anchors for anchoring to concrete

7/30/2019 Anchors for Anchoring to Concrete

1/4

PART TWO.

Types of Anchors forAnchoring to Concrete

Everything you wanted to know about types of anchorfor use in concrete (well almost everything

by Richard E. Wollmershauser, P.E., FAC

Designers and installers of anchors

for fastening attachments and structural

components to concrete face a seeming-

ly unlimited variety of anchoring prod-ucts. How is one supposed to know

which is the right anchor for a given

application? This article gives a briefoverview of the types of anchors avail-able, how they develop the capacity to

resist loads, and some general comments

about appropriateness of these anchors

for different applications.Anchors can be separated into two

major categories, those that are placed

before the concrete is cast, known as

cast-in-place anchors, and those that areinstalled into hardened concrete, known

as post-installed or drilled-in anchors.

Each of these two categories is composed

of a variety of different anchors, all ofwhich transfer loads from the attach-

ment to the concrete in a variety of ways

depending on their individual designs.

LOADS AND LOAD-TRANSFER

MECHANISMS

Before reviewing the various types of

anchors, it is appropriate to present thetypes of loads that anchors must resist

and mechanisms for load-transfer from

anchor to concrete.

The primary types of loads to be

resisted in normal construction are staticand dynamic. Load factors are spelled

out in the applicable building codes. Sta-

tic loads can be tension, shear, or combi-

nations of both. Dynamic loads can be

seismic, fatigue, wind, and shock. Themany types of anchors available have

characteristics that make them suitable

for or unsuitable for use under these var-ious types of loads. Two state-of-the-artdocuments, references 1 and 2, give

much more detailed information. Espe-

cially good are approval agency evalua-

tion reports such as ICBO ES, BOCA,and SBCCI or product listings for specif-

ic use as issued by listing agencies (e.g.

UL and FM). Manufacturers literature

may specify the applications for whichanchors are qualified and if test reports

are available to document capacities.

The primary load-transfer mecha-

nisms under tension loading are (in noparticular order of importance): keying

or bearing, friction, and bonding as well

as combinations. For shear loading, it is

keying or direct bearing.Keying is the direct transfer of load

from the anchor into the concrete by

bearing forces in the same direction of

loading of the anchor. Friction is thetransfer of load through friction between

expansion sleeves of the anchor and the

wall of the drilled hole in the concrete.

Bonding takes place when resins or

grouts are used around the anchor, usu-ally with post-installed anchors. Local

crushing occurs when the expansion

forces at the sleeve are great enough to

locally crush the concrete. In this case,

the load-transfer mechanism becomes acombination of friction and localized

keying. With many bonding systems, key-

ing is also part of the load-transfer as theresin or grout fills the local pores in thewall of the hole. See Figure 1 for illustra

tions of the basic mechanisms.

CAST-IN-PLACE SYSTEMS

Traditionally, design engineers specify

cast-in-place anchors if they know before-

hand where anchors are to be installed

Table 1 provides a brief overview of themajor types of cast-in-place anchors avail-

able, all of which use keying as a load

transfer means, in tension, shear, or

both.

Table 1. Cast-in-place Anchors

Standard fasteners Headed bolts

J and L bolts

Stud-welded plates

Proprietary shapes Threaded inserts

Proprietary anchors

and shapes

Through bolts Usually sleeved

Special shapes Shear lugs

Channel bars

FIGURE 1. Load-Transfer Mechanisms

Friction Keying Bonding


2/4

Many of these types of anchors have

special uses. Shear lugs and stud-welded

plates provide large shear resistance,

while channel bars give specific attach-ment capability. J and L bolts are typical-

ly used for anchoring sill plates to

foundations, but have a tendency to

straighten and pull out under high ten-sion loading. Headed bolts under

ASTM A 490 and F 1554 specifications

are veritable workhorses for structural

engineers.But what if the cast-in-place anchor is

mislocated, or if the location of the

anchor is not known before construc-

tion, or additional anchorages need tobe made after the concrete is cast? The

post-installed anchor industry has devel-oped a wide variety of products for these

situations.

POST-INSTALLED ANCHORS

With the development and improve-

ments of rotary hammer drills and car-bide-tipped bits, the user has the

capability to install many different kinds

of post-installed anchors in hardened

concrete virtually anywhere that is acces-sible to the drills.

Post-installed anchors can be divided

into two major types, depending on themethod of transferring load into theconcrete. They are mechanical systems

and bonded or adhesive systems.

Anchors can also be cros s-classi fied

according to their load carrying capabili-ty; heavy-duty, medium-duty and light-

duty. Table 2 provides an overview of the

major types of post-installed anchors cur-rently available as well as a generalized

load categorization.

