MACALLOY BARS FOR USE IN POST-TENSIONING APPLICATIONS

DESIGN DATA

P O Box 71, Hawke Street, Sheffield, S9 2LN. Telephone: (0114) 2426704, Fax: (0114) 243 1324

Macalloy is a registered trade mark and trading name of McCalls Special Products.

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CONTENTS

1. INTRODUCTION ........................................................................................................................................................................................ 3

2. LOSS OF PRESTRESS .............................................................................................................................................................................. 3 2.1 RELAXATION OF THE STEEL......................................................................................................................4 2.2 ELASTIC DEFORMATION OF THE CONCRETE....................................................................................................5 2.3 SHRINKAGE OF THE CONCRETE..................................................................................................................5 2.4 CONCRETE CREEP.................................................................................................................................5 2.5 LOSS AT THE ANCHORAGE ON TRANSFER OF LOAD FROM THE JACK........................................................................6 2.6 FRICTION IN THE JACKS...........................................................................................................................6 2.7 FRICTION IN THE ANCHORAGE ..............................................................................................................7 2.8 FRICTION DUE TO WOBBLE OF THE DUCT.................................................................................................7 2.9 FRICTION DUE TO CURVATURE OF THE TENDON PROFILE....................................................................................7 3. TENDON AND ANCHORAGE ARRANGEMENT .............................................................................................................................. 8 3.1 DIMENSIONAL SPACING .........................................................................................................................8 3.2 ANGULAR MISALIGNMENT ....................................................................................................................10 4. ANCHORAGE ZONE REINFORCEMENT..........................................................................................................................................10

5. CURVATURE OF TENDONS.................................................................................................................................................................12 5.1 CALCULATION OF BAR AND THREAD LENGTHS ............................................................................................12 5.2 BAR LENGTHS...................................................................................................................................12 5.3 THREAD LENGTHS............................................................................................................................13 6. CALCULATION OF EXTENSION .........................................................................................................................................................14

7. CALCULATION OF FRICTION LOSS, BAR ELONGATION AND JACKING FORCE ..............................................................14

8. STRESSING RECORD.............................................................................................................................................................................17

9. TORQUE LOADING.................................................................................................................................................................................17

10. MISCELLANEOUS DATA.................................................................................................................................................................18 10.1 MODULUS OF ELASTICITY......................................................................................................................18 10.2 FATIGUE RESISTANCE..........................................................................................................................19 10.3 NOTCH DUCTILITY ..............................................................................................................................19 10.4 EFFECT OF CHANGE OF TEMPERATURE ......................................................................................................20 10.5 ELECTRICAL RESISTIVITY ......................................................................................................................20 10.6 TRANSVERSE STRESSES .......................................................................................................................20 10.7 CUTTING OFF EXCESS BAR THREAD..........................................................................................................20 10.8 SHEAR STRENGTH ..............................................................................................................................20 10.9 MACALLOY AT CRYOGENIC TEMPERATURES................................................................................................20 10.10 WELDING....................................................................................................................................21 10.11 BENDING ....................................................................................................................................21

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1. Introduction Macalloy bars complying to BS4486:1987 grade 1030 are supplied in 25, 26.5, 32, 36 and 40 mm nominal diameters. Bars having the same mechanical properties are available in both 50 mm and 75 mm diameters; other diameters can be supplied by arrangement. ( Note: BS4486 only covers specific bar diameters up to 40mm. ) All diameters are offered with a smooth or fully threaded surface and with the exception of 75 mm bar, in standard lengths up to 11.8m (by arrangement bars up to 17.8m long can be produced). 75 mm bars are supplied in lengths up to 8.4 m. All bars are anchored or joined using a coarse pitch, cold rolled thread and threaded nuts or couplers. The standard range of bars and the related characteristic failing load and design forces are given in Table 1 - DESIGN DATA.

