Concrete Construction
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Curing and Sustainability
Curing is the process in which the concrete is protected from loss
of moisture and kept within a reasonable temperature range. The
result of this process is increased strength and decreased permeability.
Curing is also a key player in mitigating cracks in the concrete,
which severely impacts durability.
In addition to curing being a fundamental
step in the concreting process, the drive for sustainability makes
paying particular attention to curing all that more important. When
smart, suitable, and practical curing is used, the amount of cement
required to achieve a given strength and durability can be reduced
by either omission or replacement with supplementary cementitious
materials. Since the cement is the most expensive and energy intensive
portion of a concrete mixture, this leads to a reduction in the
cost as well as the absolute carbon footprint of the concrete mixture.
It is also necessary that the concrete be
allowed to properly mature to provide the full service life. It
is this service life, which can extend to hundreds or thousands
of years, that provides concrete with the inherent ability to span
the needs of many generations. More on
curing concrete.
Focus
on Floors (Click
here or on individual topics.)
 The
following sections will help you through the various issues relating
to concrete slabs and floors (click on topic). You will find that
there is much more than you imagined to that hard surface you walk
on every day.
Building Tips for Trouble-Free Concrete Slabs
Concrete is the material of choice for driveways,
sidewalks, patios, steps, and for garages, basements, and industrial
floors. It is relatively inexpensive to install and provides an
attractive, durable surface that is easy to maintain. Proper attention
to the standard practices and procedures for constructing exterior
or interior concrete can yield a concrete surface that will provide
long-lasting, superior performance. Click
here for some building tips that will aid in the construction
of quality concrete projects.
Placing Contraction/Control Joints in Concrete
Flatwork
 The
most widely used method to control random cracking in concrete slabs
is to place contraction/control joints in the concrete surface.
As concrete hardens, there is a reduction in volume, often resulting
in cracking of concrete. Joints produce an aesthetically pleasing
appearance since the crack takes place below the finished concrete
surface. The concrete has still cracked, which is normal behavior,
but the absence of random cracks at the concrete surface gives the
appearance of an un-cracked section.
Contraction joints should be placed to produce panels that are as
square as possible and never exceeding a length to width ratio of
1 ½ to 1. Joints are commonly spaced at distances equal to
24 to 30 times the slab thickness and established to a depth of
¼ the slab thickness. Joints should be sawed as soon as the
concrete will withstand the energy of sawing without raveling or
dislodging aggregate particles. For most concrete mixtures, this
means sawing should be completed within the first 6 to 18 hours
and never delay more than 24 hours. More
about contraction joints.
Isolation/Expansion Joints:
Why and Where they are Used
 Isolation
joints are used to relieve flexural stresses due to vertical movement
of slab-on-grade applications that adjoin fixed foundation elements
such as columns, building or machinery foundations, bridge abutments,
light standards, drop inlets, and so on. The isolation joint material
allows the slab-on-grade to move up or down with the changes of
soil support conditions. Heaving/settling of moist soils due to
freeze/thaw cycles and long-term settlement are the primary causes
of changes in soil support conditions. In addition, an isolation
joint may be used in slabs that require a change in contraction
joint layout, which would create T intersections. The isolation
joint would be considered a free edge allowing the termination of
a contraction joint at a T intersection.
Expansion joints are used primarily to relieve stress due to confinement
of a slab. If the slab is placed adjacent to structures on more
than one face of the slab, an expansion joint should be placed to
relieve stress. For example, if a slab were placed between two buildings,
an expansion joint should be placed adjacent to the face of at least
one of the buildings. Confinement on three faces would normally
be handled by placing expansion joints on all three faces. Confinement
on four faces should be isolated on all faces. This allows for thermal
expansion and contraction without inducing stress into the system.
