2 high strength concrete

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High Strength Concrete

CVLE 519

Concrete Technology

Dr. Adel El Kordi

Professor

Civil and Environmental

Engineering Department

Faculty of Engineering

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A concrete that meets special combinations of

performances and uniformity requirements that cannot

always be achieved using conventional and normal

mixing, placing and curing.

High-strength concrete by definition is:

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• High-strength concrete has a compressive

strength greater than 40 MPa. In the UK, BS EN

206-1 defines High strength concrete as concrete

with a compressive strength class higher than

C50/60.

• High-strength concrete is made by lowering the

water-cement (W/C) ratio to 0.35 or lower. Often

silica fume is added to prevent the formation of

free calcium hydroxide crystals in the cement

matrix, which might reduce the strength at the

cement-aggregate bond.

High-strength concrete

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• High-strength concrete is typically used in high-rise

structures. It has been used in components such as,

shear walls, and foundations. High strengths are also

occasionally used in bridge applications as well. A

high-rise structure suitable for high-strength concrete

use is considered to be a structure over 30 stories.

• High –strength concrete is occasionally used in the

construction of highway bridges. High-strength

concrete permits reinforced or prestressed concrete

girders to span greater lengths than normal strength

concrete girders. Also, the greater individual girder

capacities may enable a decrease in the number of

girders required. Thus, an economical advantage is

created for concrete producers in that concrete is

promoted for use in a particular bridge project as

opposed to steel.

Applications of High-Strength Concrete

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Generally 28 days–compressive. Strength

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This ultra high strength concrete specimen suffered

from a shear failure, where one small section

completely separated from the rest

7shear failure

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Time-dependent probability of concrete cover spalling in a typical reinforced

concrete bridge deck. (NC=normal-strength concrete; HPC=typical high

performance concrete; HPC-IC=high performance concrete with internal curing.

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Temperature distribution at various depths during fire exposure

in normal-strength concrete (NSC) and high-strength concrete

(HSC) columns

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Punching Shear Resistance of High-Strength

Concrete Slabs

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Punching Shear Resistance of High-Strength

Concrete Slabs

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types W/C Fc28，MPa notes

HSC with good

mobility0.25～0.40 50.0～70.0

15～20cm slump

large amount of

cement

high-strength

with normal

consistency

0.35～0.45 55.0～80.0

5～10cm slump

large amount of

cement

high-strength

without slump0.30～0.40 55.0～80.0

< 25mm slump

normal amount of

cement

high-strength

with low W/C0.20～0.35 100.0～170.0 admixture

RPC 0.05～0.20 70.0～240.0 70.0Mpa or above

Classifications of High Strength Concrete

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High-Strength Concrete Materials

Cement

• Use cement yielding highest concrete strength at extended ages (91-days)

• Cement should have min. 7-day mortar cube strength of 30 MPa

• Cement contents between 400 and 550 kg/m3

• All types of cement are applicable.

• To maintain a uniform high strength conrete:

1- Tricalcium silicate content varies by< 4%

2- Ignition loss varies by < 0.5%

3- Fineness varies by < 375 cm2/g (Blaine)

4- Sulphate (SO2) level should be maintained at optimum with variations limited to ± 0.20%.

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It would be difficult to produce high-

strength concrete mixtures without

using chemical admixtures. A

common practice is to use a

superplasticizer in combination with a

water-reducing retarder. The

superplasticizer gives the concrete

adequate workability at low water-

cement ratios, leading to concrete

with greater strength. The water-

reducing retarder slows the hydration

of the cement and allows workers

more time to place the concrete.

Chemical admixtures

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Chemical admixtures

• All type of HRWR, Superplasticizers or PolyCarboxylates can be used.

• Air-entraining admixtures are not necessary or desirable in high-strength concrete as it decreases the value of the required compressive strength.

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Aggregates

• Coarse aggregate: 9.5 - 12.5 mm (3/8 -

1/2 in.) nominal maximum size gives

optimum strength

• Combining single sizes for required

grading allows for closer control and

reduced variability in concrete

• For 70 MPa and greater, the FM of the

sand should be 2.8 – 3.2. (Lower may

give lower strengths and sticky mixes)

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Supplementary Cementing Materials

• Finely divided mineral admixtures, consisting mainly of

fly ash, silica fume and slag cement have been widely

used in high-strength concrete.

