Sohila Bemanian, Nevada Department of Transportation
As a result of the state’s fast growth rate, the Nevada Department of Transportation (NDOT) is faced with the largest work program in its history. NDOT will spend $1 billion in the next several years constructing and reconstructing major freeways and urban arterial systems. These projects include major bridges such as the Galena Bridge in northern Nevada, which will be the second longest concrete arch structure in the United States.
Implementation
In 1999, NDOT created a High Performance Concrete (HPC) Task Force consisting of personnel from the Materials, Bridge, and Construction Divisions; the Federal Highway Administration; and an experienced consultant, who provides many concrete mix designs for NDOT. The mission of the HPC Task Force was to develop HPC specifications utilizing local aggregates.
Based on a research study conducted by the University of Nevada-Reno, none of the local aggregates resulted in concrete that met all the HPC requirements suggested by FHWA. Therefore, the HPC Task Force selected permeability and modulus of elasticity as requirements for the northern part of the state and only permeability for the southern part of the state. Modulus of elasticity was included because field values were much lower than values assumed in design. Several mix designs were made to determine how HPC properties could best be achieved utilizing local aggregates. In addition to permeability and modulus of elasticity for the northern part of the state, creep and shrinkage properties were identified as important parameters in the concrete mix design for the Galena Bridge. Preliminary mix designs using these parameters were developed prior to project bidding.
A 10-day wet curing period was added to the specifications and became a mandatory requirement for all bridge decks in order to reduce plastic shrinkage cracking and to make bridge deck curing practices more consistent throughout the state. Continuous fogging is required prior to placement of the burlap. The second addition to the specifications was a maximum rapid chloride permeability requirement of 2000 coulombs at 56 days. Several meetings were organized with contractors to discuss the importance of wet curing and constructing crack free bridge decks. During mandatory pre-bid meetings and pre-placement conferences, the HPC requirements were discussed again.
Life Expectancy
The main objective of HPC is to increase the life of a structure. In a greater sense, the objective is to reduce the life-cycle cost. Any increase in the cost of the material and workmanship used to create the structure is expected to be regained by less maintenance and longer times between rehabilitation and replacement. At this time, we expect a 35 to 50 percent increase in life expectancy of the structures.
Cost
Implementation of the HPC can increase the cost of a concrete bridge deck by 30 percent. The cost of HPC for materials and placement is approximately $415/cu yd ($543/cu m). This is an increase of $100/cu yd ($131/cu m) over traditional concrete costs. The Galena Bridge structure is anticipated to cost about $600/cu yd ($785/cu m) because of additional requirements and very complex falsework. However, this cost increase is insignificant compared to the overall cost of the project and the potential for less frequent rehabilitation in the future. In addition, as contractors become familiar with the process of producing quality HPC, the cost is expected to become more competitive.
