CIV3284: Concrete Mix
Design and Testing
Lecturer: Ye Lu
Semester 1, 2019
Group 18:
Laila Halim (27830500)
Athan Giuliani (27835510)
Prahn Nicholson (28753968)
Benjamin Di Petta (28751647)
Mustafa Mirza (27937666)
Farhaan Essa (25656805)
Due date: 21/05/2019
i
Executive Summary
The British Method for mix design was adopted to create a 0.02𝑚" batch concrete with a target
slump of 120mm and target compressive strength of 35 MPa. This method involved a design for
strength through free water to cement ratio, design for workability through free water content, as
well as cement and total aggregate content and finally the proportion of fine and coarse aggregates.
As the British method assumes that aggregates are in SSD state, further calculations were performed
to adjust for the additional water uptake into the aggregate pores. The final mix design had 7.86kg
cement, 13.69kg fine aggregate, 22.64kg coarse aggregate and 4.84kg water.
The concrete batch was then prepared in the laboratory and tested in accordance with AS1012
parts 3-5 for its properties. This involved being subjected to a slump test, compaction factor test, air
content test and checking the density of the concrete. The concrete sample was subsequently made
and cured in accordance with AS1012, Part 8 before the sample compressive strength was tested. The
concrete was found to have an average slump of 215mm, a density of 2382 kg/m3 , a compacting factor
of 0.995, an air content of 1.4% and an ultimate compressive strength of 45 MPa. The density aligned
well with expected density of normal strength concrete, however, the slump indicated that the
concrete was far too fluid, and the compressive strength was higher than desired which would result
in unnecessary expense.
Future direction should see the reduction in strength as well as reduction in workability. A
decrease in water to cement ratio would achieve a lower slump, however, this would additionally
increase the strength of the concrete. Since the current slump does not comply with slump test
standards, this must be prioritised and as such a compromise must be reached between the properties
of slump and compressive strength.
ii
Table of contents
1.0 Introduction
1
2.0 Assumptions
2
3.0 Week 2 Mix Design
2
4.0 Moisture Adjustments and Final Mix Proportions
3
5.0 Mix design procedure
4
6.0 Mix design testing
5
6.1 Slump test
5
6.2 Compacting factor test
6
6.3 Air content test
6
6.4 Density of fresh concrete
7
6.5 Compression test
7
7.0 Testing Results
8
7.1 Slump Test
8
7.2 Density of Fresh Concrete
8
7.3 Compacting Factor
8
7.4 Air Content
9
7.5 Compressive Strength
9
7.6 Summary
9
8.0 Observations during mixing
10
9.0 Discussion
11
9.1 Explanation of Test Results
11
10.0 Analysis and alterations to mix design and actual mixing process
14
11.0 Conclusion
16
11.0 References
17
Appendix A - Week 2 Mix Design Calculations
18
iii
List of Figures and Tables
Table 1. Concrete Desired Properties..................................................................................................... 2
Table 2. Mixture Proportions before adjustment .................................................................................. 3
Table 3. Results of Oven-Drying Aggregates .......................................................................................... 3
Table 4. Proportion of Laboratory trial Batches, Including Weight Adjustment for Aggregate Moisture
................................................................................................................................................................ 4
Table 5. Measured slump for each specimen......................................................................................... 8
Table 6. Density Test Data and Result .................................................................................................... 8
Table 7. Compacting Factor Test Data and Result .................................................................................. 8
Table 8. Failure Loads for Compressive Strength Test ........................................................................... 9
Table 9. Summary of Test Results .......................................................................................................... 9
Table 10. Approximate compressive strength for different aggregate and cement types [2] ............. 18
Table 11. Approximate free-water content (kg/m3) for various maximum aggregate sizes, aggregate
types and slump values [2] ................................................................................................................... 19
Table 12. Sieve Analysis Results ........................................................................................................... 21
Figure 1. Abrams cone dimensions [2] ................................................................................................... 5
Figure 2. Illustrative procedure for concrete slump test [3] ................................................................... 5
Figure 3. compaction factor test apparatus [4] ...................................................................................... 6
Figure 4. Compressive Strength v Age for 100% Moist Curing [7]........................................................ 12
Figure 5. Image of concrete cylinder after compressive testing .......................................................... 12
Figure 6. Stress-strain relationship of individual components [7]........................................................ 13
Figure 7. Relationship between curing temperature and % effect compared to 28-day strength [4]. 15
Figure 8. compressive strength versus free water/cement ratio [2].................................................... 18
Figure 9. Relationship between wet density, free-water content and specific gravity [2]................... 20
Figure 10. percentage of aggregate and free-water-cement ratio relationship [2] ............................. 20
iv
1.0 Introduction
It is crucial that engineers have an understanding of the underlying principles and procedures
involved in creation of the materials that they are working with. As such, this report involves the mix
design of a concrete with specified slump and compressive strength, the creation of concrete cylinders
using the calculated mix proportions, and finally property testing of the samples.
