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In construction projects, concrete, along with steel, wood, glass, etc, is one of the most essential materials that are needed for a successful manufacture of a structure. It one of the most common materials on a construction site and accounts for billions of pounds everywhere across the world. Due to ever-increasing machinery and technological advancements concrete can now be made of a mixture of compound materials, nevertheless the necessary components of concrete are course or fine aggregates, Portland Cement and water. In the current times, concrete structures are manufactured every day and to sustain a safe environment for people, so it is vital that that the structures that are built are sturdy, durable and do not cause any hazards to people. It is therefore a huge task for construction companies to guarantee that the structures that are built are done so to meet all the specific safety codes, British Standards or the Euro Code Standards. The properties of concrete are very vital as they provide the necessary stability that structures are dependent on to maintain their sturdiness. As a result it is essential to research and be aware of the distinctive components of concrete and its properties, and how in this experiment these might affect the way that concrete performs when changing some variables.
The workability of a concrete mix gives a measure of the ease with which fresh concrete can be placed and compacted. The concrete should flow readily into the form and go around and cover the reinforcement, the mix should retain its consistency and the aggregates should not segregate.
There are four factors that can affect the workability are:
1. Consistency: The degree of consistency is depended on the nature of works and type of compaction.
2. Water/cement Ratio or Water Control of a concrete: Water/cement ratio is the ratio of water in a mix to the weight of cement. The quality of water that required for a mix is depended on the mix proportions, types and grading of aggregate.
3. Grading of Aggregate: The smooth and rounded aggregate will produce a more workable concrete than the sharp angular aggregate.
4. Cement Content: The greater workability can be obtained with the higher cement content.
The aim of this experiment was to establish the effects of water to cement ratio on the
fresh properties of concrete (workability), and its effect on the hardened properties
of concrete (strength). Furthermore to increase the understanding in making a concrete mixture and working out the water content that needs to be added to the mixtures. And last to expand on the understanding of the importance of fresh and hard properties of concrete.
The objectives of the experiment were to make three concrete mixtures by altering their water/cement ratios (0.47, 0.55 & 0.65) and to find out the water content to use for the three mixtures. To do a variety of tests such as the slump test, compacting factor test on fresh concrete and to carry out compressive and flexural strength tests of hardened concrete. Then finally to discuss how features such as variation in the water/cement ratio affects the workability
and strength of concrete.
Concrete Production, concrete is a mixture that is made up of three components, cement, water and aggregate. The water and cement are mixed together to produce a thick paste, to which then measured out aggregates are added to. The aggregates that are added are mainly composed of usual materials such as sand, gravel and crushed rocks, however due to the latest advanced technology; it has been known that other materials such as car tyres and crushed glass to be also used as aggregates. The cement is produced by blending limestone and clay, and burning it in a rotary kiln, this results in the formation of a clinker, to which gypsum is added. The mix is then ground down to fine powder cement, in which the most common is called Portland Cement. The cement/water slurry solidifies through a chemical reaction called 'hydration', the reaction produces immense heat so fresh concrete must by no means be handled with unprotected bare hands. During the winter season, temperatures drop below 2°C, so the chemical 'hydration' reaction may be very slow as heat is needed as a catalyst to speed up the collision of the particles. Therefore concrete pours during these seasons are not suitable as the concrete will not set. Initially this reaction is slow to start with, so this allows for the concrete to be transported and poured before it is hardened, and the theory states that complete 100% hydration takes place after 28 days.
Properties of Concrete: There are four key properties that are desired in fresh concrete i.e.
good workability, compactability, mobility and stability. The most desired properties for
hardened concrete are strength and durability. The concrete should have compressive strength
(resist squeezing), tensile strength (resist stretching) and flexural strength (resist bending).
All these strengths are highly dependent on the water/cement ratio and aggregate used in the
mixture, the degree of compaction and the age of the concrete. Curing concrete under water
over time allows hydration to continue hence giving it strength.
