TRANSPORT AND ROAD RESEARCH LABORATORY Department of Transport

RESEARCH REPORT 255

MEASUREMENTS OF PRESTRESS AND LOAD TESTS TO

FAILURE ON SEGMENTAL CONCRETE BEAMS

by R J Woodward

The views expressed in this Report are not necessarily those of the Department of Transport

Bridges Division Structures Group Transport and Road Research Laboratory Crowthorne, Berkshire, RG11 6AU 1989

ISSN 0266-5247

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on I st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

CONTENTS

Abstract

1. Introduction

2. Details of beams and loading arrangement

2.1 Description of beams

2.2 Concrete properties

2.3 Wire properties

2.4 Loading arrangement

3. Measurement of prestressing force

3.1 Strain relaxation--concrete

3.2 Strain relaxation--tendons

3.3 Centre hole technique--tendons

4. Load test

4.1 Instrumentation

4.2 Loading sequence

4.3 Load deflection

4.4 Temperature

4.5 Strain measurements

4.6 Cracking and joint opening

4.7 Condition of tendons

4.8 Degree of grouting

4.9 Prestressing force

5. Discussion

5.1 Failure moment

5.2 Failure mode

5.3 Prestressing force

5.4 Monitoring joint opening

6. Conclusions

7. Acknowledgements

8. References

Page

1

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6

6

6

7

8

8

11

17

19

19

19

21

21

22

22

23

24

24

24

Page

Appendix A--Descr ipt ion of test rig 25

Appendix B--Optical fibre and crack propagation gauges 28

B.1 Optical Fibre Gauges 28

B.2 Crack Propagation Gauges 28

© CROWN COPYRIGHT 1989 The views expressed in this Report are not

necessari ly those o f the Department o f Transport. Extracts f rom the text may be reproduced,

except for commerc ia l purposes, prov ided the source is acknowledged

MEASUREMENTS OF PRESTRESS AND LOAD TESTS TO FAILURE ON SEGMENTAL CONCRETE BEAMS

ABSTRACT

Two edge beams from a 32 year old segmental post-tensioned concrete bridge were load tested to failure. The failure mode of the two beams differed considerably. The f i rst beam tested failed at a maximum moment of 2730 kNm at a deflection of 105 mm. The maximum moment applied to the second beam was 2090 kNm and this caused a deflection of 54 ram. As the deflection was increased further the applied moment reduced but the beam did not fail until i t reached a deflection of 260 mm.

Several methods were used to measure the prestressing force in the tendons. The mid-span losses for the two beams were of the order of 10 per cent to 15 per cent, although losses were higher at some of the joints in the second beam tested. This was attr ibuted to corrosion of the tendons at the joints which was more severe than in the f irst beam.

Optical fibre and crack propagation gauges were used to detect the opening of the joints between the segments to determine whether they would be suitable for on-site monitoring. Neither type of gauge gave a reliable indication of joint opening.

1 INTRODUCTION

In December 1985 Ynys-y-Gwas bridge collapsed, apparently wi thout warning, and only the edge beams were left in posit ion (Plate 1). The bridge was a single span, segmental, post-tensioned concrete structure which had been built in 1953 and carried a minor road over the river Afan in South Wales. Collapse was due to the ingress of de-icing salts at the joints which had corroded the prestressing tendons (Woodward and Wil l iams 1988). This caused concern about the condit ion of other bridges of similar construction and whether they could also fail w i thout warning.

To obtain more information on the mode of failure of segmental post-tensioned beams containing partially corroded tendons the edge beams from Ynys-y-Gwas bridge were delivered to TRRL for load testing to failure. In addition a number of techniques for detecting the opening of joints between segments were applied to the beams to investigate their potential for routine monitoring of structures in service.

The oppor tun i ty was also taken to measure the prestressing force in the tendons. Several methods were used; they included measurement of: decompression moments, relaxat ion of strain on and around cores dri l led f rom the concrete, strain relaxat ion in wi res as they were cut and re laxat ion of strain around holes dril led into exposed wires.

2 DETAILS OF BEAMS A N D L O A D I N G A R R A N G E M E N T

The t w o beams wi l l be dist inguished as fo l lows:

Beam A -- the downst ream edge beam Beam B -- the upstream edge beam.

2.1 DESCRIPTION OF BEAMS The edge beams were 19.6 m (64 f t 4 in) long and made up of eight 2 .44 m (8 ft) long segments • longi tudinal ly post- tensioned together by ten Freyssinet tendons. Each tendon consisted of twe l ve 0 .2 in (5 ram) d iameter wires and was housed in an unlined duct. The segments were of box sect ion and the joints be tween them, wh ich were approx imate ly 25 mm wide, had been packed w i th mortar. Cardboard tubes had been used to form the ducts where they passed between the segments. A sketch of a beam is shown in Figure 1 and the sect ion propert ies are given in Table 1.

The modular ratio of 12 assumed at the jo ints was based on an est imate of the s t i f fness of the mortar (Section 4.5) .

A f te r the beams had been load tested grout was found in the ho l low sect ion of the box. There was only a thin layer, about 13 mm deep, in beam A but in beam B it was approx imate ly 230 mm deep. It had presumably leaked into the boxes during construct ion. A l though it increased the we igh t of beam B by approx imate ly 18 kN, its presence was ignored when determin ing the sect ion propert ies as the results of the load tes ts indicated tha t it was not act ing s t ruc tura l ly (Sect ion 4.5) .

2.2. CONCRETE PROPERTIES Eight 100 mm d iameter cores were dri l led f rom each beam and their s t rength, dens i ty and modulus were measured in accordance wi th BS1881 Parts 114, 120 and 121 (Brit ish Standards Inst i tu t ion 1983) . The resul ts obtained are summar ised in Table 2.

T i~i~ ̧, i~ ̧I

Plate 1 Ynys-y-Gwas bridge after collapse

(Photographs provided by courtesy of West Glamorgan County Council)

91 A "~ '1 19600

I I I ' I 3 I • i

d

, I s ~ l

610

- c "

279

Inner web

787

B

All dimensions mm

A ~-J 51 102 __o I I - - I

:,J Elevation

279 127 51

q

I

m

152

o o

o o ]4- 0 0 0 0 0 0 ~ 152

Section A - - A

1 762

I

1 --8 Segment numbers

Outer web

0 Duct

Fig.1 Sketch of beam

Area -- concrete 0 .412 m 2

-- tendons 2 .432 x 10-3m 2

T A B L E 1

Sect ion propert ies

Section

Jo in t 1/2 2/3 3/4 4/5

Segment 5 (centre)

Modular ratio

12 12 12 12

7

Height of neutral

axis mm

Eccent r ic i ty of t endons

mm

536 530 525 525 534

178 2 7 3 351 351 3 5 9

Second momen t of area

mm 4 ( x 101°)

