Heon-Soo Chung

Influence of Type and Replacement Level of Recycled Aggregates on Concrete Properties

The study reports results of tests, using only natural aggregates, of a control concrete and nine recycled aggregate concretes. The recycled aggregates were classified according to measured specific gravity and water absorption into three different types, namely: RS II for recycled fine aggregate having 2.36 specific gravity and 5.4% water absorption; RG III for recycled coarse aggregate having a 2.4 specific gravity and 6.2% water absorption; and RG I for recycled coarse aggregate having a 2.53 specific gravity and 1.9% water absorption. Both recycled coarse and fine aggregate replacement levels in separate mixtures were 30, 50, and 100%. For fresh concrete, slump loss and bleeding amount with time were recorded. For hardened concrete, there was also measurement of unrestrained shrinkage strain, moduli of rupture and elasticity, and compressive and tensile strengths. Fresh and hardened concrete properties tested, together with a literature-reported comprehensive database, were evaluated with respect to relative aggregate water absorption combined with recycled aggregate quality and volume. Hardened concrete properties, in addition, with different recycled aggregate replacement levels and quality were compared with ACI 318-05 design equation and empirical equality for natural aggregate concrete proposed by Oluokun, whenever possible. That the properties of recycled aggregate-containing fresh and hardened concrete were dependent on the aggregate relative water absorption was clearly shown in test results. In addition, the moduli of recycled aggregate concrete rupture and elasticity was lower than ACI 318-05-specified design equation, when relative aggregate water absorption is respectively above 2.5% and 3.0%.

Shear characteristics of high-strength concrete deep beams without shear reinforcements

Based on the strength at the first diagonal crack of normal-strength concrete and normal beams without the consideration of size effects, the ACI code specifies the shear strength of deep beams. It is necessary to evaluate whether the ACI equation for deep beams is applicable to high-strength concrete deep beams with reinforcement ratio less than 1% and to consider size effects. Twenty-one beam specimens were tested to investigate their shear characteristics with the variables of concrete strength, shear span/depth ratio, and overall depth. The decrease in shear span/depth ratio and the increase in overall depth under the same shear span/depth ratio led to more brittle failure, with wide diagonal cracks and high energy release rate related to size effects. The high-strength concrete deep beams exhibited more remarkable size effects with regard to brittle behavior. It was also shown that the ACI code gives similar safety factors on the shear strength at the first diagonal crack of high-strength concrete deep beams, but do not specify a high enough safety factor on their ultimate strength due to the size effects.

Stress–strain curve of laterally confined concrete

Abstract The objective of this study is to present a stress–strain relation of confined concrete from an empirical study of 65 columns. Experimental parameters include the strength of concrete, the volumetric ratio, strength, and confinement type of rectilinear ties, and the distribution of longitudinal bars. For the purpose of investigating confinement effects, an effectively confined distance ratio was introduced and the effects according to each parameter were analyzed. This paper provides an equation to determine the tie stress caused by lateral concrete pressure as an important index to measure confinement degree. Analyzing the experimental data by nonlinear multiple regression method, this study provides the empirical equations to determine the peak stress and its corresponding strain of confined concrete expressed by the tie stress, the effectively confined distance ratio, the strength of concrete, and the configuration of ties. Starting from the model proposed by Popovics, an empirical model for stress–strain curve of laterally confined concrete is developed by three coordinates, (fcc,ecc), (0.85fcc,e0.85), and (0.3fcc,e0.3). Comparison with other stress–strain curves illustrates the validity of the proposed relation.

Influence of Shear Reinforcement on Reinforced Concrete Continuous Deep Beams

This paper presents results from tests of 24 two-span reinforced concrete deep beams that were performed to study the influence of shear reinforcement on structural behavior. The main variables studied were concrete strength, shear span-to-overall depth ratio (a/h) and the amount and configuration of shear reinforcement. The results show that the load transfer capacity of shear reinforcement was much more prominent in continuous deep beams than in simply supported deep beams. The ratio of the load capacity measured and that predicted by the strut-and-tie model recommended by American Concrete Institute 318-05 dropped against the increase of a/h. This decrease rate was more remarkable in continuous deep beams than that in simple deep beams. The strut-and-tie model recommended by ACI 318-05 overestimated the strength of continuous deep beams having an a/h of more than 1.0. Horizontal shear reinforcement was always more effective than vertical shear reinforcement for beams having an a/h of 0.5. However, vertical shear reinforcement was more effective for an a/h higher than 1.0.

Explicit Motion of Dynamic Systems with Position Constraints

Although many methodologies exist for determining the constrained equations of motion, most of these methods depend on numerical approaches such as the Lagrange multiplier’s method expressed in differential/algebraic systems. In 1992, Udwadia and Kalaba proposed explicit equations of motion for constrained systems based on Gauss’s principle and elementary linear algebra without any multipliers or complicated intermediate processes. The generalized inverse method was the first work to present explicit equations of motion for constrained systems. However, numerical integration results of the equation of motion gradually veer away from the constraint equations with time. Thus, an objective of this study is to provide a numerical integration scheme, which modifies the generalized inverse method to reduce the errors. The modified equations of motion for constrained systems include the position constraints of index 3 systems and their first derivatives with respect to time in addition to their second derivatives with respect to time. The effectiveness of the proposed method is illustrated by numerical examples.

