A. Using high dosages of next-generation synthetic structural/macro fibers has been beneficial in this type of bridge application, says Dan Biddle of Forta Corp. He says their use has helped to reduce or eliminate much of the steel reinforcement in projects such as the Dalhousie (TUNS) steel-free bridge deck projects.
Nick Carter, of PGI/FabPro, agrees that synthetic fiber admixtures should be considered for small concrete bridges. However, he notes, it is important to take into account a number of factors when considering this subject. Does the particular bridge application require control of cracking during curing, impact resistance during installation or use, or enhanced flexural response due to differing strains resulting from the types of loads carried by the structure? Are you casting at the site or precasting in part, or whole, and then moving it to the location for installation? It is much easier to address the “design, constructability and economics” concerns when specific needs are considered, he says, such as whether the traffic is pedestrian or equine or light vehicular.
Two efforts currently under way in the synthetic fiber industry are worth noting, Carter says. The first involves tuning existing structural fiber products to improve performance. Second is the development of entirely new synthetic fiber systems targeting the same issues that existing products address. Factors to consider include the accuracy of dispensing systems and how efficiently the fibers are distributed within each batch of concrete, which goes beyond the straight cost of the fibers.
The use of fibers, limited to the wearing course and not as a replacement for positive moment steel such as that in a girder, can provide for a more durable and economic structure, agrees Michael Mahoney, structural fiber technology manager for the Euclid Chemical Company. Fiber use would be. He says the use of fibers in a “steel free” bridge system is a good idea provided the proper fiber type and dosage is used, and that the fibers are used for crack control only where the arch system will require them to perform.
Another suggestion, from consultant Don Hourahane, is the use of textile concrete as permanent formwork for small bridges. Such formwork can be made 100% vapor tight, he says, reducing the need for concrete cover. Additionally, the textile concrete formwork is further strengthened through strain hardening after the first cracking.