1.5

FIGURE 2. Cast-in-place Anchors

Table 2. Post-Installed Anchors

MECHANICAL SYSTEMS HEAVY- MEDIUM- LIGHT-

DUTY DUTY DUTY

Undercut Anchors

Torque-controlled expansion anchors

Heavy duty sleeve anchor

Wedge anchors x

Coil anchors x

Screw anchors

Sleeve anchors

Light duty metal anchors

Plastic anchors

Displacement-controlled expansion anchors

Drop-in x x

Self-drilling x x

BONDED OR ADHESIVE SYSTEMSUsing threaded rods or reinforcing bars

Capsule systems x

Cartridge systems x

Bulk-injection systems x

Cemetitious or epoxy grouted systems x

Indicates normal categoryx Indicates possible but not primary category

Traditionally, design

engineers specify

cast-in-place anchors if

they know beforehand

where anchors are to

be installed.

Headed Bolt L-Bolt J-Bolt Stud-welded Plate

MECHANICAL

ANCHORING SYSTEMS

While mechanical anchors have beenavailable for several decades, there have

been significant advances in the typesand capabilities of these systems. They

cover the range from heavy duty to lightduty capacities. The major concern is to

select the appropriate anchors that will

resist the expected loads for the given

application. Is the anchor expected toexperience seismic or fatigue loads, or

merely static loads? While some anchors

will withstand all of these types of loads,

others may fail abruptly under dynamicloading at a load level less than the static

capacity.

s UNDERCUT ANCHORS

These anchors have been on the mar-ket for about 20 years. They are excel-

lent for use under both static and

dynamic loads. They obtain their hold-ing capacity through keying, that is,direct bearing on the concrete, and,

under proper installation, can withstand

very high loads without slipping out of

the drilled hole. They are the preferredanchors for use where cracks in tension

zones of the concrete can be expected to

occur.


3/4

s EXPANSION ANCHORS

Expansion anchors have been avail-

able for at least 30 years. There are two

basic types that are distinguished by theiroperating principles. The first, torque-

controlled expansion anchors, are

installed by inserting the anchor into thedrilled hole, and applying the prescribedsetting torque to the head or nut. A cone

at the bottom of the anchor is pulled up

into an expansion sleeve that expands

into the concrete with local crushing,and providing both friction and local-

ized keying as load-transfer mechanisms.

Most have a beneficial follow-up expan-

sion characteristic that under tensionloading, the more the anchor bolt dis-

places upward out of the hole, the

greater the expansion forces.

There are several types available thatvary significantly in their ability to resist

static and dynamic loads. The heavy-duty

sleeve anchor (not to be confused with

the light duty sleeve anchor composed ofa sheet metal sleeve around a cone) can

resist dynamic loads as well as function

well in expected cracks in concrete.

Wedge anchors are the most commontype of torque-controlled expansion

anchor. Many have been tested for resist-

ing seismic loads and have receivedICBO ES Evaluation Report listings for

such service.

Sleeve anchors are generally consid-ered light duty, and rely more on frictionand less on local crushing and keying of

the concrete. They can be used in hol-

low masonry.

The second major type of expansionanchor is the displacement-controlled

expansion anchor. Two primary exam-

ples are the drop-in and the self-driller.

Drop-in anchors are installed in thepredrilled hole by use of a setting tool

that drives a plug into the expansion

portion of the anchor. The lower section

of the anchor is expanded into the con-crete, which experiences local crushing.

The second type has cutting teeth on the

lower end and drills its own hole. The

anchor is driven onto an expansion plugthat expands the lower portion of the

anchor into the concrete. These anchors

derive their holding capacity from fric-

tion and keying. The drawback is thatthey do not have any follow-up expan-

sion. Once set, they do not expand fur-

ther under loading. If the plug is not

sufficiently set, the anchor may experi-

ence reduced capacity.

s OTHER TYPES OF

MECHANICAL ANCHORS

Screw anchors of smaller diameters

and used for light duty applications havebeen available for many years. They are

installed into holes drilled with matched-tolerance bits. Recently, manufacturers

have been introducing larger diameterscrew anchors in the heavy-duty range,

either installed into holes drilled with

matched-tolerance bits or using bits

meeting the requirements of ANSIB212.15. They distribute the load into

the concrete through load transfer from

the threads to the concrete (keying).

Coil anchors have also been intro-duced into the market in the heavy-duty

load range. A coil anchor is composed of

a coil near the end of a specially

designed lag bolt. As the bolt is torquedthe screw advances through the coil,

which expands into the concrete. High

load capacities can be developed.

s LIGHT DUTY ANCHORS

There are many light duty anchorsavailable, made of metals and plastics.

They function in a variety of base materi-als, concrete, masonry, wood, drywall

etc. Manufacturers publish test data to

document their capacities in these basematerials.

