Sung-Gul Hong

Shear Testing of Steel Fiber-Reinforced Lightweight Concrete Beams without Web Reinforcement

This study investigates the effect of steel fibers on the shear strength of lightweight concrete beams without web reinforcement. Twelve beams were tested under four-point loads, including three normalweight steel fiber-reinforced concrete (SFRC) beams and six steel fiber-reinforced lightweight concrete (SFRLC) beams. The other variables include the shear span-depth ratio (a/d) (2, 3, and 4) and steel-fiber volume fraction (Vf = 0, 0.5, and 0.75%). Results show that the addition of steel fibers with Vf of 0.75% increased the shear capacity by 30% and promoted a ductility of 5.3 or higher. The findings also indicate that the a/d adversely affects the shear capacity. The compressive strength of SFRLC was increased by 13% for Vf =0.5% and 20% for Vf=.75%, indicating that the effectiveness of steel fibers is greater in lightweight concrete than in normal weight concrete. The shear capacity of the SFRC beam was greater than that of the SFRLC beam at a given deflection due to the increased material properties of SFRC. A design shear strength model for SFRLC beams without web reinforcement is proposed based on these results.

Effective Capacity of Diagonal Strut for Shear Strength of Reinforced Concrete Beams without Shear Reinforcement

The appropriate evaluation of the effective capacity of a concrete strut is an important factor in the analysis and design of concrete members using the strut-and-tie model. Current design codes for the strut-and-tie model introduce this factor using the effective compressive strength of concrete in a strut or nodal zone. This study considers that the mechanism of diagonal cracking reduces the width of a concrete strut and hence causes a reduction in the capacity of the concrete strut. Based on this approach, models for predicting the diagonal cracking strength and ultimate shear strength of simply supported beams without shear reinforcement are proposed, with the concrete strength, shear span-depth ratio (a/h), and longitudinal reinforcement ratio as the primary parameters. The predicted values are compared with proven test data from various published experiments and codes of practice to show the validity of the proposed models.

Acceleration of Intended Pozzolanic Reaction under Initial Thermal Treatment for Developing Cementless Fly Ash Based Mortar

Without using strong alkaline solution or ordinary Portland cement, a new structural binder consisting of fly ash and hydrated lime was hardened through an intensified pozzolanic reaction. The main experimental variables are the addition of silica fume and initial thermal treatment (60 °C for 3 days). A series of experiments consisting of mechanical testing (compressive and flexural strength, modulus of elasticity), X-ray diffraction, and measurements of the heat of hydration, pore structure, and shrinkage were conducted. These tests show that this new fly ash-based mortar has a compressive strength of 15 MPa at 91 days without any silica fume addition or initial thermal treatment. The strength increased to over 50 MPa based on the acceleration of the intensified pozzolanic reaction from the silica fume addition and initial thermal treatment. This is explained by a significant synergistic effect induced by the silica fume. It intensifies the pozzolanic reaction under thermal treatment and provides a space filling effect. This improved material performance can open a new pathway to utilize the industrial by-product of fly ash in cementless construction materials.

Microstructural Investigation of Heat-Treated Ultra-High Performance Concrete for Optimum Production

For optimum production of ultra-high performance concrete (UHPC), the material and microstructural properties of UHPC cured under various heat treatment (HT) conditions are studied. The effects of HT temperature and duration on the hydration reaction, microstructure, and mechanical properties of UHPC are investigated. Increasing HT temperature accelerates both cement hydration and pozzolanic reaction, but the latter is more significantly affected. This accelerated pozzolanic reaction in UHPC clearly enhances compressive strength. However, strength after the HT becomes stable as most of the hydration finishes during the HT period. Particularly, it was concluded that the mechanical benefit of the increased temperature and duration on the 28 day-strength is not noticeable when the HT temperature is above 60 °C (with a 48 h duration) or the HT duration is longer than 12 h (with 90 °C temperature). On the other hand, even with a minimal HT condition such as 1 day at 60 °C or 12 h at 90 °C, outstanding compressive strength of 179 MPa and flexural tensile strength of 49 MPa are achieved at 28 days. Microstructural investigation conducted herein suggests that portlandite content can be a good indicator for the mechanical performance of UHPC regardless of its HT curing conditions. These findings can contribute to reducing manufacturing energy consumption, cost, and environmental impact in the production of UHPC and be helpful for practitioners to better understand the effect of HT on UHPC and optimize its production.

Behavior of Concrete Members at Elevated Temperatures Considering Inelastic Deformation

A constitutive model of concrete subjected to elevated temperature is suggested in this study. The model is composed of four strain components: free thermal strain, mechanical strain, thermal creep strain, and transient strain due to moisture. The thermal creep strain of concrete is derived from the modified power-law relation for steady state creep. Mathematical description on the multi-axial creep behavior of concrete is also presented. The transient strain component is made in order to consider rapid irreversible strain change of moisture diffusion and evaporation. Some applications for the proposed model are carried out by a nonlinear analysis and compared with the test results. The comparisons with the test results show that the proposed model gives a good agreement and the influences of inelastic strain changes at elevated temperatures are very important for the structural response at elevated temperatures.

