This report outlines the experimentally obtained results on (1) a special admixture for use with the ECL (Extruded Concrete Lining.) method capable of maintaining the flowability of concrete for 2-4 hours after mixing and of developing excellent strength, and (2) agents used to improve the set acceleration and the strength development of concrete used with the wet shotcrete method. The purpose of these experiments was to improve the quality of lining concrete.
As reinforced concrete structures have become large, cracking caused by thermal stress is becoming a serious problem. For reducing heat of hydration, Super Low Heat Cement has been developed. This cement has extremely low heat of hydration, that is approximately 55cal/g at age of 28 days, which is about 35 and 20cal/g lower than that of normal portland cement and moderate heat portland cement, respectively. The strength of the concrete using this cement was found to be about the same as that using normal portland cement. Nevertheless the adiabatic temperature rise of the Super Low Heat Cement concrete was approximately 10°C lower than that of normal portland cement. The same tendency was recognized when used for concrete with segregation resistivity in water, too. Therefore, it is evident that use of this cement is very effective for reducing the temperature rise of mass concrete members, in the atmosphere or in water.
The authors have proposed a visco-plastic finite element method (VFEM) to estimate the deformation of fresh concrete by using a homogeneous continuum model. In this paper, a new method which is called the suspension element analysis (SEM) is proposed as another approach to the flow simulation of fresh concrete. The simulation method by SEM supplements the weakness of one phase model in VFEM by using a two phase model consisting of aggregate and matrix. SEM can consider the effects of aggregate, segregation and more complex behaviors of concrete such as collapse, separation, and mixing, and some examples of analytical results obtained by SEM are shown herein.
A new unique concrete production procedure has been developed utilizing small ice pieces. The concrete is made with small ice pieces substituted for the mixing water, at the start of mixing. The ice must completely melt prior to concrete placement, as a general rule. Various required properties of the concrete during production, including mixing efficiency, placement performance and consolidation ability are greatly improved with the use of ice. Also presented in this paper are practical applications of this new method to ready mixed concrete for actual buildings. The data demonstrate that the workability of fresh concrete with small ice pieces is much better than that of conventional concrete. As an application of this method using small ice pieces, a two-step hydration technique for high quality concrete products has been developed. The first step hydration is carried out with a very small amount of water which is dispersed uniformly on a macroscopic scale among the cement particles. After the relative positions of the cement particles have been fixed, water absorption and the second step hydration are carried out. By this two-step hydration technique, very high quality concrete products with little variation in quality can be easily made.
A water reducing agent P, an agent for extremely stiff consistency and lean mix concrete R, an agent for lean mix concrete Y and an air entraining agent V were used as the admixtures. The consistency and compaction characteristic of extremely stiff consistency and lean mix concretes with these admixtures were investigated by comparison. The VC value of the concrete decreased linearly with increasing unit admixture content. The decrement degree of VC value was about 5 to 10 seconds per 1kg/m3 of admixture content and it was almost the same for all the admixtures. The flow value of mortar comprised in the concrete increased with increasing unit admixture content. The increment degree of flow value for the mortar with admixture R or Y was considerably higher than that of mortar with P or V. The flow values of mortars with admixture R, Y, P and V were 160, 140, 117 and 116, respectively, for the concrete with the VC value 20 seconds. The optimum compaction periods to obtain the efficient compaction degree of concrete with admixture R, Y, P and V were 120, 160, 190 and 270 seconds, respectively, for the VC value 20 seconds. The VC values of the concretes with admixture Y, R, P and V changed from 20 seconds to 30, 38, 51 and 51 seconds, respectively, at the elapsed time after mixing 90 minutes.
It has been recognized that the use of superplasticizer is very effective to improve the workability of concrete. This study was planned to make clear the properties of fresh and hardened artificial lightweight aggregate concrete with superplasticizer, and to provide useful information for the mix proportion and the practice of such concrete. The artificial lightweight coarse and fine aggregates were used. Also, the normal crushed stone and natural sand were used as the control aggregate. The effects of the type of aggregate, the type and kind of chemical admixture, the sand/aggregate ratio and cement content on the properties of super -plasticized lightweight aggregate concrete were experimentally investigated. The slump, air content and compressive strength under various conditions were measured. The workability of superplasticized lightweight and normal aggregate concrete was evaluated by V.B. test, compacting factor test, rheological test and segregation test. It has been proved that the workability of superplasticized lightweight aggregate concrete could be improve as high as that of superplasticized normal aggregate concrete by choosing a suitable mix proportion.
Cement concrete and mortar have some disadvantages such as low tensile strength, large drying shrinkage and low extensibility. Cracks frequently occur in various concrete structures and mortar walls. In particular, the large drying shrinkage is the most important factor affecting crack formation. Recently, surfactant-type shrinkage-reducing agents for concrete and mortar have been developed in Japan. The shrinkage-reducing agents (SRA) are not only added to fresh concrete and mortar, but also applied on hardened concrete and mortar surfaces. However, the effectiveness of SRA treatments for thin mortar walls in wooden buildings has not been reported till now. The purpose of this study was to make clear the effect of drying shrinkage reduction on the thin mortar by treating with SRA. Mortar specimens with or without SRA were prepared with thicknesses of 10, 20 and 40mm, and covered with an epoxy resin paint except for one side of each specimen. Then four types of SRA were treated at coverage rates of 100 and 300g/m2 on the surfaces of the mortar specimens without SRA, and the penetration depth of SRA was measured. The drying shrinkage of the mortar specimens was also measured up to 364 days of dry curing periods. The conclusions obtained from the test results are summarized as follows: (1) The drying shrinkage of the mortars is considerably reduced by treating with the shrinkage -reducing agents, and depends on the coverage rate of the agents and the thickness of the mortars. The drying shrinkage can be predicted as a function of the coverage rate. (2) The effectiveness of the shrinkage-reducing agent treatments increases with increasing coverage rate of the agents and with decreasing thickness of the mortars. (3) On the basis of the above results, the shrinkage-reducing agent treatments are applicable to reduce the drying shrinkage of the finishing mortars troweled on the metal lath bases of wooden buildings.