• Dosage rate varies from 5% to 30% or higher by mass of

cementing material depending on the type of mineral

used .

Fly ash (Class C ) - Metakaolin (calcined clay) - Silica fume -

Fly ash (Class F) – Slag - Calcined shale (from left)

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Pozzolans, such as fly ash and silica fume, are

the most commonly used mineral admixtures in

high-strength concrete. These materials impart

additional strength to the concrete by reacting

with portland cement hydration products to

create additional C-S-H gel, the part of the paste

responsible for concrete strength.

Pozzolans

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Chemical Analysis of Fly Ash, Slag and Silica Fume

Class F

fly ash

Class C

fly ash

Ground

slag Silica fume

SiO2, % 52 35 35 90

Al2O3, % 23 18 12 0.4

Fe2O3, % 11 6 1 0.4

CaO, % 5 21 40 1.6

SO3, % 0.8 4.1 9 0.4

Na2O, % 1.0 5.8 0.3 0.5

K2O, % 2.0 0.7 0.4 2.2

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Properties of Fly Ash, Slag and Silica Fume

Class F

fly ash

Class C

fly ash

Ground

slag

Silica

fume

Loss on

ignition, %2.8 0.5 1.0 3.0

Blaine

fineness,

m2/kg

420 420 400 20,000

Relative

density2.38 2.65 2.94 2.40

ASTM C 150 → L.O.I. ≤ 3% for O.P.C. Typical value = 1.4

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Quantity of

Fly ash, slag and Silica fume in Concrete

by Mass of Cementing Materials

Fly ash

15% to 40%Class C

15% to 20%Class F

30% to 45%Slag

5% to 10%Silica fume

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Silica fume slurry consists of approximately 50 percent silica

fume and 50 percent water, by mass. When first introduced to

the market, slurried silica-fume products often contained water

reducers or high-range water reducers. Today, slurry is available

without any such additions.

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Effects Of Supplementary Cementing Materials On

@Freshly Mixed ConcreteReduced Increased No/Little Effect/Varies

Fly ash SlagSilica

Fume Nat. Pozzolans

Water Requirements

Workability

Bleeding & Segregation

Air Content

Heat Of Hydration

Setting time

Finishing

Pump ability

Plastic Shrinkage

Cracking

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Effects Of Supplementary Cementing Materials On

Hardened ConcreteReduced Increased No/Little Effect/Varies

Fly ash SlagSilica

Fume

Nat.

Pozzolans

Strength Gain

Abrasion Resistance

Drying Shrinkage &

Creep

Permeability

Alkali-Silica Reactivity

Chemical Resistance

Carbonation

Concrete Color

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1- For high strength concrete, the indirect tensile

strength may be about 5 percent of the compressive

strength.

2- At low strengths, the indirect tensile strength may be

as high as 10 percent of the compressive strength.

Tensile splitting strength

3-The tensile splitting strength

was about 8 percent higher

for crushed-rock-aggregate

concrete than for gravel-

aggregate concrete.

4- The indirect tensile strength

was about 70 percent of the

flexural strength at 28 days.

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Mix Design Procedure:

Fig-1 :Relation between compressive strength and reference number

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Fig-4: Relation between compressive strength and reference number

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Fig-5: Relation between water-cement ratio and Reference Number

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Table – 1: Aggregate cement ratio (by weight) required to give four degrees of workability with different water –cement ratios using ordinary Portland cement

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Example 1Design a high strength concrete for use in the production of precast

prestressed concrete to suit the following requirements:

Specified 28-day works cube strength = 50 MPa

Very good degree of control; control factor = 0.80

Degree of workability = very low

Type of cement = ordinary Portland cement

Type of coarse aggregate = crushed granite (angular) of maximum size

10mm.