5 .33 5 .38 5 .47 5 .47 5 .30

T A B L E 2

Concrete propert ies

Beam A Beam B

Elastic Elast ic Position Strength modulus Densi ty St rength modu lus Dens i ty of core kN/mm 2 kN/mm 2 kg/m 3 kN/mm 2 kN/mm 2 kg/m 3

Deck

Web

59.0 63 .0 66 .0 65.5 66.5 60.5 64.5 67 .0

40 .0 47 .0

38 .5 44 .0

2330 2330 2380 2310 2310 2330 2300 2360

59 .5 71 .0 54 .0 79 .5 52 .0 57 .5 54 .5 65 .5

29 .7

32 .2 37 .7

4 7 . 5 56 .5 3 9 . 0

2 3 2 0 2 3 3 0 2 3 3 0 2 3 4 0 2 3 4 0 2 3 4 0 2 3 7 0 2 3 2 0

2.3 WIRE PROPERTIES The tens i le s t rengths of four teen uncorroded wires we re measured; t hey were in the range 1 5 5 0 N /mm 2 to 1 7 0 0 N/ram 2 w i t h an average of 1 6 0 0 N/ram 2 and a standard dev ia t ion of 46 N/ram 2. Load strain curves were measured on three w i res . They had an elast ic modu lus of 195 N/ram 2 and a breaking strain of 0 . 0 3 2 per cent .

2.4 LOADING ARRANGEMENT The beams were loaded at the th i rd po in ts . A schemat i c diagram of the loading ar rangement is s h o w n in Figure 2. Loads were appl ied under de f lec t ion cont ro l using a d isp lacement t ransducer moun ted at mid-span. A detai led descr ip t ion of the tes t rig and loading system is g iven in A p p e n d i x A.

I-.~ 19.6m ~.-I I ~"L v I

II

I~.. 6m ~.~1 I~" r l

~ App l ied load ~

,I Bearing

18m Bearing ,~1

Fig.2 Loading arrangement v I

3 MEASUREMENT OF PRESTRESSING FORCE

3 . 1 S T R A I N R E L A X A T I O N - - C O N C R E T E

3 . 1 . 1 Test procedure

The prestrain in the concrete on the sof f i t of beam B was measured at four posit ions by dri l l ing 75 mm diameter cores and measuring the relaxat ion of strain on and around each core. Strains were measured using electrical resistance and vibrat ing wire strain gauges w i th gauge lengths of 50 mm. The posit ions of the gauges and cores are shown in Figure 3.

The procedure used was to take strain gauge readings w i th the beam unloaded and then loaded to 96 kN and 171 kN, respect ively. Cores were then drilled in four increments to a depth of approx imate ly 50 mm. Af ter each increment two more sets of strain readings were taken; one w i th the beam unloaded and the other w i th it loaded to 160 kN.

3 . 1 . 2 Strain relaxat ion on cores

The applied load did not cause any s igni f icant strain change on the cores after they had been dri l led. This indicates that the strain on the surface had been to ta l ly relieved. The prestrain in the concrete under zero applied load was therefore assumed to equal the strain relief after the core had been dril led.

I - r l - 2.45 A

X A,B,C,D Position of cores

X X

I I

1.23 r l ~ 0 . 7 5 I t , ~ 3.31 B C

Sof f i t of beam

12ram T 1 2 m m

D

| |

51.4mm Vibrating wire strain gauge

I I 50mm Electrical resistance

strain gauge

Posit ion of gauges around each core

Fig.3 Position of cores and gauges for measurement of strain relaxation

4

3.1.3 Strain relaxation adjacent to cores As the cores were drilled there was a relief of strain in the adjacent concrete. For example at position A drilling the core to a depth of 50 mm caused a strain relief at the centre of the longitudinal gauge of 102 microstrain under zero load and 87 microstrain under an applied load of 160 kN.

Under zero load the strain in the concrete was due to the prestressing force in the tendons and the dead load, due to the beam and test rig. At an applied load of 160 kN the strain in the concrete was due to the above plus that due to the applied load. Therefore the difference of 15 microstrain between the strain relieved under zero load (102 microstrain) and that relieved under an applied load of 160 kN (87 microstrain) was the relieved portion of the strain caused by the applied load of 160 kN.

The strain due to an applied load of 160 kN was determined by loading the beam. At position A it was 34 microstrain so the ef fect of dril l ing the hole to a depth of 50 mm was to relieve 43 per cent of the strain ie 15/34. This implies that the strain of 102 microstrain which was relieved under zero load was 43 per cent of the prestrain in the concrete which gives a prestrain of 240 microstrain. The prestrains at the other three positions were calculated in the same way.

3.1.4 Calculation of prestressing force The preceding sections described how the results obtained from the vibrat ing wire and electrical resistance strain gauges were used to est imate the prestrains in the concrete at sections A, B, C and D. The moments required to produce these prestrains and thus reduce the stress in the

concrete to zero were obtained by ext rapolat ing the loads and strains measured before the cores were dril led. These decompression moments were then used to calculate the prestressing force in the tendons at the four sect ions. The results are g iven in Table 3.

3 . 2 STRAIN R E L A X A T I O N - - T E N D O N S Small slots, 150 mm long by 75 mm deep, were cut into the webs in the centre of seven of the segments in beam B, to expose the upper tendons. Small holes were then dril led into three or four wi res at each posi t ion and used as Demec gauge locat ion points. A datum set of Demec readings was taken before the beam was load tested. Af ter the load tes t the wires were cut and the Demec measurements repeated. The stresses in the wires were obtained f rom their load strain curves and the equivalent tendon forces are g iven in Table 4.

There is a considerable d i f ference in stress in the t w o tendons. Some of the wires in the outer web were subsequent ly found to be severely corroded. It is possible that these wi res had f ractured and lost most of their prestressing force.

If i t is assumed tha t the uncorroded wi res were typ ica l then the prestressing force in the tendon in the inner web would have been of the order of 240 kN, 285 kN, 235 kN, 240 kN, 235 kN, 245 kN and 235 kN wi th increasing d is tance along the beam (Figure 4).

Electrical resistance strain gauges were mounted on t w o wires in the end segment in beam A. A f te r the beam had been load tested the wi res were cut. Their strains were 5 9 0 0 and 5 4 5 0 microstra in wh ich cor responded to prestressing forces of 280 kN and 260 kN per tendon, respect ive ly .

