Thin concrete overlays are widely used to protect decks against corrosion caused by the infiltration of chloride ions. These overlays are designed to have low permeability and to be free of cracks that may adversely affect the performance of the deck by facilitating the penetration of chloride ions. Sixteen high-performance concrete overlays with 13 concrete mixtures placed on two bridges were evaluated, with an emphasis on shrinkage characteristics. One overlay was constructed with concrete containing a new shrinkage-reducing admixture. Specimens prepared with this admixture had the lowest shrinkage. Based on a visual survey, all overlays were performing satisfactorily at 1 year after placement.

Concrete containing various supplementary cementitious materials (SCMs) such as silica fume, fly ash, and slag has improved properties. Nanomaterials, new SCMs with possible applications in concrete, have the smallest particle size (less than 100 nm). Nanomaterials are reactive because of the small size and large surface area of the particles, and they have great potential in improving concrete properties such as compressive strength and permeability. This study evaluates the use of a variety of nanomaterials in concrete compared with conventional concrete and concrete containing common SCMs. The potential benefits of using nanomaterials over other SCMs are high reactivity and cost-effectiveness; in addition, smaller amounts are necessary, resulting in less cement replacement. Concretes containing nanosilica and nanoclay were prepared in the laboratory. They were compared with concretes containing silica fume, fly ash, slag, or only portland cement. Specimens were tested for compressive strength and permeability. The microstructure of selected concretes with improved compressive strength and permeability was analyzed by using an atomic force microscope and nanoindenter to explain the improvements. The results of this study indicate that some of the nanomaterials tested have potential in concrete applications. The microstructure of the nanosilica concrete was denser and more uniform than the conventional concrete microstructure. In addition, the nanosilica had the largest improvement in both compressive strength and permeability among the nanomaterials tested.

No abstract

Durable concrete exposed to the environment requires a proper air-void system for protection against cycles of freezing and thawing. Durable concretes also must have low permeability to resist the infiltration of harmful solutions into concrete. Work was conducted by the Virginia Department of Transportation (VDOT) on the permeability of concretes. Information is provided on permeability, recent VDOT efforts with a low-permeability specification, and the field application of the low-permeability specification.

Results are presented of a laboratory investigation to determine the properties of fiber-reinforced concretes (FRCs) with steel (hooked-end), polypropylene (monofilament and fibrillated), and the recently introduced polyolefin fibers (monofilament) for application in pavements and bridge deck overlays. Concrete properties in the unhardened and hardened states were evaluated and compared. Although the ultimate splitting tensile strength, compressive strength, and first-crack strength were higher in most of the FRCs, when strength values were adjusted for changes in air content, only a few batches had higher strengths. The addition of fibers resulted in great improvements in flexural toughness and impact resistance. Three FRC pavement overlays were applied in Virginia in 1995. The FRCs used in the projects were similar to those used in the laboratory investigation, with similar fiber volumes, types, and sizes. To implement the findings of the study successfully, the performance of the FRC pavement overlays is being monitored.