The concrete constituents that will be included in the design include cement, water and
aggregates. Cement and water form the paste that bonds the aggregates together. The water-cement
ratio (w/c) is a crucial parameter as it has a significant effect on the workability and strength of the
mix. Enough water is needed so that the concrete is flowable and able to be manipulated, but too
much can be detrimental to strength. The aggregates are inert fillers in concrete, taking up
approximately 60-80 percent of the mix due to their significant influence on cost reduction [1].
Aggregate grading, size and shape also has an impact on the strength, dimensional stability and
durability [1] of the concrete. It is for these reasons that a mixture of both fine and coarse aggregates
will be utilised in the design.
After selecting the component materials, it is necessary to conduct theoretical mixture
proportioning. Standard procedures are available for calculating the required proportions of each
constituent, and the “British Method” (or “DOE Method”) will be utilised in this report. The British
Method calculates the following constituents of the mix:
❖
❖
❖
❖
❖
❖
❖
Water/cement ratio
Water content
Combined Specific Gravity
Fineness Modulus
Wet concrete density
Total aggregate content
Percentage of fine aggregates
The use of this standard method will allow for an economic design that meets strength and
workability requirements. It is not enough, however, to simply calculate the required amounts. It is
inevitable that unforeseen laboratory conditions will have an influence on the resultant properties of
the concrete, as well as the assumptions made during the theoretical design process. As such, test
batches should be created and tested in the laboratory by standard methods. The test methods used
in this report include compressive strength testing, the slump cone test for workability, the
compacting factor test, a measurement of concrete density and air content. All tests will be conducted
in accordance to the specification of AS1012. By completing standard tests, adjustments can be made
to the mix design in an iterative manner until the desired properties are obtained and the engineer
can be confident that the concrete will be satisfactory in service.
This report will detail the mix proportion calculations for a 0.02𝑚" batch of concrete with a slump
of 120mm, and compressive strength of 35 MPa, as well as results from laboratory mixing and testing,
comparisons between the theoretical and actual properties and suggestions for future mix
improvements.
1
2.0 Assumptions
There were several key assumptions that were made in order to conduct the concrete mix design,
and these are detailed below:
●
●
●
●
●
●
The selected cement type and type of coarse aggregate are assumed as GP (AS3972) and
crushed aggregate consisting 14 mm maximum particle size of Granite material.
The state of aggregate in the concrete mix was under the saturated and surface dry (SSD)
condition until the adjustment is made.
Adding chemical admixtures are negligible in concrete mix design. They are not considered in
this report.
The coarse aggregates were used as clean, hard, strong and chemically inert with unknown
shape and surface texture.
Specific gravity of fine aggregates, coarse aggregates and Portland GP cement are 2.65, 2.95
and 3.15 respectively.
The combined SG of the aggregates in the mix design assumes a 50/50 proportion of fine and
coarse aggregate by weight.
3.0 Week 2 Mix Design
The initial Mix Design completed in week 2 followed the British Method. A summary of the
parameters found during the mix design process as well as the mix proportions are tabulated below.
For full calculations see Appendix A.
Table 1. Concrete Desired Properties
Desired Compressive strength
35 MPa
Desired Slump
120mm
Water to Cement Ratio
Combined Specific Gravity
Fineness Modulus
Cement Content
0.575
2.8
2.104
391.3 kg/m3
Water Content
225 kg/m3
Wet Density
2450 kg/m3
Total Aggregate Content
Percentage of Fine Aggregate
1833.7kg/m3
37.8 %
2
Table 2. Mixture Proportions before adjustment
Mass for 1 m3 mixture (kg)
Material
Free-Water
Mass 0.02 m3 (kg)
225
4.50
Cement
393.1
7.86
Fine Aggregate
693.1
13.86
Coarse Aggregate
1140.6
22.81
Total Mass (kg)
2451.8
49.04
4.0 Moisture Adjustments and Final Mix Proportions
The British Method utilised for mix design is based on the assumption that the aggregates are in
saturated and surface dry (SSD) conditions. A correction needs to be made to account for the actual
moisture conditions of the aggregate. The process to determine the moisture content of the
aggregates is done through oven drying them and then attributing the difference in mass as water. To
ensure that all the pores are empty, the aggregates are oven-dried at 105OC for 24 hours until they
maintain a constant weight. Before and after readings are recorded, the moisture content is
calculated, and the values are recorded below.
Table 3. Results of Oven-Drying Aggregates
Type
Mass
Container
(g)
Container and
aggregate wet
(g)
Mass wet
aggregate (g)
Container and
aggregate dry (g)
m.c. %
Coarse
48.1
460.5
412.4
458.6
0.458
Fine
47.5
570.6
523.1
569.2
0.264
𝑚. 𝑐. % = (𝑀𝑤𝑒𝑡 − 𝑀𝑑𝑟𝑦)/(𝑀𝑤𝑒𝑡 − 𝑀𝑐𝑜𝑛𝑡𝑎𝑖𝑛𝑒𝑟) ∗ 100
𝐶𝑜𝑎𝑟𝑠𝑒 𝑚. 𝑐. % = (460.5 − 458.6)/(412.4) ∗ 100 = 0.458
𝐹𝑖𝑛𝑒 𝑚. 𝑐. % = (570.6 − 569.2)/(523.1) ∗ 100 = 0.264
The values in column 2 of table 4 are calculated values for the amount of aggregate required
assuming SSD condition. From this, the actual weight is adjusted to account for the moisture that is
absorbed by the aggregates. The negative values in column 6 indicate the aggregates are partially dry,
which is expected as they are stored indoors with exposure to drafts. To get them to SSD condition,
the aggregates will absorb some of the water from the mixture, thus there needs to be more water
added so that w/c ratio is not lowered (which would result in a lower rate of hydration and less
workability).
