WHITE CAP
Course 8 Script
The Elements of Design and Construction of a Concrete Slab on Ground
Slide 1: Welcome
Thank you for your interest in the design and construction of concrete slabs on ground. Because
excessive moisture in concrete slabs causes failures of impermeable flooring, we’ll discuss the
importance of vapor retarders and allowing the slab to dry. Other important features of a durable slab
come from a well-cured surface, proper levelness, and using proper shear transfer devices to keep joints
aligned.
This session will also introduce you to how moisture enters a slab, flooring failures that result from
excessive vapor transmission, tests for measuring a slab’s moisture content, the use and installation of
joints and joint fillers, and safety equipment recommended for working with concrete slabs.
Slide 2: Instructions
Before we begin, let me point out a few features.
On the left, you’ll see two tabs. Use the Notes tab to follow this narration. Click on the Outline tab to track
your progress through this course. You can click on a section or page to navigate back to it.
On the top right, you’ll see Attachments and Exit. Click on Attachments to download additional resources
for this program. Click on Exit to leave this program at any time. Your progress through the course will be
saved. When you open the course again, you’ll start at the slide you last completed.
On the bottom, there are three buttons: the Play/Pause button and the Forward and Backward buttons.
Use the Play or Forward buttons to advance to the next page. Use the Backward button to go back to a
previous page. Use the Pause button to pause the program briefly (for instance if you need to stop to take
a note). Click Pause again to resume the program.
Click the Play or Forward button to advance to the next page. Please note that you cannot skip sections
of this program. You must view them in order, and you must complete each page before advancing to the
next. As you progress through the course, you’ll know you can advance when the Play/Pause button
blinks light blue and the narration ends.
Slide 3: Recommended Connection
To ensure a successful connection as you complete the course, we recommend the following:
 Use a recommended Web browser: Microsoft Internet Explorer 7 or Microsoft Internet Explorer 8.
 Use a wired broadband connection. Using a laptop with a wireless broadband connection may
contribute to a loss of connection.
 Keep ALL windows associated with the course open while completing the course. If a window in
your browser opens during the process of starting the course, then leave it open.
 Close all other programs.
Slide 4: Course Progress
PLEASE NOTE: This program tracks your course progress so that you can save your session and return
at a later time. If you are presented with an error message while completing or exiting the course, or if you
believe that you have completed a course but do not see the course list on the “My Training” page under
“Your Courses Completed,” please email wctrade@hdsupply.com. The TRADE Center will still record
your progress, which will be used to confirm course completions.
Slide 5: Course Agenda
This course is separated into four sections: mitigating moisture transfer through concrete slabs, using
shear transfer devices to help create better joints in slabs, understanding the use and installation of
control joints, and safety equipment when working with concrete slabs. We recommend that you take
notes while viewing this presentation to help you learn the information presented.
After each section, there will be a short learning check, or assessment. You’ll have to pass the
assessment before advancing to the next section.
Slide 6: Learning Objectives
After completing this course, you will be able to properly answer customer questions about the design
and construction of slabs on ground.
By the end of this course, you will understand:
1. How to test for moisture in a slab and why vapor barriers are important.
2. Why stable joints are important and the different types and uses of shear transfer devices.
3. How to use and install joints and the different types of joint fillers.
4. Safety equipment when working with concrete slabs.
SECTION 1: MITIGATING MOISTURE TRANSFER THROUGH CONCRETE SLABS
Slide 7
Section 1 will discuss the importance of mitigating moisture transfer through concrete slabs. During this
program, you will learn how moisture enters a building through the subgrade and concrete slab; what
happens when flooring is placed on concrete slabs with high moisture levels; and how to test a slab’s
moisture content.
Slide 8
Excessive moisture content in concrete slabs is a major cause of flooring failures.
A hardened Class 4 industrial concrete floor may look solid and impermeable, but there are thousands of
small interconnected pores that initially held water when the concrete was placed. After placement, some
of the water combines with the cement (hydration) and the rest slowly evaporates from the slab and the
pores dry out.
Slide 9
Moisture in concrete may exist as either water (when the concrete is wet and the pores are saturated) or
water vapor, which provides a level of relative humidity within the concrete material.
