Post-cracking Load Research Articles

Seismic performance of confined prestressed hollow core wall panels Part I: Experiment

Concrete prefabrication techniques have been popularized and utilized in building construction industry for years. To further improve the construction efficiency and quality for low-rise buildings, a confined prestressed hollow core wall system is proposed. The new system employs the standardized prestressed hollow cores that are commonly used for floors to be vertical structural elements to resist gravity and lateral loads. The panels are confined by a precast constructional frame that is composed of precast concrete ring beams and tie columns to improve its ductility and strength. A series of experimental tests were conducted to study the axial and in-plane/out-of-plane lateral behaviors of the new proposed system. The test included 19 specimens with various parameters of geometric properties, prestressing forces and binding details between panels and constructional frames. The experimental results showed that prestressed hollow core walls experienced shear-flexural behaviors at small lateral drifts and were controlled by a braced-frame deformation mode at large drifts with opening and splitting of horizontal and vertical joints between panels and frames. The confining frames were able to significantly increase the structural ductility, prevent hollow core units from collapse as well as maintain a considerable post-cracking load bearing capacity. The ultimate lateral drift of prestressed hollow core walls generally exceeded 1%–2%, and thus the structural system was preliminarily proved feasible for engineering practice. Numerical simulation and analysis of the proposed system is discussed in Part II of this study.

Experimental investigation on the blast resistance of framed PVB-laminated glass

PVB-laminated glass has been widely adopted for building windows as safety glass due to its significant post-crack load bearing capacity. To achieve a better understanding of the mechanical behaviour of framed PVB-laminated glass under explosion, a series of field blast tests were carefully designed and conducted aiming to study the dynamic responses and failure modes of framed PVB-laminated glass subjected to various blast loadings. The reflected overpressure and displacement time histories were measured in the tests. In particular, DIC technology was adopted to record the 3-D deflection field of the glass panel, which is essential for analyzing the dynamic response feature of laminated glass panels under blast loading. Three typical failure modes, i.e. global flexural failure, local punching failure and mixed failure, were identified and corresponding failure mechanisms were analyzed. Finally, the test results were used to check the effectiveness of different design approaches. The results from this study can provide reference for further blast resistant analysis and design for framed PVB-laminated glass.

Flexural behaviour and theoretical prediction of lightweight ferrocement composite beams

Sixteen full-scale simply supported composite beams of the same dimensions, breadth = 100 mm, thickness, 200 mm, and length of 2000 mm, subjected to flexural loading, were experimentally tested and their structural parameters, namely, pre-crack stiffness, serviceability loads, post-cracking loads, energy absorption, ultimate load-to weight ratio, and compressive strains were investigated. In addition, theoretical prediction of ultimate loads was carried out to adopt a theoretical approach as a design methodology for ferrocement elements. Experimental results showed that pre-crack stiffness, maximum service loads, maximum values of energy absorption and ultimate loads to weight ratios of ferrocement beams are higher than that of lightweight control beam by up to 46%, 32%, 64.4% and 32.8%, respectively. Higher post-cracking loads were exhibited by beams reinforced by expanded metal mesh, compared to those reinforced by welded wire mesh regardless of the core filling type. For post cracking load indicator, confinement with expanded metal mesh was the decisive factor affecting the post-cracking load capacity. Beams reinforced with expanded metal mesh showed higher energy absorption than those of the other beams reinforced with welded wire mesh or fiberglass mesh. Increasing the amount of mesh reinforcement results in higher energy absorption for beams made of Autoclaved Aerated Lightweight Brick Core (AAC). Generally, the maximum compressive strains of the ferrocement composite beams were generated at higher loads compared to those of the control beams. Theoretical calculations, based on the assumption of strains and forces distribution block, results in acceptable prediction of the ultimate loads. The ratio of experimental to theoretical ultimate loads ranges from 0.91 to 1.26. This study showed that ferrocement composite beams may be used as an alternative to traditional reinforced normal or lightweight concrete beams after careful choice of the combination of core and mesh types to suit the application in question.