Finishing Air-Entrained Concrete
 Used
in many applications, air-entrained concrete is concrete that uses
air-entraining cement or an air-entraining admixture to produce
a system of small voids within the hardened cement paste. These
voids develop during the mixing process and stabilize through action
by the air-entraining mechanism. The primary use of air-entraining
concrete is for freeze-thaw resistance. The air voids provide pressure
relief sites during a freeze event, allowing the water inside the
concrete to freeze without inducing large internal stresses.
Hard troweling is a process by which a finisher
uses a steel trowel to densify the surface of the concrete. This
finish is optional and produces a hard, smooth surface. Hard-troweled
surfaces are not recommended for exterior concrete slabs, because
the smooth finish becomes slippery when wet. Hard troweling is also
not recommended for air-entrained concrete for several reasons.
More
on finishing air-entrained concrete.
Safety Measures for Concrete
Construction
 Concrete
construction is no exception to the importance of construction safety.
Although claiming one of the lower jobsite-injury rates, dangers
associated with both the material aspects and construction practices
of concrete construction must be addressed to continue the industry’s
focus on safety. Heightened awareness, improved safety training
programs, and diligent enforcement are the keys to improving safety
on the jobsite. More.
Bugholes
 One
of the primary influences affecting the surface aesthetics of concrete
is bugholes. Bugholes, pinholes, blowholes, surface voids –
they are recognized by various names, but all refer to a common
problem that contractors want to minimize. Bugholes, are small,
regular or irregular cavities (usually not exceeding 15 mm [9/16
in.]) resulting from entrapment of air bubbles on the surface of
vertically formed concrete structures during placement and consolidation.
More.
Conductive Concrete for Bridge Deck Deicing
 Heated
deck of Roca Spur Bridge in Nebraska is the world's first implementation
using conductive concrete for deicing. To read full article by Christopher
Y. Tuan, Ph.D., P.E., Associate Professor of Civil Engineering,
University of Nebraska click
here.
First Use of Ultra-High Performance Concrete
for an Innovative Train Station Canopy
 The
Shawnessy Light Rail Transit (LRT) Station, constructed during fall
2003 and winter 2004, forms part of a southern expansion to Calgary's
LRT system and is the world's first LRT system to be constructed
with ultra-high performance concrete (UHPC). To read the full article
by V. H. Perry and D. Zakariasen, Lafarge Canada Inc., click
here.
Pervious Concrete Placement
 Pervious
concrete mixtures are stiff, zero-slump mixtures requiring placement,
finishing, and curing requirements falling outside of normal concrete
flatwork processes. Due to the dry nature of the mixture (w/cm <
0.35) and high surface area, it is important to consider rapid concrete
placement methods. As these mixtures are not appropriate for pumping,
rapid placement methods may include chute placement directly from
the truck mixer, wheel barrow or buggies, conveyors, or dump placement
into an asphalt type paving machine. Regardless of the placement
method, remember that the quicker the placement, the better.
More on placing pervious concrete pavement.
Pervious Concrete and Durability
 Pervious
concrete can become clogged, which directly affects the hydrologic
performance and may indirectly affect other aspects of durability,
such as freeze-thaw resistance, deicer salt scaling resistance,
and sulfate resistance. Abrasion resistance of pervious concrete
is also of concern, particularly in locations that use snow plows
or have turning traffic. Carbonation and corrosion resistance are
not concerns with pervious concrete as it is neither recommended
nor necessary to use reinforcing steel bars or welded wire reinforcement.
More on concrete
and durability.
"Bendable Concrete" Replaces Bridge
Expansion Joints
 University
researchers have collaborated with a state Department of Transportation
to apply bendable concrete in a local bridge project. Engineered
Cementitious Composites (ECC) have been shown to have all of the
characteristics sought by highway designers and structural engineers
for a highly durable concrete material. The distinctive property
of ECC is the ability to bend while maintaining its compressive
strength. These properties make the material a good fit for use
in place of bridge expansion joints as demonstrated in this innovative
project. More.
