Concrete Mix Proportioning
In this laboratory, two different methods of concrete mixture proportioning, trial batch method
and ACI method, will be explored. The trial batch method is a simple, empirical approach to mixture
design. In the trial batch method, an appropriate water-to-cementitious materials ratio (w/cm) is selected
to obtain the desired strength and durability. In this laboratory, the cementitious materials to be used may
be simply Type I portland cement or a proportioned blend of portland cement and fly ash, depending upon
the lab party. A mixture is made with that w/cm incorporating fine and coarse aggregate to achieve the
desired plastic consistency (slump and workability). In practice, this is often an iterative process, where
several batches are made to achieve the most economical mixture with the desired properties.
The ACI design method is a standardized design procedure developed by the American Concrete
Institute. Using graphs, charts, and tables provided by ACI and based on experimental data, proportions
of cement, water, SSD aggregate, and air are calculated to achieve the strength, durability, and
workability desired. In this laboratory, a mixture will be prepared using the trial batch procedure, and the
resulting mixture proportions will be compared to proportions generated through the ACI design
procedure. If the mixture prepared contains supplementary cementitious materials such as slag or fly ash,
some minor adjustments to the standard ACI mix design procedure are necessary, as the standard
procedure does not include such materials.
The objectives of this experiment are: (1) to use the trial batch mix method to determine
optimum proportions of aggregates, cement, and water for concrete to meet specified slump requirements,
(2) to verify the effectiveness of the trial batch method of mix design by comparison with ACI mixture
design (3) to learn concrete mixing practice in a laboratory environment, (4) to observe the characteristic
properties of fresh concrete, and (5) to prepare seven 3x6in. concrete cylinders for later evaluation.
Essential Apparatus
Seven 3 in. diameter by 6 in. tall cylinder molds
Two containers for stocking aggregates
Mix equipment: large mixing pan, trowels, slump cone, large and small tamping rods, scoops
12 in. ruler
Scale sensitive to 0.1 lb.
Material Characteristics and Mixture Specifications






Cement:
Fly Ash:
Slump:
Air content:
w/c:
Coarse Agg:

Fine Agg:
Type I; SG = 3.15
Type F; SG = 2.25
3.5  0.5 in.
Non air entrained; assume 1.5% entrapped air.
As stated below
#67 crushed granite, specific gravity (bulk SSD)=2.65;
AC= 0.58%; MC=TBD; MSA = 3/4”; dry rodded unit weight = 100 pcf
Natural sand, specific gravity (bulk SSD)=2.63; AC=0.40%; MC=TBD; FM=2.2
The following quantities of the materials will be used for each party. The materials should be sufficient
to produce enough concrete for casting seven 3 in. diameter by 6 in. cylinder specimens.
1
Initial Quantity of Materials for Laboratory Concrete Batching (lb.)
Party
No.
C1-1
C1-2
C1-3
C1-4
C2A-1
C2A-2
C2A-3
C2A-4
C2B-1
C2B-2
C2B-3
C3-1
C3-2
C3-3
C3-4
C4-1
C4-2
C4-3
C4-4
C5A-1
C5A-2
C5A-3
C5A-4
C5B-1
C5B-2
C5B-3
C2B-1
Test
Age
7 days
7 days
7 days
7 days
7 days
7 days
7 days
7 days
14 days
14 days
14 days
7 days
7 days
7 days
7 days
7 days
7 days
7 days
7 days
7 days
7 days
7 days
7 days
14 days
14 days
14 days
14 days
W/CM1
Curing
Fog
Fog
Fog
Fog
Fog
Fog
Fog
Fog
Fog
Fog
Fog
Ambient
Ambient
Ambient
Ambient
Ambient
Ambient
Ambient
Ambient
Fog
Fog
Fog
Fog
Fog
Fog
Fog
Fog
0.45
0.50
0.55
0.60
0.45
0.50
0.55
0.60
0.45
0.50
0.55
0.45
0.50
0.55
0.60
0.45
0.50
0.55
0.60
0.45
0.50
0.55
0.60
0.45
0.50
0.55
0.45
Cement
6.7
6.0
6.0
5.7
4.7
4.2
4.2
4.0
4.7
4.2
4.2
6.7
6.0
6.0
5.7
4.7
4.2
4.2
4.0
5.7
5.1
5.1
4.85
6.7
6.0
6.0
4.7
Initial Wt. Per Batch (lb.)
