Mix designs for concrete block
Proportioning using the fineness modulus method
By Neal Jablonski
roportioning the mix
components for a concrete masonry unit
(CMU) is an important
step in producing high-quality
units. A well-proportioned mix can
improve a unit’s physical properties
(compressive strength, unit weight,
absorption) so they meet or exceed
ASTM C 90 Standard Specification
for Load-Bearing Concrete Masonry
Units. Additionally, the right mix
proportions can improve CMU
durability and appearance.
This article gives general guidelines for designing a concrete mix
with locally available aggregates,
and focuses on determining aggregate fineness modulus, proportioning aggregates, and choosing a cement-aggregate ratio.
P
The fineness modulus method
The most commonly used
method for designing mixes for
concrete masonry is called the FM
method. FM stands for fineness
modulus, an index number roughly
proportional to the average size of
the particles in a given aggregate.
The coarser the aggregate, the
higher the FM.
While all block mixes require at
least one aggregate, a producer, for
a variety of reasons, might decide
to use as many as four aggregates
in a mix. Such reasons include lack
of well-graded aggregates nearby
(and increased cost to obtain such
aggregates) and the desire to produce a unit with better physical and
aesthetic properties.
However, because two-aggregate
mixes are the most common, that
combination is covered in this article. Manually calculating three- and
four-aggregate blends is laborious,
therefore spreadsheet software
should be used.
Following are the six steps involved in designing a mix using the
FM method:
Step 1: Determine the FM of each
aggregate.
To determine the FM of a single
aggregate, follow the procedure
outlined in ASTM C 136, “Standard
Test Method for Sieve Analysis of
Fine and Coarse Aggregates.” The
steps are as follows:
1a. Sieve approximately 500
grams of oven-dry aggregate
through 3⁄8 inch, No. 4, No. 8, No.
16, No. 30, No. 50, and No. 100
sieve sizes.
1b. Starting with the largest sieve,
add the percent retained on each
successive sieve to arrive at the cumulative percent retained up to and
including that sieve.
1c. Add the cumulative percent
retained on each sieve and divide
the sum by 100. Don’t include the
pan in the sum. This will give you
the fineness modulus. [See the example below.]
Step 2: Proportion aggregates for
proper FM.
Aggregates are blended to obtain the desired FM for a specific
Sieve analysis and
calculating fineness modulus
Using the sieve analysis result below, the fineness modulus can be calculated. The fineness modulus is the sum of the total percentage retained
on each of a specified series of sieves divided by 100.
A sample of fine aggregate weighing 508.5 grams is passed over the
sieves shown below and the weights retained on each sieve are as shown.
Sieve
Weight
Individual
retained, grams % retained
Cumulative
% retained
3
8
⁄ inch
No. 4
No. 8
No. 16
No. 30
No. 50
No. 100
Pan
0
9.2
67.6
101.2
104.2
122.5
95.3
8.5
0
2
13
20
20
24
19
2
0
2
15
35
55
79
98
(100)*
Total
508.5
100
Sum = 284
284
Fineness Modulus = --------- = 2.84
100
*Pan not included in fineness modulus
class of unit. The industry-recommended FMs for various types of
units and aggregates are: normalweight CMU, 3.70; medium-weight
CMU, 3.67; and lightweight CMU,
3.84. (Grading charts are shown in
Figure 1.)
Aggregate blends shouldn’t contain excess fines or coarse particles.
Blends with excess fines require
more cement to coat the added surface area, while mixes with excess
coarse aggregate will contain large
interconnecting voids and be harsh.
Calculate the proportion of fine
and coarse aggregate for the desired unit using the following
equation:
X=
A-B
(_____
A - C)
where
X = percent of fine aggregate
Y = percent of coarse aggregate
A = FM of coarse aggregate
(determined per step 1)
B = FM of desired blended
aggregate for block class
C = FM of fine aggregate
(determined per step 1)
Step 3: Determine aggregate batch
proportions.
Calculate the batch weight of
each aggregate by multiplying the
total design batch weight (usually
dictated by mixer size) by the percent of fine and coarse aggregate
(per Step 2) in the mix.
100
Step 4: Determine moisture content
in aggregate.
Most aggregate contains moisture
Y = 100 - X
when it’s batched. Because design
weights are for dry aggregate, batch
weights must be increased to maintain the same dry cement-to-aggregate ratio. If aggregate batch weights
aren’t adjusted to account for the
moisture, you’ll get a lower yield
(fewer units per pound of cement)
and thus a less economical mix. To
adjust aggregate batch weights, the
moisture content of each aggregate
must first be determined.
4a. Weigh a representative sample of each aggregate (500 to 2,000
grams, depending on particle size)
and record the initial weights.
4b. Oven dry each sample to remove the moisture, then re-weigh
and record this final weight.
4c. Determine the moisture content of each aggregate. Subtract the
final (dry) weight of the aggregate
Designing a block mix
Following the steps outlined in the article, let’s design a two-aggregate, normal-weight block mix with
an industry-recommended FM of 3.70. The mixer can
handle a total dry aggregate batch weight of 4,500
pounds.
Step 4: Moisture content. Moisture content of the
sand is 5% and the gravel 2%. We determine the adjusted batch weights per Step 4d as follows:
(for sand) 2,430 lbs.
1.05 = 2,552 lbs.
(for gravel) 2,070 lbs.
1.02 = 2,111 lbs.
Step 1: Aggregate FMs. As indicated in the table below, our coarse aggregate (gravel) has an FM of 4.82
and our fine aggregate (sand) has an FM of 2.75.
Step 5: Cement content. We desire a moderate cement-to-aggregate ratio of 1:10 (per ratio listings on
page 368). For the required aggregate design batch
weight of 4,500 pounds the required cement is:
1 = 450 lbs.