The concrete used in this experiment was a C30 concrete grade and according to B.S. 5328
the compressive strength for this grade at 28 days is 30.0 N/ sq mm which can also be written
as 30 MPa which is adequate for use in beams, however this is only an estimation as there
are other factors (mentioned above) that affect concrete strength. In this experiment the slump
test and the compacting factor test were used to assess the workability and uniformity of
concrete. The deflection/ flexural strength test was carried out to evaluate the strength of the
concrete beam (mini beam sample) and find the failure load of the mini beam (100mm by
100mm by 500mm). The compressive strength was carried out to determine the maximum
failure load of the cube samples (150mm by 150mm) and the cylinder samples (150mm by
300mm) (Barnes 1992).
MATERIALS AND EQUIPMENT
1. Concrete mixer
Figure 1: Shows Pictures of concrete mixer closed (left) and open (right)
2. Bucket (average size)
3. Measuring Cylinder
5. Wheel Borough
7. Figure 2: Shows Compaction Factor Apparatus. (used to determine workability of concrete
Taken from; http://www.enkaymachine.com/eke337.htm [Online] (Accessed on 4/11/09).
8. Figure 3: Slump Test Apparatus
Taken from; http://www.intec.com.my/slump_test_apparatus.htm [Online] (Accessed on
â€¢ B.S. Slump cone (300mm high, tapering from a 100mm diameter top to a 200mm
â€¢ Slump rod (or steel tamping rod) (16 mm diameter, 600mm long, with rounded ends)
â€¢ Flat metal base plate (600 sq mm)
9. Metal Rule (300mm long)
10. Metal Scoop
11. Levelling Trowel
12. Waste rag
13. Vibrating Table
· 6 no. Cube Moulds (150mm by 150mm)
· 3 no. Cylinder Moulds (150mm by 300mm)
· 3 no. Mini beam Moulds (100mm by 100mm by 500mm)
· Course Aggregates (Stones)
· Fine Aggregates (Sand)
· Water (Tap)
*Note: Aggregate used was natural aggregate used was from London. Therefore no need for
determining aggregate moisture content as aggregate is assumed to be laboratory dry to
SSD. Hence no considerable effect on water-cemet ratio.
1. Pressure pipe (for striking cubes and cylinders)
2. Brushes (Soft and Hard metal brushes)
3. Oil, oil brush and rugs (for cleaning moulds before storing)
4. Crayon (for labelling concrete samples)
5. Curing room
1. Compressive test machinery
Figure 4: Shows the Compressive test machine used to apply loads on cubes and cylinder
2. Deflection test machinery (Picture shown in figure
3. Load reader/display
4. Concrete samples
5. Digital Camera
*Personal Protective Clothing was worn on all days of the experiment (Safety boots and
Coats, individuals handling concrete wore protective gloves).
1. Aggregates were readily weighed and placed into buckets. Quantities (constants) used in
all Concrete Mixes are shown below:
Material Quantitative Weight (Kg)
Cement (CEM1) 6.50
Fine Aggregate (Sand) 16.55
Natural Course Aggregate (Stones) 26.00
2. The amount of water required was determined by using the formulae shown below.
· Water content = (water/cement ratio) x cement weight.
3. Water was measured into a bucket using measuring cylinders.
4. The water/cement ratio was set as the variable between 3 Concrete Mixes (to determine
the effect of water/cement ratio on the strength and workability of the concrete). Water
content quantities used are shown on table 1.
Table 1: Water/Cement Ratio (variable) for Concrete Mixes 1, 2 &3
Concrete Mix Water/Cement Ratio Water Content (litres)
1 0.47 3
2 0.55 3.6
3 0.65 4.25
*See Appendix 1 for Actual Calculations Carried Out.
5. The concrete mixer paddles and pan were lightly dampened before aggregates were
placed in the mixer.
6. Course and fine aggregates were placed into the mixer and mixed for 30seconds.
7. Half the water required for the mix was added to the mixture and the contents were
further mixed for 1 minute.
