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Can someone suggest if it is cheaper to mix on-site or do RMC? As per my calculations it is cheaper to go for RMC. We are using M25 for columns and slabs.
Please advice. Currently I have price of about Rs 4500 for M25 from a reputed RMC supplier. No extra charges for pump etc.

I estimated rmc to be costlier by 5% but if you add value to the time, effort and hassle saved, rmc is definitely cheaper. Also rmc produces consistent concrete mix which is very important.

Are you considering M20 or M25? Was this calculation before 1 month or in past 1 week ( cement prices have shot up by Rs 50 to 80 since mid-june). I see that in site mixing usually if materials is from owners side and it is labor contract, then they end up using more cement than we suggest. Any other feedback?

RMC is way cheaper and superior in quality. it was proved to me and I am convinced.
a) Schmidt hammer tests were conducted on my site with RMC for column and a sample of site mixed concrete column , Site mixed turned out to be of lower strength to the scale of 5 to 10 MPa (after 28 days). Even though the site mixed ratio was accurate. I have used RMC for footing,slabs,lintels,Columns, thanks to my RMC provider , he could service our small requests.
b) Last week I compared RMC to Site mix Cost(cause I had a small patch of 3m3, which i decided to do a manual mix for cost comparison) , you could save up to Rs 700 per m3 if you go for RMC(only material wise). Site mix was highly controlled.
(with cost of cement and sand so high , I wonder why would any one do on site mixing ,even for small quantities).

There are few techinical things to be considered while comparing the RMC Vs Site mix.

1. In RMC they are using 50% of cement only ( For m20 we are using 300 kg for cmt ) But In RMC they are using 150 kgs of cement only .
2. We are using sand in site mix .. But In RMC they are using Robo sand or Another artificial Concrete .
3. For any RMC vehicle it will take 1 to 2hrs to reach the concrte to the site .. It may happen for some sites not all the sites . But if it get delayed we wont get required streangth . But in site mix we will pour the concrete instantly.
4. All the RMC people are not supplying the correct qty ... There is no cross checking about the Qty they are supplying .

I am not telling the all the RMC or bad .

But Site mix is always good .

Regards
Murali

I feel RMC is expensive than onsite mixing. As it was pointed out, RMC is using 40% river sand and 60% M-sand. This is a significant saving for the RMC supplier. For M20 concrete, I did some research and found that there should be:

1. 7.66 bags of cement/m3,
2. 10.9 cubic feet of sand/m3
3. 16.55 cubic feet of jelly/m3

for 1:1.5:3

(refer M20 mix design at http://elearning.vtu.ac.in/12/enotes/Ad ... t3-MCN.pdf )

This roughly translates to around Rs. 4000 per cubic meter for material alone. If we add labour to it, definitely it will go upto 4300-4400 per CMT approximately.

Today the average cost of M20 concrete is around 4800 where they might not use the best materials.

Can someone tell me what are the labour risks involved if we decide to go for onsite mixing for Roofing

Regards,
Praveen

Labour productivity and workmanship is a grey area. Imagine a roof slab, poured with 100 small batches of site mix , each batch the w/c ratio , sand\aggregate\cement ratio , homogeneity( sometimes the batch might need 50 more rotations and lazy operator poured it too early).

I was thinking on the same lines as pomksi initially, After visiting a RMC plant and experimenting with site mix with strength tests and doing a detailed cost comparison, I personally feel RMC guy makes money through smart scientific ways and not by cheating. My doubts about yield , cheating , time and quality management(time to deliver the mix to site without losing strength) all vanished. Its mandatory to get the water they use to be tested,dispensing equipment to be calibrated periodically and everything is automated. i recommend a visit, i am sure it will clear many misconceptions.

M Sand, retardants, GGBS,Flyash, plastizers ,these are wiser(cost wise) and better engineered materials for concrete , unavailable for site mixing at reasonable prices. But people just perceive this as cheap only by looking at cost but not its value.

there are several places we overlook while doing a comparison both cost and quality wise. to achieve the same strength you will have to add more cement in site mixing and the miscalculations in the sand/cuft price.
a)Mixing and w/c ratio is what is very difficult to control , In site they typically add too much water owing to reduction in strength. whether the mix is homogeneous or not cannot be merely determined by eyeballing the mix color. RMC manufactures have better ways to deal with plasticity and mixing.
b) As I learnt while constructing my house , Calculating the cost per cuft of sand and quality of sand is a grey area, its not just ( Amount / Lorry Size). cost incurred could exceed 30% and there are reasons.

pomski wrote:I feel RMC is expensive than onsite mixing. As it was pointed out, RMC is using 40% river sand and 60% M-sand. This is a significant saving for the RMC supplier. For M20 concrete, I did some research and found that there should be:

1. 7.66 bags of cement/m3,
2. 10.9 cubic feet of sand/m3
3. 16.55 cubic feet of jelly/m3

for 1:1.5:3

(refer M20 mix design at http://elearning.vtu.ac.in/12/enotes/Ad ... t3-MCN.pdf )

This roughly translates to around Rs. 4000 per cubic meter for material alone. If we add labour to it, definitely it will go upto 4300-4400 per CMT approximately.

Today the average cost of M20 concrete is around 4800 where they might not use the best materials.

Can someone tell me what are the labour risks involved if we decide to go for onsite mixing for Roofing

Regards,
Praveen

They are not using 7.6 bags of cement per m3 . They are using 150(3 bags ) Kgs of cement only . You an check the Batching slip of any RMC plant . it Clearly written that they are using 50% of the required cement .

Thanks for the quick reply Nivas. It really shows lot of ground work and experience you have undergone. Can you please tell me the RMC supplier you engaged? Our contractor is sourcing from AVS RMC from Attibele.

Hi Nivas,

Please share the RMC supplier name.

Regards,
Raghuveer