Concrete
Concrete anchor and fastener installation field guide
What actually holds an anchor in concrete: cast-in vs post-installed, the mechanical and adhesive types, the hole cleaning that makes or breaks epoxy, edge distance, and the cracked-concrete rating.
Direct answer
Concrete anchoring attaches equipment, steel, and pipe to concrete using cast-in or post-installed anchors. The holding strength comes from the concrete, the embedment, the edge distance, and the install, not the bolt alone. A poorly installed or wrongly placed anchor pulls out or breaks the concrete cone. ACI 318 Chapter 17, the manufacturer instructions, and the engineer of record control.
Key takeaways
- Concrete anchor strength comes from the concrete, embedment depth, edge distance, and install quality, not the bolt itself.
- A dirty hole is the number one cause of adhesive anchor failure; clean every hole by the manufacturer's blow-brush-blow sequence.
- Cracked-concrete-rated anchors are code-required for seismic and tension-zone connections; a crack can cut anchor capacity by a third to a half.
- Design anchors to fail in the steel (ductile, with warning), not concrete breakout (brittle, sudden); tension capacity tracks embedment, shear tracks edge distance.
- ACI 318 Chapter 17 only covers anchors qualified by testing (ACI 355.2 mechanical, 355.4 adhesive) with a current evaluation report (ICC-ES ESR); substituting a different anchor voids the design.
What concrete anchoring is, and why the bolt is the least of it
Concrete anchoring is the act of transferring a load from something bolted on top, a piece of equipment, a steel column, a pipe rack, a guardrail, into the concrete underneath it. The anchor is the path that load takes. People look at the bolt and assume the bolt is the strength. It almost never is. The strength of a concrete anchor comes from the concrete around it, the depth it reaches, how far it sits from an edge, and whether it was installed the way it was designed to be installed.
Think about what the anchor is actually fighting. Pull up on it hard enough and one of two things gives: the steel snaps, or a cone of concrete tears out around the anchor and comes with it. On a properly designed anchor in sound concrete away from an edge, the steel is the limit and you have a ductile, predictable failure. Crowd it next to an edge, set it too shallow, or pour it into weak concrete and the concrete lets go first, which is sudden and brittle and gives no warning.
So the load matters, the concrete matters, and the geometry matters as much as the hardware. The strength of the slab is the strength of the mix that went in it, which is why the anchor question and the concrete question are the same question. For how that concrete got its strength and where the reinforcing steel sits, see the rebar and mix-design guides. The anchor designer needs both: the concrete's compressive strength and where the steel is, because you are about to drill into it.
Anchors get designed by the engineer of record against a real load, not picked off a shelf because they looked stout. The rest of this guide is what it takes to put the designed anchor in correctly, because a perfect design installed wrong holds like a bad design.
Cast-in anchors vs post-installed anchors
Cast-in anchors are set in the formwork before the pour, so the concrete hardens around them. Post-installed anchors go into hardened concrete later, into a hole you drill. That is the whole split, and it drives almost every other decision.
The cast-in anchor is the strongest and the one to plan for. A headed anchor bolt, an L or J bolt, a threaded rod with a nut and plate, or a weld plate with studs becomes part of the concrete mass, and it carries load by bearing the head against solid concrete rather than by gripping the side of a hole. When the bolt locations are known before the pour, cast-in is stronger, simpler, and cheaper than drilling later. The cost is planning. The template has to be right, the bolts have to be held in position through the pour, and a bolt set 2 in off pattern is a problem you discover when the equipment shows up.
Post-installed is what you reach for when the concrete is already there: a retrofit, a piece of equipment nobody knew about at design, a column base where the cast-in bolts came out wrong. It is the workhorse of anchoring because most anchoring happens after the fact. It comes in two families, mechanical and adhesive, covered next. The trade-off is honest: post-installed gives you placement freedom in cured concrete, and in exchange it depends entirely on the hole and the install for its strength.
What is the difference between a mechanical and an adhesive anchor?
A mechanical anchor holds by physical grip inside the drilled hole, friction or interlock against the concrete. An adhesive anchor holds by bond, a threaded rod or rebar glued into the hole with epoxy or another chemical adhesive. Both are post-installed. They fail differently and they install differently, and choosing wrong is a common source of trouble.
