Ju-Hyun Mun

Tests on Cementless Alkali-Activated Slag Concrete Using Lightweight Aggregates

Five all-lightweight alkali-activated (AA) slag concrete mixes were tested according to the variation of water content to examine the significance and limitation on the development of cementless structural concrete using lightweight aggregates. The compressive strength development rate and shrinkage strain measured from the concrete specimens were compared with empirical models proposed by ACI 209 and EC 2 for portland cement normal weight concrete. Splitting tensile strength, and moduli of elasticity and rupture were recorded and compared with design equations specified in ACI 318-08 or EC 2, and a database compiled from the present study for ordinary portland cement (OPC) lightweight concrete, wherever possible. Test results showed that the slump loss of lightweight AA slag concrete decreased with the increase of water content. In addition, the compressive strength development and different mechanical properties of lightweight AA slag concrete were comparable with those of OPC lightweight concrete and conservative comparing with predictions obtained from code provisions. Therefore, it can be proposed that the lightweight AA slag concrete is practically applicable as an environmental-friendly structural concrete.

Flow and Engineering Properties of Fiber Reinforced Hwangtoh Mortars

In this study, six mortar mixes were tested in order to examine the significance and limitations of hydrophilic fiber in terms of its capacity to enhance the tensile resistance of Hwangtoh mortar. Lyocell, polyamide and polyvinyl alcohol (PVA) fibers were selected for the main test parameters. The tensile capacity of mortars tested was evaluated based on the splitting tensile strength and the modulus of fracture, while their ductility was examined using the toughness indices specified in ASTM. Test results showed that the addition of lyocell and PVA fibers had little influence on the flow of the Hwangtoh mortars. To enhance the tensile capacity and toughness index of Hwangtoh mortar, it is proposed that fiber spacing above 0.0003 is required, regardless of the type of fiber.

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...

Flexural Capacity and Stress in Unbonded Tendons of Post-Tensioned Lightweight Concrete Beams

The present study reports the effect of the amounts of mild longitudinal tensile reinforcement, effective prestress and prestressing strands on the flexural performance of the prestressed lightweight concrete (LWC) beams. Test data were compared with those compiled from the post-tensioned normal-weight concrete (NWC) beam specimens and prediction models derived based on the ACI 318–11 provision. Test results showed that the effect of the investigated parameters on the flexural behavior of the post-tensioned LWC beams was usually similar to the typical trends observed in the post-tensioned NWC concrete beams. With the increase of the reinforcing index, the incremental stress of the unbonded strands at ultimate and displacement ductility ratio of the beams decreased, while flexural capacity of the beams increased. The incremental stress of the unbonded strands at ultimate strength of the post-tensioned LWC beams was commonly higher than that of the post-tensioned NWC beams with the same reinforcing index values and the predictions obtained from the ACI code equations. Alternatively, the normalized flexural capacity of the post-tensioned LWC beams was comparable to that of the post-tensioned NWC beams and higher than the predictions based on the ACI code provision.

Simplified Model for the Stress-Strain Relationship of Confined Concrete

This study aims to develop a simple and rational stress-strain model of confined concrete in compression. Considering the confinement effect provided by the lateral reinforcement, the specified properties of confined concrete was formulated using regression analysis of 123 data sets as follows: modulus of elasticity, strength gain factor, strains at peak stress and 85% of the peak stress on the descending branch. Numerical and parametric analysis were carried out to derive equations for the key parameter determining the slopes of the ascending and descending branches of the stress-strain curves of confined concrete. The reliability of the developed model is examined using a normalized root mean-square error obtained from a comparison of model estimates with the experimental data. The proposed model gives superior accuracy and mathematical simplicity.

Generalized Lateral Load-Displacement Relationship of Reinforced Concrete Shear Walls

This study generalizes the lateral load-displacement relationship of reinforced concrete shear walls from the section analysis for moment-curvature response to straightforwardly evaluate the flexural capacity and ductility of such members. Moment and curvature at different selected points including the first flexural crack, yielding of tensile reinforcing bar, maximum strength, 80% of the maximum strength at descending branch, and fracture of tensile reinforcing bar are calculated based on the strain compatibility and equilibrium of internal forces. The strain at extreme compressive fiber to determine the curvature at the descending branch is formulated as a function of reduction factor of maximum stress of concrete and volumetric index of lateral reinforcement using the stress-strain model of confined concrete proposed by Razvi and Saatcioglu. The moment prediction models are simply formulated as a function of tensile reinforcement index, vertical reinforcement index, and axial load index from an extensive parametric study. Lateral displacement is calculated by using the moment area method of idealized curvature distribution along the wall height. The generalized lateral load-displacement relationship is in good agreement with test result, even at the descending branch after ultimate strength of shear walls.