3
Table 4. Proportion of Laboratory trial Batches, Including Weight Adjustment for Aggregate Moisture
1.
Material
Coarse
Aggregate
(CA)
2.
3.
4.
5.
6.
7.
Calculated Trial Batch Absorption Moisture
Free
Trial batch
weight, SSD Weight, SSD of aggregate, content of moisture
weight in
for 1m3
(kg)
ac
aggregate,
on
moisture
3
(kg/m )
(%)
mc, (%)
aggregate, condition for
Mca
mixing (kg)
(kg)
1140.6
22.81
1.2
0.458
- 0.169
22.641
Fine
Aggregate
(FA)
Cement (C)
693.1
13.86
1.5
0.264
- 0.171
13.689
393.1
7.86
7.86
Water (W)
225
4.5
4.84
Total Weight
2451.8
49.04
49.03
The final column in the table lists the final adjusted weights of the different materials required to
make the desired concrete cylinders.
5.0 Mix design procedure
The preparation of the concrete was done under supervision to ensure it was carried out as laid
out in the Australian Standards, AS1012. As the calculations for the elements had been completed, it
was just to ensure that they were mixed correctly.
1. Firstly, both aggregates and about ⅓ of the water were added to the mixer which was then
operated for 30 seconds to wet the aggregate.
2. Secondly, the cement powder was added slowly to ensure no dust was produced when the
mixer continued. The remainder of the water was slowly added within the next 2 minutes.
3. The mixer was then stopped for another 2 minutes and the mix was allowed to rest to ensure
hydration occurred.
4. The concrete was mixed for a further 2 minutes to create the final product.
4
6.0 Mix design testing
6.1 Slump test
The slump test was conducted according to AS1012 part 3. An inverted cone known as an
“Abrams cone” was utilised with dimensions given in figure 1 below.
Figure 1. Abrams cone dimensions [2]
A fresh layer of concrete was placed in the cone until it was approximately one third filled. This
layer was then compacted using a steel rod with 25 blows. A second layer was subsequently added
filling the cone up to two thirds of its capacity and only the second layer was compacted with the steel
rod and the same number of blows. The cone was then filled up to maximum capacity and compacted
in the top one third of the cone using the steel rod and the same number of blows. After the cone was
filled with concrete, the top of the cone was levelled using the steel rod.
The cone was then gently lifted vertically over a 3 second period to allow the concrete to slump
evenly and also reduce the chances of the concrete becoming a work hazard. The cone was then
turned upside down and placed next to the concrete with the rod laid on top, and the slump was
measured from top of the concrete to the rod. This process is illustrated in figure 2 below.
Figure 2. Illustrative procedure for concrete slump test [3]
5
6.2 Compacting factor test
The compacting factor test was carried out in accordance with AS1012 Part 3. The testing
apparatus is shown in figure 3 below.
Figure 3. compaction factor test apparatus [4]
In order to carry the compacting factor test, the apparatus was first cleaned. The cylinder was
then weighed, and the mass recorded. Following the mass measurement, the cylinder was placed
directly under the hoppers and the hopper doors were secured shut.
Fresh concrete was carefully poured into the top hopper up to capacity and the top was levelled
off. The trap door of the first hopper was opened allowing the concrete to fall into the second hopper.
The trap door of the second hopper was then opened allowing the concrete to flow into the cylinder.
The impact upon hitting the cylinder compacts the concrete. The concrete was levelled at the top of
the cylinder, all excess concrete on the edges of the cylinder was wiped off and the cylinder was again
weighed. The initial mass of the cylinder was subtracted from this value and this was considered as
the mass of partially compacted concrete.
The cylinder was again cleaned and weighed. Fresh concrete was then poured into the cylinder in
layers and vibrated until no bubbles appeared on the surface. Any excess concrete was wiped off and
the cylinder was reweighed. The initial mass of the cylinder was subtracted from this value to give the
mass of fully compacted concrete. The compacting factor was found by dividing the mass of the
partially compacted concrete by the mass of the fully compacted concrete.
6.3 Air content test
The air content test was conducted as per AS1012 Part 4. Fresh concrete was poured into a
container to one third capacity of the container and compacted by vibration. This process was
repeated for the next third and then the full capacity of the container. The excess from the top was
cleared and the top of the concrete was levelled out. The edges of the container were thoroughly
cleaned and wiped with a damp cloth to ensure the rubber seal is effective. A calibrated air
entrainment meter was clamped down onto the bowl of concrete. The air entrainment meter
contained two valves which were used to remove the air. This was done by pushing water through the
left valve until the water had displaced all the air between the lid and the concrete surface, causing
water to be pushed out of the right valve. The air valves were then shut. The hand pump at the top of
the air lid was then used to pressurise a separate container. The pressure was then released to allow
6
air to flow from the pressure container into the test container. This then provided a calibrated value
as a percentage.
