Note: The following is based on the presentation I gave at the 2017 CRAG-VT Spring Kickoff event. The presentation, entitled “99 problems, but is this pitch one?: Why the climbing community is talking about replacing bolts” addressed the macro view of the history and science that got us to the point of needing to replace bolts en masse. I did my best to limit the technical information in that presentation and make it as relevant to the average climber who only clips bolts,
In the nearly 100 years of bolting in North America, climbers have watched various iterations of fixed anchors age and degrade, each at their own rates. As time has gone on we’ve learned more as a community about the metallurgy, chemistry, and physics that affect bolts over their service lives and we’ve been able to draw conclusions on what does and does not work. Bolt replacement is principally focused on identifying all the types of fixed protection that we’ve learned don’t work and replacing them with materials that conform to today’s better informed best practices.
Reasons to replace bolts
The most obvious reason to replace a bolt that comes to mind is an improperly installed bolt, but thankfully these are rare. So we can group the remaining reasons into 4 main issues.
- Wrong material
- Wrong size
- Wrong type
The principal issue plaguing many of the old bolts is corrosion. Corrosion in steel is the oxidation of the iron in the steel. More specifically this is oxygen molecules binding to the iron molecules and changing them into iron oxide. This change from iron to iron oxide causes a structural change in the steel, what we would call rust. This oxidation/rusting process happens in the presence of water (i.e. rain and humidity). Often rain water flowing down cliffs picks up/dissolves mineral content from soil and on the rock creating an electrolyte solution which accelerates the corrosion process via electrochemical means. Over time, as more iron changes to iron oxide, the macro structure of the steel begins to change and the overall strength of the bolt decreases until it no longer can bear the typically expected forces. This is when a bolt fails.
Corrosion is the pressing concern for the majority of our older bolts because in the past, bolts that were made out of plated steel were used heavily. Plated steel is so-called “plated” because a thin layer of zinc is applied to the steel for protection. Unfortunately zinc oxidizes rather quickly when exposed to the elements in exterior installations, particularly in wetter areas. The end result is that after a short period of time, the zinc has been fully oxidized and oxygen is now able to begin reacting with the under-lying steel (and more importantly the iron in the steel). It is not uncommon to see wide spread uniform corrosion occurring on plated steel bolts in a matter of years in Vermont. In order to solve the corrosion problem we need to look to different alloys of steel that provide increased resistance to oxidation of the iron.
An alloy of steel refers to the particular ratio of elements relative to one another in the steel. By changing the chemical composition of the steel the performance and physical characteristics of the steel can be tailored. This is most interesting for us when attempting to solve the corrosion problem and so our attention turns towards the so-called “stainless steels”. Stainless steels are a family of alloys that contain higher ratios of elements that help provide corrosion resistance. While many stainless steels provide enhanced corrosion resistance, the 300 series stainless steels are the most common encountered; and the primary element that helps provide our corrosion resistance in the 300 series steels is chromium. When chromium reacts with oxygen (i.e. oxidizes) it forms a very thin (2-3 nanometers) layer of chromium oxide on the surface of the steel that effectively shields the iron from further reaction with oxygen. Chromium oxide is self-repairing; i.e if the chromium oxide layer were damaged, the chromium will form a new layer of chromium oxide when exposed to oxygen.
This makes stainless steel a good choice for climbing anchors which are exposed to mild corrosive conditions. More aggressive environments which see susceptibility to stress corrosion cracking should be looking to more “exotic” alloys and materials such as titanium or various “2nd generation” alloys that bolt manufacturers are starting to produce for these environments. It should be noted that titanium likely offers the absolute best corrosion resistance for any environment where humans are climbing at a relatively modest price point given its extreme corrosion resistance and the fact that in these environments it’s the only sure solution.
Bolts resist the forces imposed upon them by means of some type of mechanical force, typically some type of mechanical expansion. In simple concept typically this is achieved by bringing two opposing components together that engage to cause expansion. If this expansion effect is somehow impeded or degraded, then the bolt likely will begin to fail at loads below the advertised values.
Non-functioning expansion is difficult to identify without tools. A torque wrench will pretty clearly tell one if a bolt is functioning properly as a non-functioning bolt will never achieve the required torque setting when being tightened. Any bolt identified in this manner needs to be replaced as typically they can be removed disturbingly easily, see the video below.
In the early decades of bolting, power tools did not exist and bolts were drilled by hand using hand-operated percussive drilling tools. Drilling by hand leads one to want to drill the smallest possible hole required, as a modern day bolt can take 10-20 mins to drill by hand. A mainstay of the hand drilling days were 1/4″ diameter bolts which when originally installed were fairly strong, but no where approaching the margins of safety we’ve come to expect from fixed anchors. For comparison, the below table lists relative strengths in shear and tensile strength for 1/4″, 3/8″, and 1/2″ diameter anchors of roughly the same length.
|Anchor θ||Tension (kN)||Shear (kN)|
Source: Powers PowerStud technical data sheet. Numbers for plated steel fastener in 6,000 psi concrete
Over the decades, many different types of bolts have been used by climbers with varying degrees of success. Before we consider some of the commonly installed types of bolts we’ve identified as being inadequate, we should take a moment to note that at this point any bolt purchased at a local hardware store is likely inadequate for climbing. Most consumer grade fasteners lack the manufacturing quality and quality control to be used in life critical safety applications like fall protection.
Some of the notable types of bolts installed on the East Coast are listed below with a breakdown of their short-falls that makes them ill-suited for climbing.
- Compression / buttonhead / split-drive: This bolt’s myriad names reflect various facts about the bolt. Referred to as split drives because the shaft is split down much of the length which allows the bolt body to compress when hammered into the hole. This compression fit is the mechanical action that allows the bolt to resist forces and stay in the hole. The buttonhead name comes from the rounded striking surface of the head. The principal problems for this bolt are that it typically was installed in a 1/4″ diameter and very short length (1 1/4″ – 1 3/4″). As well they are typically made of plated steel and thus are susceptible to corrosion. However, the real deal breaker is that they are susceptible to hairline fractures when installed that can be exploited by corrosion and caused to propagate. This leads to a ticking time-bomb anchor waiting for the convergence of factors to lead to a critical failure.
- Self-drive: Named for the fact that the body of the bolt is the drilling mechanism, eliminating the need for a drill bit when used in conjunction with a special adapter. These require a precise hole depth be drilled, a tapered cone inserted into the hole and the expansion body hammered in place to mate with the cone. These bolts are still widely seen in Europe (particularly Southern France and Eastern Spain). Once briefly popular in the U.S., they soon fell out of favor when climbers realized the cutting teeth usually dull before the required depth is met, resulting in a significantly weaker placement or the need to change out bolt bodies while on lead. The short length, lack of corrosion resistance, relatively weak strength, and finicky installation requirements make these ill-suited for use in climbing.