concrete technology part 1

Concrete Technology

Concrete

Concrete is the most commonly used construction material today. The versatility and mouldability of this material, its high compressive strength, and the discovery of the reinforcing & prestressing techniques have contributed to its widespread use.

Reasons for widespread use of concrete

Excellent resistance to water: The ability of concrete to withstand the action of water without serious deterioration makes it an ideal material for building structures to control, store, and transport water.

Itaipu dam, Brazil


Ease with which structural concrete elements can be formed into a variety of shapes and sizes: Freshly made concrete is of a plastic consistency, which enables the material to flow into prefabricated formwork.

Fountain of Time: a sculpture in concrete Fountain of

Time by Lorado Taft — a massive 120 by 18 by 14 ft work of art in concrete on the south side of the University of Chicago campus.


The cheapest and most readily available material: The principle components for making concrete are relatively inexpensive and are commonly available in most parts of the world.

Concrete vs Steel

Maintenance Fire resistance Resistance to cyclic loading

Materials for Concrete

Cement Fine Aggregate Coarse Aggregate Water Admixtures*

Chemical composition of cement

The raw materials used for the manufacture of cement mainly consist of

a) Lime b) Silica c) Alumina d) Iron oxideThe relative proportions of these oxides

composition are responsible for influencing the various properties of cement.

Approximate oxide composition limits of Ordinary Portland Cement

Oxide Per cent, content

CaO 60-67

SiO2 17-25

Al2O3 3.0-8.0

Fe2O3 0.5-6.0

MgO 0.1-4.0

Alkalies (K2O,Na2O) 0.4-1.3

SO3 1.0-3.0

Bogue’s compounds

At high temperature these oxides present in the raw materials combine with each other to form complex compounds. These compounds are identified based on R.H.Bogue’s work and hence it is called as “Bogue’s compounds”.

Bogue’s compounds(In abbreviated formula C –CaO, S-SiO2, A-Al2O3, F-Fe2O3)

Name of compound

Formula Abbreviated Formula

Tricalcium Silicate

3 CaO.SiO2 C3S

Dicalcium Silicate

2 CaO.SiO2 C2S

Tricalcium Aluminate

3 CaO.Al2O3 C3A

Tetracalcium Aluminoferrite

4 CaO.Al2O3.Fe2O3 C4AF

Oxide composition & Compound composition

Oxide Per cent Compound Per cent

CaO 63 C3S 54.1

SiO2 20 C2S 16.6

Al2O3 6 C3A 10.8

Fe2O3 3 C4AF 9.1

MgO 1.5

SO3 2

Alkalies (K2O,Na2O)

1.0

Types of cement - I Ordinary Portland Cement

33 Grade – IS 269:1989 43 Grade – IS 8112: 1989 53 Grade – IS 12269:1987

Rapid Hardening Cement – IS 8041: 1990 Extra Rapid Hardening Cement Sulphate Resisting Cement – IS 12330: 1988 Portland Slag Cement – IS 455: 1989 Quick Setting Cement Super-Sulphated Cement – IS 6909: 1990 Low Heat Cement – IS 12600: 1989

Types of cement - II

Portland Pozzolana Cement – IS 1489:1991(Part I & II) Air-entraining Cement Coloured Cement: White Cement-IS 8042:1989 Hydrophobic Cement – IS 8043: 1991 Masonry Cement – IS 3466:1988 Expansive Cement

Types of cement - III Oil-well Cement – IS 8229: 1986 Redi-set Cement Concrete Sleeper grade Cement – IRS-T40:1985 High Alumina Cement – IS 6452:1989 Very High Strength Cement

ASTM Classification

Type I - concrete construction where the special properties specified for other types are not required (OPC).

Type II – concrete construction exposed to moderate sulphate action, or where moderate heat of hydration is required.

Type III – when early high strength is required (Rapid Hardening Cement).

ASTM Classification

Type IV – when low heat of hydration is required (Low Heat Cement).

Type V – when high sulphate resistance is required (Sulphate Resisting Cement).

Other minor types like Type IS, Type IP and Type IA, IIA & IIIA.

Ordinary Portland Cement Ordinary Portland cement is the most

commonly used cement for a wide range of applications. These applications cover dry-lean mixes, general-purpose ready-mixes etc.

After 1987 higher grade cements were introduced in India.

Grades: 33, 43, 53 – Strength of the cement at 28 days when tested as per IS 4031-1988.

Rapid Hardening Cement - I

As the name indicates it develops strength rapidly and more appropriate to call it as high early strength cement.

RHC should not be confused with Quick setting cement which only sets quickly.

RHC at the age of 3 days develops the same strength of OPC at 7 days.

Rapid Hardening Cement - II

RHC gives out much heat of hydration and hence should not be used in mass concrete construction.

This higher rate of development of strength attributes to higher fineness of grinding and higher C3S and lower C2S content.