Table 1 - DESIGN DATA

Diameter mm

25

26.5

32

36

40

50

75

Characteristic Failing Load kN

506

569

828

1049

1295

2022

4310

Max. Design Load (at 70% of Characteristic Failing Load KN)

354

398

580

734

906

1415

3017

The following sections detail factors which must be considered in the design and detailing of a structure. Specific information useful in the calculation of extension and jacking forces is provided together with data on properties needed only in unusual applications.

2. Loss of Prestress The effective prestressing force in service is less than the force applied by the jack. The various sources of loss are as follows:

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(1) Relaxation of the steel. (2) Elastic deformation of the concrete ( or stressed member. ) (3) Concrete ( or stressed member ) shrinkage (4) Concrete ( or stressed member ) creep (5) Loss at the anchorage on transfer of load from the jack (6) Friction in the jacks (7) Friction in the anchorage

(8) Friction due to wobble ( unintentional variation in profile ) of the duct (9) Friction due to curvature of the tendon profile

2.1 Relaxation of the steel

The maximum stress relaxation loss of alloy steel bars is specified in BS4486 : 1987. The maximum loss for Macalloy steel, in all diameters, stressed to 70% of the failing stress after 1000 hours is 3.5%. This value must be allowed for in design. A typical relaxation curve is shown below:

2

3

456789

1.0

10987654

3

2

0.100

12 3 4 5 6 7 8 9 2 3

106 7 8 94 5 2 3 54 9876

100032

1006 7 8 95

0.1

Loss

of s

tress

-%

Time - H

3.5 (BS 4486)

Stress relaxation for 1000 hours at 70% UTS for 40mm dia bar 70% breaking load 978kN

Figure Figure Figure Figure 1111 ---- TYPICAL STRESS RELAXATION CURVE FOR 40MM DIA BAR TYPICAL STRESS RELAXATION CURVE FOR 40MM DIA BAR TYPICAL STRESS RELAXATION CURVE FOR 40MM DIA BAR TYPICAL STRESS RELAXATION CURVE FOR 40MM DIA BAR

5

2.2 Elastic deformation of the concrete

There is no loss of force in a tendon due to elastic deformation of the concrete ( or stressed member ) when that particular tendon is being stressed, as the shortening of the concrete ( or stressed member ) is included in the travel of the jack ram. However, when several tendons are stressed in succession there is a progressive loss of prestress. This can be calculated on the basis of half the product of the modular ratio and the stress in the concrete adjacent to the tendons averaged along their length. Note : It is usually sufficiently accurate to assume that the tendons are located at their centroid.

Hence loss = fEEco

s

c.2

where f co = average concrete stress as defined above Es = modulus of elasticity of steel Ec = modulus of elasticity of concrete at the time of stressing For most applications, it is sufficient to calculate the total movement of the jack ram, ie. the sum of the bar elongation and the concrete shortening based on the full prestressing force. This will initially result in a force in the bars slightly greater than the design value, falling to the design value as subsequent bars are stressed.

2.3 Shrinkage of the concrete

Concrete shrinkage is due to drying out of the concrete. The amount of shrinkage loss in the bars will depend upon the age of the cement paste at the time of prestressing and the relative humidity of the environment. For jacking between 7 and 14 days after concreting, the loss of stress in the tendon will be approximately 70 x 10-6 x Es in humid conditions (90% relative humidity) and up to 200 x 10-6 x Es in normal conditions (70% relative humidity). It may be assumed that half the total shrinkage takes place during the first month after jacking, and three quarters in the first six months after jacking.

2.4 Concrete Creep

The concrete member will shorten when subjected to compressive stress by an amount additional to that caused by shrinkage. The resulting loss of stress in the tendon is obtained from the product of the modulus of elasticity of the steel and the creep in the concrete adjacent to the tendons. The value of the creep is proportional to the stress in the concrete provided that the stress does not exceed one third of the cube strength at the time of jacking.

