JoAnn Browning and David Darwin, University of Kansas and Kenneth F. Hurst, Kansas Department of Transportation
A pooled-fund study is being implemented in two phases under the direction of the Kansas Department of Transportation in conjunction with 18 other state departments of transportation and the Federal Highway Administration to construct 40 low-cracking, high performance concrete (LC-HPC) bridge decks in Kansas and partner states. In the first phase of the project, new concrete materials and construction specifications were developed and implemented to construct the first 20 LC-HPC bridge decks. These decks, which did not contain any supplementary cementitious materials, are now being evaluated and compared with conventional decks for cost and cracking performance.
The LC-HPC specifications require the use of a concrete with a paste content (total volume of water and cement) less than 25%, low slump of 1.5 to 3.0 in. (40 to 75 mm), moderate water-cement (w/c) ratio of 0.43 to 0.45, controlled concrete placement temperature of 55 to 70°F (13 to 21°C), and an elevated air content of 6.5 to 9.5%. In combination with an optimized combined aggregate gradation, the mix must be workable, placeable, and finishable on the bridge deck. The construction specifications focus on the implementation of a thorough 14-day wet cure. This is started within 10 minutes of concrete strike-off. The concrete is protected following the 14-day wet curing period through the application of a curing compound to slow the rate of drying for the next 7 days at least. This limits the rate of development of tensile stresses in the young concrete. A qualification slab with dimensions equal to the bridge width, full depth, and 33 ft (10 m) in length is cast by the contractor prior to bridge placement to demonstrate competency in working with the concrete and meeting the curing guidelines with the available equipment. The requirement for the qualification slab may be waived on a case-by-case basis if the contractor has cast an LC-HPC bridge deck within the previous few months. More details about the specifications and goals of Phase I of the project can be found HPC Bridge Views Issue No. 46. This article focuses on the lessons learned during the first phase of the project and the broadened scope of work for Phase II.
Low-Cracking, High Performance Concrete Specifications
Two of the primary lessons from Phase I are that the concrete specifications can be implemented at a reasonable cost and that the low-paste concrete mix is workable, placeable, and finishable in the field. For the first 13 bridges bid, the costs of LC-HPC decks and conventional bridge decks in Kansas were similar when the same coarse aggregates having a maximum absorption of 0.7% were used. In terms of the concrete properties, working within the parameters of the concrete specifications has led to good workability and placeability, even with pumping, in most cases. Working with manufactured sands, however, which tend to be more angular than natural aggregates, may hamper the pumpability of an LC-HPC mix.
Practices that promote higher concrete strengths in the field are undesirable as they lead to increased cracking in bridge decks. Experience has shown that the target w/c ratio 0.43 to 0.45, when combined with the prescribed lower paste content, provides concrete strength in the range of 4000 psi (28 MPa) in the field. Any practice that lowers this ratio (e.g., inaccurate moisture contents of the aggregate or holding water out of the mix at the plant) may not only prevent easy placement of the concrete but also raise the concrete strength to undesirable levels. High strength concrete is not needed in bridge decks and results in increased cracking. Higher strength concretes creep less than moderate strength concretes as tensile stresses develop due to restrained drying shrinkage and thermal contraction. The use of high-range water-reducing admixtures is also discouraged as their usage leads to increased concrete strength.
Construction Methods
The relationships developed between owners, inspectors, contractors, and concrete suppliers are of prime importance. It might be argued, in fact, that they are the most important factors in successfully constructing an LC-HPC bridge deck. All participants need to be onboard to achieve the project goals and work to clearly communicate their expectations and needs to successfully meet the specifications. The construction parameter that leads to the most successful placements of LC-HPC bridge decks is a consistent, uninterrupted supply of concrete that meets the specifications. Only in this way can the consolidation and finishing operations proceed in a manner that will allow curing to start within 10 minutes of strike-off. Delaying the process by 10 to 30 minutes has been shown to increase the surface crack density by 0.06 to 0.08 ft/ft2 (0.20 to 0.25 m/m2), a value that is more than five times the total crack density of a properly executed LC-HPC deck.
Many elements contribute to the efficiency of the LC-HPC bridge deck placement operation. A clear understanding of the concrete testing schedule and agreement on how to handle out-of-specification concrete is the first element. In addition, the contractor needs to have two pumps or conveyor belts on site in case of equipment failure and to avoid delays from relocating equipment during placement. Over-finishing of the concrete should be discouraged. Minor corrections are applied by grinding the surface of the hardened deck. If diaphragms or abutments are cast integrally with the bridge deck, crews should begin filling the forms ahead of the finishing equipment as the deck placement approaches these larger concrete elements to limit delays in the finishing operation. Finally, careful estimates of the total quantity of concrete needed to complete the structure will eliminate costly delays when placing the last concrete on the bridge deck.
Experience continues to show that construction of the qualification slab yields great returns in terms of successful placement of LC-HPC bridge decks. For one reason, contractors have the opportunity to demonstrate competency in meeting the 10-minute curing initiation requirement with their crews and equipment. But an even more critical aspect has proven to be the ability to check the equipment that the contractor intends to use at the site with the qualified concrete mix. The pumpability of the mix can be verified and finishing techniques improved or simplified during the placement of the qualification slab.
Post-construction procedures have also been improved based on experiences during the first phase of the project. Formwork should be removed within 2 to 4 weeks after the end of the 14-day curing period. This is to ensure even drying of the concrete from the bottom as well as the top surface and thus avoid large moisture gradients through the deck. And for ease of application and inspection, the curing compound that is applied to slow the rate of drying of the top surface should be opaque rather than clear.
The implementation of the LC-HPC specifications has worked in the field by producing decks with less than 10% of the cracking found in traditional bridge decks. Phase II of the study will expand the scope with the construction of another 20 bridge decks, some of which will include the use of supplementary cementitious materials, internal curing agents, and shrinkage-reducing admixtures.
Further Information
For further information about this project, please contact the first author at [email protected].