3-D finite element modelling of prestressed hollow-core slabs strengthened with near surface mounted CFRP strips

A non-linear finite element model (FEM) was constructed using a three-dimensional software (ATENA-3D) to investigate the effect of strengthening on the behavior of prestressed hollow-core (PHC) slabs with or without openings. The slabs were strengthened using near surface mounted (NSM)-carbon fiber reinforced polymer (CFRP) strips. The constructed model was validated against experimental results that were previously reported by the authors. The validated FEM was then used to conduct an extensive parametric study to examine the influence of prestressing reinforcement ratio, compressive strength of concrete and strengthening reinforcement ratio on the behavior of such slabs. The FEM results showed good agreement with the experimental results where it captured the cracking, yielding, and ultimate loads as well as the mid-span deflection with a reasonable accuracy. Also, an overall enhancement in the structural performance of these slabs was achieved with an increase in prestressing reinforcement ratio, compressive strength of concrete, external reinforcement ratio. The presence of openings with different dimensions along the flexural or shear spans reduced significantly the capacity of the PHC slabs. However, strengthening these slabs with 2 and 4 (64 and 128 mm2 that represent reinforcement ratios of 0.046 and 0.092%) CFRP strips was successful in restoring the original strength of the slab and enhancing post-cracking stiffness and load carrying capacity.

DETERMINING THE RESIDUAL STRENGTH OF SFRC FLEXURAL MEMBERS / LENKIAMŲJŲ PLIENO PLAUŠU ARMUOTŲ BETONINIŲ ELEMENTŲ LIEKAMOJO STIPRIO NUSTATYMAS

The application of steel fibre reinforced concrete (SFRC) isconstantly increasing. However, due to the absence of universallyaccepted guidelines for SFRC, the application fields of this materialare still limited. Post-cracking load carrying capacity is aproperty of concrete most benefited by steel fibre reinforcement,usually designed by the residual strength of SFRC in tension, ffr.However, the quantification of ffr is one of the most critical issues.The research reported in this paper is dedicated to determining theresidual strength of SFRC in tension. The experimental results often notched beams with the fibre content of 0.5% and 1.0% byvolume subjected to a three-point loading scheme are presented.Considering SFRC as a homogeneous material, a mechanicallysound method for the inverse analysis of SFRC was proposed.The adequacy of the calculated results was validated using experimentaldata on standard flexural specimens. Computationaleffectiveness was proved employing derived residual strengthmodels into the nonlinear finite element analysis program. Santrauka Dispersiškai armuotas betonas vis dažniau naudojamas statybos pramonėje, deja, platesnį šios medžiagos taikymą riboja visiems priimtinų ir patikimų projektavimo normų stoka. Pagrindinė to priežastis – sudėtingas supleišėjusio dispersiškai armuoto tempiamojo betono elgseną apibūdinančio parametro, t. y. liekamųjų įtempių, nustatymas. Liekamieji įtempiai nusako elemento įtempius plyšyje, kuriuos plyšį varžantis plieno plaušas perduoda betonui. Šiame straipsnyje nagrinėjami plieno plaušu armuoti lenkiamieji betoniniai elementai. Aptarti dažniausiai naudojami supleišėjusio tempiamojo dispersiškai armuoto betono elgseną aprašantys modeliai. Pasiūlytas alternatyvus liekamųjų įtempių nustatymo būdas, naudojant standartinių sijų bandymų rezultatus. Nagrinėtų skaičiavimo modelių adekvatumas tikrintas baigtinių elementų programa ATENA. Apskaičiuotos sijų apkrovos ir plyšio pločio priklausomybės palygintos su eksperimentinėmis, gautomis lenkiant plieno plaušu armuotas sijas viena koncentruota apkrova.

Flexural response of hybrid carbon fiber thin cement composites

It is evident that the carbon-fiber-reinforced cementitious composites are being used in the structural and construction works owing to the synergetic action from two components viz. fiber and mortar matrix. Incorporation of a very nominal percentage of carbon fibers into a mortar mixture produces a strong and durable composite that leads the product of smart material properties. Flexural behavior of cement-based matrices carrying carbon fibers reinforcement of different percentage and size is studied in this paper. Influence of fiber content and length of the fiber is quantified using load–deflection curves. Specimens containing fiber of 0.0, 0.5, 1.0 and 1.5% with 3 mm (0.12 in.), 6 mm (0.36 in.), and their combination are prepared and tested. It is demonstrated that combination of 3 mm (0.12 in.) and 6 mm (0.36 in.) fibers enhances the bearing capacity to crack- and ultimate-stresses as well as the Young’s modulus of the fiber reinforced cement composites. The paper emphasizes the desired performances after the initiation of cracks and discusses the pre- and post-cracking load–deflection characteristics of the composites.