Type of fine aggregate = natural sand

Specific gravity of sand = 2.60

Specific gravity of cement = 3.15

Specific gravity of coarse aggregates = 2.50

Fine and coarse aggregates contain 5 and 1 percent moisture

respectively and have grading characteristics as detailed as follows:

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IS sieve size Percentage Passing

Coarse aggregate Fine aggregate

20mm 100 100

10mm 96 100

4.75mm 8 98

2.36mm - 80

1.18mm - 65

600 micron - 50

300 micron 10

150 micron - 0

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Fig-6: Combining of Fine aggregates and Coarse aggregates

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DESIGN OF MIX

Mean strength = (50 / 0.80) = 63 MPaReference number (fig.1)= 25Water cement ratio (fig 5) = 0.35For a 10mm maximum size aggregate and very low workability, the aggregate-cement ratio for the desired workability (table-1) =3.2The aggregates are combined by the graphical method as shown in figure 6, so that 30 percent of the material passes through the 4.75 mm IS sieve.Ratio of fine to total aggregate = 25%Required proportions by weight of dry materials:Cement – 1Fine aggregates – [(25/100)x3.2] = 0.8Coarse aggregates – [(75/100)x3.2)] = 2.4Water = 0.35If C = weight of cement required per cubic meter of concrete, then

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F=50MPa

Control factor = 0.8

Workability = very low

N.M.S = 10mm

sand crusted stone

GS 2.6 2.5

Moisture 5% 1%

Mean strength = 50/0.8 =63 MPA

R.N (fig1) = 25

W/c = 0.35

A/C = 3.2 (fine =0.25, coarse =0.75)

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C/3.15x1000 + W/1000 + F/2.6x1000 + Cs /2.5x1000 = 1

C/3.15X1000 + 0.35C/1000 + 0.25X3.2C/2.6X1000 +

0.75X3.2C/2.5X1000 = 1

(3.17 + 3.50 + 3.08 + 9.6) X 10-4C = 1

C = 104/19.35 = 520 KG

W = 0.35 X 520 = 182 L

Fine = 0.25 x 3.2 x 520 = 416 kg

Coarse = 0.75 x 3.2 x 520 = 1250

Unit weight = 2368 kg

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Dry aggregate Moist aggregate

Cement 520 520

Water 182 182-21-13 = 148

Fine 416 416x1.05 = 437

Coarse 1250 1250X1.01 = 1263

2368 2368

Batch Quantities per cubic meter of concrete

A high strength concrete mix proportions for one cubic meter and the

materials properties are as follows:

Assume air entrained = 2%

Calculate:

1. The mix proportion as ratios to cement weight.

2. The unit weight of concrete.

3. The concrete yield.

4. The cement factor.

5. The amount of cement, sand and gravel required to produced

500 m3 of concrete.

Coarse aggregateFine aggregateWaterCement

1085

2.43

1.59

600

2.57

1.65

165

1

--

550

3.15

--

Weight by kg

Specific gravity

Unit weight t/m3

Example 2

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Air = 2%

• Unit weight = 550 + 165 + 600 + 1085 = 2400 kg/m3

•Yield =

• 50 15 54.55 98.64

•Y = 50/1000x3.15 + 15/1000 + 54.55/2.57x1000 + 98.64/2.43x1000

• = 0.0159 + 0.0150 +0.0212 + 0.0406 = 0.0927

•Y = 1.02 x y = 0.0946 m3

•Cement factor = 1/y = 10.57 bag. *c= 529kg

Cement Water Fine Coarse

kg 550 165 600 1085

Ratio 1 0.30 1.09 1.97

Bag of

cement

50 15 54.55 98.64

ɣ kg/m3 1.65 1.59

Gs 3.15 1 2.57 2.43

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•For 500m3 concrete

Mi x proportion

C w fine coarse

529 159 577 1042

V= 529/3.15x1000 + 0.159 +577/2.57x1000 + 1042/2.43x1000 + 0.02

= 0.1679 + 0.1590 + 0.2245 + 0.4288 + 0.02

V= 1.002 m3

For 500 m3 of concrete

C= 500x 529 = 264500 kg

Fine = 500x 577 = 288500 kg/1.65 = 174.9 m3

Coarse = 500 x 1042 = 521000 kg/1.59 = 327.7 m3

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Design a high strength concrete mix using. The 28-day characteristic cylinder

compressive strength is 600kg/cm2. The required slump is 200mm. The water/

cement ratio = 0.30 and the total aggregate /cement ratio = 2.70. The ratio of sand

to all in aggregate = 0.32. The properties of aggregates are given in table below

(N.M.S of coarse aggregate is 20 mm.).