TABLE 3

Prestrain in concrete and prestressing force in tendons (kN per tendon)

A B C D

Prestrain Decompression moment Prestressing force (ps) (kNm) (kN)

Position 1 2 1 2 1 2

245 170 165

240 90

120 130

8O5 1045 1065

795 685 865

1045

195 225 215

190 145 175 175

1. From strains measured on cores (Section 3.1.2). 2. From strains measured adjacent to cores (Section 3.1.3) .

5

TABLE 4

Force in tendons (kN)

Web Distance f rom end of beam (m)

1.2 3 .7 6.1 8.6 13.5 15.9 18.4

inner

outer

2 2 0 * 2 6 0

175 185

2 7 0 2 9 0 3 1 5 2 7 5

115 135

7 5 * 110

245 250 225 220

60 75 75

105 230 2 3 0 * 255

25 55*

100

240 235 230 230

235 250 220 220

250 260 235 230

220 2 2 5 * 235 220

240 230* 240

215 225* 185

*Wi res on w h i c h centre hole measurements were made (Section 3.3).

300

v

Q~

O-

O

Q .

260 f ~ ~ - - - I n n e r web

220 -

180

140

100

60 I I I I ~ I I i I I I 1 I 1 3 5 7 9 11 13 15 17 19

Distance along beam (m)

Fig.4 Average prestressing force in tendons (from strain measurements on wires) -- Beam B

varied around the c i rcumference of the wires. The presence of residual stresses and their c i rcumferent ia l var iat ion may account for the high stresses measured in some wires (Section 3.2).

TABLE 5

Comparison of results f rom centre hole measurements and Demec gauge

Tendon force kN

Centre hole 80 55 210 240 235 270 280 Demec 80 55 230 230 225 235 235

3.3 CENTRE HOLE TECHNIQUE- - TENDONS

These measuremen ts were made on seven of the exposed w i res in beam B. T w o electr ical res is tance stra in gauges were moun ted along the axis o f each w i re . A 1.6 mm d iameter hole was then dr i l led m i d w a y be tween the gauges and the change in st ra in recorded as a f u n c t i o n of hole depth . The readings were used to de termine the stress in the w i res and the resu l ts are g iven in Table 5 a long w i t h s t ra ins ob ta ined f rom Demec readings on the same wi res. Both sets of measuremen ts have been conver ted in to an equ iva len t t endon force.

A f t e r beam B had been load tes ted to fa i lure, t w o w i res we re removed, one f rom near the cent re and the o ther f rom an end segment . Centre hole measuremen ts we re made on the un loaded w i res and residual s t resses were f ound in .both. The i r magn i tude was b e t w e e n + / - 150 N/ram 2 and

4 L O A D T E S T

4.1 INSTRUMENTATION The beams were inst rumented as f o l l o w s : -

(i) Two 4 0 0 KN load cells were posi t ioned under the beams at each support.

(ii) Mid-span def lect ions were measured using displacement t ransducers posit ioned on each side of the beams.

(iii) Vert ical movement at the supports was moni tored using a precise level.

(iv) Displacement t ransducers, optical f ibre gauges, crack propagat ion gauges, and electrical resistance and vibrat ing wire strain gauges were mounted on the sof f i t of the beams across the joints (Table 6).

A

Distance from side (mm) of beam

Beam Joint

1/2 2/3 3/4 4/5 5/6 6/7 7/8

2/3 3/4 4/5 5/6 6/7 7/8

CPG CPG CPG

CPG CPG CPG

TABLE 6

Instrumentation across

75 150

OFG OFG OFG

OFG OFG OFG

oint soff i ts

Centre

VW VW DT VW DT VW VW

VW VW VW VW VW VW

150

OFG OFG OFG

ERS ERS ERS

75

CPG CPG CPG

CPG CPG CPG

DT -- Displacement transducer OFG - - Optical fibre gauge CPG -- Crack propagation gauge

VW -- Vibrating wire strain gauge ERS -- Electrical resistance strain gauge

Details of the optical fibre and crack propagation gauges are given in appendix B.

(v) Vibrating wire strain gauges were also mounted on both sides of the beams at various heights above the soffit across the mid-span joint and at the centre of one of the segments adjacent to mid-span (Figure 5). Two gauges were mounted on concrete prisms and positioned near the beam. Their purpose was to record changes in output from the gauges due to temperature. Thermally insulated covers were mounted over all gauges to minimise temperature effects.

(vi) Copper constantan thermocouples were used to monitor shade temperature and the temperature under the cover of one of the vibrating wire strain gauges.

A Gauge Technique data logger was used to energise the vibrating wire strain gauges and record their response. Outputs from the other gauges were fed into an Orion data logger.

4 . 2 LOADING SEQUENCE The maximum mid-span bending moments applied to the beams are summarised in Table 7. The dead load moment for beam A was greater than for beam B because the jacks and loading frame used were heavier and this more than compensated for the grout which had leaked into the box in beam B during construction (Section 2.1).

254

1 E j I

852

l ] Al l d imens ions in mm

I l 826

I l

305

Cross-section through the beam I I

V ib ra t i ng w i re strain gauge

Fig.5 Posit ion of vibrat ing wire strain gauges on the inst rumented sections

TABLE 7 Mid-span bending moments (kNm)

Beam Cycle Dead Live

A 1 450 2 450

B 1 440 2 440 3 440

*Max imum moment at (Section 4.3).

Loading frame Applied

135 1410 135 2175

80 1225 80 1435 80 1570

Total

1995 2760*

1745 1955 2090

oint 3/4 was 2730 kNm

4 . 3 L O A D - D E F L E C T I O N The weights of the beams were not evenly distributed on the four load cells indicating that they were sl ightly twisted, This had presumably arisen because the surfaces of the atcutments on the bridge had not been parallel. Rubber sheets were inserted under the beams in an attempt to equalise the loads. This reduced the differences considerably although there was a slight asymmetry in loading and this was reflected in the transverse bending of the beams (Section 4.5).

The moment-deflection behaviour of the two beams differed considerably (Figures 6 & 7). Beam A failed by fracturing of the tendons at joint 3/4 after it had been deflected to 105 ram. It bowed outwards as it collapsed. The maximum applied moment at the failure section was 2730 kNm and all but 13 of the 120 wires across the section fractured at the joint; the remainder broke within the segments. Localised compression failure occurred in the concrete adjacent to the joint in the final stage of collapse.

The maximum moment applied to beam B was 2090 kNm. This occurred at a deflection of 54 mm. As the deflection was increased further the load reduced but the beam did not fail until it had deflected 260 ram. By this time the applied load had almost fallen to zero. Failure occurred at mid-span and all but 28 of the 120 wires fractured at the joint; the remainder broke within the segments (Plate 2).

The stiffness of the beams prior to cracking was calculated from the average mid-span deflections and was used to estimate the elastic modulus of the concrete (Table 8).

4 . 4 T E M P E R A T U R E The temperatures recorded during each load cycle are summarised in Table 9.