6.4 Density of fresh concrete
The testing of fresh concrete density was undertaken in accordance with AS1012 Part 5. A metal
bowl was cleaned and filled with water to the brim and the mass of the container plus the water was
recorded. The bowl was then emptied and wiped clean. Next, the bowl was filled with fresh concrete
to one third capacity and compacted with a vibro-compactor until bubbling ceased. This process was
then repeated for two thirds full and maximum capacity. The bowl with the compacted concrete was
then weighed and the mass was recorded for density calculations. As the density of water is known,
the volume of the bowl could be found. The density of the fresh concrete was then calculated as the
mass of compacted concrete and bowl minus the mass of the bowl, divided by the volume of the bowl.
6.5 Compression test
Three standard concrete test cylinders of 100mm diameter and 200mm length (AS1012.8 section
5) were used to make the specimens. Each cylinder was filled to approximately the one third mark and
placed on the vibro-compactor. When sufficient compaction had occurred (but not enough to cause
segregation), the vibration was stopped. This process was repeated until the cylinders were filled. The
tops of the cylinders were then smoothed, covered with plastic and allowed to rest for 24hrs in their
respective moulds to gain strength. After 24hrs the cylinders were removed from their moulds,
marked/numbered for the group and were submerged in a curing tank filled with lime-saturate at
23 ± 2°𝐶 for a period of 40 days. The specimens were then removed from the curing tank and allowed
to air dry for a full day before testing. Unfortunately, due to laboratory timetabling, the standard
curing time of 28 days specified in AS1012 Part 8 was unable to be followed.
On the day of compression testing, the specimens were capped with a material containing
mixtures of sulphur as per AS1012.9 section 6.2. It was ensured that the ends of the specimens were
parallel within 2 degrees as they were to be in contact with the platens of the compressive testing
machine. The concrete specimens were loaded into the compressive testing machine and the top plate
was lowered so as to perform uniform loading on the specimen. Each sample was loaded to failure
and the failure loads were recorded.
7
7.0 Testing Results
7.1 Slump Test
Table 5. Measured slump for each specimen
Test
Slump (mm)
Test 1
215
Test 2
215
Average
215
7.2 Density of Fresh Concrete
𝑫𝒆𝒏𝒔𝒊𝒕𝒚 = 𝑴𝒂𝒔𝒔/𝑽𝒐𝒍𝒖𝒎𝒆
(𝟏)
As water has a known density of 1000𝒌𝒈/𝒎𝟑 , the volume of the container can first be found.
𝑽𝒐𝒍𝒖𝒎𝒆 = (𝟏𝟎. 𝟗𝟎 − 𝟑. 𝟖𝟒)/𝟏𝟎𝟎𝟎 = 𝟕. 𝟎𝟔 ∗ 𝟏𝟎_𝟑
With a known volume, the density of the concrete can be calculated as per Equation 1.
𝑫𝒆𝒏𝒔𝒊𝒕𝒚 = (𝟐𝟎. 𝟔𝟔 − 𝟑. 𝟖𝟒)/(𝟕. 𝟎𝟔 ∗ 𝟏𝟎_𝟑 )
𝑫𝒆𝒏𝒔𝒊𝒕𝒚 = 𝟐𝟑𝟖𝟐 𝒌𝒈/𝒎𝟑
Table 6. Density Test Data and Result
Weight Container
(kg)
3.84
Weight Container +
Water (kg)
10.90
Weight Container +
Concrete (kg)
Density (kg/m3)
20.66
2382
7.3 Compacting Factor
The compaction factor can be calculated as a proportion of the fully compacted concrete, as per
equation 2 below.
𝑪𝒐𝒎𝒑𝒂𝒄𝒕𝒊𝒐𝒏 𝑭𝒂𝒄𝒕𝒐𝒓
= 𝟏 − 𝑴𝒂𝒔𝒔 𝑪𝒐𝒏𝒄𝒓𝒆𝒕𝒆 / 𝑴𝒂𝒔𝒔 𝑭𝒖𝒍𝒍𝒚 𝑪𝒐𝒎𝒑𝒂𝒄𝒕𝒆𝒅 𝑪𝒐𝒏𝒄𝒓𝒆𝒕𝒆
𝑪𝒐𝒎𝒑𝒂𝒄𝒕𝒊𝒐𝒏 𝑭𝒂𝒄𝒕𝒐𝒓 = 𝟏 − (𝟐𝟑. 𝟑𝟒 − 𝟏𝟏. 𝟒)/(𝟐𝟑. 𝟒 − 𝟏𝟏. 𝟒)
𝑪𝒐𝒎𝒑𝒂𝒄𝒕𝒊𝒐𝒏 𝑭𝒂𝒄𝒕𝒐𝒓 = 𝟎. 𝟗𝟗𝟓
(𝟐)
Table 7. Compacting Factor Test Data and Result
Weight of
Container (kg)
11.4
Weight of Container +
Concrete (kg)
23.34
Weight of Container + Fully
Compacted Concrete (kg)
23.4
Compacting Factor
99.5%
8
7.4 Air Content
As in the method of testing, the air content can be read directly off the gauge. For the concrete
mix, the gauge gave a value of 1.4%.
