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D

uring pretensioning operations

for prestressed concrete, its

standard practice to measure

force in the strand by two inde-

pendent methods.

After ini tially tensioning the strand to re-

move slack, an operator applies the final

prestressing force, which is read from a hy-

draulic gauge mounted on the tensioning

jack . T he force i s then checked by measur-

ing strand elongation and comparing this

measured value wi th a theoretical (calculat-

ed) value. M easured and theoretical elon-

gation must agree within 5% . A lso, i f the

gauge force is higher than the specified

force, the combined percentage deviation

of elongation and force measurements cant

exceed 5% ( see box on page 776).

I n a well-run plant, the measurements

usually fall within the 5% tolerance. Signif-

icant elongation variations are relatively

rare, especially for straight strands, says

John K ime, vice president of operations at

D ukane Precast Inc. in Aurora, I ll. Well

investigate our system i f theres any appre-

ciable difference in measured and theoreti-

cal elongation, even i f its less than 5% .

What should you look for when investi-

gating the system? T he Precast/PrestressedConcrete Institutes quali ty control manual

( M NL-116) describes some of the conditions

that might lead to differences between mea-

sured and theoretical elongation values.

Check the jacking systemA malfunctioni ng hydraulic gauge is one

possible source of jack ing pressure error.

G auges, jacks, and pumps should be cali-

brated as a system in the same way that

theyre used in the tensioning operations.

M NL-116 requires calibration at least yearly

and any time a tensioning system yields er-ratic results.

Some producers who use multiple-strand

tensioning devices may initially calibrate

the total system, then use a master gauge

for check ing the production gauges. When

this is done, production gauges must be

checked against the master gauge at least

every six months and the master gauge

must also be i ndependently calibrated at a

six-month frequency.

Load cells can also be installed in front of

the stressing ram as a means of checking

gauge accuracy. Prestress Supply I nc. ( PSI) ,

in Lakeland, Fla., mark ets a single-strand

tensioning jack wi th a load cell between

the jack ing box and cylinder head in the

stressing ram. A ccording to PSI V ice Presi-

dent Gaylan Leeman, this load cell mak es

the hydraulic gauge redundant, providing a

double-check for jack ing force. H e adds

that the load cell i s less susceptible to dam-

age than one mounted in front of the ram.

Elongation di screpancies may also be

due to a scored hydraulic cylinder, says

James Simms, president of Simms Engineer-

ing Co. in Elk G rove Village, I ll. I n such

cases, the hydraulic cylinder would have to

be replaced.

I f there are friction forces wi thin the

jack ing system, the gauge reading reflects

those forces as well as the forces in the

Measuredandtheoreticalstrandelongationsdont agreewi thin 5%.Now what?

By Kaushal

Gupta

Handling out-of-tolerance strandelongations

During prestressing, force in the strand is measured by pressure gauge. T he gaugereading i s checked by comparing measured and theoretical strand elongation values.

KaushalGupta

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strands at one or two points near

mi dspan. Since the stressing ram has

been disconnected when the strands

are deflected, gauges in the jack ing

system cant be used to verify the fi-

nal force. Instead, when a new bed

is set up, or deflection procedures

change, stress measurements are

made on strands at each anchorage

at each end of the bed.

Stressing deflected strands. If final

stressing is done wi th strands in a

draped position, friction at the de-

flecting devices resists some of the

force applied by the jacki ng system.

T hus, force in the strand decreases

at each successive deflection point

away from the jack ing source.

H old-up or hold-down devices

should be equipped with low-fric-

tion pin-and-roller assemblies or

other means for minimizing friction.

H owever, actual elongation may

not be wi thin 5% of theoretical

elongation or the combined percent

deviation for elongation and gauge

pressure may exceed 5% .