The initial source of moisture in concrete is the mixing water used at the time of manufacture. Once the
concrete is placed, there are numerous other sources of moisture, including wet curing, wet subgrades,
condensation, and water-based adhesives.
Slide 10
A vapor barrier is a plastic sheeting used under a concrete slab to block moisture migration and capillary
action. Without a vapor barrier, moisture and dampness enter the concrete over time, causing flooring
issues, mold, and costly repairs to get a dry surface.
The sheeting product blocks moisture and dampness that is migrating through the concrete slab to
interior spaces. However, a vapor barrier does not help a newly placed concrete slab dry out. In fact,
concrete takes longer to dry when a vapor barrier is used.
Slide 11
Without a vapor barrier below the slab, the concrete acts like a sponge, continually drawing moisture from
the subgrade. This is because the relative humidity at the bottom of the slab is 100 percent, but is lower
at the surface of the slab. Problems arise when the moisture movement is excessive because this
moisture will accumulate beneath any impervious flooring placed on top of the slab.
Slide 12
The severity of the moisture problem will depend on the subgrade, the water-to-cement ratio of the
concrete placed, and whether a vapor barrier was placed below the slab. Usually, the only time moisture
rising up through the concrete slab is a concern is if flooring will be placed on the slab. But moisture can
also accumulate beneath materials stored on a slab and can discolor decorative concrete and can cause
efflorescence on the concrete surface.
If a slab’s moisture content is too high, the adhesives or mastics used to secure flooring products to the
concrete will not bond properly. Flooring manufacturers will often prescribe an acceptable range of
moisture vapor emission rate from the slab surface or relative humidity within the slab.
Slide 13
Moisture problems can be deceiving. Proper testing is the only means of detection. Newly placed
concrete slabs can require as long as four weeks of curing time per inch of slab thickness just to
evaporate the excess water used during placement. But if moisture is getting in from below, the slab will
never dry out.
Slide 14
The permeability of a concrete slab is affected by the water-to-cement ratio of the concrete when it is
placed. Placing concrete with a lower water-to-cement ratio reduces the permeability, i.e., reduces the
amount of moisture moving through the slab after it has fully cured.
Slide 15
Projects put on a fast track can cause moisture problems. It’s important to allow enough time for concrete
to cure and dry out before flooring is installed. Ideally, this will occur with the building closed in and the
air-handling system operating to reduce the relative humidity in the air above the slab. To ensure
accurate floor moisture test results, the slab should be exposed to its finished environment for a sufficient
amount of time prior to testing, usually up to 28 days.
Slide 16
When a concrete slab has a high relative humidity, the moisture (water vapor) will diffuse through the slab
until it reaches the impermeable floor coating at the surface. Moisture condenses and is trapped beneath
the floor covering, often causing defects such as bubbles, blisters, and delaminations. The key to
successfully placing floor coverings on a concrete slab is to wait until the slab has dried sufficiently and to
have a vapor barrier beneath the slab to prevent moisture from migrating in from the ground below.
Slide 17
Some obvious signs of excessive vapor transmission are:
 bubbled coatings or toppings
 soft or re-emulsified adhesives
 curled or cracked tiles
 carpets that mold and mildew
 buckled or warped wood floors
Slide 18
Vapor barriers are recommended to be installed below the slab. Most experts today recommend that the
concrete be placed directly in contact with the vapor barrier. In the past, vapor barriers were sometimes
placed and then topped with a granular fill to protect the barrier during construction and to allow the slab
to also dry from the bottom. This practice may reduce curling of the slab at the joints. New, thicker
moisture barriers (10 to 15 mil) are much less susceptible to damage during construction and proper mix
design can eliminate excess curling.
Slide 19
Changing ambient conditions above the concrete surface will affect the moisture in the slab. If flooring
contractors install their products too early on a concrete slab that is still drying, the products often fail. It’s
preferable to test the slab after the HVAC has been activated and the slab has equilibrated with the
environment. Assuming there is a good vapor barrier in place, the drier air in the building will maximize
drying in the slab.
Slide 20
On-site concrete moisture management relies on an understanding of changeable conditions, testing
methods, and timing. To ensure accuracy, a slab’s relative humidity should be tested in the environment
in which the flooring will be installed. The International Concrete Repair Institute has a certification
program for moisture testing technicians.