Calculate:

1. The concrete mix proportions.

2. The unit weight of concrete.

3. The concrete yield.

4. The cement factor.

5. Calculate the indirect tensile strength, and the flexure strength of concrete.

6. Adjust mix proportions, if the coarse aggregate can absorb 1.5% of its weight,

and the fine aggregate has moisture of 0.50%.

Coarse aggregateFine aggregate

2.51

1.65

2.55

1.70

Specific gravity

Unit weight t/m3

Example 3

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W/C = 0.3

A/C = 2.7 (Fine =0.32, coarse = 0.68 )

Fine coarse

Gs 2.55 2.51

1.70 1.65

C/3.15 x 1000 + 0.3C /1000 + 2.7x0.32c/1000x2.55 + 2.7x0.68 c/1000x2.51 =1

(3.175 + 3 + 3.388 + 7.315) x 10-4 c = 1

Concrete mix proportion : C = 1000/16.878 = 593 kg

W = 0.30 x 593 = 178 kg

Fine = 0.32 x 2.7 x 593 = 512 kg

Coarse = 0.68 x 2.7 x 593 = 1089 kg

Unit weight of concrete = 2372 kg/ m3

C W fine coarse

593 178 512 1089

50 15 43.17 91.82

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Y = 50/3.15x1000 + 0.15 + 43.17/1000x2.55 + 91.821/1000x2.51

= 0.0159 + 0.015 + 0.0169 + 0.0366 = 0.0844

Cement factor = 1/y = 11.85 bag

Fc = 600 kg/ cm2

Ft = 5/100 x 600 = 30kg/ cm2

Ff = 30/0.7 = 43kg/cm2

Fine = 512 + 0.005 x 512 =512 + 3 = 515 kg (moisture )

Coarse = 1089 - 0.015 x 1089 = 1089 - 16 = 1073 kg

W = 178 – 3 + 16 = 191 kg

C W fine coarse

593 191 515 1073

Unit weight = 593 + 191 + 515 + 1073 = 2372 kg/ m3

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@HIGH-EARLY-STRENGTH CONCRETE• The time period in which a specified strength should be

achieved may range from a few hours to several days.

• High-early-strength can be obtained by using one or a

combination of the following:

1. Type III or HE high-early-strength cement.

2. High cement content 400 to 600 kg/m3.

3. Low water-cementing materials ratio (0.20 to 0.45).

4. Higher freshly mixed concrete temperature.

5. Higher curing temperature.

6. Chemical admixtures.

7. Silica fume (or other supplementary cementing materials).

8. Steam or autoclave curing.

9. Insulation to retain heat of hydration.

10. Special rapid hardening cements.

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46Applications for Fly ash and Slag Cement.

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Synthetic fibers improve toughness and Plastic

Shrinkage of High Strength Concrete.

Synthetic fibers

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Placing, Consolidating and Curing

1. Delays in delivery and placing must be eliminated

2. Consolidation very important to achieve strength

3. Slump generally 180 to 220 mm (7 to 9 in.)

4. Little if any bleeding—fog or curing agent have to be

applied immediately after strike off to minimize plastic

shrinkage and 7 days moist curing.

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Curing

•Curing is the process of maintaining a satisfactory moisture content

and a favorable temperature in concrete during the hydration period so

that desired properties of the concrete can be developed.

•Curing is essential in the production of quality concrete; it is critical to

the production of high-strength concrete.

•The potential strength and durability of concrete will be fully

developed only if it is properly cured .

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Precast concrete

Prestressed concrete

Precast - Prestressed concrete

Advantages of high strength concrete

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Advantages of high strength concrete

2- Reduced initial construction costs

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3- Wider girder spacing and longer spans

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4- Significant savings in concrete quantities

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5- Reduced long-term costs due to fewer repairs

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6- Significant savings in construction depth

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Strength-Weight ratio becomes comparable to steel

0

5

10

15

20

25

30

35

40

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Structural steel Concrete High strength

concrete

Lightweight HSC

Strength-Weight Ratio

7- Cost Saving

%

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Example 3

The photographs given in Figures from 1 to 6 show different things related

to high strength concrete construction. Discuss what you understand from

each photograph.

Figure2

Figure1

Figure3

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Figure 5

Figure 6

Figure 4