5-DAY STRENGTH OF CONCRETE BLOCK 1

[zaes73]

if the compressive strength of a concrete block after 5-days is "452psi" what may be the approximate value after 28-days.
And what is a good strength value of hollow concrete block and solid concrete block, at 28days compressive test.

 

[JKW05]

An ASTM C-90 block must have a minimum net area compressive strength of 1900 psi. Verify if the test results are net or gross area.

As to the testing, that is an interesting question, in that ASTM C-140 does't appear to have a requirement for the age of the block when tested. It only requires the specimen be tested within 72 hours of receipt of the specimen.

The strength gain curve is probably different for concrete block, but if it is similar to normal concrete, I think ~70% of strength at 7 days is normal.

 

[concretemasonry]

zaes73 -

I would seriously question the testing lab about the reporting AND the testing procedures. The results are so low and bizzare, there must be some very obvious reason.

Could non-standard shapes have been tested or could you be looking at a prism report? While 5 days is not a factor for block, it is not reasonable and probably not even verifiable and indicates an unusual practice. For a prism, it could be an indicator, if it was possible to handle a prism without damage at 5 days.

In the 30 years I was was involved in the manufacture of concrete masonry units we were never able to make a block that weak. The use of non-loadbearing block has ceased because the manufacturing process makes the manufacture and inventorying of non-bearing units uneconomical and impractical. The biggest compliants in the industry is that the strength requirement (1900 psi) is rediculously low and actually dates back to the 1920's.

I have made block out of sand and rice husks/hulls and out of sand and wood chips/sawdust that were over 2000 psi, which is far more than you are seeing.

ASTM C90 requires a compressive strength of 1900 psi based on the net area. About 20 years ago, the strength requirement was based on the gross area, which was really not a measure of the strength of the concrete. ASTM C140 is a testing procedure and not product standard and as such, an age requirement has never been considered and block are not identified by manufacturing dates, except in the case of optional identification (usually on the pallet) for architectural units where the block are made in special colors for a specific project.

There is no requirement for the age in the ASTM C90 standard since it is a performance requirement based on the properties when the units are used. The type of concrete in a concrete block is a "zero slump" mix and not a wet cast concrete with curing in the plant. Typically, this type of concrete will be closer to 90% when used. The cheapest and easiest way to increase the strength of a concrete masonry unit is to add extra water to this type of mix, but contractors would not buy the block.

If a sound unit is selected for testing, it is very difficult to get a low value. As an example, capping with gypsum board scraps or plywood scraps only decreases the indicated compressive strength by 10-15%. In the case of very high strength block (4000 psi to 8000 psi), some testing labs do not have adequate platen thicknesses (too much deflection/bending) to accurately test these units. This is because the typical cylinder testing is not adeqaute without swapping out the platens. For lower strength units, this would not be a factor.

I would suggest also contacting the testing lab to look at the history for that supplier for clues.Many jurisdictions require independant testing for licensing and certification of block producers.

As you point out, the strength of 452 psi does not seem correct and does not comply with any standard.

Dick

 

[DarthSoilsGuy]

all the questions above +

Does the testing lab perform this test often? I use to work at one and in 4 years we only had to break blocks for two jobs. i could see how it could get screwed up, but i would never let someone give me one destructive test sample for a test that i don't normally do. I'm having trouble imagining how that break got to you without any explanation or discussion of the significance. Because, i certainly would try to figure out what was going on as the ITL materials manager.
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On a side note, i think i've seen a block like that. As a Peace Corps volunteer, i rejected a town CMU manufacturer as the supplier for my school construction project. i made one site visit and broke a bunch of their 8" standard stockpile blocks into tiny pieces with my bare hands, yargg... (insert emoticon flexing his biceps here)

 

[concretemasonry]

zaes73 -

My comments were made as a professional courtesy to the lab. They should be made aware of erratic results, so they can determine if there is an internal problem. Quality control is a team effort between engineer, supplier, contractor and lab.

As DarthSoilsGuy mentioned, a single sample is virtually worthless. A proper set of samples is 6 units - 3 for compression testing and 3 for moisture/density/absorption. You cannot use the compression units for other purposes because of the turn-around time for the moisture/density tests and the remote possibility of adding curing due to the wetting and oven drying.

Testing block is like testing cylinders, which is not always closely monitored internally - in the summer at a large lab, the summer/temporary "grunts/students" do the actual capping, weighing and breaking (I did it one year). When school is over, the regulars usually do the work and reporting. Because of a lack of familiarity with the normal range of results, some gross errors can get through.

Dick

 

[msucog]

something is wacky. what is the 5-day break all about? are we talking about masonry prisms or cmu units? i don't think we're all talking about the same thing. 452psi seems illogical if it is a cured block (still trying to figure out the 5 day thing). if it's not green block, then it sounds like the thing was cracked all to heck before testing, the strength was calculated based on the gross area instead of the net area, or the wrong specimen was tested. i've never seen a break that low but i wouldn't say it's absolutely impossible (but not very probable being that low). i did see some 1200psi block come to a job not too long ago and it turned out the plant sent the wrong block, so anything is possible.

 

[concretemasonry]

msucog -

I agree on the results and timing presented, but prefer the term "bizzare".

In some areas of the world, there are some unusual products referred to a "concrete block" that are produced in the same cities as quality block made to the conventional ASTM C90 specifications. These unusual products perform adequately for the intened purpose (one story homes), but should not be confused with "real" concrete masonry units.

Some of these unique units use aggregates that are non-complying (improper gradations, high amounts of fines and organics) or use excessive replacement amounts (40 to 70%) of low grade fly ash.

As an example, in India, there are CMUs with a net strength of 2000-5000 psi made daily in the same cities as "concrete block" with strengths of only 1000 psi for the same size and shape.

Because of this, anything is possible if the definition of loadbearing concrete masonry units is stretched. Proper specifications, sampling and testing is the key to reliable construction when you are in some areas. - Europe, Russia and most of South America have similar standards to the U.S. and use similar terms.

Dick

 

[civilperson]

These results require rejection of the product. Any results under 1000 psi for 5 day tests are a real problem. 70-80% of ultimate strength usually occurs for seven day cure. F'c of masonry is ASTM specification.

 

[concretemasonry]

civilperson -

The time or age of the product is immaterial if the project is under the U.S. model codes (and almost all international standards) or ASTM standards. The ASTM C90 specifications refers to the properties of the units when delivered to the purchaser. Any concerns about the curing of the products after delivery is a moot point.

To my knowledge, there has been independant or credible relationship between the strength out of the plant in comparison to ultimate strength because the units are usually cured in the plant using different conditions. That is the reason the ASTM standard applies to the properties at the time of delivery to the purchaser. - All other property requirements also relate to the same sampling time.

While on the voting committe for ASTM masonry units and mortar, I have never seem any suggestions to relate the compressive strength to anything other than delivery date or date of use since the strength gain on site is so small and variable, depending on the conditions.

The 1000 psi is probably a carry-over from the old standards that referred to the gross area strength. For most size units, the old 1000 psi equates to the current standard of 1900 psi based on the net area.

"Engineered masonry" design is not related to f'c. It related to f'm that is determined either by a prism test or by a conservative method of using a table referring to the net compressive unit strength and the mortar type.

I agree that the poster's strength (apparently based on one sample) renders the product to be unusable for use under a code referencing ASTM C90 and ASTM C140. - I suspect the product tested was someting similar to a conventional U.S./western hemisphere CMU, but made using non-complying materials or low-tech manufacturing (molding and handling) methods.

It would be interesting to find out what the application for the product is.

Dick