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For jacking between 7 and 14 days after concreting when the cube strength is greater than 40 N/mm2, the creep of the concrete per unit length should be taken as 36 x 10-6 per N/mm2. For lower values of cube strength at time of jacking, the creep per unit length should be taken as:

36 x 10-6 x 40f ci

per N/mm2

where f ci is the actual cube strength at the time of jacking. If the compressive stress anywhere in the section at the time of jacking exceeds one third of the cube strength, up to a maximum of one half of the cube strength, the values for creep should be increased. The creep value at a stress of one half of the cube strength is to be 1.25 times the above value, and at intermediate levels between one half and one third values of creep should be interpolated linearly. Hence the loss is fco x 36 x 10-6 x Es N/mm2 or as modified to take account of the concrete strength at time of jacking and the compressive stress in the concrete.

2.5 Loss at the anchorage on transfer of load from the jack

Any loss of stress at the anchorage on transfer of load from the jack to the nut is due to dirt or angularity between the bearing faces of the plate, washer and nut, and to the take up of the thread tolerances between the bar thread and the nut. These are negligible when compared with the total elongation for bars over 4m in length, and can be minimised even further by ensuring that the bearing surfaces are clean and parallel. For short tendons, ie. less than 2m long, the loss on anchoring is reduced by using a greater than normal torque to transfer load to the Macalloy nut before releasing the jack, and by cycling the jack three or four times from zero to full load to ensure that all bearing surfaces are bedded down before finally releasing the jack. The loss of elongation can be assumed to be as below: Single stressing Two or more stressing cycles 25mm - 36mm 1.5mm 0.7mm 40mm - 75mm 2.0mm 0.7mm

2.6 Friction in the jacks

All jacks are calibrated against a master gauge before despatch and the loads exerted by the ram are tabulated against the pressure gauge readings. Any friction on the jack is allowed for if the calibration readings are used to control the applied load. Electrical or mechanical load cells are available for the recalibration of jacks and gauges on site, or to control loading with greater accuracy than that provided by commercial pressure gauges. Loads calculated from pressure gauge readings based on the jacks ram areas do notinclude an allowance for friction in the jack. A range of typical ram areas and friction losses are listed in Table 2. Actual values should be obtained from the jack supplier.

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Table 2 - TYPICAL RAM AREA/FRICTION LOSSES

Ram Area mm2

7600

12890

19510

31500

Typical friction loss in jack kN

20

30

40

50

2.7 Friction in the anchorage

There is no friction loss in single bar anchorages as the bar does not deviate in direction.

2.8 Friction due to wobble of the duct

The prestressing force Pxat any distance x from the jack may be calculated from

P P ex okx= −

where K x. .≤ 0 2, e Kx− may be taken as 1 - K x. , ie P P K xx o= −( . )1

where: Po is the prestressing force in the bar at the jacking end e = 2.718 K is a constant depending on the type of duct or sheath, the nature and condition of the inside surface and the extent of unintentional contact between the bar and the sheath. The value of K per metre length for Macalloy bars in closely supported semi rigid steel sheaths may be taken as 12 x 10-4 to 18 x 10-4 depending on the degree of rust of the bar and sheath, the former being appropriate for clean, non-rusted bar and sheath.

2.9 Friction due to curvature of the tendon profile

The loss of force in a tendon is dependent on the angle turned through and the coefficient of friction, µ, between the bar and the sheath or duct. The prestressing force Px at any distance x along the curve from the tangent point may be calculated from

P P ex ox r= − µ /

where : Po = the prestressing force in the tendon at the tangent point near the jacking end. r = the radius of curvature of the tendon profile, and

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µχ / .r < 0 2 , (e x r− µ / may be taken as 1 - µx r/ ) The value of µ for processed Macalloy bars on bright steel sheathing may be taken as 0.2. Where rust is present, the value of µ will be between 0.25 and 0.30. The equations given above may be combined so that if

( / ) .Kx x r+ <µ 0 2 , then

( )e Kx x r− + µ / may be taken as 1- ( / )Kx x r+ µ Hence the complete equation would be

( )[ ]P P Kx x rx o= − +1 µ /

An example of the method of calculating jacking forces and extensions is given in Section 0

3. Tendon And Anchorage Arrangement

3.1 Dimensional Spacing

The recommended duct and end plate sizes are shown in table 3 To suit particular requirements, the dimensions can be varied provided that concrete cover, load transfer and stress conditions are satisfactory.