BONDED ANCHOR SYSTEMS

Bonded resin or adhesive anchorswere generally introduced into the con

struction market about twenty years ago

Sleeve Anchor Torque-controlledExpansion Anchor

Heavy-dutySleeve Anchor

UndercutAnchor

FIGURE 3. Mechanical Anchors

Drop-in Anchor Self-drilling Anchor Stud Anchor

FIGURE 4. Displacement-controlled Expansion Anchors


4/4

Bonded systems use a combination of

adhesive bond and micro keying into the

pores of the concrete. Early systems used

polyester resin, epoxies, and later, vinylester resins. In recent years, a larger vari-

ety of resins have been developed that

have individual advantages, such as usein high temperatures, low temperatures,damp and wet holes, etc. For two compo-

nent epoxy systems, the ratio of harden-

er to resin is critical. Prepackaged

cartridge systems assure that the propermixing is obtained. While a variety of

installation methods are used, most are

two component resin systems that

anchor threaded rod into predrilledholes. Most will resist dynamic loads,

both seismic and fatigue, but documen-

tation in the form of test reports should

be obtained. Bonded or adhesiveanchoring systems are not well suited for

cracked tensile zones of concrete since

about 1/2 the bonding is lost, unless one

uses special anchoring systems devel-oped for use in cracked concrete. One

should also use the recommended drill

bit size, since many of the adhesives are

sensitive to the size of the gap betweenthe anchor rod and the concrete.

s CAPSULE SYSTEMS

The original capsules were made of

glass, with an internal glass capsule that

contained the hardener. Recent versionsuse plastic foil instead of the glass cap-

sules. The capsule is inserted into the

hole. The threaded rod or reinforcing

bar is then rotary-hammered into the

glass capsule, breaking the glass and mix-ing the resin and hardener. Capsule

anchors provide very good bonding as

the rotary mixing action wipes andcleans the surface of the hole, allowingthe adhesive to bond well.

s CARTRIDGE SYSTEMS

Introduced also about 20 years ago,cartridge systems are the most popular

delivery method since they are also the

most versatile. The resin and hardener

are packaged in separate but connectedside-by-side plastic or foil tubes. A mixing

nozzle is placed on the end of the tubes

and the resin and hardener dispensed by

a hand operated or a pneumatic dis-penser. Also used for heavy-duty applica-

tions, hole cleaning is very critical since

dust on the concrete hole surface limits

the bond and micro keying.

s BULK INJECTION SYSTEMS

For high volume applications such as

highway contracts or large buildingwhere repetitive installation of anchors is

required, bulk mechanical and hydraulic

dispensing systems are available. Atten-

tion to the proper mixing ratio for epoxyresins is critical since bulk equipment

can go off ratio.

s GROUTED SYSTEMS

Predating resin systems are cemeti-

tious grouts. There are a variety of pro-

prietary systems available. Non-shrink orslightly expansive grouts are preferred to

prevent shrinkage and lack of bonding

to the existing concrete.

IN SUMMARY

A wide variety of anchoring systems

are available. The correct selection

depends on several factors, includingtypes of loads to be resisted, magnitude

of the load, type of base material (con-

crete or masonry), and environmentconditions (damp, wet, dry), among oth-ers. The manufacturers literature pro-

vides a good starting place, but also look

for evaluation reports and product list-

ings, since they may be required undermany building codes. s

REFERENCES

1. State-of-the-Art Report on Anchorage toConcreteACI 355 .1R-91, AmericanConcrete Institute, Detroit, 1991.

2. Fast enings to Concre te and Masonr yStructures; State of the Art Report,Comit Euro-Intrnational du Bton

Thomas Telford Services Ltd., Lon-

don, 1994.

3. ANSI B212.15-1994, American NationalStandard for Cutting ToolsCarbide-Tipped Masonry Drills and Blanks forCarbide-Tipped Masonry Drills, Ameri-can National Standards Institute, NewYork, N. Y., 10 pp.

Richard Wollmershauser is Director Tech-nical Services for Hilti, Inc. of Tulsa,Oklahoma. He has been active in the con-

crete anchor industry for the past 20 yearsHe is a registered PE in Ohio andOklahoma and a Fellow of the AmericanConcrete Institute. He currently serves asVice-President of CAMA, The ConcreteAnchor Manufactures Association, Chair-man of ASTM Subcommittee E06.13,Performance of Connections in BuildingConstruction, and just completed 7 years asChairman of the American ConcreteInstitute Committee 355, Anchorage toConcrete. He was editor of the ACI State-of-the-Report on Anchorage to Concrete (ACI

355R1-93). He also serves on the Federa-tion International du Beton Task GroupSAG 4 Fastening to Concrete and MasonryHe served on three committees of the recentNEHRP 2000 Seismic Provisions Updateprogram where he was concerned with theintroduction of anchoring design methods.

Adhesive CapsuleAnchor

Adhesive CartridgeSystem

FIGURE 5. Adhesive Bonded Anchor Systems

anchors for anchoring to concrete

Documents