GB2198745A

GB2198745A - Prestressing steel bar and prestressed concrete pile

Info

Publication number: GB2198745A
Application number: GB08702986A
Authority: GB
Inventors: Hiroyuki Kozuma; Masamitsu Takei; Tadashi Nishino
Original assignee: Kawatetsu Wire Products Co Ltd; Daido Concrete Co Ltd
Current assignee: Daido Concrete Co Ltd; JFE Techno Wire Corp
Priority date: 1986-12-11
Filing date: 1987-02-10
Publication date: 1988-06-22
Anticipated expiration: 2007-02-10
Also published as: GB8702896D0; HK78991A; JPH0541684B2; JPS63151720A; KR930001137B1; GB2198745B; KR880007879A

Abstract

A high strength prestressing steel bar has excellent delayed failure property and contains alloy steel elements including, by weight, 0.25-0.45% C, 1.0-2.0% Si, not less than 1.0% Mn with the ratio of [Si]/[Mn] being 1.20+/-0.50 when the weights of Si and Mn are represented by [Si] and [Mn] respectively. The alloy steel is subjected to being or drawing of not less than 0.8% strain rate during tempering after hot rolling and subsequent quenching to provide a tensile strength of at least 160 kgf/mm<2>. Also, a high strength prestressed concrete pile including the aforementioned prestressing steel bars and a stirrup fixed around the steel bars is disclosed. The prestressing steel bar and stirrup is embedded in concrete and pretensioned to a stress of not less than 0.7 times the tensile strength of the bar before hardening of the concrete. The prestressed concrete pile has an effective prestress of at least 50 kgf/cm<2> after steam curing.

Description

HIGH STRENGTH PRESTRESSING STEEL BAR AND HIGH STRENGTH PRESTRESSED CONCRETE PILE The present invention relates to a high strength prestressing steel bar and more particularly to a super high strength prestressing alloy steel bar having a low stress relaxation and also relates to a high strength prestressed concrete pile using such a high strength prestressing steel bar.

Hitherto, high strength prestressing steel bars have been used for manufacturing high strength prestressed concrete piles, prestressed concrete poles and the like (hereinafter generally referred to as prestressed concrete piles). It is well known that a prestressing steel bar having highest strength according to Japanese Industrial Standard No. G 3109 is a deformed prestressing bar D-SBPD 130/145 having mechanical properties of an ultimate tensile strength of not less than 145 kgf/mm2 and 0.28 proof stress of not less than 130 kgf/mm2.

In view of an effective prestress in the concrete pile as will be mentioned hereinafter it is essentially advantageous that the strength of the prestressing steel bar used for the prestressed concrete pile is as high as possible.

However, in practice the higher the strength of the prestressing steel bar the higher the risk of delayed failure arised before and after application for the prestressed concrete pile. Accordingly, when the prestressing steel bar according to the deformed prestressing bar D-SBPD 130/145 prescribed in the above mentioned Japanese Industrial Standard G 3109 is used, the strength is limited to about the lower limit of 145 kgf/mm2, even at the highest to 155 kgf/mm2.

While, in recent years there has been popularly used the high strength prestressed concrete pile for which the prestressing steel bar is mainly used.

In manufacturing of the high strength prestressed concrete a process of steam curing at high temperature and under high pressure is generally employed in view of productivity and quality of the concrete pile. In such a steam curing process usually a curing treatment is carried out in an autoclave at a temperature not higher than 2000C within three hours after a steam curing treatment is carried out at a temperature not higher than 9O0C for three to five hours. However, at this time, since the effective prestress mentioned hereinafter is largely affected by a value of stress relaxation loss of prestressing steel bar, a prestressing steel bar having low stress relaxation during the steam curing at high temperature and under high pressure is required.Under the above circumstance, a prestressing steel bar having a rate of stress relaxation of not higher than 15% and more particularly not higher than 88 is recently becoming to be practically used.