Mechanical anchors set fast, take load right away, and need no cure time, which makes them the choice when the schedule is tight or the connection has to carry weight immediately. They are sensitive to edge distance and to the concrete being sound right at the hole. Adhesive anchors reach deeper, distribute load along the full embedment instead of at one point, and often win where you need higher capacity, closer edge spacing, or a watertight set. The price of adhesive is discipline: the hole has to be clean, the temperature has to be in range, and the adhesive has to cure before the anchor sees load.
The blunt version: mechanical when you need it to hold now and the geometry is generous, adhesive when you need more out of the concrete and you can control the install. The engineer of record and the manufacturer's evaluation report settle which one the connection actually calls for.
The mechanical anchor types and how each grips
Mechanical post-installed anchors fall into three working groups: expansion anchors, screw anchors, and undercut anchors. Each grips the concrete by a different mechanism, and the mechanism tells you where it belongs.
Expansion anchors, the wedge and the sleeve, hold by friction. A wedge anchor is a stud with a tapered end and an expansion clip; you tighten the nut to a set torque, the clip is forced up the taper, and it presses hard against the wall of the hole. The friction is the strength. Sleeve anchors work the same way over a longer body. Expansion anchors are everywhere because they are quick and cheap, but they generate splitting force as they expand, so they want edge distance and they can lose grip in cracked concrete unless the anchor is rated for it.
Screw anchors, the concrete screw, cut their own threads into a drilled hole slightly smaller than the screw. They hold by the threads bearing into the concrete, install fast with an impact driver, and many are removable, which makes them handy for temporary work and for anchoring where you cannot tolerate the expansion force of a wedge. Undercut anchors are the high end. They seat into a small cavity reamed at the bottom of the hole and carry load by bearing, the same way a cast-in headed bolt does, instead of by friction. That bearing mechanism is why a good undercut anchor does not fail in pullout and why it is the post-installed choice for the heaviest and most critical connections. Verify the specific anchor against its evaluation report, because grip mechanism, cracked-concrete rating, and capacity all live there.
| Mechanical type | How it grips | Where it fits |
|---|---|---|
| Wedge / expansion | Friction from a clip forced against the hole wall | Fast, general-duty, with edge distance |
| Sleeve | Friction over a longer expansion body | Lighter fixtures, brick and block too |
| Screw anchor | Threads cut and bear into the concrete | Fast, removable, low expansion force |
| Undercut | Bearing in a reamed cavity, like a cast-in head | Heaviest and most critical connections |
Adhesive and epoxy anchors
An adhesive anchor is a threaded rod or a length of rebar bonded into a drilled hole with a structural adhesive, an epoxy or an acrylic, injected from a cartridge. The chemical anchor carries load along the full depth of the bond, from the surface of the rod, through the cured adhesive, into the concrete. There is no expansion force, so it generates no splitting as it sets, which is why it can run tighter to an edge and deeper than an expansion anchor.
That bond is where adhesive beats mechanical. You can develop high tension capacity, you can set rebar dowels to tie new concrete into old, and you can place anchors closer to an edge than an expanding anchor would allow because nothing is pushing the concrete apart. Adhesive doweling is the standard way to connect a new slab, a new wall, or a new footing to existing concrete when there was no cast-in provision.
The catch is that the bond only exists if the hole was prepared right and the adhesive cured. An adhesive anchor has no mechanical backup. A wedge anchor that was set in a slightly dirty hole still grips by friction. An epoxy anchor in a dirty hole bonds to dust and holds almost nothing. That is why the next section is the most important one in this guide for anyone running adhesive.
Why does the hole have to be cleaned for an adhesive anchor?
Because the adhesive bonds to whatever is on the wall of the hole, and if that is drilling dust, it bonds to dust, not to concrete. Drilling concrete makes a fine powder that coats the inside of the hole. Leave it and the epoxy grabs a loose dusty layer that shears off under load. A dirty hole is the number one cause of adhesive anchor failure, and it is invisible once the anchor is set. Nobody can see a bad clean six months later when the connection lets go.