T A B L E 8

Elastic moduli from moment-deflection and moment-strain measurements

Cycle

Beam A 1

Beam B 1 2 3

Applied moment kNm

O- 285 2 8 5 - 600 6 0 0 - 900 9 0 0 - 1 0 5 0

0 - 300 3 0 0 - 600 6 0 0 - 870 8 7 0 - 990

O- 545 0 - 540 0 - 495

Moment-deflection (kN/mm 2)

43.1 43.3 40.5 38.1 44.7 43.0 43.1 41.4

39.7 39.4 39.0

Moment-strain (kN/mm 2)

51.9 52.0 48.0 45.1 50.0 51.8 50.3 51.5

42.2 42.2 40.3

Mortar* (kN/mm 2)

22.0 16.6 15.7 11.9 18.2 17.4 17.8 12.0

8.4 8.9 8.4

* Calculated from the average strains across the joints--excluding joint 4/5 in beam B (Section 4.5).

8

2.8

A

=o

2.4

2.0

1.6

1.2

0.8

- f~ " - "

- - Beam A /

- /

\ Cycle 1

Cycle 2

Cycle 3

Beam B

0 40 I I I I I

80 120 160 Deflection (mm)

Fig.6 M o m e n t versus def lec t ion

I I I 200 240

2.8

2.4

2.0

=o F-

1.6

:~ 1.2

0.8

0.4

- - ~ ~ ~ ~ ~ " "

_ _ Beam A . A ~ ~

-

-

- z _

Cycle 3 I

I I I I 20 40

Deflection (mm)

Fig.7 M o m e n t versus def lect ion (0 - - 5 0 m m )

9

~'deg No CR192 88 8

, ! ~ t

° t t

m w _ ~

rjeg r,,~o CR192,88/9

! t

Neg.No. CR192 88 10

Plate 2 Load test on beam B

10

TABLE 9 Shade temperature variat ions during test (°C)

Beam A Start of test Maximum End of test

Beam B Start of test Minimum Maximum End of test

* Cycle 3 day 1 * *Cyc le 3 day 2.

Load cycle

1 2 3* 3 * *

18.8 13.7 - -

2 3 . 9 21.0 -- 20.2 18.2 --

m

8.5 10.2 5.2 8.5 7.5 8.3 5.1 8.5

10.7 10.7 9.8 9.5 10.7 8.4 7.5 9.4

4 . 5 STRAIN M E A S U R E M E N T S The apparent changes in strain recorded by the gauges mounted on the concrete prisms were less than 10 microstrain which was consistent wi th the small changes in shade temperture recorded during each test (Table 9). For this reason no at tempt was made to correct the strain readings for changes in temperature.

Inspection of the strain profiles showed that, prior to cracking, the centroid of the section in the concrete and across the mid-span joints was of the order of 550 mm above the sof f i t (Figures 8 & 9). This is in good agreement with the calculated value (Table 1). There was only a marginal difference between the two beams which indicated that the thick layer of grout found in the bottom of the box in beam B was not acting structural ly.

The elastic modulus of the concrete was estimated from the strain profiles through the section and the results are summarised in Table 8. The values for beam B are in good agreement wi th the moduli obtained from the moment-def lect ion curves and the direct measurements on the cores but considerably lower than those for beam A. Differences between elastic moduli obtained by different methods have been reported before (Buchner and Lindsell 1987). There are several possible reasons for such differences. Measurements of the global behaviour of the beams included the response of the mortar joints as well as the concrete, the loading applied to cores differs from that applied to the concrete in the beam and there is also a t ime factor, cores are loaded in a few minutes whereas the load tests on the beams took several hours.

An est imate was also made of the elastic modulus of the mortar (Table 8). This was done by assuming that the strain in the concrete adjacent to an instrumented jo int was equal to tha t in the centre of the segment. This enabled the strain in the mortar and thus its modulus to be calculated. The average values were 16 kN/mm 2 for beam A and 9 N/mm 2 for beam B. There was considerable var iat ion from joint to jo int in beam B. In jo int 3 /4 it was 11 kN/mm 2, in jo int 5/6 i t was 6 kN/mm 2 and in joint 4 /5 i t was less than 5 kN/mm 2. The values for joint 4 /5 have not been included in Table 8 because of the anomalous behaviour of the jo int (see below).

Strain measurements were used to give an indicat ion of the onset of cracking wi th in the concrete and the moments required to open the joints. Plots of moment versus strain in the concrete on the sof f i t of the beams are shown in Figure 10. They show quite c lear ly that for beam A cracking occurred adjacent to the gauge causing a reduction in measured strain whereas for beam B there was a rapid increase in strain fo l lowed by the appearance of a crack wi th in the gauge length.

For most jo ints there was a clearly def ined moment at wh ich there was a sharp increase in strain which was at t r ibuted to the opening of the joints (Figures 11 -1 5). The except ion was joint 4 /5 in beam B where it was ex t remely d i f f icu l t to est imate the momen t required to open the joint.

As the load applied to beam B reduced during cycle 3 the strains in the concrete and across joints 3/4, 4 /5 and 5/6 cont inued to increase whereas the strains across joints 2/3 and 6/7 decreased (Figures 16 and 17). Strains in the concrete and across joints 3 /4 and 5/6 reached a max imum at a momen t of 1885 kNm wh ich corresponded to a def lect ion of 75 mm. As the def lect ion was increased fur ther the deformat ion was concentrated at the mid-span joint, the w id th of wh ich cont inued increasing to fai lure.

The strain measurements showed that both the beams were bending t ransverse ly as load was applied. This was also ref lected in their crack patterns (Section 4.6) and the fai lure mode of beam A, as described above.

11

v

o

o

T=

-1-

1.0

0.8

0.6

0.4

0.2

0

--160 --120

\ 19 50kN }

A 95kN IC°ncrete

O 200kN!

• 50kN I • 95kN Mortar

• 200kN

-80 --40 0 40 80 Microstrain

Fig.8 Strain profi les Beam A f irst load cycle

120 160

E

o

O

e~

T=

-1-

1.0

0•8

0.6

0.4

0.2

0 --200

m

m

I -100 0 100

Microstrain

Fig.9 Strain prof i les Beam B first load cycle

[] 50kN I A 95kN Concrete

O 180kN

• 50kN

• 95kN Mortar

• 180kN

200

12

2.8

2.4

2.0

o

1.6

°E 1.2

0.8

0.4

(

; \ BeamA

- _

/ / ~ Cyc,e 3

I I I I 0 200 400

Microstrain

Fig. 10 M o m e n t versus strain on the so f f i t o f the i n s t r u m e n t e d segment

2.0

" o E

O . E F- v

g v

O

1.8

1.6

1.4

1.2

1.0

0.8

0.6

I I 0.2

Fig. 11

Beam A

Beam B

~ f

J

Cycle 1

Cycle 2

Cycle 3

I I I I i I 0.4 0.6 0.8

Microstrain (Thousands)

