Recently, resin concrete, which is a kind of composite materials made of synthetic resins, sands, gravels and so on, has become recognized as a regular structural material. But, as this material has not been in service for a long time unlike cement concrete which has been applied to structural purposes for about two centuries, it is very necessary to investigate its durability. Particularly, the durability study in water is most important for the case of resin concrete made of unsaturated polyester resin, from the point of view of the property of the resin.
From this point of view, we have carried out some accelerated experiments on its durability in water and on its chemical resistance, and tried to clarify the ageing behavior of this material by deriving an experimental equation for durability in its property from these experiments. In addition, the same material was immersed in water at room temperature for about six years. The results showed that this resin concrete has very excellent durability in comparison with cement concrete.

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The rebar splicing method discussed in this report consists of a double-tapered frustum-shaped cylinder with an annular-grooved inner wall, which is filled with high-strength non shrink grout, LL-602, to effect splicing between separate bars butted in the sleeve. It can be applied with best advantage to connections between precast structural members in the vertical direction. This report covers a part of our development research on the Splice Sleeve method, including the tests on isolated rebar splices and structural columns containing splice sleeve connections in them.
It was found from the testing of sleeve length vs. splice strength and compressive strength of grout vs. splice strength, that the sleeve length (L) must be 10 fold of nominal diameter (D) in order to obtain the 125% of the nominal yielding load of the bar used. It was also found that, to obtain the splice strength higher than JIS specified tensile strength, L must be about 14D and the minimum compressive strength of grout must be higher than 650kg/cm². No adverse effects were recognized on the sleeve properties according to the fire-resistance tests. No difference in the splice strength was observed between the pre- and post-grouting. Misalignment between upper and lower rebar did not affect on the splice strength.
Structural member tests were performed to evaluate the effectiveness of the splice sleeve connection in reinforced concrete columns in comparison with the column monolithically casted without splicing of reinforcing bars. The tests were conducted on Y-Type and U-Type column specimens, each provided with different combinations of the factors such as axial forces, types of transverse reinforcement around splice sleeves, shear keys at connections, grouting methods, etc.
The results of these tests showed equal or superior performance of the splice sleeve connections as compared with the columns without connections.

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Thermo-dependency is one of the most important properties of resin concretes when they are used as structural members.
Compressive strength, compressive modulus of elasticity and tensile strength as well as compressive creep properties of resin concrete were investigated at various temperatures. Atmospheric temperatures adopted in the tests were 5, 20, 40, and 60°C.
The results obtained are summarized as follows.
(1) The compressive strength of polyester resin concrete decreases almost linearly as atmospheric temperature rises from 5°C to 60°C. And this tendency is very pronounced in the mixes with higher resin content.
(2) The compressive modulus of elasticity also decreases linearly as atmospheric temperature rises from 5°C to 60°C.
(3) The splitting tensile strength decreases as atmospheric temperature rises. However, in this test, it attained the maximum value at 40°C, probably because of the internal stress caused by setting shrinkage.
(4) The resin content has appreciable influence on thermo-dependency of creep deformation.
(5) The creep curve shows a portion proportional to the time, and it can be supposed to be due to viscous flow. The creep rate of this portion increases rapidly when the atmospheric temperature rises.
(6) The primary creep, which is the remainder of total creep deformation minus viscous flow, seems to show the minimum at 40°C.
(7) 60-100 percent of the creep deformation at 20°C recovers in about 40 days after unloading.

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The temperature dependence of the optimum exposure dose for polymerization of polymer-impregnated concrete (PIC) was examined on the mortar specimens containing MMA monomer cured by gamma-rays at the dose rate of 0.9×10⁶R/hr. In addition, the strength of PIC irradiated with gamma-rays in water was compared with that of the one polymerized in air. Furthermore, the effect of size on the attenuation of gamma-rays in mortar was discussed in the present paper.
Judging from the strength of PIC, the optimum total exposure dose is dependent upon the dose rate and temperatute. It is necessary to increase the exposure dose when PIC is polymerized at lower temperatures (Fig. 9). If the polymerization is carried out at the optimum exposure dose, the strength of PIC takes almost the same value, regardless of the dose rate and temperature (Fig. 7). No significant difference is found between the strengths of PIC polymerized in air and in water (Fig. 10). As the exposure dose attenuates about half per 4cm thickness of mortar, the exposure dose of over one megaroentogen at the dose rate of 2.5×10⁵R/hr is required at the center of cross section or at the shadow face of mortar.