The Effect of Radial Crack Locations on Load Resistance in C1550 Panel Tests

Abstract ASTM C1550 has been used for post-crack performance assessment of fiber reinforced concrete (FRC) and shotcrete (FRS) for several years and has proven to be an excellent tool for quality control testing due to the very low level of within-batch performance variability typically obtained using this test method. Experience in the field indicates that the positions of the three radial cracks generated in a test vary between specimens and concerns have been raised by users of this test that specimen performance may change as a result of this variation in crack location. The significance of this aspect of specimen behavior has been investigated to determine the characteristic range of crack rotations experienced in C1550 specimens as a result of variations in crack location and the magnitude of their corresponding influence on post-crack load resistance.

Durability of date palm fibres and their use as reinforcement in hot dry climates

Abstract This paper concerns the durability of male date palm surface fibres (MDPSF) immersed in three alkaline solutions: calcium hydroxide, sodium hydroxide and Lawrence solution with a pH ranging between 12.5 and 12.95. In addition the durability of MDPSF as reinforcement in small scale concrete beams has been studied. The concrete specimens were cured for six months in two different ways: 28 days in water followed by curing in wet chamber; and 14 days in water followed by a hot dry environment. The durability of MDPSF is evaluated by the loss of tensile strength and elongation, occurred over time, of fibres submitted in alkaline solutions using the tension test. The durability of the fibre-reinforced concrete is estimated by the variation of the first cracking strength, maximum post-cracking load and the maximum load ratio obtained from bending test. Some scanning electron microscopy (SEM) images of fibres extracted from alkaline solutions, as well as from the matrix at the fibre–matrix interface, have been examined to characterize the microstructure of the fibres. The results of the investigation have shown that MDPSF are easily attacked by alkaline solution under both curing conditions. Hence the durability of MDPSF-reinforced concrete is poor.

Crack Offset Corrections to Post-crack Performance Parameters Obtained from Third-point Loaded Fiber Reinforced Shotcrete Beams

Abstract Fiber Reinforced Shotcrete (FRS) is used as an economically competitive method of ground support in tunnel excavations throughout the world. Fibres are incorporated into this material to impart post-crack load capacity. Beam tests are commonly used to assess the post-crack performance of FRS but high within-set variability has hindered the usefulness of beam test data. A method of correcting performance data to account for the offset between the position of the crack and the center of the beam has been suggested as a means of improving within-batch variability for post-crack performance parameters obtained from third-point loaded beam tests. In this paper, an offset correction method has been applied to a large set of beam test data obtained as part of Quality Control testing for a tunnel-lining project to determine whether the method is effective in reducing within-batch variability.

Behaviour of round steel fibre reinforced concrete panels under point loads

The objective of this investigation was to examine methods of testing round Steel Fibre Reinforced Concrete (SFRC) panels to obtain empirical estimates of post-crack load capacity under point loads. The performance of steel fibre reinforced concrete panels under point loads is of interest because such tests are commonly used to determine the suitability of a concrete mix for use in sprayed tunnel lining construction. This investigation has focussed on experimental obstacles to the reliable determination of post-crack load capacity. To examine possible differences in behaviour between continuousin situ tunnel linings and specimen panels representing portions of a lining, four modes of edge support were used to restrain round specimens that varied in both thickness and diameter. Determinate and simple support conditions were examined as possible modes of panel support for routine laboratory testing. Fully clamped and quasi-continuous edge restrained panels were used as approximations of anin situ lining. Performance was measured in terms of peak load capacity and energy absorption.

Service, Post-Cracking, and Ultimate Response of Continuous, Post-Tensioned Voided Slab Bridges

Two quarter-scale continuous, voided slab bridge models post-tensioned longitudinally and transversely were tested to determine their service, post-cracking and ultimate responses under simulated AASHTO truck loading. The models were designed to comply with current practice and their response is indicative of bridges currently in use. Eleven tests were conducted to investigate the elastic response. The response was linear and the applicability of superposition confirmed. The applied post-cracking load was between 1.67 and 2.0 times the cracking load. Although the response was non-linear there was negligible residual deflection. The ultimate load tests indicated great ductility and reserve strength with ultimate moment capacities far in excess of designed values.