High strength concrete

HIGH STRENGTH CONCRETE – CONCRETE OF TOMORROW

Vinay Kumar, Addl. Director General, CPWD, MumbaiDr K M Soni, Chief Engineer, CPWD, Nagpur

Concrete Has become an essential item of civil engineering construction. Weak in tension hence used in a combination with reinforcement

but if reinforcement bars could have been avoided or concrete could have tensile strength, it could have reduced cost, time of construction and structure would be durable due to no corrosion problem.

Concrete strength is gained through curing due to heat of hydration but if curing could have been accelerated, it could gain early strength which can help in early construction.

Compaction plays an important role in uniformity but achieving uniform compaction through vibrators becomes difficult, particularly when reinforcement is used with concrete. If compaction could be achieved without vibrators or concrete could be cast without reinforcing bars, it could provide uniformity.

Low w/c ratio leads to high compressive strength but also to low workability but if w/c ratio could be reduced with required workability, it could give high compressive strength.

High strength concreteLow w/c or w/b ratioAddition of fine pozzolanic

materials/silica fumesAddition of superplasticizers

Ettringite- calcium sulfoaluminate (delays rapid setting of cement)

Portlandite is the major bonding agent in cement and concrete, formed duringcuring process, when elemental calcium reacts withwater to form calcium hydroxide.

(Calcium – silicate- hydrate)

High Strength concreteCompressive strength;Normal structural concrete :20-

50 MPa, High strength concrete(HSC): 50-

100 MPaUltra Strength concrete: 100-150

MPa, andEspecial strength concrete > 150

MPa.

High-Value Concrete

High-Strength Concrete Materials

• 9.5 - 12.5 mm 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)

Aggregates —

High-Value Concrete

High-Strength Concrete Materials

• Fly ash, silica fume, or slag often mandatory

• Dosage rate 5% to 20% or higher by mass of cementing material.

Supplementary Cementious Materials -

• Superplasticisers for workability

“Stronger concrete mixtures would be more durable” did not prove to be right, hence, performance criterion was adopted

High Performance Concrete(HPC)Possessing high workability, high durability and

high ultimate strength. As per ACI, HPC is defined as a concrete meeting

special combination of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing, and curing practices.

The Strategic Highway Research Program (SHRP) in the United States defined HPC for highway structures by three requirements, namely a maximum w/cm, a minimum durability factor to cycles of freezing and thawing (ASTM C 666, Method A), and a minimum early-age or ultimate compressive strength.

U.S. Federal Highway Administration (FHWA) has revised the definition of HPC for highway structures. HPC is a concrete that has been designed to be more durable and if necessary, stronger than conventional concrete.

Characteristics of HPCHigh early strengthHigh strengthHigh modulus of elasticityHigh abrasion resistanceHigh durability and long life in

severe environmentsLow permeability and diffusionResistance to chemical attack

Characteristics of HPCHigh resistance to adverse

climatic conditionsToughness and impact resistanceVolume stabilityEase of placementCompaction without segregationInhibition of bacterial and mold

growth

High-Performance?◦High-Early Strength Concrete◦High-Strength Concrete◦High-Durability Concrete◦Self-Consolidating Concrete◦Reactive Powder Concrete

Material Primary Contribution/Desired

PropertyPortland cement Cementing material / DurabilityBlended cement

Cementing material / Durability /

High strength

Fly ash / Slag / Silica fumeCalcined clay/ MetakaolinCalcined shaleSuperplasticizers FlowabilityHigh-range water reducers Reduce water-cement ratioHydration control admix. Control setting

Materials used in HPC

Material Primary contribution/Desired property

Retarders Control settingAccelerators Accelerate settingCorrosion inhibitors Control steel corrosionWater reducers Reduce cement and water

contentShrinkage reducers Reduce shrinkageASR inhibitors Control alkali-silica activity

Improve workability/reduce pastePolymer/latex modifiers

HPCFour types of HPC were

subsequently developed:◦Very Early Strength (14 MPa in 6

hours),◦High Early Strength (34 MPa in 24

hours),◦Very High Strength (69 MPa in 28

days),◦High Early Strength with Fiber-

reinforcement.

High-Value Concrete

High-Early-Strength Concrete

• High-early-strength cement• High cement content 400 to 600 kg/m3 • Low water-cementing materials ratio (0.20

to 0.45 by mass)• Higher freshly mixed concrete temperature• Higher curing temperature

May be achieved by;

High-Value Concrete

High-Early-Strength Concrete

• Chemical admixtures• Silica fume (or other SCM)• Steam or autoclave curing• Insulation to retain heat of hydration• Special rapid hardening cements

May be achieved by -

High-Value Concrete

High-Strength Concrete Materials

• Use of water reducers, retarders, or superplasticizers — mandatory in high-strength concrete

• Air-entraining admixtures not necessary or desirable in protected high-strength concrete.– Air is mandatory, where durability in a freeze-

thaw environment is required (i.e.. bridges, piers, parking structures)

