বিল্ডিং কনস্ট্রাকশন কাজে
প্লাস্টার একটি অন্যতম কাজের নাম ।
এটি আমাদের কারও অজানা নয় ।
তবে প্লাস্টার কিন্তু কয়েক প্রকারের হয়ে থাকে,
সেটি কি সবার জানা আছে ???
যারা জানেন না, তারা জেনে নিন-
১। সিমেন্ট প্লাস্টার
২। জিপসাম প্লাস্টার
৩। মোজাইক বা টেরাজো প্লাস্টার
৪। মাড প্লাস্টার
৫। লাইম প্লাস্টার
৬। সুরকী প্লাস্টার
৭। লাইম-সুরকী প্লাস্টার
৮। স্টাকো প্লাস্টার
৯। চুনাম প্লাস্টার
১০। স্যান্ড রাবিং
১১। মোগল প্লাস্টার
১২। লাইম পানিং বা নিরু বা লাইম পুটিং
A good formwork should satisfy the following requirements:
1. It should be strong enough to withstand all types of dead and live loads.
2. It should be rigidly constructed and efficiently propped and braced both horizontally and vertically, so as to retain its shape.
3. The joints in the formwork should be tight against leakage of cement grout.
4. Construction of formwork should permit removal of various parts in desired sequences without damage to the concrete.
5. The material of the formwork should be cheap, easily available and should be suitable for reuse.
6. The formwork should be set accurately to the desired line and levels should have plane surface.
7. It should be as light as possible.
8. The material of the formwork should not warp or get distorted when exposed to the elements.
9. It should rest on firm base.
Causes of Cracks in concrete structures:
The principal causes of occurrence of cracks in a building are as follows:
1. Permeability of concrete.
As deterioration process in concrete begins with penetration of various aggressive agents, low permeability is the key to its durability. Concrete permeability is controlled by factors like water-cement ratio, degree of hydration/curing, air voids due to deficient compaction, micro-cracks due to loading and cyclic exposure to thermal variations. The first three are allied to the concrete strength as well. The permeability of cement paste is a function of water-cement ratio given good quality materials, satisfactory proportioning and good construction practice; the permeability of the concrete is a direct function of the porosity and interconnection of pores of the cement paste.
2. Thermal movement:
Thermal movement is one of the most potent causes of cracking in buildings. All materials more or less expand on heating and contract on cooling. The thermal movement in a component depends on a number of factors such as temperature variations, dimensions, coefficient of thermal expansion and some other physical properties of materials. The coefficient of thermal expansion of brickwork in the vertical direction is fifty percent greater than that in the horizontal direction, because there is no restraint to movement in the vertical direction.
Thermal variations in the internal walls and intermediate floors are not much and thus do not cause cracking. It is mainly the external walls especially thin walls exposed to direct solar radiation and the roof which are subject to substantial thermal variation that are liable to cracking.
Thermal joints can be avoided by introducing expansion joints, control joints and slip joints. In structures having rigid frames or shell roofs where provision of movement joints is not structurally feasible, thermal stresses have to be taken into account in the structural design itself to enable the structure to withstand thermal stresses without developing any undesirable cracks.
Concrete when subjected to sustained loading exhibits a gradual and slow time dependant deformation known as creep. Creep increases with increase in water and cement content, water cement ratio and temperature. It decreases with increase in humidity of surrounding atmosphere and age of material at the time of loading. Use of admixtures and pozzolonas in concrete increases creep. Amount of creep in steel increases with rise in temperature.
4. Corrosion of Reinforcement
A properly designed and constructed concrete is initially water-tight and the reinforcement steel within it is well protected by a physical barrier of concrete cover which has low permeability and high density. Concrete also gives steel within it a chemical protection. Steel will not corrode as long as concrete around it is impervious and does not allow moisture or chlorides to penetrate within the cover area. Steel corrosion will also not occur as long as concrete surrounding it is alkaline in nature having a high pH value.
