A special cement admixture for making high strength concrete has been developed. The basic properties of concrete made with this admixture of 5-10wt% and some water reducing agent were reported in this note. The compressive strengths of about 800kgf/cm2 and 1000kgf/cm2 were achieved at the ages of one day and 28 days, respectively, by steam curing at normal pressure. The flexural strength, tensile strength, elastic modulus, creep, drying shrinkage and frost resistance of the concrete were also determined, and they were found to be superior.
Flowing concrete was produced by adding some superplasticizers and their properties were examined experimentally. The results obtained are as follows. (1) The most suitable sand percentage in flowing concrete can be approximately determined from the amount of slump. (2) There is such trend that the flowing properties improve with increasing unit amount of cement. It is desirable that concrete contains more than 420kg/m3 of fine particles of less than 0.3mm size. (3) The flowing property and slump loss of flowing concrete vary depending upon the temperature and timing of the addition of superplasticizers. (4) The compressive strength develops up to three years in the same trend as in the case of low slump concrete. (5) The relationship between compressive strength and tensile one or static modulus of elasticity is the same as that of conventional concrete. (6) The drying shrinkage and the durability to freezing and thawing are the same as those of low slump concrete. Also, several examples of job site application of the flowing concrete were given along with the mix proportion used and the reason of adopting the present method.
Recently, much attention is being paid to new methods utilizing concrete admixtures. Perhaps it is because many kinds of concrete have been used in different fields of construction and chemical admixtures have been recognized as a necessary material in concrete. In this study, in order to improve the properties of fresh concrete, several kinds of concrete were made in the following ways. (1) To improve the effectiveness of water reduction, the timing of addition of admixtures was delayed for a short moment, such as 30, 45, 60 and 120 second, after mixing was finished. (2) To recover the consistency of concrete, a water reducing admixture was added 60min. after mixing was finished. (3) To produce a flowing concrete, a superplasticizer was added to the base concrete with a water reducing admixture 60min. later. The measurements of slump, bleeding, setting time, entrained air content, compressive strength were carried out on these specimens. The results, obtained showed that a high quality concrete could be achieved economically without deteriorating significantly the subsequent hydration of cement and the final products.
Slump loss of concrete has been attributed ambiguously to the chemical hydration reaction. On the assumption that the physical coagulation of cement particles during the dormant stage is responsible for the slump loss to a large extent, the phenomenon was analyzed in this paper by the theory of colloidal coagulation. From the results obtained, it was found that the half loss time of concrete slump (S1/2) or cement paste flow (F1/2) was approximately proportional to the time during which the particle number was reduced to a half of the initial (t1/2). The technique of repeated partial dosing of superplasticizers applied in this test was found to be a useful way to keep slump for a long time. A newly developed automatic metering and injecting device installed on a truck agitator was quite effective to maintain the slump before placement.
The mechanisms of segregation in mixtures under action of vibration have not been completely understood in the field of concrete technology. In the present study, the phenomenon of segregation in a mixture of ideal simple system consisting of the bed of fine particles and a single large sphere particle was investigated experimentally, and the degree of segregation defined by the segregation time was evaluated by the motion of a single large particle through a dry fine particle bed. The frequency of vibration was selected to be 50 to 150Hz with the acceleration ranging from 2 to 15g, and the effect of acceleration at a constant frequency of vibration on the segregation time was measured. The effects of the size, shape and density of the large particle, the particle size and the grading of fine granular were also studied. The results obtained provided useful informations to define the fundamental mechanism of segregation in fresh concrete.
This paper deals with slag cement concrete without utilizing portland cement. The results of the experimental studies are summarized as follows. The slag cement can be made conveniently with industrial wastes of granulated slag and gypsum from the desulfurization process. The most preferable slag-gypsum ratio was found to be 9:1. With the slag cement, the compressive strength of concrete ranging from 250kg/cm2 to 450kg/cm2 can be obtained easily. When W/C is in the range of 45∼65%, the nature of σ28∼C/W curve is linear, which is the same as ordinary concrete. The slag cement concrete has an advantage of low heat of hydration and disadvantages of long setting time, rapid carbonation and small air content in the case of AE concrete. The rest of the properties are similar to those of C-PB cement concrete.
