Abstract
Research studies in the last 20 years allowed to obtain reliable rules for designing structures made of fiber reinforced concrete (FRC). However, design aspects like the long-term behavior of FRC, especially when synthetic fibers are adopted, require further research. Long-term behavior includes aging and creep. Aging represent the change of fiber properties into the concrete environment, which may reduce the structural bearing capacity; when present, it is an important issue for the structural safety, especially when fibers are the only reinforcement. Aging of fibers must be proven by experimental tests. Creep is a complex phenomenon, roughly considered by building codes even for traditional reinforced concrete (RC) structures. The introduction of fibers do not change anything in concrete matrix and, before cracking, in the material concrete creep behavior is not expected any change. After cracking, the structural effect of FRC creep depends on the degree of structural redundancy and on the presence of rebars since creep produces a stress redistribution in the structure or from FRC to the rebars. When FRC post-cracking resistance is necessary for equilibrium requirements, in structures with cracked sections in service conditions the structural deferred response has to be analyzed by considering the FRC creep behavior. When FRC is used for resisting secondary actions and rebars are present for equilibrium requirements, the response of a FRC element (with rebars and fibers) will be identical to a conventional RC; FRC contributes by controlling the crack development under both short and long term loading.
Highlights
Fiber reinforced concrete (FRC) is entering extensively into the market due to the availability of structural codes as the fib Model Code 2010 (MC2010) [1], the German Code [2], the Italian Code [3] the ACI 318 [4] and incoming codes under development in several Countries, including the Eurocode 2 [5]
Fiber reinforcement effects in structural elements could be affected by long-term behavior of FRC related to fiber aging and creep behavior of FRC
Creep behavior of FRC may represent in principle a major concern, real FRC structures of practical interest nowadays are not affected by FRC creep
Summary
Fiber reinforced concrete (FRC) is entering extensively into the market due to the availability of structural codes as the fib Model Code 2010 (MC2010) [1], the German Code [2], the Italian Code [3] the ACI 318 [4] and incoming codes under development in several Countries, including the Eurocode 2 [5]. There are the fiber efficiency, due to both orientation and distribution, as well as the long-term behavior of FRC [9, 10] The b coefficient can be either 0.5 or 1 for short or long term loading, respectively Another case were creep is considered in design is related to the crack width, where another (but different) b parameter is included (see Eq 7.6-6 of MC2010), with low consideration of influencing parameters. No more creep issues are mentioned in structural codes This limited presence of creep in building codes for reinforced concrete (RC) elements (without fibers) is related to the very complex phenomena that make creep very difficult to model and, especially, hardly predictable in current structures where environmental conditions are not known a priori and continuously change. The use of fibers for different reasons as fire (spalling), shrinkage cracking, aesthetic aspects or other not structural considerations are out of the scope of this paper
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