Slide 21 INTERACTION – Test Methods for Measuring Moisture Content (TAB INTERACTION)
Instructions:
Click on the tab on the left to hear a description of each type of test method for moisture content. Please
note that the narration will provide additional information to the text that is presented on the screen. This
additional information is available to download by using the Attachments link at the top right. You can
click on the image to enlarge it, and then click on it again to shrink it.
There are three common test methods for measuring a slab’s moisture content: the calcium chloride test,
handheld concrete moisture meters, and in-situ relative humidity testers.
Calcium Chloride Test
The calcium chloride test, ASTM F-1869, Standard Test Method for Measuring Moisture Vapor Emission
Rate of Concrete Subfloor Using Anhydrous Calcium Chloride, measures moisture migration at the slab’s
surface and at a very shallow depth. The test is often conducted using a self-contained kit. Not all kits
meet ASTM requirements.
The test procedure is simple. A sample of bone-dry calcium chloride is weighed and then placed on the
slab’s surface under an air-tight hood, where it is exposed to the moisture that is in the top of the slab.
After a predetermined amount of time, the hood is removed and the calcium chloride is reweighed. Since
calcium chloride absorbs moisture, the sample will weigh more based on the amount of moisture emitted
from the slab.
The test provides a rough estimate of the volume of moisture that emits from 1,000 square feet of slab
surface in 24 hours. Typical results required by flooring manufacturers are 4 to 6 pounds. Note that this
test only indicates the moisture in the top ½ inch of the concrete slab.
Handheld Concrete Moisture Meters
Handheld concrete moisture meters use electromagnetic fields to detect the presence of moisture in a
concrete slab. However, their signal can penetrate only the very thin surface of the slab, and the signal
can be distorted by interacting with metal in the concrete, giving a false reading of the slab’s moisture
condition.
Since handheld moisture meters cannot accurately measure relative humidity, they are rarely used to
determine when to install a floor covering. They do, however, indicate variations in moisture in the floor
and can be used to locate relatively damp spots for further investigation.
Relative Humidity Testers
Many contractors prefer to use testing procedures that use in-situ probes to measure a slab’s relative
humidity. Since they are less susceptible to environmental or ambient changes at the surface, their depthspecific application is able to accurately represent moisture conditions in the slab.
ASTM F-2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using
In-Situ Probes, is the test method used. It is the best predictor of concrete moisture levels and, therefore,
is the preferred test method for monitoring moisture conditions in a concrete slab. A hole is drilled into the
slab to a depth of 40 percent of the slab’s thickness. A relative humidity probe is inserted to the bottom of
the hole and the relative humidity is measured. This provides an indication of the relative humidity of the
slab once the surface is sealed with impermeable flooring. This value should be no higher than 85
percent for a successful flooring installation.
Slide 22: Section 1 Summary
This section discussed how moisture enters a building through the subgrade and concrete slab; what
happens when flooring is placed on concrete slabs with high moisture levels; and three ways to test a
slab’s moisture level.
Slide 23: Section 1 Assessment Placeholder
This short assessment will test your knowledge of mitigating moisture transfer through concrete slabs.
You must answer eight of ten questions correctly to advance to Section 2. You will receive feedback after
answering each question.
If you do not pass on your first attempt, that’s OK. You will be directed to the beginning of the section
where you can review the concepts again before re-attempting the assessment. Click Next to begin.
SECTION 2: STABLE JOINTS AND SHEAR TRANSFER DEVICES
Slide 24
Section 2 will describe the importance of stable joints and the different types and uses of shear transfer
devices.
Slide 25
Stable joints are important because industrial floors need to be flat to provide a smooth surface; flatness
is especially crucial at transition points such as joints. Industrial floors often have to withstand heavier
floor loads (such as heavy trucks and forklifts) than residential floors, necessitating the use of joints and a
way to transfer shear loads across the joints to prevent joint breakdown. If the slab sections on each side
of a joint deflect relative to one another then that movement will break down the edges of the joint.
One way to maintain smooth transitions at joints is to install devices into the concrete slab that transfer
shear loads. These devices prevent joint breakdown by locking the slabs together on either side of the
joint.
Slide 26
Stable joints enable traffic to smoothly cross from one section of concrete to another. As a heavy vehicle
approaches a joint, the slab may deflect slightly downward from the weight of the vehicle. This deflection
causes the top edge of the loaded slab to contact the concrete slab on the opposite side of the joint. The
concrete-to-concrete contact will eventually degrade the joints and cause the concrete to break.
