ABOUT THE INTERNATIONAL CONCRETE ABSTRACTS PORTAL

  • The International Concrete Abstracts Portal is an ACI led collaboration with leading technical organizations from within the international concrete industry and offers the most comprehensive collection of published concrete abstracts.

International Concrete Abstracts Portal

Showing 1-5 of 248 Abstracts search results

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_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.


Document: 

SP-343_12

Date: 

October 1, 2020

Author(s):

Barros, J.A.O.; Foster, S.J.

Publication:

Symposium Papers

Volume:

343

Abstract:

For the development of reliable physical-mechanical models for predicting the behaviour of fibre reinforced concrete structures at service and strength limit conditions, constitutive models simulating comprehensibly the governing phenomena must be used. In this context, simulating the post-cracking mechanisms of the fibres, and their symbiotic relationship with the cementitious matrix that surrounds them, is required for the development of realistic modelling approaches that accurately represent empirical observations. Several experimental test setups and inverse analysis procedures have been proposed to derive the fundamental stress-crack width ( –w) law, but a consensus still does not exists on the best strategy for its determination. In structures governed by shear, fibre reinforcement increases the stiffness and shear stress transfer across a crack, but a methodology to capture the contribution of fibres in this regards is challenging. To overcome this, a clear strategy is needed in deriving relationships that simulate fibre reinforcement mechanisms in the mobilized fracture modes and, also, develop design approaches capable of capturing the relevant contributions of the fibres. This study firstly reviews current inverse analysis models used to describe the tensile (Model I fracture) relationship for FRC and, secondly, discusses a newly proposed model, referred to as the integrated shear model (ISM). The ISM is developed from mesoscale observations from gamma- and X-ray imaging on FRC elements under Modes I and II fracture conditions. The resulting model is compared to test data reported in the literature and a good correlation is observed.


Document: 

SP-343_05

Date: 

October 1, 2020

Author(s):

Galeote, E.; Blanco, A.; Cavalaro, S.H.P.; de la Fuente, A.

Publication:

Symposium Papers

Volume:

343

Abstract:

The influence of size effect becomes an issue particularly relevant during the characterization stage of concrete at laboratory scale. The dimensions of the specimens have a direct influence on the strength measured and, therefore, selecting an appropriate specimen size is of utmost importance in order to obtain representative results to be used in the design of a structure. The notched beam test to characterize FRC is among the most extended methods to determine the design parameters of the residual strength, mainly associated to crack openings of 0.5 mm and 2.5 mm (fR1 and fR3, respectively). The main objective of this study was to determine the influence of the beam dimensions on these two parameters. For this, an analytical model able to reproduce the flexural behavior of FRC was developed. Additionally, an experimental program involving high performance fibre reinforced concrete (HPFRC) beams of dimensions 40x40x160, 100x100x400 and 150x150x600 mm was conducted. The parameters of the analytical model were calibrated for each specimen dimension using the results of the experimental program. The results show a clear influence of the size of the specimen, mainly attributed to the area of fracture and the crack crosssection.


Document: 

SP-339_10

Date: 

March 1, 2020

Author(s):

John S. Ma

Publication:

Symposium Papers

Volume:

339

Abstract:

The U.S. Nuclear Regulatory Commission (NRC) defines seismic Category 1 structures as the structures (buildings) that should be designed and built to withstand the maximum potential earthquake stresses for the particular region where a nuclear plant is sited. Seismic Category 1 structures have been designed for ground-shaking intensity associated with a safe-shutdown earthquake (SSE) – the intensity of the ground motion that will trigger the process of automatic shutdown of the reactor in operation. The SSE generates floor response spectra at different floor elevations in a building, and these spectra and their associated forces are used for the design of piping and piping anchors and equipment and equipment anchors at their floor locations. The NRC policy requires that the seismic Category 1 structures whose collapse could cause early or/and large release of radioactive materials into the atmosphere to be analyzed/designed for “no collapse” during the ground-shaking intensity of a review-level earthquake (RLE), which is 1.67 times that of an SSE. Most seismic Category 1 concrete structures, such as containment and shield buildings (curved cylindrical wall; see Figs. 1 and 2 in the next section) and containment internal structures (straight wall; see Fig. 1), use walls to resist earthquakes. This paper presents guidelines for the performance-based seismic design for these wall-typed structures that could meet the NRC policy. The method consists of (1) proportioning wall thickness based on shear stress of 6√fc’ (0.5√fc’ megapascals (MPa)) generated by SSE ground motions, (2) limiting vertical compressive stress in walls to less than 0.35 fc’, (3) providing minimum percentage of reinforcement of 1.0 percent to prevent steel reinforcing bar fracture, (4) subjecting the building design to nonlinear dynamic response analyses under RLE ground motions, (5) identifying any members and their connections in the building that have failed or collapsed during the RLE ground motions, (6) increasing reinforcement or wall thickness, or both, to provide additional strength or/and ductility for the failed or collapsed members and their connections, and (7) resubjecting the revised building design to the nonlinear dynamic response analyses as stated in step (4) until no collapse of the building and its members and their connections. This performance-based seismic design method is a direct, transparent, and scientific answer to whether these important seismic Category 1 structures meet the NRC’s policy that they will not collapse during the RLE ground motions. Examples of using the nonlinear dynamic response analyses are cited and described. Guidelines for the performance-based seismic design of seismic Category 1 concrete Structures are listed at the end of this paper.


12345...>>

Results Per Page 




Please enter this 5 digit unlock code on the web page.