Dong-Uk Choi

Bar Pullout Tests and Seismic Tests of Small-Headed Bars in Beam-Column Joints

Headed reinforcing bars increasingly are being used in reinforced concrete structures. This paper reports on a study to evaluate the applicability of these headed bars in exterior beam-column joints. A total of 12 pullout tests were first performed to examine anchorage behavior of headed bars subjected to monotonic and repeated loading. Variables such as the head size, shape, and head attaching technique were examined. Reversed cyclic tests of two full-scale exterior beam-column joints were conducted to assess seismic performance. The pullout test results revealed that all types of heads and head-attaching techniques performed almost equally well, while the seismic test results indicated that the joint using small-headed bars showed better seismic performance than the joint using hooked bars in terms of damage extent, joint behavior, lateral drift capacity, and energy dissipation. In particular, the joint with headed bars generally satisfied ACI 374 acceptance criteria. The findings also indicate that small-headed bars can be effectively anchored in exterior beam-column joints under inelastic deformation reversals and that they perform well with a development length shorter than that needed for hooked bars.

Splice Length of GFRP Rebars Based on Flexural Tests of Unconfined RC Members

Glass fiber reinforced polymer (GFRP) bars are sometimes used when corrosion of conventional reinforcing steel bar is of concern. In this study, a total of 36 beams and one-way slabs reinforced using GFRP bars were tested in flexure. Four different GFRP bars of 13 mm diameter were used in the test program. In most test specimens, the GFRP bars were lap spliced at center. All beams and slabs were tested under 4-point loads so that the spliced region be subject to constant moment. Test vari- ables were splice lengths, cover thicknesses, and bar spacings. No stirrups were used in the spliced region so that the tests result in conservative bond strengths. Average bond stresses that develop between GFRP bars and concrete were determined through non- linear analysis of the cross-sections. An average bond stress prediction equation was derived utilizing two-variable linear regres- sion. A splice length equation based on 5% fractile concept was then developed. As a result of this study, a rational equation with which design splice lengths of the GFRP bars can be determined, was proposed.

Test of Headed Reinforcement in Pullout II

A total of 32 pullout tests were performed for the multiple headed bars relatively deeply embedded in reinforced concrete column-like members. The objective was to determine the minimum embedment depth that was necessary to safely design exterior beam-column joints using headed bars. The variables for the experiment were embedment depth of headed bar, center-to-center distance between adjacent heads, and amount of supplementary reinforcement. Regular strength concrete and grade SD420 reinforcing steel were used. The results of the test the indicated that a headed bar embedment depth of was not sufficient to have relatively closely installed headed bars develop the pullout strength corresponding to the yield strength. All the experimental variables, influenced the pullout strength. The pullout strength increased with increasing embedment depth and head-to-head distance. It also increased with increasing amount of supplementary reinforcement. For a group of closely-spaced headed bars installed in a beam-column joint, it is recommended to use column ties at least 0.6% by volume, 1% or greater amount of column main bars, and an embedment depth of or greater simultaneously, to guarantee the pullout strength of individual headed bars over 125% of and ductile load-displacement behavior.

Seismic Performance of Circular RC Columns Retrofitted Using Ductile PET Fibers

An experimental research was performed using fibers for the purpose of retrofitting existing reinforced concrete circular columns. Glass fiber (GF) and polyethylene terephthalate (PET) were used as well as combined GF+PET (HF). PET has high tensile strength (over 600 MPa) and high ductility (about 15%), but has very low elastic modulus (about 1/6 of GF). A total of four columns was tested against laterally applied reverse cyclic load: control column, GF-, PET-, and HF-strengthened columns. All columns retrofitted using fibers demonstrated improved moment capacity and ductility. Moment capacity of GF-, PET-, and HF-strengthened columns was 120%, 107%, and 120% of the control column, respectively. Drift ratio of all retrofitted columns also increased by 63 ~ 83% over the control column. The final failure mode of the control column was main bar buckling. The final failure mode of the GFand HF-strengthened columns was GF rupture while that of the PET-strengthened column was main bar rupture in tension. No damage was observed for PET at the ultimate stage due to excellent strain capacity intrinsic to PET. Current test results indicate that PET can be effectively used for seismic retrofit of RC columns. It is noted that the durability characteristics of PET needs to be investigated in the future.