Slide 27
To prevent this movement, the two sections of concrete are tied together at the joints one-eighth of an
inch apart vertically so that one side cannot deflect independently. Having good shear transfer will ensure
the integrity of the joints, thus prolonging the life of the floor.
Slide 28 INTERACTION: Shear Transfer Devices (TAB INTERACTION)
Instructions:
Click on the tab on the left to hear a description of each type of shear transfer device. Please note that
the narration will provide additional information to the text that is presented on the screen. This additional
information is available to download by using the Attachments link at the top right. You can click on the
image to enlarge it, and then click on it again to shrink it.
There are three common types of shear transfer devices: keyways, dowels, and diamond-shaped load
plates.
Keyways
The American Concrete Institute (ACI) defines a keyway as an interlocking tongue-and-groove design
that provides shear strength at a joint. The keyway’s design keeps the slabs on either side of the joint
aligned vertically.
Keyways come in two types: trapezoidal and half-round. The main difference is in the shape of the
groove. Both types perform in essentially the same way. It is important to position keyways in the center
of the slab. If they are too close to the surface, loads along the keyway can create concrete spalling.
Keyways also reduce edge stress, which prevents possible cracking along the edge.
Contractors create keyways as they install the forms for slab. They attach a wooden or metal strip on the
form. When the form is removed following concrete placement, the hardened concrete has a groove.
When the concrete is placed on the other side of the joint, it fills into the groove, locking the sides into
position. Be aware that keyways are not recommended for floors that will handle heavy loads. The
shrinkage of the concrete will open the keyway enough that the joint will not maintain enough stability to
prevent damage of the joint edges.
Dowels
Dowels are steel bars that are inserted into concrete at joints to transfer shear loads. Dowels can be
round, square, or rectangular in cross-sectional shape. Dowels must be smooth, with no sharp edges that
could catch in the concrete.
Dowels are used in contraction joints. When the anticipated loading is heavy, dowels are used rather than
keyways. Doweled contraction joints provide tighter joints, often leaving no gap between two sections of
concrete.
To be effective, dowels must be properly aligned: perpendicular to the joint both vertically and
horizontally. Misaligned dowels can lead to joint deterioration. The best way to keep dowels properly
aligned in a construction joint is to drill holes through the bulkheads, or use a proprietary bulkhead form
that aligns the dowels.
Dowel baskets are prefabricated jointing devices that are ideal for saw-cut contraction joints. They look
like baskets in that the dowels are held in place by steel wires so that they are parallel to the slab’s
surface. These devices are placed on the subgrade prior to concrete placement. After the concrete
hardens, workers return to saw the joints directly above the embedded dowels. The steel positioning
wires help maintain the dowel’s position during concrete placement.
Diamond-Shaped Load-Plates
Diamond-shaped load-plates are a newer, and very effective, shear load transfer device. Embedded in
the concrete during placement, the devices transfer loads across a construction joint without creating
restraint in any direction, meaning the slab is free to shrink at the joints in both horizontal directions.
ACI also refers to these devices as deformed plates thicker than one-quarter inch that have horizontal
deformations or corrugations.
ACI 302, 1R-04, Guide for Concrete Floor and Slab Construction, and ACI 360R-06, Design of Slabs on
Ground, provide information on sizes and spacing for dowels and diamond-shaped load plates.
Slide 29: Section 2 Summary
This section discussed the importance of stable joints and the different types and uses of shear transfer
devices, such as keyways, dowels, and diamond-shaped load plates.
Slide 30: Section 2 Assessment Placeholder
This short assessment will test your knowledge of the importance of stable joints and shear transfer
devices. You must answer six of eight questions correctly to advance to Section 3. You will receive
feedback after answering each question.
If you do not pass on your first attempt, that’s OK. You will be directed to the beginning of the section
where you can review the concepts again before re-attempting the assessment. Click Next to begin.
SECTION 3: JOINTS AND JOINT FILLERS
Slide 31
Section 3 will describe the use and installation of joints and the different types of joint fillers.
Slide 32
Concrete expands and contracts with moisture and temperature changes. These volume changes may
produce random cracks in hardened concrete unless they are properly controlled.