Uses of RHC

Prefabricated concrete construction. Where formwork is required to be

removed early for re-use elsewhere. Road repair works. In cold weather concreting.

Extra Rapid Hardening Cement - I

It is obtained by inter-grinding Calcium Chloride with RHC.

The normal addition of Calcium Chloride should not exceed 2% by weight of RHC.

Concrete made by using ERHC should be transported, placed, compacted and finished within 20 minutes.

Extra Rapid Hardening Cement - II

ERHC should not be stored for more than a month.

Very suitable in concreting in cold weather.

Sulphate Resisting Cement OPC is susceptible to the attack of sulphates,

in particular to the action of MgSO4. Sulphates in solution permeate into concrete

and cause disruption and this is known as Sulphate attack. This is accelerated if accompanied by alternate wetting and drying which normally takes place in marine structures due to tidal variations.

Cement with low C3A & C4AF is known as sulphate resisting cement. In other words, this cement has a high silicate content.

C3A content limits to 5%.

Uses of sulphate resisting cement Marine structures In foundation and basement where soil

is infested with sulphates. Fabrication of pipes which are likely to

be buried in marshy region or sulphate bearing soils.

Construction of sewage treatment works.

Portland Slag Cement (PSC) Obtained by mixing Portland cement clinker, gypsum

and granulated blast furnace slag and grinding the mixture or separately grinding and later mixing them intimately.

Low heat of hydration. So can be used in mass concreting.

It cannot be used in cold weather. Resistant to soils and water containing sulphates or

alkali metals, alumina and iron, to acidic waters. So can be used in marine structures.

Slag mixed with Portland cement clinker will range from 25-65%. Hence blast furnace slag, which is waste product from blast furnaces can be used effectively.

Quick Setting Cement As the name indicates it sets very early.

The early setting property is brought out by reducing the gypsum content at the time of clinker grinding.

It is used mainly under water construction where pumping is involved. Use of quick setting cement reduces the pumping time and makes it economical.

It may also find its use in some typical grouting operations.

Super Sulphated cement - I Manufactured by grinding together a mixture of

80-85 % granulated slag, 10-15 % hard burnt gypsum, and about 5% Portland cement clinker. The product is ground finer than that of PC.

Specific surface must not be less than 4000cm2/gm.

It is more sensitive to deterioration during storage than PC.

It has a low heat of hydration of about 40-45 calories/gm at 7 days and 45-50 calories/gm at 28 days.

Super Sulphate cement - II

Because of high sulphate resistance it is recommended for use in

i) Foundation, where chemically aggressive conditions exist

ii) marine works iii) Fabrication of R.C. pipes which are

likely to be buried in sulphate bearing soils.

Low Heat Cement Hydration of cement is exothermic action

which produces large quantity of heat. Formation of cracks in large body of concrete necessitated the production of cement with less heat of hydration.

A low-heat of hydration is achieved by reducing the contents of C3S and C3A which are the compounds evolving the maximum heat of hydration and increasing C2S.

The feature of low-heat cement is a slow rate of gain of strength but ultimate strength is the same as that of OPC.

Three-Gorges Dam Three-Gorges Dam,

People’s Republic of China Largest concrete dam in the

world The 185 m high concrete

gravity dam has a total length of 2300 m along its axis.

To minimize the thermal stresses in the mass concrete, low-heat cement and forty percent of fly ash were used

Portland Pozzolana Cement Manufactured either by grinding together

Portland cement clinker and pozzolana with addition of gypsum, or by intimately and uniformly blending PC and fine pozzolana.

It produces less heat of hydration and offers greater resistance to the attack of aggressive waters than OPC. It reduces the leaching of CaOH when used in hydraulic structures.

It is useful in marine and hydraulic construction and other mass concrete construction.

Air-Entraining Cement Made by mixing a small amount of air-

entraining agent with OPC clinker at the time of grinding.

Air-entraining agents i) Alkali salts of wood resins ii) Synthetic detergents iii) Calcium lignosulphate iv) Calcium salts of glues & other proteins Produces air bubbles in the body of

concrete which will modify properties w.r.to workability, segregation, bleeding.

Coloured Cement Coloured cement consists of Portland cement with 5-10 %

of pigment. For manufacturing various coloured cements either white

cement or grey Portland cement is used as a base. Though the use of white cement as a base is costly, a wide

range of colours is obtainable. With the use of grey cement only red or brown cement

could be produced. Pigment cannot be satisfactorily distributed throughout the

cement by mixing. Hence it is usual to grind the cement and pigment together.

Hydrophobic cement In India places such as Assam, Shillong etc. get plenty

of rainfall in the rainy season and have high humidity in other seasons. The transportation and storage of cement in such places cause deterioration in the quality of cement.

