| Concrete Design & Production
Concrete is a mixture of two components: aggregates
and paste. The paste, comprised of cement and water, binds the aggregates
(usually sand and gravel or crushed stone) into a rocklike mass
as the paste hardens because of the chemical reaction of the cement
and water. Supplementary cementitious materials and chemical admixtures
may also be included in the paste.
For more on concrete basics, click
Materials for Use in Concrete
set and harden by reacting chemically with water. During this reaction,
called hydration, cement combines with water to form a stonelike
mass, called paste. When the paste (cement and water) is added to
aggregates (sand and gravel, crushed stone, or other granular material)
it acts as an adhesive and binds the aggregates together to form
concrete, the world’s most versatile and most widely used
construction material. More.
cementitious materials are generally divided into:
- Pozzolans (fly ash, silica fume, and natural
pozzolans, such as calcined shale, calcined clay or metakaolin)
- Slag cements
These materials when used in conjunction with portland or blended
cement, contribute to the properties of the hardened concrete through
hydraulic activity, pozzolanic activity, or both.
Cementing Materials For Use in Concrete CD
Fly Ash, Slag,
Silica Fume, and Natural Pozzolans
Design and Control
of Concrete Mixtures
Materials for Use in Blended Cements
Benefits of Ternary Mixtures
are classified by ASTM C33 (AASHTO M 6/M 80) as fine or coarse.
Fine aggregate consists of natural sand, manufactured sand, or a
combination thereof with particles that are typically smaller than
5 mm (0.2 in.). Coarse aggregate consists of either (or a combination
of) gravel, crushed gravel, crushed stone, air-cooled blast furnace
slag, or crushed concrete,
with particles generally larger than 5 mm (0.2 in.). The maximum
size of the coarse aggregates is generally in the range of 9.5 to
37.5 mm (3/8 to 1 ½ in.).
More on why we use aggregates in concrete.
More on recycled aggregates,
Content: Why Less Is More
The quality of hardened concrete is greatly influenced by the amount
of water used in relation to the amount of cement. Higher water
contents dilute the cement paste (the glue of concrete). Here are
some advantages of reducing water content:
- Increased compressive and flexural strength
- Lower permeability, thus increased watertightness and lower
- Increased resistance to weathering
- Better bond between concrete and reinforcement
- Less volume change from wetting and drying
- Reduced shrinkage and cracking
ASTM Specification for Mixing
More about Adding Water On-Site.
are those ingredients in concrete other than portland cement, water,
and aggregates that are added to the mixture immediately before
or during mixing.
For basics on chemical admixtures, click
A wide range of admixtures are available. The table below provides
a list of common types of chemical admixtures. The effectiveness
of an admixture in concrete depends upon many factors including
cementitious materials properties, water content, aggregate properties,
concrete materials proportions, mixing time and intensity, and temperature.
Type of Admixture
|ASTM C260 and C233 (AASHTO M 154 and T 157).
||To stabilize microscopic bubbles in concrete, which can provide
freeze-thaw resistance and improve resistance to deicer salt
|Water reducing admixture (WR)
||ASTM C494 (AASHTO M 194)
||Reduce the water content by 5 to 10%, while maintaining slump
|Mid-range water reducer (MRWR)
||ASTM C494 (AASHTO M 194)
||Reduce the water content by 6% to 12%, while maintaining slump
and avoiding retardation.
|High-range water reducer (HRWR)
(also called superplasticizer)
|ASTM C494 (AASHTO M 194),
|Reduce the water content by 12% to 30%, while maintaining
||ASTM C494 (AASHTO M 194)
||To decrease the rate of hydration of cement.
||ASTM C494 (AASHTO M 194)
||To increase the rate of hydration of cement.
||Reduce drying shrinkage (and related cracking) in concrete
||Reduce or eliminate deleterious expansion due to alkali-silica
||Minimize steel reinforcement corrosion
Identifying Material Incompatibilities
The wide variety of materials options and mix proportions possible
in concrete allows it to be customized for a wide range of applications
and placement and service environments. However, the cementitious
materials (cements, fly ashes, slag cements, etc.) and chemical
admixtures (accelerators, retarders, water reducers, etc.) are all
chemically complex and this complexity can lead to problems when
they don’t work together properly. Even when all materials
meet and exceed their specification requirements individually, problems
can arise under field conditions.
