The Use of Synthetic Blended Fibers to Reduce Cracking Risk in High Performance Concrete
Ideker, Jason H.
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Transportation departments have observed varying degrees of cracking in their concrete structures. Cracking of high performance reinforced concrete structures, in particular bridge decks, is of paramount concern to Pacific Northwest Departments of Transportation. Cracking at early ages (especially within the first year after placement) results in additional costs and a significant maintenance burden to ODOT. The causes behind cracking in high performance concrete are well known and documented in the existing literature. Recent research at Oregon State University under two separate projects: SPR 711 and SPR 728 has elucidated the propensity for shrinkage in high performance concrete mixtures mainly used for concrete bridge decks. These previous projects identified 1) mitigation methods to reduce shrinkage (e.g. internal curing using fine lightweight aggregates –FLWA, and/or the use of shrinkage reducing admixtures SRA) and 2) proposed shrinkage measurement techniques and thresholds/limits using those techniques that should reduce shrinkage and early-age cracking risk. The aim of this project was to investigate a relatively new technique to control early-age cracking; the use of blended size polypropylene fibers in high performance concrete mixtures. The key findings from this work were that the use of drying shrinkage test methods alone, without the capture of cracking risk, showed that the inclusion of fibers did not reduce drying shrinkage in unrestrained specimens. However, in restrained testing (where the possibility of crack formation is promoted) the fibers were able to 1) reduce the rate of stress generation in specimens 2) prolong the time to cracking in the restrained ring test (ASTM C 1581) and 3) reduce the crack widths and the growth of cracks once cracking did initiate. As a result the use of blended fibers in high performance concrete points to another viable solution for reducing the risk of cracking in service.