Er. Appasaheb Bhosale, B G Shirke Construction Technology Pvt. Ltd, Pune Prof. Dr. Ganesh Hinge, TSSM’s BSCE&R, Pune.
Building Construction is one of the labour-intensive construction activities. Still many processes in building construction are carried out conventionally compromising on quality and safety. Day by day, it is becoming difficult to get the skilled manpower to carry out the skilled building activity such as brickwork, plaster, bar bending and tying etc. Cement plaster is widely used in India on construction project to protect the wall material like bricks, cement blocks etc. Plastered surface adds the aesthetic value in building and provides uniform and strong base for painting. Surface finish of plaster is mainly depending on mason’s expertise & experience. Therefore, plastering process is a more of an art than work and its quality is mainly depend on skill of mason. Good quality plaster must be free from cracks or has minor cracks that are aesthetically acceptable (since it is rare not to have cracks in cement plaster). The plaster must have a good adhesion to the walls so that they do not sound when knocked (Debonding). The exterior plaster is a protective layer that helps prevent rain water or other water sources from entering the building through the walls, so the exterior plaster must be water-resistant and of a low permeability. Following are the few standard type of defects normally we observe in plaster. Non-Structural cracks may occur in the following ways:
Crazing cracks: Crazing cracks is a network of fine cracks, usually in a hexagonal pattern. They are usually due to over-trowelling a rich mix render.
- Map crazing: Map crazing is similar to crazing except that it is usually deeper (sometimes going through the plaster) and the hexagons of the pattern may measure up to 200 mm across.
- Drying shrinkage cracks: These cracks are the result of moisture loss after the plaster has hardened. When water is not enough in the mix, then plaster cracks will also occur.
For the modification of the plastering technique, lots of work had been done in the past by many people, but due to some problems or deficiencies in that some machines have not been that much in working condition. Following are some vital points which we have been taken from the recorded literature and it is very much important to use plastering machine for plastering work. Putzmeister company has launched the RUMA 1, Putzmeister 1 and Putzmeister Gipsomat machines in 16th century. With shortage of skilled manpower there is increase in demand for plastering machine in global market. [13,14] Trowel Technique: Mahesha P.K, etal  Experimented the trowel operation procedure which is a customary plastering method. The exact plastering procedure is dependent on how the trowel is being used and apply coat of mortar on wall. Receiving a good finish is the grouping of compact force combined with the right angle of trowel, how distant the foremost edge is with the wall. The foremost edge will be approximately 10-15 mm away apart from the wall.
Strength of Cement Plaster
There are different type plasters and are mainly categorised based on the material used, texture layers and mix proportions followed. Cement plaster in two layers is widely used for internal & external plaster respectively. Double coat external plaster in 1:4 Cement mortar (10 to12 mm + 8 to 10 mm) and internal plaster (1:6 cement mortar) 10 mm thick with Neeru finish or with White Cement Putty [8,9]. Strength of plaster is not governing criteria that define the good quality plaster. Strength of plaster material is measured by the “Compressive Strength” test in European specifications EN 998-1, where there are four categories of plaster according to the compressive strength, as follows:
|CS I||0.4-2.5 MPa|
|CS II||1.5 – 5 MPa|
|CS III||3.5 – 7.5 Mpa|
|CS IV||More than 6 Mpa|
Do we really need strength to cement plaster?
The prevailing notion that greater the compressive strength of a plaster, the better the quality is a misconception. Rather; it is contrary to the correct engineering analysis of the function of the plaster material and the stresses that it is subject to. Heavy weights are rarely applied to cement plaster and the main stress that occurs in cement plaster is tensile stress; mainly caused by plastic shrinkage or dry shrinkage; which results in cracks called plastic shrinkage cracks and drying shrinkage cracks respectively. If we discuss in depth the behaviour of the raw materials used in the manufacture of cement plaster in relation to shrinkage-induced stress, we find the following:
- Cement: PPC cement is generally preferred for plastering. PPC has a lower heat of hydration and it is prone to fewer cracks compared to OPC. PPC has lower strength than OPC but PPC provides better workability and finishing than OPC. Higher the cement content in the mixture, the greater the compressive strength. In contrast, the shrinkage-induced stress increases, then mixture becomes more susceptible to cracking. This may be accompanied by a greater tendency for debonding to occur in the applied plaster.
- Water Content: The higher water content, the less compressive strength and the greater shrinkage-induced stress. Therefore, the least amount of water must always be used, which gives good workability to the mixture when applying it.
- Sand: Sand must be clean, sand containing clay must not be used “or should be washed well before use”. The presence of clay significantly increases shrinkage-induced stress and may lead to the occurrence of deep and wide cracks in the plaster. Therefore silt content should not be more than 5%. 
