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-cement ratio
(w/c) is selected to obtain the desired strength and durability, and a mixture is made with that
w/c 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.
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:
Slump:
Air content:
w/c:
Coarse Agg:
Fine Agg:
Type I; Sp gr. = 3.15
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
Based on the properties determined in previous lab.
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.
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Initial Quantity of Materials for Laboratory Concrete Batching (lb.)
Party
No.
C1-A1
C1-A2
C1-B1
C1-B2
W/C
C2-A1
C2-A2
C2-B1
C2-B2
0.60
0.55
0.5
0.45
0.45
0.5
0.55
0.60
Initial Wt. Per Batch (lb.)
Cement
Water
C. Agg
Sand
6.7
3.0
20
15
6.0
3.0
20
15
6.0
3.3
20
15
5.7
3.4
20
15
5.7
6.0
6.0
6.7
3.4
3.3
3.0
3.0
20
20
20
20
15
15
15
15
Note: Adjust the amount of coarse aggregate and sand during batching to achieve adequate workability and the
slump for the concrete mix.
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. If the aggregate is dry, add water
(typically about 0.7% by wt, of the aggregate) separately to bring the aggregates to
about SSD condition.
2. Weigh the quantity of cement given in the table above in a separate container.
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".
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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 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 them in a moisture curing room at 72º F. (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/c 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.
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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.)
(E) weight /
yd3
cement
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: _____________________
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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 =
Laboratory Instructor: _____________________
7
__________ psi
Results (combined for Concrete Mixing and Testing)
1. Present the calculated mixture proportions using the ACI procedure and those
obtained by trial batching.
2. 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.
3. 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.
4. Provide sketches of the failure modes in the compression test, and note the number
of your group’s cylinders to fail in each manner.
5. Using data from all of the mixtures prepared in your lab section, generate a plot
comparing the compressive strength of the concrete (y-axis) at 7 days and 14 days
of age with the w/c of the mixture (x-axis). (note: calculate average strength and
standard deviation for this plot)
6. Using data from all of the mixtures prepared in your lab section with different w/c,
generate a plot comparing tensile strength (y-axis) at 7 days and 14 days of age with
the w/c of the mixture (x-axis). (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.
3. 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.
4. For the mixture prepared by your lab group, compare the average compressive
strength with predicted or design compressive strength (see ACI graphs in the text,
which shows 28 day strengths for various w/c; note that in Chapter figure 7-1, the
x-axis should have the w/c values of 0.3, 0.4, 0.5, 0.6 and 0.7). Comment on how
sources of error or how your party’s mix design may have affected the strength of
the trial batch concrete.
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5. Using the plot generated in Results (5), discuss the effect of w/c and age (or time of
curing) on the compressive strength of the concrete.
6. For the mixture prepared by your lab group, compare the average tensile strength
with the average compressive strength. Compare this relationship to the estimates
provided in the Lai text. (Use data contained in the “Tensile Strength” section of
Ch. 7 Hardened Concrete Properties and equation 2 in that chapter, t=kt(fc)1/2.)
7. 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.
8. Using the plot generated in Results (6), discuss the effect of w/c and age (or time of
curing) on the tensile strength of the concrete and the effect of any sources of error
on the strength.
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