Cost of RMC vs On-Site Mixing

[Nivas]

Yes you are right, RMC typically do not use cement determined by the ratio of M20 , reason being concrete of desired strength and properties can be made with lot more materials other than just sand,water,aggregate,opc . But generally books and internet do not discuss other recipes of concrete. Generally people are not aware of other recipes of concrete , and speculate RMC and often with suspicion of cheating and foul play with chemicals. I would put myself in that suspicious category of people,till I checked out the facts myself.

You can talk to RMC and get your mix with 100% opc with no addition of pozzolon if you like, they give the choice of recipe to you.(with different cost tag).I feel achieving M20 with 7.6 bags OPC is not the smartest way. it may yield M20 28 day strength, but when you compare with RMC 90 day compressive strength test, site mixed or 100% opc is very inferior. I did one test after 3 or 4 months to test this fact.
RMC strength increases with time . The site mixed 100% opc is very exothermic develops lot of heat, leads to cracks compared to other recipes.
RMC exploits the pozzolon reaction to cut costs(or make money), that means you can achieve M20 with lot better properties with reduction of OPC upto 50% with careful choice pozzolon materials like GGBS and flyash.

I would like to share some thing about Admixtures\chemicals that your RMC may add to your batch. RMC guy needs to input few parameters in software and rest of batching\dispensing is controlled by computer.
RMC guy would typically know the approx distance and time from the batching plant to your place , the inspection of site is first done before they commit to you. computer will add retarder if required. that will slow the setting.
Depending on the height to which it has to be pumped , slump is decided (flowability \ work-ability of concrete) and accordingly plasticizer is added. without adding additional water or cement. (cost reduction). Slump can be measured at site , typically RMC truck will carry a slump cone which you can ascertain yourself. you can also cast the concrete brick for compression test , get it tested at civil aid , if you doubt his lab. RMC will do a 7th day strength test and he needs to furnish it when you ask for it.


I used http://www.rdcconcrete.com/ for my house.

[pomski]

That's very insighful Nivas. I did speak to many RMC suppliers and specifically with ACC directly. They mentioned the same which you have noted. No one uses 100% OPC and they said almost 60% will be slag cement. And it will add up to the strength of the concrete over the long run.

Is there any technical way to find out that the concrete poured in the site is actually as per the measurement he is billing us? Yesterday we poured concrete to our footing and it was 21 CMT. What is the way for me to ensure that they have indeed poured 21 CMT of M20 concrete?

[zahid]

@Nivas: Thank you, for sharing those details. Really helpful. I also used M20 RMC for all slabs of my home, which is currently in its final stages of completion. I believe there are many misconceptions about RMC. Even I had to do lot of research before opting for RMC. I feel RMC definitely works out cheaper than site mix and the RMC is uniform. Though my requirement per slab was around 17 Cu. Mt, I opted for RMC. Even with pumping charges of 5K, I think RMC works out cheaper. They don't charge for pumping if the quantity is more than 30 Cu. Mt.

@pomski - I will tell you the easiest way to ensure you have got the right quantity of RMC. Before ordering, I checked with my engineer on the quantity (in Cu. Mt.) of concrete required for a slab. I just ordered 1/2 Cu. Mt. of RMC extra when compared the quantity given by the engineer (since calculations on paper always fall on the lesser side). What I observed is that the RMC was always sufficient and around 1/4 Cu. Mt. of RMC used to be left out (the quantity that resides in the pumping pipes), which I used for filling up parking area, etc.

[Nivas]

Yield of RMC is something that you need to trust your RMC and its quality processes , less than 2 to 4% error ,we need to live with (i think its negligible).Go with reputed RMC suppliers and you need to worry less about RMC.
air en-trainer admixture added to increase the yield and also to increase the homogeneity of concrete. The air en training is accounted in the batching software and all materials are automatically weighed before mixing.
Similarly to zahids experience, every time we could estimate it correctly.The pump and truck operators are not that technical, do not let them add too much water at site. They will say pump gets heated , but hold it as much as you can.

[rrprakash]

@ Nivas,

Thanks for the insight. Can you suggest the best RMC for JP Nagar 8th phase considering location of plant, travel time, quality of RMC and reliability?

Regards

RR Prakash

[riturajmnit]

Reinvoking this thread:

with this sand strike, I am kind of getting inclined to go for RMC
Any suggestions on what all precaution i should take,

I am planning to use it for footing, slab casting

[pgrsridhar]

With all the headaches of managing the manual gang for proper maxing ratio and time and effort it takes, I see gradual growth in usage of RMC nowadays. My only suggestion is go with the branded RMC companies like ACC or Ultratech rather than local RMC players even though branded companies charge a bit higher but you are assured of the quality.

Regards

Sridhar
BuildHomeSmart.com

[riturajmnit]

Thanks Sridhar.

Somehow my architect is still not convinced with RMC.
How much does it costs end to end including pumping and labor for RMC with river sand ?

[Nivas]

Please share with the forum if your architect has any scientific facts and data.
Pumping is not charged unless your quantity is very less. while casting roof slabs , you may require 3 masons and 4 helpers for half a day.
Any reputed RMC may not do a River sand mix as the quality of River sand cannot be controlled and is not consistent and not economical.
Composition wise
River sand = Crushed Rock(M.Sand) + Silt \ Mud + Organic Matter + Shells\Bones ( Add a million years ,forces of nature, which will render River Sand particles round and makes the gradation haphazard) .
Now does the million years or the round ness of sand or silt\mud organic matter add any strength to concrete , The answer is no,infact detrimental.
a) Gradation of sand is very important , it tells you the percentage of various size of particles like 4.75mm,2.36mm, 1.15mm,600,150,75 microns in a batch. Gradation of M.Sand can be highly controlled. infact reputed RMC source M.Sand from reputed companies only and they provide this data for every batch.
b)M.Sand grains are cubical compared to rounded sand particles. and better moisture retaining adding to strength of concrete.
more technical data is available on M.Sand vendor websites.

[jeet]

@Nivas,

What's the approx cost of M20 and M25 Grade Ready Mix concrete per cubic feet?

thanks.
Jeet.