Introduction Concrete bridge decks are inherently exposed to harsh environment. They are known to undergo degradation due to numerous distresses, of which the most common are corrosion of reinforcement (caused by ingress of chloride through bulk concrete), freeze-thaw damage, shrinkage cracking (aggravating both corrosion and freeze-thaw problems) and surface scaling due to deicing salts. Recently, high performance concrete (HPC) has become widely utilized in the applications where severe

more »

... nment leads to premature deterioration. The premise behind the development of HPC was to address the durability issues typical of normal concrete. HPC almost always implies incorporation of mineral admixtures, such as fly ash (FA) or silica fume (SF). These materials are added to concrete for variety of reasons, of which the most important is improvement (compared to plain ordinary portland cement (OPC) concrete) of certain durabilityrelated properties. However, there are also some downsides of utilizing them in concrete. For example, fly ash is known to reduce the early age strength and early age resistance to chloride-ion penetration, as well as resistance to salt scaling and carbonation. Furthermore, fly ash concrete has been reported to be fairly sensitive to curing conditions compared to plain cement concrete. The shortcomings of incorporating silica fume, on the other hand, include increased susceptibility to shrinkage cracking and potentially reduced resistance to freezing and thawing. In the view of the aforementioned side effects associated with the use of fly ash or silica fume in binary mixtures, the ternary cementitious systems have been introduced as a potentially viable solution to address those durability issues. Superior properties of this kind of cementitious systems are commonly attributed to so-called synergistic effect taking place when both pozzolanic materials are utilized. The intuitively obvious benefit of the use of OPC/FA/SF mixtures is that the presence of fly ash in concrete compensates for the deficiencies of silica fume, and vice versa. For example, FA offsets the increase in water demand, heat of hydration and alkalinity of pore solution resulting from addition of SF. On the other hand, SF compensates for the low early age strength and sensitivity to curing (with respect to development of both mechanical and durability-related properties of FA concrete). From the economical perspective, a relatively low cost of FA counterbalances higher cost of SF. Although a fairly large amount of research has been conducted to evaluate the properties of ternary cementitious mixtures containing fly ash and silica fume, relatively little was done with the emphasis on the durability-related issues encountered when these binder systems are used in the bridge deck concrete. Up to date, no data exists on the actual performance of ternary concrete containing class C fly ash and silica fume in bridge decks. The purpose of this research was to examine the applicability of ternary binder systems containing ordinary portland cement, class C fly ash and silica fume for bridge deck concrete. This was accomplished in two parts, the laboratory part and a field application part. During the laboratory studies, four ternary mixtures, each containing either 20% or 30% FA and either 5% or 7% SF were subjected to four different curing regimes (air drying, 7 days curing compound application and 3 or 7 days wet burlap curing). The properties studied included water and chloride solution controlling properties, compressive strength, free shrinkage and 25-1 4/10 JTRP-2008/29-2 INDOT Division of Research West Lafayette, IN 47906 resistance to shrinkage cracking, salt scaling and freezing-thawing resistance. The field application part of the research was conducted on the bridge carrying SR-23 over US-20 in South Bend, Indiana. This bridge deck was constructed in two phases, one constructed in late fall 2004 and the other constructed during spring 2005. Test specimens were collected for laboratory evaluation from both construction phases and visual observations of the deck performance were conducted at 6, 10 and 24 months after construction. Findings During the laboratory part of the study it was observed that all four ternary mixtures exhibited very good water and chloride solution transport-controlling properties (resistance to chloride-ion penetration, chloride diffusivity and rate of water absorption). However, it was concluded that in order to ensure adequate strength, good freezing and thawing resistance, satisfactory resistance to salt scaling, and adequate shrinkage cracking resistance the FA content should not exceed 20%, SF content should not exceed 5% (by total mass of binder) and paste content should be kept below 24% by volume of concrete. Further, wet burlap curing for a minimum of 3 days was required to achieve satisfactory performance and to obtain a reliable assessment of in-situ compressive strength (up to 28 days) using maturity method. The properties developed by field concrete from construction phase 2 (which took place in spring of 2005) were comparable to those of concrete which was made and cured under laboratory conditions. However, the chloride-ion penetrability (evaluated using rapid chloride permeability (RCP test) of concrete from phase 1 (which took place in fall of 2004) remained relatively high for prolonged period of time (4000 coulombs at 90 days). Using maturity method developed for the purpose of this study, it was determined that the unexpectedly high RCP values were mostly attributed to low ambient temperature.