Fly Ash
Water
C. Agg
Sand
-3.0
20
15
-3.0
20
15
-3.3
20
15
-3.4
20
15
2.0
3.0
20
15
1.8
3.0
20
15
1.8
3.3
20
15
1.7
3.4
20
15
2.0
3.0
20
15
1.8
3.0
20
15
1.8
3.3
20
15
-3.0
20
15
-3.0
20
15
-3.3
20
15
-3.4
20
15
2.0
3.0
20
15
1.8
3.0
20
15
1.8
3.3
20
15
1.7
3.4
20
15
1.0
3.0
20
15
0.9
3.0
20
15
0.9
3.3
20
15
0.85
3.4
20
15
-3.0
20
15
-3.0
20
15
-3.3
20
15
2.0
3.0
20
15
Note: Adjust the amount of coarse aggregate and sand during batching to achieve adequate workability and the slump
for the concrete mix.
1 Water-to-cementitious materials ratio, W/CM = W
water/(Wcement + Wfly ash)
Trial Batch Procedure (Detailed information concerning the sample preparation procedures is
given in ASTM C-192.)
1. Weigh quantities of coarse aggregate and fine aggregate and store them in separate
containers. Record the weights on the data sheet. (normally, the aggregate moisture content
would be adjusted to SSD, but in lab this often causes enormous batch-to-batch variations;
this step is skipped.)
2. Weigh the quantities of cement and fly ash (if any) given in the table above in a separate
container.
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3. Weigh the quantity of mixing water given in the table above in a container.
4. Take approximately 2/3 each of the coarse aggregate and the fine aggregate obtained in Step
1 into the large, just dampened mixing pan. Pour all the cement obtained in Step 2 on top of
the aggregates in the mixing pan. Use a square end shovel to thoroughly mix the aggregates
with cement, then gradually pour all the water obtained in Step 3 into the aggregate-cement
mix and thoroughly mix the dry components with water to form a uniform (but rather soupy)
mass. Test the slump (see Step 6 for slump test). At this point the slump of the mixture
should be greater than 4 in. Gradually add small quantities of fine or coarse aggregate (or
both) and thoroughly remix the concrete and test the slump. Repeat this process until the mix
reaches the desired slump. As the slump is approaching 5 to 6 in., the batch should be
carefully examined in order to judge which aggregate to add to bring the concrete to the
desired slump. The correct amount of sand is the minimum which will produce enough
mortar to fill the space between pieces of coarse aggregate. In general, too little sand in the
mixture creates harshness (lack of troweling ability) and shows lack of cohesiveness whereas
too much sand decreases "yield".
NOTE, if measured slump is less than the desired range: cement, fly ash, and water must be
added in the w/cm specified in the table for the lab party. Adding water alone, to increase
slump, will increase the w/cm, decreasing strength.
5. When the batch is judged to be satisfactory by the slump test, weigh the remaining
aggregates and record in the data sheet, and compute the actual amounts of coarse and fine
aggregates used in the concrete mix.
6. In testing the slump, dampen the slump cone and place in the mixing pan. Hold the slump
cone down firmly against the pan. Fill the slump cone with concrete in three layers, each
approximately one-third of the volume of the slump cone for each layer. Rod each layer with
25 strokes, distributed uniformly over the cross-section of the cone. The rod should slightly
penetrate into the previous layer. After the top layer has been rodded, strike off excess
concrete with the tamping rod so that the cone is exactly filled. Immediately remove the
cone from the concrete by raising it carefully in a vertical direction. Measure the "slump" of
the concrete by determining the difference between the height of the mold and the height of
the subsided concrete. After the slump measurement is completed, tap the side of the
concrete frustum gently with the tamping rod. The behavior of the concrete under this
treatment is a valuable indication of the cohesiveness. A well-proportioned mix will
gradually slump to lower elevation and retain its original form while a poor mix will
crumble, segregate and fall apart.
7. Before filling the concrete into the cylinder molds, determine the weight of all seven cylinder
molds together and record the weights in the data sheet. Form the cylindrical concrete
specimens by placing the concrete into the cylinder mold in three layers of approximately
equal volume. Rod each layer with 25 strokes using a small tamping rod (1/4 in. diameter
rod). Distribute the strokes uniformly over the cross-section of the mold. Also, gently tap
the cylinder mold filled with concrete with the tamping rod. This treatment will help to
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consolidate the concrete and to drive out the air bubbles trapped in the concrete. After the
top layer has been rodded, strike off the surface of the concrete with a trowel. Fill a total of
seven concrete cylinder specimens. Measure the weight of all seven cylinder molds filled
with concrete and record the weight on the data sheet. Cover the concrete cylinders with a
plastic bag to prevent the loss of moisture from the concrete.