Concrete normally provides excellent protection to reinforcing steel. Notwithstanding this, there are large number of cases in which corrosion of reinforcement has caused damage to concrete structures within a few years from the time of construction. One of the most difficult problems in repairing a reinforced concrete element is to handle corrosion damage. Reinforcement corrosion caused by carbonation is arrested to a great extent through repairs executed in a sound manner. However, the treatment of chloride-induced corrosion is more difficult and more often the problem continues even after extensive repairs have been carried out. It invariably re-occurs in a short period of time. Repairing reinforcement corrosion involves a number of steps, namely, removal of carbonated concrete, cleaning of reinforcement application of protection coat, making good the reduced steel area, applying bond coat and cover replacement. Each step has to be executed with utmost care. When chlorides are present in concrete, it is extremely difficult to protect reinforcing steel from chloride attack particularly in cases where chlorides have entered through materials used in construction and residing in the hardened concrete.
This increase in volume causes high radial bursting stresses around reinforcing bars and result in local radial cracks. These splitting cracks results in the formation of longitudinal cracks parallel to the bar. Corrosion causes loss of mass, stiffness and bond and therefore concrete repair becomes inevitable as considerable loss of strength takes place
Corrosion of steel in a canopy
Corrosion of steel in a canopy
Reinforcement steel in concrete structures plays a very important role as concrete alone is not capable of resisting tensile forces to which it is often subjected. It is therefore important that a good physical and chemical bond must exist between reinforcement steel and concrete surrounding it. Due to inadequacy of structural design and /or construction, moisture and chemicals like chlorides penetrate concrete and attack steel. Steel oxidizes and rust is formed. This results in loss of bond between steel and concrete which ultimately weakens the structure.
The best control measure against corrosion is the use of concrete with low permeability. Increased concrete cover over the reinforcing bar is effective in delaying the corrosion process and also in resisting the splitting.
5. Moisture Movement:
Most of the building materials with pores in their structure in the form of intermolecular space expand on absorbing moisture and shrink on drying. These movements are cyclic in nature and are caused by increase or decrease in inter pore pressure with moisture changes.
Initial shrinkage occurs in all building materials that are cement/lime based such as concrete, mortar, masonry and plasters. Generally heavy aggregate concrete shows less shrinkage than light weight aggregate concrete.
Controlling shrinkage cracks.
Shrinkage cracks in masonry could be minimized by avoiding use of rich cement mortar in masonry and by delaying plaster work till masonry has dried after proper curing and undergone most of its initial shrinkage. In case of structural concrete shrinkage cracks are controlled by using temperature reinforcement. Plaster with coarse well graded sand or stone chip will suffer less from shrinkage cracks and is preferred for plastering for external face of walls.
Considering the building as a whole, an effective method of controlling shrinkage cracks is the provision of movement joints. The work done in cold weather will be less liable to shrinkage cracks than that in hot weather since movement due to thermal expansion of materials will be opposite to that of drying shrinkage.
6. Poor Construction practices.
The construction industry has in general fallen prey to non-technical persons most of whom have little or no knowledge of correct construction practices. There is a general lack of good construction practices either due to ignorance, carelessness, greed or negligence. Or worse still, a combination of all of these.
The building or structure during construction is in its formative period like a child in mother’s womb. It is very important that the child’s mother is well nourished and maintains good health during the pregnancy, so that her child is healthily formed. Similarly for a healthy building it is absolutely necessary for the construction agency and the owner to ensure good quality materials selection and good construction practices. All the way to building completion every step must be properly supervised and controlled without cutting corners.
Some of the main causes for poor construction practices and inadequate quality of buildings are given below:
Improper selection of materials.
Selection of poor quality cheap materials.
Inadequate and improper proportioning of mix constituents of concrete, mortar etc.
Inadequate control on various steps of concrete production such as batching, mixing, transporting, placing, finishing and curing
Inadequate quality control and supervision causing large voids (honey combs) and cracks resulting in leakages and ultimately causing faster deterioration of concrete.
Improper construction joints between subsequent concrete pours or between concrete framework and masonry.
