Kwang-Myong Lee

Assessment of interfacial fracture toughness in concrete composites

Abstract Interfaces, such as mortar-aggregate interfaces and cement paste-fiber interfaces, affect the mechanical behavior of concrete composites. Characterization of interfacial behavior is needed to study the role of the interfaces on the global behavior of concrete composites as a basis for the development of high-performance cementitious materials. In this paper, an interface fracture mechanics-based methodology is presented to assess the fracture toughness of mortar-aggregate interfaces. First, sandwich specimens used to develop the fracture toughness curves of mortar-aggregate interfaces are described. Then, an experimental and numerical simulation study is presented of a two-phase beam composite model consisting of a mortar matrix and an aggregate inclusion to investigate the crack penetration versus crack deflection scenarios in the interfacial regions. The results from the composite model study are correlated with those obtained from testing sandwich specimens.

Characteristics of Autogenous Shrinkage for Concrete Containing Blast-Furnace Slag

The use of blast-furnace slag (BFS) in making not only normal concrete but also high-performance concrete has several advantages with respect to workability, long-term strength and durability. However, slag concrete tends to show more shrinkage than normal concrete, especially autogenous shrinkage. High autogenous shrinkage would result in severe cracking if they are not controlled properly. Therefore, in order to minimize the shrinkage stress and to ensure the service life of concrete structures, the autogenous shrinkage behavior of concrete containing BFS should be understood. In this study, small prisms made of concrete with water-binder (cement+BFS) ratio (W/B) ranging from 0.27 to 0.42 and BFS replacement level of , , and , were prepared to measure the autogenous shrinkage. Based on the test results, thereafter, material constants in autogenous shrinkage prediction model were determined. In particular, an effective autogenous shrinkage defined as the shrinkage that contributes to the stress development was introduced. Moreover, an estimation formula of the 28-day effective autogenous shrinkage was proposed by considering various W/Bs. Test results showed that autogenous shrinkage increased with replacement level of BFS at the same W/B. Interestingly, the increase of autogenous shrinkage is dependent on the W/B at the same content of BFS; the lower W/B, the smaller increasing rate. In concluding, it is necessary to use the combination of other mineral admixtures such as shrinkage reducing admixture or to perform sufficient moisture curing on the construction site in order to reduce the autogenous shrinkage of BFS concrete.

Fundamental Properties of Cement Composites Containing Lightly Burnt MgO Powders

The volume change in concrete takes place with changes in temperature and water content immediately after concrete casting. In the early age stage, the thermal and drying shrinkages can cause cracks that are very crucial to the durability of concrete. It was reported that when the cement with lightly-burnt MgO powder was used, the shrinkage of concrete can be reduced. This study investigates fundamental properties of cement composites with lightly burnt MgO powder by performing various experiments. The stability test results verified that MgO powder in cement composites does not cause any abnormal expansion. Also, the hydrate product analysis results obtained from MgO cement paste showed that MgO powder reduces the shrinkage at the longterm ages. In addition, the cement composites containing the proper amount of MgO powder could improve compressive strength. Finally, the shrinkage reduction from using MgO powder can be optimized by increasing MgO replacement level and curing temperature.

Influence of Mineral Admixtures on the Resistance to Sulfuric Acid and Sulfate Attack in Concrete

It has been well known that concrete structures exposed to acid and sulfate environments such as sewer, sewage and wastewater, soil, groundwater, and seawater etc. show significant decrease in their durability due to chemical attack. Such deleterious acid and sulfate attacks lead to expansion and cracking in concrete, and thus, eventually result in damage to concrete matrix by forming expansive hydration products due to the reaction between portland cement hydration products and acid and sulfate ions. Objectives of this experimental research are to investigate the effect of mineral admixtures on the resistance to acid and sulfate attack in concrete and to suggest high-resistance concrete mix against acid and sulfate attack. For this purpose, concretes specimens with three types of cement (ordinary portland cement (OPC), binary blended cement (BBC), and ternary blended cement (TBC) composed of different types and proportions of admixtures) were prepared at water-biner ratios of 32% and 43%. The concrete specimens were immersed in fresh water, 5% sulfuric acid, 10% sodium sulfate, and 10% magnesium sulfate solutions for 28, 56, 91, 182, and 365 days, respectively. To evaluate the resistance to acid and sulfate for concrete specimens, visual appearance changes were observed and compressive strength ratios and mass change ratios were measured. It was observed from the test results that the resistance against sulfuric acid and sodium sulfate solutions of the concretes containing mineral admixtures were much better than that of OPC concrete, but in the case of magnesium sulfate solution the concretes containing mineral admixtures was less resistant than OPC concrete due to formation of magnesium silicate hydrate (M-S-H) which is non-cementitious.