Concrete Shines as Solar Reflectance Material
Concrete does a very good job of reflecting solar
energy. That is the finding from a recent PCA study which measured
the solar reflectance of 135 concrete specimens from 45 mixes representing
exterior concrete flatwork. In fact, all concretes tested in this
study would qualify for LEED® credits for Heat Island Effect.
Solar reflectance index (SRI), a calculated
value based on solar reflectance, SR, is one way to determine how
much light energy a material reflects: stated another way, comparing
SRI or SR of different materials tells which ones absorb less solar
radiation. This is useful because darker materials absorb more heat,
which is generally considered undesirable for its effect on the
environment. This may have an immediate, local effect, like heat
gain in urban areas (heat island).
More.
Read the complete report describing test procedures,
concrete mixes, materials, and other aspects of this study, Solar
Reflectance of Concretes for LEED Sustainable Sites Credit: Heat
Island Effect (SN2982).
Hot Weather Concreting
When the temperature of freshly mixed concrete approaches approximately
25°C (77°F) adverse site conditions can adversely impact
the quality of concrete. Ambient temperatures above 32°C (90°F)
and the lack of a protected environment for concrete placement and
finishing (enclosed building) can contribute to difficulty in producing
quality concrete.
 The
precautions required to ensure a quality end product will vary depending
on the actual conditions during concrete placement and the specific
application for which the concrete will be used. In general, if
the temperature at the time of concrete placement will exceed 25°C
(77°F) a plan should be developed to negate the effects of high
temperatures. More
on the suggested precautions for hot weather concreting.
Cold Weather Concreting
 Cold
weather concreting is a common and necessary practice, and every
cold weather application must be considered carefully to accommodate
its unique requirements. The current American Concrete Institute
definition of cold-weather concreting, as stated in ACI 306 is,
“when the air temperature has fallen to, or expected to
fall below 4°C
(40°F)
during the protection period.”
This definition can potentially lead to problems with freezing
of the concrete at an early age.
Rule number one is that ALL concrete must be protected from freezing
until it has reached a minimum strength of 3.5 MPa (500 psi),
which
typically happens within the first 24 hours. In addition, whenever
air temperature at the time of concrete placement is below 4°C
(40°F) and freezing temperatures within the first 24 hours
after placement are expected, the following general issues should
be considered:
(1) Initial concrete temperature as delivered; (2) Protection while
the concrete is placed, consolidated, and finished, and (3)
Curing
temperatures to produce quality concrete. More
about cold weather concreting.
Drying of Concrete
Unwanted moisture in concrete floors routinely causes millions
of dollars in damage to buildings in the United States. Problems
from excessive moisture include deterioration and de-bonding of
floor coverings, trip-and-fall hazards, microbial growth leading
to reduced indoor air quality, staining, and deterioration of building
finishes.
The terms curing and drying are frequently used interchangeably
with regard to the moisture condition of new concrete slabs. More
on the difference between curing and drying.
The term “concrete moisture” is understood to mean
the total water used in the concrete batch, plus curing water, minus
the water bound in hardened cement due to hydration. Drying begins
when water is no longer available at the exposed surface. More on
how long it takes concrete to dry.
The moisture content of concrete must be viewed from the context
of total water content of the fresh concrete mixture and the available
moisture content of the hardened concrete. The total water content
of a fresh concrete mixture is a function of the total cementitious
materials and water cement ratio (w/cm). Read
an FAQ about the moisture content in concrete.
Vapor Retarders Protect Floor Finishes over Slabs
 Vapor
retarders are sheet materials used under concrete slabs on ground
to restrict the flow of moisture vapor from the subgrade into and
through the slab. Moisture migration through concrete slabs can
lead to microbial growths and failures of adhesives, flooring coverings,
and coatings. Therefore, all concrete slab-on-ground floors that
will receive floor coverings or coatings must have a vapor retarder
below the slab.
