Jae-Il Sim

Flexural Behavior of Reinforced Concrete Columns Strengthened with Wire Rope and T-Plate Units

Unbonded techniques to strengthen reinforced concrete columns have become increasingly popular. This study evaluates the flexural behavior of reinforced concrete columns strengthened with unbonded wire rope and T-shaped steel plate units. Seven strengthened columns and an unstrengthened column were tested to failure under constant axial load and cyclic lateral loads to explore the significance and limitations of the strengthening procedure developed for resistance against earthquakes. The main variables investigated were the volume ratio of wire rope, axial load level, and the presence of mortar cover for strengthening steel elements. In addition, the theoretical monotonic lateral load-displacement curve for strengthened columns is simply derived using the combination of section laminae method and the idealized curvature-displacement relationship. The findings show that wire rope and T-shaped steel plate units were highly effective in preventing spalling of concrete cover and buckling of longitudinal reinforcement. The flexural capacity of columns strengthened without mortar cover was slightly higher than that of the unstrengthened column, but the flexural capacity of strengthened columns with a 60 mm (2.36 in.) thick mortar cover was at least 2.5 times higher than that of the comparable strengthened columns without mortar cover. The developed strengthening procedure was particularly effective in enhancing the ductility of the columns, showing that the displacement ductility ratios and work damage indicators in the strengthened columns were much higher than in the unstrengthened column. The monotonic lateral load-displacement relationship of the column specimens predicted from the proposed numerical analysis is in good agreement with backbone curves obtained from measured cyclic lateral load-displacement relationships.

Effect of Water Content on the Properties of Lightweight Alkali-Activated Slag Concrete

The water content and lightweight aggregate proportions in lightweight alkali-activated (AA) concrete need to be carefully managed to control the quick slump loss of fresh concrete and to meet the mechanical properties that are required for structural concrete. This study tested 10 lightweight AA slag concrete specimens to evaluate the effect of the water content on the workability and various mechanical properties of the concrete. The source material, ground granulated blast furnace slag, was activated by using sodium silicate and calcium hydroxide powders to produce a cementless binder. The rate of development of the compressive strength and the shrinkage strain measured from the concrete specimens were compared with empirical models proposed by American Concrete Institute (ACI) 209 for normal-weight portland cement concrete. To examine the practical applicability of the lightweight AA slag concrete, the splitting tensile strength and the moduli of elasticity and rupture recorded from the concrete speci...

Practical Application of GGBS-Based Alkali-Activated Binder to Secondary Products of Concrete

This study examined the practical application of ground granulated blast-furnace slag (GGBS) based alkali-activated (AA) binders for the development of cementless environmental-friendly secondary products of concrete, such as brick, shore protection blocks and interlocking blocks. The addition amount and type of alkaline ion to activate GGBS varied according to the diverse qualities of the secondary products of concrete required in Korean industrial standards (KS) and other specifications. Test results showed that the secondary products of concrete using GGBS-based AA binders surpassed the demanded capacities of KS and other specifications. In addition, shore protection block had a pH value close to neutral, enabling an advantageous environment for marine life. Therefore, the GGBS-based AA binders can be effectively applied to develop eco-friendly secondary products of concrete with reduced .

Air Content, Workability and Bleeding Characteristics of Fresh Lightweight Aggregate Concrete