Development Length of GFRP Bars

The objective of this study was to propose a development length equation for GFRP bars. A total of 104 modified pullout tests were completed while the test variables were embedment length (15, 30, ), net cover thickness (), top-cast bar effect, different GFRP bar types (K2KR, K3KR and AsUS), and bar diameters (10, 13, 16 mm). Average bond stresses were determined based on modified pullout test results. Two variable linear regression analysis was performed of the average bond stresses. Utilizing 5% fractile concept, a conservative development length design equation was derived. The design equation derived in this study was compared to the ACI 440 committee equation. The cross-comparison revealed that the current equation resulted in shorter development lengths than those determined by the ACI 440 equation when the net cover thickness was large (greater than ). On the other hand, when the net cover thickness was small (equal to or less than ), the development lengths required by the current equation were larger than those by the ACI equation. The bond stresses were significantly influenced by the cover thicknesses. The current equation results in development lengths that are more economical when the cover thickness is large, and more conservative lengths when the cover thickness is small than the ACI 440 committee equation.

Development and Splice Lengths of FRP Bars with Splitting Failures

Data from beam-based bond tests for FRP bars in the literature were collected and regression analyses were conducted for the data of splitting failure. Average bond strengths obtained from splice tests were found to be lower and more affected by C/ values than average bond strengths from anchorage tests, indicating needs of new design equation for the splice length of FRP bars based on the data of splice tests only. In addition, the variation of bond strengths was greater than that of tensile strengths of FRP bars and, therefore, a new safety factor should be involved for the design equation. Five percent fractile coefficients were used to develop the design equations based on the assumption that load and resistance factors for FRP reinforced concrete structures are same to the factors for steel reinforced concrete structures. The proposed design equations give economical and reliable lengths for development and splice of FRP bars. The proposed equation for splice provides shorter lengths than the ACI 440 equation in case of C/ of 3.0 or greater. Because FRP bars are expected to be used in slabs and walls exposed to weather with thick cover and large spacing between bars, the proposed equation gives optimal splice lengths.

Bond Capacity of Pseudo-Ductile FRP Hybrid Sheet to Strengthen RC Members

12 concrete blocks, on which hybrid fibrous sheets (carbon fiber and glass fiber) had been bonded, were subjected to tensile load in order to estimate properties of the bonded interface. the sheet length was varied by 100 mm, 200 mm and 400 mm. It was found that more than 150 mm bond length is required to achieve the maximum bearing capacity of the interface. In this study, maximum bond stress τF,max, ultimate slip SFU of the interface were estimated τF,max= 3.0 MPa and SFU = 0.175 mm, respectively.

Application of PCM Technology to Concrete II : Effects of SSMA(Sulfonated Styrene-Maleic Anhydride) on the Properties of the 1-Dodecanol Micro-Capsule

Thermal storage technology used for indoor heating and cooling to maintain a constant temperature for a long period of time has an advantage of raising energy use efficiency. This, the phase changing material, which utilizes heat storage properties of the substances, capsulizes substances that melt at a constant temperature. This is applied to construction materials to block or save energy due to heat storage and heat protection during the process in which substances melt or freeze according to the indoor or outdoor temperature. The micro-encapsulation method is used to create thermal storage from phase changing material. This method can be broadly classified in 3 ways: chemical method, physical and chemical method and physical and mechanical method. In the physical and chemical method, a wet process using the micro-encapsulation process utilized. This process emulsifies the core material in a solvent then coats the monomer polymer on the wall of the emulsion to harden it. In this process, a surfactant is utilized to enhance the performance of the emulsion of the core material and the coating of the wall monomer. The performance of the micro-encapsulation, especially the coating thickness of the wall material and the uniformity of the coating, is largely dependent on the characteristics of the surfactant. This research compares the performance of the micro-capsules and heat storage for product according to molecular mass and concentration of the surfactant, SSMA (sulfonated styrene-maleic anhydride), when it comes to micro-encapsulation through interfacial polymerization, in which Dodecan-1 is transformed to melamin resin, a heat storage material using phase changing properties. In addition, the thickness of the micro-encapsulation wall material and residual melamine were reduced by adjusting the concentration of melamin resin microcapsules.