It is advantageous that the initial tension to be applied for the prestressing steel bar when it is used for the prestressed concrete pile is as high as possible in order to increase the effective prestress, while the higher the initial tension stress, the larger the stress relaxation loss is and furthermore the higher the risk of delayed failure which occurs from head portions for heading or fixing ends of prestressing steel bar or surface injured portions of the prestressing steel bar. Therefore, according to Japanese Industrial Standard G 3109, the initial tension stress is limited to a value not higher than 0.7 times of the tensile strength of the prestressing steel bar or not higher than 0.8 times of the yield strength which is 0.2% proof stress of the prestressing steel bar.

While, the value of effective prestress ace (kgf/cm2) to be introduced into the high strength prestressed concrete pile is determined by the following formula In consideration of elastic deformation, creep and drying shrinkage of concrete and relaxation of the prestressing steel bar.

opi : Initial tension stress of prestressing steel bar (kgf/cm2) #op : Loss caused by creep and drying shrinkage of concrete (kgf/cm2) Agr: Loss caused by relaxation of prestressing steel bar (kgf/cm2) Ap : Cross sectional area of prestressing steel bar Ac : Cross sectional area of concrete n' : Ratio of elastic modulus of prestressing steel bar and concrete upon introducing stress.

It can be understood from the formula (1) that the effective prestress can be effectively increased when Opi is large as well as hop and Acir are as low as possible. If the same effective prestress is required for the prestressed concrete pile, it can be provided economically by using less amount of prestressing steel.

In this case, the effective prestress can be calculated by the following simplified formulation.

herein ho'r is a rate of Aar a'p is a rate of ha ' is a rate of loss caused by elastic deformation of concrete.

However, according to the prior art as mentioned above, even if the deformed prestressing bar D-SBPD 130/145 prescribed by Japanese Industrial Standard G 3109 is used in consideration of the delayed failure and the stress relaxation, the initial tension stress of the prestressing steel bar is limited to 708 of 145 kgf/mm2, i.e, 1,015 kgf/mm2 and the above mentioned value is also used as an initial load for determining the value of relaxation.

Accordingly, the substance of the loss of effective prestress in the high strength prestressed concrete pile is as shown in Table 1.

Table 1

Loss caused by elastic deformation of concrete 3-5 (%) Loss caused by drying shrinkage of concrete 7-8 (%) Loss caused by relaxation 6-20 (%) of prestressing steel bar It can be seen in the table 1 that the conventional high strength prestressed concrete pile manufactured by using the conventional high strength prestressing steel bar has a large loss of effective prestress so that amount of the prestressing steel bar can not be reduced as expected.

An object of the present invention is to provide a high strength prestressing steel bar having such a mechanical property that the value of relaxation is low even if a high initial tension stress is applied so that loss of the effective prestress is small when using in the prestressed concrete pile.

Another object of the present invention is to provide a high strength prestressed concrete pile using a prestressing steel bar having higher strength than the prior art, being subjected to an initial tension stress of not less than 0.7 time of the tensile strength of the prestressing steel bar during steam curing process and high temperature high pressure curing process and having such a mechanical property that the value of relaxation is not higher than 8% and the effective prestress is large.

Thus, the present invention provides a high strength prestressing steel bar having excellent delayed failure property consisting of alloy steel elements including, by weight, 0.25-0.45 C, 1.0-2.0t Si, not less than 1.0% Mowith the ratio of [Si]/[Mn] being 1.20+0.50 when the weights of Si and Mn are represented by [Si] and [Mn] respectively and the alloy steel being subjected to bending or drawing of not less than 0.8% strain rate during tempering at a temperature of 3500C-5000C after the completion of hot rolling and subsequent quenching to provide a tensile strength of at least 160 kgf/mm2.

The present invention also provides a high strength prestressed concrete pile comprising high strength prestressing steel bars having excellent delayed failure property consisting of alloy steel elements including, by weight, 0.25-0.45% C, 1.0-2.0% Si, not less than 1.0% Mnwith the ratio of [Si]/[Mn] being 1.20+0.50 when the weights of Si and Mn are represented by [Si] and [Mn] respectively and the alloy steel being subjected to bending or drawing of not less than 0.8% strain rate during tempering at a temperature of 3500C-5000C after the completion of hot rolling and subsequent quenching to provide a tensile strength of at least 160 kgf/mm2 and stirrups formed of spiral or hoop reinforcements fixed around the prestressing steel bars, the prestressing steel bars with the stirrups being embedded in concrete and tensioned to a stress of not less than 0.7 time of the tensile strength before hardening of the concrete and then cured to provide an effective prestress of at least 50 kgf/cm2. The curing process may be preferably carried out by steam curing at a temeprature within a range of 600C to 900C or by a combination f steam curing at a temeprature within a range of 600C to 900C and high temperature high pressure curing as shown in Fig. 1.

