Myoungsu Shin

Seismic Design of Reinforced Concrete Beam-Column Joints with Headed Bars

Section 12.6 provisions of ACI 318-08 detail the development of headed and mechanically anchored deformed bars for the first time in the Code series. Prior to this, Joint ACI-ASCE Committee 352 published design recommendations for headed reinforcement used in reinforced concrete beam-column joints (ACI 352R-02). However, both ACI 318-08 and 352R-02 are based on quite limited experimental research. Given this concern, these ACI standards and recommendations were evaluated using an extensive database encompassing most available test data for reinforced concrete beam-column joints with headed bars subjected to reversed cyclic loading. The primary objectives of this study are to document the experimental investigations in a uniform format; provide a detailed review for the test data; and, finally, propose design guidelines to supplement ACI 352R-02 and 318-08 on the subject of headed bars anchored in beam-column joints.

Cyclic Testing for Seismic Design Guide of Beam-Column Joints with Closely Spaced Headed Bars

Given the overly strict requirements for clear headed-bar spacing in ACI 318-08 and the lack of headed bar provisions in CEB-FIB MC 90, two approximately 2/3-scale exterior beam-column connection subassemblies were tested under cyclic lateral loading. The tests primarily explored the effect on their seismic performance of using (a) small clear spacings and (b) multiple layers of headed bars in the beam. Also, previous test data were thoroughly analyzed. It was concluded that the clear bar spacing of approximately 2db or the use of two bar layers may be permitted for headed bars anchored in exterior beam-column joints.

Effect of Fly Ash on Rheology and Strength of Recycled Aggregate Concrete

ABSTRACT As the amount of construction wastes increase, reuse of demolished concrete is being considered in research areas. Reflecting these interests, this experiment was performed to clarify concrete’s mechanical property and workability using recycled aggregate as a coarse aggregate. Eleven cases of concrete specimens were produced by changing the rates of replacement of coarse recycled aggregate, replacement of fly ash, design strength, and moisture state of coarse aggregate. Compressive and tensile split strength tests were taken to study the mechanical properties of ha rdened concrete. To verify flowability of fresh concrete, a s lump test and a flow curve test using ICAR Rheometer were performed. It was found that using recycled aggregate and fly ash leads good workability by testing slump and flow curve. The yield stress of fresh concrete decreased with increase of recycled aggregate substitution rate. Through the test, it was confirmed that there is inversely proportional relationship between the slump and yield stress roughly. Recycled aggregate concrete containing fly ash ha s considerably lower plasticity viscosity than not containing fly ash. Strength test results showed that recycled aggregate tended to decrease compressive and tensile strength of concrete, when recycled aggregate was used as a coarse aggregate. Using over 30% recycled aggregate caused significant decreases in compressive and tensile strength. Replacing 30% cement with fly ash was helpful to improve the long-term strength of concrete.

Concrete Crack Identification Using a UAV Incorporating Hybrid Image Processing

Crack assessment is an essential process in the maintenance of concrete structures. In general, concrete cracks are inspected by manual visual observation of the surface, which is intrinsically subjective as it depends on the experience of inspectors. Further, it is time-consuming, expensive, and often unsafe when inaccessible structural members are to be assessed. Unmanned aerial vehicle (UAV) technologies combined with digital image processing have recently been applied to crack assessment to overcome the drawbacks of manual visual inspection. However, identification of crack information in terms of width and length has not been fully explored in the UAV-based applications, because of the absence of distance measurement and tailored image processing. This paper presents a crack identification strategy that combines hybrid image processing with UAV technology. Equipped with a camera, an ultrasonic displacement sensor, and a WiFi module, the system provides the image of cracks and the associated working distance from a target structure on demand. The obtained information is subsequently processed by hybrid image binarization to estimate the crack width accurately while minimizing the loss of the crack length information. The proposed system has shown to successfully measure cracks thicker than 0.1 mm with the maximum length estimation error of 7.3%.

Principles and Applications of Ultrasonic-Based Nondestructive Methods for Self-Healing in Cementitious Materials

Recently, self-healing technologies have emerged as a promising approach to extend the service life of social infrastructure in the field of concrete construction. However, current evaluations of the self-healing technologies developed for cementitious materials are mostly limited to lab-scale experiments to inspect changes in surface crack width (by optical microscopy) and permeability. Furthermore, there is a universal lack of unified test methods to assess the effectiveness of self-healing technologies. Particularly, with respect to the self-healing of concrete applied in actual construction, nondestructive test methods are required to avoid interrupting the use of the structures under evaluation. This paper presents a review of all existing research on the principles of ultrasonic test methods and case studies pertaining to self-healing concrete. The main objective of the study is to examine the applicability and limitation of various ultrasonic test methods in assessing the self-healing performance. Finally, future directions on the development of reliable assessment methods for self-healing cementitious materials are suggested.