Figure 2 - TENDON AND ANCHORAGE ARRANGEMENT

Table 3 - DUCT AND END PLATE SIZES

9

Tendon dia mm 25 26.5 32 36 40 50 75 Recommended duct inside dia. mm

41

41

50

50

61

71

91

Coupler sheathing inside dia. mm

59

59

66

71

75

91

125

End plate Length mm Width mm Thickness mm

100 100 40

110 110 40

125 125 50

140 140 50

150 150 60

200 175 60

300 250 75

Suggested minimum edge distances and spacings of tendons are set out in Table 4444 assuming a maximum aggregate size of 40 mm.

Figure 3 - TENDON SPACING

Table 4 - TENDON SPACING AND EDGE DISTANCES

(maximum aggregate size = 40 mm)

Tendon dia mm 25 26.5 32 36 40 50 75 Minimum Centres Horiz. mm Ducts (1) Vert. mm Horiz. mm End Plates (2) Vert. mm

85

85

125

125

85

85

125

125

90

90

150

150

95

95

165

165

100

100

175

175

110

110

200

200

135

135

275

275

Minimum Edge Distance Centre of Duct (3) mm Centre of End Plate (4) mm

75

140

75

140

75

160

80

175

80

190

85

210

100

270 Notes 1. Assuming recommended duct dia(table 3) and minimum of 40 mm between ducts 2. Assuming a minimum of 25 mm between end plates

3. Assuming recommended duct dia(table 3) and minimum cover to duct of 50 mm 4. Assuming anchorage reinforcement shown in Table 6 with 50 mm cover Where non-standard end plates are used, the minimum cover of concrete needed to prevent spalling beyond the edge of the plate is 40mm. Certain exposure conditions may call for a greater cover of concrete.

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3.2 Angular Misalignment

The bar, nut and washer should be perpendicular to the end plate. Standard nuts and washers allow for an angular misalignment of +/- 1.5o, spherical nuts and washers allow for an angular misalignment of +/- 3o. If the misalignment is greater than this then the surface of the concrete should be built up, using a high strength grout, to provide a surface perpendicular to the axis of the bar. Alternatively tapered washers or a pair of sliding wedges can be used to provide a bearing surface for the nut perpendicular to the bar.

4. Anchorage Zone Reinforcement Bursting tensile forces are induced in the concrete immediately behind the anchorage end plates due to the compressive load applied through the end plates. Reinforcement in the form of links, helices or a combination of these should be provided in each end block. Additional links are needed to enclose a group of end blocks where there are several anchorages in a member. The dimensions of the end block are given by the end concrete area geometrically similar to and concentric with the end plate bounded by the edges of the concrete or by the end block of adjoining anchorages. The design of the anchorage reinforcement is covered by Section 4.1 of BS8110 and described in greater detail by CIRIA GUIDE 1- June 1976. The bursting tensile force Fbst in an individual end block loaded by a symmetrically placed end plate may be calculated from Table 5 where: 2yo is the side of the end block 2ypo is the side of the end plate Pk is the tendon jacking load