Referred to the chemical composition, C is an element which is useful to provide the adequate high strength. If the amount is less than 0.25% by weight, no adequate high strength is obtainable after tempering, and on the other hand, if the amount exceeds 0.45% by weight, the strength and the elongation of spot welded part are adversely affected so that it is preferred to limit the C content within a range of from 0.25% to 0.45% by weight.

Si is an essential element effective to inhibit propagation of micro-crack and to increase stability against the delayed failure in high strength prestressing steel bar by including an amount of not less than 1.0% by weight, but the effect of the Si is saturated when the amount is about 2.0% by weight.

Furthermore, if the amount exceeds 2.0% by weight, the toughness is decreased. Accordingly, it is preferred a range of from 1.0 to 2.0% by weight.

Mn is an essential element effective to improve the spot weldablllty as well as the effect of deoxidation and hardening by including an amount of not less than 1.0% by weight, but within a range of alloy steel elements of both not less than 1.0% Si and not less than 1.0% Mn by weight, the Si and the Mn are micro-segregated to produce a hair line structure so that the lower portion of head becomes brittle in heading and increases the sensitivity of delayed failure. In order to prevent the above mentioned difficulties, it is necessary to control the ratio of [Si]/[Mn] within a range of 1.20+0.50 and more preferably 1.20+0.30.

The temperature and the value of strain during the tempering process may determine the strength and the value of stress relaxation of the prestressing steel bar and is very important in the present invention for achieving the low relaxation even if the prestressing steel bar has a high strength and is subjected to a high initial load. If the temperature of tempering is not higher than 3500C, no adequate relaxation is obtainable, and on the other hand if the temperature is higher than 5000C, required tensile strength can not be obtained.

Accordingly, it is preferable to limit the temperature of tempering within a range of 350-5000C. Furthermore, in order to obtain adequate relaxation, a value of strain of not lower than 0.88 is required.

In the accompanying drawings.

Fig. 1 is a graph showing a relation between the temperature and steam curing time; Fig. 2 is an elevational view of a testing jig; and Fig. 3 is a diagrammatic view of a testing apparatus.

Tables 2 and 3 show some examples of the present invention as compared with the prior art and comparative examples. The relaxation was measured by applying various initial loads after the temperature history in the steam curing. The initial load test was carried for the conventional case of 145 kgf/mm2 x sectional area of each of diameter x 0.7 and an additional case of 160 kgf/mm2 x sectional area of each of diameter x 0.75.

The delayed failure test was carried by using a jig shown in Fig. 2. Each of test pieces was subjected to the predetermined heading and then fixed under a load of 160 kgf/mm2 x 0.75 x sectional area in the jig. Thus pretensioned test pieces were held at 600C for 180 days in a saturated steam in a hot humid testing apparatus shown in Fig. 3.

Referring to Fig. 2, the reference numeral 1 designates a testing jig, 2 a protector plate, 3 a plate for supporting the head of the test piece, t a body, 5 a plate for holding the axial force, 6 a nut for holding the axial force, 7 a nut for retaining the test piece, 8 a vent bore, and 9 the test piece.

Referring to Fig. 3, the reference number 10 designates a thermostatic oven of the hot humid testing apparatus, 11 a saturated steam room, 12 oil heated at 600C, 13 an agitator, 14 water held thermostatically at 600C, 15 a steam cooler, 16 a table for supporting the testing jigs of Fig. 2 in steam within the saturated steam room 11.