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Generally, the steam curing method is adopted as an accelerated curing method of cement concrete products, but very few reports concerning the properties of steam-cured polyester resin concrete have appeared until now, in fear of hydrolysis of polyester resin by hot water or steam. In precast polyester resin concrete factories, the relatively expensive heat cure method by circulation of hot air is usually adopted for curing of products. The objective of this study is to examine the possibility of applying steam cure to polyester resin concrete in the same manner as cement concrete.
Polyester resin concrete (polymer concrete) is mixed with variable catalyst or accelerator content for liquid resin in selected mix proportion, and the resin concrete specimens are prepared under various steam-curing conditions. The cured resin concrete specimens are tested for tensile and compressive strengths. The test results indicate that, in the catalytic system for ordinary temperature cure, the strength development of polyester resin concrete is accelerated by steam cure without noticeably harmful hydrolysis of polyester resin observed.

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In future, a large-scale application of resin concrete and polymer-impregnated concrete is expected in the field of ocean engineering structures (offshore buildings, floating structures, etc.). The ocean engineering structures are exposed under more severe external forces than land structures as permanent structures, and so in such application of resin concrete and polymer-impregnated concrete it is most important to know whether they can be used safely as materials for ocean engineering structures.
In this paper, the marine exposure tests of polyester resin concrete (abbrev. REC), polystyrene-impregnated concrete (abbrev. PIC) and cement concrete (abbrev. CC) are conducted in the Inland Sea of Seto, where high needs for ocean engineering structures are expected in future. Furthermore, for the design consideration of ocean engineering structures, the change of mechanical properties, the shape change needed for wave force estimation, the weight change needed for dead load estimation, etc., are discussed.
The test results indicate no mechanical deterioration for all specimens. However, when compared with cement concrete, the degree of fouling with sea living creatures on polyester resin concrete and polystyrene-impregnated concrete is remarkably large.

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Such characteristics of fresh concrete as workability, consistency, placeability, finishability, pumpability etc. belong to complex qualities which can not be measured directly. In order to evaluate quantitatively the characteristics of fresh concrete, it is necessary to seek the help of the knowledge of rheology which is the field of science dealing with the deformation and the flow of material.
A series of studies have been designed to clarify the feasibility of applying rheology to evaluate the characteristics of fresh concrete. They included the examinations on the results obtained by means of the rotational viscometer and the tri-axial compressive test device, and on the propagation properties of dynamic wave.
The present paper describes the design of a rotation viscometer and the experimental investigation on rheological characteristics of fresh paste and mortar obtained with this apparatus.

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It is expected that a large quantity of concrete will be used for constructing the marine structures such as port facilities, with increasing utilization of ocean. When one considers the difficulties accompanied in supplying water for mixing concrete in such a large scale of construction, one can easily see several problems urgently to be solved concerning the properties of the concrete using sea water as mixing water.
This paper presents some results of the investigation on the properties of concrete using sea water, particularly the various mechanical properties of the concrete at young ages at both ordinary and relatively low temperatures.
The results obtained are summarized as follows:
(1) The unit water content in the concrete should be increased by about 2.5 percent when sea water is used.
(2) More air is entrained in supper-high-early-strength cement concrete than in normal cement concrete at ordinary temperatures. However, the relation is reversed under low temperature.
(3) When the strength index is defined as a percentage of the strength of a given concrete to that of the standard one, the strength index of normal cement concrete using sea water decreases with age, while that of the high-early-strength cement or supper-high-early-strength cement concrete remains almost unchanged.