Vapor retarders are produced to meet specifications such as ASTM
E1745, Standard Specification for Water Vapor Retarders Used
in Contact with Soil or Granular Fill Under Concrete Slabs,
or ASTM D4397, Standard Specification for Polyethylene Sheeting
for Construction, Industrial, and Agricultural Applications.
Construction practice and placement of vapor retarders has been
the subject of much debate for many years. Some experts believe
that concrete placed directly on a vapor retarder will bleed excessively,
warp and crack more frequently, and take longer to dry than a slab
placed on a compacted granular subbase. Other experts believe that
vapor retarders function best to exclude moisture when directly
below the concrete with no intervening material that can act as
plenum space for the passage of moisture. ACI Committee 302 recommends
that any floor that will receive a moisture-sensitive finish should
have a vapor retarder directly under the concrete slab with no intervening
blotter or cushion layer. More
about vapor retarders.
Designing Mixtures to Reduce Shrinkage Potential
 Producing
a concrete mixture with low shrinkage potential for flat work applications
is of high importance. Early-age volume changes can cause significant
shrinkage and cracking, but the focus of this article will be the
shrinkage caused by the drying of concrete after the slab has been
cured. As concrete shrinks, tensile forces develop and may lead
to warping and cracking of the slab.
Common factors for drying shrinkage potential include
coarse aggregate type, aggregate size and volume, water content
of the mixture, mixture temperature, cement type, and chemical admixtures.
More on reducing shrinkage potential.
Identifying and Evaluating Concrete Defects
 Concrete
structures are regularly constructed without complications. However,
defects can occur that can be traced to problems related to environmental
conditions during construction or with the concreting procedures
used. In order to determine a repair method, it is necessary to
identify what caused the defect . Evaluation of deficiencies helps
ensure that repairs will be effective and the defect will not extend
into the surrounding concrete.
Many concrete defects are immediately recognized and others are
not. Concrete defects can be broken down into four broad groups
based on visual observation: deformation of the surface, cracking
of the surface, disintegration of the surface, and other defects.
Visual examination typically does not provide
enough information to determine the cause or causes of the defect.
In some cases, it may not provide evidence of a defect at all. In
order to narrow the scope of an investigation to probable causes
and suitable repair methods, the appropriate information factors
and the proper evaluation methods need to be identified. More
on concrete defects.
The Perils of Power Washing
 It
is springtime and that means spring cleaning. Building owners and
property managers will be looking at every exposed surface on their
property and then deciding how to clean those surfaces. The cleaning
method chosen for concrete requires careful consideration: Power
washing would seem to be a good choice because, after all, it is
simple, fast, and effective. Right? Not so fast…
Power washing concrete surfaces can cause real problems. Relatively
inexpensive high-pressure power washing units are commonly available.
Some of those units can deliver water at pressures well in excess
of 6,000 psi! Moreover, it is not just high-pressure water that’s
the problem. The water exits the nozzle at both a high pressure
and a high velocity. The resulting momentum is great enough to dislodge
not only dirt and debris but also to create flakes, popouts, and
even concrete spalls. Good quality concrete will also experience
accelerated wear from high-pressure power washing. More
on power washing concrete.
Quality Control of Pavements; Issues and Test Methods
The final quality of concrete pavement may be affected
by many issues including the following:
| Cement fineness - cement fineness
effects heat of hydration, water demand, strength gain characteristics,
and workability. Changes in fineness can introduce the possibility
of incompatibilities between cement and chemical admixtures,
shorten setting times, effect setting characteristics complicating
timing for sawing of joints. |
| Aggregate gradation - changes
in aggregate gradation, increased fineness or void content,
may increase water demand, workability, and paste requirements.
These changes may increase the potential for segregation, bleeding,
and shrinkage. |
| Water-cement ratio (w/c) -
accurate control of water-cement ratio is a key component to
control of strength properties of the material. |
For a complete list of issues and test methods
that are recommended for quality control of concrete pavements,
click here.
|