Re: Cost of RMC vs On-Site Mixing
Posted: August 21st, 2014, 5:07 am
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.
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.