The standard method is blow, brush, blow, and the manufacturer's printed installation instructions, the MPII, govern the exact sequence and tool sizes. Blow the hole out to the bottom with oil-free compressed air, run a correctly sized wire brush full depth several times to scour the wall, then blow again to clear what the brush loosened. Many products call for the cycle more than once. The brush has to match the hole; a worn or undersized brush drags through the dust without touching the wall. Vacuum-bit systems that clean as they drill are an approved alternative on products listed for it, but only vacuuming a hole drilled with a plain bit is not acceptable and gives less than the published capacity.
Do it every hole, not the first one and then the rest by feel. The MPII is not a suggestion on adhesive anchors. It is the install procedure the anchor was qualified against, and the special inspector is checking that it was followed.
Embedment depth and drill diameter
The embedment depth, how far the anchor reaches into the concrete, is the single dimension that sets how much load it can develop, and it is fixed by the design, not by how deep the hole happened to go. Deeper embedment pulls a larger cone of concrete into the resistance, which is why pullout and breakout capacity climb with depth. Set an anchor short of its design embedment and you have quietly cut its strength, often by more than the depth difference suggests.
Drill the diameter the anchor calls for, no more. Adhesive anchors are sized for a specific hole diameter because the adhesive needs the right annular gap to bond, too tight and it will not flow, too loose and the bond is weak. Mechanical anchors need their matched bit so the expansion or the cut threads engage as designed. Carbide bits to the right standard, often ANSI-tolerance, keep the hole diameter where the listing assumes it.
Drill the hole deep enough to reach full embedment plus the room the manufacturer specifies for dust and adhesive at the bottom, then verify the depth with a stop or a marked bit. The common field shortcut is to drill until the fixture sits flush and call it good, which sets embedment by the bolt length instead of by the design. On a critical anchor, measure the hole. The depth gauge is faster than the failure.
How close to the edge can you anchor?
Not as close as the slab lets you drill. An anchor too near an edge, or too near another anchor, cannot develop a full cone of concrete around it, so its capacity drops and the failure shifts toward sudden concrete breakout. The minimum edge distance and spacing are set by the anchor's evaluation report and the design under ACI 318, and they exist because the concrete, not the bolt, is what runs out near an edge.
Here is the mechanism. When you pull an anchor in tension, it tries to break out a cone of concrete roughly 35 degrees from the anchor axis. That cone needs room. Put the anchor near a free edge and the cone is clipped, so there is less concrete to resist and the breakout strength falls. Crowd two anchors and their cones overlap, so they share concrete and neither develops full capacity. Get the anchor too close with deep embedment and you invite side-face blowout, where the concrete spalls off the side rather than breaking a cone off the top.
Field anchors get crowded toward edges all the time because that is where the steel and the equipment want to land, at the edge of a pad, the lip of a footing, the corner of a pier. That is exactly where capacity is lowest. When the layout pushes an anchor toward an edge, that is an engineering question, not a field call. The fix is more edge distance, deeper concrete, edge reinforcement, or a different anchor, and the engineer of record picks it.
The failure modes a design has to check
An anchor can fail several different ways, and the design has to check every one and size for the weakest. The ones a designer runs through come in tension and in shear.
In tension, the anchor can fail by the steel breaking, by the anchor pulling out of the hole, by a cone of concrete breaking out around it, or, near an edge with deep embedment, by side-face blowout where the concrete spalls off the side. In shear, the steel can break, a cone of concrete can break out toward the edge the load pushes against, or the anchor can pry out the back of a shallow embedment. The design is governed by whichever of these gives first.
The mode you want is steel failure, because steel yields before it breaks and gives warning. Concrete breakout is the one to design away from, because it is brittle and sudden with no ductility. That is the whole logic behind embedment depth and edge distance: push the failure into the steel and away from the concrete. When you read that an anchor is governed by breakout rather than steel, that is a flag that it is sitting too shallow or too close to an edge for the load it carries. Tension capacity tracks embedment depth; shear capacity tracks edge distance. Those two numbers are the levers the design pulls.
| Load | Failure mode | Driven mainly by |
|---|---|---|
| Tension | Steel failure (ductile, preferred) | Steel area and grade |
| Tension | Pullout | Anchor type, embedment, concrete strength |
| Tension | Concrete breakout (brittle) | Embedment depth, edge distance |
| Tension | Side-face blowout | Deep embedment near an edge |
| Shear | Steel failure | Steel area and grade |
| Shear | Concrete breakout toward edge | Edge distance |
| Shear | Pryout | Shallow embedment, stiff anchor |
ACI 318 Chapter 17 and qualified anchors
Anchoring to concrete is designed under ACI 318, Chapter 17, which is the provisions for anchoring in the structural concrete code. It is the framework that ties the load, the concrete strength, the embedment, the edge distance, and the failure modes together into the capacities a designer checks. The exact chapter number has moved across code cycles, so confirm it against the edition the jurisdiction has adopted before citing it on a submittal.
