concrete grades please 2

[projecticons]

i would like to know how the different concrete grades relate to concrete mixes.for example what concrete mix do i use to achieve a concrete grade 25,as well as grade 15,25,30.
concrete mixes include 1:2:4, 1:3:6, etc

 

[dicksewerrat]

More info please.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[cvg]

what country are you in?
around here we use class for our concrete such as

Class A - Min. Cement Content Lbs. Per Cu Yard 520, 3,000 psi shall be used for concrete structures, either reinforced or non-reinforced, and for concrete pavements.

Class B - Min. Cement Content Lbs. Per Cu Yard 470, 2,500 may be used for curbs, gutters and sidewalks.

Class C - Min. Cement Content Lbs. Per Cu Yard 420, 2,000 psi may be used for thrust blocks, encasements, fill or over-excavation, etc.

Class AA - Min. Cement Content Lbs. Per Cu Yard 600, 4,000 psi for structures as specified

 

[dirtsqueezer]

Ah, I believe with 1:2:4, ect. you're referring to ratio of cement:sand:water. Is this correct?

 

[BigH]

cvg: as follow up - why do they use minimum cement content AND requisite compressive strength? As to original post, concrete grades should be specified by the designer as per his requirements - higher grades might be specified in order to achieve thinner members. We've used 40MPa for precast beams. They also have the ultra-high strengths now being used with specified compressive strengths far in excess of 150MPa.

 

[cvg]

Big H, good question. When writing my own specs, I usually omit the required cement content and request that mix design be submitted for review. However, the agencies around use the double spec. I guess it does possibly provide a little more assurance of the required quality. With the specified amounts of cement, it is difficult to not meet the strength requirements. However, some contractors still manage to goof it up.

 

[concretemasonry]

There is a very serious problem with the dual specifications - performance and prescriptive that may conflict. It seems easy to spec both and eliminate responsibility, but there are many pitfalls.

What do you do when the supplier follows the perscriptive requirement and does not achieve the performance requirements?

Or what if he has to use more cement or a different aggregate to get the strength, but ends up with a change in other properties such as shrinkage, permeability, density (for lightweight concrete only), etc?

The minimum performance requirements strength, air content, slump, etc. and material standards (ASTM) are easy to verify and monitor. Once you begin to impose perscriptive requirements in addition to the performance requirements, the water gets cloudy and the specifier can incur more respomsibility. Approval of mix designs is perhaps a middle ground, since the contractor/supplier is making the sunmittal.

I have seen situations where it is impossible to satisfy both the performance and prescriptive requiremnts. ASTM C270 is a classic example, but this ASTM standard directly says to use or the other, but not both.

 

[Ussuri]

In the UK we used to specify concrete in terms of grade C15, C20, C35 etc which related to cube strength. This has now changed and the designation now has to include the cylinder strength as well (the don't use cubes in europe).

The Euronorm BS EN 206 and its UK counterpart BS8500, specify the grades along with the restrictions on mix design, minimum and maximum cement content, maximum water cement ratio, minimum aggregate size. Using this information along with required 7 and 28 day strengths the quarry will usually do the mix design. ie they do the calculations to work out exactly how much cement, sand aggregate and water goes into the mix, along with any admixtures required to meet the design parameters

You can have a number of different mixes which give the same concrete grade and meet the design requirements.

Concrete mixes batched by volume 1:2:4 etc are very rough and have no quality control attached to them. There is no way to guarantee a design strength and as such should never be used to structural concrete. They are fine for small quantities to set a kerb or haunch a manhole cover for example.

 

[GeoPaveTraffic]

BigH,

I believe the duel spec, strength and cement content, goes back to when the mix designs were provided by the local agency. Our local DOT just stopped providing the mix designs to contractors in the last two years.

