Grouting Training Q&A

Grouting Questions Received from December 1-11 Webinar   

1.  When do you recommend doing the air testing of the ducts? 

The air test is recommended after stressing, cutting the tails, and installing the grout caps are completed. In other words, just prior to grouting. The purpose of air testing is to check for leaks, cross-overs between adjacent ducts, and other issues to allow repairs to be performed and/or grouting plans to be revised prior to grouting. This is the test referred to in the PTI Specification for Grouting of Post-tensioned Structures M55.1-19 Section 5.5.4.  

2.  Do the grout discharge points always have to be above the duct level?  

Yes, the discharge points should always be above the duct level. By keeping the vents and discharge points above the duct level, it helps minimize potential for trapped air pockets. Guidance on the vent locations is provided in ASBI/PTI Specification for Multistrand and Grouted Post-tensioning M50.3-19 Section 9.9.       

3.  About time limit for grouting. When does this time begin? When you finish placing the tendons inside the element? or is it taken after tensioning? 

The time limits, referenced in ASBI/PTI Specification for Multistrand and Grouted Post- tensioning M50.3-19 Table 11.1, are after installation (i.e., placement of strand into the duct).          

4.  To avoid excessive grouting pressure: What diameter of grout hoses is suitable? 

Although the diameter may vary based on the size of the pump and other variables, 1” diameter grout hoses are typical for most operations. The use of a grout header and recirculation line may be helpful in controlling excessive grout pressures, especially in small tendons such as flat duct tendons.

5.  When you try to fix a blockage duct and drill to install a new vent to re-establish one-way grouting, how do you make sure you don't damage the tendon? 

The approach for repairs will depend on the situation, and often can be quite different. A specific tailored plan for each location and situation is recommended, however some various considerations with respect to new ports are listed below:

  • The key is to have an idea of where the tendon should be and work carefully. It is recommended the work be performed by someone experienced in similar types of repairs. It is good practice to first layout the anticipated duct location by measurements from plan locations, use of Ground Penetrating Radar (GPR) and/or other methods. The approximate expected depth should also be determined before drilling.   
  • Closely monitor the depth of the drill holes until the duct is reached then carefully measure the distance to the duct and set the drill depth to just penetrate the duct to avoid contacting the strand. Marking the drill bit or wrapping a piece of tape around the drill bit at the estimated depth to help in monitoring the depth of the hole may also be used. Drilling at the high point is often the best place to start since the strands will be in the bottom of the duct at high points and less chance of strand damage. It may be possible to drill two holes near the high point, one serving as the inlet, one as the outlet.   
  • It may be prudent to consider different approaches for drilling into concrete and the duct. Typically, a larger, stronger drill and bit are used to drill the concrete but those might be too robust around the tendon steel. Suggest stopping about 1/2” away from the anticipated duct location then using a smaller drill to find the actual duct.  Slow is the key and using a drill with a variable clutch that will stop if the drilling gets too hard may be helpful. As mentioned above, the strand location within the duct should be considered and avoid drilling where the strands may be located. In some cases, it might be necessary, or worth considering, to excavate a larger area of concrete to expose the duct without drilling right down on top of the duct.   
  • One approach assuming the original outlet is still functional is as follows. Connect air at about 30 psi air pressure on the void area. Then probe with a ¼” roto-hammer bit and air will escape as soon as the duct is pierced. Often this must be done multiple times to determine the extent of the void and to pierce the duct in a place that has no strand against the wall of the duct. Then use a larger bit with the depth to the duct marked on it. Stopping just short of this, chip the remaining concrete out and open the wall of the duct by hand. Then insert a new grout port and secure with epoxy. 
  • There is generally a balance that must be maintained in providing adequate access for repair and inspection, but also avoiding doing too much damage by drilling excess holes or chipping more concrete than necessary.
  • Vacuum grouting is a procedure that can be employed as well, but still requires at least one useable grout port.

6.  Could you please send me more information, details about grouting vertical tendons?

A resource on vertical tendons (and excellent resource for many other items as well) is the FHWA Post-tensioning Tendon Installation and Grouting Manual, May 2013, FHWA-NHI-13- 026. Sections 4.3.7 and 4.5.10 and 4.5-11 address vertical tendons. It can be downloaded here: https://www.fhwa.dot.gov/bridge/construction/pubs/hif13026.pdf.

One of the first considerations in a plan for vertical tendons is to ensure the particular grout to be used is rated for the vertical head/pressure that will be encountered. The pressure should be considered for the equipment as well. In addition, some strategies for grouting vertical tendons may include one or more of the following, especially for tall tendons: breaking the tendon grouting up into stages/lifts, grouting the top portion of the tendon as a separate operation, adding a reservoir/standpipe at the top of the tendon.

7.  Would you recommend me a document in which I can read more about thixotropic grout, its general composition, characteristics, etc.? 

A very good document which includes information on thixotropic grout is Development of High-Performance Grouts for Bonded Post-tensioned Structures (1999). The document can be downloaded here: https://fsel.engr.utexas.edu/pdfs/1405_2.pdf.

In addition, the following article includes some information on promising developments in the use of an in-line density meter that can be used on jobsites to monitor and record the quality parameters of the grout in real time. The article can be downloaded here: http://aspirebridge.com/magazine/2019Fall/CBT-AToolForContinousOnSiteMonitoring.