Chapter 17 only applies to anchors that have been qualified by testing, and that is the part the field needs to understand. A post-installed anchor is qualified under ACI 355.2 for mechanical anchors and ACI 355.4 for adhesive anchors, and the results are published in an evaluation report, commonly the ICC-ES ESR for that exact product. That report is not marketing. It is the document that gives the anchor's capacities, its minimum edge and spacing, its installation procedure, and its cracked-concrete rating, and it is what the engineer designs from.
The practical rule that follows: a generic anchor with no current evaluation report has no Chapter 17 capacity you can defend, no matter how heavy it looks. On a structural connection, the anchor on the truck has to be the anchor in the ESR the engineer used, installed the way the ESR says. Substitute a different brand or a different anchor and the design no longer applies. The report and the product go together.
What is a cracked-concrete anchor, and when seismic forces it
A cracked-concrete anchor is one that has been tested and qualified to hold its rated capacity in concrete that has cracked through the anchor location, not just in sound uncracked concrete. The distinction matters because concrete in service cracks, and a crack running through an anchor's cone cuts its capacity, often by a third to a half compared with the same anchor in uncracked concrete.
Concrete cracks wherever it goes into tension: the top of a slab over a support, the bottom at midspan, anywhere shrinkage and load put the section past its modulus of rupture. ACI 318 and the building code treat the concrete around an anchor as cracked unless analysis shows it stays in compression, because assuming uncracked is the unsafe assumption. An anchor rated only for uncracked concrete, dropped into a tension zone, is sitting on a capacity it does not have once the crack arrives.
Seismic is where this turns from a margin into a rule. The code requires anchors resisting seismic forces to be qualified for cracked concrete, because an earthquake drives the concrete into cracking and reverses the load. Using a non-cracked-rated anchor in a tension zone or in a seismic connection is one of the more dangerous shortcuts on the list, and it is invisible after install. The rating is on the anchor's evaluation report and on the box. Check it before you drill, not after the inspector asks.
Adhesive orientation, temperature, and cure
Adhesive anchors are far more sensitive to how and where they are installed than mechanical anchors, and three conditions decide whether the bond develops: orientation, temperature, and cure.
Orientation is the one crews underestimate. An anchor drilled downward lets gravity hold the adhesive at the bottom of the hole around the rod. Drill horizontally, upward, or overhead and gravity works against you, the adhesive wants to slump or sag out, and you can get voids along the bond that you cannot see. Overhead and horizontal adhesive anchors that carry sustained tension are a special case in the code for exactly this reason, and they call for piston plugs or other measures to fill the hole from the back, plus wedges to hold the anchor while it cures so it does not drift.
Temperature governs both the working time and the cure. The base material temperature, the temperature of the concrete itself, sets how fast the adhesive sets and how long until it can take load. Cold concrete cures slowly and stretches the cure time out; hot concrete shortens the working time so much the adhesive can stiffen before the anchor is seated. Every product publishes a minimum and maximum base-material temperature and a gel and cure time for each, and those are in the MPII. A wet hole is its own problem: standing water has to be removed, the hole blown out, and the product has to be one rated for the moisture condition you have, because not all adhesives are. The blunt rule on cure: the anchor holds nothing until the published cure time has passed. Load it early and you have tested the adhesive to failure on the job.
Special inspection and certified installers
Post-installed anchors on structural work usually require special inspection under the building code, and adhesive anchors get the tightest version of it. This is not optional paperwork. It is a code-mandated check that the right anchor went into a clean hole at the right depth, set the right way, and the inspector signs to it.
The IBC has required adhesive anchor installers to be certified since the 2012 edition. The specific high-risk case, an adhesive anchor installed horizontally or upwardly inclined to resist sustained tension load, has to be installed by an installer certified under the ACI/CRSI Adhesive Anchor Installer Certification program or an equivalent, and it requires continuous special inspection, meaning the inspector watches the installation rather than checking it after the fact. That overhead sustained-tension case is the one that has failed catastrophically in service, which is why the code singles it out.