Table 2

Composition (%) Tempering Relaxation (%) Delayed Tensile failure test temper- Strain Note strength rate of ature (%) (kgf/mm) 145kgf/mm 160kgf/mm occurrence C Si Mn Si/Mn ( C) x0.7 x0.75 (%) 0.29 0.49 0.78 0.63 410 0.60 149 13.9 17.2 2.0 # low Si Prior 0.30 0.26 0.86 0.30 410 0 148 21.3 24.5 0.5 low Mn art 0.27 1.68 0.79 2.13 440 0.60 148 7.6 9.2 1.0 # high Si 0.30 1.24 0.63 1.97 410 0 146 14.2 16.5 1.5 low Mn 0.30 0.45 1.40 0.32 445 0.06 147 13.2 15.9 2.5 # low Si 0.28 0.35 1.45 0.24 440 0 148 20.2 22.8 1.5 high Mn 0.29 1.25 1.29 0.97 410 0.83 163 6.8 7.8 " " " " " " 1.02 " 6.4 7.4 " Present 0.30 1.58 1.39 1.14 420 0.83 166 6.5 7.5 " inven " " " " " 1.02 " 5.9 6.8 " tion 0.30 1.87 1.52 1.23 450 0.83 165 6.1 7.0 " " " " " " 1.02 " 5.5 6.3 " Compar- 0.29 1.25 1.29 0.97 410 0.60 163 8.5 9.8 0.0 ative 0.30 1.58 1.39 1.14 420 " 166 8.0 9.2 " example 0.30 1.87 1.52 1.23 450 " 165 7.2 8.3 " Table 3

Composition (%) Tempering Relaxation (%) Delayed Prestress Tensile failure test ing steel temper- Strain strength rate of bar ature (%) (kgf/mm) 145kgf/mm 160kgf/mm occurrence diameter C Si Mn Si/Mn ( C) x0.7 x0.75 (%) (mm) Present 0.30 1.58 1.39 1.14 430 0.83 163 6.6 7.4 0.0 11 inven- " " " " 420 0.83 166 6.3 7.2 " 7.4 tion " " " " 410 0.83 162 6.7 7.7 " 13 Compar- " " " " 430 0.60 163 8.3 9.3 " 11 ative " " " " 420 " 166 7.9 8.8 " 7.4 example " " " " 410 " 162 8.6 9.8 " 13 Table 4 show the results obtained by tests for determining the limit strength and its stability under various heading conditions of the prestressing steel bar according to the present invention. It is seen from the Table 4 that the limit strength of the prestressing steel bar is within a range of 173-178 kgf/mm2 and more particularly the limit strength is very stable within a range of 160-173 kgf/mm2 even if the heading conditions are varied.

Table 4

Composition (%) Relaxation (%) Delayed failure test Tempering rate of occurrence (%) ing Tensile Type of heading machine temper- Strain Direct strength contact (%) Direct C Si Mn Si/Mn ature (kgf/mm) 145kgf/mm 160kgf/mm Holding contact ( C) x0.7 x0.75 duration constant Pressing Induction force heating currents constant 35A 55A 0.30 1.58 1.39 1.14 420 0.83 166 6.3 7.2 0.0 0.0 0.0 0.0 0.30 1.87 1.52 1.23 420 1.02 173 6.1 7.1 0.0 0.0 0.0 0.0 0.30 1.87 1.52 1.23 400 1.02 178 6.5 7.4 5.0 3.3 2.5 2.5 The sectional areas of the prestressing steel bars are shown in Table 5.

Table 5

Nominal Diameter sectional area 7.4 (mm) 40.0 (mm2) 9.2 1 64.0 11.0 90.0 13.0 125 Now, some example of high strength prestressed concrete piles used the high strength prestressing steel bars with the material of not less than 20% being saved according to the present invention is described as below.

The high strength prestressed concrete pile having an outer diameter of 500 mm, a thickness of 100 mm and an effective prestress of not less than 50 kgf/cm2 are manufactured.

The prestressing steel bars used for the concrete pile are shown in Table 6.