Table 5 - BURSTING TENSILE FORCE

ypoyo

0.3

0.4

0.5

0.6

0.7

FPbst

k

.023

0.20

0.17

0.14

0.11

11

2yo

2ypo

Anchorage 1 Anchorage 2

4a General arrangement 4b Single anchorage 4c Section through

Link 1Helix

Figure 4 - ANCHORAGE ZONE REINFORCEMENT

The force Fbst will be distributed in the zone extending from 0.2yo to 2yo from the loaded face of the end block and will be resisted by the reinforcement provided that it is acting at its design strength of 0.87 x characteristic yield stress. Mild steel reinforcement is preferable in order to limit the strain in the steel and hence possible cracking of the concrete. For rectangular end blocks, the area of reinforcement should be checked for each of the two axes and the links or helices required should be detailed on the basis of the greater area. Helical and link reinforcement which is adequate for a typical end block is detailed in Table 6. These areas are in excess of the requirements derived from Table 5, but cater for any inconsistency in concrete strength, angularity of plates, or incorrect location of the reinforcement. The first turn of the helical reinforcement should commence immediately behind the plate. The amount of reinforcement given in Table 6 is also adequate when the ribbed sleeve anchorages are used with single bars. The first link or first turn of the helix should be positioned as near to the nut end of the sleeve as possible while allowing for the specified concrete cover over the steel.

Table 6 - ANCHORAGE ZONE REINFORCEMENT (Helix And Links Used Together - See Figure 4)

MACALLOY HELIX LINKS Bar Dia

(mm) Rod dia

mm I/D mm

Pitch mm

Turns mm

Rod dia mm

CRS mm

Number

25 12 130 40 5 8 70 3 26.5 12 130 40 5 8 70 3 32 12 165 40 6 8 80 3 36 12 195 40 7 8 80 4 40 12 220 40 7 8 80 4 50 16 250 50 8 10 100 4 75 20 350 75 8 16 100 6

Note :A longitudinal length of rod may be used to attach the links together, but it is not required as part of the reinforcement.

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5. Curvature Of Tendons

Single bar tendons may be given a curved profile with a minimum radius of curvature of 200 x nominal bar diameter. The minimum radius of curvature of multiple bar tendons is governed by the longitudinal distance between spacers. Recommendations are given in Table 7

Table 7 - TENDON CURVATURE

Macalloy Bar dia, mm

25

26.5

32

36

40

50

Minimum radius of curvature (m)

5

5

6.4

7.2

8

10

The minimum radius for the 3 x 40 tendon is 25 m in the plane containing the three bars and 8 m in the plane perpendicular to the three bars.

8 mMIN. RAD

MIN. RAD25 m

a) Horizontal tendons a) Vertical tendons

Figure 5 - TENDON CURVATURE

It may be necessary to prebend bars of 40 and 50 mm diameter in suitable powered roller equipment before fixing them in the formwork.

5.1 Calculation Of Bar And Thread Lengths

5.2 Bar Lengths

Calculation of the overall length of bar is by measurement along the tendon profile and adding the thickness of both end plates plus an allowance for attaching the prestressing jack at one or both ends of the bar. When jacking at one end only, allowance must be made for a nut or tapped plate to be fitted at the opposite end. Table 8 provides details of the allowances necessary for attaching the prestressing jacks.

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X1 = LIVE END

X2 = DEAD END

X3 = TAPPED PLATE

Y = LENGTH OF BAR PAST NUT ORTHROUGH TAPPED PLATE12 FOR 6mm PITCH (25 - 36mm)16 FOR 8mm PITCH (40 - 75mm)

X1 L

X3

X2

DEAD ENDJACKING END

Y

Figure 6 - BAR LENGTH CALCULATION

Table 8 - JACKING ALLOWANCE

Tendon 25 26.5 32 36 40 50 75 Jack One end, X1 + X2 mm 133 146 164 179 203 258 351 Jack Both ends, 2 x X1 mm 166 184 212 232 262 332 470 Tapped Plate One end X1 + X3 mm 95 104 118 128 147 182 N/A

5.3 Thread Lengths

The thread length at a jacking end must allow for attaching the jack plus elongation of the bar under working load. The standard jacking thread is 250mm long which caters for tendon lengths up to 18m jacked one end or 36m jacked both ends. Additional thread length is needed for longer tendons at the rate of 25mm extra thread for each 5m of bar when jacked at one end or 10m of bar if jacked at both ends. Standard thread lengths for jacking ends, dead ends and coupled joints are listed in Table 9.