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Experimental studies were conducted on the strength and deformation properties and durability of high strength concrete having more than 800kg/cm² strength, which was fabricated by five different mixing proportions and cured under four different conditions including steam and autoclaved curing. In addition to natural sand and crushed stones, cement clinker was also used as a concrete aggregate. High strength concrete containing various level of entrained air was also examined. The compressive, flexural and tensile strengths as well as static and dynamic moduli of elasticity and Poisson's ratio of the test specimens were obtained periodically up to the age of one year.
The following experimental results were obtained: in order to produce a certain level of compressive strength and workability, the clinker aggregate concrete required a considerably less unit water level, consequently reducing the unit cement level, than that of the natural aggregate concrete. While the air entrained concrete required a large unit water level, consequently increasing the unit cement requirement.
All the concrete specimens cured under various conditions increased their strength as they were aged. However, the rate of strength increase between the age of 28 days and one year was about 20 percent for the fog room cured concrete and only few percent for the autoclaved concrete.
The modulus of brittleness, i.e., the ratio of compressive strength to tensile strength, of high strength concrete fall between 14 and 18 and was larger than that of ordinary concrete.
The freezing and thawing resistance test indicated that the durability of autoclaved concrete without entrained air was poor, while the durability of fog room cured concrete without entrained air and autoclaved air entrained concrete was excellent. Air entrainment in high strength concrete has potential usage because it yields good durability and easier handling properties in field, although the air entrainment causes negative effect on the strength of the products.
Modulus of elasticity of autoclaved concrete was smaller than that of the concrete cured under other conditions.

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The flexural failure of reinforced concrete section is generally caused by crushing of concrete at the compression zone. For the calculation of ultimate flexural capacity or corresponding deformation of a beam, the compressive fiber strain of concrete at failure should be assumed. In practice, the empirical values, for instance, ε_cu=0.25∼0.35%, are being used in the calculation. However, these values are not always clearly defined from the theoretical viewpoint.
In this study, based on the stress-strain curve of concrete having the strain softening region, the theoretical estimation was made on the compressive fiber strain of concrete at the ultimate flexural capacity as well as that at the failure or the crushing of concrete in compression zone of the reinforced concrete rectangular beam section. The results obtained are as follows:
(1) The extreme compressive fiber strain to be used for ultimate flexural strength calculation is defined as the value at which the compressive stress brock coefficient, k₂/k₁k₃, obtained from the stress-strain curve of concrete becomes minimum, where k₁k₃ and k₂ denote the ratio of the average stress of stress-strain curve up to an arbitrary strain to the compressive strength and the ratio of the location of center of gravity of the corresponding area to the arbitrary strain, respectively.
(2) It is reasonable to assume that the compression failure of concrete in compression zone takes place primarily when the distance of neutral axis from compressive fiber becomes minimum. The corresponding compressive fiber strain can be defined from the stress-strain curve of concrete as the strain at the maximum value of stress brock coefficient, k₁k₃.
(3) Practically, the compressive strain at the failure of concrete described in (2) can be used for the calculation of ultimate flexural strength or deformation of beams.
(4) The increase of negative slope of the strain softening region in concrete stress-strain curve reduces the value of compressive fiber strains, especially that for the failure of concrete. On the contrary, the increase of strain at the peak stress (that is, at the compressive strength) in stress-strain curve results in much increase of the ultimate compressive fiber strains.

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For testing the complete deformation characteristics of brittle materials such as rocks and concrete, a stiff testing machine should be employed, and the compressive force acting on a specimen should be preferably controlled by the rate of strain in the specimen. The authors firstly designed and constructed a uni-axial compressive testing machine of oil pressure type controlled automatically and accurately by the rate of strain by using a servo-control system.
Secondly, performance of this machine was examined by testing specimens of concrete with various brittleness. The tests show that merely by this control system the apparent stiffness of the machine nearly equal to the brittleness factor of the specimen can be adjusted up to 90t/mm. For specimens with higher brittleness factors, up to 140t/mm, satisfactory results can be obtained by adopting stiff columns which are subjected to compression together with specimens.
It is also proved that the rate of deformation can be controlled smoothly from 0.001 to 2mm/min for specimens with the total deformations of about 2mm, and from 0.0025 to 0.5mm/min for specimens with those of about 0.5mm. Thus, the complete stress-strain diagram of concrete can be obtained under the strain rate from 1×10^-6/sec to 140×10^-6/sec, even for high strength concrete of high brittleness.
It is found that the Young's modulus, the compressive strength and the brittleness factor of concrete increase with increasing the rate of strain, while the strain corresponding to the maximum stress decreases, and that among ordinary concrete, lightweight aggregate concrete and mortar, the brittleness of the third is greatest, and that of the second is greater than that of the first.