Table 6

Composition (%) Tempering Relaxation (%) Tensile Diameter temper- Strain (mm) strength ature (%) (kgf/mm) 145kgf/mm 160kgf/mm C Si Mn Si/Mn ( C) x0.7 x0.75 Prior 9.2 0.27 1.68 0.79 2.13 440 0 148 13.5 15.6 art 11.0 " " " " 430 0 148 14.1 16.5 Present 9.2 0.30 1.58 1.39 1.14 420 1.02 166 5.9 6.8 invention 11.0 " " " " 430 0.83 163 6.6 7.4 Number (n) of prestressing steel bars is determined as follows; in case of using conventional high strength prestressing steel bar with the relaxation of 15% and the elastic deformation loss, the creep and drying shrinkage of concrete of 11.5%.

When the diameter of the steel bar is 9.2 mm.

0.64 cm2 x n/1256 cm2 x 145 x 102 kgL/cm2 x 0.7 x (1-0.15-0.115) > 50 kgf/cm2 n > 13.2 n = 14 When the diameter of the steel bar is 11.0 mm.

0.90 cm2 x n/1256 cm2 x 145 x 102 kgf/cm2 x 0.7 x (1-0.15-0.115) > 50 kgf/cm2 n > 9.4 n = 10 in case of using the high strength prestressing steel bar according to the present invention with the relaxation of not higher than 8% at a stress of 160 kgf/mm2 x 0.75.

When the diameter of the steel bar is 9.2 mm.

0.64 cm2 x n/1256 cm2 x 160 x 102 kgf/cm2 x 0.75 x (1-0.08-0.115) > 50 kgf/cm2 n > 10.2 n = 11 When the diameter of the steel bar is 11.0 mm.

0.90 cm2 x n/1256 cm2 x 160 x 102 kgf/cm2 x 0.76 x (1-0.08-0.115) > 50 kgf/cm2 n > 7.2 n = 8 The result of the above are shown in Table 7.

Table 7

Diameter (mm) 9.2 11 number of used prior art 14 10 prestressing steel bars the invention 11 8 reduction of prestressing 21.4% 20.0% steel bars It is seen from the Table 2 that the high strength prestressing steel bar according to the present invention has a strength higher than that of the prior art, a highly improved delayed failure property, a low stress relaxation such that the relaxation is not higher than 8% at a strain of not less than 0.8% even if the initial load is 160 kgf/mm2 x sectional area x 0.75.

Accordingly, when the prestressed concrete pile is manufactured by using the prestressing steel bars according to the present invention, the amount of prestressing steel bar to be used is saved of not less than 20% as compared with the prior art.

Claims

1. A high strength prestressing steel bar having excellent delayed failure property consisting of alloy steel elements including, by weight, 0.25-0.45% C, 1.0-2.08 Si, not less than 1.0% Mnwith the ratio of [Si]/[Mn] being 1.20+0.50 when the weights of Si and Mn are represented by [Si] and [Mn] respectively and the alloy steel being subjected to bending or drawing of not less than 0.8% strain rate during tempering at a temperature of 3500C-5000C after the completion of hot rolling and subsequent quenching to provide a tensile strength of at least 160 kgf/mm2.

2. A high strength prestressing steel bar as claimed in claim 1 substantially as described herein with respect to Tables 2 to 7 set out hereinbefore.

3. A high strength prestressed concrete pile comprising high strength prestressing steel bars having excellent delayed failure property consisting of alloy steel elements including, by weight, 0.25-0.45% C, 1.0-2.0% Si, not less than 1.0% Mnwith the ratio of [Si]/[Mn] being 1.20+0.50 when the weights of Si and Mn are represented by lSi3 and [Mn] respectively and the alloy steel being subjected to bending or drawing of not less than 0.8% strain rate during tempering at a temperature of 3500C-5000C after the completion of hot rolling and subsequent quenching to provide a tensile strength of at least 160 kgf/mm2 and stirrups formed of spiral or hoop reinforcements fixed around the prestressing steel bars, the prestressing steel bars with the stirrups being embedded in concrete and pretensioned to a stress of not less than 0.7 time of the tensile strength before hardening of the concrete and then cured to provide an effective prestress of at least 50 kgf/cm2.

4. A high strength prestressed concrete pile comprising high strength prestressing steel bars substantially as claimed in claim 1 or 2.