CONSTRUCTION TECHNOLOGY MORTARS & CONCRETES BY: SANA TARIQ
GENERAL INTRODUCTION :
When a binding material, a fine aggregate and water are mixed together in suitable proportions , they form an easily workable paste which is termed as Mortar .
When a binding material, ( a fine & a coarse aggregate) and water are mixed in suitable proportions , they form an easily workable mix which is termed as plastic, wet or green concrete.
When this plastic concrete becomes hard like a stone, this is termed as hardened concrete or simply Concrete .
For preparing mortars and concretes, cement and lime are generally used as binding materials;
sand and surkhi as fine aggregates and
crushed stone and crushed bricks as coarse aggregates.
Mortars are usually named according to the binding material used in their preparation.
They are essentially required for masonry work, plastering and pointing etc.
FUNCTIONS OF MORTAR:
To bind together the bricks or stones properly so as to provide strength to the structure.
To form a homogenous mass of the structure so as to resist all the loads coming over it without disintegration.
To provide a weather resisting i.e., a durable layer between the different courses of masonry in the structure.
To hold coarse aggregate together in any concrete so as to form a solid mass. The mortar used in a concrete is termed as matrix.
To do pointing and plastering to the structure. The mortar used for plastering is known as plaster.
To fill up empty joints in brick and stone masonry. The mortar used for such purposes is a thin liquid mortar which is termed as grout and the process is known as grouting.
The sands and the cement have to be thoroughly mixed by hand or in a mechanical mixer before adding any water - do not use dirty water, or water from puddles or ponds, as this could impair the final strength of the mortar.
Similarly, keep any sugar-containing liquids, such as soft drinks, well away from the mix - sugar, even in small amounts, seriously impairs the setting ability of the cement.
PREPERATION OF MORTARS:
When mixing by hand, the sands and cement are heaped up on a mixing board or in a wheelbarrow and repeatedly turned over and over until thoroughly mixed.
The color of the dry mix will change as the cement is distributed throughout - there should be no 'streaking' of cement, and no clumps of pure sand or pure cement.
Once the dry ingredients are mixed, the water can be added.
If any additives are being used, such as a plasticizer or a frost-proofer, they are normally added to the water, and then mixed in, rather than being directly added to the dry ingredients.
A "well" is formed in the centre of the mixed heap, water added to it and then folded in.
More water is added a bit at a time and folded in until the required consistency is attained.
This should be when the mortar is thoroughly mixed but is able to stand in peaks, like whipped cream; too wet and it just makes a mess, too dry and it's almost impossible to work.
Dry sand and cement in wheelbarrow Begin to mix sand and cement Mix to evenly distribute cement The dry mix should be all one colour Add water and plasticiser Mix to required consistency
When using a mechanical mixer, add half a bucket (2 or 3 liters) of clean cold water to the empty drum before adding the dry ingredients in sequence.
Add 4 measures of sand then 1 of cement, followed by 4 sand, then another cement and so on until the required quantity is in the mixer.
This ensures a more thorough mix than adding, say, 20 measures of sand and then 5 measures of cement.
Again, the water is added to the revolving drum once the dry ingredients are thoroughly blended, a bit at a time until the required consistency is achieved.
For wide joints in paving or for stonework, either as paving or as walling, a coarser mortar is often preferred - replace half of the building/soft sand with grit/sharp sand. You will probably find that a coarse mortar such as this requires less gauging water to achieve a working consistency than does a bricklaying/general purpose mortar. 2 x Building Sand plus 2 x Grit Sand plus 1 x cement Mortar for Wide Joints
TYPES OF MORTAR & THEIR USES:
The paste is prepared by mixing cement and sand in suitable proportions in addition to water.
The general proportion is 1 part of cement to 2-8 parts clean sand.
These mortars must be use within half an hour, i.e.; before initial setting time of the cement.
This type is used for all engineering works where high strength is desired such as load bearing walls, deep foundations, flooring etc.
The paste is prepared by mixing lime and sand or surkhi in suitable proportions in addition to water.
If surkhi is to be added in lime mortar the equal proportions of sand and surkhi should be mixed with lime.
These mortars are inferior to cement mortars in strength as well as water tightness.
These mortars should not be used for underground works as they set in the presence of carbon dioxide and break up in damp conditions.
This type is used for construction work above ground level i.e. exposed positions.
Light weight mortar:
The paste is prepared by mixing wood powder, wood sawing or saw dust with cement or lime mortar.
In such mortars fibers of jute coir or asbestos fibers can also be used.
These are generally used as fiber plasters in sound and heat proof construction.
Fire resistant mortar:
The paste is prepared by mixing aluminous cement and finely crushed fire bricks in suitable proportions in addition to water.
The usual proportion are 1 part aluminous cement to 2 parts of finely crushed fire bricks.
These are generally used for lining furnaces, ovens and fire places with fire bricks.
The paste is prepared by mixing suitable clayey soil with water.
The soil which is used for preparing mud mortar should be free from grass, pebbles etc.
These are the cheapest mortars but weakest in strength.
These mortars are used for brickwork of ordinary buildings and for plastering walls in rural areas.
FUNCTION OF SAND AND SURKHI IN MORTARS:
Functions of sand:
It reduces shrinkage of the building material.
It prevents development of cracks in the mortar on drying.
