Mortar Compressive Strength Test Report 1

[wayniac3]

I am preparing to write field report and reviewing a mortar compressive strength test report and note that the 28 day compressive srength is a bit lower than the 28 day design strength (1400 psi vs. 1800 psi). The wall constructed with the sampled mortar is 8" c.m.u. reinforced with #5 @ 32" o.c. The wall is 12'-0" tall and lightly loaded (350plf DL + 400plf LL) with a portion of roof/ceiling. I am wondering what others would do given this situation.

 

[Ron]

While I hate to see such issues generally blamed on the testing laboratory, you certainly have to look there.

A common mistake made in mortar testing is to misuse 2" cube molds. Secondly, such cubes have to be carefully tested in the lab to achieve consistent results.

Actually if you achieved 1400 psi on a Type S mortar from sampling in the field and testing in the lab, that's not so bad. I've seen anything from 600 psi to an occasional 2000 psi, usually in the 800 to 1200 psi range.

The other issue is variability. If the mason did not use a calibration box (1 cubic foot box to "calibrate" the use of shovels in loading the mixer), you can easily get a lower strength mortar in the mornings (sand is damp and piles up on the shovel) as compared to the afternoon (sand is dry and slides off the shovel). So sampling time is somewhat critical as well.

I prefer the use of 3" dia. cylinder molds for mortar testing. I believe the results to be more accurate and consistent; however, the standard test is to use 2" cube molds.

Check with the lab to find out when they sampled and how they did the test. If they used large brass "gang" molds for the samples, they should be left in the field an extra day to prevent the mass of the mold from disturbing the sample during movement.

 

[weab]

wayniac - Last week, I was just writing a project spec for this issue. There is an ASTM spec (I forget the number) for testing and it says that you should test at 7 days & 28 days, maybe more often. But I know of no ASTM spec that says what you should do should the tests not meet spec. I could not answer your exact question.

I'm not able to offer help, but perhaps someone knows an applicable ASTM spec that answers this question.

 

[concretemasonry]

Mortar is easy to specify, sample, require tests for and even relatively easy to test for, but it is a very small sample in comparison in relationship to the the project and the wall itself. It is also not cured in the same manner (moisture/absorption of adjacent materials) as it is used in the real world.

This of similar where the specifications for grout which require a high slump (8"-11") to insure proper construction and rely on the absorption of the masonry units to increase the the bond and real strength. At least the grout specifications require a sample preparation and curing similar to the real application and do not specify steel or non-absorptive molds.

Mortar compressive strength is a very small factor in the strength of a masonry wall. I have made/tested HOLLOW 8" block prisms that tested at 4750 psi (f'm) on the NET area made using a mortar sample that tested 2200 psi. The fallacy is that the prism is representative of the the of the strength of the wall and not the individual materials.

A mortar cube (1:1:1 ratio or so for ease of preparation) fails in a totally different method than a prism fails since it is relatively thin in comparison to the width of the mortar bed carrying the prism load that fails in something like a shear failure similar to a concrete cylinder.

Why the emphasis on the compressive strength of a small sample when ASTM suggests in the ASTM C270 standard appendix, that the lowest strength mortar be used to carry the load (determined by prism strength), since the other properties can be more important?

Is it just a testing/specification situation?

Dick

 

[darthsoilsguy2]

the test ASTM 780 is the field test for sampling mortar for compression testing. It states in bold letters to not use it for mortar acceptance. the test for mortar acceptance are C270 and C109. the lab test is run on a controlled sample of mortar cement (or masonry cement) on a proportioned amount of mortar sand and water. The acceptance test for mortar has nothing to do with what's going on in the field. mortar is deemed acceptable by testing performed on silos of cement by the manufacturer. imagine if the same were true for concrete....

Here's the deal... ASTM use to stand for American SOCIETY for Testing and Materials, before it became ASTM International. This society is made up of members from all industries and the standards and tests go through a consensus approval and revision method. ASTM has proportionally speaking very few members who are involved in the testing for acceptance business. There are a lot of members who are involved in manufacturer QC. C780 is the best the consensus can come up with. I would personally like to see everybody get together and agree to a REDUCED ACCEPTANCE psi for both batch samples and board samples of field sampled mortar.

Here's the dance:
1.Master spec calls for C780
2.Designer (Arch, Egr, or the Secretary) leaves it in
3.Testing company takes a sample and it fails
4.Everybody starts to question constructed walls
5.The contractor takes the position that the test is invalid. Which may occur only after they have spoke with their mortar cement supplier. This means cutting prisms of constructed masonry will be taken off the table (unless you want a big change order)
6.Walls stay. No more mortar is tested. Testing company looks like a bunch of jacka$$es.

Here's a concept for design professionals out there that use the ACI masonry code in their design.... Why are you specifying so much testing? Most projects i see have Level 3 Masonry TESTING and Level 1 Masonry INSPECTION. Taking samples every 5000 sqft of wall is a LEVEL 3 testing requirement. Level 3 INSPECTION requires CONTINUOUS observation of block placement, mortar joints, and obviously the grout....My most intensive masonry projects require Level 2 inspection (periodic block, joints, reinforcing, and continuous grout), however the testing requirement by code for LVL 2 is met by pre-construction testing.

back to the point... wayniac3 should go back to the site with a camera and observe mortar batching. If there is no box (1 ft3) to measure sand, document it on your field report and take a photo of the shovel guy. If there is no box, the design team will have more leverage to demand saw-cut prism testing.

If i were on the design team, i would view it quietly as acceptable... but make a stink about it to keep the masons on their toes about proper batching.