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In this study, the initiation and the propagation behaviours of fatigue cracks emanating from a circular hole were investigated under cyclic tensile loading for 18% Ni maraging steel sheets. Fracture surfaces were observed directly by a scanning electron microscope, while the crack features on specimen surfaces were observed by an optical microscope.
The main results obtained were summarized as follows.
(1) Slopes of S-N curves were very steep and the ratio of the number of cycles to crack initiation to the total number of cycles to failure was approximately equal to or greater than 0.5 in the region of finite life. This value was fairly large comparing with the values obtained for various carbon steels and aluminum alloys.
(2) In the range of stable propagation after the crack length exceeds 0.3mm, the following exponential relation exists between the crack propagation rate and the stress intensity factor.
dL/dN=C(ΔK)^m
Parameters C and m do not depend on the stress level, and m is approximately equal to 4.
(3) In the range of stable propagation, a few small cracks were observed near the main crack tip in the surface layer, and these cracks grew individually until joined each other. However in the inner region, no small crack preceding the main crack was observed and it was thought that the fatigue damage was accumulated in a limited small region near the crack tip during a certain number of stress cycles, and this region was fractured instantaneously when the damage exceeded a certain value.

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In the fatigue test at elevated temperatures, the test pieces must be pre-heated to the prescribed testing temperature. In LTQ low carbon steel, therefore, the age-hardening takes place during such pre-heating. In the present study, the rotary bending fatigue test was performed at elevated temperatures ranging from room temperature to 500°C and under the running speed of 3500rpm, after pre-heating the specimens for 1hr. The main results were as follows.
(1) In the pre-heating at temperatures from room temperature to 150°C, the surface hardness increased with heating time. The hardness reached maximum at 150°C after 1hr. At 300°C and 400°C, the over-age hardening phenomenon was seen after 1hr. This is considered due to carbide dispersions.
(2) The hardness values at 1hr heating were Hv 135, Hv 150, Hv 165, Hv 125 and Hv 120 at room temperature, 100°C, 150°C, 300°C and 400°C, respectively. In the subsequent fatigue test at 150°C, the hardness decreased with increasing the number of stress cycles. On the other hand, in the tests at 100°C and 300°C, the hardness increased with increasing the number of stress cycles.
(3) The fatigue strength at 10⁷ cycles σ_w were 23kg/mm², 26kg/mm², 23kg/mm², 30kg/mm² and 25kg/mm² at room temperature, 100°C, 150°C, 300°C and 400°C, respectively. These fatigue strengths seem to be unrelated with the hardness values at the start of fatigue tests after 1hr pre-heating.
(4) All of the values of σ_w for LTQ steel were higher than those for annealed steel. It seems that this fact in LTQ steel is due to the combined contribution of age-hardening in pre-heating and cyclic stressing. On the other hand, in annealed steel, the effect of age-hardening is less than that in LTQ steel.
(5) In the LTQ steel as well as in the annealed steel, in the temperature range up to 500°C the fatigue strength became maximum at 100°C and 300°C, and the value at 300°C was higher than that at 100°C.

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This paper presents a method for estimating the fatigue life of S40C steel, with or without a notch, under multiple Gaussian random loadings.
First, a random S-N curve for a simple Gaussian random load is so proposed theoretically that the cumulative damage Σ(n/N) calculated from the curve equals unity, with the aid of the frequency of peak stress representing an approximate Rayleigh distribution. The curve is of the form:
N_fσ^m'=C'Γ(1+m/2),
where m, m' and C' are material constants and Γ(x) represents a gamma function.
Second, an attempt is made to estimate the fatigue life under multiple Gaussian random loads applied to the material over its whole life. It is found that the cumulative damage is given by the sum of the cumulative damage of each component Gaussian random load, and that fatigue life can be easily estimated by using the aforementioned S-N curve. This implies that the random S-N curve proposed is very useful for fatigue life estimation under random loading.

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