The prestressed concrete pile is generally cast by the centrifugal method, and immediately steam-cured until it gets the minimum compressive strength necessary for prestress transfer. After prestress transfer, it is cured sufficiently in water. It can be predicted, therefore, that the loss of prestress due to creep and shrinkage is considerably smaller than that of ordinary precast prestressed concrete unit. However, the available data are not so sufficient as to predict exactly the loss of prestress. In this paper a field method is proposed to determine the effective amount of prestress by measuring the re-opening load of the flexural crack. Several test results on prestressed concrete beams and piles are also presented in this.
Recently, concrete piles are driven almost always by blow. This method of driving simplifies the process of construction but sometimes it will have destructive effect on the piles. It is necessary to investigate the process to find out the factors for the destruction of the piles and improve that method. In this study, RC piles (φ350mm×13m) and PC piles (φ350mm×13m+φ350mm×7m), both centrifugally spun, were used as test piles, and drop hummers (1t, 2t and 3t) and diesel hummers (Delmag-12 and Delmag-22) used. The ground into which the piles were driven has layers of sand (N-value=15∼20) at the depth of 6∼7m, gravel (N-value=30∼50) at the depth of 12∼13m and silt and sand (N-value=20∼35) deeper below. The impact strains at the head, center and foot of the piles were measured by means of wire strain gauges, and the effects of cussion materials, eccentric blow and the characteristics of ground on the pile stress have also been investigated. Comparative studies were made of some of the theoretical equations for the impact stresses with their experimental results. Principal conclusions obtained are as follows. (1) Rebound of a pile increases as the ground becomes harder. (2) Set per blow decreases as the ground becomes harder and the depth of the set is greater by drop hummers than by diesel hummers. (3) Delmag-12 cannot drive a pile into the ground which is more than N-value=50. (4) The impact stresses caused by pile-driving ordinarily reach their maximum at the head of the pile and their minimum at the foot of the pile. (5) The impact stress at the foot of the pile increases as N-value of the ground increases. But the impact stress at the head of the pile is not so related with N-value. (6) The impact stress at the head of the piles by drop hummers is greater than that by diesel hummers. The former sometimes exceeds the impact strength of RC pile concrete. (7) The maximum impact stress caused by eccentric blows of both drop hummers and Delmag-22 sometimes exceeds the impact strength of the pile concrete. The tensile stress and bending moment due to the eccentric blows sometimes cause cracks in the pile. In these respects, the PC pile is superior to the RC pile, and the diesel hummer is superior to the drop hummer. It is very important to drive a pile as normally as possible and it is necessary to set a proper cussion at the head of the pile. (8) The impact stress caused by pile-driving should be analyzed by the wave equation rather than by the energy equation. Further studies are needed of the following problems. (1) The impact strength of concrete. (2) The correlation of the compressive strength by the normal test piece and that by the test piece made by the centrifugal force. (3) Estimation of the impact stress in a pile caused by driving. (4) Fundamental studies on pile-driving analysis by means of the electric computer.
This study has been carried out to decide the reinforcement method of the impacted concrete pile by the pile driving. From the analysis of measured values of the dynamic strain on the pile head, the following conclusions have been obtained. Moreover, the author wishes to promote this study and to find out the behaviours of the pile driven in the ground. (1) The relation between the dynamic energy of the hammer and the dynamic strain in the pile. εV.100=F(1-k)/EA(S+R/2) (1) εV.50=F(1-k)/EA(S+R) (2) where εV.100=the dynamic strain at the point of 100cm from the top of the pile εV.50=dynamic strain at the point of 50cm from the top of the pile F=dynamic energy of hammer A=sectional area of the pile E=Young's modulus of concrete S=penetration R=rebound (1-k)=efficiency of the dynamic energy of hammer W=hammer weight H=hammer stroke For F, (1-k) and E in Eqs. (1) and (2), the following values in Table I and Table II is available. Table I Table II Fc=ultimate compressive strength of concrete (2) The relation between the sectional area of the pile and the dynamic strain of the head of the pile. When the PC pipe pile (350mmφ or 300mmφ) is driven in the ground with the Type 22 diesel hammer, the value of εV.50 is assumed by Eq. (3) εV.50=(987-0.57A)×10-6 (3) where, A=equivalent sectional area of the pile (cm2) (3) The distribution of the dynamic strain of pile head The axial dynamic strain εV.x at the point of distance x from the top of pile is expressed as the following general Eq. (4). εV.x=ax-b (4) where, a and b=constant characteristic Then the dynamic strain of PC pile, which is driven with diesel hammer 22, is expressed in Eqs. (5) or (6). For PC pile (D=300mmφ, t=70mm) εV.x=9.02×104×x-0.0897 (5) For PC pile (D=350mmφ, t=80mm) εV.x=10.92×104×x-0.152 (6) (4) The relationship between the vertical (axial) dynamic strain εV and horizontal dynamic strain εH. The value of εH/εV at the point (such as x=10cm), near the top of pile, shows the variable value as εH/εV=0.1∼0.4. But εH/εV values at 35cm point and at 70cm point from the top of pile become the constant values as follows: For RC pile (εH/εV)=0.2 (7) For PC pile (εH/εV)=0.24 (8) (5) The suggestion for the reinfocement range of the head of pile By assuming the ultimate compressive strain of concrete as εcu=2500×10-6 and the ultimate tensile strain as εtu=125×10-6 and applying εtu and εcu to the Fqs. (5), (6), (7) and (8), the author has suggested the following idea. 1) The minimum width of the iron band to reinforce the pile head is necessary to be in the range of 20∼30cm. 2) The pile must be specially reinforced by spiral hoop at least 100cm in length from the top of the pile head.