8. Compute the Unit Weight of concrete, and the weight of materials for making one cubic yard
of concrete. Record these results in the data sheet. Submit the results to the laboratory
instructor for approval.
9. After about 24 hours, strip the disposable plastic molds from the concrete cylinder specimens
and place them in a moist (100% RH) curing room at 72º F if “fog” curing; if ambient curing,
leave stripped cylinders outside lab. (TAs will do this step.)
10. Use the ACI absolute volume method (Ch.7 Lai) to calculate mix proportions for 1 yd3
concrete, using the same w/cm as in the trial batch mixture (i.e. 0.45, 0.50, 0.55, or 0.60). Use
0.70 for the volume ratio of dry-rodded coarse aggregate per unit volume of concrete. Results
of the ACI design should be included in the laboratory report.
4
CONCRETE TRIAL MIX DATA
Name _______________________
Date _____________
Group No. _____ Section _______
Max. Aggregate Size __________
W/C _______________________
Fine Aggregate F.M. __________
Absorption:
Fine Aggregate _____ %
Coarse Aggregate _____ %
Moisture content:
Fine Aggregate _____ %
Coarse Aggregate _____ %
(A) Material
(B) Initial
weight (lb.)
(C) weight
remaining
(D) weight
used (lb.)
cement
fly ash
water
coarse aggregate
fine aggregate
other:
Total Batch Weight = ________ (lb.)
Total Wt./ yd3 = _______(lb.)
Measured Slump __________________
(1) Weight of Concrete + containers = ________
(2) Wt. of Containers
= ________
(3) Wt. of Concrete = (1)-(2)
= ________
(4) total Vol. of Containers
= _________
(5) Unit weight of concrete = (3)/(4) = ________ (pcf)
(6) Concrete batch volume = (Total Batch Wt.)/ (Unit Wt.) = ___________ (ft3)
Note: (E) = (D) x 27 / (6); Check: Total Wt./ yd3 / 27 = (5)
Comments:
Laboratory Instructor: _____________________
5
(E) weight /
yd3
Testing Hardened Concrete Properties:
Compressive Strength and Splitting Tensile Strength
The objectives of this experiment are to determine the compressive strength and tensile strength
of concrete and to observe the behavior and fracture of concrete under unconfined compression and
diametric compression loading.
Essential Apparatus
Compression machine with at least 50,000 lb. capacity
Compression Test Procedure
1. Remove the concrete cylinders from the curing room and surface dry the cylinders.
2. Select 5 cylinders for this test. Measure the diameter of the cylinders.
3. Grind the concrete cylinder ends with a mason’s rubbing stone to remove surface irregularities. Place
neoprene end cap on each end.
4. Apply compressive load slowly and continuously until the maximum load is reached. Loading rate
should be between 20 psi to 50 psi per second (150 lb. to 300 lb. per second). Failure of the cylinder
is imminent during the test when the load indicator is slowing down, and finally stops. Allow the
compressive load to continue until the cylinder is crushed. Examine closely the type of failure of the
cylinder.
5. Record the maximum load, and determine the compressive strength for each specimen tested.
Identify and record the fracture mode for each specimen according the sketches shown in Figure 1.
Modes (a) and (b) are considered the normal fracture modes.
Figure 1. Fracture of Concrete cylinder under compression
Figure 1 Fracture Mode of Concrete Cylinder in Compression
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Split Tension Test Procedure
1.
Measure the dimensions of the remaining two cylinders. Draw diametrical lines on each end of
the specimen which bisect the cylinder.
2.
Center one strip along the center of the lower bearing block of the testing machine. Place the
cylinder on the strip and align so that the lines marked on the ends of the specimen are vertical
and centered over the strip. Place a second strip lengthwise on the cylinder. Lower the upper
loading head of the testing machine until the assembly is secured in the machine, see Figure 2.
3.
Estimate the maximum loading the specimen may take and apply the compressive load slowly
(about 100 psi to 200 psi per min.) and continuously until the specimen fails in split tension.
Record the maximum applied load. Examine the fracture surface, and estimate the percentage of
aggregate that have fractured. Does the failure occur in the matrix or through the aggregate?
Figure 2 Split Tension Test
4. Compute the split tensile strength,t
t = 2Pmax/ (DL)
Where D = Diameter (in.)
L = Length (in.)
Pmax = Maximum Compressive Load (lb.)
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CONCRETE TEST DATA SHEET
Name: ______________________
Date: _____________________
Group No: _________, Session No: ___________
Age of concrete: ______________
Curing condition:______________
Compression Test Results
Specimen
No.
Diameter
(in.)
Max. Load
(lb.)