7.5 Compressive Strength
Once the three cylinders had cured for 40 days, they were taken out of the bath and left to dry
overnight. The results obtained from compression test are shown in table 8 below.
Table 8. Failure Loads for Compressive Strength Test
Load (kN)
Cylinder 1
Cylinder 2
Cylinder 3
Average
370
370
310
350
7.6 Summary
Table 9. Summary of Test Results
Test Type
Slump
Density
Compacting Factor
Air Content
Compressive Strength
Overall Result Achieved
215 mm
2382 kg/m3
0.995
1.4 %
350 kN = 45 MPa
9
8.0 Observations during mixing
It is essential that fresh concrete has sufficient workability to allow for mixing, manipulation,
placement, transportation and finishing. The concrete mix was observed to have a high water content
and this allowed for good workability. The high w/c was evident by the ease at which the fresh
concrete could be scooped and lifted from the mixer, as well as the tendency for the mixture to drip
from the scoop during transportation to the slump cones and standard cylinders. The high fluidity and
lubrication that was observed during scooping made it abundantly clear that a high slump would be
achieved.
This high water content was confirmed during the slump test as the slump was recorded at
215mm. Once the cone was lifted, the fresh concrete had no resemblance to the shape of the cone it
was contained in. With the slump being greater than 150mm, it was classified as a total collapse.
Segregation is the separation of constituents of the concrete mix. This process can have
detrimental effects on the concrete, creating a non-homogeneous mixture with non-uniform
properties. The high w/c and likely excess of water present in the concrete mix increased the likelihood
of segregation occurring. This was a concern for the compaction of the fresh concrete during the
creation of the concrete cylinders for the compression test. During vibration, it was observed that the
coarse aggregates had a tendency to drop and the cement paste a tendency to rise quite quickly. As a
result, additional care was taken during compaction by keeping the frequency of vibration at a
moderate level and ensuring that vibration was not performed for too long.
10
9.0 Discussion
9.1 Explanation of Test Results
The properties of the concrete mix found through testing were not satisfactory, which can be
expected for mix trials. Both the experimental slump and compressive strength varied significantly
with expected targets.
The target slump of 120mm indicates a workable concrete that still held its general shape,
however this was exceeded significantly. The actual slump of 215mm results in a concrete that is much
too workable which could lead to problems such as segregation, excessive bleeding and lower
strength. The higher than expected slump can certainly be attributed to the fact that, even though
supervised, the concrete mixing and testing was being performed by students for the first time which
could have led to inaccuracies from lack of experience. These human errors could have included
prodding the concrete too much/with too much force that segregation of the mix could have occurred,
or not prodding each layer consistently. The difference in slump suggests that too much water was
added to the concrete mix. The slump of concrete is directly proportional to its water content, and
therefore too much water will lead to a large slump. This could possibly be due to inaccuracies involved
in measuring how much water the aggregates would absorb to be in SSD condition, or the aggregates
not having enough time to completely reach saturated state before slump testing [5], leaving excess
water in the mix. A slump of greater than 150mm is considered a total collapse failure, suggesting that
the concrete is not suitable to be used for construction purposes. Therefore, the result obtained
indicates an undesirable mix.
The density of concrete achieved by the mix design was 2382 kg/m3 which is within the range for
normal strength concrete of 2000-2600 kg/m3 [6]. This indicates satisfactory testing performance as
expected from a basic level test.
The compaction factor test was a more practical test of workability, by reflecting how the concrete
is placed during construction. The compacting factor of 0.995 that was obtained from testing is a very
high value, given that typical concrete has a range of compaction factors from approximately 0.6<CF<1
[2]. This value for compaction factor concurs with the slump of 215mm, and further suggests that
there was too much water added to the concrete mix.
The actual compressive strength of the concrete, 45 MPa, exceeded the target strength of 35 MPa
despite having a very high workability. Concrete is usually tested 28 days after it is mixed as by this
stage is has almost reach full strength. AS-3600 specifies the characteristic compressive strength of
concrete to be at 28 days, hence the calculations are done based on this timeline. Our concrete was
tested after a period of 41 days (40 of which in the curing tank), which means that concrete will have
gained some additional strength in that time period. The influence of curing time on compressive
strength is shown in figure 4 below. It can be seen that there is roughly an 8% increase in strength for
40 days in the tank, hence one would expect a value of 38 MPa for the mix designed in this report.
Another factor that could have caused the unexpectedly high strength is the level of compaction.
Despite, compacting each layer exactly 25 times, the degree of compaction may have been better than
expected, resulting in less air voids and a higher strength.