- PP Fibre: Addition of Polypropylene fibres in mortar reduces the shrinkage cracks in plaster. Mixing of PP fibre in machine is easy and fibre disperse easily and uniformly in mortar spraying machine . The fibres restrict crack propagation and positively affect several concrete parameters. To improve the adhesion of polypropylene to cement matrix, geometrically deformed or modified fibres are commonly used. 
Importance of Flexural Strength
Flexural strength test is more important and more accurate test for measuring the quality of the cement plaster compared to the compressive strength test. The results of this test provide better predictability of the plaster’s ability to withstand tensile stresses. The higher the flexural strength values, the better flexibility and the less tendency of shrinkage cracking. A cement plaster of high flexural strength is better than a cement plaster of high compressive strength.
Sometimes special additives may be added to the cement plaster mixture which significantly increase the flexural strength. At the same time those additives may not have any significant effect on the compressive strength (some of them may even reduce the compressive strength). The use of such additives give cement plaster a better flexibility and a greater ability to resist the shrinkage stresses, thus offer a less tendency to crack. From past few years, modern devices have been developed to measure the shrinkage of the cement plaster. One of the best methods is determining the shrinkage by the laser technique. The use of this type of measuring equipment gives the product developer the ability to accurately measure the plastic shrinkage and the dry shrinkage of the tested plastering specimen and the ability of developing new types of plastering materials that have lower shrink-ability and higher quality.
The pull-off test is one of the best tests for evaluating the adhesion strength of the plaster to the walls. Lab results can be used to measure the quality of the plaster material. Field results of this test can also be used to assess the quality of the surface preparation and the quality of the plaster’s application done by the contractor.
Plastering with Spray machine
The spray plaster is a technology to provide fairly a level and smooth surface ready for further decoration. Spray plaster can be applied as a self-finished texture requiring no further decorating. Spray plaster is one of the technologies used for the protective and decorative coating of walls and for moulding of decorative elements. The benefit of spray plaster is to ensure the consistent uniform finish. The spray plaster technique can be applied mostly in large structures, and without special training workers cannot be realized all the possibilities of model materials and technology & therefore will not be achieved economic effect. Spraying and plastering equipment allow a plasterer to skim drywall five times faster than if they used the traditional hand float approach. The spraying and plastering machine is becoming more popular as a result of rapid infrastructure development, which has resulted in an increase in residential construction and a high demand for high-rise buildings.
Raw material specification and proportion:
Cement: Portland Pozolona Cement (PPC)
Sand: Manufactured (Crush) Sand
Chemical: CRYSO HS 1240
Fibre: Polypropylene Fibre
Table 1 shows the comparison of gradation of sand recommended in IS 1542 for conventional way of plater, vis-a–vis ideal requirement of gradation for mortar machine and actual sand used for plaster at site.Fig. 1 represents the graphical gradation of sand gradation mentioned in table 1. Sand available at site follows the gradation pattern of min requirement of IS1542. It is being observed that if fines available in sand is close to what recommended for machine mortar plays important role flowing of material through the pipe with minimum friction and segregation. Cement slurry act as a lubrication between the sand particles.
Numerous trials were conducted to arrive at appropriate W/C ratio and admixture doses. Behaviour of flow of mortar also depend on the vertical lift and horizontal lead of mortar flowing from pipeline. Change in grading of sand and W/C cement ratio results in either chocking of machine Nozzle or flowing down of mortar from wall surface. Therefore, control on consistent quality of sand is important and so it is advisable to use premix mortar designed for machine plaster to avoid or minimise trials and error.
Field trials were carried for internal plaster in CM (1:6) 10-12 mm thick & in double coat and for external plaster (1:4) 10-12 mm + 8 mm to 10 mm thick plaster on 14 floor residential building.
|Table 2: Mortar mix proportion|
|Method of Plaster||Cement (Kg)||Sand (Kg)||Water (Lit.)||Admixture (Ml)||PP Fibre (gm)||W/C Ratio|
|Machine mortar||50||230||27||650 to 750||100||0.54|
Surface preparation is made with dash course and 30 mm wide X 12 mm thick at 1.5-2 meter interval mortar strips to get uniform level surface during the application of mortar by machine as shown in Fig. 3.