Provision for volume changes at predetermined locations prevents a concentration of crack-producing
stress forces. Such provisions are called contraction joints. These joints help eliminate unsightly random
surface cracks by gathering, distributing, and dissipating stresses resulting from temperature and
moisture variations.
Slide 33 INTERACTION: Joints (TAB INTERACTION)
Instructions:
Click on the tab on the left to hear a description of each type of joint. Please note that the narration will
provide additional information to the text that is presented on the screen. This additional information is
available to download by using the Attachments link at the top right. You can click on the image to
enlarge it, and then click on it again to shrink it.
Contractors use joints to help control cracking in concrete. There are three common joints found in slab
construction: isolation joints, contraction joints, and construction joints.
Isolation Joints
Isolation joints are used on a concrete slab when it abuts another surface, such as another concrete slab,
a foundation, pipe, wall, or column. Isolation joints relieve stresses that result from the concrete
contracting and expanding.
To allow for contraction and expansion at these joints, materials such as cork, rubber, fiberboard, plastic,
asphalt impregnated fiberboard, and neoprene are used. Isolation joint material should run the full depth
of the slab and just below the surface of the slab, which can then, if required, be filled with a sealant.
Contraction Joints
Contraction joints are weakened planes constructed in the concrete slab for the purpose of controlling
shrinkage stresses to that specific area. The joints are placed at predetermined points of possible stress
concentration. Thin joints spaced at frequent intervals are more effective than thicker joints spaced less
frequently.
Contraction joints are installed in a number of ways. Plastic joint systems create effective contraction
joints in concrete slabs and can be easily installed during concrete placement. Plastic joint systems come
in a number of configurations and depths for different slab thicknesses.
A commonly used plastic joint system is one that is pressed into the top of the concrete slab while it is
green. Contractors often use a chalk string line to mark the slab. The plastic joint is then cut to the proper
length and laid next to the line. The contractor then presses the joint system into the concrete, creating a
straight joint.
Another style of plastic joint is a cast-in-place system. Following placement, contractors peel off the top
for a crisp-looking joint.
Sawing is the most common method of creating contraction joints on industrial floors. Contractors use
electric- or gasoline-powered circular saws fitted with either reinforced abrasive blades or metal-bonded
diamond blades. Sawed joints are uniform in width and depth and have straight sides with sharp edges.
Water is generally required as a coolant for the blades to dissipate frictional heat.
There are two techniques for sawing joints. The wet cut method requires the concrete to be sufficiently
hardened to support the weight of the saw and for the edge of the joint not to ravel during sawing. Sawing
normally occurs about 18–24 hours after placement (this time delay can increase the likelihood of random
cracking). The depth of the cut needs to be at least one-quarter the depth of the slab.
A more preferred method is early entry sawing. Contractors use a lightweight saw that allows them to cut
within six hours after placement. This method also allows the contractor to make a shallower cut. The
joint’s full depth is created, not by the saw cut, but by the shrinkage of the concrete itself. As a result of a
shallower cut, the slab interlocks better after cracking below the cuts at the contraction joints (aggregate
interlock).
Construction Joints
Construction joints are used to allow slab movement at transitional load bearing points from one concrete
slab to the next. A construction joint must allow relative horizontal transition, which is typically due to the
thermal expansion and contraction or humidity-induced shrinkage. At the same time it must not allow
relative vertical transition and relative rotation due to traffic movement and load bearing weights.
Construction joints are formed using some sort of bulkhead, made of wood, steel, plastic, or precast
concrete. These bulkheads are often used as screed rails during placement and finishing of the slab.
Construction joints should be worked into the overall joint plan, where they can also function as
contraction joints.
Slide 34
Another way to maintain a high-quality joint is to install special joint fillers. These products help prevent
the edges of joints in heavy-traffic floor areas from being broken off, and help restrict dirt or water from
entering the joint. Fillers help transfer movement and heavy loads at concrete joints but do not transfer
shear (vertical) loads. Joint fillers do not take the place of shear transfer devices and do not increase joint
stability.
The American Concrete Institute (ACI) and Portland Cement Association (PCA) recommend using semirigid fillers for filling joints in warehouse and industrial floors. Prior to installing joint fillers, contractors
must thoroughly clean the area, removing any debris from the depression. Common joint fillers are either
an epoxy resin or polyurea.
