The Multidisciplinarian

This hastily assembled piece expresses concerns over the use of concrete screws in caving. It stems from a discussion with Max Elfelt last night. Two aspects of screw use underground interact adversely: 1) screws rely on rock tensile strength while wedge bolts rely on compressive strength, and 2) the uncharacteristically large difference between tensile and compressive strength in cave-forming limestone, particularly oolitic types. Combined, they raise significant concerns. Comments are welcome, either below or privately. Find me in the NSS directory, on X, or LinkedIn.

Cave exploration demands absolute trust in permanent rigging for pits, climbs, and traverses where falls could be fatal. Anchors, the critical connection points securing ropes to cave walls, must withstand the forces of climbers, falls, and environmental wear. While concrete screws have gained attention in some climbing communities for their ease of installation, their use as permanent or semi-permanent rigging in caves and in any cases where axial loads (pullout forces along the anchor’s axis) are possible may pose unacceptable risks.

Unlike wedge bolts, which rely on the rock’s compressive strength to create a secure grip, concrete screws depend heavily on the rock’s tensile and shear strength, properties that appear, on the basis of reported mechanical properties, to be inadequate for safe use of screws in cave-forming limestones, particularly oolitic and anisotropic varieties (those having different properties in different directions, typically caused by depositional discontinuities – bedding planes – or cemented planar fractures). A quick look at the mechanical differences between these materials reveals grounds for concern – possibly grave concern.

The Mechanics of Anchors: Wedge Bolts vs. Concrete Screws

Wedge bolts work by inserting a bolt into a drilled hole, then tightening it (torquing the nut) to push a cone-shaped wedge against the sides of the drilled hole. This action generates a high clamping force called preload. Preload allows the bolt to resist axial loads without any further movement of the bolt in the hole when even large loads are applied to the hanger. The preload is verifiable during installation: if torque is felt by hand or measured with a wrench (27 Nm or 20 foot pounds for 10 mm or 3/8 inch anchors), the rock is compressing adequately, the needed friction has been generated, and the desired preload exists. The physics and mechanics of wedge bolts ensures that applied axial or perpendicular loads, if they are less than the amount of axial preload (several thousand pounds, or greater than 10KN), do not alter the stress states of the installed bolt or the adjacent rock at all. The mechanics of wedge bolts and how they are misunderstood by climbers appears in a paper by Amy Skowronski and me in the Oct 2024 Journal of Search and Rescue.

Concrete screws, in contrast, operate somewhat like screws in wood. However, this analogy has serious limitations, discussed below. Concrete screws are threaded directly into a pre-drilled hole, cutting or crushing threads into the rock. Unlike wedge bolts, preload is not a significant factor of the gripping mechanics. Their resistance to axial loads depends on the mechanical interlock between threads and rock. When a caver applies force – either axial or perpendicular loading – the threads bear against the rock, inducing shear stress as threads resist sliding. Applied axial loads, if present, induce tensile stress as the rock resists radial pulling apart. This makes the rock’s tensile and shear strengths critical, as failure at the thread-rock interface will result in pullout. Cyclic loading – each time a load is added and removed during descending, ascending, or falling – causes wear, micro-fractures, and grain dislodgement. This increases the induced stresses by reducing the size of the surfaces under shear and tensile stress. Use of concrete screws in climbing is discussed in The Bolting Bible, about which I express several concerns below.

Material Mechanical Properties: Concrete vs. Limestone

The suitability of bolts and screw anchors hinges critically on the rock’s mechanical properties. The limestone that forms most caves has highly variable strength, unlike concrete, which is engineered for consistency. More critically, the relationship between compressive strength and tensile strength is predictable in concrete, but less so in limestone. Below are comparisons of compressive and tensile strengths, focusing on cave-relevant limestones, particularly oolitic and anisotropic types.

Property	Structural Concrete	Oolitic Limestone	Anisotropic Limestone

Compressive Strength

20–200 MPa

20–60 MPa (varies with porosity and cementation)

~20–180 MPa (depending on bedding and orientation)

Tensile Strength (Direct)

2–5 MPa

~0.5–4 MPa

Highly variable: ~0.5–6 MPa depending on bedding details

Modulus of Rupture

3–7 MPa

~0.5–5 MPa

~0.5–12 MPa

Anisotropy

Highly isotropic

Weakly anisotropic

Strongly anisotropic (bedding planes matter greatly)

Failure Mode in Tension

Brittle fracture

Grain-boundary separation

Splitting along bedding or delamination

Elastic Modulus (Young’s)

25–50 GPa (typical NSC)

~5–30 GPa

~10–50 GPa, highly dependent on direction

* Granite, for comparison, often has a compressive strength of 100–200 MPa and a tensile strength of ~7–20 MPa. Tensile strength measurements are from splitting (ASTM C496) and flexure (ASTM C78) tests. Low-tensile oolite strength values from Ippolito (1975), Szechy (1966), and Paronuzzi (2009).