___
cement weight = 4,500
10
Step 6: Water content. Our experience in producing similar units indicates that a good total water content to start with is about 5.5% of total batch weight
(4,500 pounds of aggregate + 450 pounds of cement)
or in this case 272 pounds of water. We know, per
Step 4c, that the sand contains 5% moisture (122
pounds) and the gravel contains 2% moisture (41
pounds). This 163 pounds of water already contained
in the aggregate is subtracted from the required
amount of water (272 pounds), leaving 109 pounds of
water to be batched. Since water weighs 8.34 pounds
per gallon, about 13 gallons of water will be used in
the first trial batch.
Total initial batch weights are as follows:
Cement
450 lbs.
Sand (5% moisture)
2,552 lbs.
Gravel (2% moisture) 2,111 lbs.
Added water
109 lbs. (about 13 gallons)
Percentage retained on each sieve
3
8
⁄
4
8
16 30
50 100 Pan FM
Fine
—-
1
10
15 27
32
14
1 2.75
Coarse
—- 24 48
21
2
1
1 4.82
Material
3
Step 2: Aggregate proportioning. Knowing the FMs
we can calculate the aggregate proportions for the mix:
4.82 - 3.70
1.12
X = __________ (100) = _____ (100) = 54%
4.82 - 2.75
2.07
X = 54% fine aggregate
Y = 46% coarse aggregate
Step 3: Batch proportions. Our dry aggregate design batch weight is 4,500 pounds, so we calculate:
4,500
54% = 2,430 lbs. of fine aggregate
4,500
46% = -------2,070 lbs. of coarse aggregate
4,500 total design batch weight
of aggregate
from the initial (wet) weight, and
divide it by the final weight. This is
the moisture correction factor.
4d. Adjust the batch weight of
each aggregate to maintain the
proper mix proportions. Multiply
the design batch weight for each
aggregate (3a) by the moisture correction factor (4c) and add this
weight to the original design batch
weight. Or simply multiply the design batch weight for each aggregate by one plus the moisture correction factor (found in 4c).
Step 5: Determine cement content.
Cement is the final component
needed to produce a high-quality
CMU. Cement binds aggregate particles and partially fills spaces between them.
5a. Choose the cement-to-aggregate ratio that will achieve the necessary CMU properties with aggregates being used in the mix. Below
are ranges of cement-to-aggregate
ratios that can be used for various
Figure 1. Aggregate gradation for concrete masonry units
Normal-weight
Sieve size
3
8
⁄"
4
8
16
30
50
100
Pan
Min.
Max.
0
20
10
10
10
10
0.5
0.2
5
30
23
20
20
20
15
10
Ideal
0
25
15
15
15
15
10
5
FM = 3.7
Medium-weight
Sieve size
Min.
Max.
Ideal
3
8
0
12
18
16
11
5
5
7
0.3
22
27.5
25
19
13
11
13
0
17
23
20
15
9
7
9
FM = 3.67
⁄"
4
8
16
30
50
100
Pan
Lightweight
Sieve size
3
8
⁄"
4
8
16
30
50
100
Pan
Min.
Max.
Ideal
0.5
17
21
13
7.5
5
5.3
0.7
5
28
30
21
15.8
13
10.5
13.1
0.5
21
25.5
17
11.5
9
6.5
9
FM = 3.84
Source: Besser Co., “Concrete Masonry Technology Blockmakers Workshop Series”
types of aggregates. All of the ratios
are based on dry weight.
Type of
aggregate
Range of ratios
(cement:aggregate)
Sand and gravel
Limestone
Pumice
Cinders
Slag (expanded)
Slag (air cooled)
Clay (expanded)
1:8 to 1:12
1:7 to 1:12
1:4 to 1:6
1:6 to 1:8
1:5 to 1:7
1:8 to 1:12
1:6 to 1:9
5b. Determine the required total
aggregate design batch weight by
adding the design batch weight of
the fine and coarse aggregate as
found in Step 3.
5c. Determine how much cement
is required in the mix. Multiply the
weight determined in 5b by the desired cement-to-aggregate ratio.
Aggregate design batch weight
cement-to-aggregate ratio = cement
(in lbs.)
For example:
4,500 lbs.
1
—— = 450 lbs.
10
Step 6: Determine water content.
The total amount of mixing water
needed to make a high-quality unit
will vary depending upon the type
of aggregate used, cement content,
and desired appearance. The producer is left to determine this
amount through trial batches.
Just a start
The mix design techniques described above can be used to design mixes for normal-weight,
medium-weight, and lightweight
units. Extra care should be taken
when designing lightweight and
medium-weight mixes. If a calculated blend contains too much or too
little lightweight or medium-weight
aggregate, the desired density
might not be achieved.
A well-designed mix helps producers meet the increasing demand
for high-quality low-cost CMUs.
The fineness modulus method provides a good starting point when
designing a mix for any type of
unit. However, whether producers
use the fineness modulus method
or any other, they need to know
that choosing the best mix design
usually requires a trial-and-error
approach in which mixes are tested
and adjusted until a desired result
is attained. ✥
Neal Jablonski is a technical service specialist for Grace Construction Products
in Milwaukee.
References:
1. Grant, William, Manufacturer
of Concrete Masonry Units, second edition, Concrete Publishing
Corp., 1959
2. Besser Co., Concrete Masonry
Technology Blockmakers Workshop Series
3. Standard Test Method for
Sieve Analysis of Fine and
Coarse Aggregates, ASTM C
136-95, 1995
PUBLICATION #J960363
Copyright © 1996, The Aberdeen Group
All rights reserved