Jae-Sung Mun

Workability and Mechanical Properties of Heavyweight Magnetite Concrete

To provide basic data for developing mixing details and design models for structural heavyweight magnetite concrete, 18 concrete mixtures were prepared under different replacement levels with natural sand and granite coarse particles for magnetite fine and coarse aggregates, respectively. The density of concrete ranged between 3182 and 3400 kg/m³ (197.28 and 210.8 lb/ft³) for replacement by natural sand, and 2446 and 3118 kg/m³ (151.65 and 193.31 lb/ft³) for replacement by granite coarse particles. Test results revealed that the replacement by natural sand was preferable to that by granite coarse particles for enhancing the initial slump, tensile resistance capacity, shear strength and bond behavior with a reinforcing bar of heavyweight magnetite concrete, and for minimizing the decrease in concrete density due to replacement by normal weight aggregates. ACI 349 equations for moduli of elasticity and rupture were generally conservative for heavyweight magnetite concrete. The CEB-FIP equations were not conservative for the moduli of elasticity and rupture, strain at the peak stress, and splitting tensile strength of heavyweight magnetite concrete, whereas they were conservative in predicting the bond stress-slip response of heavyweight magnetite concrete. Overall, it can be concluded that the density of concrete should be considered as a critical factor, together with its compressive strength, in determining the various mechanical properties of heavyweight concrete.

Effect of Curing Temperature Histories on the Compressive Strength Development of High-Strength Concrete

This study examined the relative strength-maturity relationship of high-strength concrete (HSC) specifically developed for nuclear facility structures while considering the economic efficiency and durability of the concrete. Two types of mixture proportions with water-to-binder ratios of 0.4 and 0.28 were tested under different temperature histories including (1) isothermal curing conditions of 5°C, 20°C, and 40°C and (2) terraced temperature histories of 20°C for an initial age of individual 1, 3, or 7 days and a constant temperature of 5°C for the subsequent ages. On the basis of the test results, the traditional maturity function of an equivalent age was modified to consider the offset maturity and the insignificance of subsequent curing temperature after an age of 3 days on later strength of concrete. To determine the key parameters in the maturity function, the setting behavior, apparent activation energy, and rate constant of the prepared mixtures were also measured. This study reveals that the compressive strength development of HSC cured at the reference temperature for an early age of 3 days is insignificantly affected by the subsequent curing temperature histories. The proposed maturity approach with the modified equivalent age accurately predicts the strength development of HSC.

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.

Tests on the Compressive Fatigue Performance of Various Concretes

AbstractIn the present study, six concrete mixtures were prepared to examine the effect of the types of binder and concrete on the fatigue performance of concrete under compression. The selected binder types include ordinary portland cement (OPC); high-volume supplementary cementitious material (SCM) composed of 30% OPC, 20% fly ash (FA), and 50% ground granulated blast-furnace slag (GGBS); and alkali-activated (AA) binder composed of 50% FA and 50% GGBS activated by the combination of Na2SiO3 and Ca(OH)2. Using the prepared binders, normal-weight concrete (NWC) and lightweight concrete (LWC) with a unit weight of 1,700  kg/m3 were produced. For cyclic loading of concrete samples, the constant maximum stress level varied among 75, 80, and 90% of the static uniaxial compressive strength of concrete, whereas the constant minimum stress level was fixed at 10% of the static strength. On the basis of a regression analysis conducted using test results, the fatigue life and fatigue stress-strain curve for concre...

Evaluation of Shrinkage and Creep Behavior of Low-Heat Cement Concrete

This study examined the long-term inelastic characteristics, including unrestrained shrinkage and creep, of low-heat cement concrete under different ambient curing temperatures. To achieve the designed compressive strength of 42MPa, water-to-binder ratios were selected to be 27.5, 30, and 32.5% for curing temperatures of 5, 20, and 40°C, respectively. Test results showed that the shrinkage strains of concrete mixtures tended to decrease with the decrease in curing temperature because of the delayed evaporation of internal capillary and gel waters. Meanwhile, creep strains were higher in concrete specimens under lower curing temperature due to the occurrence of the transition temperature creep. The design models of KCI provision gave better accuracy in comparison with test results than those of ACI 209, although a correction factor for low-heat cement needs to be established in the KCI provision.

Evaluation of Shrinkage of Heavyweight Magnetite Concrete with Fly Ash

The objective of this study is to examine the drying shinkage and autogenuous shrinkage strains of heavyweight magnetite concrete. As a main parameters, cement was partially replaced by fly ash (FA) from 5% to 35%. The measured shrinkage strains were compared with predictions obtained from CEB-FIP equations and Yang et al.`s model. Test results showed that the magnitite of the autogenous and total shrinkage strains of heavyweight concrete slightly increased as the amount of fly ash increases up to 15%, beyond which the strains tended to decrease. The CEB-FIP equations considerably underestimated the shrinkage behavior of heavyweight concrete, indicating that this trend was more notable with the age. On the other hand, Yang et al.`s model predicted accurately the shrinkage of heavyweight concrete.

Evaluation on Fatigue Performance in Compression of Normaland Light-weight Concrete Mixtures with High Volume SCM

(Received December 20, 2014 / Revised December 24, 2014 / Accepted December 25, 2014)The objective of this study is to examine the fatigue behavior in compression of normal-weight and lightweight concrete mixture s withhigh volume supplementary cementitious material(SCM). The selected binder composition was 30% ordinary portland cement, 20% fly-ash, and 50% ground granulated blast-furnace slag. The targeted compressive strength of concrete was 40 MPa. For the cyclic loading, the constant maximum stress level varied to be 75%, 80%, and 90% of the static uniaxial compressive strength, whereas the constant minimum stress level was fixed at 10% of the static strength. The test results showed that fatigue life of high volume SCM lightweight concrete was lower than the companion normalweight concrete. The value of the fatigue strain at the maximum stress level intersected the descending branch of the monotonic stress-strain curve after approximately 90% of the fatigue life.키워드 : 다량치환 혼화재, 경량콘크리트, 압축 피로, 응력-변형률 관계Keywords : High volume SCM, Lightweight concrete, Fatigue in compression, stress-strain relationship* Corresponding author E-mail: whenwhere@kgu.ac.kr