HPC for Permeability and Strength
Similar to the earlier survey, the recent survey also tracked the agencies' use of HPC for permeability and strength benefits. Rapid chloride permeability values in the range of 1001 to 2000 coulombs were most commonly specified for overlays, decks, superstructures, and/or substructures.
HPC specified for permeability
A compressive strength range of 4001 to 5000 psi (28 to 34 MPa) was most commonly used for decks and substructures. The next most common range was 3001 to 4000 psi (21 to 34 MPa). For superstructures, the compressive strength range of 8001 to 10,000 psi (55 to 69 MPa) was most commonly specified. The next most common was 4001 to 5000 psi (28 to 34 MPa).
In the recent survey, the use of SCC was categorized by superstructure and other precast members, and by various substructure elements. Eleven agencies had used SCC in up to 10 bridge superstructures and/or precast members. Three used SCC in 11 to 20 bridge superstructures. One agency reported its use in 21 to 30 precast structural members, and one agency reported from 31 to 50 bridge superstructures with SCC.
SCC use in substructure elements has been much less. Only five agencies reported such usage, in up to 10 bridges over the past four years. This was for pier caps and columns, footings, piles, and drilled shafts.
As with other data noted above, the earlier survey tracked only generally whether an agency had tried lightweight HPC on an experimental basis, or whether the agency had progressed to the point of developing standard specifications for the technology.
The recent survey tracked actual project experience. This time, 11 agencies had used lightweight HPC in up to 10 bridge decks, and three used it in the superstructure of up to 10 bridges. Two agencies used lightweight HPC in 11 to 20 decks, and two agencies on more than 50. None used lightweight HPC in bridge substructures.
HPC Performance Characteristics
The survey obtained results on what HPC performance characteristics the agencies tested. Performance characteristics include freeze-thaw (F/T) durability, scaling and abrasion resistance, chloride penetration, alkali-silica reactivity (ASR), and sulfate resistance.
Strength-related performance characteristics include compressive strength, modulus of elasticity, shrinkage, and creep. Flowability can also be specified as a conventional slump value or as a flow for SCC.
Not surprisingly, compressive strength tests were specified by the highest number of agencies, with 48 out of 53 using them. The next highest was rapid chloride permeability, tested by 34 agencies, followed by shrinkage, tested by 20 agencies. Flowability, ASR, and F/T were tested by 17 agencies. Six tested scaling and sulfate resistance.
Methods of Specifying HPC
The most common methods of specifying HPC for bridges was either by (a) special provision for a particular project, or (b) a combination of special provisions and general specifications. Twenty-two agencies used Method (a) and 22 agencies used Method (b). Only eight used general specifications. Slightly more than half the agencies had neither built nor planned HPC bridge projects using end-result, performance-based specifications (ERS).
Eleven agencies had one to five bridges either planned or built using ERS. Only one agency had made substantial progress with ERS being used on more than 100 bridges.
High-Performance Corrosion-Resistant Reinforcing Bars
For many years, agencies have been experimenting with corrosion-resistant alternatives to epoxy-coated reinforcement for bridge decks. A long-term research study has been performed for the FHWA and the Florida DOT by Florida Atlantic University to evaluate alloys previously identified as candidates for corrosion-resistant reinforcement.
The author wishes to express his appreciation to Rodolfo F. Maruri and Claude S. Napier of the FHWA for synthesizing a huge amount of data from the survey in a very timely manner. For more, vist www.nationalconcretebridge.org.