Guniting

• The guniting is the most effective process of repairing concrete work which has been damaged due to inferior work or other reasons. It is also used for providing an impervious layer.
• Gunite is a mixture of cement and sand, the usual proportion being 1 : 3. A cement gun is used to deposit this mixture on the concrete surface under a pressure of about 2 to 3 kg/cm².
• The surface to be treated is cleaned and washed. The nozzle of gun is generally kept at a distance of about 75 cm to 85 cm from the surface to be treated and velocity of jet through nozzle varies from 120 to 160 m/sec.

Compressive Strength Test

IS 3495 (Part-1): 2019 lays down the following procedure to determine the compressive strength of burnt clay bricks.

• Immerse the brick specimens in water at a temperature of 27° ± 2°C for 24 hours. Remove the specimens and drain out any surplus moisture. Remove unevenness observed in the bed faces, to provide two smooth and parallel faces, by grinding.
• Use a cement/sand mortar for capping and filling the frog and all voids in the bed faces. The cement/sand mortar used must be such that its compressive strength shall be atleast that of the expected brick strength.
• Store the specimens under damp jute bags for 24 hours followed by immersion in clean water for 3 days at a temperature of 27° ± 2°C. Remove and wipe out any traces of moisture.
• Place the specimens with flat faces horizontal, and mortar filled face facing upwards between two 3-ply plywood sheets each of 3 mm thickness and carefully centered between plates of the testing machine. Apply load axially at a uniform rate of 7 ± 1 N/mm²/min till failure occurs and note the maximum load at failure. The load at failure shall be the maximum load at which the specimens fail to produce any further increase in the indicator reading on the testing machine.
• Compressive strength is given by the following formula
  Compressive strength, in N/mm² = Maximum load at failure (in N) / Average area of the bed faces in mm²
• The average compressive strength shall be reported to the nearest 0.1 N/mm².

Fire Bricks

• These are made from fire clay. The process of manufacture is same as that of ordinary clay bricks.
• The burning and cooling of fire bricks are done gradually.
• These bricks are usually white or yellowish white in colour.
• The weight of a fire bricks is about 30 N to 35 N.
• The fire bricks can resist high temperature without softening or melting.
• These bricks are used for linings of interior surface of furnaces, chimneys, kiln, ovens, fire places etc.
• The compressive strength of these bricks varies from 200-220 N/mm².
• The percentage of water absorption for these bricks varies from 5-10%.
• Following are the three varieties of fire bricks:

(i) Acidic Bricks

• These bricks are used for acidic lining.
• Following are the types of acidic bricks.

(a) Ordinary firebricks: These bricks are prepared from natural fire-clay and they provide a good material for acidic refractory lining.

(b) Silica bricks: These bricks contain a very high percentage of silica to the extent of about 95 to 97%. A small quantity of lime, about 1 to 2% is added to work as binding material. These bricks are moulded under pressure and burnt at high temperature. Silica bricks can withstand a high temperature up to about 2000°C.

(ii) Basic Bricks

• These bricks are used for basic lining and basic refractory materials are used in the manufacture of such bricks.
• Magnesia bricks are prepared from lime and magnesia rocks. Dolomite may also be adopted for the manufacture of these bricks.

(iii) Neutral Bricks

• These bricks are used for neutral lining.
• Following are the types of neutral bricks:

(a) Chromite Bricks: These bricks are prepared from a mixture of chrome, iron ore, ferrous oxide, bauxite and silica. Such bricks are unaffected by acidic or basic actions.

(b) High Alumina Bricks: These bricks contain a high percentage of alumina and they are found to be more inert to slags.