Quote:
Originally Posted by Sebring 'Cement is Cement', they're all the same. The magic of branding something as ubiquitous as limestone and sand have resulted in this perception. Two factors contribute to strength, river sand and the 'cement-sand' mix ratio. River sand is extremely hard to come by, chances are you get a mix of shore sand or 'jelly-crush'. Clay bricks baked in ovens or baked out in hot sweltering sun remains the strongest building material till-date. |
I agree while it holds true for Cement it is not equally true for steel take example of Fe 500 D steel which is most commonly used ( Though IS specifications calls for Fe 415 it is no longer available in market )
You can do a simple test take a bar of Primary Steel producer such as Tata Tiscon 500D or JSW Neo 500 D and ask bar bender to bend and straighten it
repetitively with these 500D steels you can continusly bend and make it straight again 12 to 15 times where as other
reputed brands fail in 5 or 6 bends.
The steel supplier from which I purchased steel is supplying to metro rail project and he told me that only JSW and Tata passed both chemical and physical tests other brands just pass physical tests.
Due to possibility of cracks and failures structural engieeer usually increase margin of safety in Design by 4 or 5 fold instead of 2 or 3 folds. So if you are using good quality Fe 500D steel you can tell structural engineer and total quantity of steel may reduce.
Even if structural engg does not agree to your view and keep quantity as same price difference in steel is not huge for same grade.
Example when I purchased JSW Neo 500 D was 54500/- per tonne where a other local steels were 51 to 52 ( meenakshi , kamdhenu etc ) Indus was even costlier at 56K per tonne in Bangalore.
So my take is buy a cheaper mid range brand OPC 53 Cement ( Dalmia / Birla Shakti/ A1 ) which costs 360 to 380 Rs instead of premium brand such as Ultratech or Birla Super ( 415 to 450/-) and get top quality steel ( Tata , JSW )
In all if suppose 900 to 1000 bags are used for you house construction you will save 30K rs which you can invest in buying better steel ( approx. 11 - 14 tonnes) for 40x60 site
This way you can get optimal construction for same price.
One more way to reduce costs and over engineering is use of RMC instead of mixing of concrete at site. In independent house construction at least major concreting portions such as footings and floor slabs are easily doable with RMC.
Here are my considerations
1) Quality:
While contractors have done enough work and they may have best experience the biggest problem with on site mixing is control on amount of water and maintaining proportions by labourers.
Excess water causes voids and honey combing and reduces strength of structure.
You can put the amount of cement as per M25 but workers will add water as per work-ability. If you are constructing individual small house its highly
unlikely that you can do cone slump test with every batch of mix.
So unless you can ensure strict supervision stand on the site and quality control is difficult.
2) Cost
For site mixing M20 nominal mix of 1:1.5:2 is used this was for probably M33 grade cement now a days normally M53 or M43 is used so end result is
much higher strength then required. (provided water is controlled) For M 25 the nominal ratio is 1:1:2.
Though these ratios are for OPC 33 grade cement which is no longer sold in market you can not simply reduce cement for OPC 53 or 43 grade so in absence of any technical test for margin of safety only nominal mix method is feasible for on site mixing.
So for 1 cubic meter M25 you will need 11 bags of cement, 13.5 cubic feet sand and 27.19 cubic feet aggregate. So material cost works out to 5380 Rs ( taking cement as 380 per bag which I last purchased) if you include labour,water cost , mixer rent , fuel etc it will be still higher.
Water is an important cost in Bangalore atleast because on tanker of 4000 liters cost between 800 to
1000 Rs and even if you are pumping ground water from deep bore it is your own future resource you are depleting,
RMC M25 is 4400 Rs ( A grade brand such as RDC which is present at national level but not Lafarge which is costlier)
Next question which comes naturally to mind is how can RMC cost less ? See next section
3) GGBS ( granulated blast furnace slag).can be used to replace cement up to 60% it consists of same materials which are present in cement and bind with lime to give strength ( Al2O3 , Fe2O3 , SiO2) Theoretically you can purchase GGBS and lower site mix cost however its next to impossible to train workers maintain ratio etc.
If suppose you don't need early strength but want chemical attack prevention flyash mix is also possible however GGBS is better in terms of 28 day strength. Compared to flyash though 3 months strength may be same. flyash RMC is priced 100 to 150 rs lower then GGBS.
By the way GGBS mixed in cement is sold as brands such as ACC Concrete+ cement or PSC and flyash mixed in cement is sold as PPC for plastering with almost no great difference in final retail price compared to OPC cement.
A bag of PPC or PSC cement is priced just 10 to 15 rs lower then OPC 53
so cost benefit is not passed on to end consumer fully in case of buying bags
of PPC or PSC.
4) Eco-friendly:
GGBS is waste product of steel industry.
Each KG of cement manufacturing creates 1Kg CO2 emission. So replacing Cement with GGBS is environment friendly. Similarly flayash is a waste product and its use replacing cement is ecofriendly.
5) RMC guys will cast cubes of RMC and will give test report for comprehensive strength.
6) Reduces labour and time : In my house construction pouring for 16 footings total 35 cubic meter was done in a day with manual mixing it will take many days and labour requirement will be high
Cons of RMC for small projects.
(1)One truck brings around 7 cubic meters so not suitable for small portions. Every RMC plant has
some minimum quantity usually 3 cubic meters.
(2) Pumping charges : Pump is provided free above 30 Cubic meters else pump charge is 10K.
(3) Without pumping matching speed of manual workers and RMC truck is difficult
If the work is planned carefully then required quantity can be achieved for single house.
Even if you need to pay pumping charges reduced labour and costs can offsets it if suppose you can using 15 cubic meter at a go
Bulk work such as footings and roof slab RMC can be used and for columns , chajja retaining walls etc site mix can be used.