The shrinkage of concrete is one of its greatest disadvantages for concrete structures. The expansive cement is worthy of note as it has disolved the problem of dry shrinkage. The theory of expansive cement is as follows: at early age, cured in moist atmosphere, the concrete expand to some extent, so the following shrinkage, under drying condition, is compensated. The self-stressing cement in USSR and the chemical pre-stressed cement in USA are typical expansive cements whose expansive energy is generated by the formation of Ettringite. V. V. Mikhailov, A. Klein and other investigators studied on the application of expansive cement on the pre-stressed concrete, and it was found that some expansive cement could be employed in the manufacture of pre-stressed concrete. There are many factors, however, having influences on the magnitude and the rate of expansion, the amount of expansive component, the water cement ratio of the mix, the curing condition, the method of restraining the expansion, etc. For the industrial application of the expansive cement, therefore, it is necessary to establish the technique that controls the expansion exactly.
General description on several qualities of grouting mortar for prestressed concrete members is given in the present report by reference to the results of experiments shown in foreign and domestic papers and obtained by the author. Main important qualities necessary for the cement grout are faculty of flow, quantity of bleeding, change of volume, compressive strength, bond strength and resisting power for freezing, and the author showed and explained the results of experiments on them, when some kinds of mixing agent which is produced and sold in Japan were applied to the cement grout. The author suggests that one of the mixing agents suited to the cement grout is “Pozzolith No. 8” manufactured in Japan at present, if engineers want to apply one kind of agent produced in this country to the cement grout with the aim to improve its qualites.
This paper describes some problems on high strength concrete studied in our laboratory in recent years. Summary of this report is as follows: (1) In order to make high strength concrete for prestressing, it is necessary to lower the water-cement ratio. But, in the case of water-cement ratio lower than 40%, increase of strength of concrete is little for lowering water-cement ratio. Investigation results shows that one of this cause is due to increase of entrapped air by lowering water-cement rotio and lowering consistency of concrete. Therefore, it is necessary to place compactly by vibrator of good performance for concrete with low water-cement ratio. (2) One of the causes of that the increase of strength of concrete is little for lowering water-cement ratio in low water-cemente ratio, is due to the decrease of the rate of hydration of cement. That is, the structures of floccules of cement particles in paste in so densified by lowering water-cement ratio that the spase for hydration products around cement particles is decreased, and the rate of hydration of cement is decreased. Therefore, in order make high strength concrete rationally, it is necessary to mix thoroughly so that cement particles may be dispersed. (3) In the case of structures used rich mixture, the influences of temperature rise of concrete due to heat of hydration of cement can not be ignored. To remove the deffects of this temperature rise, considerations should be given to the selection of cement, proportion of concrete, making and curing temperature and procedures of concrete. (4) It is effective to use alumina cement for concrete or mortar which is used in joint of block at post-tensioning as the case may be.
Recently, with the tendency to lighten structures coming out, some artificial light-weight aggregates having excellent qualities suitable for high strength structural use have come to be produced for trial. We tested these artificial light-weight aggregates and light-weight concrete using these aggregates, and further, checked up the effects of these light-weight concrete used in prestressed concrete members, comparing with normal-weight concrete in the same case.
Prestressed Concrete wire is used under high tensile stress in a wide variety of atomosphere, and stress corrosion will develop in this wire to some extent. We have studied on this problem using three methods: free loop bending and tensioning methods. The specimens were PC wires with a diameter of 2.9mm made of 0.7% and 0.8% carbon steel. They were cold drawn, cold drawn and stress relived and oil-tempered wires. As corroding media, we applied some chrolides, nitrates and ammonium rhodanate. The results we obtained are summarized as follows: (1) Oil-tempered wire is less stable to stress corrosion attack than cold drawn wire and cold drawn and stress relieved wire. (2) The cracks and fractures of oil-tempered wire are trans-axial, but those of cold drawn wire are axial or semi-axial. (3) As to the corroding media, rhodanate and nitrate are active, while chrolides are not so active. (4) Of oil-tempered wires, the higher the tensile strength is, the more sensitive they are to stress corrosion. (5) The amount of carbon content and stress relieving treatment have little influence on the resistivity to stress corrosion. Anyway, we could conclude that cold drawn wire and cold drawn and stress relieved wire were rather stable to stress corrosion.
Effect of induction hardening on corrosion fatigue properties of steels is studied, especially in respect to the recent development of high tension steels. Specimens, such as in Fig. 1, made of four kinds of steels as in Tables 1 and 2, are induction-hardened as shown in Table 3, and tested on rotary bending fatigue testing machines (1500rpm) in the mist of well water (as in Table 4) in the chambers shown in Fig. 2. Results of corrosion fatigue tests are shown in Figs. 3, 4, 5 and 6, and Fig. 8 inclusively. As shown in these figures, the corrosion fatigue strength of any hardened steel is higher than the fatigue limit in air of non-hardened steel, accordingly this induction hardening being a superior protection against corrosion fatigue. Especially, the strength of S 45 C steel for 107 cycles is as high as 50kg/mm2 and this stress level looks like a fatigue limit. This property is believed to be one of the highest of the steels suffered with corrosion fatigue that have ever been reported. As well known, the metallurgical improvement of quality of steel by ordinary quenching does not contribute by itself to improve corrosion fatigue strength. For example, as shown in Fig. 7, the rail steel, induction-heated but quenched in oil at 25°C, is very weak under corrosion fatigue conditions. (cf. Fig. 6) As shown in Fig. 9, on the surface of induction-hardened specimens after corrosion fatigue tests, there are many lines of corrosion products rising along the periphery of specimen (perpendicular to the direction of stress). And the fracture surfaces of induction-hardened specimens, Fig. 10 (a) and (b), appear to be flat plane with shallow cracks on their edges, and that of non-induction-hardened, (c) and (d), appear to be the cutter-like fracture surface. From these facts, induction hardening can not prevent the initiation of corrosion fatigue cracks, but can arrest their further propagation near the surface of specimens. It is supposed that this crack-propagation-arresting ability mainly attributes to the compressive residual stress near the surface of specimens that has withstood many repetitions of stresses. For better understanding, the effect of mean stress on the corrosion fatigue strength of high carbon steel for some given cycles is shown in Fig. 11. As shown in this figure, time strength diagrams for corrosion fatigue are also limited by the fatigue limit in air. Also in this respect, induction-hardened steels have excellent properties as well known.
