International Concrete Abstracts Portal

This paper presents an experimental investigation on shrinkage and creep of high strength concrete, in which 200 X 250 X 300mm specimens were used, sealed or dried and loaded at diferent ages, including earlier age after casting. For this purpose high strength concrete was made with high-early strength cement and silica fume. The effects of the age at which drying and loading started, on shrinkage and creep properties are discussed. Creep at early age of loading develops very quickly. The earlier the age when drying started, the larger the total shrinkage is, and the relation between increment of autogenous shrinkage after drying and increment of drying shrinkage is approximately linear, independent of the age when drying started. Based on the experimental results, the equations of CEB-FIP MODEL CODE 1990 are modified by using regression analysis for predicting autogenous shrinkage and creep of high strength concrete at various ages.

In recent years, especially in Japan, various types of chemical admixtures have been developed, with emphasis on making highly durable concrete and on developing the highly flowable concrete for labor-saving. In this paper, one kind of water-repellent admixture incorporating highly reactive silica powder, denoted as WRP, was examined to confirm its effect on various concrete properties. The concept is not only to prevent water penetration due to water repellency provided by the siloxane compound but also to compensate for the hindered hydration due to its adsorption to cement particles by the use of the highly reactive pozzolanic material. The efficacy of the admixture was confirmed as to its ability reducing the water permeability with little lowering of mechanical properties dependent on the dosage of WRP. From the tests for carbonation, drying shrinkage, resistance to freezing and thawing and the air-void systems, the effect of WRP was confirmed as satisfactory for those properties.

Admixtures based on sulfonate melamine formaldehyde condensate polymer (SM) have been available commercially for more than three decades. These products are widely used by the precast concrete industry, but with some limitations on slump retention and early strength. To improve these properties, synthesis of a modified sulfonate melamine formaldehyde condensate polymer (MSM) was performed. In this paper its physical and chemical characteristics are reported. Dispersibility and adsorption tests, using cement pastes, were carried out to compare the superplasticizing effect of the new MSM polymer against the SM polymer. Concrete mixtures were also made to evaluate the performances of this MSM polymer in comparison with the other types of superplasticizers such as SM and B-naphthalene sulfonate formaldehyde condensate (SN). Results of these tests show that MSM polymer has a better water reduction capacity, lower air content in the fresh concrete and higher early strength at 1 day as compared to the other superplasticizers under the tests conditions. These characteristics are very much desired by the precast industry for easy placement of concrete and quick turn around of the formworks. Field tests also reported, confirm the laboratory findings.

This paper deals with various types of silanes (alcohol or water sol-vent, polymerization degree) and silane systems, which are typical hydrophobic agent as repair materials. From the viewpoint of the deterioration mechanism, water control in concrete structures is one of the most important repair strategies. Therefore, various types of surface treatments which can control water content in concrete structures are applied for repair work. Silanes and silane systems were one of the most important for hydrophobic treatment. In this study, the effect of various types of silanes and silane treatment systems on the hydrophobicity of treated concrete is investigated. Furthermore, water control property in concrete treated with silane system is discussed.`

Portland cement contains sulfur compounds from the clinker phase and from added calcium sulfate (e.g. gypsum) which acts as a set regulator. The purpose of this investigation was to study the influence of the sulfate content in the clinker phase on the performance of superplasticized concrete mixtures in terms of initial slump level at a given water-cement ratio (0.49, slump-loss rate, and compressive strength at early and later ages. Two batches (A and B) of clinker from the same kiln source were studied, the main difference being the content of sulfate (SOs) in the clinker (0.72% and 1.40% respectively). Different percentages of natural gypsum, as set regulator, were interground in a laboratory mill to manufacture portland cements: A1 , A2 , A3 from clinker A, and B1 , B2 , B3 from clinker B. Three levels of total sulfate content in terms of SOs were set: 3.0% in portland cements A1 and B1 , 3.5% in portland cements A2 and B2 ; 4.0% in portland cements A3 and B3. At a given sulfate content in portland cement, the lower the clinker sulfate content, the more effective is the slump increase of the concrete caused by the superplasticizer addition. Moreover, the lower is the clinker sulfate content, the lower is the slump-loss rate of the superplasticized concrete mixture. Finally, at a given water-cement ratio, there is a reduction in the compressive strength at early ages (< 3 days) when the low sulfate clinker is used to manufacture portland cements. These results are related to the effect of the clinker sulfate content on the degree of cement hydration: the lower the clinker sulfate content, the lower the early cement hydration in terms of gypsum consumption, ettringite formation, and tricalcium silicate (alite) hydration.