8. The contents were covered and left for 8 minutes, to allow aggregates to absorb water,
(because aggregates are porous therefore they should soak in water into voids to get a good
mix and bonding with cementious (water/cement) paste).
9. Cement was spread evenly over the aggregates and mixed for 1 minute.
10. The remaining water was added and the contents were mixed for 2 minutes ensuring
homogeneity of the mix.
11. Workability tests were then carried out, in the order shown below.
*Note; immediately after each test the used concrete was returned into the mixer and the
contents were remixed for 30 seconds.
FRESH CONCRETE TESTS
Compacting Factor Test:
1. Trap doors of all hoppers were shut prior to beginning the test.
2. Sample of freshly mixed concrete was scooped from the mixer into the upper hopper, the
concrete sample was filled up to the brim of the upper hopper.
3. The trap-door of upper hopper was opened, to enable concrete to fall into the lower
4. After all concrete had been collected onto lower hopper, the trap-door of the lower hopper
was then opened and the concrete allowed to fall into the cylinder.
5. Excess concrete remaining above the top level of the cylinder was then cut off using a
6. The concrete collected in the cylinder was then weighed. (This weight is known as the
weight of partially compacted concrete).
7. The concrete filled cylinder was vibrated to obtain full compaction, and more concrete
was added to the cylinder as required to ensure the vibrated/compacted concrete was
filled to the brim of the cylinder.
8. The now fully compacted concrete in the cylinder was weighed.
9. The compacting factor was then obtained using the formulae shown below.
Compacting factor = (Weight of partially compacted concrete)/(Weight of fully
Figure 5: Shows steps followed during the compacting factor test.
1) Compacting factor equipment.
2) Partially compacted weight is taken on a scale,
3) The concrete is vibrated/compacted
on a vibrating table and then the contents are toped up and vibrated to the rim container and the
partially compacted weight was taken.
1. Concrete was thoroughly mixed in the concrete mixer.
2. The slump cone was dampened to prevent concrete sticking to it.
3. The slump cone/mould was placed on the centre of the metal plate and one individual was
asked to stand on the foot pieces on both sides of the mould.
4. The mould was filled in 3 equal depth layers and each layer was rod 25 times using the
steel slump rod (ensuring even spread of blows covering over the whole area).
5. Concrete was heaped over the top of the cone and with a rolling motion of the rod over
top of the mold the concrete was levelled thus removing the excess concrete.
6. The spillage was carefully removed from the sides of the mould and the base plate
7. The mould/cone was carefully and slowly lifted vertically upwards.
8. The slump cone was turned upside down and placed next to the molded concrete and the
rod was laid across the slump cone and the distance (slump) between the underside of the
rod and the highest point of the moulded concrete were read using a metal rule.
9. There are different kinds of slump a collapsed slump, sheared slump and a true slump.
The first two slump types indicate bad workability and a true slump indicates good
Concrete Beam Casting & Curing:
1. Concrete was scooped out of the mixer into oiled moulds on the vibrating table (ensuring
2. Concrete was vibrated throughout the pour to eliminate voids and to enable compaction
of concrete by switching on the vibrating table.
3. The vibrating motion also levelled the concrete.
4. The concrete was left to set on the mould for 24 hours
5. After which concrete was struck and placed in the curing room over 14 days.
HARDENED CONCRETE TESTS
Concrete Sample Testing:
1. Compressive Strength Tests; were carried out on cube and cylinder samples.
2. Flexural Strength Tests; were carried out in the mini beams.
3. The machines where loaded with concrete sample and load applied was set to zero
before running the test.
4. Base and top plates (spacers) were used to determine to provide platforms for the
concrete specimens and to also help provide even distribution of load.
Figure 6: Shows the type of spacers to be used for specific specimen shape and size
5. The load was applied by the machine till maximum failure load was reached.
6. This reading was taken and the machine cleaned off concrete debris before running tests
for other samples.
