Compressive Strength of Concrete: Debs' Question

[debwaldy]

hi just a quick question,having done materials labs we came up with a figure of 275.8kN for the compression strength of a cube of concrete acting over an area of 150mm x 150mm. i want to convert this compressive strength to the standard N/mm^2 units.To do this i multiplied 275.8 * 1000 and divided by(150*150) & got an answer of 12.26N/mm^2.Is this correct procedure, as the value seems extremely low,or did we just make exceptionally weak concrete?the test was done after only 7 days though so maybe this accounts for the low stength value?any opinions,suggestions welcome
thanks debs

 

[FredGarvin]

That's kind of low but not as low as you may think. That equates to about 70 psi. For a partially cured concrete that's not bad considering there's no guarantee as to the composition of the mix. I do know that cement compressive strength is very reliant on water to cement ratio. I had to look it up, but in all the tables I have for concrete they state a cure time of 28 days for all data. They also state that the numbers obtained are under highly controlled conditions with very good materials.

 

[Pyrrhus]

Depends. It could be because of the mix design (and components) or other. There are procedures such as the ACI Method, that people follow to design mixes for a nominal strength of hydraulic concrete. For example if it was a high-strength concrete, that value will be low.

 

[chemisttree]

If 70 psi is the correct conversion, it is an extremely low 7-day strength.

Even mortar mixes (3 parts sand to 1 part mortar cement) designed to yield only 1200 psi 28-day breaks will have strengths of several hundred psi at 7 days.

Something is very wrong unless you are performing tests on CLSM (Controlled Low Strength Materials) type mixes. Most likely the measurement itself is to blame. Strengths for cubes are very sensitive to manufacturing defects. Did you break the cube directly on the steel platen or did you break it with sulfur caps?

Did you use air entrainment? Overdosing coupled with extended mixing times can whip too much air into the sample. Does the sample density indicate more than 8% air entrainment?

Did you use a water/cement ratio greater than .45? This can cause the air content to go crazy with air entrainers and will cause laitance. If you had a weak layer due to laitance, it could initiate cracks at low strength that fool you to think that the specimen had failed. Was the sample tested to ultimate failure? Did the failed specimen have the characteristic hourglass shape for a proper break? Did the break fail any of the aggregate or is aggregate intact along the break? If aggregate is broken, it is very likely that the measurement methodology is to blame. If aggregate is intact, it is likely a manufacturing issue.

Were pozzolans used? Some pozzolans (silica fume, fly ash, bagasse ash, incinerator ash) can kill the strength of samples if they are used at a cement replacement levels greater than 25% to 30%.

What was the strain rate of the press? Strain rate can affect the measured strength of samples but not this much! Shock loading usually gives high strength values.

 

[Bystander]

That's kind of low but not as low as you may think. That equates to about 70 psi. For a partially cured concrete that's not bad considering there's no guarantee as to the composition of the mix. I do know that cement compressive strength is very reliant on water to cement ratio. I had to look it up, but in all the tables I have for concrete they state a cure time of 28 days for all data. They also state that the numbers obtained are under highly controlled conditions with very good materials.

1800 psi --- not great --- but, not bad for 7 days.

 

[radou]

Well, since the value is low, isn't the point of your assignment simply to classify the tested sample as low strength concrete?

Btw, the lowest characteristic compression strength for a concrete cube-shaped sample (a = 150 mm) found in my table is 10 [N/mm^2].

 

[Mech_Engineer]

That's kind of low but not as low as you may think. That equates to about 70 psi.

1800 psi --- not great --- but, not bad for 7 days.

Indeed, the stress is 1778 psi, not 70 psi. Fred must have missed that the result the OP gave was in N/mm^2, not N/m^2 which is a Pa. I suspect a crushing strength of 70 psi is closer to a warm block of cheese than concrete.

 

[FredGarvin]

holy crud. I was way off. That's the last time I post early in the morning.