myBDAsites.com

Hi forum members,

I have a question w.r.t M20/M25 design mix ratio. As per code book/general understanding, mix ratio by volume for M25 is 1:1:2 and M20 is 1:1.5:3. But, after going through RMC design mix ratio, i am confused about this.
Due to the well known problems in site mixing and inputs from some of the forum members, I went ahead with M25 grade RMC from one of the top brand. It's been 2 weeks now and i will get cube test report by month end. But, while going through their design mix ratio, i found out some interesting aspects.

Their design mix ratio is stated as follows,
for 1 m3,
GGBS:96kg
OPC:224kg
Sand:765 kg
20mm:689 kg
12.5 mm: 477kg
Admixture:2 kg

for rough calculation, if we consider ggbs is same as opc, then total weight of cement used is 320kg.
Fine aggregate is 765 kg and coarse aggregate is 1166kg.

To convert it to volume, i use following density values,
Cement-1440kg/m*3
Sand-1750 kg/m3
20mm/12.mm - ~1550 kg/m3

So, converting materials to volume leads to ,
Cement: 320kg/1440 = 0.2222 m3
Sand: 765kg/1750 = 0.4371 m3
20mm/12.mm: 1166kg/1550=0.7522 m3

Converting this to ratio leads to,
1:1.96:3.38 .

Now my doubts are,
1. If the above is mix ratio, then how RMC can achieve M25 grade?
2. If RMC can achieve M25 with this ratio, then why should we use 1:1:2 in site mixing which increases the cost by many folds?
3. This is roughly the same mix ratio used by gang people for site mixing like 1:2:3 or 1:2.5:3.
4. If above mix ratio can achieve m25, then what is standard volume mix which substantially increases cement consumption?

I am looking for valuable inputs from forum members.
Please highlight any mistakes in my calculation.
I am just using information available over web for this rough calculations.

Thanks

Hello I had same doubts before using RMC in my site however crash tests constantly gave around M30 for M25 mix I used.
The secret of this anamoly lies in three factors.
1. Handbooks are from the days when cement used to be OPC33 if you pickup any handbook cement strength assumed is 33 and not 53. So there is improvement in cement technology from 70s.

2. Water ratio in the RMC mix is strictly kept at 0.4 with respect to cement and admixture is used to address plasticity and workability. This is highly impossible at site even if you are standing on head of workers and ask them to pour only 20 liters water per 50kg cement bag.
Its almost impossible for them to work with water half of cement. Usually in the site mix water is double ot triple of the desired.
To compensate for variances and more water higher content of cement is assumed in hand mix ratios.

3. RMC uses a mix of 20mm, 12mm, 8mm and 6 mm crushed stone jelly. As per concrete theory strength is mainly from stone and sand fills up void between stones and cement fills void between sand particles and provide binding. Here in RMC smaller aggregates fill between larger ones and increases strength.

I got the M25 hand mix as per the code for smaller works and also M25 RMC whereever minimum 4 cubic meter pour was poasible ( min order quantity ) and found that compression strength of RMC was always superior.

Thanks Visualizer for the inputs. I totally agree with you on grade of cement. But, i am looking at the aspect of using only 320 kg of cement(224 OPC+96 GGBS) for M25 in RMC, whereas site mixing m20 itself consumes 400 kg of cement/m3.

1. If we consider theoretical m25, then it comes to 550kg of cement. There is a huge difference. Actual cost of RMC works out be much cheaper considering cement is the costliest item.

2. For columns, we used site mixing(m20) where we used 2:3:5:1 ratio, where 2 parts cement, 3 parts m sand, 5 parts 20mm , 1 part 12.5 mm which came out well. Original plan was to use 4 parts 20mm, 2 part 12.5 mm but later realised that sometimes they made it 50:50. So, used 5:1 ratio. Also, water used was 14-15 litres/mixing. 2 parts cement roughly weighed around 32-33kgs. It was difficult in the beginning, but later they followed our mixing principle.

3. If we consider 53 grade cement is giving more strength, then we can also use 1:2.5:3 ratio which is generally used by gangmen and looks to be the case of RMC as well or even worse. This leads to considerable savings compared to RMC or at-least quantity of cement used is more if we go by code book.

4. I am trying to understand what is mix ratio if we consider 53 grade cement. I had a belief that RMC guys can get the huge margin due to GGBS. But 100% opc RMC mix costs just 100-150 more when compared to 70:30 mix.So, cement used even in 100% opc will be 320 kg only for m25 which is far far away from the code book.

5. Cost analysis for RMC 25
Cement:224 KG * 8 Rs(assuming 400/bag)- Rs.1792
GGBS: 96kg* 4 Rs(not sure on this)- Rs.384
Msand: 765 kg * 1 Rs(~ 800/tonne) - Rs. 765
Aggregate: 1166 kg* 0.5 Rs(~500/tonne) - Rs.583

Total: 1792+384+765+583= Rs. 3524
But, what i paid was 4850/m3.