M o m e n t versus strain on the so f f i t across j o i n t 4 / 5

I 1.0 1.2

13

1.8

A

~3

0 . ( : }- v

z v

0

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2 0.4

Fig.12

Beam B

Cycle 1

Cycle 2

Cycle 3

0.6 0.8 1.0 1.2 1.4 1.6

Microstrain (Thousands)

M o m e n t versus s t ra in o n the s o f f i t across j o i n t 3 / 4

1.8 2.0

1.8

A

"0

2 f -

z v

0

1.6

1.4

1.2

1.0

O.B

0.6

0.4

0.2 0.4

Fig.13

Beam B

Cycle 1

Cycle 2

Cycle 3

0.6 0.8 1.0 1.2 1.4 1.6

Microstrain (Thousands)

M o m e n t versus s t ra in o n the s o f f i t across j o i n t 5 / 6

1.8 2.0

14

1.8

1.6 t

1.4

1.2

1.0

0.8

0.6

oE

0.4

D

. f , / Cyce I

I I I I I rl I 200 400 600 800

IV) icrostrain

Fig.14 Moment versus strain on the sof f i t across jo in t 2 /3

'.8 t_ 1.6

0 . c

v

z

E 0

1.4

1.2

1.0

0.8

0.6

0.4

Beam A

f

Beam B

Cycle 1

, Cycle 2

Cycle 3

1 I I 1 i 200 400 600

Microstrain

Fig.15 Moment versus strain on the sof f i t across jo in t 6/7

800

15

A ¢n E

o #-

E Z

E o

1.4

1.2

1.0

0.8

0.6

0.4

J o i n t ~ 6 / 7 / / s

0.2

v f -

• ~ ' ~ Jo in t 2 /3

Io int 4/5

I I 0.4

F ig .16 M o m e n t versus strain

I I l I I I I I 0.6 0.8 1.0 1.2

Microstrain (Thousands)

Beam B cycle 3 joints 2 /3 , 4 / 5 and 6 /7

I 1.4

¢n

¢o

Z v E

o

2.0

1.8

1.6

1.4

1.2

0.8

0.6

0.4 I 2

F i g . 1 7

J

J 1.0

Jo in t 4 /5

J f

/

f /

J Jo in t 3 /4

Jo in t 5/6

I I I I I I I 4 6 8

Microstra in (Thousands)

M o m e n t versus strain Beam B cycle 3 joints 3 /4 , 4 / 5 and 5 /6

I 10

16

4 .6 CRACKING A N D JOINT OPENING The onset of cracking in the concrete and joint opening between segments were detected using the crack propagation and optical fibre gauges, and by visual inspection. Joint opening was also estimated from the moment-deflection and moment-strain curves.

At joints--Joints opened between the mortar and the precast units. The moments at which they were first detected are summarised in Table 10.

The crack propagation gauges on beam A gave a clear indication of cracking before the cracks were

observed visually. Four of the gauges reconnected when the load was removed and were used to indicate re-opening of the cracks as the load was re-applied. The gauges on beam B did not indicate cracking until after the cracks were visible. Two of the gauges reconnected when the load was removed but only one indicated re-opening of the cracks as the load was re-applied.

The optical fibre gauges were damaged during installation. By shining light through each gauge and looking for leakage of light where fractures had occurred it could be seen that most were broken in at least one place. However it was stil l

TABLE 10

Detection of joint openings

Beam A Observed

inner side outer side inner side outer side

Optical fibre inner side outer side

Crack propagation inner side outer side inner side outer side

Vibrating wire

Displacement transducer

Beam B Observed

inner side outer side inner side outer side

Crack propagation inner side outer side inner side outer side

Vibrating wire

Load cycle

Total moment (kNm) Joint

1 1 2 2 1 2 1 2

1 1 2 2

1 1 2 2 1 2 3

ERS inner side 1 outer side 1

2/3

m

1500

m

m

D

1305

m

1160 1125 1075

m

3/4

1870 1900 1705

1750

1870 1825 1750 1705

1780 1585

1030

1000 1000

m

1595

1120 1120 1105

1030 1270

4/5

1905 1935 1800

1800

1875 1830

1740 1785 1590

1060 1060 1030 1030

1515

775

925 1180

5/6

D

1780 1900 1765

1750 1705

1735 1765 1615

1690 1615

1030

1000 1000

1595 1595 1245

1015 1000

985

1030 1270

ERS--Electrical resistance strain gauge

6/7

m

m

m

1545

m

m

B

1370

m

D

m

m

1210 1160

17

possible to transit suff icient light through four of the gauges to obtain a measurable signal. All four gave an early indication of cracking during the f irst load cycle. There was a sharp reduction in t ransmit ted light through two, but the others gave a more gradual change (Figure 18). None of these gauges could be re-used for the second loading.

The cracks were di f f icul t to see and became almost invisible when the load was removed. On both beams they were observed on the outer face before the inner face, which was consistent wi th the strain measurements and failure mode. The crack propagation and optical fibre gauges also reflected the transverse bending, gauges adjacent to the outer face detecting cracks before those adjacent to the inner face.

Crack widths and joint openings obtained from the strain and displacement gauges are given in Table 11.

Within segments--Beam A. At an applied moment of 1455 kNm hairline cracks were observed in the

concrete within the segments. The height of the cracks was greater on the outer side of the beam consistent with the transverse bending. They tended to occur over stirrups so their spacing was either 300 mm or 600 mm. With one exception, the acoustic gauges on the concrete were between cracks. The width of the crack passing through the gauge reached 0.13 mm just before collapse.

Within segments--Beam B. During the second load cycle hairline cracks were observed in the concrete within one of the segments adjacent to mid-span at an applied moment of 1200 kNm. As the load was increased more cracks appeared, and extended to heights of between 300 mm and 450 mm, respectively. A crack also appeared under the load point. During the third load cycle the cracks became visible at an applied moment of 945 kN.m. Their spacing was of the order of 300 mm and they extended to a height of 900 mm. The width of the crack under the acoustic gauge on the soff i t of the concrete is given in Table 11.