Addition of excess water in concrete and mortar mixes.
Poor quality of plumbing and sanitation materials and practices.
7. Poor structural design and specifications
Very often, the building loses its durability on the blue print itself or at the time of preparation of specifications for concrete materials, concrete and various other related parameters.
It is of crucial that the designer and specifier must first consider the environmental conditions existing around the building site. It is also equally important to do geotechnical (soil) investigations to determine the type of foundations, the type of concrete materials to be used in concrete and the grade of concrete depending on chemicals present in ground water and subsoil.
It is critical for the structural designer and architect to know whether the agency proposed to carry out the construction has the requisite skills and experience to execute their designs. Often complicated designs with dense reinforcement steel in slender sections result in poor quality construction. In addition, inadequate skills and poor experience of the contractor, ultimately causes deterioration of the building.
Closely spaced of reinforcement steel bars due to inadequate detailing and slender concrete shapes causes segregation. If concrete is placed carelessly into the formwork mould, concrete hits the reinforcement steel and segregates causing fine materials to stick to the steel, obstructing its placement and is lost from the concrete mix while the coarse material falls below causing large porosity (honeycombs).
Slender structural members like canopies (chajjas), fins and parapets often become the first target of aggressive environment because of dense reinforcement, poor detailing, less cover of concrete to the reinforcement steel. Added to all this, low grade of concrete and poor construction practices can make the things worse. It is necessary for the structural consultant to provide adequate reinforcement steel to prevent structural members from developing large cracks when loaded.
To sum up, the following precautions are required to be taken by the Architects, Structural Consultants and Specifiers:
Proper specification for concrete materials and concrete.
Proper specifications to take care of environmental as well as sub – soil conditions.
Constructable and adequate structural design.
Proper quality and thickness of concrete cover around the reinforcement steel.
Planning proper reinforcement layout and detailing the same in slender structures to facilitate proper placing of concrete without segregation.
Selection of proper agency to construct their designs.
Architects and Engineers are parents of the buildings they plan and design and therefore their contribution to the health and life of the building is quite significant. Once the plans are drawn the structural designs and specifications are prepared, it is then the turn of the agency to construct the building and bring the blue print to reality. Special care must be taken in the design and detailing of structures and the structure should be inspected continuously during all phases of construction to supplement the careful design and detailing.
8. Poor Maintenance
A structure needs to be maintained after a lapse of certain period from its construction completion. Some structures may need a very early look into their deterioration problems, while others can sustain themselves very well for many years depending on the quality of design and construction.
Leakage from roof slab
Leakage from roof slab
Spalled concrete due to corrosion of steel
Spalled concrete due to corrosion of steel
Regular external painting of the building to some extent helps in protecting the building against moisture and other chemical attacks. Water-proofing and protective coating on reinforcement steel or concrete are all second line of defence and the success of their protection will greatly depend on the quality of concrete.
Leakages should be attended to at the earliest possible before corrosion of steel inside concrete starts and spalling of concrete takes place. Spalled concrete will lose its strength and stiffness, besides; it will increase the rate of corrosion as rusted steel bars are now fully exposed to aggressive environment. It is not only essential to repair the deteriorated concrete but it is equally important to prevent the moisture and aggressive chemicals to enter concrete and prevent further deterioration.
9. Movement due to Chemical reactions.
The concrete may crack as a result of expansive reactions between aggregate containing active silica and alkalines derived from cement hydrations. The alkali silica reaction results in the formation of swelling gel, which tends to draw water form other portions of concrete. This causes local expansion results in cracks in the structure.
To control Cracks due to alkali-silica reactions, low alkali cement, pozzolona and proper aggregates should be used.
10. Indiscriminate addition and alterations.
There have been some building collapses in our country due to indiscriminate additions and alterations done by interior decorators at the instance of their clients.
Generally, the first target of modifications is the balcony. Due to the requirement to occupy more floor area, balconies are generally enclosed and modified for different usages.