Why Concrete Screws in Caves Might Be Riskier than We Think

The disparity between compressive and tensile strengths in limestone, particularly oolitic and anisotropic types, is large. Wedge bolts rely on limestone’s compressive strength (20 MPa or greater), which is sufficient to develop preload in typical cave limestones. The preload reduces reliance on the rock’s tensile/shear strength and eliminates cyclic wear concerns for expected loads.

Concrete screws depend on limestone’s tensile and shear strength, not its compressive strength) to resist axial loads. Stresses induced by cavers’ weight or falls (e.g., 5–15 kN, translating to 1–3 MPa at the thread interface, depending on thread geometry – see sample calculations at the bottom of this post) are dangerously close to (and possibly exceed in cases) the tensile strengths of the limestone in which they are placed. In some oolitic limestone, the weak matrix fails under thread stresses, causing grain dislodgement or micro-fractures, especially under cyclic loading, though specific performance data in cyclic loading in material like oolitic limestone is lacking. Unlike wedge bolts, where preload is confirmed at installation, concrete screws offer no such assurance, and their performance necessarily degrades over time because each each load application results in fretting fatigue. Common caving field-tests for limestone compressive strength (e.g., the ring of a hammer blow) are unlikely to reliably predict tensile strength, particularly when the tensile strength of the matrix is low compared to that of the grains (clasts), as is the case with many oolitic limestones.

This disparity arises because both hard and soft rocks can undergo brittle fracture and are prone to tensile failure under localized stresses, whereas they resist compression better, due to grain-to-grain contact even after the matrix yields. In oolitic limestone, the rock consists of hard, spherical grains (ooids) cemented by a softer matrix (e.g., calcite). The matrix’s tensile and shear strengths are significantly lower than the bulk compressive strength, which is dominated by the hard grains. When a concrete screw is installed, its threads cut or crush into the matrix, and under axial load, the matrix must resist shearing or tensile failure. If the matrix is weak (e.g., porous or amorphous cement), the threads can dislodge grains by fracturing the matrix, reducing pullout strength over time, especially under cyclic loading. Manufacturers like Simpson Strong-Tie (Titen HD screws) report pullout strengths of 42–52 kN in hard limestone and concrete, but these assume isotropic substrates, not cave limestones with weak matrices or bedding planes.

Another consideration gives rise to even greater concern regarding placement in oolitic limestones, particularly those with large ooids. ASTM C496 tensile strength tests seem likely to overestimate oolitic limestone’s tensile strength for predicting concrete screw pullout. Overestimation of tensile strength for the screw scenario is likely because of the large size of the test specimen (152 mm, 6 inches), the test’s bulk measurement (e.g., 2–5 MPa) versus the matrix’s lower tensile/shear strength (e.g., 1–3 MPa) and the size of ooids compared to that of the threads (1–2 mm threads and .25–1 mm ooids). The test’s homogenized results probably don’t capture localized matrix failure adjacent to threads.

Cause for Concern in the Common Caving Use Case

Our discussions with cavers who use concrete screws suggest that they are used only for aid climbs (temporary placement) and only where the nominal applied load is not axial, i.e., they use horizontal screws with vertical applied loads. This, in theory, would eliminate concerns about the low tensile strength of certain limestones. One part of the allure of concrete screws is that no hammer or wrench is needed for installation. If hammer, wrench, and wedge anchors aren’t available at the top of a climb, screws then take on a role slightly more critical than their use on the climb itself. Many scenarios at the top of climbs entail a descent setup where some degree of axial loading is inevitable. All descents entail some degree of cyclic loading. Similarly, a fall when the climber is above a single bolt (first bolt placed only) in a climb will always result in significant axial loading.

Five factors combine to raise considerable concern for these and similar scenarios.

• The unusual relationship between compressive and tensile strength of common limestones.
• The absence of evidence for how many load cycles are needed to cause significant tensile yielding of the internal threads carved by screws
• The small margin between tensile strength (ultimate tensile stress) of certain limestones and normal working loads in use
• The lack of confirmation of a successful screw installation such as the resistance to torquing with wedge bolts
• Inability to assess tensile strength of the limestone by a means such as listening to the sound of a hammer blow

Intuitions about strength properties of common materials may further exacerbate the situation. Metals’ tensile strength is typically 80 or 90 percent of its compressive strength. In limestone, this ratio is 10 to 15%. In oolitic and anisotropic limestone, it can be below 5 percent.