Fifteen lightweight concrete mixes were tested to evaluate the effect of maximum size of coarse aggregate and the replacement level of natural sand on the various properties of fresh lightweight concrete. The different properties, such as water absorption against the elapsed time, pore size distribution and micro-structure of lightweight aggregates used, influencing on the workability of fresh concrete were also measured. Test results showed that the initial slump of lightweight concrete decreased with the increase of the replacement level of natural sand. The slump of all-lightweight concrete sharply decreased by around 80% of the initial slump after 30~60 minutes. The air content and bleeding rate of lightweight concrete were significantly affected by the replacement level of natural sand as well as the maximum size of coarse aggregates. Empirical equations recommended in ACI 211 and Korea concrete standard specifications underestimated the air content of the lightweight concrete, indicating that the under- estimation increases with the decrease of the replacement level of natural sand. In addition, equations to predict the air content and bleeding rate of lightweight concrete were proposed based on the test results.Fifteen lightweight concrete mixes were tested to evaluate the effect of maximum size of coarse aggregate and the replacement level of natural sand on the various properties of fresh lightweight concrete. The different properties, such as water absorption against the elapsed time, pore size distribution and micro-structure of lightweight aggregates used, influencing on the workability of fresh concrete were also measured. Test results showed that the initial slump of lightweight concrete decreased with the increase of the replacement level of natural sand. The slump of all-lightweight concrete sharply decreased by around 80% of the initial slump after 30~60 minutes. The air content and bleeding rate of lightweight concrete were significantly affected by the replacement level of natural sand as well as the maximum size of coarse aggregates. Empirical equations recommended in ACI 211 and Korea concrete standard specifications underestimated the air content of the lightweight concrete, indicating that the underestimation increases with the decrease of the replacement level of natural sand. In addition, equations to predict the air content and bleeding rate of lightweight concrete were proposed based on the test results.

Effect of Aggregate Size on Shear Behavior of Lightweight Concrete Continuous Slender Beams

In this paper twelve continuous slender beams were tested to ascertain the effect of the maximum aggregate size on the shear behavior of concrete beams. The typical characteristics of the failure surface along the inclined cracks of the beams tested were compared according to the maximum aggregate size and the type of concrete by using a microphotograph. The test results showed that the shear strength of light weight concrete (LWC) continuous beams increased with the maximum aggregate size, though the increasing rate was lower than that of normal weight concrete (NWC) continuous beams. The microphotograph showed that the inclined crack of mortar beams with an aggregate size of 4 mm (0.16 in.) had a near-linear shape and a smooth failure surface, whereas that of the concrete beams was emboss-shaped with a failure plane partially formed along the pastes around the aggregate particles, regardless of the type of concrete. These characteristics of the failure plane contributed to the enhancement of the shear strength of LWC beams, though the shear force transferred by the aggregate interlock was much lower than that in NWC beams. The increasing rate of shear strength of LWC beams against aggregate size is similar to that predicted from the simplified compression field theory or the empirical formulas proposed by Bazant and Sun. The modification factor for shear strength of LWC specified in ACI 318-08 and EC2 is unconservative in the continuous beams tested, showing an increase of the unconservatism with the maximum aggregate size.

Generalized Equivalent Stress Block Model Considering Varying Concrete Compressive Strength and Unit Weight

In this study, a generalized equivalent Stress Block Model has been proposed that is applicable to all types of concrete, including lightweight and high-strength concrete (HSC). First, available stress-strain models for concrete were verified through an extensive database. Then, the coefficients used in the proposed stress blocks were formulated based on a nonlinear regression analysis of the values determined from the layer-by-layer integral evaluation approach. The hypothesis underlying this integral approach is that the magnitude and location of the resultant force in the equivalent stress distribution are the same as those in the actual distribution. The extreme compressive fiber strain of 0.003 and the factor of 0.85 used to compensate for the difference between the in-place strength and the cylinder strength were assumed, as was done for normalweight and/or normal-strength concrete (NSC). The reliability and safety of the proposed stress blocks were confirmed through comparisons with the flexural capacities measured from approximately 175 normalweight and 80 lightweight concrete (LWC) beams, and approximately 100 normalweight concrete (NWC) columns.