11
Figure 4. Compressive Strength v Age for 100% Moist Curing [7]
The additional 10 MPa of strength achieved for the concrete would be inappropriate in industry
as the concrete will have a higher than necessary cost. As such, the concrete mix should be adjusted
to reduce the strength, and this will be discussed in section 10.
Figure 5. Image of concrete cylinder after compressive testing
Figure y shows one of three of the cylinders after being subject to the compressive test. Based on
figure 5 failure can be seen to occur partially in the aggregates as well as the concrete paste. As the
interior of the concrete specimen could not be seen during testing, it is difficult to determine with
certainty whether the aggregates or the cement paste failed first. Considering the generally
understood behaviour of aggregates and cement paste to applied stress as shown in figure 6 below, it
is likely that the cement paste was the first to fail.
12
Figure 6. Stress-strain relationship of individual components [7]
Ultimately, the most common problem amongst the tests is the workability being too high, which
links to the water content. The water content could have been too high for several reasons. Firstly,
calculations could have been performed incorrectly during the initial mix design or in the adjustment.
Secondly, there may have been too much water directly added to the mixture through lack of care.
Finally, the water content of the aggregates could have actually been greater than calculated from the
oven-dry test, so they did not absorb as much moisture as calculated, or worse, contained free water
which added to the w/c ratio of the mixture.
13
10.0 Analysis and alterations to mix design and actual mixing process
If another mix trial was to be conducted in the laboratory there are a variety of amendments that
could be made to the mixture proportions, resulting either due to failures in the previous design or
any improvements that could be realised. These alterations will be outlined below within the context
of analysis into why such an outcome occurred.
Firstly, the concrete mix produced in the laboratory had a much larger slump than was designed
being approximately 80% higher than designed for. As such the concrete had far too much workability
with the simple amendment being to reduce the water to cement ratio of the mix. Yet the compressive
strength achieved was also 29% greater than the target strength of the concrete mix, although this
may have been affected by the sample being cured for a period longer than was accounted for in the
design stage of the project. As such any efforts to improve the slump of the material involving a
reduction in the water to cement ratio would also likely result in further discrepancies between the
actual and targeted concrete compressive strength. Hence, if the mixture proportions were to be
further adjusted to have a lower water to cement ratio, consideration would have to be given to
determining an optimal amount that reduces workability to an acceptable level without further
altering the achieved compressive strength.
Aside from altering the water to cement ratio to improve the concrete slump test closer to the
desired workability, smaller aggregate sizes could be utilised than initially proposed. With smaller
aggregate sizes there is a corresponding increase in the total surface area of the aggregates leading to
a sparser coating of cement paste and as such a reduction in the workability of the concrete. Although
this is once again offset by increased concrete compressive strength due to the greater interlocking of
particles. Another alternative concerning the aggregates is to use a type with sharp corners which will
tend to interfere with one another as concrete flows, which again will result in reduced slump and as
such workability.
The sample had a target sump of 120mm, but the sample produced greatly exceeded this amount
having a slump of 215mm. The discrepancy between the target and actual slump was extremely large
and may have occurred due to a variety of reasons including the mix design process as discussed
above, with the water to cement ratio and aggregate size as well as shape potentially contributing to
a greater than desired slump. Other potential causes of discrepancy beyond the mix design may have
been the unintentional incorrect carrying out of the test. The standards deal in approximate amounts
with the cone meant to be filled in 3 layers each with approximately one third of the cone volume,
with each of these layers then rodded 25 times uniformly over the cross section. As such poor layering
of the concrete and inconsistent rodding may have contributed slightly to the unexpected slump
results. Ultimately though it is unlikely these systematic errors were large factors in the slump
achieved and correcting for such inconsistencies in testing is not feasible as the purpose of the slump
test is a fast, on site test, render mechanising the process impractical. Despite this, all future test mixes
should be taken in ensuring the test is carried out in accordance with AS1012, Part 3 [8].
The concrete sample was found to have a compressive strength which greatly exceeded the design
goal of 35 MPa. This outcome is unexpected when observed in context with the results in the
preceding workability tests. With the concrete having greater strength and greater workability than
designed for this result was not predicted as greater strength is usually obtained at the expense of the
concrete workability. As discussed in previous sections, amending either of these issues in the mix is
likely to result in the other property of the concrete being further displaced from the design goal.
There are a variety of reasons this may have occurred.
Firstly, concrete compressive strength is usually measured after 28 days, yet the samples were
tested after 41 days. As they were in the curing tank for almost all of this time, a reduction in water
loss due to evaporation leads to an efficient hydration process, further increasing their strength [9].
14
For future mixes, it is vital the process be carried out properly and in accordance with the relevant
Australian standards. In spite of this being a potential reason for the conflicting results it would still
have been expected that for such a large slump that concrete strength would have been severely
diminished so other factors must also have affected the strength of the concrete without reducing its
workability. Other such factors relate to the curing and temperature.
In relation to the curing process, the ideal curing temperature is 230C [4]. The error margin in the
curing tank was 2 degrees. The effect of the difference in temperate is shown in figure 7 below, where
short term effects outweigh the long term. For the first few days, the difference could alter the
resultant strength by up to 5%, however as time reaches 30 days and beyond the effect of temperature
is negligible.