TABLE 1 1

Crack widths and joint openings

Mid-span moment kNm)

Beam A 1800 1935 2115 2235 2370 2520 2670 2750

Beam B 1240 1405 1570 1720 1885 2020 2090 2020 1870 1690 1420 1150

895 715

Deflection (mm)

20.4 23.4 28.5 34.8 44.1 58.8 75.5 98.0

12.4 16.5 22.9 29.0 37.3 45.8 54.1 58.3 90.8

102.6 119.3 1 57.4 173.0 227.4

Crack widths (mm)

u

w

m

0.13

0.04 0.06 0.09 0.13 0.17 0.28 0.36 0 .40 0.52 0.48 0.40 0.31 0.19 0.18

2/3

m

0.06 0.09 0 .14 0.28 0.46 1.00

m

0.05 0.08 0 .12 0.15 0.13 0.08 0 .05 0.03

3/4

0.04 0.08 0.13 0.20 0.28 0.39 0.58 0.89

0.06 0.10 0.20 0.33 0.51 0.68 0.83 0.84 0.83 0.74 0.47 0.22 0.06 0.05

Joint openings (ram) Joint

4/5

0.08 0.16 0.25 0.37 0.52 0.76 1.01 1.28

0.15 0.28 0.52 0.71 0.95 1.25 1.44

5/6

0.07 0.11 0.20 0.29 0.41 0.66 0.95 1.36

0.06 0.11 0.21 0.32 0.47 0.63 0.78 0.89 1.45 1.37 1.10 0.65 0.06

6/7

0.06 0.11 0.17 0.27 0.39

D

0.04 0.05 0.08 0.10 0.10 0.05 0.04 0.03

*The gauge across the mid-span joint in beam B broke at a deflection of 54 mm. The joint continued to open as the beam was deflected further and was greater than 50 mm wide at failure.

18

0 ..C

z

E 0

2 .0

1.8

1 .6

1.4

1.2

B

1.0 m

B

0.8 m

0.6 F

0

Joint 5/6 (outer)

Joint 4/5 (outer)

I I I I I 4 6

Output (V)

Fig.18 Outputs from optical fibre gauges

Joint 5/6 (inner)

10

4 . 7 C O N D I T I O N S OF T E N D O N S After the load tests had been completed the beams were broken up and the condition of the wires across selected joints carefully recorded (Plate 3). The degree of corrosion on each wire was assessed using the classification given in Table 12 and the results obtained are summarised in Table 13 along with the number of fractured wires.

The results were used to estimate the loss of section in the tendons by assuming average losses of section of 5 percent, 12.5 percent, 37.5

TABLE 12

Classification of corrosion

Degree of corrosion Condition of wire

None Mild

Moderate

Severe Very severe

No corrosion Slight surface corrosion with some scaling and pitting Mild corrosion plus up to 25% loss of section 25 to 50% loss of section >50% loss of section

percent and 75 percent for wires in the mild, moderate, severe and very severe categories, respectively. For beam A this gave results of 9 percent, 3 percent and 6 percent for joints 3/4, 4/5 and 7/8, respectively. The corresponding results for beam B were 11 percent, 16 percent and 17 percent for joints 3/4, 4 /5 and 5/6, respectively.

4 . 8 D E G R E E O F G R O U T I N G Voids were found in most of the def lected ducts in the end segments in beam A but at mid-span the ducts were fully grouted. In beam B, the four straight ducts were ful ly grouted at mid-span but voids were observed in the def lected ducts.

4 . 9 P R E S T R E S S I N G F O R C E The prestressing force was calculated from the moment required to open the joints. It was assumed that, during the second load cycle, the joints would open when the applied moment reduced the stress on the soff i t to zero. The second load cycle was used because the moment required to open the joints during the first cycle would also have had to overcome the tensile strength of the joints. The decompression moment was estimated from visual inspect ion and the change in slope on the moment-strain and moment-def lect ion curves. The results obtained are summarised in Table 14.

19

Severe corrosion and fracture

Neg. No CR767 88 18

Ductile fracture of a slightly corroded wire

Plate 3 Wires from Ynys-y-Gwas bridge

20

TABLE 13

Condition of wires

Number of wires in each category

Joint None Mild Moderate Severe V. Severe Fractures*

Beam A 3/4 4/5 7/8

Beam B 3/4 4/5 5/6

55 91 88

67 67 56

41 11 15

22 12 20

9 14 12

11 12 14

11 4 4

11 13 14

*It was not possible to identify which wires had fractured before the test anq the load test.

9 16 16

0 3 0

4 92 32

which had fractured during

TABLE 14

Prestressing force (per tendon)

Method

Vibrating wire gauges

ERS-wire -concrete

Beam A

Position along cable

Joint 2/3 Joint 3/4 Joint 4/5 Joint 5/6 Joint 6/7

Segment 5 Segment 1 Joint 3/4

Moment kNm

1305

1590

1370

Force kN

235

245

245

275

Moment-deflection Visual

Displacement transducer

Joint 4/5 Joint 5/6 Joint 4/5 Joint 3/4 Joint 4/5 Joint 5/6 Joint 3/4 Joint 5/6

B

1560 1810 1860 1825 1585 1615

240

240 245

Beam B

Moment Force kNm kN

1075 1105

985 1160 1230

1150 1045 1150 1330 1030 1060 1030

210 185

165 225 2O5

190 175 190 220 170 175 170

ERS--Electrical resistance strain gauge

5 DISCUSSION

5.1 FAILURE M O M E N T The ultimate moment was calculated using the theory of maximum strain. The initial force in the tendons was assumed to be 210 kN per tendon and subsequent values were taken from the load strain curves measured on wires taken from the

bridge. A rectangular stress distr ibution was assumed for the concrete. There was condsiderable uncertainty as to the correct value to be used for the maximum concrete stress because of the ef fect of the mortar so the calculations were repeated for a range of maximum concrete stresses. Several est imates of sectional area of steel were also used. The results obtained are summarised in Table 15.

21

TABLE 1 5

Calculated ult imate moment (kNm)

Maximum concrete stress (N/ram 2)

30 40 50

Assumed loss of Section of tendons (%)

0 5

2885 2770 3055 2930 3165 3025

10

2655 2805 2885

20

2415 2530 2595

Beam A failed at joint 3/4 rather than mid-span. This probably reflected differences in the condition of the tendons at the two joints. The estimated loss of section at joint 3/4 was 9 percent whereas at mid-span it was only 3 percent. A loss of section of 9 percent at the failure section would indicate an ult imate moment in the range 2680 kNm to 2915 kNm which is in good agreement wi th measured value of 2730 KNm.

For beam B, failure occurred at mid-span. Corrosion at joints 4/5 and 5/6 was similar but joint 4/5 would have been subjected to a greater bending moment. The 16 percent loss of section at the failure section would reduce the ult imate moment to the range 2500 kNm to 2700 kNm. This is still considerably greater than the measured value of 2090 kNm. The reasons for this are discussed in Section 5.2.

5.2 FAILURE MODE The test to failure on beam A has been simulated quite accurately by a mathematical model and it was shown that in principle the behaviour of beam B could also be modelled (Woodward and Wilson to be published). The results for the two beams showed that their moment-deflect ion curves were governed by the fol lowing:

- - the prestressing force; -- the bond between the tendons and the grout; - - the degree of corrosion at the joints.

