Konstantin Kovler

PREVENTION OF AUTOGENOUS SHRINKAGE IN HIGH-STRENGTH CONCRETE BY INTERNAL CURING USING WET LIGHTWEIGHT AGGREGATES

Restrained autogenous shrinkage in high-strength lightweight aggregate concrete was investigated. Effects of a partial replacement of normal-weight aggregate by lightweight aggregate on autogenous shrinkage were also discussed. The concrete with saturated lightweight aggregate exhibited no autogenous shrinkage, whereas the normal-weight concrete with the same matrix exhibited large shrinkage. A partial replacement of normal-weight aggregate by 25% by volume of saturated lightweight aggregate was very effective in eliminating the autogenous shrinkage and restrained stresses of the normal-weight concrete. It should be noted that the internal supply of water from the saturated lightweight aggregate to the high-strength cement matrix caused continuous expansion, which may be related to continuous hydration.

RILEM TC 162-TDF: Test and design methods for steel fibre reinforced concrete' - sigma-epsilon-design method - Final Recommendation

General information Publication status: Published Organisations: Section for Structural Engineering, Department of Civil Engineering Contributors: Vandewalle, L., Nemegeer, D., Balazs, L., Barr, B., Barros, J., Bartos, P., Banthia, N., Criswell, M., Denarie, E., Di Prisco, M., Falkner, H., Gettu, R., Gopalaratnam, V., Groth, P., Hausler, V., Kooiman, A., Kovler, K., Massicotte, B., Mindess, S., Reinhardt, H., Rossi, P., Schaerlaekens, S., Schumacher, P., Schnutgen, B., Shah, S., Skarendahl, A., Stang, H., Stroeven, P., Swamy, R., Tatnall, P., Teutsch, M., Walraven, J. Pages: 560-567 Publication date: 2003 Peer-reviewed: Yes

The effect of dehydroxylation/amorphization degree on pozzolanic activity of kaolinite

The effect of heat treatment parameters on the dehydroxylation/amorphization process of the kaolinite-based materials such as natural and artificial kaolin clays with different amounts of amorphous phase (metakaolin) was investigated. The procedure for quantitative estimation of amorphous phase in the heat-treated kaolinite materials was developed. The process of dehydroxylation/amorphization of kaolinite was characterized by DTA/TGA with mass-spectrometry and X-ray powder diffraction. The influence of the heat treatment temperature and content of the amorphous phase on pozzolanic activity was studied. Finally, the relationships between the chemical activity, activity strength index and the amorphous phase content were found and discussed. The results obtained are important for an optimization of the process of the metakaolin large scale production and its use as an active pozzolanic admixture.

Natural radionuclides in building materials available in Israel

Abstract Most building materials contain naturally occurring radioactive elements, the most important of which are potassium 40 K and the members of two natural radioactive series, which can be represented by the isotopes of thorium 232 Th and radium 226 Ra. The presence of these radioisotopes in the materials causes external exposure to the people who live in the building. In addition, the disintegration of radium 226 Ra increases the concentration of radon gas 222 Rn and of its daughters in the house. Concentrations of natural radionuclides ( 226 Ra, 232 Th, 40 K) in the samples of building products, building binders, in lightweight aggregates, normal-weight aggregates and in industrial by-products used in the construction industry in Israel were determined by a gamma-ray spectrometer with a Ge-detector. A methodology was introduced to regulate the use of building materials that causes an increase in indoor radiation exposure. The results indicate that a radium equivalent varies within the range of 158.8– 18.8 Bq / kg in the samples of building products, 74.3– 17.5 Bq / kg in the samples of building binders, 164.5–17.7 Bq/kg in the samples of aggregates, and 761.4– 241.6 Bq / kg in the samples of industrial by-products used in construction.

Influence of cement paste matrix properties on the autogenous curing of high-performance concrete

Abstract A novel approach to has been recently proposed mitigate self-desiccation, one of the foremost problems of high-performance concrete (HPC). It is based on incorporation of pre-soaked lightweight aggregate in the concrete mix. Such aggregate acts as an internal water reservoir preventing reduction of relative humidity in the cementitious matrix. This method is known as “autogenous” or “internal” curing. Recent studies demonstrated that this kind of curing could be successfully applied to obtain improved HPC with reduced sensitivity to cracking. However, the content of lightweight aggregate required to completely eliminate autogenous shrinkage was high, and this caused a reduction of compressive strength and an increase in the cost of the concrete. Recently, a work has been conducted to optimize the internal curing strategy by eliminating autogenous shrinkage while using the smallest possible amount of lightweight aggregate. The effect of grain size, pore structure and type of the lightweight aggregate was studied. The next step in this study––the effect of the properties of the cement paste matrix on the effectiveness of internal curing is discussed in this paper.