*Note the loading Pace Rates varied for different sample shape as shown below:
â€¢ Cylinders loading Pace Rate was set at 5.30 KN/s
â€¢ Cubes loading Pace Rate was set at 6.80 KN/s
â€¢ Mini Beams loading Pace Rate was set at 0.200 KN/s
1. FRESH CONCRETE PROPERTIES TEST RESULTS
Compacting Factor Test Results:
Observations: The Concrete Mix appeared to be dry and did not pass through when the trap
door of the upper hopper was opened. The concrete mix was helped through the trap door to
the lower hopper by pushing it with a metal rod through the first trap door. The same was
done in order to get it through the second trap door into the container. This showed that it
was a bad mix with bad flowability, mobility and workability properties due to low water
Observations: The concrete mix was passed through the hopers with better ease than mix 1,
however only ¼ of the contents went through, the rest was forced through both trap doors
with a metal rod. Therefore the flow ability and workability properties of this mix were bad,
but better than mix1, owing it to the increased water content in mix 2.
Observations: The obtained concrete mix was a wet mix (a bit too wet) with what would
appear to be good flowability properties as all contents went through the hopers and trap
doors with one sweep and much ease. Therefore the flowability and workability properties
were the best observed for all 3 mixes, but too much water content is not good either.
The compacting factor test was worked out for all the 3 Concrete Mixes and results are
shown in table 2 below.
Table 2: Shows the Compacting Factor of Concrete Mixes 1, 2, and 3 with varying
water content ratios.
Mix 1 (0.45
8.1 18.08 20.18 9.98 12.08 0.83
Mix 2 (0.55
8.12 19.7 21.80 11.58 13.68 0.85
Mix 3 (0.65
8.12 21.08 21.34 12.96 13.22 0.98
*The calculations were carried out on Microsoft Excel using the formula shown below.
Compacting factor = (Weight of partially compacted concrete)/(Weight of fully
BS 1881: Part 103 states that concrete is deemed unsuitable if its compacting factor is
below 0.70 or above 0.98. For normal concretes the compacting factor normally lies
between 0.80 and 0.92 (Jackson & Dhir 1996).
Apparent workability shown below was determined by using Compacting factor table in
Appendix 2 (Kew 2009).
Mix Compacting Apparent Workability
Mix 1 (0.45 0.83 Low
Mix 2 (0.55 0.85 Medium
Mix 3 (0.65 0.98 Very High
Slump Test Results:
Table 3: Shows Slump Results of Concrete Mixes with variable water/cement ratio
1 0.45 0 Zero
There was no slump as
the mix was too dry
2 0.55 13 True
True slump was
good workability and
was within the + 25
mm or 1/3 of required
tolerance (BS 5328)
3 0.65 150 Collapsed
Collapsed slump was
obtained and the slump
exceeded the allowable
tolerance stated in BS
5328. The slump cone
was 300mm high and
the concrete mix
slumped by half that
value to 150mm. This
indicates that the mix
was too wet and this
affected its cohesive
*Apparent workability shown above was determined by using Slump Results Table shown
in Appendix 2 (Kew 2009).
(K0837225) Page 12
Mix 1 Dry Mix/ Zero Slump Mix 2 Wet mix /13mm True Slump Mix3 Mix too wet/ collapsed slump
Figure 7: Shows the Slump Results Obtained for concrete mixes with varying water
cement ratios. (Mix 1 w/c ratio 0.45, Mix 2 w/c ratio 0.55 and Mix 3 w/c ratio 0.65).
2. HARDENEDED CONCRETE PROPERTIES TEST RESULTS
Figure 8: Shows the cube specimen being loaded into the compressing machine and on the right ,
the classical cube hour glass failure mode on one of the cube specimen.
(K0837225) Page 13
Figure 9: Shows the cylinder specimen being loaded into the compressing machine and on the
right, the failure mode on 3 of the cylinder specimens.