Table 9 - STANDARD THREAD LENGTHS

Bar dia mm 25 26.5 32 36 40 50 75 Jacking End mm 250 250 250 250 250 250 350 Dead End mm 100 100 100 100 100 100 150 Coupled Joint mm 45 50 60 65 75 85 150 Alternatively, the manufacturing details may be stated as follows: “x” No. of Macalloy bars “d ” diameter x “l ” overall length with end threads of length S1 and S2.

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6. Calculation Of Extension Assuming the bar extension is measured relative to the end plate then the extension measured during jacking is the sum of the elongation of the bar and the shortening of the concrete under load. The total extension is given by the following formula

Elongation = L fE

fE

s

s

c

c+

�

��

�

��

where L is the stressed length of the bar fs is the steel stress based on actual bar area fc is the average concrete stress along the line of the bar Es is the modulus of elasticity of the steel at the applied stress Ec is the modulus of elasticity of the concrete at the time of stressing For usual stress conditions: Es = 170 kN/mm2 (approximately) for 25-50 mm bars and 205 kN/mm2 (approximately) for 75 mm bars. Measurements of Es are obtained during routine tensile testing and the value appropriate to the Macalloy bars supplied on any particular consignment can be given on request as outlined in Section 0. Ec = 30 kN/mm2. And, as a guide, the extension of a bar (25 - 50mm) stressed to 70% of the ultimate stress will

be L

220 mm approximately when L is given in millimetres.

An appreciable amount of the total measured extension in short bar, ie less than 4m long, is due to the following: a) Elongation in the threads and draw bar b) Bedding down of jack and anchorage components c) Rotation caused by angularity of the bar relative to the end plate. It is advisable in these circumstances to control jacking by load, ensuring that the jack gauges are calibrated frequently.

7. Calculation Of Friction Loss, Bar Elongation And Jacking Force The derivation of bar elongation and jacking force from curved tendons can most simply be carried out in tabular form. The tendon is subdivided into component lengths for which the length and radius of curvature can be determined. The “wobble” factor and coefficient of friction appropriate to the design as described in Sections 0 and 0 are applied to the lengths and radii to derive the jacking force required to give the design force at the critical sections. The effective lengths for elongation calculation are also obtained. The following sample calculation illustrates the method. The values of Es and fc were taken

15

from a specific project and would vary according to the design and material supplied. ( Note the tendon is jacked from both ends, A and J. )

A

B

C D

E

F G H

J R1 R2

R3

R4 R5 R6

R7

R8

5850

4860

9180 9585 9585

5130

9700

7760 Figure 7 - TENDON PROFILE

Table 10 - TENDON PROFILE DATA

Section Length L Radius R K µ KL µL/R KL+µL/R AB BC CD DE

5850 9180 4860 9585

182081 183315 21937

163035

6X10-4

6X10-4

6X10-4

6X10-4

0.20 0.20 0.20 0.20

.00351 .00551 .00292 .00575

.0064 .0100 .0443 .0118

.0099 .0155 .0472 .0175

JH HG GF FE

7760 9700 5130 9585

189443 172462 23401

163035

6X10-4 6X10-4 6X10-4 6X10-4

0.20 0.20 0.20 0.20

.00466 .00582 .00308 .00575

.0082 .0112 .0438 .0118

.0129 .0170 .0469 .0175

Table 11 - TENSION FACTORS

Tendon Tension Section KL+µL/R 1-(KL+µL/R) Point Tendon Factor Force AB BC CD DE

.0099 .0155 .0472 .0175

.9901 .9845 .9528 .9825

A B C D E

1.0000 .9901 .9747 .9287 .9124

959

875

H HG GF FE

.0129 .0170 .0469 .0175

.9871 .9830 .9531 .9825

J H G F E

1.0000 .9871 .9703 .9248 .9124

962

875

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Table 12 - TENDON EXTENSION

Tendon Factors Actual Effective Section Jacking End Far End Average Length Length

