Report. Corrosion protection of rock bolts with Pc-Coat. 100 years lifetime. Author(s) Pc-Bolt TM. NC-Bolt. SINTEF Industry. 2018: Restricted

Transcription

1 - Restricted Report Corrosion protection of rock bolts with Pc-Coat 100 years lifetime Author(s) Ole Øystein Knudsen Pc-Bolt TM NC-Bolt SINTEF Industry

2

3 Document history DATE DESCRIPTION First version Page 2 of 7

4 Table of contents Introduction... 4 Corrosion protection of the bolts... 4 Corrosion properties of the bolts in grout... 5 Corrosion properties of the bolts in atmosphere... 5 Experiences... 5 Life time expectancy... 6 Conclusions... 6 References... 7 Page 3 of 7

5 Introduction SINTEF has been asked by Pretec to evaluate lifetime expectancy for two types of rock bolts, Pc-Bolt and NC-Bolt, which is reported here. Tunnels are typically built with lifetimes of 100 years or more. The Norwegian Public Roads Administration (NPRA) specifies that the rock bolts shall have the same lifetime as the tunnel, i.e. minimum 100 years. The Pc-Bolt and the NC-Bolt are produced according to the NPRA technical specifications [1]. This report summarizes experiences with rock bolts of various types, and what we know about lifetime of the various materials and coatings in the relevant exposure environments. Based on these data, expected lifetime is evaluated. Corrosion protection of the bolts The bolts are produced from rebar steel with certified composition, hot dip galvanized according to ISO 1461, Zn-Mn phosphated and epoxy powder coating according to NS-EN This is a socalled duplex coating, where the epoxy powder coating and metallic zinc coating supplement each other in a way that the lifetime of the duplex coating is longer than the sum of lifetimes of the individual coats applied alone. This is expressed in the following formula, based on experiences with painted and hot dip galvanized steel [2]: L T = K (L Zn + L P) Where: L T is the duplex system s lifetime in years L Zn is the estimated lifetime of zinc coating in years in the current environment L P is the estimated lifetime of powder coating in years in the current environment, if applied directly to steel K is a synergy factor, which is estimated to be: 1.5 when exposed to environmental class C5 or permanently submerged in seawater when exposed to environmental class C3 C4, or when the time that it is wet is less than about 60% when exposed to environmental class C2. For example, in a C2 environment, if a 115 µm zinc coating has a corrosion rate of 0.7 µm/year and the powder coating a lifetime of 30 years, the duplex coating will have a lifetime of more than 400 years, according to the formula (synergy factor 2.1). In a C4 environment, the zinc will have a corrosion rate in the order of 3 µm/year, while the lifetime of the powder coating likely will be in the order of 20 years. Assuming a synergy factor of 1.7 we get a lifetime of 100 years. Even though the formula is based on experiences, it should only be regarded as a guideline. The formula and the experiences it is based on are for atmospheric exposure. For grouted rock bolts, the degradation mechanisms are somewhat different, and the validity of the formula can be questioned. Luckily, the conditions in the grout cement is predictive, very stable and not very aggressive. This is further discussed in the next section. Page 4 of 7

6 Corrosion properties of the bolts in grout The rock bolts are exposed in cement-based grout, i.e. an oxygen free alkaline medium with ph around 12. In this medium, the bolt steel is passive. Hence, even unprotected steel is expected to have a lifetime of 100 years or more. The zinc coating can have a significant corrosion rate in wet cement, which has been shown both in tests and field [3]. In cured concrete, however, the corrosion rate of the zinc is low and has been measured to be in the order of 0.1 µm/year [4]. The epoxy coating is highly resistant to alkaline environments and will protect the zinc, both in the wet and the cured cement. Hence, when the cement is cured, both the duplex coating system and the bolt steel are stable and must be expected to have extremely long lifetime. Degradation of the coating is not expected until the cement is degraded. In subsea tunnels, tunnels in acidic rocks, marine environments etc., aggressive species like chloride or sulphate may diffuse into the cement grout over time. This may activate the steel and start corrosion. For such environments the duplex coating will provide essential corrosion protection, extending the lifetime of the bolt. Chlorides in the cement will increase the corrosion rate of the zinc, but field experiences has shown that the corrosion rate of galvanized rebar in concrete exposed in seawater still will be less than 1 µm/year [5]. The epoxy coating will protect both the zinc and the steel, though. The epoxy coating is stable even in grout containing aggressive species. Mechanical damages in the epoxy coating may result in corrosion of the underlying zinc, and the steel when the zinc has corroded away. An undamaged epoxy coating will have very long lifetime though. The plastic grout pipe used with the NC-Bolt will protect the cement and prevent ingress of aggressive species. Hence, even in these environments we can expect the bolt to last beyond the lifetime of the cement. Corrosion properties of the bolts in atmosphere The tip of the bolt, the plate, nut and ball are exposed in the atmosphere outside the rock and the grout. The lifetimes estimated with the formula is for atmospheric exposures, so the calculations will give an indication of expected lifetime. However, for grouted bolts the atmospherically exposed parts of the bolt are not contributing to the function of the bolt, according to the NPRA [6]. The rock is secured by the grouted part of the bolt alone, and corrosion of the atmospherically exposed parts is not critical. Experiences Muskötunneln outside Stockholm in Sweden is a subsea tunnel that was opened to traffic in In 1995, after more than 30 years, nine rock bolts of unprotected rebar steel were retrieved and investigated with respect to corrosion. Neither of the bolts showed any sign of corrosion [7]. This confirms that grouted bolts do not corrode, independent of surface treatment, in unpolluted cement. Even bolts with incomplete grouting did not show any corrosion. Kvalsund tunnel and Øksfjord tunnel are both subsea tunnels in Norway that were opened to traffic in Several grouted rock bolts without any corrosion protection were retrieved from both freshwater and seawater zones in Neither of the bolts showed any corrosion [8]. Page 5 of 7

