# MEASURING LOSS OF PRESTRESS IN PRESTRESSED CONCRETE

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Instruments for measuring loss of prestress in prestressed concrete:

The  initial  pre-stressing  force  applied  to  the  concrete  element  reduces  due to elastic shortening of the concrete, anchorage losses, frictional losses etc.  The  pre-stressing  force  further  reduces  with  the  passage  of  time  due  to creep, shrinkage, temperature effect and relaxation of steel. The loss of pre-stressing force may reduce load carrying capacity of the structure.

It will be worthwhile to note that  no direct method is available to measure loss  of  pre-stress.  However,  it  can  be  ascertain  by  knowing  the  changes in  stress  levels  in  pre-stressing  cables.  The  change  in  stress  level  can  be known  by  periodical  measurement  of  the  same.  The  force  in  pre-stressing steel can be measured by using Vibrating Wire (VW)Load Cells or Vibrating Wire (VW) Strain Gauges.

#### Vibrating Wire (VW) Load Cell:

It  consist  of  a  set  of  vibrating wire  gauges  mounted  parallel  to each  other  equally  spaced  in  a ring  in  an  alloy  steel  cylindrical housing.  The  studs  of  vibrating wire  are  attached  with diaphragm of load cell.  The load cells  are  manufactured  with  a centre  hole  to  accommodate tendons or anchor cables. These load  cells  are  available  in various  ranges  from  100  to  5000  KN.

The  change  in  pressure  on  the diaphragm  will  change  the  tension  of  vibrating  wire.  The  natural  frequency of  wire  is  proportional  to  the  tension  of  the  wire.  Thus,  by  measuring  the natural  frequency  of  wire  pressure  on  diaphragm  can  be  known.  Natural frequency  of  the  wire  can  be  co-related  with  the  diaphragm  pressure.  The natural frequency of wire can be expressed as below:

where  f   =   Natural frequency of  the wire
l   =   Length of the wire
T  =   Tension of wire
m =   Mass/unit length

The  operation  of  VW  Load  cell  is  based  on  electrical  plucking  of  the vibrating  wire.  The  wire,  vibrating  at  its  resonance  frequency,  which  varies with  the  tension  in  the  wire,  induces  an  alternative current  in  the  coil  which is detected by the read out unit.

Fig: Vibrating Wire (VW) Load Cell Arrangement

The  VW  load  cell, sandwiched  between two  plates,  is  installed at  the  pre-stressing  end of the girder.  The cable of  load  cell  is connected  to  common read out unit.  Read out units  generally measures  the  period  or frequency  of  the  wire from  which  force  in cable  can  be  obtained.

The  read  out  unit  can  be  connected  with  P.C.  and  data  can  be  analysed using  custom-made  software  for  this  purpose.  Initial  reading  is  taken  soon after  the load  cell is installed.  Subsequent readings  are taken  at  each  stage of  pre-stressing.  However,  after  cement  grouting  of  cable  it  may  not  be possible  to  measure  the  force.  If  the  cable  duct  is  filled  with  grease  or  gel, the pre-stressing force can be measured periodically  during service.

Summary:    From the above, it can be noted that V.W. load cell system is  suitable  for  new  construction  and  not  for  existing  structure.  V. W.  load cell  measures  the  force  in  pre-stressing  wire.  Once  the  cable duct  is cement  grouted  the  force  transfer  to  load  cells  does  not  take  place.

However,  if  the  cable  duct  is  filled  with  grease,  oil  etc.  load  cell  continue  to give  the  force  in  pre-stressing  steel.  Hence,  in  the  present  system  where cable  duct  is  cement  grouted  load  cell  system will  not  help.  Load  cell system  can  also  be  used  to  monitor  the force  in  pre  stressing  steel,  at the time of pre-stressing, to avoid overstressing of the same.

#### Vibrating Wire Strain Gauge:

With  the  help  of  Vibrating  Wire  Strain  Gauge,  the  change  in  strain  can  be obtained  with  periodical  measurement  of  strain.  The  stress  can  be calculated  by  multiplying  strain  with  modulus  of  elasticity.  The  pre-stress force  can  be  obtained  by  multiplying  stress  with  cross  sectional  area of  pre-stress cables.

Fig: Vibrating Wire Strain Gauge

V.  W.  Strain  Gauge  operates  on  the  principle  that  a  tensioned  wire,  when plucked,  vibrates  at  a  frequency  that  is  proportional  to  the  tension  in  the wire.  The  gauge  is  constructed  in  such  a  way  that  a  wire  is  held  in  tension between  two  end  flanges.  Force  in  concrete  changes  the  distance  between its  two  flanges  and  results  in  a  change  in  the  tension  of  the  wire.  Strain  is then  calculated  by applying  calibration  factors  to  the  frequency measurement.

Vibrating  Wire  Strain  Gauges  are  of  two  types  i.e.

i)  weldable  type  and

ii) embedment type.

Weldable type strain gauges are fixed on the surface of the  structure.  They  do  not  function  in  embedded  state.  Therefore, embedment type strain gauges are more suited for our purpose.

VW embedded strain gauge:

VW  embedded  strain gauge  is  embedded  in concrete  nearer  to  pre-stressing  cable  to  assess change  in  strain  and subsequently  change  in pre-stressing  force.  The gauge  may  be  installed  either  before  or  immediately  after  placement  of  the concrete.  Their  unique  design  provides  superior  performance.  These  are available in 6 and 10-inch lengths.

For new  construction  the  strain  gauge is  embedded in  concrete  around  pre-stressing  cable  during  placement  of  concrete.  The  leads  of  the  gauge  are taken out and connected to read out or data logger unit.  The datum reading of the gauge is recorded immediately after installation of gauge. Subsequent readings are taken at each stage of pre-stressing during construction and at pre-decided interval during service.

For  existing  structure,  bore  is  drilled  in  concrete  at  desired  locations  and strain  gauge  is  installed.  After  Installation  of  strain  gauge,  the  bore  is  again filled  up.  The  leads  of  the  gauges  are  taken  out  and  connected  to  data logger  unit.  Datum  and  subsequent  reading  of  strain  are  taken  at  pre-decided intervals.

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