To examine the relaxation phenomena of PC steel wire used for pretension-concrete method over long periods, the authors measured the relaxation of 2.9mm dia. PC steel wire taking the following factors, which are relative to the relaxation factor of prestressed-concrete, into consideration. (1) Loading time; i.e. the time from O load to the given measured load value. (2) Initial load to relax the PC steel wire (3) Bluing (one of the heat treatments of PC steel wire) (4) Prestretching of PC steel wire (5) Measuring temperature From this experiment, the authors arrived at the following results: (1) The relaxation value must be corrected in accordance with changes in the loading time. (2) The relaxation value in percent is the same for loads up to 80% of the yielding point load. (3) Bluing treatment greatly reduces relaxation. (4) Prestretching significantly reduces the relaxation, Prestretching is particularly effective in the case of 10% overload. (5) The measuring temperature is an important factor for relaxation tests. Therefore the tests should be held at a constant temperature. The authors also were able to locate the turn over point in the semi-log graph of the time-relaxation characteristic curve. The log-log plot indicates the straight line characteristics and the turn over point corresponds to the change in slope point. Based on their investigation, the authors suggest an estimation method for the relaxation phenomena over long periods. Experimental results indicate that the relaxation values estimated with this method are consistent with the actually measured values over long periods.
For the prestressed concrete steel bar, a smaller amount of relaxation is required together with higher mechanical properties. We studied on the relaxation property of prestressed-concrete steel bars which had heat treated carbon steel, spring steel, and low alloy steel to 115kg/mm2 and 130kg/mm2 in ultimate strength. In the experiment, we observed that S30C, S40C, S50C relaxed over 3% for 20 hours under the initial stress of about 80% of actural yield strength, SAE1041, SCr-2 relaxed 2 to 3% and SCM2, SCM4, SUP9 relaxed about 1% As for the relation of the relaxation with mechanical properties, it is recognized that the amount of relaxation is inclined to decrease with the increases of ratio σS/σB and of elongation. It was concluded that, the steel bar which had tempered at high temperature and was sufficient in regard to mechanical properties, was excellent also from the view point of relaxation.
NIPPON ROCLA PIPE CO. has been producing noncylinder prestressed concrete pipes since the fall of 1961. One of the products which has 1200mm inner diameter and is designed for the inner pressure of 13.0kg/cm2 was tested hydrostatically and under the external load. The results of test obtained are as follows: (1) Under the hydrostatic pressure up to 12.0kg/cm2, there was no leakage on outer surface of the pipe and at rubber joints. As the capacity of the bulkhead of the machine was not sufficiently large, the test up to 13.0kg/cm2 pressure was given up. (2) Under the external load up to 8.25 tons per meter (total load of 33.0 tons), could no visible crack was found at any point of the pipe. From analyses about the stress and measured strain on concrete surface at the end of the pipe, the prestress in the core concrete was supposed to be as much as the designed value, and the flexural strength of the concrete to be sufficient.
For the purpose of getting same curing effect both on prestressed concrete products and their specimens, three kinds of tests were carried out. All the tests were dome at the actual concreting works and classified as follows: (1) Natural curing for post-tensioning bridge beam, in summer. (2) Ditto, in winter. (3) High temperature steam curing for pre-tensioning railway sleeper, in factory. As the test results, the following facts were obtained. (1) Keeping the specimen merely under the same cover for the product is not sufficient. (2) Pretty good effects were observed when the specimens had been wrapped up with wet paper and put in a wooden box stuffed with air-insulating materials, then the box was placed under the same cover of the product. (3) As air-insulating materials, waste sack cloth and chaffs were used, which are easily available on the spot.
Recently, a new typedlift-slab construction method with prestressed concrete girders has been devised by Mr. T. Okamoto. An interesting aspect of this method is the prestressing of the connection of column to slabs and, as this result, we may obtain a higher rigid connection of column to slabs than that of flatslab typed-one. Some static bending tests were carried out to obtain the fundamental data on the prestressing, rigidity, strength and so on of the connection of column to slabs using some tests specimens with various sectioned-prestressed concrete girders and variously prestressed connection of column to two girders. As the tests results, the following aspects were obtained: (1) Observed was the tendency that F, the coefficient of shear friction, becomes smaller as the prestressing force of the connection of column to two girders get larger. But, in any test specimen, the value of F was larger than 0.45. (2) Consequently, adopting 0.4 as the value of F and prestressing the connection of column to two girders with the prestressing force correspondent to the value of F, we can obtain the rigid connection of column to slabs.
The prestressed concrete tie to be used on the Model Line of the New Tokaido High Speed Railways were designed in August, 1961. This paper describes the problems in design and manufacture of PC ties and the details of PC ties designed by pre-tensioning and post-tensioning system.