Hydrophobic cement is obtained by grinding ordinary Portland cement clinker with water repellant film-forming substance such as oleic acid, and stearic acid.

The water-repellant film formed around each grain of cement, reduces the rate of deterioration of the cement during long storage, or transport, or under unfavourable conditions.

Masonry cement Cement mortar is good when

compared to lime mortar w.r.to strength and setting properties but inferior w.r.to workability, shrinkage etc.

Masonry cement is a mixture of Portland cement and plasticizing materials such as limestone together with other materials introduced to enhance one or more properties such as workability, shrinkage & durability.

Used in brick and stone masonry construction

Expansive cement Concrete made with OPC shrinks while setting due to

loss of free water. Concrete also shrinks continuously for long time. (known as drying shrinkage)

Cement used for grouting anchor bolts or grouting machine foundations or the cement used in grouting the prestressed concrete ducts, if shrinks, the purpose for which the grout is used will be to some extent defeated.

The type of cement which suffers no overall change in volume on drying is known as expansive cement. This type of cement is made by using an expanding agent and a stabilizer.

IRS – T 40 Special Grade Cement

Manufactured as per specification laid down by M/o of Railways.

Very finely ground cement with high C3S content designed to develop high early strength.

Used for prestressed concrete elements, high rise buildings, high strength concrete

Oil-well cement Cement slurry used to seal off the annular

space between steel casing and rock strata should remain sufficiently mobile to be able to flow.

This is achieved by adjusting compound composition or by adding retarders to OPC. The common retarders are starches, cellulose products or acids.

Retarding agents prevent quick setting and retain slurry in mobile condition to facilitate penetration to all fissures and cavities.

Rediset Cement

A cement which could yield high strength in a matter of hours without showing any retrogression.

Developed by Associated Cement Company of India, equivalent to “Regset” developed by PCA laboratories of USA.

Applications: a) Very-high-early-(3 to 4 hours) strength concrete

and mortar. b) patch repairs and emergency repairs. c) quick release of forms in the precast concrete

products industry. d) slip-formed concrete construction. e) construction between tides.

High Alumina Cement Raw materials used for manufacture of high

alumina cement are limestone and bauxite. (Composition: CaO-37%, SiO2-4%, Al2O3-39%, Fe2O3-9%, FeO-6%, TiO2-2%, MgO- 1%)

Very high rate of strength development, about 80% of the ultimate strength is achieved in one day.

Used to make refractory concrete to withstand high temperature.

Shows retrogression in strength when exposed to hot and humid conditions.

Very High Strength Cement - MDF

Macro-defect-free cements (MDF) refers to the absence of relatively large voids or defects which are usually present in conventional mixed cement pastes because of entrapped air and inadequate dispersion.

In the MDF process 4-7 % water-soluble polymers such as hydroxypropylmethyle cellulose is added as rheological aid to permit cement to be mixed with very small amount of water.

At the final processing stage entrapped air is removed by applying pressure.

A strength of 300 MPa for calcium aluminate system and 150 MPa for Portland cement system can be achieved.

Very High Strength Cement - DSP

Densely Packed System (DSP): Normal Portland cement and ultra-fine silica fume are mixed.

Silica fume is added from 5 to 25%. A strength of 270 Mpa have been

achieved with silica fume substituted paste.

Very High Strength Cement – Pressure Densification & Warm pressing

A new approach has been developed for achieving very high strength by a method called “Warm Pressing” (applying heat and pressure simultaneously) to cement paste.

Compressive strength of 650 MPa and tensile strength up to 68 MPa have been obtained by warm pressing Portland and calcium aluminate cements.

Increase in strength is resulted from the removal of most of the porosity and generation of very homogeneous, fine microstructure with the porosities as low as 1.7 %.

Very High Strength Cement – High Early Strength Cement

High early strength is an important factor for repair and emergency work.

Lithium salts have been used as accelerators in high alumina cement.

Results: 4 MPa within 1 hour, 27 MPa within 3 hours and 49 MPa in one day.

Very High Strength Cement – Pyrament Cement

A high early strength and durable cement called by trade name “Pyrament Cement” for repair of Air Field Runways developed in USA.

In India, ACC in collaboration with R & D Engineers, Dighi, Pune also produced high early strength for rapid repair of airfields.

Very High Strength Cement – Magnesium Phosphate Cement (MPC)

MPC giving high early strength has been developed by CRRI, New Delhi for rapid repair of damaged concrete roads and airfield pavements.

Magnesite (MgCO3) when calcined at or above 15000C gives dead burnt magnesite (DBM). Powdered DBM is mixed with Mono ammonium Phosphate, Sodium tri-polyphosphate, di-sodium tetraborate (Borax) , fine aggregate and water.

The DBM and sand is added into cold phosphate and borax solution and applied for the purpose of repair – ready for opening traffic within 4-5 hours.

concrete technology part 1

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