Figure 10: Shows a mini beam failing when subjected to Flexural Loads. This is the classical failure
mode of beams. The beam undergoes tensile and flexural strain resulting in bending and snapping of
the beam. Concrete is generally brittle and this makes it weak in tension. Hence the need for
reinforcement of concrete, steel is good in tension so it lends that quality to concrete, resulting in
better stronger structures.
(K0837225) Page 14
Table 4: Shows Compressive Strength (N/mm2) of Cylinder Samples and Cube Samples for
Concrete Mixes 1, 2 & 3 with varying water/cement ratio.
(0.45 w/c ratio)
(0.55 w/c ratio)
(0.65 w/c ratio)
The results above are indicative of the effects of w/c ratio on the strength of concrete. At
0.45 w/c ratio the strength was 630.4< at 0.55 N/mm2, w/c ratio strength was 646.80 N/mm2
and at 0.65 w/c ratio strength was 703N/mm2. The trend observed here is that as w/c ratio
increases strength of concrete increases. However the concrete cubes show the opposite. This
may be down to the contact surface area to volume ratio being larger in cubes than in
cylinders, hence more water being absorbed during curing such that it is excess hence starts
to have reverse effect.
(K0837225) Page 15
Figure 11: Comparison between concrete specimen shapes e.g. (Cube Vs Cylinder)
(Influence of test conditions. Table above show that specimen shape and size is also
influential on the compressive strength. Therefore measured strength of concrete is also
affected by height diameter ratio. This is to just show that test conditions can also affect the
determination of concrete strength. In BS 1881: Part 116 specifies that 150mm cube test are
only used for quality control purposes. Whereas BS 1881: Part 120 indicates that cylinder
test specimens are used to carry out compressive strength tests for in situ concrete and
precast members. A correction factories usually applied to the cylinder strength to obtain an
equivalent cube strength, it takes into account the specimen height /diameter ratio (i.e.
300mm/150mm = 2.). This explains the high compressive strength results obtained in
cylinder specimens than in cube specimens despite the being made off the same batch of
concrete. It should also be considered that the loading Pace Rates for cubes (and cylinders
Table 5: Shows the effects of increasing the water cement ratio on flexural strength.
bd2 (mm3) Flexural Strength
8.68 300 1000000 2.604
12.96 300 1000000 3.888
11.55 300 1000000 3.465
The trend obtained from the results shown above indicates that increasing w/c ratio increases
flexural strength. Af hydration strengthens the bonding between the cementious material and
the aggregates. However like all other factors, too much of anything is not good. If the mix
has excess water it will result in reduced flexural strength and results in bleeding of concrete
thus a weakened structure with pours in them. Again the normal distribution curve can me
expected with extremes.
One type of test is not enough to indicate the workability of the concrete as a whole. Use of
various tests bring out various properties that determine workability, for example, the
compacting factor can indicate how workable in the concrete will be in terms of how easily
can the concrete be vibrated and compacted. It is also a good indicator of the mobility and
flowability of concrete. It Shows how easily the concrete can be pumped from a concrete
skip into shutters, how easily the concrete will pass through the skip trap door when on
casting real structure on site. On the other hand the slump best indicates how workable the
concrete is in terms of its cohesive nature and segregation of its aggregates. It is important to
carry more than one of these tests to indicate various workability factors. These tests can also
be carried out at various stages between concrete production and casting. The common
construction site test (In situ test) is the slump test, it serves as the last point of quality check
prior to casting, and all other workability factors are normally carried out on the concrete
production sites. For example, the compactability factor will be most useful on production as
other mobility enhancing admixtures may be added prior to transporting concrete to site,
hence saving time, money and other complications that may arise from delaying site
programmes. From table 2 the results obtained from all mixes had compacting factors
between 0.70 and 0.98 hence indicating that all the tested concrete mixes would be
acceptable under the BS 1881. This certainly does not mean that all mixes had good
workability properties. Jackson & Dhir (1996) state that some of the basic assumptions for
the test are not correct and should not be solely relied upon extensively as they can be
misleading. As concrete mixes can have same compacting factor but may not always require
the same amount of work to reach full compaction as compaction cannot be justified in the
true sense. From the results in table 2 it shows that changing the water/cement ratio affected
the compacting factor. Increasing the water cement ratio increased the compacting factor
therefore the workability of the concrete. All these tests have limits, for example placing
more water would have resulted in decreasing compactability factor as increasing the water
content will result in lowered compacting factors. (Compacting liquid materials do not result
in changes between partially compacted weight and fully compacted weight, hence if more
excess water is added the mix will have lower differences between partially compacted
weight and fully compacted weight. Hence giving rise to normal distribution curves for the
compressive tests. This also applies to flexural strength and durability of the concrete.