Had i gone for m20 site mixing, it would have costed me 3200 for cement alone considering 8 bag/m3.
1. Considering the input costs for bulk buyers, RMC guys have huge margin.
2. Assuming we can control water, why cannot we use same mix design used by RMC by having considerable savings.
3. If water tightness is the main criteria, then even using 1:1.5:3 or 1:1:2 may not help.
4. Even if RMC controls water, how can they drastically reduce cement quantity from 550kg to 320 kg?
5. It's high time that experts should suggest new mix ratio for 53 grade cement. Why we should overuse cement when RMCs can achieve it with lesser quantity?

Yes nothing stops you from using same mix at site and saving money however my practical experience is that enforcing any kind of discipline on masons and laborers is practically impossible. They have their own notions and beliefs and however I hard I explained to them about limiting water they will not listen.
Even enforcing ratio of cement , 20 mm jelly , 12 mm jelly and m sand was herculean effort the moment your eye is away you will find them randomly going back to usual habits. If you have time and energy to stand on their heads

so I used RMC wherever possible.

Experts do suggest changes in mix ratio if you look at published papers etc and RMC companies take advantages of research done with various admixture and construction chemicals. Procuring construction chemicals in small quantities and enforcing the exact quantities at site is difficult part.

Thanks Visualizer. I totally agree on practical problems faced at site.
But, my intriguing question is how limiting water and adding admixtures can bring down cement quantity required for m25 from whopping 550kg to 320 kg.
Even code book says we get m25 from1:1:2 ratio only when we control water. So, only differentiating factor for RMC is admixture.
As far as i know admixtures can only influence workability but cannot induce cement properties.
I dont see any mention of special chemicals and i doubt if they can influence cement quantity itself.
If that is the case, we all should take advantage of this which can reduce cement consumption thereby helping individuals and also mother nature

Well i can search and post some papers on difference in calculations between M33 and M53 cement for making M25 mix. The extra quantity prescribed in hand mix is taking the error tolerances on safer side.

The key point is RMC companies cast cube of the concrete being poured at site and show you 7 day and 28 days crushing test and in all the samples from my site exceeded the guaranteed strength.
There are several admixture such as silicon flue using which even M60 grade is produced by RMC companies awhich exceeds strength of 53 grade OPC itself .

One more point using RMC truck and pump reduces lot of wastages if calculations are done perfectly. In site mixing there is lot of wastage of material.

Check the link below

Key points as you can see
1. OPC53 Cements of all brands do not have same strength in fact big and popular brands like Birla and Lafarge have lower 7 day strength in the test not all the brands listed here are being sold in Bangalore but story will not be too different. So the handbooks prescribe thumbs of rule for lowest common denominator. Whereas RMC plant guys can test and mix as per the exact guaranteed strength they are offering. OpC33 and 53 compressive strengths (28 days) are roughly in ratio of 3:5.
2. See another table with water ratio a small variation in water is shifting cement requirement greatly. In RMC which I git they kept water at 0.4 constantly so that is another reason.

I would say unless you have acess to the test methodology and perfect computerized control like an RMC plant be cautious on side of error and take mix design as per handbook.

Buying expensive and heavily advertised brand is no guarantee of strength.

From

https://www.nbmcw.com/product-solutions/concrete-chemicals-waterproofing-repairs/concrete-articles/strength-of-concrete-vs-grades-of-cement.html

Though i agree with some of the points mentioned, I am perplexed at the amount of deviation in cement usage for site mixing vs RMC design mix i.e, around 40-45% . A simple microcontroller can control the amount of water needed . Probably, Nitin Gadkari should step in

I had got a quote from local RMC guy for 4050/m3 for M25. But, i doubted him thinking about the cement quantity required for M25.
Based on some of the threads in different forums, i had an assumption that GGBS is the only differentiating factor for RMC guys to reduce the cost other than mass procurement. But, more than GGBS, reduction in cement/ggbs quantity itself is giving them huge margin.
Also, conditions part of RMC bill/quotation is another big story. We cannot held them responsible for any deviations

That's good rate i took from rdc concrete at 4200 last year. Check of they will cast cubes at your site and give you 7 day and 28 day crush test report.
Don't go just by price alone check the following points

1. Minimum quantity for which they will provide pump free. For footings if there is space for truck to move around they can pour without pump but for higher floors pump saveas a lot of labor.
2. Pumping charges if Minimum quantity is not met. Pump is absolutely essential for roof slab cast else your labor cost will increase.
3. Distance of plant from your site and how long it will take.
4. Go for little bigger players even if 100 -200 rs more instead of small local plants.

5. Usually they supply half truck minimum ( 4.5 cubic meters in single order) however if suppose you are ordering for full truck they should be willing to supply 1 or 2 cubic meters extra of quantity falls short.

About value of RMC mix plant

RMC plants computer is also a microcontroller opening gates of different silos for mixing material in required quantities.

Microcontroller is easy part making manual labourers follow a discipline is hardest part.
Observe work of laborers for few days and try to guide them on anything. Lowest common denominator of lot comes to concrete mixing jobs because it is repetitive, zero skill job to fill up a headpan with either stone jelly or m sand and throw in a mixer at site.

Those who have little bit of intellegence and inclination to learn become masons or plaster mistri.