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Specialized fiber reinforced concretes under static and impact loading Xu, Hanfeng

Abstract

Fibers are increasingly being added to cementitious materials to achieve pseudoductility for a variety of structures. Unfortunately, there is no universally accepted test method to characterize the energy absorption capacity (toughness) of fiber reinforced concrete (FRC). Traditional beam flexural tests provide toughness assessment only under small deflections. This has not reflected the reality of most FRCs applied in slab-on-grade and tunnel/mine projects, where large deflection could be experienced. A new test method has been developed for the concrete industry: ASTM C-1550, "Standard Test Method for Flexural Toughness of Fiber Reinforced Concrete (Using Centrally Loaded Round Panel)", referred to as the RDP method. So far this technique has only been applied for static loading, and none of the previous studies have addressed the effect of high loading rates on the performance of these round panels. A unique test setup was developed in this work to determine the behaviour of round panels under impact loading. The initial load (used for accelerating the specimen) was analyzed and deducted from the total measured load to obtain the true load. Deflections up to 65 mm were achieved. It was found that it is feasible to use the RDP method to characterize fiber reinforced concrete and welded wire mesh (WWM) reinforced concrete; the tests provided data on the effect of loading rate and concrete strength on the behaviour of a reinforced concrete system. Overall, more than 260 round panels were tested for the above purposes. In this study, two polymeric fibers and one steel fiber, and three matrix strength levels were investigated to examine the influence of fiber material, hybrid reinforcement with WWM, and the matrix strength under static and impact loading. Results show that high strength matrices have negative effects on the toughening effect of fibers. Under impact loading, this tendency is more significant. Panels with hybrid reinforcement exhibited more favourable behaviour than panels containing only a single type of reinforcement. For FRC with polymer modification (PM-FRC), it was found that the toughness of FRC panels due to polymers additions were improved much more significantly under static loading than under impact loading. In addition to RDP specimens, the performance of PM-FRC was also investigated under compressive impact loading. Similar findings to those in the round panel test were noted, though there were differences in strain rate sensitivity. The damage evolution of hybrid reinforced panels was evaluated by using post-impact static testing; three damage indices were defined based on peak load, stiffness and toughness reduction. It was found that damage defined on the basis of residual toughness is much more meaningful and could be used as a standard index for evaluating hybrid reinforced systems. This necessitated modifying the existing continuous damage theory for concrete, which was mainly confined to the strength parameter or elastic modulus, and was applicable only in the elastic domain. To further understand the deformation of round panels containing fibers, for the first time a unique setup, on the basis of the current RDP method, was designed to measure the rotation and side slip of panel segments after cracking. Thereafter, the central crack width (CCMOD) could be obtained for further analysis. This study proved the importance of other parameters: rotation and side slip of broken segments other than the central deflection in the current standard, which could be used to better interpret the RDP test. In the analytical part of this work, Yield Line Theory (YLT) incorporating the CCMOD concept and friction effect analysis was used to predict the performance of RDP specimens from the moment capacity vs. rotation of FRC beams. Cast beams and panels of the same type of FRC were used to assist in validating the analytical approach for this model study. Experimental results have shown good agreement with those predicted from beam studies.

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