Types of Anchors for Anchoring to Concrete - Hilti

PART TWO...
Types of Anchors for
Anchoring to Concrete
Everything you wanted to know about types of anchors
for use in concrete (well almost everything)
by Richard E. Wollmershauser, P.E., FACI
Designers and installers of anchors
for fastening attachments and structural
components to concrete face a seemingly
unlimited variety of anchoring products.
How is one supposed to know
which is the “right” anchor for a given
application? This article gives a brief
overview of the types of anchors available,
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 are
installed 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 of
which transfer loads from the attachment
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 the
types 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 static
and dynamic. Load factors are spelled
out in the applicable building codes. Static
loads can be tension, shear, or combinations
of both. Dynamic loads can be
seismic, fatigue, wind, and shock. The
many types of anchors available have
characteristics that make them suitable
for or unsuitable for use under these various
types of loads. Two state-of-the-art
documents, references 1 and 2, give
much more detailed information. Especially
good are approval agency evaluation
reports such as ICBO ES, BOCA,
and SBCCI or product listings for specific
use as issued by listing agencies (e.g.
UL and FM). Manufacturers’ literature
may specify the applications for which
anchors are qualified and if test reports
are available to document capacities.
The primary load-transfer mechanisms
under tension loading are (in no
particular 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 the
transfer 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, usually
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 a
combination of friction and localized
keying. With many bonding systems, keying
is also part of the load-transfer as the
resin or grout fills the local pores in the
wall of the hole. See Figure 1 for illustrations
of the basic mechanisms.
CAST-IN-PLACE SYSTEMS
Traditionally, design engineers specify
cast-in-place anchors if they know beforehand
where anchors are to be installed.
Table 1 provides a brief overview of the
major types of cast-in-place anchors available,
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
Friction Keying Bonding
FIGURE 1. Load-Transfer Mechanisms

1.5
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
FIGURE 2. Cast-in-place Anchors
Many of these types of anchors have
special uses. Shear lugs and stud-welded
plates provide large shear resistance,
while channel bars give specific attachment
capability. J and L bolts are typically
used for anchoring sill plates to
foundations, but have a tendency to
straighten and pull out under high tension
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 construction,
or additional anchorages need to
be made after the concrete is cast? The
post-installed anchor industry has developed
a wide variety of products for these
situations.
POST-INSTALLED ANCHORS
With the development and improvements
of rotary hammer drills and carbide-tipped
bits, the user has the
capability to install many different kinds
of post-installed anchors in hardened
concrete virtually anywhere that is accessible
to the drills.
Post-installed anchors can be divided
into two major types, depending on the
method of transferring load into the
concrete. They are mechanical systems
and bonded or adhesive systems.
Anchors can also be cross-classified
according to their load carrying capability;
heavy-duty, medium-duty and lightduty.
Table 2 provides an overview of the
major types of post-installed anchors currently
available as well as a generalized
load categorization.
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 SYSTEMS
Using threaded rods or reinforcing bars
Capsule systems • • x
Cartridge systems • • x
Bulk-injection systems • • x
Cemetitious or epoxy grouted systems • • x
• Indicates normal category
x Indicates possible but not primary category
MECHANICAL
ANCHORING SYSTEMS
While mechanical anchors have been
available for several decades, there have
been significant advances in the types
and capabilities of these systems. They
cover the range from heavy duty to light
duty capacities. The major concern is to
select the appropriate anchors that will
resist the expected loads for the given
application. Is the anchor expected to
experience seismic or fatigue loads, or
merely static loads? While some anchors
will withstand all of these types of loads,
others may fail abruptly under dynamic
loading at a load level less than the static
capacity.
■ UNDERCUT ANCHORS
These anchors have been on the market
for about 20 years. They are excellent
for use under both static and
dynamic loads. They obtain their holding
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 preferred
anchors for use where cracks in tension
zones of the concrete can be expected to
occur.