Seismic Performance Evaluation of RC Beam-Column Connections in Special and Intermediate Moment Frames

This article presents the results of a research project aimed at investigating the seismic performance criteria of beam-column connections in reinforced concrete (RC) moment frames. A total of 112 RC beam-column connection tests were examined in detail. All the test specimens were qualified as either special moment frame (SMF) or intermediate moment frame (IMF) connections, based on the design and detailing requirements of ACI 318-08 [2008, Ch. 21]. The performance acceptance criteria, originally defined for steel moment frame connections in the 1997 edition of the AISC Seismic Provisions, were employed to evaluate the seismic performance of the RC connection specimens. Most of the specimens that satisfied the ACI 318 design requirements applying for SMFs were found to be ductile up to a plastic drift of 3% without any major degradation in strength, which satisfied the AISC performance acceptance criteria. This is most likely attributed to the stringent ACI 318-08 requirements for joints in SMFs. On the other hand, more than 40% of the IMF connections were not satisfactory based on the AISC criteria applying for steel IMF connections. Joint shear stress level showed more significant effect on their seismic performance than the column-to-beam flexural strength ratio. It is believed that inadequate joint detailing prevented these connections from developing their full beam flexural capacity, which resulted in premature strength deterioration and subsequent failure. Based on the findings, it is recommended that design requirements for beam-column connections in RC IMFs should be revisited and enhanced; otherwise, the R-factor assigned for RC IMFs should be somewhat reduced to accommodate their smaller ductility than intended by the ACI 318 requirements.

Combined Effects of Set Retarders and Polymer Powder on the Properties of Calcium Sulfoaluminate Blended Cement Systems

This study investigates the effects of set retarders on the properties of polymer-modified calcium sulfoaluminate (CSA) and Portland cement blend systems at early and long-term ages. The fast setting of the cement blend systems is typically adjusted by using retarders to ensure an adequate workability. However, how the addition of retarders influences the age-dependent characteristics of the cement blend systems was rarely investigated. This study particularly examines the effects of retarders on the microstructure and strength development of polymer-modified CSA and Portland cement blend pastes and mortars from 2 h to 90 days. The macro- and microstructural properties are characterized by compression testing, powder X-ray diffraction, mercury intrusion porosimetry, and scanning electron microscopy with energy dispersive spectroscopy. The test results reveal that the use of retarders delayed the strength development of the cement blend systems at the very early age by hindering the production of ettringite, which was cumulative to the delaying effect of polymer, but it increased the ultimate strength by creating denser and finer pore structures with the evolution of hydration products.

Experimental and numerical assessment of bonded and unbonded post-tensioned concrete members

A post-tensioned (PT) concrete member can be reinforced with either bonded or unbonded prestressing tendons. The bond condition of tendons may influence the flexural and shear performance of various types of PT concrete members. The main purpose of this study is to provide a direct comparison between members with the two different tendon systems and assess their structural performance in depth. To accomplish this, previously tested PT beams, one-way slabs, and slab-column connections are reexamined, and detailed nonlinear finite element analyses are conducted using the developed modeling techniques. A series of test results verifies that the developed model is reliable for both bonded and unbonded PT members. The results of the prior experimental and current numerical studies are used for better understanding the behavior of bonded and unbonded PT members.

Crack and Noncrack Classification from Concrete Surface Images Using Machine Learning

In concrete structures, surface cracks are important indicators of structural durability and serviceability. Generally, concrete cracks are visually monitored by inspectors who record crack information such as the existence, location, and width. Manual visual inspection is often considered ineffective in terms of cost, safety, assessment accuracy, and reliability. Digital image processing has been introduced to more accurately obtain crack information from images. A critical challenge is to automatically identify cracks from an image containing actual cracks and crack-like noise patterns (e.g. dark shadows, stains, lumps, and holes), which are often seen in concrete structures. This article presents a methodology for identifying concrete cracks using machine learning. The method helps in determining the existence and location of cracks from surface images. The proposed approach is particularly designed for classifying cracks and noncrack noise patterns that are otherwise difficult to distinguish using existing image processing algorithms. In the training stage of the proposed approach, image binarization is used to extract crack candidate regions; subsequently, classification models are constructed based on speeded-up robust features and convolutional neural network. The obtained crack identification methods are quantitatively and qualitatively compared using new concrete surface images containing cracks and noncracks.

Hysteretic Behavior Evaluation of a RC Coupling Beam using a Steel Fiber and Diagonal Reinforcement

In this paper, a bundled diagonal reinforcement using high performance steel fiber was proposed to enhance the construct ability and seismic performance. Experiments of coupling beam was composed of four specimens and the hysteretic behavior evaluated for reverse cyclic loading to specimens using high performance steel fiber. The main variables of the experiment is a amount of stirrup and bundled reinforcement, depending on whether the mix of steel fiber. Specimen which criteria was applied 100% of stirrup and bundled diagonal reinforcement of ACI318 criteria. With this, by appling same diagonal reinforcement, two specimens were created by adjusting stirrup of 75%, 50%. So, a total of four specimens were produced. When coupling beam was placed concrete, this experiment was mixed in a content of steel fiber 1%. All the specimens were produced by aspect ratio 3.5(l/h=1050/300) to a half-scale. In this result, two specimens as reduced to stirrup of 75%, 50% was no significant difference in the strength, stiffness and energy dissipation capacity, respectively compared to the stirrup of 100%.