Concrete is America’s most widely used building material. Composed of water, air,
cement, coarse aggregate, and sand, concrete is the optimum material for many
construction projects due to its durability and workability compared to other
building materials.
Though all concrete is made from the same ingredients, the concrete’s properties
vary greatly depending how those ingredients are chosen and placed. Different
construction projects call for different types of concrete. Some projects may need
high load-bearing strength, while others require durability against erosion.
Concrete Mix Design is the process by which concrete components are
proportioned, mixed, and cured for a specific construction project.
Part I: Proportioning Components
Initial Information
To create the proper mixture, the design team must decide
which material properties of concrete are most important to
the project. Designers must know the values of the project’s
critical strengths before moving forward:
 Compressive Strength – resistance to lengthwise
flattening
 Tensile Strength– resistance to lengthwise stretching
 Shear Strength – resistance to perpendicular loads
 Bending Strength – resistance to twisting forces
Figure 1. Types of material strength
Additionally, designers must know the following factors of
the concrete before moving forward:
 Maximum Size of Aggregate (MSA) – diameter
of the largest piece of gravel used based on
3
1
concrete design specifications; typically8” to 1 2”
Figure 2. Testing to determine desired
slump for concrete mixture
 Particle Size Distribution – value representing
range of diameters within aggregate batch
 Slump – change in height of concrete mixture
in cone mold over time due to moisture content
 Exposure Environment – measure of climate
and erosion conditions at site of concrete
placement
The values of these desired factors affect the decisions and calculations made
during the design process.
Water Content
Water content of a concrete mix relates to its workability when placing it on the
construction site. More moisture in the mixture makes it easier to pour and shovel
into a mold. Designers
determine the desired
water content using charts,
dependent on the MSA and
slump for the mixture.
Air Content
Concrete’s air content has a
direct effect on the future
durability of hardened
concrete. Large air voids
within concrete in colder
climates can result in
Figure 3. Empirical chart used to determine both
cracking from freezing and
water and air contents
thawing cycles. To
determine the best air content, designers use charts dependent on MSA and the
placement’s exposure environment.
Cement Content
Cement is the main source of strength in concrete; it acts as the glue holding
together the other constituents of the mix, and it must be able to withstand the
large loads of the project. Designers find the required cement ratio using equations
relating the necessary critical strengths, exposure environment, and water content.
Coarse Aggregate Content
The method for finding coarse aggregate content requires more calculation than the
methods for the other constituents in concrete. Designers use the MSA, FM, water
content, and empirical charts to develop a series of equations, which results in the
desired value for coarse aggregate content.
Sand Content
Finding the ratio of sand for the concrete mixture proves to be one of the simpler
steps in the concrete design process. With all other contents solved for, designers
now find the sand content by subtracting the other percentages from 100.
Part II: Preparing Mixture
Trial Batch Demonstration
To ensure all calculations and assumptions were formed correctly, designers will
now prepare a small batch of concrete to test their work. They use this trial batch to
check the slump, strength, unit weight, etc. of the mixture. If the batch does not
perform as the design called for, the content ratios are adjusted.
On-Site Mixing
Concrete is so widely used because it can be made on a construction site, unlike
other materials like metal, wood, or masonry. Mixing trucks combine all
components together in a drum that spins at a rate of 16 rpm for 90 minutes.
Restrictions of rotation speed and time reduce the amount of segregation of
concrete components. If rotated for too long, aggregate will gather along the bottom
while air and water will rise to the top, making placement of the mixture difficult.
Part III: Placing Mixture
Placement
Upon completion of mixing, workers will guide the
truck’s chute to the placement mold, and open the drum
for fresh concrete to exit. For smaller projects, workers
use buggies or wheelbarrows to transfer cement from
the drum to the molding.
Figure 4. Placing concrete via truck chute
Curing
Concrete does not dry through evaporation; rather, it loses its moisture through a
chemical reaction, which takes nearly a month after setting to complete. During this
time, the new concrete gains its strength and durability. Particular measures are
taken to lengthen the reaction’s duration, and therefore the concrete’s strength and
durability.
Figure 5. Heating coils to maintain
constant temperature
Maintaining constant temperature during the
curing process ensures that the concrete hardens
uniformly. The proper approach to keep
temperature constant depends on the climate of
the project. Some situations may call for heating
coils over the placement, while others may
benefit more from spraying the concrete with
steam.
Excessive evaporation of water in the mixture results
in concrete cracking. All water added into the concrete
mixture is needed to complete the chemical reaction.
To prevent evaporation, workers often cover the
concrete placement with insulating tarps.
Figure 6. Cracking due to premature drying
Summary
To create the perfect mixture, designers must first know the specific needs for their
concrete, as well as the qualities of the constituents they will be using. Next they
choose the ratios of water, air, cement, coarse aggregate, and sand to be used in the
concrete. After creating a trial batch with the calculated ratios, the full batch of
concrete is mixed on-site and placed. Proper curing procedures follow to ensure the
best quality placement.
Concrete designers use this same procedure to create the optimum concrete
mixture to fit their needs. Whether the concrete is used for a bridge, sidewalk,
building, foundation, stadium, or statue, minor changes in its design can result in
drastic differences in its quality.
Though concrete construction can be costly at times, its long-term durability saves
money in the long run. Additionally, the opportunity for on-site mixture expedites
the construction process, while other materials like metals and woods need to be
manufactured and transported to the site. Concrete’s ability to adapt for different
construction purposes renders it the best construction material for many projects.
Bibliography
Cover Page.
Appman, Timothy G. Web. 20 Mar. 2015.
Figure 1.
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Figure 2.
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Figure 3.
Qui, Tong. Air and Water Contents. 2014. Art. The Pennsylvania State University,
State College, PA.
Page 3 Footer.
Bluemetal Coarse Granite Gravel. 2009. White and McGregor. Web. 24 Mar. 2015.
Figure 4.
Permanent Concrete Formwork. 2010. Your Home. Web. 20 Mar. 2015.
Figure 5.
Concrete Curing. 2014. Civil Blog. Web. 20 Mar. 2015.
Figure 6.
Concrete that has Cracks. 2011. Robinson Landscape. Web. 20 Mar. 2015.