AB BC CD DE

1.000 .990 .975 .929

.990 .975 .929 .912

.995 .982 .952 .920

5850 9180 4860 9585

5821 9024 4627 8818

28290

JH HG GF FE

1.000 .987 .970 .925

.987 .970 .925 .912

.993 .978 .947 .917

7760 9700 5130 9585

7706 9487 4858 8789

30840 Macalloy steel - average value of Young’s Modulus = 161.5 kN/mm2 average diameter = 40.8 mm area = 1310 mm2 Elongation = 28290 x 959 = 128 mm 1310 x 161.5 + 30840 x 962 = 140 mm 1310 x 161.5 Total = 268 mm Concrete - average stress = 8.65 N/mm2 Ec = 30 kN/mm2 approx.

Shortening = 61650 8 65

30000× .

= 18 mm Per Tendon = 1 mm approx. (based on the number of bars in the concrete m member) Due to friction reversal, stress at mid span will be approximately constant. Stress near ends of tendons reduces due to shortening.

17

8. Stressing Record It is useful to set out the desired load and extension values for each Macalloy bar and record the measurements taken during jacking to provide a permanent dossier on the structure. A stressing document with the following layout would meet normal requirements.:

Contract ......................................................................................................................................................... Section or Bay Reference ............................................................................................................................... Concrete : Date Cast .................................... Date & Stage of Stressing ...................................................... Cube Crushing Strength : ....... N/mm2 at ....... days. Macalloy Steel: Modulus of Elasticity.......................................................kN/mm2 ....... N/mm2 at ....... days. Macalloy Jacks: Mark.................................. Ref. ................................................ ....... N/mm2 at ....... days.

Calculated Measure Values

Bar Mark

Dia

Extension

Initial Ram Position

Load

Final Ram Position

Load

Total Extension

Remarks

1 2 3

Figure 8 - Suggested Site Stressing Record

9. Torque Loading Macalloy and other threaded bars are also used for applications where the load required is small and does not need to be measured accurately, e.g. temporary works or to induce a small compressive stress to control cracking of new concrete. For these applications, it is possible to develop a load in a Macalloy bar up to 25% of the normal working value given in Table 1 by applying a pre-determined torque to the Macalloy nut. Toque wrenches are available which have a dial indicating the torque value exerted, or which can be preset to slip at a specified torque value. The axial tension induced by a given torque depends upon the diameter and pitch of the threads, and upon the friction within the threads and between nut, washer and end plate. Accuracy of the tensile force cannot be expected to be more than ±25%. There is little point in using a precise formula for calculating the torque in these circumstances, and a general expression is :

Torque = PDKt

Nm

18

Where P is the desired axial force in N D is the diameter of the thread in m Kt is constant measured by test N.B. This accuracy of measurement of load applied will be approximately ±25% if measured through the torque wrench

Table 13 - - Kt VALUES FOR MACALLOY COARSE THREADS

BAR DIAMETER Kt

25

26.5 32 36 40 50

4.1 4.3 4.7 4.9 4.5 4.1

10. Miscellaneous Data

10.1 Modulus of Elasticity

Macalloy bars of 25 mm to 50 mm diameter are cold worked by stretching to a load of approximately 85% of the ultimate tensile strength. After processing, there is no discernible yield point in a tensile test. The stress resulting in a permanent elongation of 0.1%, i.e. the 0.1% proof stress, is taken as equivalent to the yield stress. The modulus of elasticity at the working level of 70% of the characteristic failing stress is reduced compared with the value for a steel that has not been cold worked. An average value of Es is 170 kN/mm2, but the precise value depends on the amount of cold working to which the bars have been subjected. Routine tensile tests carried out at the rate of one test from each 5 tonnes of bars processed enable the appropriate value to be provided for any particular consignment on request. Bars of 75 mm diameter are processed by quenching and tempering the steel to achieve the desired properties. The value of the modulus of elasticity at the working load is higher than for the cold worked steel. The average value is 205 kN/mm2 for 75 mm bar.