Cement mortar is uniformly sprayed on the AAC block wall as shown in Fig. 4
After the spraying the mortar it is being levelled by trowel by mason to give smooth and uniform surface as shown in Fig. 5
Manpower used in mechanised plaster:
As shown in Table 3 by conventional way, 6 mason with 8 helpers worked for 8 hrs. plastered 153 sqmt. internal wall surface area. Thus, per mason per day output of plaster by mechanized way is around 25.5 sqmt./day. And the same area plastered using plastering machine with the help of 4 mason & 4 helper +2 supporting helper. Per mason per day output of plaster by machine is method is around 61.2 sqmt./day climbing Works Platform.
|Table 3: Manpower required for Machine and Manual method|
|Method of Plaster||Time required (Hrs.)||Mason (No)||Machine Operator (No)||Mortar Sprayer (No)||Helper (Nos.)||Total (Nos.)||Per day (8hr)/ per Mason Output (sqmt)|
|Conventional (manual) way||8||6||-||-||8||14||25.5|
Using mortar spraying machine
Material & labour rates are considered at the time of trials. Table 4 gives the cost of machine, material and manpower for machine plaster process.
|Table 4: Costing by mechanized plaster method|
|A. Machine carrying cost & maintenance cost||Amount (Rs)|
|Cost of Machine =||14,00,000.00|
|Life of machine 7 yrs. Depreciated value of machine @ 15%||2,10,000.00|
|Cost / day (250 days effective working in year)||673.00|
|Maintenance cost : Life of rotor stator is 6500 Sqmt. costing Rs. 25000/- Maintenance cost per day for 153 Sqmt||588.00|
|Total Cost of machine & maintenance||1261.00|
|Cost of machine & maintenance Per Sqmt.||8.24|
|B. MATERIAL COST (M)||Unit||Qty||Rate||Amount (Rs)|
|Cement Bags||Bag (50Kg)||22||268.8||5913.60|
|Sand (1: 4) (100 Kg)||Cumt.||3.08||2466.7||7597.28|
|Admixture / batch (150 - 200 ml)||Lit||7.7||59||454.30|
|Water / batch - (11 - 12 litres/ batch)||Lit||506||0.05||25.30|
|Chicken Mesh @ 0.2 / sqm||Sqmt.||30.58||5||152.92|
|Total Cost of Material for 153 Sqmt area||14726.84|
|Cost Per Sqmt.||96.25|
|* PP Fiber quantity required and cost is same in both manual and machine-made plaster hence cost is not considered|
|B. MANPOWER COST||Quantity (Nos.)||Rate||Amount (Rs.)|
|Total Cost of manpower for 8 hr.||7500|
|Cost of manpower for 5 Hrs||4687.5|
|Total cost of manpower per Sqmt.||11||30.63|
|Total Cost of Machine + Material+ Manpower per Sqmt. (A+B+C)||135.12|
|Add 10% for Contractor Profit & O.H||13.51|
|Total Plastering Cost / Sqmt||148.63|
Table 5 gives the area of plastered per day (8 Hr. working) for 153 Sqmt.by conventional method.
Summary of cost comparison
Summary of comparison of conventional method and machine method is presented in Table-6, which shows that plaster done using spraying machine is economical by Rs. 48.23 per Sqmt.
|Table 6: Cost comparison summary|
|Particular||Machine Made Plaster||Conventional Method|
|Machine & maintenance cost (Rs. / Sqmt.)||8.24||Nil|
|Material Cost (Rs. / Sqmt.)||96.25||88.97|
|Manpower cost (Rs. Sqmt.)||30.63||90|
|Contractor Profits (10% on M+M+M) (Rs. / Sqmt.)||13.51||17.89|
|Total (Rs. / Sqmt.)||148.63||196.86|
|Difference (Rs. / Sqmt.)||48.23|
Project duration is major element that contribute to the cost of project. Mechanizing some construction activities helps in finishing the job faster and achieve better quality compared with the conventional method. For plastering activity, mechanized way is an improved construction method. It is being found during the trials that plastering done with spray plaster machine is found cheaper over method. Skilled manpower required is also less. Mechanised plaster system is useful where large area needs to be plastered at one location and there is plain surface available without much projection on building surfaces. Main conclusions drawn from the experiment are as under:
- Initial cost of investment in machine is high.
- To achieve break-even on investment in machine there should be continuous work front required for machine.
- This process is more suitable for large plain surface area to be plastered.
- Quality of sand and controlled mix proportion play important role in machine performance.
- Mechanised process is 25% to 30% economical over conventional process.
- Mechanised process is more safe and speedy process than conventional process
- Mechanised plaster requires 30% less skilled manpower
- It reduces wastage of mortar over conventional method
- Mechanised plaster has better quality resulting in more life over conventional method
Authors are grateful to M/s B G Shirke Construction Technology, Pune for technical support and encouragement for this research work.
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