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Blending
Three Aggregates
by
Pete Alexander, Research and Training Specialist
Besser Company
Proper gradation of
aggregates is critical for the production of strong and aesthetically
pleasing concrete masonry units. Aggregates can be blended automatically
with a customized program such as the Besser Concrete Products Manager
software or blended manually. Blended gradation is extremely important
for maintaining or achieving proper masonry unit texture as well as
optimizing cement usage. Proper gradation is also critical to the
overall performance of a masonry unit when subjected to absorption and
compressive strength testing.
The process for
blending two aggregates is relatively simple with this basic formula:
X=100 (A-B/A-C)
Where:
X= % of fine
material in a blend
A= *F.M. of Coarse
Aggregate
B= F.M. of Desired
Blend
(Figure 1)
C= F.M. of Fine
Aggregate
*F.M. = Fineness
Modulas is an index number that is roughly proportional to the average
size of the particles in a given aggregate; thus the coarser the
aggregate the higher the Fineness Modulas / Index Number.
There are times when
it is necessary to blend more than two aggregates to obtain a
well-graded mix. In many cases, after performing a sieve analysis, a two
aggregate blend will lack material retained on the number 16 and number
30 screens. This is primarily due to the demands by the asphalt industry
for the number 16 and number 30 sized material, which creates a material
shortage for producers. As a result, the producer needs to locate an
aggregate supply which, when blended with a normal blend, will result in
a well-graded mix.
Another reason for
blending three aggregates is to reduce batch costs. The typical masonry
producer can reduce costs and achieve a well-graded blend by adding a
less expensive aggregate that may not be adequate as a stand-alone
material.
Blending three
aggregates utilizes the same formula noted above for the two-step
operation. For example, a sieve analysis test (Figure 2) determined that
the pea gravel has an F.M. of 5.57, sand #1 has an F.M. of 3.10, and
sand #2 has an F.M. of 2.04.
Step One:
The first step involves determining the percentage of coarse aggregate
in the final blend.
X= 100 (5.57 - 3.70
/ 5.57 - 2.75**)
X= 100 (1.87 /
2.82)
X= 100 (0.66)
X= 66% Combined
Sands
Therefore:
100% - 66% = 34%
Pea Gravel
**2.75 is used
as a reference number. It represents the median F.M. for medium graded
sand (see Sand Classification). Step Two will involve blending the
coarse sand (sand #1) and the fine sand (sand #2) to achieve this
desired F.M. of 2.75.
The results of Step
One can be visualized through the use of a pie chart (Figure 3).
Step Two:
This step involves determining what percentage of the two sands will be
in the final three aggregate blend. The same blending formula is used
however, in this instance the coarse aggregate F.M. is represented by
the coarse sand, the fine aggregate F.M. is represented by the fine
sand, and 2.75 represents the F.M. of the desired blend.
X= 100 (3.10 –
2.75 / 3.10 – 2.04)
X= 100 (0.35 /
1.06)
X= 100 (0.33)
X= 33% Fine Sand
Therefore:
100% - 33% = 67%
Coarse Sand
Step One determined
that the combined sands would total 66% of the final three aggregate
blend therefore:
66% X 0.67 = 44%
Sand #1
66% X 0.33 = 22%
Sand #2
Step One also
determined that 34% of the final blend would be Pea Gravel, therefore:
34% Pea Gravel
44% Sand #1
+ 22% Sand #2
100% Total Blend
A great deal can be
determined about a potential blend by first comparing it to a desired
blend (Figure 1). One area to look for is a gradation gap. A typical gap
in gradation can be found on the number 16 and number 30 screens
signified by a "valley" in the graph. This "valley"
indicates a higher potential for water penetration through the face
shell of the unit produced from this blend.
A second concern is
an excessive amount of material retained on the number 50 and number 100
screens and in the pan. As the overall percentage of fine material
increases, the cement requirement increases. This is due to an increase
in the aggregate surface area, which the cement must coat. This feature
can drastically affect the color of a masonry unit when using pigments.
To conduct a
comparison, the percentage of material that will be retained on each
sieve must be computed (Figure 4).
Example: Sand #1 has
8.9% retained on the number 8 sieve (Figure 2). The three aggregate
blend however, will only utilize 44% of this 8.9%. Therefore, 8.9% X
0.44 = 3.9% of the number 8 screen material, from Sand #1, will be used
in the three aggregate blend.
After adding the
percentages in each sieve column the numbers in the Total row are then
graphed against the ideal 3.70 blend (Figure 5). The numbers in the ACC
row are the accumulated sieve numbers. This function is performed as a
final check to assure that the blend being attempted (in this instance
3.70) is achieved as an end result.
Figure 5 - The green
line below represent the actual blend, compared to the ideal graph in
red.
It is proven that
gradation plays a major role in the performance characteristics of a
concrete masonry unit. However, there are many other areas to consider
when choosing aggregates for production including cost (i.e. shipping),
particle shape and soundness. Detailed information on blending, mix
design and related topics is covered in the Concrete Masonry Technology
course, which is offered as part of the Blockmakers Workshops®
educational series. This course is taught in a state-of-the-art
classroom and lab facility located at the World Center for Concrete
Technology in Alpena, Michigan.
Scheduling
information for the Blockmakers Workshops can be accessed from the
Besser web site, under the training tab at www.besser.com
or by contacting Vicki Cripps at 989.358.7238 or via e-mail at crippsv@alpena.cc.mi.us.
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