RMC vs site mix comparison

[Visualizer]

Hi ,

I have decided to go with RMC instead of.mixing
concrete at site for most part.
Here is my consideration writing down so that
it may help someone

1) Quality:
While contractors have done enough work and they
may have best experience the biggest problem with
site mixing is control on amount of water.
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.
So unless you can ensure strict supervision stand on the site and quality control is difficult.

2) 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 ratio is 1:1:2
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 cost works out to 5380 Rs ( taking cement as 380 per bag which I last purchased)

RMC M25 is 4500 Rs ( RDC Prism brand)

Next question comes how is that 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 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 grear difference in final retail price compared to OPC cement.

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

Cons:

(1)One truck brings around 7 cubic meters so not suitable for small portions.
(2) Pumping charges : Pump is provided free above
30 Cubic meters else pump charge is 10K.
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 cost offsets it.

Bulk work such as footings and roof slab
RMC can be used and for columns , chajja etc site mix has to be used.

[tvsh]

I thought RMC added more water as they have to pump the concrete mix. I went for site as mix we provide the materials hence have better control over the strength of the concrete. The cube test which is normally done is as per standard test conditions ( the cube is kept submerged in water until taken out prior to test) and not site conditions, where it is exposed to elements and cured a few times a day. So the strength they claim may not be the actual strength we would have achieved on the slab. We can also use PPC, PSC or composite cement instead of OPC if eco friendly ness is desired. However consistency is an issue when using site mix concrete.

[ragalax]

RMC testing, my 2 cents:
i have requested the RMC supplier to leave the cubes at site and i have let the cubes on the top of the newly built slab and i have also asked them to take samples of the same material to test under their standards.
the end result is not much difference both the samples have attained almost same strength (at 3 days and 28 days test).

i have done 3 to 4 times curing a day based on the weather conditions, and i have also let the concrete to cure for almost 12 days (by leaving 2 inch water on the slab) before i start building a wall on the top of it.

[Visualizer]

6 cubes are casted in my case 3 will be kept at site and 3 they will take back

. We can also use PPC, PSC or composite cement instead of OPC if eco friendly ness is desired. However consistency is an issue when using site mix concrete.

PSC cement is priced almost same as OPC (barely 10 - 20 rs difference however GGBS is cheap so cement companies are making money. In case of brands such as ACC PSC is priced at par with OPC.

However RMC M25 is 4400 per cubic meter ( A grade brand with GGBS ) and site mix M25 with OPC costs 5200 Rs. If suppose you use flyash instead of GGBS price further goes down by 100-150rs.

So for approx 40 Cubic meter for foundation you save around 32K rs same goes for slabs so may be 1 L savings for G + 2 house.