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Document: 

SP347

Date: 

March 15, 2021

Publication:

Symposium Papers

Volume:

347

Abstract:

Sponsors: Sponsored by ACI 370 Committee Editors: Eric Jacques and Mi G. Chorzepa This Symposium Volume reports on the latest developments in the field of high strain rate mechanics and behavior of concrete subject to impact loads. This effort supports the mission of ACI Committee 370 “Blast and Impact Load Effects” to develop and disseminate information on the design of concrete structures subjected to impact, as well as blast and other short-duration dynamic loads. Concrete structures can potentially be exposed to accidental and malicious impact loads during their lifetimes, including those caused by ballistic projectiles, vehicular collision, impact of debris set in motion after an explosion, falling objects during construction and floating objects during tsunamis and storm surges. Assessing the performance of concrete structures to implement cost-effective and structurally-efficient protective measures against these extreme impacting loads necessitates a fundamental understanding of the high strain rate behavior of the constituent materials and of the characteristics of the local response modes activated during the event. This volume presents fourteen papers which provide the reader with deep insight into the state-of-the-art experimental research and cutting-edge computational approaches for concrete materials and structures subject to impact loading. Invited contributions were received from international experts from Australia, Canada, China, Czech Republic, Germany, South Korea, Switzerland, and the United States. The technical papers cover a range of cementitious materials, including high strength and ultra-high strength materials, reactive powder concrete, fiber-reinforced concrete, and externally bonded cementitious layers and other coatings. The papers were to be presented during two technical sessions scheduled for the ACI Spring 2020 Convention in Rosemont, Illinois, but the worldwide COVID-19 pandemic disrupted those plans. The editors thank the authors for their outstanding efforts to showcase their most current research work with the concrete community, and for their assistance, cooperation, and valuable contributions throughout the entire publication process. The editors also thank the members of ACI Committee 370, the reviewers, and the ACI staff for their generous support and encouragement throughout the preparation of this volume.


Document: 

SP-347_02

Date: 

March 1, 2021

Author(s):

Jonathan Harman, Emmanuella O. Atunbi, and Alan Lloyd

Publication:

Symposium Papers

Volume:

347

Abstract:

Many common building materials, such as concrete and steel, are understood to experience a change in apparent material properties under high strain rates. This effect is often incorporated into impact and blast design by using dynamic increase factors (DIFs) that modify properties of the material such as strength and stiffness when subjected to high strain rates. There is currently limited guidance on dynamic properties of fiber reinforced polymer (FRP) sheets bonded to concrete. Since FRP is a common retrofit material for blast and impact load vulnerable structures, it is important to have a full understanding on the behaviour of the FRP material and of the composite action between the FRP sheet and the substrate it is bonded to. Important parameters for blast and impact resistant design of reinforced concrete structures retrofitted with surface bonded FRP include dynamic measures of debonding strain, development length, and bond stress. This paper presents the results of an experimental program measuring the dynamic properties of carbon fiber reinforced polymer (CFRP) sheets bonded to concrete under impact induced high strain rates.

A series of rectangular concrete prisms were cast and fitted with surface bonded CFRP sheets to facilitate pull-out shear tests that directly measure the FRP to concrete bond. The bonded length of the CFRP sheet was variable with three different lengths explored. A series of static tests have been conducted to measure the strain fields on the FRP sheets under load up to failure. These strain fields, which were measured with digital image correlation techniques, were used to determine development length, bond stress, and ultimate strain of the FRP sheet prior to debonding. A companion set of prisms have also been cast and will be tested under impact loading to explore the same properties at high strain rates of around 1 s-1. Initial test results indicate a potential increase in both ultimate strain and bond stress, and a decrease in development length under high strain rates. The results of the larger study will be compiled and, when compared with the static companion set, be used to propose DIFs for FRP sheets bonded to concrete for use in design in high strain rate applications.

However, the main constitutive phases of SHCC, i.e. matrix, fibers and interphase between them, are highly rate sensitive. Depending on the SHCC composition, the increase in loading rates can negatively alter the balanced micromechanical interactions, leading to a pronounced reduction in strain capacity. Thus, there is need for a detailed investigation of the strain rate sensitivity of SHCC at different levels of observation for enabling a targeted material design with respect to high loading rates.

The crack opening behavior is an essential material parameter for SHCC, since it defines to a large extent the tensile properties of the composite. In the paper at hand, the rate effects on the crack opening and fracture behavior of SHCC are analyzed based on quasi-static and impact tensile tests on notched specimens made of three different types of SHCC. Two SHCC consisted of a normal-strength cementitious matrix and were reinforced with polyvinyl-alcohol (PVA) and ultra-high molecular weight polyethylene (UHMWPE) fibers, respectively. The third type consisted of a high-strength cementitious matrix and UHMWPE fibers. The dynamic tests were performed in a split Hopkinson tension bar and enabled an accurate description of the crack opening behavior in terms of force-displacement relationships at displacement rates of up to 6 m/s (19.7 ft/s).


