Donald A. Streeter, New York State Department of Transportation
The New York State Department of Transportation (NYSDOT) developed a high performance concrete (HPC) mix in 1994 in an effort to produce longer lasting, more durable bridge decks. Positive results from the initial placements led to the issuance of specifications for use of high performance concrete. The HPC developed by NYSDOT, designated Class HP, was designed to be more durable, less permeable, more resistant to cracking, and easily placed and finished. The changes were achieved by reducing the cement content, mainly by substituting pozzolans, and lowering the watercementitious material ratio by using normal-range water-reducing admixtures. Over 125 placements have been made on approximately 100 new bridge decks. High performance concrete overlays have been placed on approximately 25 structures either for rehabilitation of the decks or as bonded wearing surfaces. With the establishment of guidelines, substructure use is now growing.
Construction
Attention to detail makes HPC perform much better than conventional concrete. The NYSDOT Materials Bureau has been working with contractors, prod u c e r s , engineers, and inspectors to ensure quality concrete. Training sessions are used to present highlights of the procedures that must be followed. This provides all the necessary information to achieve the quality concrete desired prior to any concrete batching or placement.
The control of batching and construction operations is important. Batching HPC is similar to batching conventional concrete. However, the addition of microsilica and water-reducing admixtures must be closely controlled. Cementitious materials are batched within a 1/2 percent tolerance, which is tighter than the 1 percent tolerance used for conventional concretes. Class HP concrete has no bleed water and dries out quickly. Therefore, it is important not to leave the concrete exposed for extended periods. Concrete is only placed 5 to 8 ft (1.5 to 2.4 m) ahead of a properly set-up finishing machine. If there are any delays in placement, all concrete not yet finished or textured is protected from evaporation by either covering with plastic sheeting or wet burlap. If the set-up and finishing are performed properly, the concrete requires no additional handwork. Finishing should not result in a glass-like surface since texturing and saw-cut grooving will be applied to roughen the surface. A turf drag texturing is applied immediately after finishing, with saw-cut grooving applied to the hardened concrete. Continuous curing for seven days using wet burlap is initiated immediately after texturing is complete. A burlap imprint left on the plastic concrete is of less concern than a delay in the application of curing. In future projects, 14 days of wet curing will be required.
Performance
Performance of Class HP concrete has been very good to date. The average 28-day compressive strength is 5,400 psi (37 MPa), which is an increase of about 20 percent over conventional concretes. Permeabilities in the field average 1600 coulombs at 28 days, which is 30 to 50 percent of the values for conventional concretes. Cracking has been reduced and those cracks that do form are finer than in the past. The concrete has been easy to handle and place in the field provided sufficient water-reducing or set-retarding, water-reducing admixtures are used.
Although most placements have been properly completed, some problems have occurred. Open crack surfaces have resulted when the fresh concrete was exposed to the environment for extended periods of time. This problem becomes worse when the initial slump is low due to a lack of sufficient water-reducing admixtures. In areas where excessive hand finishing has been performed in an attempt to close the surface, scaling has resulted. Occasional l y, microsilica balling has occurred, usually associated with a batching problem or improper mixing. Cracking results from a variety of reasons. If there is not sufficient retardation during placement, cracks have developed, primarily on multi-span, continuous structures. Shrinkage cracks occur if curing is delayed. Also, if fresh concrete is placed on existing concrete that is not in a saturated, surface-dry condition, shrinkage occurs. This problem is prevented by placing soaker hoses or sprinklers on the existing concrete for 12 or more hours prior to concrete placement.
Summary
Overall, NYSDOT is pleased with the performance of Class HP concrete. It can be placed and finished easily resulting in an improved concrete that is more durable and less permeable than conventional concretes. Editor’s Note: The next edition of HPC Bridge Views will contain an article quantifying the improved deck performance in New York State.