Compressive
Strength, (psi)
Fracture
Mode
1
2
3
4
5
Compressive Strength: Average
= _________ psi
Standard Deviation = _________ psi
Split Tension Test Results
Specimen
No.
Diameter
(in.)
Height
(in,)
Max. Load
(lb.)
Tensile
Strength (psi)
1
2
Average Tensile Strength =
__________ psi
Laboratory Instructor: _____________________
8
Results (combined for Concrete Mixing and Testing)
1. Present the calculated mixture proportions using the ACI procedure and those obtained by
trial batching. Follow example 7.3 in the Lai book. For those with fly ash, the procedure can
be modified after step 6 by including fly ash as an additional material, using the SG for fly
ash, but including it as part of the cementitious materials used in the mixture.
2. Create a table comparing the cost of the trial batch mixture prepared by your group with the
cost of the corresponding ACI design mixture. Make sure the quantities compared are the
same – 1 cu. yd. is standard. Assume these prices: cement is $85/ton, fly ash is $25/ton, #67
crushed granite is $11/ton, natural sand is $13/ton, and water is $0/ton.
3. Create a table showing the cost for all of the trial batch mixtures prepared.
4. Calculate the theoretical % air voids in the concrete batch made in the lab (your party’s data
only) and compare with the % air void assumed for the pre-lab mix calculations. Show the
calculations in Appendix.
5. Calculate compressive strength and tensile strength for each of the cylinders tested. Calculate
average strength values and standard deviations. Present this data in tabular form.
6. Provide sketches of the failure modes in the compression test, and note the number of your
group’s cylinders to fail in each manner.
7. Using data from all of the mixtures prepared in all lab sections, generate a plot comparing the
compressive strength of the concrete (y-axis) at 7 days with the w/cm of the mixture (x-axis).
Label each sample type. (i.e., cement concrete at 7 days with fog-curing, fly ash-cement
concrete at 7 days with fog-curing, etc.) (note: calculate average strength and standard
deviation for this plot)
8. Prepare a similar plot for the 14-day compressive strength data.
9. Using data from all of the mixtures prepared in all lab sections with different w/cm, generate
2 plots comparing tensile strength (y-axis) at 7 days and 14 days of age with the w/cm of the
mixture (x-axis). Label each sample type. (note: calculate average strength for this plot)
Discussion (combined for Concrete Mixing and Testing)
1. Discuss any sources of error that may have affected the accuracy of the concrete mixing or
testing. Comment on the standard deviation of the measured compressive strengths.
2. Compare the proportions for the mixture prepared in your group using the trial batch method
with the proportions calculated by ACI mixture design. Discuss differences in proportions of
the two mixes and reasons why they may have differed.
9
3. Compare the cost of the trial batch mixture to the ACI design mixture. Discuss differences in
the costs of the two mixtures and reasons why they may differ.
4. Comment on the workability of the mixture prepared by your lab group. Discuss how the
target slump was achieved and the cohesiveness of the mixture.
5. For the mixture prepared by your lab group, compare the average compressive strength with
predicted or design compressive strength (see Figure 9-2 in PCA’s Design and Control of
Concrete Mixtures.) Comment on how sources of error or how your party’s mix design may
have affected the strength of the trial batch concrete.
6. Using the plots generated in Results (7) and (8), discuss the effect of w/cm, age (or time of
curing), and curing conditions (fog vs. ambient) on the compressive strength of the concrete.
7. Using the plots generated in Results (7) and (8), discuss the effect of cement replacement by
fly ash on 7-day and 14-day strength. Is the influence of fly ash addition dependent on w/cm
or age?
8. Using the data generated in Results (3), (7), and (8), comment on the effect of cement
replacement by fly ash on total cost and $/psi at 7 and 14 days.
9. For the mixture prepared by your lab group, compare the average tensile strength with the
average compressive strength. Compare this relationship to (1) the estimates provided in
Figure 11.25 of the Young text and (2) the values obtained from the equation recommended
by the American Concrete Institute for predicting tensile strength t:
t=kt(fc)0.5
where fc is the average compressive strength in psi and the constant kt is 6.7 for normal
strength concrete.
10. By examining the fracture surface of the splitting tension specimens, comment on primary
mode of failure (i.e. Does the fracture primarily occur through the matrix and around the
aggregate or primarily through the aggregate?) Estimate the percent of aggregate that have
fractured. Discuss what this indicates about the strength of the aggregate relative to the
strength of the cement paste.
11. Using the plot generated in Results (9), discuss the effect of w/cm, age (or time of curing),
curing conditions, and replacement of cement with fly ash on the tensile strength of the
concrete and the effect of any sources of error on the strength.
10