"Influence of Vibration During Setting of Post-Tensioning Tendon Grout"
Research on “Influence of Vibration During Setting of Post-Tensioning Tendon Grout” to be Conducted at Penn State. At the November 17 Board meeting, the Board approved a proposal by Dr. Andrea J. Schokker, The Pennsylvania State University for research on vibration effects on grouted post-tensioning tendons. The research will be co-sponsored by the Florida Department of Transportation and ASBI. Excerpts from the research proposal are presented below.
The influence of vibration during set on grout properties remains an unanswered question in segmental bridge construction. Shocks and vibration from construction equipment has been questioned with respect to potential effects on bleed and bond.
Proposed Test Program:
The test program for bleed, visual inspection, and bond is described below. A plain water-cement ratio grout will be used for the majority of bleed testing so that bleed effects will be more evident. A prepackaged grout will be used for the bond testing to reduce variability in these specimens.
Bleed Testing and Visual Inspection:
To determine the effect on bleed, clear vertical duct specimens will be subjected to varying degrees of vibration. A small-scale shake table with a maximum frequency of 20 Hz will be used to vibrate the specimens. The vibrations expected in the field may vary considerably. It is anticipated that a frequency of 15 Hz will be used unless a background search suggests a different value. The clear tube specimens will include a 3 strand bundle. The specimens will remain in a constant environment and will remain capped. Vertical specimens will be the primary specimens for bleed evaluation since bleed values can most accurately be read in this type of specimen. Bleed values will be recorded hourly on all specimens. Horizontal specimens will also be included as a more critical case for bond. All specimens will be autopsied to observe the influence of vibration. The primary variables are duration of vibration and timing of vibration. Testing is expected to continue through 14 hours to look at effects just after set. The anticipated set time will be measured prior to full testing by the Vicat needle, and the timeframe will be modified if necessary. It is anticipated that a longer time of vibration will be more critical on bleed. However, horizontal specimens are also included to observe the effects of vibration isolated to critical times (such as near set or immediately after set). These times will be adjusted as necessary based on the actual set time of the grout. A small number of prepackaged grout specimens are included. The prepackaged grout should not bleed at this height level, but the grouts are included in case the vibration has a significant effect. If testing shows that the bleed is accentuated in the plain grout, it is likely to have a similar effect for tall vertical tendons with prepackaged grouts. If this is the case, future testing should include tall vertical specimens and pressure filter testing.
Bond will be evaluated with the PTI test method (adapted as ASTM A981) for pull-out. This test setup is already in place at Penn State for a Texas DOT funded project on the effect of emulsifiable oils for strand protection. The standard specimen consists of a single strand in a steel tube filled with grout. Initial evaluations have shown that much more consistent specimens are achieved with the use of prepackaged grouts. The bond specimens will determine the effect vibration has on the grout matrix and grout strength.
The outcome of the proposed testing program will indicate if future work is necessary in this area. Potential future work might include large-scale vertical duct testing for prepackaged grouts and large-scale testing of post-tensioned specimens to evaluate the effect of vibration on bond in an actual structural member.
It is estimated that the work and final report can be completed within two months from the start of the project. The primary time factors are the cure time for the bond specimens (28 days) and autopsy documentation.