Good Mortar

PROPERTIES OF A GOOD MORTAR

A good mortar should have the following properties:

• It should be capable of developing good adhesion with the building units such as bricks, stones etc.
• It should be capable of developing the designed stresses.
• It should be cheap, durable and easily workable.
• It should set quickly so that speed in construction may be achieved.
• It should be inert, i.e, it should not affect the durability of materials with which it comes into contact.
• The joints formed by mortar should not develop cracks and they should be able to maintain their appearance for a sufficiently long period.
• It should be water resistance and should have the capability of resisting the penetration of water.

IMPURITIES IN LIMESTONES

Magnesium Carbonate

• The magnesium limestones are hard, heavy and compact in texture.
• The magnesium limestones display irregular properties of calcination, slaking and hardening.
• Upto 5%, magnesium oxide imparts excellent hydraulic properties to the lime.

Mixing

• Objective of mixing is to obtain homogenous, uniform colour and consistent concrete of desired strength.
• Mixing time depends on the type and capacity of mixer but IS-456 suggest an approximate mixing time of 2 minutes.
• Generally 20 revolutions of concrete in the mixture provide sufficient mixing.
• If mixing time is increased upto 2 minutes then, the compressive strength of concrete produced is enhanced, and beyond this time the improvement in compressive strength is insignificant, and prolonged mixing may cause segregation as due to longer mixing periods, the water may get absorbed by the aggregates or evaporate, resulting in loss of workability and strength.
• The mixing is done in two ways i.e. Hand mixing and Machine mixing.

(A) Hand Mixing

• Hand mixing is adopted for small jobs where the quantity of concrete involved is small and the approximate time is 2 minutes and should never exceed 3 minutes.

(B) Machine Mixing

• When a large quantity of concrete of the desired quality is to be produced, then, the machine mixing becomes imperative as concrete can be produced at a faster rate with better quality.
• Concrete mixers are specified by the volume of mixed concrete discharged after mixing of each batch expressed in m3 (such as 0.25, 0.38, 0.57, 0.75, 1.5, 2.25 and 3m³).
• Sometimes the total volume of the unmixed ingredients in m3 is given as a prefix. i.e. 1.0/0.75 mixer takes 1 m³ of unmixed material and gives 0.75 m³ of mixed concrete in each batch.
• The machine mixing is done by using Tilting type mixture, and Non tilting type mixture.
  (i) Tilting type mixture:
  • In this, mixed concrete is discharged by tilting the drum about the horizontal axis.
  • Tilting mixers are useful for large construction works.
  • It gives better results even with dry concrete. dry concrete
  • It can be used for aggregate size more than 75 mm. aggregate size more than 75 mm
  • Tilting mixers are easier to clean and can discharge the mix quickly and with minimum segregation.
  • The tilting type mixtures are represented as 85T, 100T, 140T, 200T (where 85, 100, 140 are in litres.)
  (ii) Non-tilting type mixture:
  • Non-tilting mixers are suitable for small works.
  • Non-tilting mixer is equipped with a drum rotating about a horizontal axis.
  • Non-tilting mixers cannot be used when aggregate size more than 75 mm. aggregate size more than 75 mm
  • Non-tilting mixers are represented as 200 NT, 280 NT, 340 NT, 400 NT, 800 NT.

Transportation

• Specification states that the process of mixing, transporting, placing and compacting the concrete should not take time more than initial setting time of cement (30 minutes using OPC).
• It must also ensure that segregation not took place.
• The transportation of concrete can be done by following methods:
  (i) Pans: Recommended only for small jobs.
  (ii) Power Buggies: These have speed up to 24 km/h.
  (iii) Chutes : When concrete is to be deposited below ground level at a higher depth, it can be
  discharged through a steel shaft called chute.
  (iv) Concrete Pumps: It is Concrete Pumps: used commonly for tunnel works and on locations which are not easily accessible, where concrete can be pumped for a distance of about 400 m horizontally and 80 m
  vertically and having slump value 50 mm to 100 mm and the pipe used in concrete pump are of diameter 10 cm to 20 cm.
  (v) Transit Mixer: Transit Mixer is a truck on which a concrete mixer is mounted and useful in built up areas.
  (vi) Belt-conveyer: A Belt-conveyer: belt conveyer is used when the concrete is to be transported  continuously and to an inaccessible area.

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