Effect of Substituting Normal-Weight Coarse Aggregate on the Workability and Mechanical Properties of Heavyweight Magnetite Concrete

The objective of this study is to evaluate the workability and various mechanical properties of heavyweight magnetite concrete and examine the reliability of the design equations specified in code provisions. The main parameters investigated were the water-to-cement ratio and substitution level of normal-weight coarse aggregate (granite) for magnetite. The oven-dried unit weight of concrete tested ranged between 2446 and 3426 kg/m 3 . The measured mechanical properties included compressive strength development, stress-strain curve, splitting tensile strength, moduli of elasticity and rupture, and bond stress-slip relationship of concrete. Test results revealed that the initial slump of heavyweight magnetite concrete increased as the substitution level of normal-weight coarse aggregate increases. The substitution level of normal-weight coarse aggregate had little influence on the compressive strength and tensile resistance capacity of heavyweight concrete, while it significantly affected the modulus of elasticity and stress-strain curves of such concrete. The design equations of ACI 349-06 and CEB-FIP provisions mostly conservatively predicted the mechanical properties of heavyweight magnetite concrete, but the empirical equations for modulus of elasticity and splitting tensile strength need to be modified considering the unit weight of concrete.

Effect of Aggregate Size on the Shear Capacity of Lightweight Concrete Continuous Beams

Twenty-four beam specimens were tested to examine the effect of the maximum aggregate size on the shear behavior of lightweight concrete continuous beams. The maximum aggregate size varied from 4 mm to 19 mm and shear span-to-depth ratio was 2.5 and 0.6 in each all-lightweight, sand-lightweight and normal weight concrete groups. The ratio of the normalized shear capacity of lightweight concrete beams to that of the company normal weight concrete beams was also compared with the modification factor specified in ACI 318-05 for lightweight concrete. The microphotograph showed that some unsplitted aggregates were observed in the failure planes of lightweight concrete beams, which contributed to the enhancement of the shear capacity of lightweight concrete beams. As a result, the normalized shear capacity of lightweight concrete continuous beams increased with the increase of the maximum aggregate size, though the increasing rate was lower than that of normal weight concrete continuous beams. The modification factor specified in ACI 318-05 was generally unconservative in the continuous lightweight concrete beams, showing an increase of the unconservatism with the increase of the maximum aggregate size. In addition, the conservatism.

Ductility Evaluation of Heavyweight Concrete Shear Walls with Wire Ropes as a Lateral Reinforcement

This study examined the feasibility of wire ropes as lateral reinforcement at the boundary element of heavyweight concrete shear walls. The spacing of the wire ropes varied from 60 mm to 120 mm at an interval of 30 mm, which produces the volumetric index of the lateral reinforcement of 0.126~0.234. The wire ropes were applied as a external hoop and/or internal cross-tie. Five shear wall specimens were tested to failure under constant axial load and cyclic lateral loads. Test results showed that with the increase of the volumetric index of the lateral reinforcement, the ductility of shear walls tended to increase, whereas the variation of flexural capacity of walls was minimal. The flexural capacity of shear walls tested was slightly higher than predictions determined from ACI 318-11 procedure. The displacement ductility ratio of shear walls with wire ropes was higher than that of shear wall with the conventional mild bar at the same the volumetric index of the lateral reinforcement. In particular, the shear walls with wire rope index of 0.233 achieved the curvature ductility ratio of more than 16 required for high-ductility design.

Shear behavior of reinforced concrete basement walls with consecutive convexo-concave-shaped bars as shear reinforcement

ABSTRACT This study examined the significance and shortcomings of the consecutive convexo-concave shaped shear reinforcement (CCSR) developed as an alternative for transverse crossties in basement walls. Three wall specimens prepared under a full scale were tested under constant axial load and out-of-plane lateral load. In the tests, the CCSR was classified into two types including S-type for the S-curve bending and B-type for approximately 90° bending. To investigate the shear transfer capacity of the CCSR, a mechanism model was driven on the basis of the upper-bound theorem of concrete plasticity. Test results revealed that the CCSR is more favorable to restrict the opening of the inclined cracks and enhancing the shear capacity of the basement wall when compared with the conventional crossties. Wall S (with S-type of CCSR) and Wall B (with B-type of CCSR) possessed 1.21 times and 1.12 times higher shear capacity, respectively, than wall C (with the conventional crossties). The wall S exhibited least rapid descending branch after the peak load, maintaining 80% of the peak load until the wall drift ratio reached 5.4%. The American Concrete Institute (ACI) 318-14 equations extremely underestimate the shear capacity of the basement wall, whereas the predictions obtained from the proposed mechanical model are in better agreement with the test results.