Numerical evaluation of interface fracture parameters using ADINA

Abstract The problem of cracking at interfaces is pertinent not only to composite materials but also to structures with adhesive joints, thin films and coatings. Criteria are needed for the study of the crack growth scenarios in the interfacial region where relative magnitudes of the fracture energy for the constitutent materials and that of the interface play an important role in cracking behavior. Interface fracture occurs when the interfacial energy release rate G is equal to the interfacial fracture energy Γi, that is characterized as a function of the loading phase ψ. Therefore, for the analysis of interfacial fracture both the energy release rate (G) and the loading phase angle (ψ) must be evaluated. In this paper, a numerical method is presented to obtain the values of these two parameters at the tip of an existing interface crack. Numerical analyses of three models containing interface cracks are performed using the ADINA finite element program. In general, good agreement between the results from the numerical calculation and the analytical solutions is obtained.

Autogeneous Shrinkage Characteristics of Ultra High Performance Concrete

본 연구에서는 고성능 콘크리트에 흡수율이 높은 경량 잔골재를 이용하여 시멘트 수화과정에 추가적인 수분을 공급하는 내부양생을 적용하여 수축을 저감하고자 하였다. 이를 위해 내부양생 재료로 경량골재를 사용하였으며, 잔골재 대체율에 따른 자기 및 건조수축에 대한 실험을 진행하였다.

A Study on the Carbonation Characteristics of Fly Ash Concrete by Accelerated Carbonation Test

The increase of industrial carbonic dioxide emissions has accelerated the carbonation of reinforced concrete struc- tures, which drops off their durability. Although advanced countries have already taken safety control measures against the car- bonation of RC structures, it is still difficult now to accurately predict the actual carbonation depth. Additionally, it requires much time and efforts. Recently, it is possible to get the data more rapidly through accelerated carbonation test with the CO2 concentration of 100%. In this paper, the carbonation test results obtained by two test methods such as the normal carbonation test method and the accelerated carbonation test method, were compared to investigate the carbonation characteristics of fly ash concrete. The accel- erated carbonation test on concrete specimens with the pre-curing age of 180 days was also carried out to examine the carbonation characteristics of fly ash concrete at long-term age. Consequently, fly ash concrete at early age was vulnerable to carbonation and however, its carbonation resistance at long-term ages was improved compared with OPC concrete.

Probability-Based Durability Analysis of Concrete Structures under Chloride Attack Environments

Recently, a variety of researches has been carried out to obtain a more controlled durability and long-term performance of concrete structures under chloride attack environments. In particular, new procedures for probability-based durability analysis/design have been noticed to be very valuable for the enhancement of service life of concrete structures. Although there is still a lack of relevant data, this approach has been successfully applied to some new concrete structures. In this paper, the diffusion equation based on Ficks second law has been solved with a time dependent diffusion coefficient and the probabilistic analysis of the durability performance has been carried out by using a Monte Carlo Simulation. From the results, the influence of each parameter on the durability of concrete structures was investigated and the new procedure for durability analysis was demonstrated in terms of chloride penetration data from various concrete structures. The new procedure might be very useful in designing important concrete structures and help to predict the remaining service life of existing concrete structures under chloride attack environments.

Durability Characteristics of Concrete Containing Lightly Burnt MgO Powder

MgO concrete containing lightly burnt MgO powder at may have a long-term expansibility characteristic. Such expansibility of MgO concrete can compensate the shrinkage at later ages since the hydration of the MgO is very slow. However, the addition of MgO delays the initial hydration of cement and increases the setting time of cement. Also, the porosity and pore-size distribution of the MgO concrete are different from OPC concrete. Therefore, in order to use MgO in practice, both mechanical and durability properties of MgO concrete should be carefully examined. In this study, durability tests on carbonation, freezing-thawing, and diffusion of chloride were carried out after 56 days of underwater curing at to compare durability characteristics of 5% MgO-mixed concrete with those of OPC concrete. The results showed that MgO concrete shows a greater durability than the concrete with no MgO, because the micro structure in the MgO concrete is much denser due to its expansibility characteristic.

Microcapsule-Type Self-Healing Protective Coating for Cementitious Composites with Secondary Crack Preventing Ability

A microcapsule-type self-healing protective coating with secondary crack preventing capability has been developed using a silanol-terminated polydimethylsiloxane (STP)/dibutyltin dilaurate (DD) healing agent. STP undergoes condensation reaction in the presence of DD to give a viscoelastic substance. STP- and DD-containing microcapsules were prepared by in-situ polymerization and interfacial polymerization methods, respectively. The microcapsules were characterized by Fourier-transform infrared (FT-IR) spectroscopy, optical microscopy, and scanning electron microscopy (SEM). The microcapsules were integrated into commercial enamel paint or epoxy coating formulations, which were applied on silicon wafers, steel panels, and mortar specimens to make dual-capsule self-healing protective coatings. When the STP/DD-based coating was scratched, self-healing of the damaged region occurred, which was demonstrated by SEM, electrochemical test, and water permeability test. It was also confirmed that secondary crack did not occur in the healed region upon application of vigorous vibration to the self-healing coating.