Civil Engineering Materials 267
Concrete Mix Design
Dr. Faiz Shaikh
Concrete Mix Design
Aim is to achieve set of physical properties in
finished mix
Strength- most important property required of hardened concrete
• Water/cement ratio
• Compaction
– Workability
– Cohesiveness
Proportions of materials required to achieve maximum
strength oppose the proportions required to achieve
high workability.
In the design of concrete mix both requirements needs
to be considered.
1
Strength
• Characteristic strength f’c
• 28 day strength
• 95% of samples greater
Target strength, fcm
• average value about which test
results will lie, Quality control affects range
• higher than the characteristic
strength used by the structural The greater the degree of quality
designer control the steeper the curve and
• Average ~ 50% of samples greater the smaller the range of results.
Normal distribution curves can be produces for a particular mix.
Often test data is not available, and standard statistical data is
used to find the relationship between f ’c and fcm
Strength tests
Strength of samples known after 28 days
Continual monitoring of plant performance gives confidence
Average test results give target strength fcm
Test program
Commercial batching plant operation – continual testing 1
per truck
User – subcontract third party testing – up to one per truck
f cm f 'c k
where f cm target strength
f 'c characteristic strength
k statistical factor used to calculate the 95 percent confidence limits
standard deviation
2
Grade Characteristic Assumed standard
designation strength, f’c (MPa) deviation, (MPa)
Compression
Grades
20 20 4.5
25 25 4.8
32 32 5.3
40 40 5.9
50 50 6.2
When is used from the above table, take k=1.65
Number of specimens in the test sample k
2 6.31
3 2.92
5 2.13
10 1.83
20 1.73
30 or more 1.65
Strength requirements
AS3600 Concrete structures
Environment – internal, external, coastal, …
Structural role – footing, column, slab, beam
Loads carried – high, low, permanent, constr
Strength water/cement ratio
Net water:
•Total water
including mc of
. aggregates
•Not incl. absorbed
water in aggregates
3
Proportioning aggregates
Grading of aggregates affects final strength, slump
Use range of grading to reduce voids, maximise the density –
minimise cement paste requirements.
Concrete containing single size aggregate- low surface area of
agg., many large voids and require more cement paste to fill.
High proportion of fine aggregates – dense concrete but require
more cement paste to coat all surfaces – need to rationalise
Material proportions
Require relative
For a give w/c ratio, the higher the
aggregate/ cement ratio, the quantities for
higher the strength and lower Strength
the workability. Workability
British method
So there needs a balance between Free-water/cement from
strength and workability. curves
Select free-water content
Calculate cement content
Calculate total aggregate
Select fine and coarse
aggr
4
British Method – 1 water cement ratio
Type of Type of Compressive strengths
cement coarse (MPa)
aggregate Age (days)
3 7 28
Type GP uncrushed 17 24 38
orSR crushed 22 31 45
HE uncrushed 24 36 47
crushed 31 39 52
Cement & aggr
type give curve
fcm gives w/c
Use min for diff
aggregate types Max value
British method – 2 water content
Function of
Maximum aggregate size
slump Max. size Type of Slump
of aggregate 0-10 10-30 30-60 60-180
Free-water aggregate
(mm)
content (kg/m3) uncrushed 150 180 205 225
10
crushed 180 205 230 250
Fine uncrushed uncrushed 135 160 180 195
Coarse crushed 20
crushed 170 190 210 225
2 1 uncrushed 115 140 160 175
W f Wc 40
3 3 crushed 155 175 190 205
5
British method – 3 cement content
Divide free-water content
by free-water cement ratio
British method – 4 aggregate
Estimate density of compacted concrete
Total aggregates =
D–C–W
D density of
compacted conc
C cement cont
W water cont
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British Method – 5 fine /coarse
aggregate
Shape & size
coarse aggr
Fineness mod
of fine aggr
Free-water /
cement ratio
Required slump
British Method – 5 fine /coarse
aggregate
Fine aggregate content
Aggregate content x percentage fine
Coarse aggregate content
Aggregate content – fine aggregate
content
Mix of coarse aggregates
10mm:20mm ratio 1:2
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Moisture content of aggregates
Must allow for mc of aggregates in free-
water content
Reduce overall water content
– (wf Fagg) – (wc Cagg)
Increase aggregate contents
• Fine + (wf Fagg)
• Coarse + (wc Cagg)
Design with blended cements
Fly ash & blast furnace slag
Act as water reducing agents – can
decrease free-water content
Treat fly ash and BFS as cementitious
material
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Mix design exercise
Design Mix
Interior columns of a building
Characteristic compressive strength 25MPa
The materials to be used are:
• Type GP Portland cement 3150 kg/m3
• Coarse agg – 20mm and 10 mm crushed quartzite
density 2680 kg/m3
• Fine agg – washed river sand density 2610 kg/m3
fineness modulus 2.74, 43% passing 600m sieve
British Method – 1 water cement ratio
Type of Type of Compressive strengths
cement coarse (MPa)
aggregate Age (days)
3 7 28
Type GP uncrushed 17 24 38
orSR crushed 22 31 45
HE uncrushed 24 36 47
crushed 31 39 52
fcm = 25 + 1.65 x 4.8
= 32.9 MPa
Use 0.63 as max w/c ratio Max value
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British method – 2 water content
Max. size Type of Slump
of aggregate 0-10 10-30 30-60 60-180
aggregate
(mm)
uncrushed 150 180 205 225
10
crushed 180 205 230 250
uncrushed 135 160 180 195
Free-water 20
crushed 170 190 210 225
content (kg/m3) uncrushed 115 140 160 175
40
crushed 155 175 190 205
Fine uncrushed
Coarse crushed
2 1 0.66 * 195 + 0.33 * 225 = 203 Kg/m3
W f Wc
3 3
British method – 3 cement content
Divide free-water content
by free-water cement ratio
C = 203 / 0.63 = 322 kg / m3
10
British method – 4 aggregate
Estimate density of compacted concrete
(2390 kg/m3)
Total aggregates =
D–C–W
= 2390 – 322 – 203
= 1865 kg/m3
Note: Specific gravity of combined
aggregate is between 2.6 and 2.7
British Method – 5 fine /coarse
aggregate
45% fine
Fine =
0.45 x 1865
= 840 kg/m3
Coarse =
1865-840
= 1025 kg/m3
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British Method – 5 fine /coarse
aggregate
Mix of coarse aggregates
10mm:20mm ratio 1:2
10mm = 1/3 x 1025 = 342 kg/m3
20mm = 2/3 x 1025 = 684 kg/m3
sand = 840 kg/m3
cement = 322 kg/m3
water = 203 kg/m3
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