In conclusion it is clear that too little w/c ratio reduces the strength of concrete just as well as
too much w/c ratio will result in porous concrete. Therefore adequate amounts need to be
used to gain the best results. The best way of getting accurate assumptions on concrete is to
consider various factors. Increasing the water content ratio generally increases the strength
but may also result in shrinkage of the concrete hence altering durability and permeability
Q1: Report all the results - fresh properties (slump value and the shape of the slump) and
hardened properties (strength) of the concrete and comment on the results. See Results
Section for Answers.
Q2: Why the need to measure the fresh and hardened properties of the concrete?
Fresh properties are only of much importance in the stages of the concrete mix. These
help concrete producers spot problems early on the stage before structures are cast thus
potentially saving money, time and preventing unstable structures form being built by
spotting and correcting problems with concrete at an early stage. Also this helps prevent
the need to strike down newly built structures due to instability of concrete mixes used.
Fresh properties can help indicate how much work labours will have to do on site and
consequently the energy and money that will be required when casting concrete on site.
On the other hand hardened concrete properties are important in determining and the life
span of the concrete in the form of s concrete structure. The hardened properties are
important in observing and maintaining the strength of the structure and its durability.
Other hardened factors are permeability and shrinkage of the concrete structures after
being built due to harsh weathers and conditions. The latter factors are of much
importance in structures like dams which require high water retaining properties.
Therefore both properties help in the development and maintenance of a good quality
structures and ensuring long life span. Whilst providing adequate safety to the habitats of
Q3: Concrete is usually tested at 28-Days for its compression strength. Why at 28-Days?
The specimens should be cured under water and for normal concrete they should have
reached maximum strength at 28 Days. Concrete hardening process (Hydration) is
thought to reach its final strength in 28 Days as the reaction slows to a halt and adding
more water or curing concrete past that stage will sure minute or no further significant
changes in concrete strength.
Q4: As for reinforced concrete beam, describe the need to place reinforced steel in
concrete beam, the purpose of cover/spacing, the diameter of the steel used and why
concrete beams need to be reinforced?
Concrete is good in compression meaning it has high resilience to compressive forces but
is very weak in tension. As noted in the results the beams failed at much lower loads than
both cubes and cylinders, although there are other factors that play a role here that is the
general observation. Hence concrete reinforcement is required, it has good tensile
resilience and when concrete and steel are combined they result in components strong in
both tensile and compressive properties. The purpose of concrete cover is to protect steel
from corrosion, due to air reacting with steel and prevent rust formation due to water.
Corrosion and rust results in weakened concrete structure as may result in loss of
resilience to tensile forces. So the concrete cove4r provides protection and a neutral
environment for steel. Concrete cover usually ranges around 500mm from the steel bars.
Excess cover is not good as it makes the structure more susceptible to chipping and hence
weakens the cover itself and increases chances of steel corrosion taking place. The
diameter of steel used can vary according to the purpose of the structure but over
reinforcement can also bring about imbalances to the structural stability and may result in
a weakened structure. The normal diameter used ranges between 10-30mm, this makes it
easier to bend and alter on site as well as provide ease of manual handling for steel fixers.