3 PSI Strength Questions

First of all, I think that you are confusing psi with load. Simplistically, you can think of load as the weight that you are placing on the concrete sample. To convert this to stress, in your case psi, you would divide the force by the area that is is applied to. So, if you had a concrete sample that is 1 inch by 1 inch in cross-sectional dimension by 1 foot long with a 100 pound weight placed on the end, the stress will be 100 psi. If the cross-sectional area was 4 square inches, then the stress will 25 psi.

Conversely, if you had a concrete sample with a strength of 3,000 psi. that has a cross-sectional area of 4 square inches, then the weight that can be suppported is 12,000 lbs. But, there is more to determining the required strength of concrete than just compressive strength, you must also consider buckling, load eccentricity, and moments, etc.

Testing the compressive strength of a concrete sample is relatively easy. The samples must be of a standard diameters and heights as described in the various standards. The load must be uniformly and slowly applied to the sample until fracture occurs. The way is fractures is very important, and can invalidate a test result. If you are using the conrete for a structural member, then you must hire a lab for testing.

Your questions don't make a whole lot of sense which tells me you need to do some more researching. You need to search the internet, and don't look for answeres to the questions you listed, because you will not find them. Concrete does not have one nice neet "Modulus" like typical mild steel, it does not react like steel does to different loads. Example concrete is incredably strong in compression; however, in tension it is no good.

I have four answers to your three questions.

Geometry geometry geometry; and reinforcing material.

Is this all for some theoretical thing, or are you actually trying to build something? If you are building something you need to tell more about it so we can attempt to help you formulate good questions so you can get good answeres, that hopefuly lead to good solutions.

Some typical examples of concrete uses.

"slab" floor, driveway, etc. you drive a car on them, but not loaded dump trucks repeatedly. these are typicaly 4-6" thick with either rebar (probably 3/8" nominal diameter) or have wire cemented in. Typicaly the metal reinforcement is in the middle protecting the cement from loads from above and frost heaving from below.

industrial flooring or highway/bridges. in this application the cement is always engineered to ensure saftey. I worked in a business that dealt with plating solid bar in diameters from 1-6" all 24 feet long, different types of steel/size/plating were all inventoried, many tons of steal on cement floor, and not dispersed very well. The cement was 8" thick with 5/8" rebar about 3" from the bottom and wire about 3" from the top. Bridges I think are over 12" thick with several layers of cris-crossed steel rebar.

"footers" thick, wide and deep chunks of concrete that support a load from sinking, moving, or vibrating. Typically don't have as strong dependance on reinforcement as slabs,or walls.

my favorite is spaning concrete beams that are precast and prestressed. At a facility these concrete beams are poured typically they have holes runing lengthwise through the center and large steal bar near the bottom. After the concrete has cured the bars are tightened with nuts on the threaded ends. This preloads the concrete on the lower section that would otherwise be in tension if the beam is simply supported from the ends. Instead, as the beam is lifted from the ends and force applied in the middle the concrete in the lower section may be under less compression as the steel bars are taking the tension load and straining ever so slighly, but the goal is to load the concrete only in compression.

Anyone else please jump in.

If you need real psi, like in a civil project, hire a lab!

If it is not that critical, psi can be estimated by a slump test, which is pretty easy. If you are pouring slabs or just want to estimate psi, you can order the concrete at a requested psi - the supplier should be able to get it pretty close. But if you are doing civil projects, especially bridges or anything pre-stressed, you are better off getting a PE or hiring the lab.

First, concrete strength is determined typically from a number of samples collected during a concrete pour. The samples are allowed to cure under laboratory controlled conditions for period of 7-, 14-, 28- and (sometimes) 56-days. These samples after curing are destroyed in a compressive strength test to determine stress/strain to critical failure. The 28-day strength is used as the accepted estimator for the strength of concrete. These results are retained by concrete companies to establish a statistical history for a specific mix design. ACI/ASTM has procedures outlined for all the information you seek.

Reputable concrete companies will have specific mix designs to meet or exceed speicifed compressive strength requirements, i.e. a 3000 psi mix will meet or exceed 3000 psi most of the time. This result is based on significant 28-day compressive strength test results to verify the mix design statistically meets or exceeds the specified strength. There is an acceptable range of samples that may fall upto 10% below the specified requirement. Large concrete companies should have a history of tests for their mix design that you could estimate a mean F'c and the standard deviation for that mix. In the field you can then use a schmidt hammer or other non-destructive test to determine the estimated strength is consistent with expected.

All concrete strengths are statistical estimators of the material used, even direct sample testing of the pour, cores of the hardened concretein-place, and non-destructive tests. As a estimator of mix design the water:cement ratio is frequently used, e.g. 0.82 is about 2000 psi and 0.68 is about 3000 psi mixes for non entrained mixes. However, the type of cement, admixtures, and rock effect the strength. Also, faster curing concretes tend to have less ultimate strength.

You have to hire a certified testing laboratory for accurate results.

1.) PSI is calulated load/area;

2.) No see 1;

3.) You have to hire a certified testing laboratory for accurate results, they have the certified technician and calibrated testing equipment.

ASTM C39 details the steps to casting concrete cylinders for compressive strength, and concrete beams for flexural strength.

maybe someone needs to ask what part of the world are you located in? The strength of concrete will not change much as you move about the globe, but regulations and availability of certified test labs may vary.