Kevin R. Pruski, William R. Cox, and Mary Lou Ralls, Texas Department of Transportation
The Texas Department of Transportation (TxDOT) specifications for high performance concrete (HPC) have evolved through TxDOT’s experience. The first two HPC projects in Texas in the 1990s were guided by researchers at the University of Texas, Austin, in conjunction with demonstration projects sponsored by the Federal Highway Administration (FHWA). Although these two projects focused on high-strength HPC, the cast-in-place concrete deck on one of the two adjacent bridges in each project was constructed with normal strength HPC. Concrete strengths of 9000 psi (62 MPa) and higher with good durability characteristics are readily achieved in pretensioned concrete beams in Texas. Now, the priority in Texas is to improve the durability of normal strength cast-in-place concrete. Therefore, subsequent HPC bridge projects have focused on normal strength cast-in-place HPC. This article discusses activities that address this priority.
For early HPC bridge projects in Texas after the first two, the contractor was expected to obtain durable HPC by adherence to TxDOT’s standard specifications with only slight modification and additional performance-related requirements. This approach relied on the contractor’s knowledge and experience to satisfy contract requirements. However, bridge projects following this approach demonstrated contractors’ inexperience at designing concrete to meet performance-based durability requirements. Consequently, noncompliance with the new performancerelated criteria as well as higher concrete costs resulted.
Specifications were, therefore, adjusted to shift responsibility to TxDOT to specify more durable concrete. The first two HPC bridge projects in Texas, HPC bridge projects in other states, and the results of various HPC research projects convinced TxDOT that inclusion of supplementary cementitious materials (SCM) makes concrete more durable. With this knowledge, TxDOT added prescriptive requirements to include specific amounts of SCM in concrete that was required to have increased durability; this concrete includes the designation “HPC” in its bid item. By forcing the use of SCM, TxDOT anticipates that Texas contractors, materials suppliers, and engineers will develop the experience to obtain HPC with the needed durability requirements.
TxDOT HPC specifications now require the contractor to develop strengthversus-time curves for the concrete with strengths measured at 4, 7, 28, and 56 days for mix design approval. This was introduced to address concerns about prescriptively specifying SCM when the materials supplier and the contractor lack experience with the effects of SCM on strength gain of concrete.
Fly ash or ground granulated blast-furnace slag can slow strength gain, especially in cooler weather, and requiring the contractor to plot this curve helps synchronize the concrete mix with the construction schedule. Additional concrete samples are taken and sent to the central laboratory for AASHTO T 277 rapid chloride permeability (RCP) tests and AASHTO T 259 salt ponding tests.
Sample Bridge Project
TxDOT recently replaced two deteriorated concrete bridges in the Lubbock District in the Texas panhandle. HPC was specified because of the significant use of deicing chemicals related to the 70 annual freeze-thaw cycles.
Ordinarily, the Class C concrete used for a bridge substructure in TxDOT projects has a minimum specified compressive strength of 3600 psi (25 MPa) at 28 days and a maximum water-cementitious materials (w/cm) ratio of 0.53. The Class C (HPC) provision required replacing 4 percent of the cement with silica fume and 26 percent with Class F fly ash, and limiting the w/cm ratio to a maximum of 0.47. The Lubbock District has only recently started using fly ash routinely, and it had never used silica fume. Although the average rapid chloride permeability (RCP) was not specified, the average measured value was 676 coulombs at 56 days.
TxDOT specified Class S (HPC) concrete for the bridge deck. Class S concrete normally has a minimum compressive strength of 4000 psi (28 MPa) at 28 days and a maximum w/cm ratio of 0.44. The Class S (HPC) provision required replacing 30 percent of the cement with Class F fly ash. There were concerns about attaining the 4000 psi (28 MPa) compressive strength at 28 days due to slower strength gain from the Class F fly ash, especially in cold weather. Therefore, the 28-day strength requirement was lowered to 3000 psi (21 MPa) but a requirement of 4000 psi (28 MPa) at 56 days was added. Although 3000 psi (21 MPa) is sufficient to resist design loads on adjacent box beam structures, 4000 psi (28MPa) was required to be consistent with the Standard Specifications. The average RCP test value for the HPC was 1057 coulombs at 56 days compared to 4000 coulombs for concrete without fly ash.
The Future of HPC Use in Texas
TxDOT continues to specify normal strength HPC to increase the durability of its bridges. The use of prescriptive HPC specifications has allowed contractors and fabricators to gain experience with the production and placement of concrete containing various types of SCM. TxDOT has now begun allowing the contractor the option of using the prescribed mix or another mix that meets the durability performance requirements. Ultimately, the department is moving toward the full implementation of performance-based durability specifications.