Balconies and canopies are generally cantilever RCC slabs. Due to additional loading they deflect and develop cracks. As the steel reinforcement in these slabs have less concrete cover and the balcony and canopy slab is exposed to more aggressive external environment, corrosion of steel reinforcement takes place and repairs become necessary.
The loft tanks are generally installed in toilets or kitchens, which are humid areas of the buildings. The structure in addition to being overloaded is also more prone to corrosion of reinforcement steel in these areas and therefore deteriorates and if not repaired, part of the building can even collapse.
TYPES OF CONCRETE VIBRATORS FOR COMPACTION
Since concrete contains particles of varying sizes, the most satisfactory compaction would perhaps be obtained by using vibrators with different speeds of vibration. Polyfrequency vibrators used for compacting concrete of stiff consistency are being developed. The vibrators for compacting concrete are manufactured with frequencies of vibration from 2800 to 15000 rpm. The various types of vibrators used are described below:
(i) Immersion or Needle Vibrators:
Immersion or needle concrete vibrators
This is perhaps the most commonly used vibrator. It essentially consists of a steel tube (with one end closed and rounded) having an eccentric vibrating element inside it. This steel tube called poker is connected to an electric motor or a diesel engine through a flexible tube. They are available in size varying from 40 to 100 mm diameter. The diameter of the poker is decided from the consideration of the spacing between the reinforcing bars in the form-work.
The frequency of vibration varies upto 15000 rpm. However a range between 3000 to 6000 rpm is suggested as a desirable minimum with an acceleration of 4g to 10g.
The normal radius of action of an immersion vibrator is 0.50 to 1.0m. However, it would be preferable to immerse the vibrator into concrete at intervals of not more than 600mm or 8 to 10 times the diameter of the poker.
The period of vibration required may be of the order of 30 seconds to 2 minute. The concrete should be placed in layers not more than 600mm high.
(ii) External or Shutter Vibrators
External or Shutter Concrete Vibrator
These vibrators are clamped rigidly to the form work at the pre-determined points so that the form and concrete are vibrated. They consume more power for a given compaction effect than internal vibrators.
These vibrators can compact upto 450mm from the face but have to be moved from one place to another as concrete progresses. These vibrators operate at a frequency of 3000 to 9000 rpm at an acceleration of 4g.
The external vibrators are more often used for pre-casting of thin in-situ sections of such shape and thickness as can not be compacted by internal vibrators.
(iii) Surface Vibrators
Surface concrete vibrator
These are placed directly on the concrete mass. These best suited for compaction of shallow elements and should not be used when the depth of concrete to be vibrated is more than 250 mm.
Very dry mixes can be most effectively compacted with surface vibrators. The surface vibrators commonly used are pan vibrators and vibrating screeds. The main application of this type of vibrator is in the compaction of small slabs, not exceeding 150 mm in thickness, and patching and repair work of pavement slabs. The operating frequency is about 4000 rpm at an acceleration of 4g to 9g.
(iv) Vibrating Table
Vibrating table for concrete compaction
The vibrating table consists of a rigidly built steel platform mounted on flexible springs and is driven by an electric motor. The normal frequency of vibration is 4000 rpm at an acceleration of 4g to 7g. The vibrating tables are very efficient in compacting stiff and harsh concrete mixes required for manufacture of precast elements in the factories and test specimens in laboratories.
Functions of admixtures to modify fresh concrete properties:
a) To increase workability without increasing water content or to decrease the water content at the same workability.
b) To retard or accelerate both initial and final setting times.
c) To reduce or prevent settlement.
d) To create slight expansion in concrete and mortar.
e) To modify the rate or capacity for bleeding or both.
f) To reduce segregation of concrete, mortars and grouts.
g) To improve penetration and or pumpability of concrete, mortars and grouts.
h) To reduce rate of slump loss.
