In many construction applications, concrete anchors
must be fastened to a concrete block to hold a fixture or item of some
kind. When installing these concrete anchors, special care must be taken
to ensure safe and proper fastening. When fastening to concrete blocks,
a number of specific considerations must be taken into account. These
include what part of the block to drill into as well as which type of
anchor would work the best in the specific application.
What is a concrete block?
A concrete block, cement block, foundation block or concrete
masonry unit (CMU), is a large rectangular block used primarily in the
construction of walls. Concrete blocks are made from precast concrete,
i.e. Portland cement, water and aggregate. The aggregate used for
high-density blocks is usually sand and/or fine gravel. Lower density
blocks, more commonly known as cinder blocks, may be produced using
cinders as aggregrate. These blocks are then layered and stacked
vertically to form a wall, and are held together using concrete mortar.
Determine where in the block the anchor should be placed
Before choosing the correct anchor for an application, it is
necessary to consider where in the block the fastener should be
positioned. Will the fastener be placed in (a) the mortar joint, (b) the
solid section of the block, or (c) the hollow section of the concrete
block? Some applications may allow each fastener to be placed in the
same part of the block while other installations require that each
anchor be placed in different sections of the block.
All anchors except for concrete screws exert an
outward force. An anchor exerting this pressure against an unsupported
edge decreases the anchor’s holding values.
Each section has unique advantages and disadvantages. Fastening
into the mortar joint can be an excellent option depending on the
quality and quantity of the mortar used to set the block in place. The
solid sections of the block usually offer the best quality fastening.
This will depend on the distance of the fastener to the unsupported edge
of the cavity and the quality of the material from which the block is
made. The hollow section of the block is the least desirable section of
the block in which to fasten but sometimes can not be avoided. When
fastening into the hollow section of a block, there are a few considerations to keep in mind:
(1) When drilling into the block with a hammer drill, the block
will spall. This leads to a cone shaped space, with the point of the
cone closest to the surface of the block.
(2) When inserting an anchor into this area, whether it's a
fastener that exerts pressure or a concrete screw that taps threads into
the base material, the holding values will be diminished or become
(3) The fastener must not penetrate the wall into the cavity itself.
Determine the type of load to be fastened
The next step in the process is to ascertain the type of load that is going to be anchored. Tech Tip:
Vibratory load: An application that has constant movement. Example: a sign posted on a block wall outside in the wind.
Shock load: An application where a load will be added and
taken away. Example: a dock bumper where a truck will back up to the
dock and hit the bumper.There are three questions that will help determine the type of load each application produces:
(1) How heavy is the item that is being fastened to the block wall?
(2) Will the load be a vibratory load?
(3) Is the load going to be placed where a shock load could occur?
Determine the type of anchor required
Lastly, the type of fastener required for the application must be
determined. There are four options available that can be used in
(1) Stud-type anchor: where the stud will stick out from the wall. Example: Sleeve Anchor
(2) Hammer-type anchor: where the stud does not protrude from the wall, for a finished look. Example: Hammer Drive, Nylon Nail-It
(3) Female-type anchor: where a screw is inserted into the anchor for a finished look. Example: Machine Screw Anchor, Lag Shield Anchor, Single Expansion Anchor, Double Expansion Anchor, Leadwood Screw Anchor
(4) Concrete screw: screwed directly into the block for a finished look
If using a stud-type anchor, female type anchor, or the concrete
screw- the fixture, or item being fastened, can be removed at any time
with little or no damage to the concrete block. However, hammer-type
anchors are permanent installations. Removing these fasteners will
greatly damage the concrete block and possibly the fixture as well.
Anchor types that can be used in concrete block applications
Stud-Type Anchor - Fixture will be removable
come in the widest variety of sizes for applications that fall into
this category. Anchors with diameters ranging from 1/4" to 5/16" are
sufficient for most light duty applications while the 3/8" and 1/2"
diameters are adequate for most medium duty situations. Sleeve anchors
with diameters of 5/8" and 3/4" are usually considered heavy duty
anchors. An engineer should be consulted every time concrete anchors are
used for heavy loads. This anchor is very versatile because there is a
wide range of lengths available allowing materials of many different
thicknesses to be attached to a block wall. The sleeve anchor also comes
in a variety of head styles - hex head, flat countersunk and acorn or
round head for aesthetics.