The prestressing force governs the load at which the beam's behaviour becomes non-linear. The bond strength determines the distance over which the tendons are strained across the joints. The degree of corrosion determines the ult imate bending capacity of the beam.

The failure of beam A was attr ibuted to the simultaneous fracture of tendons in the bottom flange of the box. The low maximum deflection of 105 mm was due to a high bond stress between the tendons and grout which concentrated the deformation at the joints. This has serious implication since it indicates that sudden failure could occur in segmental structures wi th well bonded tendons.

The failure mode for beam B differed considerably from that for beam A. Several possible reasons are listed below:

-- all of the tendons at a particular level may not have failed simultaneously;

- - the first tendons to fail in beam B may not have been those in the lower flange;

-- there may have been more debonding than in beam A.

The consequences of the first two points are that, after the first fractures occurred, there may still have been sufficient load carrying capacity to carry the beam's dead load as well as a reduced level of superimposed load. This would lead to the kind of failure observed with the moment- displacement curve descending in steps as each successive wire breaks. The effect of increased debonding is to allow increased deflections.

5.3 PRESTRESSING FORCE West Glamorgan County Council provided copies of preliminary design calculations which gave an initial prestressing force of 263 kN and an allowance for losses of 15 percent. A paper describing the construction of the bridge gave the same initial force but stated that an allowance of 20 percent had been made for losses (Kaylor 1953). Design calculations in accordance with BS5400 gave mid-span losses of just under 20 percent (British Standards Institution 1984). These figures indicate that the initial prestressing force of 263 kN per tendon would be reduced to between 210 kN and 223 kN.

The prestressing forces calculated from the change in slope of the moment-deflection curves for the two beams gave values of 240 kN and 220 kN per tendon. This corresponds to mid-span losses of 10 percent and 15 percent for beams A and B, respectively. Estimates of prestressing force from other measurements are discussed below.

5 . 3 . 1 B e a m A

The decompression moments obtained from the moment-strain curves across the joints indicated a

22

prestressing force of the order of 240 kN per tendon which is in good agreement with that obtained from the moment-deflection curves.

5.3.2 Beam B The moments required to open the joints indicated a prestressing force of the order of 170 kN to 190 kN per tendon at the centre three joints whereas at the outer joints it was 210 kN. These figures are lower than those obtained from the moment-deflection curves. This may be due to corrosion causing a local loss of prestressing force at the joints, although even without corrosion, loss of prestress at the joints might be greater than within the segments because of the different properties of mortar and precast concrete.

The average strains measured in wires indicated that the prestressing force within the segments was 230 kN to 245 kN per tendon with a maximum value of 285 kN in segment 2. Possible reasons for these high values are discussed in Section 5.3.4.

The strains measured on the cores indicated a prestressing force of between 195 kN and 225 kN per tendon. The results are similar to those obtained from the decompression moment. The results from the strains measured adjacent to the cores gave lower estimates of prestressing force.

5.3.3 Other measurements Ad-hoc measurements of prestress loss have been made on post-tensioned beams and losses have generally been in excess of 30 percent (Felstead and Lindsell 1981, Buchner and Lindsell 1987). A possible explanation for the low losses measured in the Ynys-y-Gwas beams was that flood damage is known to have delayed progress and the contract period had to be extended. So the precast units may have been several months old before being stressed which would reduce losses due to creep. In addition, it is not clear from the records whether the initial stress referred to was before or after grip set losses had occurred. If it was the latter then this would have increased the initial force at mid-span by approximately 1 5 kN per tendon.

5.3.4 Measurement techniques A variety of techniques were used to estimate the prestressing force in the two beams. Measurements of strain in individual wires using the centre hole technique were in good agreement with the results obtained using a Demec gauge. Some of the readings however were rather high. This may have been because of the residual stresses in the wires. Their origin is uncertain; they may have been introduced during manufacture or possibly during handling before

being stressed. Whatever their origin it is clear that direct measurement of strain in individual wires may not be a reliable method of measuring prestressing force.

Values of prestressing force obtained from strain relief around cores drilled into the concrete were lower than those obtained using other methods. Strain measurements on the cores gave values which were in good agreement wi th those obtained from the moment-deflect ion curves. However because of the variation of prestressing force along the beam and at the joints it is not possible to comment on the accuracy of these measurements.

5 . 4 M O N I T O R I N G J O I N T OPENING Following the collapse of Ynys-y-Gwas bridge there is a need for techniques for detecting signs of deterioration of segmental bridges before collapse. The work described in this report has focused on techniques for detecting the opening of joints between segments. Mounting vibrating wire gauges across the joints would appear to be the most reliable solution. Unfortunately cracks which opened under load and subsequently closed when the load was removed would not be detected unless the bridge was load tested. This would be t ime consuming, disruptive to traff ic, expensive and possibly cause irreversible damage. The alternative is to use gauges which are permanently affected by the opening of a crack.

Two types of gauge were investigated: optical fibre and crack propagation. The crack widths at which these gauges responded are summarised in Table 16; they were obtained from the displacement transducers and strain gauges mounted across the joints. The figures given are

TABLE 16

Sensit iv i ty to joint openings (microns)

Gauge

Optical fibre

Crack propagation

Joint

3/4

4/5

5/6

3/4

4/5

5/6

Side

inner outer inner outer inner outer inner outer inner outer inner outer

Load cycle

60

70 50 60

110 90

1 O0 8O 55 7O

2

D

m

B

m

80 60

6O 6O

23

only approximate as the crack widths were not uniform across the soff i t because of the transverse bending.

The techniques are not particularly sensitive, the gauges require careful surface preparation and there are doubts about the long term reliabil i ty of their bond to the concrete. The most disappointing feature was that some of the gauges, in particular the crack propagation gauges, closed up with the crack and would therefore require continuous monitoring.

6 CONCLUSIONS The modes of failure of the two beams differed considerably. Beam A failed close to the third point under an applied moment of 2730 kNm at a deflect ion of 105 mm. The low deflection was attr ibuted to a high bond strength between the tendons and grout which reduced the length over which the tendons were strained.

Beam B reached a maximum moment of 2090 kNm at a deflection of 54 mm, as the deflection was increased further the load reduced. The beam eventual ly failed at mid-span at a deflect ion of 260 mm by which time the applied load had almost fallen to zero. The lower maximum moment for the second beam was attr ibuted to progressive failure of the tendons.

The change in slope of the moment-deflect ion curves for the two beams indicated prestressing forces of 240 kN and 220 kN. This corresponds to mid-span losses of 10 percent and 1 5 percent, respectively. The low losses were attr ibuted to the segments being several months old when stressed.

The decompression moments at the joints in beam A indicated a prestressing force of 240 kN whereas in beam B it was only of the order of 180 kN over the middle three joints and 210 kN at the adjacent joints. The higher losses at the joints in beam B may have been due to corrosion of the tendons which was more severe in beam B than in beam A.

