Basile G. Rabbat, Portland Cement Association, Suneel N. Vanikar, Federal Highway Administration
As part of the implementation of the Strategic Highway Research Program (SHRP) High Performance Concrete (HPC) products, several states have started to use HPC for bridge construction. Applications include all bridge components: superstructures (decks and girders) and substructures (piers and abutments).
What is HPC?
The term HPC is used to describe concretes that are made with carefully selected high quality ingredients, optimized mixture designs, and which are batched, mixed, placed, compacted and cured to the highest industry standards. Typically, HPC will have a water-cementitious materials ratio (w/cm) of 0.4 or less. Achievement of these low w/cm concretes often depends on the effective use of admixtures to achieve high workability, another common characteristic of HPC mixes.
According to the American Concrete Institute, high performance concrete is defined as concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing and curing practices.
A high performance concrete is a concrete in which certain characteristics are developed for a particular application and environment. Examples of characteristics that may be considered critical for an application are:
• Ease of placement
• Compaction without segregation
• Early-age strength
• Long-term mechanical properties
• Permeability
• Density
• Heat of hydration
• Toughness
• Volume stability
• Long life in severe environments.
In What Ways Will HPC Improve Bridges?
Because a lower w/cm is used, HPC typically has a higher strength than conventional strength concrete. However, high strength is not always the primary requirement. HPC is valuable where any of the following properties are required: high strength, high early strength, low permeability, resistance to freeze-thaw damage, resistance to chemical (e.g. sulfate) attack, abrasion resistance, low absorption, high resistivity, high modulus of elasticity, and volume stability.
Given these improved material characteristics, bridge decks built with HPC can be expected to last much longer than those built with conventional concrete. High strength concrete girders can span longer distances and/or can be used at wider spacings than conventional concrete girders, thus reducing the number of required girders and resulting in economies. Alternatively, bridge designers have the option of selecting shallower girders to increase clearances without changing grades. Again, the net result is economy. HPC can be used to both reduce the size and extend the service life of superstructure and substructure elements, particularly in severe environments. HPC also allows for more graceful structures; aesthetics, although hard to measure can be an important benefit.
Which States Are Building HPC Bridges?
The States listed to the below are participating in the Federal Highway Administration (FHWA) sponsored HPC Bridge Showcase program. The intent of this program is to give the States an opportunity to see how they can benefit from the use of HPC. So far, the program appears to be a success: Virginia, for example, has completed two additional HPC bridges on their own, they have another one under construction and five in design. A number of other States including Delaware, Indiana, Louisiana, Minnesota, Missouri, and New York aren’t participating in the FHWA program but are using HPC.
Summary
HPC is clearly a great improvement on previous formulations of high quality concrete made possible by the use of mod e r n admixtures and supplementary cementitious materials. In almost all forms of construction, HPC offers a superior solution that should have lower service-life costs than conventional concrete. The ingrained traditional first cost approach to construction and lack of reliable and accurate models for predicting life-cycle costs may make it difficult to obtain widespread or rapid acceptance of this material. Ultimately, however, the superior qualities of HPC will result in its increased acceptance on the basis of life-cycle costs. In some instances, initial economies will result even though the material itself may be more expensive. In such applications, HPC offers a clear advantage.
REFERENCE
- Jobse, H. J., and Moustafa, S. E., “Applications of High Strength Concrete for Highway Bridges,” PCI Journal, Vol. 29, No. 3, May/June 1984, pp. 44-73.