Finally, the prominence of analyses of climbing gear – gear used on the surface – consisting mainly of dramatic measurement of ultimate strength may work against us. Pull-tests, while entertaining, are not particularly useful in evaluating the utility of any mechanical component. Cavers may face increased risk by extending conclusions drawn from non-representative test scenarios common on YouTube, for example.

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This concludes the thrust of my concern about concrete screws in caves. I welcome comments and corrections. Below are sections on calculating expected pullout loads of screws, discussion of fatigue and fretting fatigue, and specific concerns on the discussion of concrete screws in the Bolting Bible. These are engineering/physics oriented, and the above concerns hold independent of the topics below. Stop here if you’re not drawn math, physics, or nit picking.

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The Bolting Bible

The Bolting Bible offers much useful instruction and information on bolts and screws used in climbing. The article/chapter “The Book of Concrete Screws” by HowNOT2 aims to educate climbers about concrete screws as temporary or sometimes permanent anchors in rock climbing. While it provides practical insights and test data, it has, in my opinion, several shortcomings in terms of physics and engineering rigor. That rigor seems to me something that should be present in coverage of a relatively new technique, especially when any discussion of permanent rigging is present. The Bolting Bible explicitly disclaims scientific rigor but its comprehensiveness is understood by many to imply authority.

The article describes concrete screws as functioning “similarly to normal wood screws” where “threads bite into the sides of the hole.” The analogy may aid visual intuition, but it risks conflating fundamentally different material behaviors – elastic-plastic deformation in wood versus brittle fracture and micro-spalling in rock. It fails to explain how the threads interact with the rock substrate, particularly the stress concentrations at the thread-rock interface.

The article mentions that concrete screws are “not safe in soft rock” and can “strip the threads” in hard rock, but it provides no quantitative or mechanical reasoning. Hard and soft don’t adequately cover the variations in mechanical properties of rock to orient readers toward meaningful distinctions in substrate performance.

The article briefly notes that cyclic loading in soft rock can cause concrete screws to “lose grip” due to threads dislodging material. “Losing grip” is a casual phrase that erases the actual failure modes involved: local pulverization of rock around threads, progressive thread shearing, and fretting fatigue under micro-motion (see discussion of fatigue at bottom). The language reduces complex, cumulative damage to a vague sensation.

The article cites break tests (e.g., Simpson Strong-Tie Titen HD screws achieving 42–52 kN [9400–11700 pounds] before head shear. The head-shear test is meaningless to climbing or caving use cases and might create a false sense of safety margin. While empirical data is valuable, the absence of a theoretical framework (e.g., thread shear strength, rock compressive strength) limits the generalizability of the findings and utility of the conclusions.

The suggestion to “spit or squirt water,” though possibly derived from field experience, should be followed by some observed benefit with even a speculative mechanism: Does moisture reduce dust, increase thread cutting efficiency, or initiate micro-hydration in porous substrates? Without this, it reads as lore rather than low-tech science.

As is the case with most gear analyses that measure ultimate strength, the article’s focus on ultimate strength (e.g., head shear at high loads) misses the more relevant issue for climbers: long-term reliability under repeated sub-failure loads, particularly given the key difference in mechanism between wedge bolts, with which most cavers are familiar, and concrete screws, which have come onto the scene fairly recently.

The article emphasizes ease of installation and removal for temporary anchors but does not adequately address the ethical implications of leaving potentially compromised holes in rock, especially in public climbing areas. Drilling holes is a “permanent deformation to the rock,” as noted elsewhere in The Bolting Bible, yet the article does not discuss how concrete screw holes might affect future bolting. “Bolt-farm” problems of the past might shift to hole-farm problems.

An assoicated HowNot2 video mentions permanent rigging using screws, stating that the screws should be stainless steel. Stainless eliminates the problem of screw deterioration from corrosion. This is a distraction from the more relevant concern of hole degradation. The video mentions that repeated use has led to “spinners” without noting that if a screw spins, the threads it carved into the rock are completely compromised. If a screw spins, unlike the case with a wedge bolt, it has no resistance to pullout.