Mechanical and other post-installed anchors generally need special inspection too, often periodic rather than continuous, per the design and the code. The schedule of what gets inspected and how is on the structural drawings in the statement of special inspections. If you are setting structural anchors and nobody has mentioned inspection, that is a gap to close before you drill, not after.
Drilling without hitting the rebar
Every hole you drill into structural concrete is a gamble on what the bit finds, and the thing you do not want to find is reinforcing steel. Cut a bar with a hammer drill and you have weakened the member you are anchoring to, sometimes severely, and the anchor still has to land somewhere. On a post-tensioned slab the stakes are higher: hit a tendon and you can release it violently and damage the structure, which is why post-tensioned slabs get scanned and marked before anyone drills.
Locate before you drill. A rebar locator or, better on congested or post-tensioned work, ground-penetrating radar maps the steel and the tendons so you can mark them and set the anchor in the clear. When the designed anchor location lands on a bar, the move is to shift it within the tolerance the engineer allows or to go back to the engineer, not to drill through the steel and hope. Some adhesive systems are qualified to set into a hole that nicks a bar, but that is a design decision with its own rules, not a field improvisation.
The rookie version of this failure is drilling on layout without a scan because the slab looked plain, cutting a bar, and burying it. The bar is gone and nobody knows until the structure tells them. Scan first. For where the steel is supposed to be and why its cover matters, the rebar placement guide carries the detail.
Setting torque and proof-load testing
A mechanical anchor only develops its rated capacity if it is set correctly, and for most expansion anchors that means tightening to the installation torque the manufacturer specifies. The torque is what drives the wedge up the taper and creates the friction; under-torque it and the clip never fully expands, so it holds short of rating and can pull loose under load. Over-torque it and you can strip the threads or crush the concrete at the surface. Use a calibrated torque wrench on anchors that call out a value, and follow the setting-tool requirement on the drop-in and self-drilling types, which are set by a tool, not by feel. Screw anchors are torqued to seat without spinning out the threads they cut.
Proof-load testing, a tension pull test on the installed anchor, is how you confirm in place what the design assumed. The engineer or the specification sets a proof load, usually a fraction of the design or ultimate capacity, and a calibrated hydraulic ram pulls the anchor to that load and holds it without the anchor moving or the concrete cracking. Adhesive anchors and anchors in questionable concrete get tested more often, and the test is run after the adhesive has fully cured, never before. A failed proof test on one anchor is a question about the whole population, the concrete, the cleaning, the cure, so it triggers investigation, not just replacing the one anchor. Record the proof load, the result, and the gauge calibration.
Anchoring equipment, racks, and vibrating loads
Anchoring equipment to a concrete floor or a housekeeping pad is the most common reason a crew drills concrete, and it has its own failure pattern. Generators, pumps, transformers, air handlers, server racks, PDUs, and switchgear all get anchored, and the load is rarely a clean static pull. It vibrates, it cycles, and that is hard on a friction anchor.
Vibration is the enemy of the wedge anchor. Cyclic load works the nut loose, the clip relaxes, and the friction that held it bleeds off over months until the anchor is loose in the hole. On rotating and reciprocating equipment, that is why screw anchors, undercut anchors, or adhesive sets are often specified over plain expansion anchors, and why nuts get a locking method and a documented torque. A housekeeping pad raises the equipment and gives the anchor concrete to bite, but the pad has to be tied into the structural slab or be thick enough to develop the anchor on its own, or the whole pad lifts as a unit. That is a detail the engineer has to confirm, not an assumption.
On a data center floor, rack and PDU anchoring is also a seismic question, which loops back to cracked-concrete-rated anchors and the special-inspection schedule. The load the equipment puts on the anchor, static, vibratory, and seismic, comes from the equipment data and the project, and the anchor is sized to it by the engineer, not guessed in the field from how heavy the cabinet feels.
Corrosion: matching the anchor to the environment
An anchor is only as durable as its weakest part against the environment it sits in, and a carbon-steel anchor in a wet, coastal, or chemically exposed location rusts, swells, and either splits the concrete around it or wastes away to nothing. The material choice is part of the design, not a stock decision.
