendations:
- Air temperature during placement of 45°-80°F;
- Immediate and continuous fogging until concrete surface is covered and protected;
- Finishing and texturing as soon as possible to allow curing to begin;
- Minimum seven-day curing for silica-fume concrete; and
- Keep evaporation rate to a maximum of 0.1 lb/cu ft hr.
Actual performance to date on the I-225 bridge deck indicates 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.
CDOT believes that new construction materials can make a major difference in the cost and effort of maintaining bridge decks and roads in severe climates.
Life expectancy rises
Imagine designing bridges with an expected service life of 100 to 120 years. Those who plan, design and build bridges and highways think of 40, 50 or maybe 75 years for the expected service life of structures. ODOT said, “Why not consider longer time frames?”
The drive to extend bridge life is driven by the following:
- Oregon’s harsh environment on the coast;
- Program to preserve historic coast bridges;
- Cathodic protection for indefinite life; and
- Cannot afford to replace every 40 years.
The key technical challenges to extending the life of bridges are corrosion resistance, freeze-thaw durability and surface abrasion. Most of Oregon’s highways have two-lane bridges, and the climate requires extensive use of ice-melting chemicals. In addition, ODOT highways receive significant studded-tire traffic during the snow season.
To achieve the extended bridge life, ODOT developed a clear and focused approach to bridge deck design and construction.
First, they specified that all bridge deck concretes must have silica fume at 3 to 4% of the total cementitious materials for maximum durability and best resistance to abrasion.
Second, for complete mitigation of corrosion, ODOT specified solid stainless steel reinforcement. With one mat of stainless steel at the top and one at the bottom of the deck, the stainless steel reinforcement will not corrode, even if the concrete becomes contaminated with chlorides.
ODOT’s philosophy was to build it once not three times.