– Recent studies: • w/cm ≥ 0.30—air required • w/cm < 0.25—no air needed

Admixtures —

High-Value Concrete

High-Strength Concrete

• Delays in delivery and placing must be eliminated

• Consolidation very important to achieve strength

• Slump generally 180 to 220 mm• Little if any bleeding—fog or evaporation

retarders have to be applied immediately after strike off to minimize plastic shrinkage and crusting

• 7 days moist curing

Placing, Consolidation, and Curing

High-Value Concrete

High-Durability Concrete• 1970s and 1980s focus on —

High-Strength Concrete • Today focus on concretes with high

durability in severe environments resulting in structures with long life — High-Durability HPC

High-Value Concrete

High-Durability Concrete

• Abrasion Resistance• Blast Resistance• Permeability• Carbonation• Freeze-Thaw Resistance• Chemical Attack• Alkali-Silica Reactivity• Corrosion rates of rebar

Durability Issues That HPC Can Address

High-Value Concrete

• Cement: 398 kg/m3

• Fly ash: 45 kg/m3

• Silica fume: 32 kg/m3

• w/c: 0.30• Water Red.: 1.7 L/m3

• HRWR: 15.7 L/m3

• Air: 5-8%• 91d strength: 60 Mpa

High-Durability ConcreteConfederation Bridge, Northumberland Strait, Prince Edward

Island/New Brunswick, 1997

High-Value Concrete

Self-Consolidating/compacting Concrete

• flows and consolidates on its own• developed in 1980s — Japan• Increased amount of

– Fine material i.e. fly ash or limestone filler

– Superplasticizers• Strength and durability same as

conventional concrete

High-Value Concrete

Self-Consolidating Concrete

High-Value Concrete

Portland cement (Type I) 297 kg/m3

Slag cement 128 kg/m3 Coarse aggregate 675 kg/m3 Fine aggregate 1,026 kg/m3 Water 170 kg/m3 Superplasticizer ASTM C 494, Type F (Polycarboxylate-based) 1.3 L/m3 AE admixture as needed for 6% ± 1.5% air content

SCC for Power Plant in Pennsylvania—Mix

Proportions

Self compacting concreteExtreme fluidity No need for vibrators to compact

the concretePlacement being easier.No bleed water, or aggregate

segregation

REACTIVE POWDER CONCRETE RPC is composed of very fine

powders (cement, sand, quartz powder and silica fume), steel fibres (optional) and superplasticizer. ◦A very dense matrix is achieved by

optimizing the granular packing of the dry fine powders. This compactness gives RPC ultra-high strength and durability. Reactive Powder Concretes have compressive strengths ranging from 200 MPa to 800 MPa.

High-Value Concrete

Reactive-Powder Concrete (RPC)• Properties:

– High strength — 200 MPa (can be produced to 800 MPa)

– Very low porosity

• Properties are achieved by:– Max. particle size 300

m– Optimized particle

packing– Low water content– Steel fibers– Heat-treatment

High-Value Concrete

Mechanical Properties of RPCProperty Unit 80 MPa RPC

Compressive strength MPa 80 200Flexural strength MPa 7 40Tensile strength MPa 8Modulus of Elasticity GPa 40 (5.8 x

106)60 (8.7 x

106)Fracture Toughness 103 J/m2 <1 30Freeze-thaw RDF 90 100Carbonation mm 2 0Abrasion 10-12 m2/s 275 1.2

High-Value Concrete

• Cement• Sand• Silica quartz• Silica fume• Micro-Fibres - metallic or poly-vinyl acetate• Mineral fillers - Nano-fibres• Superplasticizer• Water

Raw Materials

â uctal

High-Value Concrete

The typical Ductal® mix

230 kg/m3

710 kg/m3

210 kg/m3

40 - 160 kg/m313 kg/m3

140 kg/m3

1020 kg/m3

Cement

Silica fume

Crushed Quartz

Sand

FibresSuperplasticizer

Total water

No aggregates !

â uctal

High-Value Concrete

The typical Ductal® mix

9 – 10%

28 - 30%

8.5 – 9%

1.7 – 6.5%0.6%

5.5 – 6%

42 –43%

Cement

Silica fume

Crushed Quartz

Sand

FibresSuperplasticizer

Total water

No aggregates !

â uctal

w/c = 0.20

Principles in developing RPCElimination of coarse aggregatesUtilization of the pozzolanic properties of

silica fumeOptimization of the granular mixture for

the enhancement of compacted densityThe optimal usage of superplasticizer to

reduce w/c and improve workabilityApplication of pressure (before and during

setting) to improve compactionPost-set heat-treatment for the

enhancement of the microstructureAddition of small-sized steel fibres to

improve ductility

Thus, High performance concrete is going to replace normal conventional concrete in future once, codes and guidelines are available.

Thank you