STRAIN

In the physical sciences and engineering, a number that describes the relative deformation of elastic, plastic, and fluid materials under applied forces.

It arises throughout the material as the particles of the material are displaced from their usual position. Normal strain is caused by forces perpendicular to planes or cross sections of the material, such as in a volume that is under pressure on all sides. Shear strain is caused by forces that are parallel to, and lie in, planes or cross sections, such as in a short metal tube that is twisted about its longitudinal axis.

STRESS

In the physical sciences and engineering, the force per unit area within materials that arises from externally applied forces, uneven heating, or permanent deformation.

Normal stress refers to the stress caused by forces that are perpendicular to a cross-section area of the material. Shear stress arises from forces that are parallel to the plane of the cross section. Stress is expressed as the quotient of a force divided by an area.

 

COLUMNS

In architecture, a vertical element, usually a slender shaft, that provides structural support by carrying axial loads in compression; columns are also subject to buckling.

Columns may be exposed or hidden in walls; constructed of precast concrete, masonry, stone, or wood or of steel wide-flange, pipe, or tubular sections; they may be plain, fluted, or sculpted, with or without a capital and base. Columns may also be nonstructural, used for decorative or monumental purposes.

BEAM
In building construction, a horizontal member spanning an opening and carrying a load.


The load may be a wall above the opening or it may be a floor or roof. Beams may be of wood, steel or other metals, reinforced or prestressed concrete, plastic, or even brick with steel reinforcement. For weight reduction, metal beams are I-shaped, having a thin vertical web and thicker horizontal flanges where greater stress occurs. A joist is any of a series of small parallel beams supporting a floor or roof. 

POLYMER CONCRETE

Continuous research by concrete technologists to understand, improve and develop the

properties of concrete has resulted in a new type of concrete known as, “Polymer Concrete”.

It is referred time and again in the earlier chapters that the concrete is porous. The porosity

is due to air-voids, water voids or due to the inherent porosity of gel structure itself. On

account of the porosity, the strength of concrete is naturally reduced. It is conceived by many

research workers that reduction of porosity results in increase of strength of concrete.Therefore, process like vibration, pressure application spinning etc., have been practised mainly

to reduce porosity. All these methods have been found to be helpful to a great extent, but

none of these methods could really help to reduce the water voids and the inherent porosity

of gel, which is estimated to be about 28%. The impregnation of monomer and subsequent

polymerisation is the latest technique adopted to reduce the inherent porosity of the concrete,

to improve the strength and other properties of concrete.

The pioneering work for the development of polymer concrete was taken up by United

States Bureau of Reclamation (USBR). The initial exploratory works carried out at the

Brookhaven National Laboratory (BNL) in cooperation with USBR and US in Atomic Energy

Commission (AEC) revealed great improvement in compressive strength, permeability, impact

resistance and abrasion resistance.

The development of concrete-polymer composite material is directed at producing a new

material by combining the ancient technology of cement concrete with the modern

technology of polymer chemistry.

Type of Polymer Concrete

Four types of polymer concrete materials are being developed presently. They are:

(a) Polymer Impregnated Concrete (PIC).

(b) Polymer Cement Concrete (PCC).

(c) Polymer Concrete (PC).

(d) Partially Impregnated and surface coated polymer concrete.

Polymer Impregnated Concrete (PIC)

Polymer impregnated concrete is one of the widely used polymer composite. It is nothing

but a precast conventional concrete, cured and dried in oven, or by dielectric heating from

which the air in the open cell is removed by vacuum. Then a low viscosity monomer is diffused

through the open cell and polymerised by using radiation, application of heat or by chemical

initiation.

Mainly the following types of monomer are used:

(a) Methylmethacrylate (MMA),

(b) Styrene,

(c) Acrylonitrile,

(d) t-butyl styrene,

(e) Other thermoplastic monomers.

The amount of monomer that can be loaded into a concrete specimen is limited by the

amount of water and air that has occupied the total void space. It is necessary to know the

concentration of water and air void in the system to determine the rate of monomer

penetration. However, the main research effort has been towards obtaining a maximum

monomer loading in concrete by the removal of water and air from the concrete by vacuum

or thermal drying, the latter being more practicable for water removal because of its rapidity.

Another parameter to consider is evacuation of the specimen prior to soaking in

monomer. This eliminates the entrapment of air towards the centre of the specimen during

soaking which might otherwise prevent total or maximum monomer loading. The application

of pressure is another technique to reduce monomer loading time.Polymer Cement Concrete (PCC)

Polymer cement concrete is made by mixing cement, aggregates, water and monomer.

Such plastic mixture is cast in moulds, cured, dried and polymerised. The monomers that are

used in PCC are:

(a) Polyster-styrene.

(b) Epoxy-styrene.

(c) Furans.

(d) Vinylidene Chloride.

However, the results obtained by the production of PCC in this way have been

disappointing and have shown relatively modest improvement of strength and durability. In

many cases, materials poorer than ordinary concrete are obtained. This behaviour is explained

by the fact that organic materials (monomers) are incompatible with aqueous systems and

sometimes interfere with the alkaline cement hydration process.