The values of prestressing force estimated from the relaxation of strain on cores drilled into the concrete were similar to those obtained from the decompression moment. The estimates derived from strains measured around the cores were lower. Because of the scatter in the results it is not possible to comment on the accuracy of the technique.

The wire strains measured using the centre hole technique were in good agreement wi th direct measurements of strain.

The strains in some of the wires were up to 25 percent above the values expected. Subsequent measurements showed that there were residual stresses in the wires which varied around their circumference. This may account for the high levels of strain measured and shows that measurements of strain in wires may not be a reliable measure of prestressing force.

The crack detection sensitivities of the crack propagation and optical fibre gauges were 55 to 110 and 60 to 80 microns, respectively. They could be used to monitor segmental bridges for cracking between segments but there are doubts about the long-term bond between the gauge and the concrete and they would require continuous monitoring.

7 ACKNOWLEDGEMENTS The help, advice and assistance of my colleagues Messrs Gent, Grainger, Tabrett, Ball and Crew is gratefully acknowledged.

The beams were provided by West Glamorgan County Council.

The core drilling and associated measurements, and the centre hole measurements on wires were carried out by Stress Engineering Services under contract to TRRL.

The work described in this report was carried out in the Bridges Division of the Structures Group of the TRRL.

8 REFERENCES BUCHNER S H and LINDSELL P (1987). Prestressed concrete beams: controlled demolition and prestress loss assessment. Technical Note 127. CIRIA. London, pp 1-42.

BRITISH STANDARDS INSITUTION (1983). BS1881. Methods of testing concrete. Part 114. Method of determination of density of hardened concrete. Part 120 Method of determination of the compressive strength of concrete cores. Part 121 Method of determination of the static modulus of elasticity in compression. British Standards Institution. London.

BRITISH STANDARDS INSTITUTION (1984). BS5400. Steel, concrete and composite bridges. Part 4. Code of practice for design of concrete bridges. British Standards Institution. London.

FELSTEAD A E and LINDSELL P (1981). Controlled demolition of a post-tensioned concrete beam. Concrete, September.

24

KAYLOR H (1954). Recent ly built road bridge in prestressed concrete. Civil Engineering and Public Works Review. 49 (580), October, pp 1 0 6 1 - 1 0 6 3 .

WOODWARD R J and WILLIAMS F W (1988). Collapse of Ynys-y-Gwas bridge, West Glamorgan. Proc. Instn. Civ. Engrs, Part 1, August , pp 6 3 5 - 6 6 9 .

WOODWARD R J and WILSON D L S. (1990). Deformation of segmental post- tensioned precast bridges due to corrosion of the tendons. (To be published•)

APPENDIX A:

DESCRIPTION OF TEST RIG The test rig was mounted on a 24 m long by 4 m wide by 1.5 m deep reinforced concrete slab. It contained sixteen 750 kN capacity Macal loy anchor points posit ioned on a 3 m by 2 m grid. The rig was designed to w i ths tand upward loads of 3000 kN on any t w o pairs of anchors (ie 750 kN per anchor).

The beams were suppor ted on four rocker bearings, t w o at each end, wh i ch were mounted on 4 0 0 KN load cells. Rubber pads were inserted be tween the upper bearing plates and the sof f i t of the beam and PTFE plates were inserted at one end to a l low hor izonta l movement . This assembly was mounted on 1 m high concrete suppor ts on the concrete slab (Plate 1A).

Load was appl ied th rough crossbeams posi t ioned at the third po in ts (Figure A1 , Plate A2) . Rubber pads were placed under the crossbeams to spread the load evenly across the surface of the beam. T w o ho l low ram jacks were posi t ioned on each crossbeam and reacted against Maca l loy bars wh i ch were connec ted to the Maca l loy anchors embedded in the concrete slab. Brass sleeves were used to pro tec t the threads on the bars where they passed th rough the crossbeams. Universal jo ints were posi t ioned above the slab and under the crossbeams to a l low ro ta t ion of the crossbeams and prevent the bars f rom bending. A preload was appl ied to the anchors to ensure tha t they were on ly subjected to axial loads. The preload was reacted against a steel washer to d is t r ibute the load over the concrete surface.

A Keelavi te hydrau l ic pump was used to supp ly oil to the jacks via a t rans fo rmer wh i ch was used to

Rubber pad

Steel

Steel blocks ~ F

Plate A1

#

\ Bearing arrangement

, Load

i ~ i i~.~!

Neg.No. R283/87/1

Concrete support

25

~per nut

0.am

I -~ Hollow ram jack Lower nut

1 .lm

Crossbeam

Rubber pad

50mm dia Macalloy bar

I ~..... 40mm dia Macalloy bar

\ Concrete beam

Universaljoints

1.5m /

Bearing plates on ~ / I ~ steel washer I l • l •

7 Concrete slab containing embedded anchorages

Not to scale Fig.A1 Jacking assembly

equalise the load applied by the four jacks. The pressure at the jacks was controlled using a Moog servovalve positioned between the pump and the transformer. A Dartec controller was used to

operate the servovalve and the load was applied under deflection control using a mid-span displacement transducer. A schematic diagram of the loading system is shown in Figure A2.

26

Cro~e~ ' ~

i , leg. No. R 3 3 5 / 8 7 / 1 6

Universal joints

i',Jeg. No. C R 2 9 4 87 13

Plate A2 Test rig

27

t Transformer

Contro l ler ~ Servo valve

!

[::! ,ack !

I Pump* i

* For retracting the jacks Pump Hydraul ic line

Fig.A2 Schematic diagram of the loading system

APPENDIX B

OPTICAL FIBRE AND CRACK PROPAGATION GAUGES

B.1 OPTICAL FIBRE GAUGES Optical f ibre gauges consist of a number of parallel glass f ibres bonded to the surface to be moni tored. If a crack develops i t causes the f ibres to f racture and the in tens i ty of l ight passing through them wi l l be at tenuated. Thus by moni tor ing the in tens i ty of l ight passing through a f ibre i t is possible to detect the onset of cracking. Tests on concrete have shown tha t crack w id ths of 20 to 40/~m can be detected.

B.2 CRACK PROPAGATION GAUGES Crack propagation gauges consist of one or more strands of wire which are bonded to the structure under test. The wires used had a nominal thickness of 0 .043 ram. If a crack develops under the wires they fracture and do not pass an electric current. Therefore by wiring the gauges into an electrical circuit the onset of cracking can be detected.

For beam A gauges which consisted of a single strand of wire were used so fracture of the wire would cause an open circuit. For beam B the gauges used consisted of twen ty wires wired in parallel, so fracture of a single wire would reduce the resistance of the gauge.

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