Match the anchor to the exposure. Zinc-plated carbon steel is for dry interior use and nothing more. Hot-dip galvanized buys outdoor and damp service. Stainless steel, type 304 or the more resistant 316 near salt water or chemicals, is for exterior, marine, wet, and corrosive environments where the anchor has to last the life of the structure. Mixing metals is its own trap: a stainless anchor against dissimilar metal hardware in a wet location sets up galvanic corrosion at the connection, so the washers and nuts have to match the anchor.
The failure here is slow and it shows up far from the install date, as rust staining, a spalled and cracked pad, or an anchor you can wiggle by hand. Specify the corrosion class for the real environment, not the price, and on exterior and wet work confirm the stainless or galvanized grade against the project specification.
What to document
Once an anchor is set, the hole cleaning, the embedment depth, and the cure vanish behind a bolt head, and a fixture that later works loose puts every one of them in question. Adhesive and structural anchors get buried in the work, so the record is the only proof later that the hole was clean, the embedment was right, and the cure had happened before load.
Capture, per anchor or per group, the anchor product and its evaluation report number, the type, the diameter and embedment, the hole cleaning method and that it was done, the edge distance and spacing achieved, the setting torque or the adhesive lot and cure conditions, and any proof-load result. On adhesive anchors record the base-material temperature and the installer's certification where it applies, because the inspector is signing to those. Tie it to a location on the drawing so the next person knows which anchor the record describes.
| Field to record | Why it matters |
|---|---|
| Anchor product and ESR number | Ties the install to the qualified, designed anchor |
| Type, diameter, embedment depth | Sets the capacity the design relied on |
| Hole cleaning method, done yes/no | The make-or-break step for adhesive bond |
| Edge distance and spacing | Governs breakout and side-face blowout capacity |
| Setting torque or adhesive lot and cure | Proves the anchor was set or cured to spec |
| Base-material temperature (adhesive) | Controls working time and cure validity |
| Proof-load result, gauge calibration | Confirms the installed anchor in place |
Common mistakes
- Not cleaning the drilled hole, so the adhesive bonds to dust and the anchor holds almost nothing.
- Setting an anchor too close to an edge or another anchor, cutting breakout capacity and inviting sudden concrete failure.
- Using a non-cracked-rated anchor in a tension zone or a seismic connection.
- Setting embedment by the bolt length instead of the design depth, leaving the anchor shallow.
- Drilling without scanning and cutting a reinforcing bar or a post-tension tendon.
- Loading an adhesive anchor before the published cure time has passed.
- Substituting a different brand or anchor than the one in the evaluation report the engineer designed from.
- Skipping special inspection or using an uncertified installer on overhead sustained-tension adhesive anchors.
- Under-torquing a wedge anchor so the clip never fully expands, or putting a friction anchor under vibration without a locking method.
- Using a plain zinc anchor outdoors or near salt, where it rusts and spalls the concrete.
Field checklist
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Standards and references
ACI 318, in the anchoring-to-concrete provisions of Chapter 17, is the design framework: it ties the load, the concrete strength, the embedment, the edge distance, and the failure modes into the capacities a designer checks. The chapter and section numbers have shifted across code cycles, so confirm them against the adopted edition before citing them.
Anchors earn a place in that design by being qualified. ACI 355.2 covers the testing and qualification of post-installed mechanical anchors and ACI 355.4 covers adhesive anchors, and the results are published in an evaluation report, commonly the ICC-ES ESR for the specific product, which carries the capacities, the minimum edge and spacing, the cracked-concrete rating, and the install procedure. The International Building Code (IBC) is where special inspection and the adhesive-anchor installer certification (ACI/CRSI) are required, with continuous inspection for overhead and horizontal sustained-tension adhesive anchors.
Above all of these sits the manufacturer's printed installation instructions, the MPII, which govern the actual install, the hole cleaning, the drill diameter, the cure, and the temperature limits. The engineer of record selects the anchor and the load. Where any of these conflict, the project specification and the manufacturer's listing control over a rule of thumb, and the adopted code edition and local amendments control over a remembered number.
Units, terms, and conversions
Anchor work spans a few terms and unit systems that read differently across a drawing, an evaluation report, and a manufacturer sheet, so the same idea can show up under more than one name.