Functions of admixtures to modify hardened concrete properties:
a) To retard or reduce heat generation during early hardening.
b) To accelerate the rate of strength development.
c) To increase the strength of concrete or mortar (Compressive, tensile or flexural).
d) To increase the durability or resistance to severe conditions of exposure including the application de-icing salts.
e) To decrease the capillary flow of water.
f) To decrease the permeability to liquids.
g) To control the expansion caused by the reaction of alkalis with certain aggregate constituents.
h) To produce cellular concrete.
i) To increase the bond of concrete to steel reinforcement.
j) To increase the bond between old and new concrete.
k) To improve impact resistance and abrasion resistance.
l) To inhibit the corrosion of embedded metal.
m) To produce coloured concrete or mortar.
While modifying any particular property, care should be taken to ensure that other properties of concrete are not affected adversely.
What are Concrete Admixtures?
Ingredients other than cement, water, and aggregates that impart a specific quality to either plastic (fresh) mix or the hardened concrete (ASTM C496) is called concrete admixture.
Why use Concrete Admixtures?
1. Reduce cost of concrete construction
2. Achieve specific concrete properties more effectively
3. Ensure quality of concrete during mixing, transporting, placing, and curing in adverse weather condition
4. Overcome emergencies during operations
Classification of concrete admixtures:
1. Air entrainers
2. Water reducers
3. High-range water reducers-superplasticizers
6. Fine minerals
7. Specialty admixtures
Air entraining concrete admixture:
Air entraining concrete admixtures produce tiny air bubbles in the hardened concrete to provide space for the water to expand upon freezing.
How do they work?
They are anionic (water hating) agents that form tough, elastic, air filled bubbles. These bubbles reduce stresses caused by movement or freezing of water. They provide more volume for expansion and shorter flow path.
Benefits of air entraining admixture:
Increases workability of fresh concrete
Increased durability; Better resistance to freeze thaw cycles, de-icers, salts, sulfates, and alkali-silica reactivity
Effect can be reduced in moderate strength concrete by lowering water cement ratio and increasing cement factor
Composition of Air Entrainers:
Salt of wood resins (Vinsol resin)
Salts of sulfonated lignin (by product of paper production)
Salts of petroleum acids
Salts of proteinaceous material
Fatty and resinous acids
Salts of sulfonated hydrocarbons
Usually liquid meets ASTM C260 specifications.
Water Reducing concrete admixture:
Water reducers can result in 3 things:
1. Increased slump at constant w/c
2. Increased strength, by lowering the water content
3. Reduced cost of the cement
How do they do this?
Water reducers increase the mobility of the cement particles in the plastic mix, allowing same workability to be achieved at lower water contents.
Superplasticizers are “high-range” water reducers. Superplasticizers are used when placing:
1. Thin sections or around tightly spaced reinforcing steel
2. concrete underwater
3. concrete by pumping
4. consolidating the concrete is difficult
Note: When super plasticizers are used, the fresh concrete stays workable for only a short period of time (30 min to 60 min), which is why they are usually added at the site
Retarding concrete admixture:
Used to delay the initial set of concrete. Why do we use them?
1. To offset the effect of hot weather
2. Allow for unusual placement or long haul distances
3. Provide time for special finishes
Possible adverse effects of retarders
1. Reduce early age strength
2. Reduction of time between initial and final set
Possible advantages of retarders
1. Air entrainment
2. Increased workability
3. Reduction of time between initial and final set
Note: The use of retarders must be evaluated experimentally before incorporation in mix design
Accelerating concrete admixture:
Used to reduce the time required to develop final strength characteristics in concrete
Possible reasons for using accelerators:
1. Reduce the amount of time before finishing operations begin
2. Reduce curing time
3. Increase rate of strength gain
4. Plug leaks under hydraulic pressure efficiently
5. Offset effect of cold weather
Calcium Chloride is the most widely used accelerator. Initial and final set times reduced;
CaCl2 by weight Initial Set Time in Hrs.
