Self-compacting concrete was first developed in 1988 to achieve durable concrete structures. Since then, various investigations have been carried out and this type of concrete has been used in practical structures in Japan, mainly by large construction companies. Investigations for establishing a rational mix-design method and self-compactability testing methods have been carried out from the viewpoint of making self-compacting concrete a standard concrete.
This paper summarizes the relation between the molecular structure and the dispersion-adsorption mechanisms of 3 types of comb-type superplasticizers used in Japan. The action mechanisms of comb-type superplasticizers and the compatibility of cements and superplasticizers are influenced by the molecular structure of polymers such as copolymer components and the grafted chain length of poly (ethylene-oxide) (PEO). Many reports regarding concrete research have investigated the influence of comb-type superplasticizers on the fluidity of concrete and the production of selfcompacting concrete. However, many have not considered the effect of the molecular structures of comb-type superplasticizers. This paper should be useful for engineers and researches studying the action of comb-type superplasticizers in the production of concrete with comb-type superplasticizers, and for understanding any new properties of such concrete.
More than ten years have elapsed since Self-Compacting Concrete (abbreviated as SCC), which does not require consolidation, made its appearance. SCC has been applied in many construction projects, including ultra-large structures. To achieve excellent self-compactability in heavily reinforced areas, SCC should deform well under its own weight without segregation of ingredients. Therefore, research on SCC had been focused on improving both deformability and resistance to segregation. However, there was still no standard test for evaluating self-compactability and resistance to segregation of SCC. The authors developed evaluation test methods for simple application in the manufacturing stage and at construction sites. At present, these evaluation test methods are recommended as standard test methods in Japan Society of Civil Engineers specifications. At the time, it was still difficult to produce SCC of constant quality at ordinary ready-mixed concrete plants because facilities and quality control of materials had not reached the required level. For improved manufacturing and handling, an SCC with a viscosity agent (β-1,3-Glucan) was developed. It is possible to reduce quality fluctuations in a fresh state for commonly available materials and production facilities. At present, this type of SCC is defined as "Combination-type" in a recommendation of the Japan Society of Civil Engineers. This paper summarizes these evaluation test methods and introduces a Combination-type SCC developed to facilitate production and handling.
The authors succeeded to make a liquefied viscosity agent Welan gum with AE superplasticizer. When the Welan gumis added into a given concentration of AE superplasticizer and agitated well, Welan gum particles swell in the AE superplasticizer, resulting in stable suspension without much viscosity increase.
The bonding of moderately-long anchorages to ordinary and high-performance silica-fume concrete is studied here with reference to size effect. To this purpose, 24 anchorages (L/db = 10), consisting of a quasi-smooth, micro-roughened bar embedded in a concrete cylinder, were cast and tested up to the pull-out of the bars, which had 4 different diameters (db= 5, 12, 18 and 26 mm). For each of the 8 cases examined here (4 diameters × 2 mixes), 3 nominally-identical specimens were tested. Though the primary objective of this study is to investigate whether a general-type size-effect law applies to bond in high-performance concrete, the modeling of an anchorage by means of a local elastic-fracturing frictional bond-slip law is also carried out, and two approaches are adopted for the description of size dependency, the first based on a stress criterion for the debonding, and the second on an energy criterion. The former approach leads to a size-dependent formulation of the local bond strength, while the second approach makes it possible to evaluate the debonding energy, which is size independent. In both approaches the values of the fundamental parameters of the model are identified by means of a bidimensional or mono-dimensional, least-square regression procedure. The results confirm that size effect in bond should be taken into consideration directly in the codes, and pave the way to the study of size effect in high-bond bars.
This paper reports the results of investigation on the chloride diffusivity and corrosion resistance of volcanic pumice(VP) blended cement mortars with varying curing age of up to one year. The mortars had 0, 15% and 30% VP as cement replacement and water/binder ratio of 0.55. Tests were conducted to determine the chloride ion migration coefficient(Di) of the mortars. In addition, electrical resistivity, mercury intrusion porosimetry, and differential scanning calorimetry (DSC) tests were conducted. Electrochemical measurement was used to monitor the corrosive behaviour of the embedded steel bars. It was found that blending cement with VP significantly reduced the long-term Di and hence increased the long-term corrosion resistance of mortars. This fact was also supported by the presence of lower quantity of Ca(OH)2 and higher quantity of Friedel's salt in the VP blended mortars as observed from the DSC tests. Mortars with 30% VP showed better performance in terms of chloride ion diffusivity, chloride ingress and passivation period of embedded steel compared to control mortar with 0% VP.
Marine durability of 30-year-old concrete specimens made with ordinary portland cement (OPC), high early strength portland cement (HES), moderate heat portland cement (MH), slag cement of type B (SCB), and alumina cement (AL) was investigated. Other parameters include sulfate content in cement, mixing water, and different exposure zones. Compressive strength, chloride ingress, corrosion of steel bars in concrete, microstructure, mineralogy of concrete, and steel-matrix and aggregate-matrix interfaces were investigated. Chloride ingress in concrete was sequenced as OPC, HES, MH > SCB > AL. However, for AL mixed with tap water, corrosion on steel bars in concrete was higher. For SCB and AL, the pore volume at the outer region of the specimensis reduced due to the ingress of chloride and other ions from seawater.
A detailed experimental investigation was carried out to understand the change in mechanical properties of concrete, surface strain over the specimens and the steel bars embedded in concrete, crack growth, and change in mechanical properties of steel bars under sustained stress induced by the Alkali-Silica Reaction (ASR) in concrete. Also, chloride diffusion in concrete, corrosion over the steel bars, investigation on ASR gel at the porous matrix region by SEM, and visual examination of the ASR gel on the cut surface of the specimens were carried out. For this, beam specimens of size 250x250x600 mm and cylinder specimens of diameter 100 mm and length 200 mm were made. Beam specimens were made with and without reinforcement. Various restrained conditions by the reinforcement were provided. Additional NaOH was added to raise total Na2O equivalent alkali content to 6 kg/m3 in concrete. The specimens were submerged in seawater at temperature 40°C in a closed container. The investigation was carried out for 383 days. Young's modulus of concrete drops significantly due to ASR immediately after cracking. However, it becomes stable later. The reduction in compressive strength was not as significant as Young's modulus. Internal restraint provided by the steel bars results in the reduction of surface strain in the restraint direction. The degree of restraint has a significant influence on the surface strain as well as strain over the bars. A good linear relation between the surface strain and the strain over the steel bars is found, especially for the cases with highly restrained conditions. Less surface strain is observed if the steel bars are placed near the concrete surface, however, the strain over the bars is increased for this arrangement. Placing longitudinal steel bars near the surface also results in the significant lateral surface strain as well as higher strain over the stirrups, if any.
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