7 Bømlafjord tunnel is a subsea tunnel on the west coast of Norway that was opened to traffic in In 2015 rock bolts with duplex coatings were retrieved from both freshwater and seawater zones of the tunnel during upgrading of the tunnel. None of the retrieved bolts showed any sign of corrosion. Figure 1. Picture of retrieved rock bolts exposed in Øksfjord tunnel for 22 years. The bolts were without any corrosion protection, but fully exposed in grout. Picture: Norconsult [8]. Life time expectancy Corrosion properties of steel and zinc in cement has been thoroughly investigated both in lab and in field. The results show that the corrosion rates are very low for zinc, and that steel is passive (i.e. no corrosion). Epoxies have been shown to be stable in cement. Hence, neither of the materials in modern rock bolts are expected to degrade after grouting. Hence, the rock bolts will have longer lifetime than the grouting. Ingress of aggressive species from seawater or acidic rocks, like chloride and sulphate, may initiate corrosion of the steel. However, the duplex coating will protect the steel from these species. The plastic grout pipe used with the NC-Bolts will prevent the aggressive species from reaching the rock bolt. Hence, even in these environments, the NC-Bolt is expected to have longer lifetime than the grouting. The epoxy coating will also protect against the aggressive species, so it is highly likely that also Pc-Bolts will have the same lifetime, given that the epoxy coating is undamaged. Conclusions Based on the considerations above we can make the following conclusions: Grouted rock bolts are expected to have longer lifetime than the grouting cement. For subsea tunnels and tunnels in acidic rock, the cement may be polluted by aggressive species that can make the cement more aggressive. The plastic grout pipe used with the NC- Bolt will protect the cement and prevent ingress of aggressive species, ensuring the same lifetime as in less aggressive conditions. Corrosion of the atmospherically exposed parts of grouted bolts is not critical. The rock is secured by the grouted part of the bolt alone. Grouted steel bolts without any corrosion protection have been retrieved after more than 30 years exposure. The bolts showed no corrosion, verifying that the grout itself is protecting the bolt. A lifetime of more than 100 years must then be expected. Page 6 of 7

8 References 1. Statens Vegvesen, Road tunnels - Handbook R , Norwegian Public Roads Administration: Oslo. (In Norwegian) 2. Eijnsbergen, J.F.H.v., Duplex systems. Hot-dip galvanizing plus painting. 1994, Amsterdam: Elsevier Science Zhang, X.G., Corrosion and Electrochemistry of Zinc. 1996, New York: Springer. 4. Treadaway, K.W.J., B.L. Brown, and R.N. Cox, Durability of galvanized steel in concrete, in Corrosion of reinforced steel in concrete. 1980, American Society for Testing and Materials: Philadelphia. p Stark, D., Measurement techniques and evaluation of galvanized reinforcing steel in concrete structures in Bermuda, in Corrosion of reinforced steel in concrete. 1980, American Society for Testing and Materials: Philadelphia. p Knudsen, O.Ø., Corrosion protection in tunnels, Report no. 410, 2015, Statens vegvesen, Oslo. (In Norwegian) 7. Bergab, Beständighet hos ingjutna bergbultar, Report no. 1998, Vägverket, Avdelningen för Bro och Tunnel, Gothenburg. (In Swedish) 8. Ongstad, A., Etatsprogrammet Moderne vegtunneler Levetid på bergbolter, Report no. 164, 2012, Statens vegvesen, Oslo. (In Norwegian) About SINTEF SINTEF is among Europe's four largest independent research organisations. Every year we perform more than 9000 research projects for some 3000 clients We deliver innovation by developing knowledge and technologies that are applied and brought into practical use and contribute to value creation and increased competitiveness within the private and public sectors. As an independent third party we offer new perspectives on challenging projects. Our experience in research-based consultancy, project review and analytical services provides us with a unique insight into the challenges facing different industries and sectors. Page 7 of 7