Page - 707 - in Book of Full Papers - Symposium Hydro Engineering

2.2.4 Corrosion and Corrosion Protection References [1] and [2] provide an analysis of the PTI Recommendations to that point (i.e., including the 2004 version) and a summary of over 400 individual case histories. Note that the provisions of the PTI 2014 edition are sensibly the same as in 2004, while the database has recently been further updated [3]. It is extremely important in the context of legacy anchors to gain perspective of contemporary attitudes, and the development of these attitudes with time. In summary, the anchor grout itself was regarded, during the early decades of practice, as an acceptable barrier to corrosion, for both the bond and free lengths. It was only with the 1996 version of the PTI Recommendations that full- length protection by at least one plastic sheath was required for permanent anchors, i.e., Class I protection, so matching contemporary standards of care in Europe (Figure 1). 3. CHALLENGES TO EVALUATING LEGACY ANCHORS There are many challenges relating to the design and the construction of such anchors, and to practical/logistical aspects of condition assessment. These challenges are common to dam Owners and Regulators throughout the dam anchor world. Firstly, it is clear that the majority of dam Owners and their consultants simply did not contemplate at the time of design and construction that long-term monitoring would be advantageous, let alone fundamental to risk management. The general philosophy was that anchors designed and constructed to the standards of the day would not corrode since the steel would be encased in grout, and placed into boreholes whose immediate proximity was pregrouted (to prevent water ingress and to prevent anchor grout loss). The anchor head itself was generally encased in concrete. Further, in acknowledgement that the tendon would naturally suffer long-term stress loss (principally due to relaxation), the post-tensioning specialists selected Lock-Off/Transfer Loads, which included an allowance for these losses, the magnitude and rate of which were well known from structural post-tensioning projects. Such elevated tendon stress levels, while (usually) still within the elastic response range (i.e., ≤ 80% fpu), would however increase the tendency for stress-induced corrosion to be initiated or accelerated under certain conditions. No provisions were made for installing restressable anchor heads. Secondly, it was exactly these old methods which have now rendered lift-off testing unfeasible: without a developed free length, it is impossible to directly measure the residual load in the tendon, or to determine its distribution within the free length or, of course, the bond length. Thirdly, it is often the case that the anchor heads are, for all practical purposes, inaccessible for any type of inspection or testing. In some cases, the functional anchor heads could be rendered accessible, but there would be a natural reticence to “probe too deeply” for fear of disturbing the anchor head – not so much from a load holding viewpoint, as the tendon is fully bonded below, but from the corrosion protection viewpoint. This is particularly the case with legacy anchors which do feature a greased and sheathed coating as a bond 707