Although these problems are relatively rare, the resulting construction
delays, performance issues, and loss of confidence in concrete as
the preferred construction material are unacceptable. FHWA and PCA
co-sponsored research into these phenomena, with the goal of minimizing
or preventing these problems in the field. The project developed
relatively simple protocols for evaluating concrete material combinations
both pre-construction and during construction. More
on identifying material incompatibilties.
cracking can be a significant problem in concrete. Early age for
concrete is the first seven days starting with final set, which
is when the concrete has obtained a benchmark level of stiffness.
During this time, concrete undergoes a significant amount of volume
change caused by many variables, such as the hydration reaction
(chemical shrinkage), water content (drying shrinkage and swelling),
and temperature changes (thermal dilation).
Volume changes in concrete will drive tensile stress development
when they are restrained, which is the case with most concrete.
Tensile stresses are forces trying to pull apart the concrete and
are opposite from compressive stresses. Cracks can develop when
the tensile stress exceeds the tensile strength. While concrete
is strong in compression, the tensile strength is generally only
10% of the compressive strength. At early ages, this strength is
still developing while stresses are generated by volume changes.
Controlling the variables that affect volume change can minimize
cracking and create a higher quality concrete placement. More
on early-age cracking.
Mass concrete is a hot topic. Owners desire long
service lives so engineers design concrete mixtures for low permeability.
These mixtures typically have high cementitious materials contents,
which results in high temperatures within the concrete. To avoid
cracking and other temperature related damage to the concrete, contractors
must control the temperature and temperature difference in the concrete.
This can pit the schedule against the service life.
When all involved parties work
together, appropriate changes can be made to achieve the desired
service life with minimal impacts to the schedule. The key is an
understanding of mass concrete. Selection of an appropriate concrete
mixture is the first step. More on mass concrete.
Flow with Show: Self-Consolidating Concrete
Offers New Opportunities for Architectural Concrete
What is Self-Consolidating Concrete (SCC)
and how is it tested?
Concrete As a Carbon Sink
The topic of global climate change is frequently
in the news. The International Panel on Climate Change (IPCC) reports
that the increase in the concentration of many compounds in the
atmosphere will impact global climate. The most notable of the long-lived
greenhouse gases are carbon dioxide and methane. Using concrete
for building structures and infrastructure can contribute to the
emission of carbon dioxide. Almost all construction processes from
manufacturing, through transportation of materials and installation
use energy, and much of this energy may come from the burning of
What most people do not realize
is that the release of CO2 from calcination in the manufacture
of portland cement may be part of a cyclic process and is partially
carbon neutral in smaller timeframes such as decades. It
may be fully carbon neutral in longer timeframes. Concrete can absorb
carbon dioxide and store it in a process commonly referred to as
carbonation. This may be viewed simply as an additional,
alternative loop of the complex carbon cycle. Carbon dioxide may
be absorbed by concrete in its many forms such as buildings, bridges
and pavements. Concrete does not even necessarily have to be directly
exposed to the atmosphere for this process to occur. Underground
concrete piping and foundations can absorb CO2 from air
in the soil, and underground and underwater applications might absorb
dissolved carbon dioxide (carbonates) present in groundwater, freshwaters
and saltwaters. Read the complete article
by Liv Haselbach.
buildings and pollution-reducing roadways: These may sound like
futuristic ideas, but they are realities of some of today’s
concrete. Recently introduced formulations of cement are able to
neutralize pollution. Harmful smog can be turned into harmless compounds
and washed away. Anything made out of concrete is a potential application,
because these cements are used in the same manner as regular portland
cements. These products provide value through unique architectural
and environmental performance capabilities.
Proprietary technology (based on particles of titanium dioxide) is what makes this cement special—capable
of breaking down smog or other pollution that has attached itself
to the concrete substrate, in a process known as photocatalysis.
on self-cleaning concrete.
Significance of Tests and Properties of
Concrete and Concrete-Making Materials (STP169D) (LT205)
a very real sense, specifications are the letter of the law. But
for the professional that needs to know the how and why behind the
development, this encyclopedic reference is the place to turn. Ever
wonder how much strength a test cylinder will lose from rolling
and bumping around in the back of a pickup truck or from being dropped
from waist level? How about the effect of bearing strips on test
cylinders used for splitting tensile strength? Which DOT performed
the original research for strength determination using maturity?
How much heat contribution should I expect from each of the four
major cement compounds? This is the type of information that can
help avoid headaches and avert disasters. This peer-reviewed work
is a must have for the cement and concrete professional producing,
using, or testing materials in conformance with ASTM specifications.
Read a review of the
More information or