With such considerations in mind, future test samples should be cured for the correct amount of
time, and if possible, in an environment with a temperate above 130C in order to make the curing
effects negligible to strength.
Figure 7. Relationship between curing temperature and % effect compared to 28-day strength [4].
Amendments relating to the durability of the concrete have not been considered as testing of the
sample could not reveal any necessary changes to improve this aspect of the proposed concrete mix.
The density achieved is expected for normal strength concrete. If future mixes require a higher or
lower density than adjustment of the type of aggregate would be required. Ultra -lightweight concrete
for instance utilises aggregates with a low density of < 500 kg/m3 [6].
As previously noted, the concrete displayed a compressive strength result greater than the target
strength. Whilst it is unconventional to attempt to reduce the compressive strength of the concrete
in order to comply with the design specifications of the project this has been briefly considered. As
workability and strength are inversely related, achieving this improved workability has been prioritised
due to its clear failure to meet not only design but slump test standards. As a result, any amendments
to the concrete mix outlined above would be adopted, but not without consideration for attempting
to strike a balance between rectifying the two discrepancies in workability and strength.
When analysing the performance of the concrete overall it appears the most critical downfall of
the mix design proposed is its failure to meet maximum workability requirements, as well as exceeding
the design strength. As such the concrete mix failed to meet design specifications and further revisions
to the concrete mix design must be made to achieve a sample that is in line with the project
specifications.
15
11.0 Conclusion
The project concrete mix design representing a 0.02m3 volume of concrete was conducted over
two lab sessions, once for mixing and fresh concrete test and the other for compression test. The
British method was adopted and then adjusted to target a specified slump of 120mm and compressive
strength of 35 MPa. The final mixing proportions of batch concrete comprised 7.86 kg cement content,
13.69kg of fine aggregates, 22.64 of coarse aggregates and 4.84kg of water that were estimated during
the lab experiment.
Considering all the assumptions of saturated and surface dry condition and relevant information
provided, the obtained result for slump test was expectedly higher than the target value, a percentage
error of 79.2% between the target and actual height in slump [10]. Experimental errors likely
influenced the slump, as well as aggregates possibly not reaching SSD state, meaning that the
workability of concrete increased through a greater amount of free-water in mixtures. Additionally,
the compressive strength testing trial was performed following the moist-curing process, and the
obtained strength was again higher than the target strength (with 28.6% percentage error [10]). This
higher than expected result was likely due to the compressive test being conducted after 41 days,
rather than the 28 days recommended by Australian Standard AS1012.
In conclusion, it is necessary to adjust the mix design in order to better suit the project
requirements and ensure that the concrete has a workability that is compliant with industry standards
and a lower compressive strength to ensure cost efficiency. This could be achieved by adjusting the
water-cement ratio, or size and shape of the aggregates. It is understood that a compromise must be
made between the workability and the strength as they are inversely proportional, and it is suggested
that the workability requirements should be prioritised. The results from the concrete testing
additionally highlight the importance of following the exact specifications within AS1012 during the
preparation and testing processes, as well as having experienced personnel conduct these processes
to reduce the margin for human errors.
16
11.0 References
[1] P. Mehta, P. Monteiro (2014). Concrete: Microstructure, Properties, and Materials, Fourth
Edition. McGraw-Hill Education.
[2] F. Azhari (2019). Concrete Mix Design Lecture. Monash University.
[3] The Constructor. (2019). Concrete Slump Test for Workability -Procedure and Results.
[online]
Available
at:
https://theconstructor.org/concrete/concrete-slump-test/1558/
[Accessed 20 May 2019].
[4] The Constructor. (2019). Compaction Factor Test for Concrete Workability - Method and
Procedure. [online] Available at: https://theconstructor.org/concrete/compaction-factortest/1565/ [Accessed 20 May 2019].
[5] R. Dhir et al. (2019). Fresh Concrete Properties. Sustainable Construction Materials.
[6] F. Azhari (2019). Concrete Aggregates Lecture. Monash University
[7] Azhari, F (2019). Properties of Hardened Concrete Lecture. Monash University
[8] Australian standards, AS1012.3.1, 2014, Methods of testing concrete - Determination of
properties related to the consistency of concrete - Slump test
[9] Australian standards, AS1012.8.1, 2014, Methods of testing concrete - Method for making and
curing concrete - Compression and indirect tensile test specimens
[10] C. Foundation, "Percent Error", CK-12 Foundation, 2019. [Online]. Available:
https://www.ck12.org/chemistry/percent-error/lesson/Percent-Error-CHEM/. [Accessed: 21May- 2019].
17
Appendix A - Week 2 Mix Design Calculations
Stage 1 - Design for Strength
The water/cement ratio is found through its relationship with the compressive strength over time.
First estimating a w/c ratio of 0.5 as per convention and utilising Table 10, a value of 45 MPa after 28
days can be found for General Purpose Crushed Cement.