I f the measurements dont agree

within 5% , the jack or a load cell

may be used at the dead end to

check force in the strand at that loca-

tion. I f force measurements from the

gauge at the live end and the jack or

load cell at the dead end agree with-

in 5% , the requirement for elonga-

tion agreement can be waived.

When draped strands are jacked

from one end of the bed, M NL-116

requires measuring force on at least

two strands at the far end of the

bed, even when actual elongation

is wi thin 5% of theoretical elonga-

tion. T his can be done with dead-

end load cells or by moving the

jack to the dead end and just barely

lifting off the chuck from the abut-

ment. T he measured force must not

be below the theoretical values by

more than 5% . I f it is, force may be

added from the dead end and the

chucks reset. Elongation from this

added force must be recorded and

added to that measured at the live

end to check tolerances.

If the theoretical strand elongation

isnt attained when the gauge read-

ing exceeds specified force by 5% ,

strand must be jacked to theoretical

elongation from the other end of the

bed. I f this requires an overstress ex-

ceeding 5% , friction-reducing modi -

fications of the hold-up or hold-

down hardware may be needed.

Alternatively, deflectors can be re-

moved at some locations until the

required elongation can beachieved with not more than a 5%

overstress. Strand is then lifted or

depressed at these locations to pro-

duce final deflection.

When a bed with draped strands

is initially set up, or when changes

are made in the deflecting equip-

ment, its a good idea to determine

distribution of strand stress along

the bed. T his is done by mark ing off

a 10-foot or greater length of strand

after the ini tial stress has been ap-

plied. After final tensioning, the dis-tance between marks is remeasured.

Forces calculated from these elonga-

tion measurements must be within

5% of specified values.

Casting bed movementand strand slippage

Bed movements caused by abut-

ment rotation, anchorage move-

ment, or elongation of anchoring in

abutment beds are sometimes small

enough to be ignored. H owever, if

needed, movements can be checked

by setting stringlines parallel to and

some distance away from both abut-

ments, then measuring the distances

before and after stressing strands.

T he procedure is to check move-

ment periodically, preferably when

members with the maximum num-

ber of strands (highest prestressing

force) are produced.

Seating in the gripping devices

duri ng strand stressing can also af-

fect measured elongation, requiring

a small correction. G enerally, the

seating corrections are very consis-

tent, says John D ick, di rector of

certification programs at PC I . Peri-

odically, though, quality control

personnel need to go out and mea-

sure the amount of seating that oc-

curs in the chucks.

Keep accurate recordsT ensioning records are an invalu-

able resource when measured elon-

gations dont fall within allowable

tolerances. For all pretensioning,

M N L-116 requires quali ty control

personnel to record:

Required total load per strand

Initial tension

Anticipated and actual gauge pres-

sure for each strand or group ofstrands stressed in one operation

Anticipated elongation for each

different jack ing load

D ifferences in measured and theo-

retical elongation values, if recorded

daily, can give an early warning

when the 5% tolerance is being ap-

proached. Recording these differ-

ences also increases awareness of

the tolerances for plant personnel.

Reference

Manual for Quality Control forPlants and Production of Precast

and Prestressed Concrete Prod-ucts, MNL-116-85, Precast/ Pre-

stressed Concrete Institute, 1985.

In addition to their quality con-

trol manual, PCI offers two useful

training manuals for technicians.

TM -101, Qua lity Control Tech-

n ic i an / In spec to r Level I & I I

Tra in in g Manual , provides in-

formation on common plant pro-

cedures, including the tensioning

of straight strands.

TM -103, Qua lity Control Tech-ni cian /In spector Level III Man u -

al , is a continuation of topics

presented i n T M -103 and in-

cludes information on prestress-

ing deflected strands.

T hese manuals can be pur-

chased from Precast/Prestressed

Concrete Institute, 175 W. Jack-

son Blvd. , Chicago, I L 60604.

Training aidsfor prestressed

concrete producers

PUBLICATION #J960775Copyright 1996, The Aberdeen GroupAll rights reserved