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Showing 1-5 of 976 Abstracts search results

Document: 

SP-343_07

Date: 

October 1, 2020

Author(s):

Javadian, A.; Mahdavi, A.; Benamrane, O. ;Majeed, M.; Aoude, H.

Publication:

Symposium Papers

Volume:

343

Abstract:

This study examines the effect of fiber properties, single fiber type and hybrid fibers on the fresh-state and hardened-state properties of self-consolidating fiber-reinforced concrete (SCFRC). As part of the study, 16 mixtures are examined with variables including the effect of fiber type, length, aspect ratio, and hybrid use of fibers (short and long fibers). Properties in the fresh state are studied using standard SCC tests including: slump flow, V-funnel and visual stability index (VSI) tests. Mechanical properties are studied by testing prisms under four-point flexural loading in accordance with the ASTM C1609 standard. The results demonstrate that self-consolidating FRC mixtures are possible at moderate fiber contents, however, once the limiting fiber contents are exceeded workability and mix uniformity are lost. The results also show the effects of fiber content, fiber type, fiber properties and hybrid fibers on the flexural toughness of SCFRC.


Document: 

SP-343_01

Date: 

October 1, 2020

Author(s):

Plückelmann, S. ;Breitenbücher, R.

Publication:

Symposium Papers

Volume:

343

Abstract:

According to the actual European standard EN 14487-1 the potential of steel fiber reinforced sprayed concrete is characterized by flexural strength tests (first peak, ultimate and residual). In most cases, this is performed by a four-point bending test on beam specimens, specified in EN 14488-3. As an alternative test method, a three-point bending test on square panels with notch is recommended by EFNARC. It is argued as main benefit of the latter test method, that the geometry and dimensions of the panels are equal to those of specimens used for measuring the energy absorption capacity according to EN 14488-5. Hence, the specification of the ductility of fiber reinforced concretes according to EN 14487-1 in terms of residual strength and energy absorption capacity can be achieved preparing only one type of specimen. Furthermore, the EFNARC guideline points out a smaller scatter of test results, compared to the beam tests according to EN 14488-3. Before the EFNARC method will be considered in EN 14487-1, the relevant CEN TC 104/WG10 requests for adequate proofs. These were performed within a round-robin test (RRT) on testing the flexural behavior of steel fiber reinforced sprayed concrete by the standardized EN 14488-3 method as well as by the proposed EFNARC method. The aim of this RRT was to investigate the comparability and correlation between the two test methods. Furthermore, the scatter of both methods was assessed. The RRT has been organized by Ruhr University Bochum, in whose labs the steel fiber reinforced sprayed concrete specimens (beams according to EN 14488-3 and square panels according to EFNARC guideline) were produced using a robotic spraying machine. The specimens were then tested in five independent laboratories Europe-wide. The results of this RRT are presented in detail. With regard to the residual strength, the relevant material parameter for steel fiber reinforced sprayed concrete, a tendency towards a slightly lower scatter was detected for the EFNARC test method.


Document: 

SP-343_18

Date: 

October 1, 2020

Author(s):

Yao, Y.; Bakhshi, M.; Nasri, V.; Mobasher, B.

Publication:

Symposium Papers

Volume:

343

Abstract:

Precast concrete segments are the predominant support method used in tunnels dug by Tunnel Boring Machines (TBM) in soft ground and weak fractured rock, providing the initial and final ground support. Conventionally, steel bars are used in concrete segments to resist tensile stresses due to all loading cases from the time of casting through service condition. With traditional reinforcement, a significant amount of time and labor are needed to assemble the cages and place the reinforcing bars. Fiber reinforced concrete (FRC) has become more attractive for its use in tunnel lining construction as a result of improved post-cracking performance, crack control characteristics and capability of partial replacement of steel bars. Due to the strength requirements in large-diameter tunnels, which are subjected to embedment loads and TBM thrust jack forces, the use of FRC is not adequate as the sole reinforcing mechanism. Therefore, the hybrid fiber-reinforced concrete (HRC) combining both rebars and steel fibers is frequently used in practice. Tunnel segmental linings are designed for load cases that occur during manufacturing, transportation, installation, and service conditions. With the exception of two load cases of TBM thrust jack forces and longitudinal joint bursting load, segments are subjected to combined axial force and bending moment. Therefore, P-M interaction diagrams have been used as the main design tool for tunnel engineers. Standard FRC constitutive laws recently allow for a significant residual strength in tension zone below the neutral axis. However, design capacity of HRC segment is significantly underestimated using conventional Whitney’s rectangular stress block method, especially for tension-controlled failure, since the contribution of fibers in tension zone is ignored. Methods that currently incorporate contribution of fibers on P-M diagrams are based on numerical and finite-element analyses, which are normally more complicated and not readily to be implemented for practical design tools. Closed-form solutions of full-range P-M interaction diagram considering both rebar and fiber contributions are presented in this paper for HRC segments. The proposed model is verified with experimental data of compression tests with eccentricity as well as other numerical models for various cases of HRC sections. Results show that using appropriate material models for fiber and reinforcing bar, engineers can use the proposed methodology to obtain P-M interaction diagrams for HRC tunnel segments.