The PCA (Portland Cement Association) recommends against using calcium chloride when:
1. concrete is prestressed
2. concrete contains embedded aluminum such as conduits
3. concrete is subjected to alkali-aggregate reaction
4. concrete is in contact with water or soils containing sulfates
5. concrete is placed during hot weather
6. Mass application of concrete
Alternatives to CaCl2
1. high early strength cement (type III)
2. increase cement content
3. cure at higher temperature (if feasible)
4. triethanolamine, sodium thiocyanate, calcium formate, calcium nitrite, or calcium nitrate
Fine Minerals as concrete admixtures:
Fine mineral admixtures added in large amounts (20% to 100% of cement weight) to improve the characteristics of plastic and hardened concrete. Classification based on chemical and physical properties
Have hydraulic cementing properties Example: blast furnace slag, natural cement and hydraulic hydrated lime
Siliceous and aluminous material
Little or no cementitious value
In presence of moisture, will react with calcium hydroxide to form compounds with cementitious properties 15% of PC weight is hydrated lime. A
The protection of reinforcement against corrosion can be done by any of the following method:
a) Anti corrosive treatment using acid / alkali / other agents based on CECRI technology (as per IS: 9077).
b) Fusion bonded Epoxy coating (as per IS:13620).
c) Corrosion resistant reinforcement as rolled in the factory and commercially available.
Anticorrosive treatment (using CECRI technology)
The procedure for anti-corrosive treatment of reinforcement is as follows:
The reinforcement rods is immersed in the pickling tank, containing Derusting Solution.
The reinforcements are then left in the derusting solution until all the rust is satisfactorily removed and a bright surface is obtained. This may take about 15-30 minutes. Typically, to prepare 100 litres of Derusting Solution, mix 5 liters of Inhibitor – Derusting solution, 50 liters of Hydrochloric acid and 50 liters of water in the pickling tank (as per manufacturer’s instruction).
The rods is then removed from the pickling tank cleaned with wet waste cloth and immersed in the ALKALINE CLEANING TANK. This tank contains Alkaline Powder in the ratio of 1.5 kg. of powder to 400 liters of water. The rods is left for about 5 minutes, cleaned and removed.
Phosphating Jelly is then immediately applied on the surface of the rods by means of a fibre brush. The jelly shall be allowed to react with the rod surface for 45-60 minutes and the jelly removed by means of rinsing in water or wet cloth.
Corrosion inhibitor solution is then applied on the reinforcement surface by brushing/dipping.
The corrosion inhibitor solution is mixed with ordinary portland cement in the ratio of 500 CC of inhibitor to 1 kg of OPC and a brushable slurry is prepared. This slurry is then be applied on the rod surface by brushing.
Note: All above steps should be applied in the same day and the steel allowed to air dry for 12 – 24 hours.
Corrosion sealing solution is then be applied by brushing / dipping.
Inhibitor is mixed with ordinary portland cement in the ratio of 600 CC of inhibitor to 1 kg. of cement and a brushable slurry is prepared. This slurry is immediately applied on the reinforcement surface. The coating is then allowed to dry for 12 – 24 hours.
Corrosion sealing solution is applied on the reinforcement surface. This coating is again repeated after 4 hours of air drying.
The above Anticorrosive treatment is given after the rods are cut and bent to shape. The treatment is done in a covered area. The treated rods is stored above ground in a covered area on wooden / masonry supports.
Typical Mechanical Properties of anti corrosive coating are:
a) Average thickness of coating = 0.3 ± 0.1 mm
b) Rockwell superficial hardness = 40 HR 15 N
Typical Corrosion Resistance properties are as under:
a) Tolerable limit for Chloride In 0.04 N, NaOH as per Anodic Polarization Technique – 5000 ppm
b) Corrosion resistance as per salt fog test – > 2000 hrs.
c) Corrosion resistance as per
d) alternative wetting and drying test – > 450 days
Fusion Bonded Epoxy Coating
This process shall be done as per IS: 13620. This process is carried out by the specialized agency in their Plant.
Commercially available corrosion resistant reinforcement:
The reinforcement are purchased from well-known brand of corrosion resistant rods. The mechanical properties like tensile strength, elongation etc. should conform to the requirements of the corresponding class of bars, like Fe 415, Fe 500 etc. as per IS-1786.