Influence of inclined web reinforcement on reinforced concrete deep beams with openings

The relation between the amount of inclined reinforcement and geometrical condition of beams must be examined in order to understand the influence of inclined reinforcement on the structural behavior of deep beams with openings. This study tested 15 reinforced concrete deep beams with openings that all had the same overall geometrical dimensions. The main variables considered were the opening size and amount of inclined reinforcement. An effective inclined reinforcement factor combining the influence of the amount of inclined reinforcement and opening size on the structural behavior of the beams tested is proposed. It was observed that the diagonal crack width and shear strength of beams tested were significantly dependent on the effective inclined reinforcement factor that ranged from 0 to 0.318 for the test specimens. As this factor increased, the diagonal crack width and its development rate decreased, and the shear strength of beams tested improved. Beams having effective inclined reinforcement factors of more than 0.15 had higher shear strengths than that of the corresponding solid beam. A numerical procedure based on the upper bound analysis of the plasticity theory was proposed to estimate the shear strength and shear transfer capacity of reinforcement in deep beams with openings. The mechanism analysis developed to predict the shear strength of deep beams with openings and shear transfer capacity of inclined reinforcement showed good agreement with experimental results.

Analytical method for constrained mechanical and structural systems

The objective of this study is to present an accurate and simple method to describe the motion of constrained mechanical or structural systems. The proposed method is an elimination method to require less effort in computing Moore-Penrose inverse matrix than the generalized inverse method provided by Udwadia and Kalaba. Considering that the results by numerical integration of the derived second-order differential equation to describe constrained motion veer away the constrained trajectories, this study presents a numerical integration scheme to obtain more accurate results. Applications of holonomically or nonholonomically constrained systems illustrate the validity and effectiveness of the proposed method.

Structural and Mechanical Systems Subjected to Constraints

The characteristics of dynamic systems subjected to multiple linear constraints are determined by considering the constrained effects. Although there have been many researches to investigate the dynamic characteristics of constrained systems, most of them depend on numerical analysis like Lagrange multipliers method. In 1992, Udwadia and Kalaba presented an explicit form to describe the motion for constrained discrete systems. Starting from the method, this study determines the dynamic characteristics of the systems to have positive semidefinite mass matrix and the continuous systems. And this study presents a closed form to calculate frequency response matrix for constrained systems subjected to harmonic forces. The proposed methods that do not depend on any numerical schemes take more generalized forms than other research results.

Influence of Inclined Reinforcement around Openings on the Shear Behavior of Reinforced Concrete Continuous Deep Beams

Twelve reinforced concrete continuous deep beams having web openings within interior shear spans were tested to failure. All beams tested had the same geometrical dimensions. The main variables investigated were the opening size and amount of inclined reinforcement around openings. An effective inclined reinforcement factor combining the influence of the opening size and amount of inclined reinforcement on the structural behavior of the beams tested is proposed. It was observed that the load distribution, diagonal crack width, and load capacity of beams tested were greatly dependent on the effective inclined reinforcement factor which ranged from 0 to 0.171 for the test specimens. The higher this factor, the smaller the diagonal crack width and its development rate. A higher load capacity also developed in beams having effective inclined reinforcement factor above 0.077 than in the corresponding solid deep beams. A numerical technique based on the upper bound analysis of the plasticity theory is proposed to evaluate the load capacity of continuous deep beams having openings within interior shear spans. Predictions obtained from the proposed formulas are in good agreement with test results.

Shear Behavior of Reinforced Concrete Beams Strengthened with Unbonded-Type Wire Rope Units

The present study reports a simple unbonded-type shear strengthening technique for reinforced concrete beams using wire rope units. Fifteen beams failed in shear were repaired and strengthened with wire rope units, and then retested to failure. Influence of the prestressing force, orientation and spacing of wire rope units on the shear behavior of strengthened beams having shear span-to-depth ratios of 1.5, 2.5, or 3.25 were investigated. Test results showed that beams strengthened with wire rope units exhibited a higher shear strength and a larger post-failure deformation than the corresponding original beams. Inclined wire rope units was more effective for shear strength enhancement than vertical wire rope units. The increase of the prestressing force in wire rope units causes the decrease of the principal tensile stress in concrete, as a result, the diagonal tensile cracking strength of strengthened beams was higher than that of the corresponding original beams. Shear capacity of strengthened beams is compared with predictions obtained from ACI 318-05 and EC 2. Shear capacity of strengthened beams having shear span-to-depth ratio below 2.5 is reasonably predicted using ACI 318-05 formula. On the other hand, EC 2 overestimates the shear transfer capacity of wire rope units for beams having shear span-to-depth ratio above 2.5.