■ EXPANSION ANCHORS
Expansion anchors have been available
for at least 30 years. There are two
basic types that are distinguished by their
operating principles. The first, torquecontrolled
expansion anchors, are
installed by inserting the anchor into the
drilled hole, and applying the prescribed
setting 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 localized
keying as load-transfer mechanisms.
Most have a beneficial follow-up expansion
characteristic that under tension
loading, the more the anchor bolt displaces
upward out of the hole, the
greater the expansion forces.
There are several types available that
vary 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 of
a 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 common
type of torque-controlled expansion
anchor. Many have been tested for resisting
seismic loads and have received
ICBO ES Evaluation Report listings for
such service.
Sleeve anchors are generally considered
light duty, and rely more on friction
and less on local crushing and keying of
the concrete. They can be used in hollow
masonry.
The second major type of expansion
anchor is the displacement-controlled
expansion anchor. Two primary examples
are the drop-in and the self-driller.
Drop-in anchors are installed in the
predrilled 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 concrete,
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 plug
that expands the lower portion of the
anchor into the concrete. These anchors
derive their holding capacity from friction
and keying. The drawback is that
they do not have any follow-up expansion.
Once set, they do not expand further
under loading. If the plug is not
Sleeve Anchor
FIGURE 3. Mechanical Anchors
Torque-controlled
Expansion Anchor
sufficiently set, the anchor may experience
reduced capacity.
■ OTHER TYPES OF
MECHANICAL ANCHORS
Screw anchors of smaller diameters
and used for light duty applications have
been available for many years. They are
installed into holes drilled with matchedtolerance
bits. Recently, manufacturers
have been introducing larger diameter
screw anchors in the heavy-duty range,
either installed into holes drilled with
matched-tolerance bits or using bits
meeting the requirements of ANSI
B212.15. They distribute the load into
the concrete through load transfer from
the threads to the concrete (keying).
Coil anchors have also been introduced
into the market in the heavy-duty
Heavy-duty
Sleeve Anchor
Undercut
Anchor
Drop-in Anchor Self-drilling Anchor Stud Anchor
FIGURE 4. Displacement-controlled Expansion Anchors
load range. A coil anchor is composed of
a coil near the end of a specially
designed lag bolt. As the bolt is torqued,
the screw advances through the coil,
which expands into the concrete. High
load capacities can be developed.
■ LIGHT DUTY ANCHORS
There are many light duty anchors
available, made of metals and plastics.
They function in a variety of base materials,
concrete, masonry, wood, drywall,
etc. Manufacturers publish test data to
document their capacities in these base
materials.
BONDED ANCHOR SYSTEMS
Bonded resin or adhesive anchors
were generally introduced into the construction
market about twenty years ago.

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, vinyl
ester resins. In recent years, a larger variety
of resins have been developed that
have individual advantages, such as use
in high temperatures, low temperatures,
damp and wet holes, etc. For two component
epoxy systems, the ratio of hardener
to resin is critical. Prepackaged
cartridge systems assure that the proper
mixing is obtained. While a variety of
installation methods are used, most are
two component resin systems that
anchor threaded rod into predrilled
holes. Most will resist dynamic loads,
both seismic and fatigue, but documentation
in the form of test reports should
be obtained. Bonded or adhesive
anchoring 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 developed
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 between
the anchor rod and the concrete.
■ CAPSULE SYSTEMS
The original capsules were made of
glass, with an internal glass capsule that
contained the hardener. Recent versions
use plastic foil instead of the glass capsules.
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 mixing
the resin and hardener. Capsule
anchors provide very good bonding as
the rotary mixing action wipes and
cleans the surface of the hole, allowing
the adhesive to bond well.
■ 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 connected
side-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 dispenser.
Also used for heavy-duty applications,
hole cleaning is very critical since
dust on the concrete hole surface limits
the bond and micro keying.
■ BULK INJECTION SYSTEMS
For high volume applications such as
highway contracts or large building
where repetitive installation of anchors is
required, bulk mechanical and hydraulic
dispensing systems are available. Attention
to the proper mixing ratio for epoxy
resins is critical since bulk equipment
can go “off ratio.”
■ GROUTED SYSTEMS
Predating resin systems are cemetitious
grouts. There are a variety of proprietary
systems available. Non-shrink or
slightly 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, including
types of loads to be resisted, magnitude
of the load, type of base material (concrete
or masonry), and environment
conditions (damp, wet, dry), among others.
The manufacturers’ literature provides
a good starting place, but also look
for evaluation reports and product listings,
since they may be required under
many building codes. ■
REFERENCES
1. State-of-the-Art Report on Anchorage to
Concrete ACI 355.1R-91, American
Concrete Institute, Detroit, 1991.
2. Fastenings to Concrete and Masonry
Structures; State of the Art Report,
Comité Euro-Intérnational du Béton,
Thomas Telford Services Ltd., London,
1994.
3. ANSI B212.15-1994, American National
Standard for Cutting Tools—Carbide-
Tipped Masonry Drills and Blanks for
Carbide-Tipped Masonry Drills, American
National Standards Institute, New
York, N. Y., 10 pp.
Richard Wollmershauser is Director Technical
Services for Hilti, Inc. of Tulsa,
Oklahoma. He has been active in the concrete
anchor industry for the past 20 years.
He is a registered PE in Ohio and
Oklahoma and a Fellow of the American
Concrete Institute. He currently serves as
Vice-President of CAMA, The Concrete
Anchor Manufactures Association, Chairman
of ASTM Subcommittee E06.13,
Performance of Connections in Building
Construction, and just completed 7 years as
Chairman of the American Concrete
Institute Committee 355, Anchorage to
Concrete. He was editor of the ACI State-ofthe-Report
on Anchorage to Concrete (ACI
355R1-93). He also serves on the Federation
International du Beton Task Group
SAG 4 Fastening to Concrete and Masonry.
He served on three committees of the recent
NEHRP 2000 Seismic Provisions Update
program where he was concerned with the
introduction of anchoring design methods.
Adhesive Capsule
Anchor
Adhesive Cartridge
System
FIGURE 5. Adhesive Bonded Anchor Systems