Mechanical Properties of Lightweight Aggregate Concrete according to the Substitution Rate of Natural Sand and Maximum Aggregate Size

The effect of the maximum aggregate size and substitution rate of natural sand on the mechanical properties of concrete is evaluated using 15 lightweight aggregate concrete mixes. For mechanical properties of concrete, compressive strength increase with respect to age, tensile resistance, elastic modulus, rupture modulus, and stress-strain relationship were measured. The experimental data were compared with the design equations specified in ACI 318-08, EC2, and/or CEB-FIP code provisions and empirical equations proposed by Slate et al., Yang et al., and Wang et al. The test results showed that compressive strength of lightweight concrete decreased with increase in maximum aggregate size and amount of lightweight fine aggregates. The parameters to predict the compressive strength development could be empirically formulated as a function of specific gravity of coarse aggregates and substitution rate of natural sand. The measured rupture modulus and tensile strength of concrete were commonly less than the prediction values obtained from code provisions or empirical equations, which can be attributed to the tensile resistance of lightweight aggregate concrete being significantly affected by its density as well as compressive strength.

Influence of Specimen Geometries on the Compressive Strength of Lightweight Aggregate Concrete

Dept. of Architectural Engineering, Kyonggi University, Suwon 443-760, KoreaABSTRACT The current study prepared 9 laboratorial concrete mixes and 3 ready-mixed concrete batches to examine the sizeand shape effects in compression failure of lightweight aggregate concrete (LWC). The concrete mixes were classified into threegroups: normal-weight, all-lightweight and sand-lightweight concrete groups. For each concrete mix, the aspect ratio of circularor square specimens was 1.0 and 2.0. The lateral dimension of specimens varied between 50 and 150 mm for each laboratorial con-crete mix, whereas it ranged from 50 to 400 mm with an incremental variation of 50 mm for each ready-mixed concrete batch. Testobservations revealed that the crack propagation and width of the localized failure zone developed in lightweight concrete spec-imens were considerably different than those of normal-weight concrete (NWC). In LWC specimens, the cracks mainly passedthrough the coarse aggregate particles and the crack distribution performance was very poor. As a result, a stronger size effect wasdeveloped in LWC than in NWC. Especially, this trend was more notable in specimens with aspect ratio of 2.0 than in specimenswith that of 1.0. The prediction model derived by Kim et al. overestimated the size effect of LWC when lateral dimension of spec-imen is above 150 mm. On the other hand, the modification factors specified in ASTM and CEB-FIP provisions, which are usedto compensate for the shape effect of specimen on compressive strength, were still conservative in LWC.Keywords : size effect, compressive strength, lightweight aggregate concrete, shape of specimen

Size Effect of Concrete Compressive Strength Considering Dried Unit Weight of Concrete

Since the size effect law announced currently has been based on the normal weight concrete, for light weight concrete having different fracture characteristics, its application is questionable. Accordingly, in this study, a model equation to predict the effect of dried unit weight of the concrete on size effect of its compressive strength was developed and a database using existing research results was created. After determining the experimental constants of prediction models of Ba?ant based on nonlinear fracture mechanics, Kim and Eo, and this study using the database, their results are mutually compared. Finally, it was found that the prediction model of this study considered dried unit weight of concrete predicted well the test results for light weight concrete than that of the models of Ba?ant and Kim and Eo.

Axial Behavior of Reinforced Concrete Columns Externally Strengthened with Unbonded Wire Rope and T-Shaped Steel Plate

An improved unbonded-type column strengthening procedure using wire rope and T-shaped steel plate units was proposed. Eight strengthened columns and an unstrengthened control column were tested under concentric axial load. The main variables considered were the volume ratio of wire rope and the flange width and configuration of T-shaped steel plates. Axial load capacity and ductility ratio of columns tested were compared with predictions obtained from the equation specified in ACI 318-05 and those of conventionally tied columns tested by Chung et al., respectively. In addition, a mathematical model was proposed to evaluate the complete stress-strain relationship of concrete confined by the wire rope and T-plate units. Test results showed that the axial load capacity and ductility of columns increased with the increase of the volume ratio of wire rope and the flange width of T-plates. In particular, at the same lateral reinforcement index, a much higher ductility ratio was observed in the strengthened columns having the volume ratio of wire rope above 0.0039 than in the tied columns. A mathematical model for the stress-strain relationship of confined concrete using the proposed strengthening procedure is developed. The predicted stress-strain curves were in good agreement with test results.