Hammer-type Anchor - Fixture will be non-removable
The hammer drive and nylon nail-it anchors
are two fasteners that fall into the hammer type category. Neither of
these anchors results in a stud protruding from the wall. Both of these
anchors are basically non-removable. They are simple to use but should
only be used in light duty applications because the only diameter
available is 1/4". The hammer drive anchor is produced with a low
profile, mushroom head while the nylon nail-it anchor is available in
three head styles - mushroom, round and flat countersunk.
Female-type Anchor - Fixture will be removable
There are a number of different female type
anchors. The anchor chosen will depend on the type of bolt being
inserted into the anchor as well as the quality of the base material.
The sizes of these anchors are designated by the inside diameter of the
anchor, which is the size of the bolt that will be inserted into it. The
different female type anchors and their unique characteristics are
The machine screw anchor
requires a machine threaded bolt with national coarse threads. For
example, 1/4"-20 machine screws require a 1/4" bolt with 20 threads per
inch. This anchor also needs to be set in the base material using an
appropriately sized setting tool. The 1/4" and 5/16" machine screw
anchors should be considered for light duty applications and the 3/8"
and 1/2" for medium duty. The 5/8" and 3/4" diameter machine screws are
typically reserved for heavy duty use.
Lag shield anchors
require the use of a lag screw. Lag shield anchors are available in
short and long lengths. The length of the lag shield is generally
determined by the quality of the base material. The long lag shield is
usually used when the quality of the base material is suspect. The extra
length of the long lag shield increases its ability to perform
properly. The 1/4" and 5/16" lag shield anchors can be used in light
duty applications, the 3/8" and 1/2" for medium duty and 5/8" and 3/4"
for heavy duty.
The single expansion anchor and double expansion anchor
require the use of a machine threaded fastener with national coarse
threading. Both anchors work well in most base materials. The double
expansion anchor should be used in applications with questionable base
material quality. Both the single and double expansion anchors come in a
variety of diameters to allow for usage in many different applications.
The 1/4" and 5/16" should be used for light duty type loads, the 3/8"
and 1/2" for medium loads and the 5/8" and 3/4" for heavier loading
Leadwood screw anchors
were originally made from lead. These concrete anchors are now produced
from a material called Zamac - a softer, non-corrosive metal. This
fastener is used in light duty applications since it is only available
up to 5/16" in diameter. The leadwood screw anchor requires the use of a
sheet metal screw. Like sheet metal screws, these anchors are also
designated by the gauge wire used to produce the screw - #6, #8 and so
on. Different lengths are available in each diameter and can be used in
applications with many embedment requirements.
Concrete Screw- Fixture will be removable
are designed to tap threads into base materials made of concrete, brick
or block. They work well in mortar joints as well as in the solid and
hollow sections of a block for light duty loads. They are availabe in a
wide array of lengths in both 3/16" and 1/4" diameters. Two different
head styles are available - hex head and flat philips for
All of the fasteners described above are suitable for use in
concrete block- whether it be the solid section of the block, the hollow
section or the mortar joint. These fasteners may be used in light or medium load applications with a non vibratory, non-shock load.
Although some of the fasteners listed above may be appropriate for heavy duty loads, it is not recommended to fasten a heavy load to concrete block.
The strength and integrity of the block will be limiting and will most
likely fail with a load that is too heavy. Heavy duty, non-vibratory,
non-shock loads would require a through-bolted fastening technique or a
concrete epoxy type fastener. Vibratory and shock loads would also
require a through-bolted fastening system, a concrete epoxy type anchor
or a cast-in place fastener. An engineer should always be consulted if your application requires any of these fastening techniques.
Many construction projects and applications will require
fastening to concrete block. Before getting started, it is essential to
choose the correct type of concrete anchor as well as determine in what
part of the block to fasten the load. Taking the time to make the right
decisions will ensure safe and effective anchoring. There are anchors
available for use in a wide range of applications. Load type will also
be a determining factor during the installation of concrete anchors.
With the right knowledge, research and a bit of patience, anchoring
concrete fasteners to block can be a simple process.
As with any anchoring project, it is important to keep safety in mind and follow instructions carefully. Always remember to wear safety goggles, handle all tools with extra care and follow all technical specifications. This article is meant to serve only as a basic explanation of concrete fasteners. Always refer to manufacturer's instructions or consult a contracting expert during any anchoring project.
Article Written By:
Bob Carlisle, President of Concrete Fasteners
Concrete Fasteners, Inc. has over 40 years of experience selling concrete fasteners. We can ship out one box or a whole pallet of concrete anchors. Our products are of the highest quality... "your satisfaction is guaranteed". We ship all orders the same day the order is received.