It helps in making mortars and concretes of desired strength by varying its proportions with the binding material.
A well graded sand adds to the density of mortars and concretes.
Functions of surkhi:
It provides brick color and make the mortar economical
An artificial stone resulting from hardening of a mixture of a binding material, fine aggregate, coarse aggregate and water in suitable proportions is called concrete.
Cement and sand are generally used as binding materials whereas sand, surkhi, cinder are used as fine aggregates , and crushed stones, gravel, broken bricks, etc., are used as coarse aggregates in preparing different types of concrete.
A good Portland cement or lime , clean sand and strong coarse aggregates are necessary for making a good concrete.
The required strength of concrete can be obtained by careful selection of its ingredients, accurate water measurements and adopting good workmanship in mixing, transportation, placing, compaction, finishing and curing of concrete in the construction works.
CHARACTERISTICS OF GOOD CONCRETE:
Crushing strength : it should have adequate crushing strength.
Durability : it must be durable enough to resist the effects of weathering agencies.
Impermeability : it should have sufficient impermeability or water tightness..
Resistance to abrasion : It should be sufficiently hard and provide enough resistance to abrasion. This property is very essential when the concrete is to be used for making road pavements and steps.
Resistance to fire : It should have minimum thermal expansion so as to provide good resistance to fire.
Workability: It should have good workability so that it can be readily deposited in position in a uniform layer and also adaptable for ornamental moldings
Compactness : It must be sufficiently dense. The concrete with greater density will be more compact.
Shrinkage : It should have minimum shrinkage when it hardens.
Creep : The continuous strain with time which the concrete undergoes due to application of external load is called creep, time yield or plastic flow., this should be minimum.
Economy: It should be economical for the desired strength.
Appearance : It should provide the required finish to the concrete structure.
TYPES OF CONCRETES & THEIR USES:
Concrete are classified into different types as follows:
According to binding material used in the preparation of concrete.
According to design of concrete.
According to purpose of concrete.
Classification according to the binding material:
The concrete consisting of cement, sand and coarse aggregate mixed in suitable proportions in addition to water is called cement concrete.
Cement is used as binding material , where proportions of the ingredients are 1 part Portland cement: 1.5 to 8 parts clean sand ; 3 to 16 parts coarse aggregate.
Cement concrete is generally used for buildings and other important engineering works where strength and durability is of prime importance.
The concrete consisting of lime, a fine aggregate and coarse aggregate mixed in suitable proportions in addition to water is called lime concrete.
In this type of concrete, hydraulic lime is generally used as a binding material; sand, surkhi and cinder are used as fine aggregates and broken bricks and stones etc., as coarse aggregates.
It has less strength but is cheaper than cement concrete.
It is generally used for the sake of economy in foundations works, under floors, over roofs etc.
Classification according to design:
Plain cement concrete :
The cement concrete in which no reinforcement is provided is known as plain cement concrete.
It is commonly used for foundation work and flooring of buildings.
Reinforced cement concrete ( R.C.C):
The cement concrete in which reinforcement is embedded for taking tensile, excessive compressive or shear stresses is called reinforced cement concrete.
The steel reinforcement generally used is in form of round bars.
Usual proportions of reinforced concrete are 1 part of Portland cement: 1 to 2 parts clean sand: 2 to 4 parts shingle or crushed stone.
It is commonly used for construction of slabs, lintels, beams, columns and their footings, raft or mat foundations, precast or cast- insitu concrete piles etc.
Pre-stressed cement concrete ( P.C.C):
The cement concrete in which high compressive stresses are artificially induced before their actual use is called pre- stressed cement concrete.
The high compressive strength is induced by pre-tensioning the reinforcement before placing the concrete and the reinforcement is released when final setting of the concrete takes place.
It is used where high stresses develop and where it is uneconomical to use reinforced cement concrete.
Classification according to purpose:
Vacuum concrete :
The cement concrete from which entrained air and excess water are removed after placing it, by suction with the help of a vacuum pump is called vacuum concrete.
The excess water which is added to increase workability but not required for hydration of cement of the concrete is removed .
This concrete can be used for all reinforced concrete works with better results.
Air entrained concrete :
The cement concrete prepared by mixing aluminum in it is called air- entrained, cellular or aerated concrete.
In this concrete, bubbles of hydrogen gas are liberated which makes it light weight and spongy.
It is used in lining walls and roofs for heat and sound insulation.
Light weight concrete :
The concrete prepared by using coke breeze, cinder or slag as coarse aggregate is called light weight concrete.
It possess heat- insulating properties and is used in making precast structural units for partitions and wall lining.
Saw dust concrete :
The concrete prepared by mixing Portland cement with saw dust in specified proportions in addition to water is called saw dust concrete.
It expands when it becomes wet and contracts when dry.
This concrete is used as a heat and sound insulating material.
High early strength concrete :
The concrete in which high early strength cement is used as a binding material is called high early strength concrete.
This concrete sets and hardens quickly as compared to ordinary cement concrete.
It expands when it becomes wet and contracts when dry and is used as a heat and sound insulating material.
It is used for construction works in cold weather as it sets quickly.
White and colored concrete :
The concrete in which white cement is used as a binding material is called white concrete, and if colored cement is used as a binding material along with colored aggregate is called colored concrete.
These concretes can be used for decorative purposes of flooring, skirting etc., in buildings.