Embedment depth is written hef (effective embedment) in design and reports, in inches or millimeters. Edge distance is ca1 and ca2; spacing between anchors is s. Tension and shear capacities show as Nn and Vn. Setting torque is given in lbf-ft or N-m, and concrete strength as f'c in psi or MPa. Cast-in anchors are also called anchor bolts, J-bolts, L-bolts, or headed studs; post-installed adhesive anchors are also called epoxy or chemical anchors; concrete screws are also called screw anchors. An ESR is the ICC-ES evaluation report; the MPII is the manufacturer's printed installation instructions.
- hef (embedment)
- Effective embedment depth, how far the anchor reaches into the concrete, in inches or mm
- ca1 / spacing
- Edge distance from the anchor to a free edge, and the spacing between anchors
- Cast-in anchor
- Anchor set in the forms before the pour: headed bolt, J or L bolt, or weld plate with studs
- Post-installed anchor
- Anchor placed in hardened concrete in a drilled hole, mechanical or adhesive
- Cracked-concrete rated
- Qualified to hold rated capacity in concrete cracked through the anchor, required for seismic
- MPII
- Manufacturer's printed installation instructions, which govern the install procedure
- ESR
- ICC-ES evaluation report, the qualification document with capacities and install rules
FAQ
What is the difference between a mechanical and an adhesive anchor?
A mechanical anchor holds by physical grip in the drilled hole, either friction from an expansion clip or threads cutting into the concrete. An adhesive anchor holds by bond, a rod or rebar glued in with epoxy. Mechanical sets fast and loads immediately; adhesive reaches deeper and needs a clean hole and cure time.
Why do you clean the hole for an epoxy anchor?
Because the epoxy bonds to whatever coats the hole, and if that is drilling dust it bonds to dust instead of concrete and shears off under load. A dirty hole is the top cause of adhesive anchor failure. Use the blow-brush-blow sequence in the manufacturer's instructions, every hole, then verify it is clean and dry.
What is a cracked-concrete anchor?
A cracked-concrete anchor is qualified by testing to hold its rated capacity in concrete that has cracked through the anchor location, not just in sound uncracked concrete. A crack can cut capacity by a third to a half, so the code requires cracked-concrete-rated anchors in tension zones and in any seismic connection.
How close to the edge can you anchor in concrete?
No closer than the minimum edge distance in the anchor's evaluation report and the ACI 318 design. Near an edge the concrete breakout cone is clipped, so capacity drops and failure turns brittle. When the layout crowds an anchor toward an edge, treat it as an engineering question, because the concrete runs out before the bolt does.
Are cast-in anchors stronger than post-installed anchors?
Yes. A cast-in anchor set before the pour becomes part of the concrete mass and carries load by bearing, which makes it the strongest and the choice when bolt locations are known in advance. Post-installed anchors hold by grip or bond in a drilled hole and depend on the install, but they anchor into concrete that already exists.
How deep does a concrete anchor need to be set?
To the embedment depth the design specifies, because that depth sets the capacity by controlling how much concrete resists pullout and breakout. Set it shallow and you cut the strength, often more than the depth difference suggests. Drill to full embedment plus the manufacturer's allowance, and verify the hole depth instead of setting it by the bolt length.
Does an adhesive anchor need special inspection?
Usually yes. The IBC requires special inspection for adhesive anchors, and the high-risk case, overhead or horizontal anchors resisting sustained tension, requires a certified installer under the ACI/CRSI program plus continuous inspection where the inspector watches the install. The statement of special inspections on the structural drawings sets exactly what is required.
What is the strongest concrete anchor failure to design for?
Design for the weakest failure mode, but aim the failure into the steel, which yields with warning, and away from concrete breakout, which is brittle and sudden. Embedment depth and edge distance are the levers: tension capacity tracks embedment, shear capacity tracks edge distance, and getting both right pushes the failure into the ductile steel.
Can post-installed anchors replace cast-in anchor bolts?
Often, but only by design, not by swapping in the field. A post-installed anchor sized and qualified for the load can replace a cast-in bolt that came out wrong or was never placed. The engineer of record checks the capacity, edge distance, and cracked-concrete rating against the connection, because the two anchor types fail differently.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.