Recently Russian authors have reported the production of a superior Polymer cement

concrete by the incorporation of furfuryl alcohol and aniline hydrochloride in the wet mix. This

material is claimed to be specially dense and non-shrinking and to have high corrosion

resistance, low permeability and high resistance to vibrations and axial extension.

Washington State University in cooperation with Bureau of Reclamation tested the

incorporation of several monomers into wet concrete for preparing PCC for fabrication of

distillation units for water disalination plants. However, it is reported that only epoxy resin

produced a concrete that showed some superior characteristics over ordinary concrete.

Polymer Concrete (PC)

Polymer concrete is an aggregate bound with a polymer binder instead of Portland

cement as in conventional concrete.

The main technique in producing PC is to minimise void volume in the aggregate mass

so as to reduce the quantity of polymer needed for binding the aggregates. This is achieved

by properly grading and mixing the aggregates to attain the maximum density and minimum

void volume. The graded aggregates are prepacked and vibrated in a mould. Monomer is

then diffused up through the aggregates and polymerisation is initiated by radiation or

chemical means. A silane coupling agent is added to the monomer to improve the bond

strength between the polymer and the aggregate. In case polyester resins are used no

polymerisation is required.

An important reason for the development of this material is the advantage it offers over

conventional concrete where the alkaline Portland cement on curing, forms internal voids.

Water can be entrapped in these voids which on freezing can readily crack the concrete. Also

the alkaline Portland cement is easily attacked by chemically aggressive materials which results

in rapid deterioration, whereas polymers can be made compact with minimum voids and are

hydrophobic and resistant to chemical attack. The strength obtained with PC can be as high

as 140 MPa with a short curing period.

However, such polymer concretes tend to be brittle and it is reported that dispersion of

fibre reinforcement would improve the toughness and tensile strength of the material.

LIGHT WEIGHT CONCRETE

One of the disadvantages of conventional concrete

is the high self weight of concrete. Density of the

normal concrete is in the order of 2200 to 2600 kg/m3.

This heavy self weight will make it to some extent an

uneconomical structural material. Attempts have been

made in the past to reduce the self weight of concrete

to increase the efficiency of concrete as a structural

material. The light-weight concrete as we call is a

concrete whose density varies from 300 to 1850 kg/m3.

There are many advantages of having low density.

It helps in reduction of dead load, increases the

progress of building, and lowers haulage and handling

costs. The weight of a building on the foundation is an

important factor in design, particularly in the case of

weak soil and tall structures. In framed structures, the

beams and columns have to carry load of floors and

walls. If floors and walls are made up of light-weight

concrete it will result in considerable economy. Another

most important characteristic of light-weight concrete is most important characteristic of light-weight concrete is the relatively low thermal conductivity, a property which improves with decreasing density. In

extreme climatic conditions and also in case of buildings where air-conditioning is to be

installed, the use of light-weight concrete with low thermal conductivity will be of considerable

advantage from the point of view of thermal comforts and lower power consumption. The

adoption of light-weight concrete gives an outlet for industrial wastes such as clinker, fly ash,

slag etc. which otherwise create problem for disposal.

Basically there is only one method for making concrete light i.e., by the inclusion of air

in concrete. This is achieved in actual practice by three different ways.

(a) By replacing the usual mineral aggregate by cellular porous or light-weight aggregate.

(b) By introducing gas or air bubbles in mortar. This is known as aerated concrete.

(c) By omitting sand fraction from the aggregate. This is called ‘no-fines’ concrete.

Light-weight concrete has become more popular in recent years owing to the

tremendous advantages it offers over the conventional concrete. Modern technology and a

better understanding of the concrete has also helped much in the promotion and use of light weight concrete. A particular type of light-weight concrete called structural light-weight

concrete is the one which is comparatively lighter than conventional concrete but at the same

time strong enough to be used for structural purposes. It, therefore, combines the advantages

of normal normal weight concrete and discards the disadvantages of normal weight concrete.

Perhaps this type of concrete will have great future in the years to-come. Out of the three main

groups of light-weight concrete, the light-weight aggregate concrete and aerated concrete

are more often used than the ‘no-fines’ concrete. Light-weight concrete can also be classified

on the purpose for which it is used, such as structural light weight concrete, non-load bearing

concrete and insulating concrete. The aerated concrete which was mainly used for insulating

purposes is now being used for structural purposes sometimes in conjunction with steel

reinforcement. The aerated concrete is more widely manufactured and used in the

Scandinavian countries; whereas in U.K., France, Germany and U.S.A. owing to the

production of large scale artificial industrial light-weight aggregate, light-weight aggregates

concrete is widely used. In some countries the natural dense graded aggregate are either in

short supply or they are available at a considerable distance from the industrial cities. In such

cases the use of locally produced light-weight aggregates in the city area offers more

economical solutions. These factors have led to the development and widespread use of

considerable varieties of industrial light-weight aggregates of varying quality by trade names

such as Leca (expanded clay), Aglite (expanded shale), Lytag (sintered pulverised fuel ash),

Haydite (expanded shale).