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(i) 40 mm Macalloy bar (typical of 25-50 mm bar) (ii) 75 mm Macalloy bar

Figure 9- TYPICAL STRESS/STRAIN CURVES FOR COLD WORKED;

QUENCHED AND TEMPERED MACALLOY

Measurements of Modulus are made on plain bars and the effect of elongation in the threads and in jack components must be allowed for in practice. The contribution of these factors is only significant for tendons shorter than 4m.

10.2 Fatigue Resistance

Test reports are available to demonstrate that Macalloy fatigue performance exceeds the current UK and European code requirements.

10.3 Notch Ductility

Macalloy bars fail in a brittle manner at room temperature as the transition temperature is well above ambient levels. Charpy “V” notch impact results at 20o Centigrade are typically 5 Joules for Cold worked steels (25-50mm dia) and 18 Joules for Quenched and tempered steels (75mm dia) See also section 10.7.

STR

ESS

IN N

/mm

0

200

100

0.1

500

400

300

MOD. OF ELAST. - 205 kN/mm

0.30.2STRAIN %

0.4 0.5 0.6 0.7

900

800

700

600

0.2% proof

0.1% proof

0.1% proof stress - 835 N/mm

UTS - 1030 N/mm

2

2

2

2

0

100

200

300

400

500

600

700

800

900

0.1 0.2 0.3 0.4 0.70.5 0.6

MOD. OF ELAST. - 170 kN/mm

0.1% proof stress - 835 N/mm

UTS - 1030 N/mm

2

STR

ESS

IN N

/mm

STRAIN %

0.1% proof

0.2% proof

70% OF U.T.S.

2

2

2

20

10.4 Effect of Change of Temperature

The coefficient of linear expansion of Macalloy steel is 11 x 10-6 per 1 degree Centigrade.

10.5 Electrical Resistivity

Table 14 gives the values of electrical resistivity at various temperatures measured on the Absolute (Kelvin) scale.

Table 14 - ELECTRICAL RESISTIVITY

TEMPERATURE o K

RESISTIVITY ohm/m

273.2 373.2 573.2 973.2

17 23.2 39.8 93.5

10.6 Transverse Stresses

Poisson’s ratio for Macalloy steel is 0.29.

10.7 Cutting off Excess Bar Thread

Excess bar thread may be cut off after stressing by sawing or disc cutting. When disc cutting, a liberal supply of water is needed over the bar during the operation to limit the heat developed and surrounding bars should be protected from sparks or spatter. Flame cutting can be performed but extreme caution should be used. If flame cutting is employed, an asbestos shield must be provided over the nut, and the cut must not take longer than 10 seconds. Bars must not be cut closer than 10 mm to the nut, and adjoining bars must be protected from the effects of heat.

10.8 Shear Strength

Where Macalloy is subject to shear loads, the shear strength of the steel should be assumed to be half its tensile strength, i.e. yield = 417 N/mm2 , ultimate = 515 N/mm2. Combined shear and tension should be checked using an appropriate formula with the above values used as the shear strength.

10.9 Macalloy at Cryogenic Temperatures

Test data is available for the Macalloy bar at temperatures down to -196oC. This shows that its strength increases by 17% between room temperature and -100 oC, but thereafter it declines slightly to give a residual increase of 11% at -196 oC. The results for elongation and reduction of area show a sharp drop at around -75 oC; this corresponds with a change in the nature of the fracture from partially brittle to wholly brittle at this temperature. Charpy impact tests average 4J at -160 oC compared to 5J at ambient temperature.

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10.10 Welding

Macalloy prestressing bar must not be welded, subjected to high local heating or splashed with weld metal.

10.11 Bending

Macalloy prestressing bar can be bent ( cold ) through 180o about a former with a radius of 6 times the bar diameter. End - August, 00