 

[BigH]

I've seen both and the one seems to defeat the other. Concrete, given the admixtures today, can achieve the requisite strengths with less cement than that specified - by using both you are basically costing the contractor and also the project money. I think that all mix designs should be contractor submitted, reviewed and given "no objections." (as GeoPaveTraffick's local DOT has seemed to learn) The mix is that of the contractor and if he dosn't come up to snuff in the testing, it is his problem - his mix, his benefit or his detriment. Throw into the mix that poor consolidation in the field can reduce the insitu concrete strength by 10 to 15% - this is the oxymoron of the "lab" strength vs in situ. As we have posted before, "lab" strength is the potential and reproducable strength. . . . mmmmmm

 

[cvg]

Bigh - in theory, I do agree with you. In practice, I see a recurring theme of contractors who pour concrete, not meeting the requirements because they didn't bother to read the spec. I see city inspectors sitting in the truck with the air conditioning on, because it is too hot to stand outside. I then see substandard concrete which according to the specs, can be left in place as long as the contractor pay is reduced by some arbitrary percentage. I rarely see contractors feet held to the fire to remove the substandard work and do it right. Most inspectors don't have the cajones to do that and risk a claim. So, I believe the agencies in an effort to get more quality with less effort required (by them) continue to specify both prescriptive and performance clauses in the specification. It is difficult for an engineer working in public works to buck that trend, as that is the defacto standard practice.

 

[BigH]

cvg - you bring up another great point that is a red herring in many cases. While we are striving for better more efficient design with cutting edge software, 4th order equations (many times, though, with only first order data), we still have to build it and many of the builders/construction types are still out there in 1960s mentality. Good at the time (heyday of engineering) but not good necessarily good enough for those complex 'designs' put out today. It is interesting how many 'engineers' get brought up on charges of incompetence, etc. (see PEO of Ontario for instance) and yet you never see the construction contrators/engineers put up to the same mark!

 

[equimo]

The grade 15, 25, 30 means the concrete compression resistance at 28 days. It is done in kg/cm2. Usually for most of the qualities of Portland Cement, sand, and gravel, the proportion is done in terms of volume: 1:2:3 for 1 cement sack volume:2 sand volume:3 gravel volume. This proportion variable depends of the size of the aggregates, physical characteristics, and quality of cement and water used. For important projects you have to send samples of these components to the lab where they can determinate the right proportion. However, in most cases fro small projects is a practice to use 1:2:3 for grade 30; 1:2:4 for grade 25; and 1:2:5 for grade 15.

 

[cvg]

equimo - what country is this grade specification used in?

 

[projecticons]

thanks equimo i think your reply was just what i was looking for.
the concrete grade actualy specifies the cube strenght in 28 days.grade 30(G30)means 30N/mm2 strenght whereas the concrete mix ratios stands for cement ratio to sand to coarse aggregate.but i still wonder if we can achieve such cube strenghts by merely following these mix ratios .

 

[equimo]

To make a correction about the concrete strength required typically in specifications and because we have still problems world wide with the imperial and SI units:
1 Newton (N)= 0.1019716 Kg force
1 Pascal = 0.1019716 Kg/m2= 1 N/m2
1 Megapascal = 101971.6 Kg/m2 = 0.1019716 kg/mm2 = 1 N/mm2
1 Megapascal = 145.0377 psi = 10.19716 kg/cm2

The usual requires of the concrete are:
So Strong structures:
30 Megapascal(N/mm2)=4351 psi = 305 kg/m2;generally 4500 psi
Strong structures:
25 MP (N/mm2) = 3625 psi = 255 kg/m2; generally 4000 psi
20 ,, = 2900 psi = 203 kg/m2; gen. 3000 psi
Blinding concrete or walkways:
15 ,, = 2175 psi = 152 kg/m2; gen. 2000 psi

If you have concerns how to meet the spcs strength, you must send the material samples to the soil lab. Have an advise from any of them. If you can proceed without that, try the best safety for so strong: 1:2:2 or 1:2:3 if sand's not so fine grains and gravel is angular shape

I hope it could help, good luck!

 

[ceanil2005]

can any one tell me how grades of concrete are specified?