School of Civil Engineering and Construction

Published:

Introduction

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.

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(Richardson, 2002).

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.

Aims

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.

Objectives

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.

Theory

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.

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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

Casting Equipment

1. Concrete mixer

Figure 1: Shows Pictures of concrete mixer closed (left) and open (right)

2. Bucket (average size)

3. Measuring Cylinder

4. Shovel

5. Wheel Borough

6. Scale

7. Figure 2: Shows Compaction Factor Apparatus. (used to determine workability of concrete

mixture)

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

4/11/09)

• B.S. Slump cone (300mm high, tapering from a 100mm diameter top to a 200mm

diameter bottom)

• Slump rod (or steel tamping rod) (16 mm diameter, 600mm long, with rounded ends)

• Flat metal base plate (600 sq mm)

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9. Metal Rule (300mm long)

10. Metal Scoop

11. Levelling Trowel

12. Waste rag

13. Vibrating Table

14. Moulds

· 6 no. Cube Moulds (150mm by 150mm)

· 3 no. Cylinder Moulds (150mm by 300mm)

· 3 no. Mini beam Moulds (100mm by 100mm by 500mm)

15. Materials

· Course Aggregates (Stones)

· Fine Aggregates (Sand)

· Cement

· 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.

Striking Equipment

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

Testing Equipment

1. Compressive test machinery

Figure 4: Shows the Compressive test machine used to apply loads on cubes and cylinder

samples

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).

METHODOLOGY

Concrete Production:

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

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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

hopper.

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

plane blade.

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

compacted concrete)

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.

Slump Test:

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

workability.

Concrete Beam Casting & Curing:

1. Concrete was scooped out of the mixer into oiled moulds on the vibrating table (ensuring

even spread).

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:

• Cylinders loading Pace Rate was set at 5.30 KN/s

• Cubes loading Pace Rate was set at 6.80 KN/s

• Mini Beams loading Pace Rate was set at 0.200 KN/s

RESULTS

1. FRESH CONCRETE PROPERTIES TEST RESULTS

Compacting Factor Test Results:

Mix 1

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

content.

Mix 2

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.

Mix 3

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

Description

Weight of

Empty

Container

(Kg)

Partially

Compacted

concrete +

container

(Kg)

Fully

Compacted

concrete +

container

(Kg)

Partially

Compacted

concrete

(Kg)

Fully

Compacted

concrete

(Kg)

Compacting

Factor

Mix 1 (0.45

w/c ratio)

8.1 18.08 20.18 9.98 12.08 0.83

Mix 2 (0.55

w/c ratio)

8.12 19.7 21.80 11.58 13.68 0.85

Mix 3 (0.65

w/c ratio)

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

compacted concrete)

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).

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Apparent workability shown below was determined by using Compacting factor table in

Appendix 2 (Kew 2009).

Mix Compacting Apparent Workability

Description Factor

Mix 1 (0.45 0.83 Low

w/c ratio)

Mix 2 (0.55 0.85 Medium

w/c ratio)

Mix 3 (0.65 0.98 Very High

w/c ratio)

Slump Test Results:

Table 3: Shows Slump Results of Concrete Mixes with variable water/cement ratio

Concrete

Mix

Water/Cement

Ratio

Slump

Results

(mm)

Type of

Slump

Observations Apparent

workability

1 0.45 0 Zero

slump

There was no slump as

the mix was too dry

therefore indicating

poor mobility,

flowability and

workability

Not workable

at all

2 0.55 13 True

Slump

True slump was

obtained indicating

good workability and

was within the + 25

mm or 1/3 of required

value allowable

tolerance (BS 5328)

Low

3 0.65 150 Collapsed

Slump

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

properties.

Very high

*Apparent workability shown above was determined by using Slump Results Table shown

in Appendix 2 (Kew 2009).

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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.

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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.

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Table 4: Shows Compressive Strength (N/mm2) of Cylinder Samples and Cube Samples for

Concrete Mixes 1, 2 & 3 with varying water/cement ratio.

Mix

Sample

Type

Sample

Name

Maximum Failure

Load (KN)

Surface Area

(mm2)

Compressive

Strength

(N/mm2)

1

(0.45 w/c ratio)

Cube

Cube

Cylinder

2U

H6

Z7

730.9

759.8

296.9

22500

22500

471.24

32.48

33.77

630.04

2

(0.55 w/c ratio)

Cube

Cube

Cylinder

AZ

7G

H35

638.9

568.9

304.8

22500

22500

471.24

28.40

25.28

646.80

3

(0.65 w/c ratio)

Cube

Cube

Cylinder

Z7

OQ

W9

584.1

596.6

331.6

22500

22500

471.24

25.96

26.52

703.68

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.

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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

were varied.

Table 5: Shows the effects of increasing the water cement ratio on flexural strength.

Mix

Description

Maximum Failure

Load (KN)

Length between

supports (mm)

bd2 (mm3) Flexural Strength

(N/mm2)

Mix 1(0.45

w/c ratio)

8.68 300 1000000 2.604

Mix 2(0.55

w/c ratio)

12.96 300 1000000 3.888

Mix 3(0.65

w/c ratio)

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.

DISCUSSION

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.

CONCLUSION

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

factors.

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

those structures.

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.