Civil Engineering April 2021 | Vol 29 No 3

44 April 2021 Civil Engineering Corrosion protection of hollow bar SDAs is a highly debatable topic (FHWA-CFC/TD-10-002) as double corrosion protection (DCP) is required on strand grouted anchors and in some passive applications, and cannot be achieved during the rotary installation method used to install hollow bar SDAs. The reader should bear in mind that the exposed area of wires that composes a strand cable is significantly larger than that of a comparable bar. Sacrificial loss of 1 mm on a R51N bar amounts to 15% of the cross-sectional area, but 65% loss of cross-sectional area on a high tensile cable. Therefore, one could theoretically be more lenient in allowances for sacrifi- cial thicknesses when using high strength reinforcement bars if other mechanisms aren’t at play. The service life of steel reinforcement can normally be increased by sacrificial thickness allowances (SCP), sufficient grout or concrete cover, reduced stress applied to bars to reduce crack widths, coatings applied to the steel (ASCP), cor- rugated sheeting (DCP), stainless steel, or a combination of these measures. These measures are discussed below with special consideration for the use of SDA/HBSN. Sacrificial thickness (SCP): Many suppliers state that sacrificial thickness allowances should be made on hollow bar SDAs, which implies any coating or grout cover has been rendered ineffective. This is typically also done on soil nails in non- highly aggressive environments according to EN14490 (See Table 6). The authors’ main concern with this approach is possible corrosion inside the coupler. Threaded ribs typically have a height of 1.5 mm to 1.7 mm. Corrosion over the bar’s thread and coupler’s thread of more than ± 0.75 mm could result in significant reduction in the coupler’s ability to transfer tensile load (corroded over full length). This could potentially also result in expansive corrosion pressure inside the coupler-bar connection, re- sulting in the splitting of the coupler. This is worsened by reduced grout cover over the couplers, increased stress concentra- tions, and possible tensile splitting of the grout body at this location (Figure 8a). Conventional couplers used in SDA applications are not close tolerance threads such as metric thread. The thread’s fit is extremely important to prevent corrosion inside the thread (com- munication from W. Van der Heiden, 2020). This could prevent the oxide that forms and expands from sealing the entrance to the mating thread connection. Considering corrosion only over the outer surface of the coupler might therefore not be justified. These bars have only been used in lateral support applications in South Africa since around 2005, according to Schwartz (2016), and are therefore not necessarily a proven system. One of the authors recently tried to apply grease impregnated tape and heat shrink sleeve over the coupler on a pro- ject. This is typically applied over couplers in bridge and building applications. These bars were exhumed immediately after installation to confirm if this measure was still intact. The protection system was however damaged due to the migration of the centraliser to the coupler posi- tions and hammering against the heat shrink sleeve. Further research is warranted to assess the effect of corrosion inside/over the coupler mating connection (SCP). Sufficient cover: EN14490 states that sufficient cover can be used as corrosion protection for soil nails, except in highly aggressive soil conditions (Class I) where DCP is required. Consideration should also be given to atmospheric corrosion at the facing. Achieving cover using SDAs is difficult especially in varying soil profiles (smaller drill bit required in harder mate- rial). A maximum cover of 50 mm can typically be achieved over the couplers (ignoring any widening as this is not guar- anteed over the full length). The central- isers are also not the exact diameter of the hole and therefore cover is typically less over the bottom of the SDA. Centralisers also interfere with grout circulation and cause snags during rotation (Dywidag, 2012). The grout body’s quality can also be contaminated by material cuttings, collapse, or groundwater due to the lack of control associated with installation. There are also no integrity and quality checks one can employ to guarantee that there are no defects in the grout cover or if continuity has been achieved. The authors believe the greatest risk to corrosion due to cover is posed at coupler locations and just below the thrust plate at the wall face. At the wall face grout might slump horizontally leaving the top of the bar exposed (see Figure 7) and often this zone is not properly filled after stressing with grease or grout. This zone is also the closest contact the bar would have to the atmosphere and free oxygen exchange. Reduced stress applied to bar and crack widths: EN1992 provides guid- ance on the maximum stress that can be applied to bars of different diameters to ensure that only cracks of a certain thickness result. Limiting stresses in bars to limit crack widths are however highly dependent on the grout cover and will also be considered problematic at abrupt cross-sectional changes at coupler positions. It is unclear if suppliers who undertook crack width tests on their bars included the couplers in these tests. Crack widths may be limited over lengths of the anchor, but it cannot be ensured over the couplers and just below the thrust plate. Coatings applied to steel (ASCP): Coatings typically consist of zinc diffused bars (Sheradised), galvanising bars, or epoxy coated bars. Epoxy and galvanised coatings are normally damaged due to abrasion during installation and are there- fore questionable. Zinc diffusion only has a finite life, especially in coastal areas, and corrosion of the base metal is inevitable. The use of zinc diffusion would however increase the design life depending on the atmospheric corrosion conditions but remains a sacrificial layer. Coatings aren’t considered as DCP but advanced single corrosion protection (ASCP). Stainless steel SDA: Stainless steel can be considered for use in highly Figure 9 Splitting force developing at coupler location T