Autogenous shrinkage and induced restraining stresses in high-strength concretes

Development of internal stresses induced by restrained autogenous shrinkage in high-strength concretes at early ages was investigated. Effects of water/binder ratio and the presence of silica fume on the stress developed were evaluated and considered in conjunction with the creep behavior of the concretes. The restrained autogenous shrinkage resulted in a relatively high stress that sometimes caused premature cracking in the high-strength concrete. This occurred mainly when the ratio between the restraining stress and the tensile strength approached 50%. The stresses were not as high as expected from the autogenous shrinkage, since considerable relaxation took place due to creep. Thus, the viscoelastic nature of the concrete at early age has a considerable influence on the stresses generated.

Pumice Aggregates for Internal Water Curing

A novel concept in internal curing of High Performance Concrete is based on dispersing very small, saturated lightweight aggregates (LWA) in the concrete, containing sufficient water to counteract self-desiccation. With this approach, the amount of water in the LWA can be minimized, thus economizing on the amount of the LWA. In this study, the pore structure of different size fractions of pumice aggregates was characterized by different techniques. The different size fractions show differences in porosity, sorption behavior, and pore-size distribution. The smaller size fractions have lower water absorption, but they release a greater percentage of their absorbed water at the equilibrium relative humidity of practical interest in early-age concrete, above 90%. Additionally, early-age properties of mortars with different contents of saturated pumice were investigated: a reference mix without pumice and mixes with 4% and 8% pumice by volume of mortar. By addition of pumice, mortars with improved strength, enhanced degree of hydration and reduced autogenous shrinkage were obtained. An important obstacle to the application of this kind of pumice for actual concrete production is saturation of the particles, which can be achieved only by immersion in boiling water or by vacuum saturation.

Evaluation of early age cracking characteristics in cementitious systems

Early age cracking has re-emerged as an important issue in modern concretes, and it has its impact in developing new formulations for high strength and repair. Adequate performance with respect to cracking cannot be properly assessed on the basis of free shrinkage tests. More advanced methodologies need to be developed and applied, to consider in addition to the strains obtained also the stresses and stress relaxation. The present paper presents an overview of the testing techniques and methodologies of early age cracking. It classifies the tests into four categories: ring, plate, longitudinal and substrate restraint and assesses their significance and limitations.RésuméLa fissuration au jeune âge apparaît à nouveau comme un problème capital dans les bétons modernes. Elle a un impact sur le développement de nouvelles formules déterminant la haute résistance et la réparation. Les bonnes performances concernant la fissuration ne peuvent être évaluées comme il le faut sur la base d’analyses du retrait libre. Il faut développer et appliquer des méthodologies plus avancées pour pouvoir prendre en considération, en plus des déformations obtenues, les tensions ainsi que les contraintes de relaxation. Cet article présente une vue d’ensemble des analyses techniques et des méthodologies de la fissuration au jeune âge. Il classifie les essais en quatre catégories: anneau, plaque, longitudinal et base de contrainte, et il les classe selon leur signification et leurs limites.

Setting and Hardening of Gypsum-Portland Cement-Silica Fume Blends, Part 2: Early Strength, DTA, XRD, and SEM Observations

The present work describes a study of setting and hardening of fresh blends of gypsum binder, Portland cement, and silica fume. Such blends can possess the advantages of gypsum (early hardening, high early strength, enhanced workability) and Portland cement (improved durability in moist conditions), but are free of the deleterious effect of ettringite and thaumasite, which are formed when gypsum and Portland cement interact. In part 1, the temperature and setting expansion dependencies in time are described. The influence of superplasticizer on setting and hardening of gypsum, gypsum-cement, and gypsum-cement-silica fume mixes is analyzed. In part 2, the data of early strength, DTA, XRD, and SEM observations are reported and discussed.

Setting and Hardening of Gypsum-Portland Cement-Silica Fume Blends, Part 1: Temperature and Setting Expansion

Abstract The present work describes a study of setting and hardening of fresh blends of gypsum binder, Portland cement, and silica fume. Such blends can possess the advantages of gypsum (early hardening, high early strength, enhanced workability) and Portland cement (improved durability in moist conditions), but are free of the deleterious effect of ettringite and thaumasite, which are formed when gypsum and Portland cement interact. In part 1, the temperature and setting expansion dependencies in time are described. The influence of superplasticizer on setting and hardening of gypsum, gypsum-cement, and gypsum-cement-silica fume mixes is analyzed. In part 2, the data of early strength, DTA, XRD, and SEM observations are reported.