Standard mountings for VW strain gauges are:
Surface mount – arc weldable blocks, groutable or bondable anchors. Embedment – only available with end flanges
Spot weldable – tabs which can be spot welded or bonded onto surfaces.
Re-bar strain meter/Sister bar – varying length of rebar which can be welded or tied into the reinforcing cage.
Mountings are available in different materials to suit the application.
What should I use to measure concrete strain?
There are two main products for measuring concrete strain, the embedment gauge or the sister bar. The embedment gauge comes in three sizes 50mm, 150mm and 250mm, the standard gauge is 150mm and accounts for 90% of the embedment gauges we sell it can be used in pile cages, diaphragm walls. The 250mm is suitable for direct installation into mass concrete where the 150mm gauge would never survive installation. The 50mm gauges are two small for most applications and have some installation and accuracy issues due to this but can be used where the other gauges cannot fit.
The sister bars are individually calibrated and are much more robust than embedment gauges but are of course more expensive. The only disadvantage is size but special versions are made for the bottom of piles with one anchor side being replaced by a flange.
Sister Bar or Rebar Strain gauge?
There seems to be some confusion through the industry on the correct terminology for a sister bar or a rebar strain meter, so this should only be taken as the authors understanding of the subject. Rebar strain meters come in many different sizes to match the different size reinforcement bars used in piles, diaphragm walls etc. The rebar strain meters are intended to be welded into the reinforcement cage to take the place of a piece of reinforcement of equal size. The rebar strain meter will now measure the strain in the reinforcement cage by becoming a load carrying part of the structure.
A sister bar is typically either a 12mm or 16mm version of the same thing but installed in the cage such that it is not fixed in such a way as it will measure the strain in the reinforcement bar but in such a way that it will measure the strain in the concrete. Sister bars are usually fixed in with some tie wire or cable ties in effect floating in the cage structure until such time as it is surrounded by concrete. Once embedded in the concrete the gauges strain matches (Sisters) that of the concrete. The reason that either 12mm or 16mm are used is that they do not have a sufficient cross section in the measuring zone to impact the accuracy of the strain readings measured.