In establishing JIS for chemical admixtures it is required to carry out quality control over admixtures (Air entraining and water reducing agents) now on the market. To that end we should: 1) Determine such specific values as will constitute the properties of admixtures to ensure production of high quality concrete: 2) Establish the method of testing the values of the properties. 3) Establish the standard for controlling the values of the properties. For the purpose of meeting the above mentioned requirements the writer picked up 2 lot each of various specimens of Chemical admixtures now on sale to test their specific gravity, solid content. PH value, foaming capability, relative surface tension and suspension value for the purpose of checking the variation of these specimens. The test was conducted successfully and I hope to explain in detail the testing method and publish the results later on.
To examine the effectiveness of water-reducing agents, typical one air-entraining agent, eight water-reducing admixtures and ten brands of normal portland cement now widely distributed throughout our country have been selected and used with some combinations. Comparative tests have been performed of fresh concrete with respect to its water-reduction, bleeding and initial hardening, and of hardened concrete with respect to its strength gain, all these with the above-mentioned admixture, and, for contrast, of the control concrete having the same cement content of 300kg/m3 and consistency of 7.5cm without the critical admixture. The results of the tests show that the good quality of water-reducing admixtures can reduce water requirement by 10 to 15 percent, bleeding per unit area by more than 30 percent and give higher compressive strength without any harmful effect on setting.
Cracks appearing on concrete or mortar of structures would reduce the life of structure and destroy its functions, because they cause permeation or penetration of air and moisture. For example, water permeation and air penetration through such cracks promote a corrosion of steel frames and bars. As a procedure to avoid such unfavorable consequences, cement paste or natrium silicate anhydride of inorganic material has been hitherto used to repair such cracks. Those materials have fulfilled the purpose of repairing such cracks to some degree, but not to the fullest satisfaction. In recent years, thanks to rapid development in the use of high polymer materials, they have been introduced into the field of concrete engineerings, and found not only to be applicable to, but most useful for, repair works of concrete cracks. This paper is intended to refer to the studies made on the use of Epoxy resin as a filler for concrete and mortar cracks instead as an adhesive agent widely being employed for concrete and mortar works.
Shield construction is employed on a large scale for the subway construction project of Osaka Municipality in which water leakage from the joints of shield segments is a serious problem during the construction as well as maintenance after completion. For this reason, an adhesive mainly composed of tar modified epoxy resin is coated onto the contact surfaces of the segments by which adhesion of the segments is reinforced, the segment being then clamped by means of bolts. However, there are many points unclarified regarding the primer, the coating method of the adhesive, the curing method and the setting time required. Experiments were conducted by varying the method applying the adhesive, and the respective effects produced on the adhesion effectiveness were studied. The results obtained are as follows. (1) Pre-coating with primer is considerably effective in improving the adhesion effectiveness of the adhesive in case of using tar-epoxy resin adhesive to adhere segment joints. This fact is quite favourable for the job at sites where the segments are liable to be wetted due to underground water such as found in shield construction. (2) In the above case, coating with tar-epoxy adhesive must be performed after the primer is hardened, so as to make it possible to use a rapid curing type of primer. (3) Adhesion effectiveness can be increased by inserting a sheet of glass cloth as reinforcement between the two coats of tar-epoxy adhesive. (4) In case of adhesion being made in water with the use of tar-epoxy adhesive, the following method is considered to be practical after primer pre-coated onto the concrete surface is cured. The surface of one concrete will be coated with tar-epoxy adhesive, and glass cloth will be applied to it. Then it will be coated again with tar-epoxy adhesive, and then it will be pressed against the other surface which is submerged in water. (5) The two-component type of primer consisting of epoxy and polyamide, which is low in viscosity and with curing time of around 24 hours as well as long in pot life and oderless, is considered to be suitable for use as the primer for segment joints.
Resin concrete is now beginning to be used in our country as light weight structural material, electrical insulating material, or mixed materials such as adhesive mortar for repairing roads and anti-corrosive lining, but there has so far been no case of its having been used for structural purposes in civil engineering. In its use as civil engineering structural material the major problems are concerning the manufacturing technique of large molds, its durability and fire resistance and the high cost of the materials. We have investigated its composition and manufacturing process, and the solution of these problems will soon be found. Resin concrete is superior to cement concrete in mechanical strength, chemical resistance, quick hardening and in many other respects. With the cooperation of NTT (Nippon Telephone and Telegraph), we are now testing the utility of resin concrete as materials for making manhole covers for communication cables, and are carrying out research and development projects of its application to many other practical uses. In this paper we will show mainly the results of our research in polyester resin concrete.
Studies have been made by various researchers in the attempt to improve the brittleness of cement mortar by admixing polymer emulsion or fibers independently, but the method to admix both polymer emulsion and fiber has not yet been developed. This experiment is based upon the inference that there would be multiple effects by admixing the three components, polymer emulsion, fibers and cement in what is termed a three component system. This was proven by the experiment. In the study of the three component system conducted by the authors, it was found first of all that the sorrounding fibers were remarkably improved in bonding strength with simultaneous improvement in the equilibrium condition of the elasticity between the fibers and the matrices sorrounding the fibers. Consequently the stress distribution of the fibers was remarkably improved. According to these effects, it made it possible for the tensile strength of the fibers to contribute to fully serving the purpose of reinforcement. This experiment suggested one more important way in the improvement of brittleness in hardened cement materials. The present study, while it deals with some fundamental problems in the polymer emulsion-fiber-cement system, aims at its fruition extensively in possible application in the industrial field, giving new outlooks to the studies in this direction.