Document: 

SP-345_14

Date: 

February 1, 2021

Author(s):

Angelo Savio Calabrese, Tommaso D’Antino, Pierluigi Colombi, Carlo Poggi, and Christian Carloni

Publication:

Symposium Papers

Volume:

345

Abstract:

Externally bonded fiber-reinforced cementitious matrix (FRCM) composites are applied to the tension side of reinforced concrete (RC) beams to increase their flexural strength. Composite action is often prematurely lost because of the debonding of the composite, which for most of the available FRCMs occurs at the matrix-fiber interface. The bond behavior is studied at the small-scale by means of single- and double-lap direct shear tests. An alternative small-scale test configuration is the beam test. Beam tests can be performed using a single notched prism with a composite strip attached to the face where the notch is located (notched beam test) or by two prisms connected by a cylindrical hinge on one side and by a composite strip on the opposite side (modified beam test). As the scientific community is discussing the best test configuration, the goal of this paper is to shed light on the differences between the two test methods. In this paper, an FRCM composite comprising polyparaphenylene benzo-bisoxazole (PBO) fibers, which exhibits debonding at the matrix-fiber interface, is subjected to single-lap shear and modified beam tests. Load responses and failure modes are compared in an attempt to provide guidance on the selection of the test method.


Document: 

SP-345_7

Date: 

February 1, 2021

Author(s):

Houman Hadad, Davide Campanini, and Antonio Nanni

Publication:

Symposium Papers

Volume:

345

Abstract:

Fabric Reinforced Cementitious Matrix (FRCM) is an established technology for strengthening and rehabilitation of existing concrete and masonry structures. In the United States, material characterization of the FRCM composites is in accordance with ICC-ES acceptance criteria AC434. The acceptance criteria recommend tensile testing the FRCM coupons with clevis-grips to obtain the mechanical properties for design purposes. The current test method, however, neglects some of the critical factors affecting the test outcome such as the effect of bonded length or number of fabric layers. The effect of bonded length on the FRCM properties tested per AC434 Annex-A is discussed in this paper. Carbon-FRCM coupons of 2, 3, 6, 9, and 12 inches (50.8, 76.2, 152.4, 228.6, and 304.8 mm) bonded length were prepared and tested in direct tension. The other test variable was the number of fabric layers. The tests were conducted with one- and two-layer fabrics for different bonded length. The results discussed in terms of ultimate stress, ultimate strain, and modulus show that the material characterization of the FRCM composites depended on the bonded length and number of fabric layers of the tested specimens. Moreover, the effect of number of fabric layers on the material characteristics was more pronounced in specimens with shorter bonded length. The experimental results are used to make suggestions for improving the FRCM characterization test methods as currently stated in AC434.


Document: 

SP-345_06

Date: 

February 1, 2021

Author(s):

Marco Carlo Rampini, Giulio Zani, Matteo Colombo and Marco di Prisco

Publication:

Symposium Papers

Volume:

345

Abstract:

Fabric-reinforced cementitious matrix (FRCM) composites are promising structural materials representing the extension of textile reinforced concrete (TRC) technology to repairing applications. Recent experiences have proven the ability of FRCMs to increase the mechanical performances of existing elements, ensuring economic and environmental sustainability. Since FRCM composites are generally employed in the form of thin externally bonded layers, one of the main advantages is the ability to improve the overall energy absorption capacity, weakly impacting the structural dead weights and the structural stiffness and, as a direct consequence, the inertial force distributions activated by seismic events. In the framework of new regulatory initiatives, the paper aims at proposing simplified numerical approaches for the structural design of retrofitting interventions on existing reinforced concrete structures. To this purpose, the research is addressed at two main levels: i) the material level is investigated on the uniaxial tensile response of FRCM composites, modeled by means of well-established numerical approaches; and ii) the macro-scale level is evaluated and modeled on a double edge wedge splitting (DEWS) specimen, consisting of an under-reinforced concrete substrate retrofitted with two outer FRCM composites. This novel experimental technique, originally introduced to investigate the fracture behavior of fiber-reinforced concrete, allows transferring substrate tensile stresses to the retrofitting layers by means of the sole chemo-mechanical adhesion, allowing to investigate the FRCM delamination and cracking phenomena occurring in the notched ligament zone. It is believed that the analysis of the experimental results, assisted by simplified and advanced non-linear numerical approaches, may represent an effective starting point for the derivation of robust design-oriented models.


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