Mathew Royce, New York State Department of Transportation
New York State’s Department of Transportation (NYSDOT) first used high strength, high performance concrete (HSHPC) for bridge beams in 2001. Initial experience showed that concrete with a compressive strength of 10,150 psi (70 MPa) allowed the design of bridge beams for significantly longer spans compared to conventional strength concrete. Higher performance requirements for durability held the promise of a maintenance-free service life of 100 years or more for bridge beams even in aggressive environments. Based on the success of the initial applications during 2001, 2002, and 2003, NYSDOT began specifying HSHPC for all precast, prestressed concrete beams in 2004.
Specifications
The NYSDOT HSHPC specification is performance based using seven criteria for design efficiency and durability as follows:
Compressive strength (f’c) at 56 days by AASHTO T 22: >10,150 psi (70 MPa)
Modulus of elasticity at the concrete age when f’c ≥ 10,150 psi (70 MPa) by ASTM C 469: ≥ 4350 ksi (30 GPa)
Shrinkage after 56 days of drying by AASHTO T 160: < 600 millionths
Specific creep after 56 days of loading by ASTM C 512: ≤ 0.41 millionths/psi (60 millionths/MPa)
Freeze-thaw relative dynamic modulus after 300 cycles by AASHTO T 161 Proc. A: ≥ 80%
Scaling resistance by ASTM C 672 visual rating: ≤ 3 Chloride penetration by AASHTO T 259 (modified) increase in chloride ion by weight: < 0.025% at 1 in. (25 mm)
In addition, certain requirements are established for the mixes:
- Minimum entrained air content of 3 percent
- Minimum silica fume content of 5 percent by weight of the cementitious materials
- Maximum water-cementitious materials ratio of 0.40
- Calcium nitrite corrosion inhibitor at a dosage rate of 646 fl oz/cu yd (25 l/cu m)
Eight precasters in and around New York State, including most of the precasters who produce beams for NYSDOT, have completed the mix design and pre-production testing. Most of these precasters have already used their approved mixes for producing concrete beams for NYSDOT. In addition, most of the local governments within the state, including the New York City Department of Transportation, are also using NYSDOT High Performance Concrete Specifications and NYSDOT approved concrete mixes for precast, prestressed concrete bridge beams.
Design Efficiency
NYSDOT bridge designers, as well as consultants working for the DOT, are achieving substantial design efficiency improvement by using HSHPC. With the use of 10,150 psi (70 MPa) concrete and 0.6-in. (15.2-mm) diameter strands, span-to-beam depth ratios have been increasing. This is helping the designers use precast, prestressed concrete beams for replacement bridges where, in the past, it was not feasible due to site limitations on maximum allowable beam depths.
Lower creep and shrinkage in HSHPC beams helps to reduce secondary stresses in indeterminate structures, such as superstructures made continuous for live load. Reduced positive moment in beams at the intermediate supports usually produces more efficient designs as well as increased durability due to the reduced potential for cracking in the closure pours between beam segments. The camber and camber growth of HSHPC beams are more predictable. Camber growth control is important in design and construction of bridges built in stages. Usually, concrete decks for the first stage of these bridges are thickened to accommodate the expected camber growth of beams to be used for the second stage. Since beams using HSHPC have lower camber growth, unnecessary increases in deck thickness can be avoided, resulting in a more efficient design.
Durability
Resistance to chloride penetration of HSHPC meeting NYSDOT specification is many times higher than the conventional concrete used for bridge beams fabricated prior to 2004. The calcium nitrite corrosion inhibitor also elevates the corrosion initiation threshold significantly. NYSDOT is confident that the combination of HSHPC and corrosion inhibitor will provide a service life of 75 to 100 years for precast, prestressed concrete beams.
Cost of High Performance Concrete
Based on the cost data for concrete bridges constructed during the last five years, there has not been any significant cost difference between bridges with HSHPC beams (cost of mix design and initial testing included) and bridges with conventional concrete beams. Use of HSHPC in precast, prestressed concrete bridge beams is very advantageous to bridge owners due to significantly reduced life-cycle costs.
High Performance Self-Consolidating Concrete (HPSCC)
Some of the precast concrete producers have started using HPSCC in products used by the Department. NYSDOT is allowing the use of HPSCC when requested by the contractor. In general, pre-production testing is similar to conventional HSHPC except that additional quality control tests are performed to ensure no segregation of aggregates during or after the placement of HPSCC. All precast concrete components produced using HPSCC for NYSDOT have shown very good results.
Conclusion
In general, the use of HSHPC and HPSCC for precast, prestressed concrete bridge beams has been a remarkable success. With no apparent increase in construction cost, NYSDOT is building bridges that are much more durable and less expensive to maintain. For a copy of the Engineering Instruction and HSHPC Specifications, go to www/dot/state/ny.us/cmb/consult/eib/files/ei03037.pdf.
Editor’s Note
This article is the fifth in a series that describes how the use of HPC has progressed since it was first introduced into a State’s program. Other articles appear in Issue Nos. 30, 35, 36, and 37