Compressive Strength of Concrete: Debs' Question

In summary, the test was done after only 7 days though so maybe this accounts for the low stength value. The original value for the compressive strength of concrete was 275.8 kN, but after multiplying by 1000, the result was 12.26 N/mm^2.
  • #1
debwaldy
38
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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:biggrin:
 
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  • #2
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.
 
  • #3
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.
 
  • #4
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.
 
  • #5
FredGarvin said:
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.
 
  • #6
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].
 
  • #7
FredGarvin said:
That's kind of low but not as low as you may think. That equates to about 70 psi.

Bystander said:
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.
 
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  • #8
holy crud. I was way off. That's the last time I post early in the morning.
 

1. What is compressive strength of concrete?

Compressive strength of concrete is the ability of concrete to resist compression or being squeezed together without breaking or deforming. It is an important property that determines the quality and durability of concrete structures.

2. How is the compressive strength of concrete tested?

The compressive strength of concrete is tested by subjecting concrete cubes or cylinders to a compressive load until they fail. The load at which the concrete fails is recorded and used to calculate the compressive strength in megapascals (MPa).

3. What factors affect the compressive strength of concrete?

The compressive strength of concrete can be affected by several factors such as the water-cement ratio, aggregate type and size, curing conditions, and the use of additives or admixtures. The quality and proportion of materials used in the concrete mix also play a crucial role in determining its compressive strength.

4. What is the typical compressive strength of concrete?

The typical compressive strength of concrete varies depending on the type of concrete and its intended use. However, for most structural applications, the compressive strength of concrete ranges from 20 MPa to 40 MPa, while high-strength concrete can have compressive strength of up to 100 MPa.

5. Why is compressive strength important in concrete construction?

Compressive strength is important in concrete construction because it ensures that the concrete can withstand the loads and stresses it will be subjected to without failing. It also helps to determine the appropriate thickness and reinforcement needed for concrete structures, ensuring their safety and longevity.

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