• International Journal of Highway Engineering
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• v.16 no.2
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• pp.91-98
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• 2014

PURPOSES : The objective of this study is to investigate the effect of asphalt and geotextile interlayer on the fracture behavior of unbonded concrete overlay through a laboratory composite beam test. METHODS : In order to evaluate the effect of interlayer materials on the fracture behavior of unbonded concrete overlay, a laboratory test of composite beam was conducted with different types of interlayer. The test results of the composite beam using two types of geotextile interlayer with different thicknesses were compared to the test results of the composite beam using the tradition type of asphalt interlayer. The unbonded concrete overlay on the existing concrete pavement without interlayer was set for the control condition. RESULTS AND CONCLUSION : Overall, the laboratory composite beam test results did show the effect of asphalt and geotextile interlayer on the fracture behavior of composite concrete beams. The three-layer geotextile interlayer and HMA layer both increase the peak load when the first macrocrack occurs in the top concrete beam, while the HMA interlayer causes the smallest load drop percentage after the first macrocrack. The three-layer geotextile did show better performance than the single-layer geotextile through the greater peak load and smaller load drop percentage. It indicates that the thickness of geotextile interlayer will affect the fracture behavior of unbonded concrete overlay and the thicker geotextile interlayer is recommended.

• Journal of the Korean Society of Safety
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• v.27 no.5
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• pp.141-147
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• 2012

Recently, the external prestressed concrete structures are increasingly being built. The mechanical behavior of prestressed concrete beams with external tendon is different from that of normal bonded PSC beams in that the increment of tendon stress was derived by whole member behavior. By this reason, the ultimate stress of external tendon is smaller than that of bonded tendon or internal unbonded tendon. The purposes of the present paper are therefore to improve the mechanical behavior of external unbonded tendon by using partially bonded external tendon and to evaluate the flexural behavior of partially bonded external tendon by the flexural member experiment. From the experimental results, before flexural cracking, there was no difference between external unbonded, partially bonded and bonded tendons. However, after cracking, yielding load of reinforcement, ultimate load, and tendon stress were increased in the sequence of external unbonded, partially bonded and bonded tendon members. The equation of ACI-318 and AASHTO 1994 were not matched with test results and had no correlations. So the newly proposed equation will be needed including the consideration of tendon profile, tendon bonded type, and so on. The proposed partially bonded external tendon in this paper will be a effective basis for the evaluation of external tendons in construction and design.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2011.05b
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• pp.91-96
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• 2011

The flat slab is well-known as a structural system to reduce the story height, so it is broadly used for recent building. However, the normal RC flat slab is not appropriate for the long span, and the quantity of reinforcement bar and concrete is raised. Recently, the post-tensioning system has been introduced and used widely as an alternative method. Nevertheless, in Korea, it is not used broadly due to lack of the understanding and field experience. Especially, the post-tensioning system is hesitated to use due to uncertainty of construction ability and economics. This paper will introduce applicability to site and economics of unbonded post-tensioning system through construction examples.

• International journal of steel structures
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• v.18 no.4
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• pp.1336-1349
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• 2018

Generally, a precast prestressed concrete (PSC) beam is used as girders for short-to-medium span (less than 30 m) bridges due to the advantages of simple design and construction, reduction of construction budget, maintenance convenience. In order to increase the span length beyond 50 m of precast PSC girder, PSC hollow box girder with unbonded prestressed H-type steel beam placed at the compressive region is proposed. The unbonded compressive prestressing in the H-type steel beams in the girder is made to recover plastic deflection of PSC girder when the pre-stressing is released. Also, the H-steel beams allow minimization of depth-to-length ratio of the girder by reducing the compressive region of the cross-section, thereby reducing the weight of the girder. A quasi-static 3-point bending test with 4 different loading steps is performed to verify safety and plastic deflection recovery of the girder. The experimental results showed that the maximum applied load exceeded the maximum design load and most of the plastic deflection was recovered when the compressive prestressing of H-type steel beams is released. Also using prestressed H-type steel as compression reinforcements in the upper part of cross section, repair and restoration difficulty and cost of PSC girders should be significantly reduced. The study result and analysis are discussed in detail in the paper.

• Structural Engineering and Mechanics
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• v.49 no.3
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• pp.395-410
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• 2014

Fiber Reinforced Elastomeric Bearings (FREBs) are a relatively new type of laminated bearings that can be used as seismic/vibration isolators or bridge bearings. In an unbonded (U)-FREB, the bearing is placed between the top and bottom supports with no bonding or fastening provided at its contact surfaces. Under shear loads the top and bottom faces of a U-FREB roll off the contact supports and the bearing exhibits rollover deformation. As a result of rollover deformation, the horizontal response characteristics of U-FREBs are significantly different than conventional elastomeric bearings that are employed in bonded application. Current literature lacks an efficient analytical horizontal stiffness solution for this type of bearings. This paper presents two simplified analytical models for horizontal stiffness evaluation of U-FREBs. Both models assume that the resistance to shear loads is only provided by an effective region of the bearing that sustains significant shear strains. The presented models are different in the way they relate this effective region to the horizontal bearing displacements. In comparison with experimental results and finite element analyses, the analytical models that are presented in this paper are found to be sufficiently accurate to be used in the preliminary design of U-FREBs.