Table 10. Approximate compressive strength for different aggregate and cement types [2]
As shown in Figure 8, the second curve is then used to find the w/c ratio of the same cement
that has a 35 MPa strength after 28 days. The w/c is estimated to be 0.575.
Figure 8. compressive strength versus free water/cement ratio [2]
Stage 2 - Design for Workability
After the aggregate has absorbed enough water to become a saturated and surface dry condition,
the leftover water is classified as free-water. It is used in the process of cement hydration - to produce
C-S-H - and workability. To find the free-water content, Table 11 is used.
18
Table 11. Approximate free-water content (kg/m3) for various maximum aggregate sizes, aggregate types
and slump values [2]
As shown in table 11, only free-water content values are supplied for maximum aggregate sizes of
10mm and 20mm and thus linear interpolation is required. With a maximum aggregate size of 14mm,
that being crushed and a slump of 120mm, the approximate free-water is between 210 and 235 kg/m3
. Linearly interpolating these values gives a free-water content of 225 kg/m3 as shown in the
calculation below.
𝑓𝑟𝑒𝑒 𝑤𝑎𝑡𝑒𝑟 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 = 235 +
210 − 235
(14 − 10) = 225 𝑘𝑔 𝑚_"
20 − 10
Stage 3 - Cement Content
To find the cement content, equation 3 is utilised.
𝐶𝑒𝑚𝑒𝑛𝑡 𝐶𝑜𝑛𝑡𝑒𝑛𝑡 =
lmnn opqnm rstqntq
o ⁄r
(3)
𝐶𝑒𝑚𝑒𝑛𝑡 𝐶𝑜𝑛𝑡𝑒𝑛𝑡 = 225 ÷ (0.575) = 391.3 𝑘𝑔 𝑚_"
Stage 4 - Determine Total Aggregate Content
To calculate the total aggregate amount, Equation 4 was utilised.
𝑇𝑜𝑡𝑎𝑙 𝐴𝑔𝑔𝑟𝑒𝑔𝑎𝑡𝑒 𝐶𝑜𝑛𝑡𝑒𝑛𝑡 = 𝑊𝑒𝑡 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 − 𝐶𝑒𝑚𝑒𝑛𝑡 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 − 𝐹𝑟𝑒𝑒 𝑤𝑎𝑡𝑒𝑟 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 (4)
From Stages 2 and 3, the Cement content is 391.3 kg/m3 and Free-Water Content is 225 kg/m3.
It is known that the specific gravity of coarse aggregates is 2.95 and fine aggregates is 2.65.
Assuming a 50/50 ratio of course and fine aggregate, the specific gravity of the total aggregate can be
calculated,
𝑆𝐺 =
2.95 + 2.65
= 2.8
2
19
Figure 9. Relationship between wet density, free-water content and specific gravity [2]
Reading off Figure 9, a value of 2450 kg/m3 for the Wet Density is found, and the total aggregate
content can be calculated.
𝑇𝑜𝑡𝑎𝑙 𝐴𝑔𝑔𝑟𝑒𝑔𝑎𝑡𝑒 𝐶𝑜𝑛𝑡𝑒𝑛𝑡 = 2450 − 225 − 391.3 = 1833.7 𝑘𝑔 𝑚_~
Stage 5 - Proportion of Fine/Coarse Aggregate
The proportion of fine aggregate used in the cement mixture is dependent on w/c ratio, maximum
aggregate size, slump and fineness modulus. The relationships are shown on the graphs of Figure 10.
Figure 10. percentage of aggregate and free-water-cement ratio relationship [2]
20
The only unknown to be worked out is the fineness modulus, which is determined based on the
sieve analysis, for which the data is given. The sum of the cumulative percentage retained is divided
by 100 which results in a Fineness Modulus of 2.104. These results are shown in table 4 below.
Table 12. Sieve Analysis Results
Sieve Size
(mm)
% Retained
Cumulative %
4.75
0.6
0.6
2.96
3.2
3.8
1.18
13.2
17
0.6
21.6
38.6
0.3
22.2
60.8
0.15
28.8
89.6
0.075
9.4
-
<0.075
1
-
SUM
210.4
Now, the Figure 10 graphs can be utilised. For a maximum aggregate size of 10mm the percentage of
fine aggregate is found to be 41% and for 20mm the fine aggregate percentage is 33%. These two
values are then interpolated for the situational maximum aggregate size of 14mm to find a percentage
of fine aggregate of 37.8% as shown below.
𝑝𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑓𝑖𝑛𝑒 𝑎𝑔𝑔𝑟𝑒𝑔𝑎𝑡𝑒 = 33 +
33 − 41
(14 − 10) = 37.8%
20 − 10
This leaves 62.2% of the total aggregate mass to be coarse.
Thus, for a volume of 1m3, there would be 391.3kg cement, 693.1kg of fine aggregate, 1140.6 kg of
coarse aggregate and 225kg of free-water. In the lab, there the batch volume will be 0.02m3. This
would require 7.86kg cement, 13.86kg of fine aggregate, 22.81 kg of coarse aggregate and 4.50kg of
free-water.
21