The vibrating compaction, pressure compaction and pressure curing of fiber reinforced concrete were investigated for the purpose of utilizing steel fiber to concrete products. In the vibrating compaction, the adoption of high frequency or high vibrating accelerator was profitable to improve the quality of fibrous concrete. The pressure compaction and pressure curing of steel fiber reinforced concrete were very effective to obtain high flexural strength and to improve the impact resistance. As the effect of reinforcement is increased by the pressure method, it is possible to reduce the fiber content for a given required strength.
Recently polyester resin concrete has become widely used in the field of construction works in Japan, but a method of predicting its strength has hardly been proposed yet. The purpose of this study was to examine the properties of orthophthalate-type polyester resin concrete with various styrene-unsaturated polyester ratios (ST/UP), and then to find the method for strength prediction. Polyester resin concrete was prepared with variation of ST/UP, and the effects of ST/UP on the properties such as the compressive, tensile and flexural strengths, stress-strain relationship, modulus of elasticity and length change were discussed. It is concluded that the strength prediction of orthophthalate-type polyester resin concrete is possible to some extent by applying the ST/UP-strengths relationships, and that the properties such as the stress-strain relationship, modulus of elasticity and length change are considerably affected by ST/UP.
Many studies on resin concrete (REC) have been made so far, and now more detailed investigations for use of REC as structural members are gradually becoming important. The objective of this study was to obtain the mechanical properties of REC structural members, especially in relation to the following two points: (1) effectiveness of using low shrinkage resin or glass fiber rod (GF) having low modulus of elasticity in order to decrease internal tensile stress due to shrinkage restriction of reinforcing bar, (2) improvement of cracking moment and ultimate moment of REC beam by using low shrinkage resin or glass fiber. The results showed that in normal type REC beams with GF the internal tensile stress decreased, preventing a reduction in cracking moment. In addition, bonding between GF and REC was good, and therefore high strength of GF could be used effectively. But GF beams showed some brittleness in cracking behaviors and deformation. On the other hand, low shrinkage resin concrete beams with SBPD were superior because of their low internal tensile stress, good bonding property, high cracking and ultimate moments. But beams of this type had similar brittleness in deformation behaviors. These features were considered due to characteristics of REC having high strength but low ductility. So, allowance should be made in using REC beam for service load or safety factor corresponding to the purposes and places for the members to be used in service.
It is well known that the strength and the modulus of elasticity of some materials are greater for rapid application of load than for slow loading. The theoretical interpretation of this fact has been given in the past based on the linear viscoelastic theory. The present experimental study was carried out to investigate the effects of strain or stress rate the compressive ultimate strength, the modulus of elasticity and the deformation characteristics of cement mortar and concrete at age of 1 to 28 days. The test variables were: the mean rate of stress, σ=0.01 to 35 (kgf/cm2·sec), the mean rate of strain, ε=0.15 to 450 (×10-6/sec); the test age of 1, 3, 7, and 28 days; the water-cement ratio W/C=0.45 to 0.75. 10φ×20cm size cylindrical specimens were used for all tests. The test results were summarized as follows. The dependence of the ultimate strength on strain rate for concrete was more remarkable at an early-age than at the age of 28 days. On the contrary, this dependence for cement mortar was remarkable at the age of 28 days. The strain at the ultimate strength of concrete at an early-age was influenced by strain rate, but it was not influenced at 28 days. The coefficient of viscosity of concrete λ is expressed by the equation as λ=aε-b, and the relationship between stress rate and strain rate is given by σ=αεβ. The variation of concrete strength σu, which is brought by various rates of loading, can be expressed by the following equation based on the rheological theory, where σu, s is the statical ultimate strength. σu=σu, s+λε(t)
It is well known that the pads used in splitting tests have a great influence on the splitting tensile strength of concrete. This study was aimed at clarifying the effect of the padding materials on the splitting tensile strength of concrete in relation to the fracture mechanism of concrete by carrying out the experiments and the FEM analyses on both the actual and model concrete. The experimental results of the actual concrete showed that the influence of the padding materials on the splitting tensile strength depended on the types of fine and coarse aggregate, the volume fraction of coarse aggregate in concrete and the friction between pad and specimen. The FEM analyses showed that the stress distribution within the model concrete was not so much affected by the padding materials before cracking, while, after cracking, the padding materials influenced the initiation and development of cracks as well as the stress distribution of the model concrete. These facts suggest that the effects of the padding materials on the splitting tensile strength of concrete are closely related to the complicated structure of concrete.
