Seite - 705 - in Book of Full Papers - Symposium Hydro Engineering

length are based on uniform bond distribution. Although inaccurate, this is a conservative assumption. 2.2.2 Construction Practices While most drilling was done in early projects by coring (for fear of somehow damaging the dam, and to extract samples of bedrock in the bond zone), current drilling practice features rotary percussive methods, and especially Down-the-Hole Hammers (DTHH). Grout mix designs, grout blending (in a high speed, colloidal mixer) and placement (always via tremie) have changed only insofar as equipment has improved. Water pressure testing (and pregrouting if necessary) has always been of a high and conscientious standard, and this is of great importance when trying to evaluate the reliability of legacy anchors. Equally important are the changes to the grouting/stressing sequence: (i) Prior to the use of sheathing on the free length of the tendon components, the sequence was:  drill, water test (and pregrout and redrill if necessary),  place the tendon,  grout the bond zone,  stress, test and lock-off,  grout the free length. (ii) After the use of sheathing on the free length of the tendon components, the sequence was:  drill, water test (and pregrout and redrill if necessary),  place the tendon,  grout the tendon completely,  stress, test and lock-off,  complete underhead grouting. Most of the early anchors had tendons comprising a multitude (“bundle”) of 6 mm diameter wires (Figure 1). These wires were typically fully bonded to the concrete of the dam by the free length (secondary) grouting after prestressing and lock-off. Thus, such anchors have only grout for corrosion protection and cannot be lift-off tested to establish residual load in the tendon. 2.2.3 Stressing and Testing Early stressing and testing procedures were surprisingly simple (given the sophistication of the original European post-tensioning specialists), and remained so in the U.S. until the 1996 Recommendations. Thus, it is typical that legacy anchors were quickly stressed to the Test Load in progressive steps, held there for a short period (but without a quantified creep criterion) and then destressed to the lock-off (or “transfer”) load. The benefits of progressive cycle loading, with well-defined criteria for creep and elastic analyses, cannot be exploited when evaluating contemporary records of legacy anchors, while Lift-Off Testing was optional and, in any case, of quite short-term duration (say 48 hours). 705