International Concrete Abstracts Portal

new method of producing high-strength porous concrete has recently been developed in Japan. In this method, porous concrete is produced by coating the coarse aggregates with a high-strength mortar, then applying vibration to fuse them. Porous concrete produced bythis method has the following properties. It has a continuous void system with a volumeko-total-void ratio exceeding 85% and a compressive strength of 20 MPa or greater when water permeability is 2 crnkc. The porous concrete produced exhibited good durability in freezing and thawing tests (ASTM C666, Procedure B) and while the amount of drying shrinkage is about 60%, or smaller than that of conventional concrete, such shrinkage occuring very rapidly.

Recently, accompanying the increase in higher strength and flowability of concrete, polycarboxylate-based superplasticizers have attracted much attention. In order to develop an advanced superplasticizer accommo-dating new functions, we attempted to develop polycarboxylate-based high-range water reducing superplasticizers with high durability as a new function. To increase the durability of concrete, we focused on shrinkage reduction, one of the factors affecting durability, and studied various compounds exhibiting shrinkage reduction. We investigated the relationship between their chemical structures and shrinkage reducing effects. In addition, we synthesized a new series of superplasticizers in which these compounds were attached to the structure of various polycarboxylate polymers, and determined the shrinkage reduction effect by testing concrete with these agents. As a result, we have developed a polycarboxylate-based advanced superplasticizer exhibiting excellent shrinkage reduction.

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.