The purpose of this work was to establish a method of estimating impact bending capacity for brittle materials. The test materials used in this paper were cement mortar and concrete. A pendulum type impact bending testing machine built as a trial was employed. The main results obtained are as follows. (1) In the region of impact speed V0≤3m/sec, the apparent kinetic energy absorbed in a specimen increases in proportion to impact energy, and this increase is nearly equal to inertial resistance of the specimen. (2) The just breaked energy is better than the kinetic energy absorbed in a specimen as the basic value to estimate impact bending capacity, since the latter depends on impact energy but the former is not influenced by it. (3) When the general equation on impact (eq. (1)) is applied on a similar specimen which has a constant bending span ratio, the impact resistance of the specimen is given by the form of impact energy in unit volume ΔU0/BDl. There is a close correlation between this value and the static tensile strength. This shows that impact bending fracture is more brittle.
The present paper describes the stress distribution around an anchorage rib which was constructed at an intermediate point along the length of a post-tensioned prestressed concrete member and a method of reinforcement for the concrete around the anchorage. The effects of several influencing factors on local tensile stress were investigated in detail by using the finite element method. Based on the results obtained and the limitation of the variables studied here, the following conclusions were drawn. (1) The distribution of bursting tensile stress (σy) in case of the rib-anchorage is similar to that of the normal anchorage without a rib which is used at one of the ends of a prestressed concrete member. Therefore, the informations on the stress σy and its resultant tensile force P for the normal anchorage may be utilized to rib-anchorage. (2) The direction of a principal tensile stress (σk) at the re-entrant corner of the rib-anchorage inclines by about 40° to the plane on which the prestressing force (P1) is applied. This direction is little changed even though the angle of inclination of P1 with respect to the axis of the member (θ=5°∼10°), the size of an anchor plate and the depth of the member (H=17∼23cm) are changed. (3) The resultant tensile force (Zk) of σk is equal to about 15% of P1, and it is little affected by the variables considered in this study. In addition, it is pointed out that the corner zone of the rib can be designed by full prestressing.
The object of this study was to examine the mechanical behaviors of prestressed concrete slabs from their cracking to final stage. The effects of the loading position, the level of prestressing and the width/span ratio of slab specimens were investigated on the cracking and failure load, failure pattern and deflections. The specimens were simply supported along two opposite edges. A uniformly distributed load having an 8×8cm loading area was applied on the center of a specimen (Case C) or on a position apart from the center (Case E). Flexural failure occured in the Case C, while both flexural and punching shear failures were observed in the Case E. Punching failure occured especially in the wide specimens subjected to small longitudinal but no transverse prestressing. Prestressing in the transverse direction was found to improve the flexural rigidity and to change the cracking and failure pattern from a shear to a flexure type. However, the prestressing did not seem to affect the cracking load of specimens. After cracking, the central deflection of the specimens with transverse prestressing was much larger than that of those specimens without prestressing. The ultimate strength in flexure could be conservatively estimated by “Yield-Line Theory”, while that in punching shear could be safely predicted by Hawkins' equation.