Slide 35
If the filler is an epoxy resin, contractors will use either a caulk gun or a specially designed hand pump.
Epoxy installations require two applications. It is recommended to wait at least 30 to 90 minutes between
applications, depending on ambient temperature. This allows the caulking contractor ample time to
inspect the first-course installation for any gaps on the side of the joint filler.
Slide 36
Polyurea joint fillers are placed using a pump machine in a one-step process. These materials set rapidly.
Slide 37
To construct a flush surface across the joint with either joint filler material, contractors are instructed to
overfill the joint. Once the joint filler has hardened, the contractor cuts off the material at about the level
plane of the floor.
There can, however, be a problem with this technique. When joints are overfilled with either epoxy or
polyurea, the material removed from the concrete may leave stains. If so, contractors can use stainpreventing products on the concrete surface.
ACI and PCA recommend allowing the slab to cure for two to three months prior to using joint filler. Doing
so will allow the slab to stabilize, reducing the possibility that the filler will crack. It is also recommended
that a joint filler be applied only after the slab has stabilized, within the first two to three months, in the
climate in which it will be used.
Slide 38: Section 3 Summary
This section discussed the use and installation of joints, and the different types of joint fillers. We learned
that contractors use joints to help control cracking in concrete, and that there are three common joints
found in slab construction: isolation joints, contraction joints, and control joints.
Slide 39: Section 3 Assessment Placeholder
This short assessment will test your knowledge of joints and joint fillers. You must answer eight of ten
questions correctly to advance to Section 3. You will receive feedback after answering each question.
If you do not pass on your first attempt, that’s OK. You will be directed to the beginning of the section
where you can review the concepts again before re-attempting the assessment. Click Next to begin.
SECTION 4: SAFETY EQUIPMENT
Slide 40
Section 4 will describe safety equipment when working with concrete slabs.
Slide 41
When working with concrete materials, workers need to wear the right clothes and protective gear. White
Cap sells a number of different eye, ear, head protection gear in addition to gloves and boots.
Workers should wear long pants, long sleeve tops, shoes, waterproof gloves, and protective eyewear
when working with concrete slabs. Powdered concrete can produce a lot of dust that can irritate lungs
and eyes; blowing dust, splattering concrete, and other foreign objects can also irritate eyes, so workers
should wear goggles and face masks. Respirators may be needed in poorly ventilated areas.
Slide 42
Do not let concrete come in direct contact with skin as it may cause irritations. Waterproof pads should be
used between fresh concrete surfaces and knees, elbows, and hands to protect the body during finishing
operations. Waterproof boots should be worn when standing in wet concrete. Ensure safety by isolating
all electrical equipment through a transformer.
Slide 43: Section 4 Summary
This section reviewed safety equipment workers will need when working with concrete slabs. We learned
that the body should be protected from direct contact with concrete, and that eye, ear, and head
protection is recommended, in addition to waterproof gloves and boots when working with wet concrete.
We also reviewed isolating all electrical equipment through a transformer.
Slide 44: Learning Game: Safety Equipment
Instead of a formal assessment, we will complete a learning game on safety equipment. You must reveal
the hidden answer one letter at a time, before time runs out. You will have 1 minute to answer each
question. You will be given 2 attempts before you can advance to the next question.
Slide 45: Congratulations
Congratulations! You have completed The Elements of Design and Construction of a Concrete Slab on
Grade. Before you go, there are a few things we’d like to share with you.
Slide 46: Course Summary
We learned that well-cured and stable concrete slabs are crucial for successful flooring installation. We
also learned that excessive moisture in concrete slabs can cause floor failures and that shear transfer
devices help keep slabs aligned. We followed the progression of how moisture enters a slab, explained
how vapor barriers work, discussed flooring defects that result from excessive vapor transmission,
reviewed the tests for measuring a slab’s moisture content, described the use and installation of joints
and joint fillers, and reviewed safety equipment when working with concrete slabs.
Slide 47: Recap
You should now be able to:
1. Test for moisture in a slab and recommend vapor barriers.
2. Explain why stable joints are important and list the different types and uses of shear transfer
devices.
3. Understand the use and installation of joints and different types of joint fillers.
4. Recommend safety equipment.