• Journal of the Korea Concrete Institute
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• v.13 no.1
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• pp.62-68
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• 2001

The purpose of this paper was to investigate the relations between the unbonded tendon stress and the influential parameters which were bonded reinforcement ratio, span/depth ratio, and loading type. To this end, first, the influence of parameters were examined with twenty eight test results obtained from references. Then, an experimental study was carried out with 21 test specimens. The investigation with previous and current tests revealed the followings; (1) The bonded reinforcement ratio and prestressing ratio were proved to be an important variables on the unbonded tendon stress. (2) The ratio of span to depth and type of loading affected partially the unbonded tendon stress although their effects varied depending on bonded reinforcement ratio. (3) AASHTO LRFD Code and Moon/Lim\`s design equation predicted the test results well with some safety margins.

• Structural Engineering and Mechanics
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• v.72 no.5
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• pp.631-642
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• 2019

The objective of the present study is to examine the flexural behavior of two-span post-tensioned lightweight aggregate concrete (LWAC) beams using unbonded tendons and the reliability of the design provisions of ACI 318-14 for such beams. The parameters investigated were the effective prestress and loading type, including the symmetrical top one-point, two third-point, and analogous uniform loading systems. The unbonded prestressing three-wire strands were arranged with a harped profile of variable eccentricity. The total length of the beam, measured between both strand anchorages, was 11000 mm. The test results were compared with those compiled from simply supported LWAC one-way members, wherever possible. The ultimate load capacity of the present beam specimens was evaluated by the collapse mechanism of the plasticity theorem and the nominal section moment strength calculated following the provision of the ACI 318-14. The test results showed that the two-span post-tensioned LWAC beams had lower stress increase (Δf_ps) in the unbonded tendons than the simply supported LWAC beams with a similar reinforcement index. The effect of the loading type on Δf_ps and displacement ductility was less significant for two-span beams than for the comparable simply supported beams. The design equations for Δf_ps and Δf_ps proposed by ACI 318-14 and Harajli are conservative for the present two-span post-tensioned LWAC beams, although the safety decreases for the two-span beam, compared to the ratios between experiments and predictions obtained from simply supported beams.

• Magazine of the Korea Concrete Institute
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• v.11 no.1
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• pp.209-219
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• 1999

This paper describes an experimental study planned to examine the effects of important design variables which were used in the proposed equation by authors. Fourteen beams and slabs were designed with the variables, such as effective prestress, concrete strength, amount of tendons, amount of bonded reinforcements, loading type, and span/depth ratio. Then, deflections and tendon stresses were measured and compared according to the parametric variations of the selected variables. It was found that the tendon stress increment decreases as the level of effective prestress or the amount of unbonded tendons and bonded reinforcements increases. Also, the contributions of concrete strength, and loading type were observed to affect on the tendon stresses. However, the stress increments of unbonded tendon were minimal alt high values of span/depth in contrast with the ACI code.

• Proceedings of the Korea Concrete Institute Conference
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• 1998.04b
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• pp.519-524
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• 1998

The current study is a part of series of research about the evaluation method of the unbonded tendon stress in prestressed concrete member at flexural failure. As the experimental study, a test program with 14 beams and slabs was planed to identify the contribution of each important variable. The variables are (1) the effective prestress, (2) the concrete strength, (3) the amount of tendons (4) the amount of bonded reinforcements, (5) the loading type, (6) the span/depth ratio. It was found that the tendon stress increment decreases as the effective prestress increases. Also, the contributions of concrete strength, amount of tendons, bonded reinforcements, and loading type were observed to affect on tendon stresses. However, the tendon stress increments were minimal at high values of span/depth in contrast with the ACI Code.

• Proceedings of the Korea Concrete Institute Conference
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• 2004.05a
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• pp.80-83
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• 2004

The FRP plate or sheet bonding technology was widely used for strengthening deficient RC structures. The strengthened structure using FRP bonding scheme, however, experience the complex interfacial behaviour which is difficult to predict. Therefore, the unbonded scheme using some anchorage device can be is an alternative for more reliable design. In this study, wedge-type anchor for FRP plate is developed for the unbonded flexural strengthening scheme. Some parameter study using 2D finite element method is performed. The analysis parameters are taken as wedge-guide friction coefficients, wedge- FRP ,.friction coefficient and wedge inclination angle. Based on the parameter study, more efficient anchors are designed and tested. The test result show that the developed anchor assure about $80\%$ FRP strength, which is higher performance than typical bonding scheme. Last, 3D finite element analysis is performed.