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Expected Pullout Load in Low-Tensile Oolitic Limestone

To calculate the effective contact area of a 3/8-inch diameter, 3-inch long concrete screw in tension, we must estimate the area over which the threads engage the concrete and can transfer tensile load. This is not the same as the root cross-sectional area of the screw (which governs tensile failure of the screw itself); you’re asking about the interface area between the screw threads and the concrete, which governs pullout or bond failure.

Nominal Parameters

• Screw outer diameter: 3/8 inch = 0.375 in
• Length embedded in concrete: 3 inches
  (Assuming full 3-inch embedment and no voids)
• Thread type: typical concrete screw (e.g., Tapcon) with a coarse thread (approx. 7–8 threads per inch)

Basic Bond Area (Cylindrical Surface)

This is a simplification that treats the bond area as a cylinder with the mean thread diameter:
Total contact area = π ⋅ mean diameter ⋅ embedded length where:
mean diameter = (outer diameter – root diameter) / 2

For a 3/8″ 3-inch Tapcon-style screw:
Outer diameter ≈ 0.375 in.
Root diameter ≈ 0.275 in.
Mean diameter ≈ (0.375 + 0.275)/2 = 0.325 in
Therefore, total contact area ≈ π ⋅ 0.325 ⋅ 3 ≈ 3.06 sqin.

Thread Engagement Area

Threads only contact the concrete at the flank surfaces, not over the full cylindrical shell. Accounting for thread flank angle and spacing typically reduces effective contact area to 40–60% of the full cylindrical surface, depending on thread profile.

Assuming 50% effective contact: Contact area = 1.5 sqin.

Tensile strength of weak but not weakest oolitic limestone = 1MPa = 145 psi. Note that reports on limestone tensile tests indicate that strengths lower than 1MPa could be encountered in oolitic and anisotropic limestones. West Virginia’s Pickaway Limestone comes to mind.

Expected pullout load in limestone having 1MPa (145 psi) tensile strength = 150 psi ⋅ 1.5 sqin = 225 pounds (or 112 pounds for 0.5 MPa tensile limestone)

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Fatigue

The term fatigue, as used in engineering, has specific technical meanings that might differ from how it is used casually in discussing gear. The most general use describes gradual reduction in strength after many cycles (typically 10,000 to 1 million cycles) of stress application where the applied stress involves superficially elastic deformation (as opposed to plastic deformation in which measurable yielding of material occurs).

Low cycle fatigue in metals describes strength reduction from cyclic loading up to about 10,000 cycles where measurable plastic (doesn’t bounce back) deformation occurs.

Both high and low-cycle fatigue in metals happen when, at the microscopic level, even when the macroscopic stress is elastic, dislocations move within favorably oriented grains of the crystalline metal. 304 stainless steel typically has an ASTM grain size number of 5 to 10, corresponding to grain diameters of about 15 to 65 microns. Repeated dislocation during cyclic loading can form persistent slip bands, which are localized zones of plastic deformation that can eventually break through the surface. These act as initiation sites for fatigue cracks.

Localized plastic deformation around slip bands creates intrusions and extrusions on the surface. Over thousands to millions of cycles, these become tiny microcracks, usually less than a grain diameter long. Initially, crack growth follows crystallographic planes (especially {111} slip planes in body-centered cubic or face-centered cubic structures – for those who are into crystals). This is called Stage I (shear-mode) propagation.

Once a crack crosses a few grains, it becomes long enough to be governed by stress intensity rather than just local slip. The crack then grows perpendicular to the maximum principal stress (Stage II, or tensile-mode crack growth). When the crack becomes long enough that the remaining material cross-section can’t support the applied load, sudden ductile or brittle fracture occurs, depending on material properties (particularly its toughness) and loading rate.

From this description, you can see that fatigue is unlikely to be a failure mode in either wedge bolts or concrete screws as used in caves or in rock climbing (in contrast to what is stated in climbing literature here and here); the applied loads are too small and the number of cycles is too low.

Above I used the term fretting fatigue to describe rock wear along the threads carved into the rock by a concrete screw. I want to be careful in using a term possibly in a non-standard way. This is a very different use of the general term fatigue in mechanics. Here I refer to a form of fatigue failure that occurs when a component (rock in this case, not the screw) experiences both cyclic loading and simultaneous small-amplitude oscillatory motion (microslip) at the contact surface with another component (the screw). This microscale rubbing involves very high contact forces and greatly reduces component life – to a degree that the term fatigue does not apply like it does in the cyclic loading of metal. Fretting fatigue usually describes a condition found at bolted joints, riveted connections, spline shafts, blade roots, or where two components are clamped but subjected to vibration or relative motion.