Restrained shrinkage behavior of high performance concrete reinforced with hybrid fibers
Creators: name: Habib, Mina
Publisher: name: No Publisher Supplied
Year: 2019

description: High-performance concrete (HPC) is characterized as a type of concrete that has superior strength and higher durability than typical Class A concrete. These qualities are what makes its application very common in slab-on-girder bridges. A stronger concrete mix will have a higher tensile strength, one that requires a higher tensile strain to induce cracking. Although HPC has higher strength than typical Class A concrete, it is still susceptible to cracking due to different types of shrinkage. There are ways to combat this shrinkage, some of which include, but are not limited to, utilizing shrinkage-reducing admixtures, wet-curing, and implementing fibers into the mix design. Fiber-reinforced concrete is being implemented in structures all over the world, and its practice in the industry is more possible than ever due to the introduction of high-range water reducing admixtures.Fibers are added to concrete for a plethora of reasons, some of which are highly favorable in particular circumstances. The natural brittle tendency of concrete is a common concern among designers, and adding fibers into their mix design increases tensile strength by mitigating crack propagation. Once a concrete member is subjected to tensile stress, the fibers distributed among the member will intercept the cracks as they are made and as they grow in depth and width. Even if a concrete element is designed to be in compression, tensile stresses can still be induced as a result of restrained shrinkage. In bridge decks, the bottom surface of the slab is typically held in place by a steel deck, while the top surface remains exposed. This differential drying shrinkage induces tensile stresses that eventually lead to transverse cracking. As deicing salts seep through the cracks, the reinforcement becomes corroded. This ultimately leads to bridges with lower lifespans that require costly rehabilitation.A case study was done in New Jersey by implementing 5 lbs per cubic yard of macro polypropylene fibers in the mix design for bridge decks. It improved transverse cracking frequency by reducing the number of cracks by 16.7% and reducing maximum crack width by 33.3%. However, the cracking frequency can still be improved by further implementing hybrid fibers into the mix design. The objective of this experiment is to improve these cracking frequencies by implementing hybrid combinations of fibers into the mix design. Of the hybrid mixes done in this study, supplementing 5 lbs per cubic yard of macro polypropylene fibers with 0.5 lbs per cubic yard of micro polypropylene fibers had favorable effects. The tensile strength was increased by 8.5% in comparison to macro fibers alone. In addition, free shrinkage was improved by 20.8%. Cracking frequencies were also surpassed in the AASHTO ring test: the number of cracks in the test was reduced by 14.6%, average microcrack width was reduced by 1.0% and the cracking area was reduced by 15.7%. In conclusion, it is proven that hybrid fiber-reinforced concrete performs better than single fiber-reinforced concrete.