Document: 

SP-343_31

Date: 

October 1, 2020

Author(s):

Poveda, E.; Ruiz, G.; Cifuentes, H.; Yu, R.C.; Zhang, X.X.

Publication:

Symposium Papers

Volume:

343

Abstract:

This work proposes a new strain-based failure criterion for compression fatigue in steelfiber reinforced concrete. It is based on the Spark and Menzies’ relationship between the logarithm of the secondary strain rate per cycle and the specimen life expressed as the logarithm of the number of cycles until failure. This relationship permits calculating the critical strain at the failure of the specimen as the sum of two terms. The first one is the maximum strain in the first cycle due to the maximum compression stress. The second term is the increase of strain due to the remaining cycles until failure. Thus, failure occurs when the strain reaches a critical level during fatigue loading. On the contrary, the material continues resisting while its accumulated strain is lower than the critical one. This criterion is validated against a series of low-cycle fatigue tests in five types of concrete with different amounts of fiber that share the same concrete matrix. Besides, the experimental results show that the fibers delay the deformation and deterioration processes caused by fatigue. They also show that there is an optimum fiber content that maximizes fatigue life.


Document: 

SP-343_43

Date: 

October 1, 2020

Author(s):

Plückelmann, S.; Breitenbücher, R.

Publication:

Symposium Papers

Volume:

343

Abstract:

In special cases, concrete members are exposed to high locally concentrated loadings. Such concentrated loadings lead to a multi-dimensional stress state beneath the loaded area. Due to the load diffusion, large splitting tensile stresses are generated in the upper regions of the concrete member (i.e. St. Venant disturbance zone) and spread along directions perpendicular to the load. In order to resist these splitting tensile stresses, the state of the art is to reinforce concrete members with transverse steel reinforcement. An alternative approach is to add steel fibers to the concrete matrix. However, regarding economic concerns it may not appropriate to reinforce the entire concrete member with an adequate high amount of steel fibers, rather only those zones where high splitting stresses are expected. The main objective of the presented experimental study was to investigate the load-bearing and fracture behavior of hybrid concrete elements with splitting fiber reinforcement under concentrated load. For this purpose, in a first step, hybrid specimens were produced containing both plain and fiber concretes. The reference specimens consisted exclusively of plain concrete, while the hybrid specimens were partially strengthened with various types of steel fibers only in the St. Venant disturbance zone, instead of a full range fiber reinforcement. The thickness of the reinforcement layer was varied in order to determine the optimal configuration of fiber reinforcement. Taking into account the influence of the casting direction on the fiber orientation and consequently on the bearing and fracture behavior, the hybrid specimens were cast either in standing or in lying molds by means of a “wet-on-wet” casting technique. These hybrid elements were then tested under concentrated load. The test results showed that under concentric loads the maximum bearing capacity of the hybrid specimens increased progressively with growing thickness of the fiber reinforced concrete layer. In contrast to the plain concrete specimens, the fiber reinforcement led to a remarkable improvement in the post-cracking ductility. Compared to the fully reinforced specimens, the hybrid specimens that were only reinforced in the St. Venant disturbance zone exhibited - besides an almost identical bearing capacity - a similar local behavior in the postcracking zone. Furthermore, a significant impact of the casting direction on the bearing as well as fracture behavior could be proved.


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