A fundamental research was carried out on silica fume concrete in order to obtain some properties such as workability, air entrainment, porosity, development of strength and durability to freezing and thawing action. A silica fume containing 85% of silica by weight, produced in Japan, was used in this study. Three levels of silica fume content, 0, 15 and 25% of replacement ratio to cement by weight, and five levels of water-to-cementitious materials ratio (W/(C+SF)) from 30% to 70% were chosen. The decrement in slump caused by the incorporation of silica fume was adjusted by the use of superplasticizer. The dosage of superplasticizer needed to keep the same workability as that of the control concrete increased linearly with silica fume content. It is difficult in practice to entrain the air of more than 4% in the mix of low W/(C+SF) ratio where the silica fume replacement ratio is 25%. The optimum replacement ratio of silica fume to cement was 15% as far as the development of strength and the durability to freezing and thawing action were concerned. The air-entrained concrete with the silica fume replacement ratio of 15% showed satisfactory result for durability to freezing and thawing action up to 300 cycles, although the air-entrained concrete with the silica fume replacement ratio of 25% did not show any good result for durability to freezing and thawing action, even though it had the entrained air of 4.4% and W/(C+SF)=50%.
This report presents the results of the investigation on the influence of steam curing on GFRC using GFRC cement of low alkaline type. The glass fiber content and condition of steam curing were varied. Prism specimens were tested for compressive strength, flexural strength, shear strength, drying shrinkage and load-deflection curve. These investigations provided the following results; The strength of GFRC using GFRC cement became higher with the increase in the presteaming period and in the wet curing period after steam curing. Not only the early-age strength development but also the later strength development for steam-cured GFRC using GFRC cement were improved compared with GFRC using ordinary portland cement. The adoption of steam curing was effective in decreasing the drying shrinkage of GFRC. The toughness of GFRC using ordinary portland cement was not so good after the curing period. Especially, the wet-cured GFRC gave the lowest value. It was considered that the alkali resistant properties of GFRC could be improved by the use of GFRC cement.
This study was carried out as a part of the research program to develop a tesing method by using a concrete specimen for determining the occurrence of alkali aggregate reaction. In order to prevent the cracking damage of concrete structure by alkali-aggregate reaction and to devise a counterplan, it is important that the reactivity of aggregate is evaluated in advance, and the characteristics of expansion and crack occured in concrete using reactive aggregate are precisly understood. In this study, the several conditions affecting the expansion and crack in concrete, that is, length change with the lapse of time, alkali contents in concrete, storage conditions, and characteristics of crack, were investigated experimentally. Moreover, the degree of cracking damage was evaluated from the correlation between the crack development and the deterioration of concrete.
Chloride permeability of concrete is one of the major factors that influence the corrosion of steel reinforcement in concrete. It has been confirmed that the chloride permeability of cement paste and mortar varies widely depending on the water: cement ratio, type of cement, curing condition and so on. However, there are only a few reports concerning the chloride permeability of concrete. This study aims at revealing the effect of curing condition on the chloride permeability of concrete in which Portland cement was replaced in various amounts by flyash and blastfurnace slag. The chloride permeability of concrete was determined by the accelerated chloride permeability test using a diffusion cell similar to that described in Federal Highway Administration report (FHWA/RD-81/119). In order to relate the porosity or pore size distribution of concrete to its chloride permeability, the mercury intrusion porosimetry measurements were also conducted. The experimental results showed that the chloride permeability of concrete containing flyash and blastfurnace slag was high when the concrete was exposed to a low humidity at the early stage of curing. It was also found that the chloride permeability of concrete increased proportionally with increasing volume of pores larger than 0.1μm in diameter.
In recent years, it became possible to detect feeble electric signals supersensitively owing to the development of electric measuring techniques. Now, an advanced technique using ultrasonic pulses is being applied in the concrete field too. The waveform of ultrasonic pulses through concrete contains various informations related to the internal structure of material. So, it may be possible to clarify the internal structure of material by analyzing the measured waves in detail. Thus, ultrasonic spectroscopy is useful to evaluate the quality of material by measuring the frequency characteristics of ultrasonic pulse through material. In this study, the ultrasonic spectroscopy was applied to evaluate the quality of concrete exposed to high temperature conditions, as the first step to establish a new non-destructive testing technique for concrete and mortar. The main results obtained in this study are summarized as follows: 1) Compressive and flexural strengths of concrete decreased according to temperature rise. Especially the flexural strength of mortar was sensitively affected by temperature rise. 2) The ultrasonic pulse velocity, maximum amplitude and energy of measured ultrasonic pulse waves decreased according to temperature rise. 3) The maximum amplitude and energy of frequency transfer function of concrete decreased with increasing temperature, and these decreasing rates were not affected very much by the water-cement ratio and heating duration. 4) The local maximum amplitude and energy of the frequency transfer function in the frequency ranges of 50-100kHz and 200-250kHz were closely related to the change of internal structure of concrete exposed to high temperature.