Computer-supported evaluation for seismic performance of existing buildings

A seismic performance evaluation is one of the highly complicated multi-criteria evaluation problems. This study presents a hierarchy of performance criteria that consists of performance categories such as strength, stiffness, configuration, and age of structures at a higher level while their subcategories considering the capacity of subsystems and components at lower levels mainly focus on the stage of preliminary evaluation. Two methods for determination of performance index are proposed; a simple composition method and a fuzzy inference method. Both methods use weighting factors to represent relative importance of multi-criteria in the determination of performance index. The simple composition method that operates on definite index values and assumes independence between performance criteria is easy to use but difficult to address the cases where complicity and uncertainty are of important problems. The fuzzy inference system uses fuzzy concepts where uncertainties are inherently unavoidable due to insufficient information and engineering assumption and exploits fuzzy rules in the multi-criteria evaluation by replicating heuristic knowledge and experience in logic-based descriptive rules. Moreover, the single performance index obtained by defuzzification enables us to draw a representative and comprehensive seismic performance index from a set of individual performance indexes at multi-levels.

Cyclic Lateral Testing of Precast Concrete T-Walls in Fast Low-Rise Construction

An innovative precast concrete (PC) T-wall panel system was developed to enhance constructibility and lateral load resistance of fasttrack, low-rise buildings. The system consists of bolt-type connections between PC wall panels and emulated cast-in-place joints between the flange-wall and web-wall components. To confirm its lateral-load-resisting and seismic performance, reversed cyclic tests of two two-thirds-scale PC T-walls with and without diagonal reinforcing bars were conducted under displacement control. Test results showed that the T-wall specimen without diagonal reinforcement performed reasonably well in terms of lateral stiffness, strength, and ductility, except for slip behavior. On the other hand, the use of supplementary diagonal reinforcement in each panel adversely affected the lateral ductility and energy dissipating capacity. All the details of the bolt-type connections between the lower and upper panels proved to be robust and practical. Finally, simplified prediction methods for strength and displacement are presented that can be used to develop design guidelines.

Performance of Fresh and Hardened Ultra High Performance Concrete without Heat Treatment

This study investigates the relationship between the performance of fresh and hardened Ultra-High Performance Concrete (UHPC) without heat treatment. The performance of fresh UHPC is determined by the slump flow test related to the fluidity of concrete mixtures, and the air content test. The variables of these tests are the water to binder ratio, superplasticizer dosages and volume fractions of steel fiber. Generally, insufficient fluidity and excessive air contents in concrete mixtures lead to the insufficient packing density related to the performance of harden concrete. The performance of hardened UHPC is determined by the compressive and flexural tensile tests. The results of the fresh UHPC tests show that there is the linear correlation between each variable and the slump flow diameter, and that the slump flow diameter is linearly decreased as the air content ratio increase. Using these results, the formula is developed to predict the fresh performance before mixing UHPC. The results of the hardened UHPC tests show that the hardened performance is not influenced by the air content ratio in the range of 3.2 to 4.2 per cent. However, the flexural tensile strength dominantly influenced by the volume fractions of steel fiber.

Application of entity-based approach for unified representation of design alternatives for structural design

Abstract To support the design process fully from preliminary to detailed design stages in a natural way, a computer-integrated design system is needed. In early design stages more than one design alternatives are considered as possible solutions. The representation of design alternatives must be uniform and unbiased to be equally treated. In this study, an entity-based approach has been adopted to develop product and process models for representing design alternatives, which is more desirable for top–down design process because it allows high-level abstraction in representing design information and design activities. The entity-based approach has several benefits: (1) a unified representation of design alternatives; (2) a consistent development of product and process models based on the entity-based concepts; and (3) an easy integration of the product and process models. The work toward product and process models for structural design presented in this paper is a useful step toward integrated computer-aided design systems.

Impact Resistance of Steel Fiber-Reinforced Concrete Panels Under High Velocity Impact-Load

This paper describes the evaluation of the impact performance of steel fiber-reinforced concrete based on high-velocity impact experiments using hard spherical balls. In this experimental study, panel specimens with panel thickness to ball diameter (h/d) ratios of 3.5 or less were tested with variables of steel fiber volume fraction, panel thickness, impact velocity, and aggregate size. Test results were compared with each other to evaluate the impact resistance. The results showed that the percentage of weight and surface loss decreased as the steel volume fraction increased. However, the penetration depth increased with up to steel fiber volume fraction of 1.5%. Particularly the results of specimens with 20 mm aggregates showed poorer performance than those with 8 mm aggregates. The results also confirmed that the impact performance prediction formulas are conservative with (h/d) ratios of 3.5 or less. Despite the conservative predictions, the modified NDRC formula and ACE formula predict the impact performance more consistently than the Hughes formula.