Question:
Should high-strength concrete be used for cast-in-place concrete bridge decks?
Answer #1:
by Kevin R. Pruski, Texas Department of Transportation
Generally, concrete bridge decks do not need the higher design strengths offered by high-strength concrete. Common construction problems for bridge decks can be exacerbated by requiring it unnecessarily.
Cracking caused by shrinkage is a concern for any bridge deck. Cast-in-place bridge decks are particularly vulnerable to plastic shrinkage cracking because of high evaporation rates from their large surface areas. Drying shrinkage is greater for high-strength concrete because of its higher cementitious materials content, which provides a higher percentage of paste in the concrete matrix. More paste means more shrinkage.
The current AASHTO Standard Specifications for Highway Bridges and AASHTO LRFD Bridge Design Specifications call for a minimum strength of 4000 psi (28 MPa) for concrete in bridge decks. Design strengths greater than 4000 psi (28 MPa) are rarely needed for bridge decks. Contractors typically provide considerably higher strength concrete than the design calls for to ensure that minimum contract requirements are met. A design unnecessarily requiring high-strength concrete risks even higher levels of paste, which would further increase shrinkage.
The 4000 psi (28 MPa) requirement for concrete in bridge decks is easily met by a water-cementitious materials ratio of 0.45. Lower water-cementitious materials ratios may be required to obtain low permeability. Replacing some of the cement with fly ash or ground granulated blast-furnace slag while maintaining the water-cementitious materials ratio at 0.45 produces concrete with a lower permeability but with slower early strength gain. While it may take longer to reach its specified strength, thereby prolonging the length of a project, the actual later-age strength is typically relatively higher. The addition of silica fume may be used to assist in achieving early-age strength gain for projects under tight construction schedules. Mix designs should always be developed to ensure that the constraints of each specific project are met.
Answer #2:
by Jerry L. Potter, Federal Highway Administration
High-strength concrete is generally not necessary with current bridge deck design practices and support spacings. The use of high performance concrete (HPC) for improved durability, however, generally results in concrete strengths greater than used for many current bridge deck designs. The mix proportions are controlled by the specific durability requirements, such as permeability and freeze-thaw resistance. The higher concrete strength is then a by-product.
A specified concrete strength greater than that normally achieved from HPC, when specified for durability, is ineffective for routine decks and necessitates additional construction controls. In limited situations, use of the higher concrete strengths, achieved as a by-product of the HPC use, may be cost effective. Some cost efficiency may be possible by increasing the design strength from the normal range of 4000 to 4500 psi (28 to 31 MPa) to the strengths achieved from the HPC specified for durability. However, the specified compressive strength should not exceed the expected strength from the durability requirements.
High-strength concrete should be specified for special projects that can effectively use the high compressive strength to achieve economy or other benefits. It may also be used for projects that are large enough to justify the added costs for more complex construction processes needed to successfully control and place the higher strength concrete. This would be applicable for structures with unusual framing systems and decks that require high early strengths for construction acceleration or early opening to traffic.